Coronal Restoration










Coronal
Restoration
17
2
De nition and Classi cation
Single-tooth rehabilitation is a signi cant area of dental technology work. When an in-
dividual tooth is so damaged by caries, fractures, or other harmful factors that other
dentistry measures are no longer able to preserve the tooth, an arti cial crown may be
placed on the prepared tooth like a cap. With this type of prosthetic single-tooth resto-
ration, the masticatory function and health of the existing teeth can be maintained and
restored.
An arti cial dental crown must take on the functions of a natural crown and must ac-
curately reproduce the ideal functional form of the natural tooth shape. Accurate knowl-
edge of tooth morphology is therefore an essential requirement for dental technicians
when fabricating a coronal restoration. Every tooth has speci c functional shape char-
acteristics that must be addressed by coronal restoration. Arti cial crowns have many
important functions, as outlined below. Figure 2-1 provides an overview of the various
types of arti cial crowns.
The occlusal surfaces of arti cial crowns adapted to the antagonists should do the
following:
• Achieve full functional contact
• Stop jaw movement
Allow transfer of forces to the periodontium during mandibular movements with tooth
contact (Fig 2-2)
• Allow interference-free gliding without overloading the periodontium

18
Coronal Restoration
Fig 2-1 Diagram of crown types.
Three-quarter crowns
Placed on occlusal, lingual,
and approximal surfaces
outside visible area
Full crowns
Protective, replacement, and anchoring crowns
– Completely cover tooth preparation
– Retained by static friction and gripping effect
Partial crowns
Replacement crowns, rarely as anchoring crowns
– Partially cover the tooth
Retained by static friction to parallel tooth
preparation
Post crowns
Replacement and protective crowns, rarely as
anchoring crowns, placed into the opened
root canal
– Retained by screw thread and static friction
Metal full-coverage
crowns
Massive or reduced full-
cast crowns on tooth
preparations with chamfer
From one
material
All-ceramic crowns
(jacket crowns)
On tooth preparations
with shoulder
Acrylic resin crowns
(jacket crowns)
On tooth preparations
with shoulder
From material
combinations
Cast full-coverage
crowns
Veneered with
tooth-colored material
Metal framework
With cured-on acrylic
resin mainly
partial veneer
Metal framework
With  red-on ceramic
partial or full veneer
Half-crown or
open-face crown
Occlusal and lingual tooth
surfaces are encased
in metal
Four- fths crowns
Placed on occlusal, lingual,
and approximal surfaces
with a vestibular window
Core crown or
buildup
Used as cast posts
with core buildups
Buildups
Placed with prefabri-
cated posts and
plastic buildups
Cap crown
Cast post with
veneered crown
Dowel crown
Veneered metal
framework; outdated
Seven-eighths crowns
For molar crowns;
mesial vestibular surface
remains clear

19
Denition and Classication
The precise anatomical surface curvature of the
articial crown does the following:
• Protects the marginal periodontium (Fig 2-3)
• Creates approximal contacts
• Protects the interdental papilla (Figs 2-4 to 2-7)
• Guarantees support in the dental arch
Aids self-cleaning of the masticatory system
(Figs 2-8 and 2-9)
• Fullls esthetic demands
• Supports phonetic functions
A precise and accurate t enables the articial
crown to do the following:
• Form a unit with the prepared tooth
• Preserve the tactile sense (Fig 2-10)
• Prepare food for digestion
Fig 2-2 When fabricating an
articial occlusal relief, the
functional surfaces must be
adapted to the antagonists to
ensure precise transfer of forc-
es. Incorrect contouring of oc-
clusal surfaces leads to faulty
contacts and harmful trans-
verse thrust. A lack of occlusal
contacts can result in displace-
ment of teeth.
Fig 2-3 The anatomical surface bulges are functional shape character-
istics that must be reproduced in articial crowns. The so-called vertical
curvature characteristics serve to protect the marginal periodontium;
excessive curvatures produce undercuts that inhibit self-cleaning of the
teeth. The horizontal curvatures of the vestibular surfaces also have to
be reproduced to ensure that no niches are formed where contami-
nants may accumulate.
Fig 2-4 The approximal surfaces form the approxi-
mal contact point that covers and protects the in-
terdental papilla. When this contact is reproduced
in articial crowns, space must be created for the
interdental papilla.

20
Coronal Restoration
Fig 2-5 If the interdental papilla is well
preserved, punctate shaping of the ap-
proximal contact is enough to maintain
the protective function.
Fig 2-6 If the interdental papilla is re-
duced, the approximal contact must have
a wider shape to secure the protective
function.
Fig 2-7 If the interdental papilla is re-
duced and shaping of the approximal
contact is punctate, food is no longer de-
ected away from the interdental space.
Fig 2-8 The approximal contact points viewed
occlusally lie in the direction of the buccal cusps
so that smaller niches are formed on the vestibu-
lar rather than the lingual side. The lingual areas of
the teeth are easier to clean because of the action
of the tongue.
Fig 2-9 Contact points that overhang too much will create large interdental
niches that are no longer lled with tissue. Deposits can accumulate because
self-cleaning is prevented. This can result in chronic inammation, and dam-
age to the periodontal tissue cannot be ruled out.
Fig 2-10 Accuracy of t is not merely a require-
ment that sets the standard for technical expertise
but also a functional necessity. Precise accuracy
of t allows the tooth to preserve a tactile sense
and, as a mechanical unit, allows smooth transfer
of forces. In the marginal area, accuracy of t pre-
vents damage to the marginal periodontium.

21
Denition and Classication
All of these functions are equally important and
must be fullled within the broad range of appli-
cations for articial crowns.
Classication of articial crowns is based on
their specic range of functions:
Replacement crowns replace lost hard substance
of the tooth, which can no longer be restored by
other (conservative) dentistry measures.
Protective crowns protect the tooth preparation
against harmful inuences (caries or defects
caused by clamps) by completely covering the
organic dental substance (Fig 2-11).
Supportive or anchoring crowns support and
anchor xed partial dentures and partial pros-
theses as abutments or carriers for attachments
or prosthetic auxiliaries (Fig 2-12); they are xed
to the prepared tooth.
Full crowns cover the clinical tooth preparation
completely, while xation (retention) is achieved
by static friction resistance and a gripping effect
(Fig 2-13).
Fig 2-11 Articial crowns replace lost hard tissue from teeth.
If natural dental crowns are partly destroyed by caries and a
tooth needs to be protected against further harmful inuences,
a protective crown should be fabricated that fully covers the
natural dental crown.
Fig 2-12 Articial crowns can be used to bear retention parts.
They are also used to protect teeth that will receive clamps.
Generally speaking, anchoring crowns are associated with par-
allel ts, tapered designs, or prosthetic auxiliaries.
Fig 2-13 Retention of articial crowns onto the tooth preparation can be achieved by two physical mechanisms: static friction and a
gripping effect. Both are produced by the complete enclosure involved in a full crown and guarantee secure retention of the crown.

22
Coronal Restoration
Partial crowns only partially cover the prepared
tooth—usually lingually, occlusally, and approx-
imally—in order to preserve the natural dental
substance and color in the labial and buccal
area (Fig 2-14). Retention is achieved by static
friction resistance of parallel surfaces, grooves,
and pins.
Post crowns come in the form of dowel crowns
and cast coping crowns or (root) buildups. These
constructions involve inserting a post into the
exposed pulp canal, which seals the root canal
and bears a core buildup for the actual crown
(Fig 2-15). The post is held in the root canal via
a screw thread, static friction resistance, and a
gripping effect.
Full and partial crowns are made from a variety
of materials:
• Metal (full-coverage cast)
• Ceramic (red, pressed, milled)
• Acrylic resin (polymerized)
Full and partial crowns (including post crowns)
made from combinations of materials are ve-
neered metal frameworks with red-on ceramic
or cured-on acrylic resin.
Indications for Coronal
Restoration
Coronal restoration is indicated whenever the
biomechanical and hence supportive function
within the dental arch has to be secured. Simi-
larly, coronal restoration is performed as a thera-
peutic function to stop any deterioration of the
dentition that has already begun. A coronal res-
toration should also fulll prophylactic functions,
stop secondary damage, and prevent disease-
related changes, thus taking on a protective func-
tion. Coronal restoration can improve or restore
the function of the masticatory system and there-
fore has a regulating function (see Fig 1-10).
Fig 2-14 If a dental crown is destroyed by fractures only in
isolated places, the missing hard substance can be replaced
by a partial crown. These partial crowns do not cover the tooth
completely but only individual surfaces of the dental crown.
Fig 2-15 If the dental crown is completely destroyed, a post
crown can be fabricated by inserting a post that will bear the re-
placement crown into the opened root canal. A long, accurately
tting post creates adequate static friction. The post with the
core buildup and the replacement crown are fabricated sepa-
rately.

23
Preparation of a Tooth
Substance loss from a tooth due to caries or
fractures makes coronal restoration necessary.
The remaining tooth preparation must be stable
and offer adequate retention, while the periodon-
tium must not be damaged. In this situation, the
articial crown mainly performs a protective
function against advancing organic decline and
replaces lost tissue. Abrasion of the incisal edges
and the occlusal relief requires coronal restora-
tion if the entire occlusal eld needs to be correct-
ed. Here the articial crown takes on a replace-
ment and supporting function if missing supports
from centric occlusion need to be built up again.
Completing an interrupted dental arch with a
xed or removable partial replacement can be
done by coronal restoration. The articial crown
may be a xed partial denture abutment, a tele-
scopic anchoring crown, a protective crown for
clamps, or an anchoring crown for attachments.
In this process, the articial crown takes on a
protective or supporting function in combination
with several teeth.
Coronal restoration is also carried out when
there are esthetic concerns (eg, morphologic de-
fects, discoloration, or positional anomalies) if
orthodontic measures are ruled out. In this situa-
tion, the articial crowns take on regulating tasks
because they fulll replacement, protective, and
supportive functions. Faulty tooth shapes are
always associated with functional deciencies,
while discolorations may be an indication of de-
stroyed pulp. Positional corrections help to pre-
serve the periodontium, support the self-cleaning
function, and restore the function of the closed
dental arch.
A risk-benet assessment should be under-
taken for every dental procedure. If an articial
crown is being fabricated, a sufcient quantity of
natural dental tissue has to be removed, which
means pulp damage may have already occurred
when the tooth was being prepared. Thus, coro-
nal restoration is contraindicated, for instance,
for adolescent teeth with a large pulp cavity and
wide dentinal tubules as well as incomplete root
growth. Coronal restoration is also contraindicat-
ed if the tooth has characteristics of disease such
as pathologic apical processes, incomplete end-
odontic treatment, or inammatory changes to
the marginal periodontium. These problems must
rst be treated and cured.
Tooth mobility, gingival and bony pockets, and
resorption of a socket beyond the apical third of
the root are also regarded as contraindications,
as is excessive loss of substance from the tooth
preparation, because there is no longer sufcient
mechanical retention to hold the crown in place.
For esthetic reasons, fabrication of a full-metal
crown may be contraindicated in the anterior re-
gion. Certain types of crown may thus be contra-
indicated, although this is subject to the dentist’s
own judgment and is not necessarily discernible
on a working model. Determining when a coronal
restoration is indicated remains a matter of the
dentist’s expert diagnosis, but dental technicians
should be aware of the general criteria for using
coronal restorations.
Poor oral hygiene is always a contraindication
for coronal restoration because deposits (plaque)
lead to caries and periodontal disease. Before
coronal restoration is considered, the patient must
rst be motivated to take adequate oral hygiene
measures after receiving suitable dental educa-
tion and instruction.
Fabrication of a crown is the result of collab-
orative teamwork between the dentist and the
dental technician. The tooth is rst prepared, and
an impression is taken so that the technician can
prepare a working model; adjust it in an articula-
tor; and carve, cast, and nish the articial crown.
The dentist cements the nished crown in place
on the tooth preparation.
Preparation of a Tooth
If a tooth is to be tted with a crown, it must be
prepared; that is, enough dental tissue must be re-
moved to enable the articial crown to be pushed
over the tooth. Pushing a thin metal sleeve over
an unprepared tooth would enlarge the natural
tooth by the thickness of the metal plate. It would
then interfere with the opposing dental arch,
protrude out of the arch, and, if the sleeve could
actually be pushed over the approximal contact
points, cause faulty relationships. Furthermore,
the sleeve would not t closely in undercut areas.
This is why a tooth needs to be prepared before
receiving a crown.

24
Coronal Restoration
The aim of preparation is to remove the dam-
aged dental substance and, if necessary, remove
enough healthy dental tissue to ensure that the
widest circumference of the tooth lies at the low-
est point on the tooth preparation. An attempt is
made to smooth the area around the tooth with-
out creating any undercuts.
One approach to preparation is to create a cylin-
dric preparation that would have almost parallel
walls and would result in the least amount of hard
tissue loss (Figs 2-16 and 2-17). As exact parallel-
ism is impossible to see with the naked eye, this
kind of fabrication would be at the limit of what
is technically feasible. Furthermore, errors would
arise when taking an impression and cementing
the crown in place because of the piston effect of
a parallel t.
A better preparation design is a slightly conical
(ie, tapered) preparation with a preparation angle
between 3 and 8 degrees toward the occlusal sur-
face. This design allows interference-free, accu-
rate impression-taking and guarantees adequate
retention due to static friction and a gripping ef-
fect (Figs 2-18 to 2-21). When the restoration is
being cemented in place, the cement is able to
ow away more easily until tactile contact of all
surfaces is achieved, leaving a minimum thick-
ness of cement equivalent to its grain size, which
increases the friction and gripping resistance.
A basic tooth preparation has the following
characteristics:
At its widest circumference, it meets the prepa-
ration margin.
There is sufcient height for mechanical reten-
tion.
The shape is slightly conical and has an angle of
3 to 8 degrees.
• The preparation surfaces are not undercut.
• An interocclusal rest space is prepared.
Fig 2-16 Preparation of a cylindric stump is at the limit of what is technically feasible because precise parallelism cannot be seen
with the naked eye. The preparation shapes illustrated here are achieved by chance.
Parallel preparation Inverted cone preparation Conical preparation
Fig 2-17 A preparation with parallel walls requires
less loss of hard substance than a tapered prepara-
tion, and the risk to the pulp is also lower.

25
Shaping the Preparation Margin
Shaping the Preparation
Margin
The preparation margin is the border between
the prepared and unprepared tooth surface (Figs
2-22 to 2-24). It is the responsibility of the dentist
to make a clear preparation margin, and it is the
responsibility of the dental technician to ensure
that the crown margin ends exactly at the prepa-
ration margin. Thus, the technician is less focused
on how far a tooth is prepared at the neck or how
deep or high the preparation margin is in relation
to the marginal periodontium. However, if the
crown margin lies above the preparation margin,
caries will ensue. If it lies below the preparation
margin (ie, in the gingiva) damage to the gingival
margin will occur.
For the purposes of accurate and safe fabrica-
tion, a supragingival preparation margin is cho-
sen, which is markedly above the gingival crev-
ice. This means that the crown margin is kept
clear of the periodontium and cannot damage the
Fig 2-18 A tall tapered preparation with
a preparation angle up to 6 degrees of-
fers the best retention for a coronal res-
toration. The crown cannot tip off the
preparation even with eccentric loading.
Fig 2-19 A very steeply tapered prepa-
ration with a preparation angle greater
than 6 degrees offers less retention for
the coronal restoration. The crown can
tip off the preparation if exposed to ec-
centric loading.
Fig 2-20 A very tapered and very short
preparation offers the least retention
for a coronal restoration. Even slight ec-
centric loading will tip the crown off the
preparation.
Fig 2-21 A cylindric preparation with parallel walls will result
in a so-called piston effect during impression-taking and crown
placement, making these processes extremely difcult if not
impossible. An impression can easily be taken of a tapered
preparation, and the crown can be placed and cemented in
place with equal ease, without causing any buildup of cement.

26
Coronal Restoration
periodontal tissue, the critical area can be kept
clean, and it is easier to take an impression of the
preparation margin.
However, supragingival preparation has seri-
ous drawbacks: If the margin is placed too high,
the tooth preparation becomes too short and
does not offer enough mechanical retention for
the crown. If the nish line is in the visible region,
the esthetics will be unsatisfactory. For cervical
defects, the preparation must follow an infragin-
gival course, just as infragingival nish lines offer
better caries protection in dentitions with active
caries.
During crown fabrication, special attention
must always be paid to the crown margin, which
is dened by the preparation margin. The quality
Fig 2-22 A gingival preparation margin
runs level with the gingival margin but
not deep in the gingival crevice. As a
result, the marginal periodontium is un-
touched and undamaged. The eventual
crown margin can be clearly seen and is
usually esthetically satisfactory.
Fig 2-23 A subgingival preparation mar-
gin runs deep in the gingival crevice and
affords very good caries prevention pro-
vided the position of the crown margin
ts accurately. Because it is difcult to
check the crown margin, however, irri-
tation of the marginal periodontium and
even gingival retraction may occur.
Fig 2-24 A supragingival preparation
margin lies well above the gingival mar-
gin. This course is very benecial for
periodontal hygiene but unsatisfactory
esthetically, and it provides no protection
against caries in the presence of plaque.

27
Forms of Preparation Margin
of the transition from tooth to coronal restorative
material is determined by how accurately the
crown margin ts the preparation margin. The ac-
curacy that is achievable depends on the shap-
ing of the nish line. At the preparation margin,
the crown material should not overlap the tooth
preparation but should be sunk into the tooth.
Three forms of preparation margin are used: (1)
tangential preparation, (2) chamfer preparation,
and (3) shoulder preparation.
Forms of Preparation Margin
A tangential preparation is ground like a tapered
margin (Fig 2-25). Tangents can be placed in a
vertical direction all over the basic conical shape
of the tooth preparation. Ideally, the preparation
margin should describe the root cross section
and the crown margin nish line. The preparation
margin nish line should lie at the bottom of the
gingival crevice; subgingival preparation is only
done for protective crowns in dentitions with ac-
tive caries.
Given this preparation nish line, the crown
margin must taper off sharply and evenly while
tting closely; the crown must then be given a
convex shape above the gingival crevice. The
crown itself must have an occlusal support so it is
not pushed beyond the preparation margin.
The coronal restorative material overlaps the
tooth preparation. However, as it has to taper
thinly, it may become frayed and bend upward.
The articial crowns have to be overcontoured at
the preparation margin, thereby forming a step
that encourages plaque accumulation.
The tangential nish line is easy to prepare
and requires the least loss of dental substance.
This preparation nish line is barely visible in the
mouth and not at all visible on the working mod-
el. A precise crown margin cannot be created and
Fig 2-25 In tangential preparation, the
widest circumference of the conical
preparation and the preparation margin
coincide. The coronal restorative material
overlaps the tooth and can displace the
marginal periodontium in some circum-
stances. If the crown margin is shaped
like a tapered margin, it runs razor-sharp
into the depth of the gingival crevice. This
tapered margin is rarely stable enough
not to deform when the crown is tted.

28
Coronal Restoration
only happens by chance. This approach is used
for young teeth with a large pulp chamber so that
acrylic resin crowns can be made as temporary
replacements.
Owing to the lack of available space, this form
of preparation is not suitable for metal-ceramic
restorations, all-ceramic restorations, or for full-
cast crowns. The tangential form of preparation
cannot offer static support against occlusal forces.
Shoulder preparation forms a circular step
around the conical tooth preparation. This offers
static support to the coronal restorative mate-
rial, which is particularly valuable for ceramic or
acrylic resin crowns. The material for the articial
crown is sunk into the dental tissue and forms a
ush transition between tooth and coronal resto-
ration (Fig 2-26).
The preparation margin can be clearly seen on
the model and thus allows for precise working. A
shoulder preparation is easy to produce because
the preparation tools have a cervical guide.
In the case of shoulder preparation, the tooth
preparation becomes much smaller than the root
cross section, and there is considerable loss of
substance from the tooth. The stability of the tooth
preparation may be reduced, putting the pulp at
risk (Fig 2-27). To prevent a buildup of cement on
insertion, the start of the shoulder is chamfered to
the tooth preparation.
The edge of the shoulder can be beveled slight-
ly to the gingival crevice to optimize the marginal
Fig 2-26 To sink the coronal restorative
material into the tooth preparation, a
shoulder is created for the preparation
margin. The coronal restorative material
ends ush with the tooth, usually in the
depth of the gingival crevice. The crown
margin does not occupy any more space
than the natural dental substance. To en-
sure stability, the shoulder slopes slightly
inward toward the tooth. If the crown is
pressed onto the tooth by masticatory
pressure, the crown material will slip
inward like an inclined plane and will be
pressed against the tooth.
Fig 2-27 The thickness of dental hard tissue over the pulp of
a canine. The dental tissue is considerably reduced by shoul-
der preparation, which can damage the pulp. In general, this
means the tooth is mechanically weakened.

29
Forms of Preparation Margin
Fig 2-29 If a shoulder preparation is
straight, a vertical marginal gap defect
will occur because the crown cannot
be lowered as far as the shoulder due
to the thickness of the cement. The ce-
ment will also be washed out by normal
teeth-cleaning in a horizontal shoulder ar-
rangement.
Fig 2-30 With a 45-degree bevel to the
shoulder, the vertical marginal gap defect
is markedly reduced, and the cement
cannot be washed out as easily. This bev-
el is sufciently identiable in the mouth
and is visible enough on the model so
that the preparation margin can be read-
ily exposed.
Fig 2-31 The vertical marginal gap de-
fect is the smallest with a very steep
bevel (more than 45 degrees). The exact
path of the preparation margin of the
bevel is very difcult to identify, similar to
tangential preparation.
t, reduce the interface between tooth prepara-
tion and crown, and move it out of the visible
area (Figs 2-28 to 2-31). Preparation of the bevel
is difcult and can blur the exact borderline on
the model.
Figs 2-28 Modern shoulder preparation involves shaping the transi-
tion to the preparation wall as a chamfer to avoid any buildup of ce-
ment on insertion of the crown, which can happen with a sharp-edged
preparation. The shoulder margin is also beveled to reduce the mar-
ginal gap.

30
Coronal Restoration
Shoulder preparation is generally necessary
when jacket crowns are being fabricated from
acrylic resin or ceramic. These materials need a
specic minimum thickness at the crown margin
for reasons of stability and color quality. Shoul-
der preparation is also appropriate for veneer
crowns. The crown margin always ends ush with
the shoulder, does not protrude horizontally, and
does not sit too narrowly on the shoulder. Under
no circumstances should the crown margin ex-
tend beyond the shoulder.
Chamfer preparation or veneer preparation
happens when the conical tooth preparation is
worked with suitably shaped abrasive tools so
that a gentle circular chamfer is created (Figs
2-32 and 2-33). As a result, the coronal restorative
material is moved into the tooth and ends ush.
More dental substance is removed than with tan-
gential preparation; consequently, the prepara-
tion nish line is clearly visible in the mouth and
on the working model.
The chamfer offers adequate material thickness
for metal frameworks, but there is not enough
space for full crowns made of acrylic resin or ce-
ramic. This preparation is suitable not only for
veneer crowns—when a pronounced shoulder
must be dispensed with because of the greater
loss of substance—but also for full-cast crowns.
Fig 2-32 Chamfer preparation is a prov-
en compromise between tangential and
shoulder preparation. This form of prepa-
ration margin is used for full-cast crowns
and veneer crowns, although the area to
be veneered is prepared in a shoulder
design. Chamfer preparation does not
require as much substance loss as the
shoulder form and produces a similarly
precise marginal course that is clearly
visible on the model and allows precise
working.
Fig 2-33 Chamfer preparation does not provide good static support to the crown
over the crown margin and cannot prevent a reduced crown from sinking, so an oc-
clusal stop is added for a reduced full-cast crown. This crown will not sag occlusally
on loading.
Fig 2-34 For veneer crowns, the tooth
structure in the area of the veneer sur-
faces should be prepared with a shoulder
that turns into a chamfer approximally
and incorporates the lingual surfaces. As
a result, less tooth substance is removed
lingually, and the tooth preparation is
more stable. This mixed form of chamfer
and shoulder preparation is the usual ap-
proach for anterior and posterior teeth.

31
Phases of Tooth Preparation
The mixed form of chamfer and shoulder prep-
aration results from a shoulder preparation that
follows a vestibular course and turns into a cham-
fer approximally, which is then continued lingual-
ly (Fig 2-34). This method results in less loss of
substance from the tooth preparation than with
shoulder preparation alone. Mixed preparation is
mainly used for veneer crowns to sink the mate-
rial into the tooth in the visible vestibular region.
Phases of Tooth
Preparation
Tooth preparation is not part of the work of a den-
tal technician. The following description is for in-
formational purposes only; it gives an overview
of what a dentist does and should enhance un-
derstanding of the collaborative relationship be-
tween dentist and dental technician.
The tooth is prepared under water cooling with
specially designed rotary instruments, usually
diamond-tipped burs of varying grit size. Stan-
dardized preparation kits contain cylindric prepa-
ration tools with working parts approximately 4
to 8 mm long and special shapes such as pointed
conical, needle-shaped separator, rounded bud-
shaped, and ball-shaped burs. The instrument
shafts are designed for micromotor-driven contra-
angle handpieces or for ball-bearing or air-bearing
turbines.
Preparation is done in the high-speed range
(160,000 to 450,000 rpm), at which only minimal
working pressure is required, no vibrations occur,
and the treatment time can be very short. High-
speed instruments require water-spray cooling.
Use of a rotary instrument without water cooling
would result in pulp damage due to friction heat.
Even briey exceeding a temperature of 51.6°C
causes protein coagulation. Following is a brief
outline of the phases of tooth preparation.
1. Preparation of the approximal
surfaces
A needle diamond (separator) is used to sepa-
rate the approximal surfaces. The purpose of this
phase is to separate or clear a space around the
tooth being prepared in the dental arch so that
the adjacent teeth are not accidentally damaged
during subsequent preparation (Figs 2-35 to 2-37).
2. Marking of depth
In order for the dentist to know how much den-
tal substance to remove in subsequent phases
of preparation without injuring the pulp, depth is
marked with a groove-cutting or step bur. These
instruments have a depth marking or a depth
stop. They are used to create one or more 1-mm-
deep grooves over the entire occlusal surface and
onto the vestibular and lingual surfaces as far as
the height of contour.
Figs 2-35 and 2-36 Preparation is done under water cooling, and the buccal and
lingual vertical surfaces are prepared rst.
Fig 2-37 Separation: The approximal
surfaces are prepared to obtain a gently
conical tooth preparation. The adjacent
teeth must not be damaged in the pro-
cess.

32
Coronal Restoration
3. Preparation of the occlusal
surface
The occlusal surface is removed as far as the
depth marking without injuring the adjacent teeth
(Figs 2-38 and 2-39). Depending on the design
and the material used for the articial crown,
adequate interocclusal rest space should be cre-
ated. The occlusal relief should retain its basic
morphology (eg, cusp size, ssure conguration).
The dental arches are checked in working and bal-
ancing positions to ensure that the preparation is
adequate.
4. Preparation of buccal and oral
surfaces
A cylindric bur that is angled or rounded at the tip
is used for gentle conical reduction of the buccal
and oral surfaces to just above the gingival crev-
ice. This vertical circulatory preparation follows
the curved path of the gingival attachment.
5. Preparation of the cusp bevel
The cusp bevel at the transitional surfaces to the
vertical surfaces is prepared with special tapered
Fig 2-38 Occlusal preparation: The tran-
sitions between the occlusal and vertical
surfaces are ground, creating an occlusal
bevel.
Fig 2-39 The occlusal surface is ground
down. Adequate interocclusal rest space
should be created and the angle of cus-
pal inclination maintained.
Fig 2-40 Lowering and precise regrind-
ing of the path of the preparation margin
is done with a suitable abrasive tool, in
this case a tapered torpedo.
Fig 2-41 (left) The preparation target
is checked with a probe. There must be
no undercuts on the vertical surfaces.
A conical preparation with a maximum
preparation angle of 6 degrees is cre-
ated.
Fig 2-42 (right) Insertion groove parallel
to the path of insertion for clear-cut xa-
tion of the crown on eventual insertion.

33
Impression of the Prepared Tooth
instruments. This allows the cusp ridges, which
have been displaced outward by the occlusal sur-
face preparation, to be corrected into their proper
position. Cusp ridges and cusp tips are moved to-
ward the central ssure.
6. Preparation of the nish line
The nish line at the gingival margin is prepared
with suitable instruments. For tangential prepara-
tion, the nish line is relocated into the oor of
the gingival crevice with a pointed tapered abra-
sive (Fig 2-40). For chamfer preparation, a thicker
conical tool (tapered torpedo) is used to create a
clear nish line in the oor of the gingival crevice.
The shoulder is mimicked with a rounded roller to
create a chamfered transition to the vertical prep-
aration surfaces and an almost horizontal step. In
this phase, the gingival margin must be pushed
back. Retraction cords have proved effective for
this, or the pressure from the cooling water jet
can also push back the gingival margin.
7. Smoothing of surfaces
The tooth preparation and transitions are smooth-
ed using a cylindric nishing diamond with a very
ne grit. The surface of the tooth preparation is
checked for undercuts using a probe held cross-
wise (Fig 2-41). For safety, an insertion groove
can be cut onto a vertical surface, allowing clear-
cut xing of the crown on insertion (Fig 2-42).
Impression of the
Prepared Tooth
After preparation is completed, a check impres-
sion can be taken to produce a preparation check
model. This allows the dentist to make an accu-
rate check of the tooth preparation and identify
any corrections needed.
An accurate model of the prepared tooth is re-
quired to fabricate articial crowns (Fig 2-43). This
means that a specic impression needs to be tak-
en using a ring-supported single-preparation im-
pression or an integrated complete impression,
which is subdivided into single-phase (mono-
phase) and dual-phase impression-taking.
Single-phase impression techniques use either
one owable material or two owable materials
simultaneously (double-mix technique). In dual-
Gingival situation
Path of preparation
margin
Unprepared dental
arch
Prepared abutment
Edentulous jaw
segment
Positional
relationship of
abutments
Fig 2-43 When a coronal
restoration is produced, the
impression needs to provide
an accurate representation
of the intraoral situation, and
this must be reproduced in a
working model. The impres-
sion and model must accu-
rately depict the prepared
abutments with the path of
the preparation margin and
the gingival situation as well
as the unprepared dental
arch, the positional relation-
ship of the abutments, and
the edentulous segments of
the jaw.

34
Coronal Restoration
phase impression-taking, an initial impression
made of harder silicone is corrected with a low-
viscosity second material (eg, correction impres-
sion, double impression). The entire occlusal area
of the jaw and the antagonists are included.
A ring-supported impression is taken in two
working phases:
1. The actual impression of the tooth preparation
is taken with a copper-ring impression made of
thermoplastic impression material or silicone,
where the ring acts as a carrier for the impres-
sion material.
2. An impression of the entire jaw is taken with
the ring in place in a combined impression with
a prefabricated impression tray.
A matching ring is cut to t the path of the cer-
vical margin. Enough space for the impression
material is left between the tooth preparation
and the ring. When the cervical edge of the ring
is adapted to the gingival contour, it is drawn in
toward the prepared tooth. After a dry eld is cre-
ated and the prepared tooth is isolated, the ring
is pushed as far as the preparation margin, and
the heated plastic impression material is pressed
occlusally as far as the edge of the ring. If low-
viscosity silicone is used for the impression, the
ring is sealed occlusally with wax, the impression
material is poured in, and the ring is placed on
the tooth preparation. A combined impression is
usually taken with viscous silicone materials over
the ring that is already in place. A ring-free im-
pression is an integrated complete impression. It
requires widening of the gingival crevice by suit-
able methods; otherwise, the impression material
will not penetrate the gingival crevice. The gingi-
val margin can be displaced by placing a retrac-
tion cord or elastic ring or by using a medication
that reduces the tissue tension in the gingival
margin.
A dual-phase correction impression rst in-
volves taking a primary impression out of viscous
silicone; this takes on the function of a custom
tray. The primary impression can be taken before
preparation so that the pre-preparation space can
be exploited to ensure tension-free reception of
the low-viscosity correction material.
If the primary impression is taken after prepa-
ration in order to prevent compressive stresses
from the primary material, the impression should
be cut out slightly in the preparation area, and
Fig 2-44 Taking an impression of a prepared
tooth and the whole jaw is a precondition for
an accurate working model. One method is a
double-mix impression in which two materials
are used to take an impression. Before taking
the impression, a low-viscosity material is applied
to the object from a disposable syringe. The gin-
gival crevice is widened beforehand so that the
impression material can reach the preparation
margin.
Fig 2-45 An impression tray is used to push the more solid second material
over the low-viscosity material. Both materials set at the same time but have
no effect on each other. The possibility of the impression materials warping
is ruled out. This method is suitable for silicone and hydrocolloid materials.

35
Impression of the Prepared Tooth
drainage grooves should be created for the low-
viscosity correction material. Only then is the
low-viscosity correction material poured into the
primary impression, which is briey pushed over
the prepared tooth under pressure but held in
place without pressure until it sets. Under pres-
sure, the secondary material may deform the pri-
mary material and distort the complete impres-
sion because of the latter’s recovery properties.
In the double-mix impression technique, two
impression materials are mixed at the same time:
a low-viscosity material from a special impres-
sion syringe and a higher-viscosity second ma-
terial for the impression tray (Fig 2-44). First the
gingival crevice is lled throughout with the sy-
ringed material, then the impression tray with the
high-viscosity silicone is immediately placed over
it (Fig 2-45). The two materials set simultaneously
without displacing each other; no elastoplastic
stresses arise between the two materials. This
working method is also used for hydrocolloid im-
pression material.
A split model can be produced with the impres-
sion: The dental arch is cast in dental stone, and
dowel pins are placed in the position of the pre-
pared tooth (Fig 2-46) with the aid of a light point
indicator (Pindex, Coltene). The dental arch is iso-
lated, and a model base is fabricated from stone
(Fig 2-47). After the plaster has set, the dies must
be cut out and the saw cuts taken down to the
plaster base (Fig 2-48). The separated die can be
lifted off the model and placed back into the mod-
el by means of the dowel pin (Figs 2-49 and 2-50).
Figures 2-51 to 2-56 illustrate the steps of taking
a ring impression. Figures 2-57 and 2-58 provide
helpful hints for marking the dies.
Fig 2-46 During impression-taking, the
dental arch and part of the alveolar ridge
(approximately 1.5 cm high) are cast. A
dowel pin is placed precisely in the pre-
pared tooth.
Fig 2-47 The other part of the dental
arch is tted with retention rings, the
plaster is isolated, and the dental arch is
based.
Fig 2-48 On the nished model, the
tooth die is sawn out, and the prepara-
tion margin is exposed.
Fig 2-49 In a split model, a preparatory
working step is to grind the preparation
margin clear from the adjacent gingival
area.
Fig 2-50 The saw cut should run paral-
lel so that the sawn die can be smoothly
taken out; the nish line of the gingiva on
the adjacent teeth must remain visible.

36
Coronal Restoration
Fig 2-51 The principle of a ring impres-
sion: A prefabricated ring that ts the
preparation circumference is adapted to
the path of the preparation margin.
Fig 2-52 The ring is pushed slightly be-
yond the preparation margin below the
gingiva; it lies close to the widest circum-
ference of the preparation.
Fig 2-53 Thermoplastic impression ma-
terial is pressed into the ring; the ring is
removed, checked, reheated, and rmly
put in place.
Fig 2-55 A multipart working model
can be produced with a ring-supported
impression. In what is known as a die
(preparation model), the prepared tooth
can be removed while the adjacent tis-
sues remain unharmed.
Fig 2-54 A combined impression is taken over the ring impression with silicone im-
pression material in a prefabricated impression tray.
Fig 2-56 (left) In the ring impression, a
tooth preparation with a conical root part
and dowel pin is rst prepared in model-
ing cement or dental stone. The prepa-
ration is isolated, and the combined im-
pression is cast.
Fig 2-57 (right) Marking a die involves
precisely depicting the path of the prepa-
ration margin and the spatial relationship
to the gingival margin, which must not be
ground free.

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Coronal Restoration172De nition and Classi cationSingle-tooth rehabilitation is a signi cant area of dental technology work. When an in-dividual tooth is so damaged by caries, fractures, or other harmful factors that other dentistry measures are no longer able to preserve the tooth, an arti cial crown may be placed on the prepared tooth like a cap. With this type of prosthetic single-tooth resto-ration, the masticatory function and health of the existing teeth can be maintained and restored.An arti cial dental crown must take on the functions of a natural crown and must ac-curately reproduce the ideal functional form of the natural tooth shape. Accurate knowl-edge of tooth morphology is therefore an essential requirement for dental technicians when fabricating a coronal restoration. Every tooth has speci c functional shape char-acteristics that must be addressed by coronal restoration. Arti cial crowns have many important functions, as outlined below. Figure 2-1 provides an overview of the various types of arti cial crowns.The occlusal surfaces of arti cial crowns adapted to the antagonists should do the following:• Achieve full functional contact• Stop jaw movement• Allow transfer of forces to the periodontium during mandibular movements with tooth contact (Fig 2-2)• Allow interference-free gliding without overloading the periodontium 18Coronal RestorationFig 2-1 Diagram of crown types.Three-quarter crownsPlaced on occlusal, lingual, and approximal surfaces outside visible areaFull crownsProtective, replacement, and anchoring crowns– Completely cover tooth preparation– Retained by static friction and gripping effectPartial crownsReplacement crowns, rarely as anchoring crowns– Partially cover the tooth– Retained by static friction to parallel tooth preparationPost crownsReplacement and protective crowns, rarely as anchoring crowns, placed into the opened root canal– Retained by screw thread and static frictionMetal full-coverage crowns Massive or reduced full- cast crowns on tooth preparations with chamferFrom one materialAll-ceramic crowns (jacket crowns)On tooth preparations with shoulderAcrylic resin crowns (jacket crowns)On tooth preparations with shoulderFrom material combinationsCast full-coverage crownsVeneered with tooth-colored materialMetal frameworkWith cured-on acrylic resin mainly partial veneerMetal frameworkWith  red-on ceramic partial or full veneerHalf-crown or open-face crownOcclusal and lingual tooth surfaces are encased in metalFour- fths crownsPlaced on occlusal, lingual, and approximal surfaces with a vestibular windowCore crown or buildupUsed as cast posts with core buildupsBuildups Placed with prefabri-cated posts and plastic buildupsCap crownCast post with veneered crownDowel crownVeneered metal framework; outdatedSeven-eighths crownsFor molar crowns; mesial vestibular surface remains clear 19Denition and ClassicationThe precise anatomical surface curvature of the articial crown does the following:• Protects the marginal periodontium (Fig 2-3)• Creates approximal contacts• Protects the interdental papilla (Figs 2-4 to 2-7)• Guarantees support in the dental arch• Aids self-cleaning of the masticatory system (Figs 2-8 and 2-9)• Fullls esthetic demands• Supports phonetic functionsA precise and accurate t enables the articial crown to do the following:• Form a unit with the prepared tooth• Preserve the tactile sense (Fig 2-10)• Prepare food for digestionFig 2-2 When fabricating an articial occlusal relief, the functional surfaces must be adapted to the antagonists to ensure precise transfer of forc-es. Incorrect contouring of oc-clusal surfaces leads to faulty contacts and harmful trans-verse thrust. A lack of occlusal contacts can result in displace-ment of teeth.Fig 2-3 The anatomical surface bulges are functional shape character-istics that must be reproduced in articial crowns. The so-called vertical curvature characteristics serve to protect the marginal periodontium; excessive curvatures produce undercuts that inhibit self-cleaning of the teeth. The horizontal curvatures of the vestibular surfaces also have to be reproduced to ensure that no niches are formed where contami-nants may accumulate.Fig 2-4 The approximal surfaces form the approxi-mal contact point that covers and protects the in-terdental papilla. When this contact is reproduced in articial crowns, space must be created for the interdental papilla. 20Coronal RestorationFig 2-5 If the interdental papilla is well preserved, punctate shaping of the ap-proximal contact is enough to maintain the protective function.Fig 2-6 If the interdental papilla is re-duced, the approximal contact must have a wider shape to secure the protective function.Fig 2-7 If the interdental papilla is re-duced and shaping of the approximal contact is punctate, food is no longer de-ected away from the interdental space.Fig 2-8 The approximal contact points viewed occlusally lie in the direction of the buccal cusps so that smaller niches are formed on the vestibu-lar rather than the lingual side. The lingual areas of the teeth are easier to clean because of the action of the tongue.Fig 2-9 Contact points that overhang too much will create large interdental niches that are no longer lled with tissue. Deposits can accumulate because self-cleaning is prevented. This can result in chronic inammation, and dam-age to the periodontal tissue cannot be ruled out.Fig 2-10 Accuracy of t is not merely a require-ment that sets the standard for technical expertise but also a functional necessity. Precise accuracy of t allows the tooth to preserve a tactile sense and, as a mechanical unit, allows smooth transfer of forces. In the marginal area, accuracy of t pre-vents damage to the marginal periodontium. 21Denition and ClassicationAll of these functions are equally important and must be fullled within the broad range of appli-cations for articial crowns.Classication of articial crowns is based on their specic range of functions:• Replacement crowns replace lost hard substance of the tooth, which can no longer be restored by other (conservative) dentistry measures.• Protective crowns protect the tooth preparation against harmful inuences (caries or defects caused by clamps) by completely covering the organic dental substance (Fig 2-11).• Supportive or anchoring crowns support and anchor xed partial dentures and partial pros-theses as abutments or carriers for attachments or prosthetic auxiliaries (Fig 2-12); they are xed to the prepared tooth.• Full crowns cover the clinical tooth preparation completely, while xation (retention) is achieved by static friction resistance and a gripping effect (Fig 2-13).Fig 2-11 Articial crowns replace lost hard tissue from teeth. If natural dental crowns are partly destroyed by caries and a tooth needs to be protected against further harmful inuences, a protective crown should be fabricated that fully covers the natural dental crown.Fig 2-12 Articial crowns can be used to bear retention parts. They are also used to protect teeth that will receive clamps. Generally speaking, anchoring crowns are associated with par-allel ts, tapered designs, or prosthetic auxiliaries.Fig 2-13 Retention of articial crowns onto the tooth preparation can be achieved by two physical mechanisms: static friction and a gripping effect. Both are produced by the complete enclosure involved in a full crown and guarantee secure retention of the crown. 22Coronal Restoration• Partial crowns only partially cover the prepared tooth—usually lingually, occlusally, and approx-imally—in order to preserve the natural dental substance and color in the labial and buccal area (Fig 2-14). Retention is achieved by static friction resistance of parallel surfaces, grooves, and pins.• Post crowns come in the form of dowel crowns and cast coping crowns or (root) buildups. These constructions involve inserting a post into the exposed pulp canal, which seals the root canal and bears a core buildup for the actual crown (Fig 2-15). The post is held in the root canal via a screw thread, static friction resistance, and a gripping effect.Full and partial crowns are made from a variety of materials:• Metal (full-coverage cast)• Ceramic (red, pressed, milled)• Acrylic resin (polymerized)Full and partial crowns (including post crowns) made from combinations of materials are ve-neered metal frameworks with red-on ceramic or cured-on acrylic resin.Indications for Coronal RestorationCoronal restoration is indicated whenever the biomechanical and hence supportive function within the dental arch has to be secured. Simi-larly, coronal restoration is performed as a thera-peutic function to stop any deterioration of the dentition that has already begun. A coronal res-toration should also fulll prophylactic functions, stop secondary damage, and prevent disease-related changes, thus taking on a protective func-tion. Coronal restoration can improve or restore the function of the masticatory system and there-fore has a regulating function (see Fig 1-10).Fig 2-14 If a dental crown is destroyed by fractures only in isolated places, the missing hard substance can be replaced by a partial crown. These partial crowns do not cover the tooth completely but only individual surfaces of the dental crown.Fig 2-15 If the dental crown is completely destroyed, a post crown can be fabricated by inserting a post that will bear the re-placement crown into the opened root canal. A long, accurately tting post creates adequate static friction. The post with the core buildup and the replacement crown are fabricated sepa-rately. 23Preparation of a ToothSubstance loss from a tooth due to caries or fractures makes coronal restoration necessary. The remaining tooth preparation must be stable and offer adequate retention, while the periodon-tium must not be damaged. In this situation, the articial crown mainly performs a protective function against advancing organic decline and replaces lost tissue. Abrasion of the incisal edges and the occlusal relief requires coronal restora-tion if the entire occlusal eld needs to be correct-ed. Here the articial crown takes on a replace-ment and supporting function if missing supports from centric occlusion need to be built up again.Completing an interrupted dental arch with a xed or removable partial replacement can be done by coronal restoration. The articial crown may be a xed partial denture abutment, a tele-scopic anchoring crown, a protective crown for clamps, or an anchoring crown for attachments. In this process, the articial crown takes on a protective or supporting function in combination with several teeth.Coronal restoration is also carried out when there are esthetic concerns (eg, morphologic de-fects, discoloration, or positional anomalies) if orthodontic measures are ruled out. In this situa-tion, the articial crowns take on regulating tasks because they fulll replacement, protective, and supportive functions. Faulty tooth shapes are always associated with functional deciencies, while discolorations may be an indication of de-stroyed pulp. Positional corrections help to pre-serve the periodontium, support the self-cleaning function, and restore the function of the closed dental arch.A risk-benet assessment should be under-taken for every dental procedure. If an articial crown is being fabricated, a sufcient quantity of natural dental tissue has to be removed, which means pulp damage may have already occurred when the tooth was being prepared. Thus, coro-nal restoration is contraindicated, for instance, for adolescent teeth with a large pulp cavity and wide dentinal tubules as well as incomplete root growth. Coronal restoration is also contraindicat-ed if the tooth has characteristics of disease such as pathologic apical processes, incomplete end-odontic treatment, or inammatory changes to the marginal periodontium. These problems must rst be treated and cured.Tooth mobility, gingival and bony pockets, and resorption of a socket beyond the apical third of the root are also regarded as contraindications, as is excessive loss of substance from the tooth preparation, because there is no longer sufcient mechanical retention to hold the crown in place.For esthetic reasons, fabrication of a full-metal crown may be contraindicated in the anterior re-gion. Certain types of crown may thus be contra-indicated, although this is subject to the dentist’s own judgment and is not necessarily discernible on a working model. Determining when a coronal restoration is indicated remains a matter of the dentist’s expert diagnosis, but dental technicians should be aware of the general criteria for using coronal restorations.Poor oral hygiene is always a contraindication for coronal restoration because deposits (plaque) lead to caries and periodontal disease. Before coronal restoration is considered, the patient must rst be motivated to take adequate oral hygiene measures after receiving suitable dental educa-tion and instruction.Fabrication of a crown is the result of collab-orative teamwork between the dentist and the dental technician. The tooth is rst prepared, and an impression is taken so that the technician can prepare a working model; adjust it in an articula-tor; and carve, cast, and nish the articial crown. The dentist cements the nished crown in place on the tooth preparation.Preparation of a ToothIf a tooth is to be tted with a crown, it must be prepared; that is, enough dental tissue must be re-moved to enable the articial crown to be pushed over the tooth. Pushing a thin metal sleeve over an unprepared tooth would enlarge the natural tooth by the thickness of the metal plate. It would then interfere with the opposing dental arch, protrude out of the arch, and, if the sleeve could actually be pushed over the approximal contact points, cause faulty relationships. Furthermore, the sleeve would not t closely in undercut areas. This is why a tooth needs to be prepared before receiving a crown. 24Coronal RestorationThe aim of preparation is to remove the dam-aged dental substance and, if necessary, remove enough healthy dental tissue to ensure that the widest circumference of the tooth lies at the low-est point on the tooth preparation. An attempt is made to smooth the area around the tooth with-out creating any undercuts.One approach to preparation is to create a cylin-dric preparation that would have almost parallel walls and would result in the least amount of hard tissue loss (Figs 2-16 and 2-17). As exact parallel-ism is impossible to see with the naked eye, this kind of fabrication would be at the limit of what is technically feasible. Furthermore, errors would arise when taking an impression and cementing the crown in place because of the piston effect of a parallel t. A better preparation design is a slightly conical (ie, tapered) preparation with a preparation angle between 3 and 8 degrees toward the occlusal sur-face. This design allows interference-free, accu-rate impression-taking and guarantees adequate retention due to static friction and a gripping ef-fect (Figs 2-18 to 2-21). When the restoration is being cemented in place, the cement is able to ow away more easily until tactile contact of all surfaces is achieved, leaving a minimum thick-ness of cement equivalent to its grain size, which increases the friction and gripping resistance.A basic tooth preparation has the following characteristics:• At its widest circumference, it meets the prepa-ration margin.• There is sufcient height for mechanical reten-tion.• The shape is slightly conical and has an angle of 3 to 8 degrees.• The preparation surfaces are not undercut.• An interocclusal rest space is prepared.Fig 2-16 Preparation of a cylindric stump is at the limit of what is technically feasible because precise parallelism cannot be seen with the naked eye. The preparation shapes illustrated here are achieved by chance. Parallel preparation Inverted cone preparation Conical preparationFig 2-17 A preparation with parallel walls requires less loss of hard substance than a tapered prepara-tion, and the risk to the pulp is also lower. 25Shaping the Preparation MarginShaping the Preparation MarginThe preparation margin is the border between the prepared and unprepared tooth surface (Figs 2-22 to 2-24). It is the responsibility of the dentist to make a clear preparation margin, and it is the responsibility of the dental technician to ensure that the crown margin ends exactly at the prepa-ration margin. Thus, the technician is less focused on how far a tooth is prepared at the neck or how deep or high the preparation margin is in relation to the marginal periodontium. However, if the crown margin lies above the preparation margin, caries will ensue. If it lies below the preparation margin (ie, in the gingiva) damage to the gingival margin will occur.For the purposes of accurate and safe fabrica-tion, a supragingival preparation margin is cho-sen, which is markedly above the gingival crev-ice. This means that the crown margin is kept clear of the periodontium and cannot damage the Fig 2-18 A tall tapered preparation with a preparation angle up to 6 degrees of-fers the best retention for a coronal res-toration. The crown cannot tip off the preparation even with eccentric loading.Fig 2-19 A very steeply tapered prepa-ration with a preparation angle greater than 6 degrees offers less retention for the coronal restoration. The crown can tip off the preparation if exposed to ec-centric loading.Fig 2-20 A very tapered and very short preparation offers the least retention for a coronal restoration. Even slight ec-centric loading will tip the crown off the preparation.Fig 2-21 A cylindric preparation with parallel walls will result in a so-called piston effect during impression-taking and crown placement, making these processes extremely difcult if not impossible. An impression can easily be taken of a tapered preparation, and the crown can be placed and cemented in place with equal ease, without causing any buildup of cement. 26Coronal Restorationperiodontal tissue, the critical area can be kept clean, and it is easier to take an impression of the preparation margin.However, supragingival preparation has seri-ous drawbacks: If the margin is placed too high, the tooth preparation becomes too short and does not offer enough mechanical retention for the crown. If the nish line is in the visible region, the esthetics will be unsatisfactory. For cervical defects, the preparation must follow an infragin-gival course, just as infragingival nish lines offer better caries protection in dentitions with active caries.During crown fabrication, special attention must always be paid to the crown margin, which is dened by the preparation margin. The quality Fig 2-22 A gingival preparation margin runs level with the gingival margin but not deep in the gingival crevice. As a result, the marginal periodontium is un-touched and undamaged. The eventual crown margin can be clearly seen and is usually esthetically satisfactory.Fig 2-23 A subgingival preparation mar-gin runs deep in the gingival crevice and affords very good caries prevention pro-vided the position of the crown margin ts accurately. Because it is difcult to check the crown margin, however, irri-tation of the marginal periodontium and even gingival retraction may occur.Fig 2-24 A supragingival preparation margin lies well above the gingival mar-gin. This course is very benecial for periodontal hygiene but unsatisfactory esthetically, and it provides no protection against caries in the presence of plaque. 27Forms of Preparation Marginof the transition from tooth to coronal restorative material is determined by how accurately the crown margin ts the preparation margin. The ac-curacy that is achievable depends on the shap-ing of the nish line. At the preparation margin, the crown material should not overlap the tooth preparation but should be sunk into the tooth.Three forms of preparation margin are used: (1) tangential preparation, (2) chamfer preparation, and (3) shoulder preparation.Forms of Preparation MarginA tangential preparation is ground like a tapered margin (Fig 2-25). Tangents can be placed in a vertical direction all over the basic conical shape of the tooth preparation. Ideally, the preparation margin should describe the root cross section and the crown margin nish line. The preparation margin nish line should lie at the bottom of the gingival crevice; subgingival preparation is only done for protective crowns in dentitions with ac-tive caries.Given this preparation nish line, the crown margin must taper off sharply and evenly while tting closely; the crown must then be given a convex shape above the gingival crevice. The crown itself must have an occlusal support so it is not pushed beyond the preparation margin.The coronal restorative material overlaps the tooth preparation. However, as it has to taper thinly, it may become frayed and bend upward. The articial crowns have to be overcontoured at the preparation margin, thereby forming a step that encourages plaque accumulation.The tangential nish line is easy to prepare and requires the least loss of dental substance. This preparation nish line is barely visible in the mouth and not at all visible on the working mod-el. A precise crown margin cannot be created and Fig 2-25 In tangential preparation, the widest circumference of the conical preparation and the preparation margin coincide. The coronal restorative material overlaps the tooth and can displace the marginal periodontium in some circum-stances. If the crown margin is shaped like a tapered margin, it runs razor-sharp into the depth of the gingival crevice. This tapered margin is rarely stable enough not to deform when the crown is tted. 28Coronal Restorationonly happens by chance. This approach is used for young teeth with a large pulp chamber so that acrylic resin crowns can be made as temporary replacements.Owing to the lack of available space, this form of preparation is not suitable for metal-ceramic restorations, all-ceramic restorations, or for full-cast crowns. The tangential form of preparation cannot offer static support against occlusal forces.Shoulder preparation forms a circular step around the conical tooth preparation. This offers static support to the coronal restorative mate-rial, which is particularly valuable for ceramic or acrylic resin crowns. The material for the articial crown is sunk into the dental tissue and forms a ush transition between tooth and coronal resto-ration (Fig 2-26).The preparation margin can be clearly seen on the model and thus allows for precise working. A shoulder preparation is easy to produce because the preparation tools have a cervical guide.In the case of shoulder preparation, the tooth preparation becomes much smaller than the root cross section, and there is considerable loss of substance from the tooth. The stability of the tooth preparation may be reduced, putting the pulp at risk (Fig 2-27). To prevent a buildup of cement on insertion, the start of the shoulder is chamfered to the tooth preparation.The edge of the shoulder can be beveled slight-ly to the gingival crevice to optimize the marginal Fig 2-26 To sink the coronal restorative material into the tooth preparation, a shoulder is created for the preparation margin. The coronal restorative material ends ush with the tooth, usually in the depth of the gingival crevice. The crown margin does not occupy any more space than the natural dental substance. To en-sure stability, the shoulder slopes slightly inward toward the tooth. If the crown is pressed onto the tooth by masticatory pressure, the crown material will slip inward like an inclined plane and will be pressed against the tooth.Fig 2-27 The thickness of dental hard tissue over the pulp of a canine. The dental tissue is considerably reduced by shoul-der preparation, which can damage the pulp. In general, this means the tooth is mechanically weakened. 29Forms of Preparation MarginFig 2-29 If a shoulder preparation is straight, a vertical marginal gap defect will occur because the crown cannot be lowered as far as the shoulder due to the thickness of the cement. The ce-ment will also be washed out by normal teeth-cleaning in a horizontal shoulder ar-rangement.Fig 2-30 With a 45-degree bevel to the shoulder, the vertical marginal gap defect is markedly reduced, and the cement cannot be washed out as easily. This bev-el is sufciently identiable in the mouth and is visible enough on the model so that the preparation margin can be read-ily exposed.Fig 2-31 The vertical marginal gap de-fect is the smallest with a very steep bevel (more than 45 degrees). The exact path of the preparation margin of the bevel is very difcult to identify, similar to tangential preparation.t, reduce the interface between tooth prepara-tion and crown, and move it out of the visible area (Figs 2-28 to 2-31). Preparation of the bevel is difcult and can blur the exact borderline on the model.Figs 2-28 Modern shoulder preparation involves shaping the transi-tion to the preparation wall as a chamfer to avoid any buildup of ce-ment on insertion of the crown, which can happen with a sharp-edged preparation. The shoulder margin is also beveled to reduce the mar-ginal gap. 30Coronal RestorationShoulder preparation is generally necessary when jacket crowns are being fabricated from acrylic resin or ceramic. These materials need a specic minimum thickness at the crown margin for reasons of stability and color quality. Shoul-der preparation is also appropriate for veneer crowns. The crown margin always ends ush with the shoulder, does not protrude horizontally, and does not sit too narrowly on the shoulder. Under no circumstances should the crown margin ex-tend beyond the shoulder.Chamfer preparation or veneer preparation happens when the conical tooth preparation is worked with suitably shaped abrasive tools so that a gentle circular chamfer is created (Figs 2-32 and 2-33). As a result, the coronal restorative material is moved into the tooth and ends ush. More dental substance is removed than with tan-gential preparation; consequently, the prepara-tion nish line is clearly visible in the mouth and on the working model.The chamfer offers adequate material thickness for metal frameworks, but there is not enough space for full crowns made of acrylic resin or ce-ramic. This preparation is suitable not only for veneer crowns—when a pronounced shoulder must be dispensed with because of the greater loss of substance—but also for full-cast crowns.Fig 2-32 Chamfer preparation is a prov-en compromise between tangential and shoulder preparation. This form of prepa-ration margin is used for full-cast crowns and veneer crowns, although the area to be veneered is prepared in a shoulder design. Chamfer preparation does not require as much substance loss as the shoulder form and produces a similarly precise marginal course that is clearly visible on the model and allows precise working.Fig 2-33 Chamfer preparation does not provide good static support to the crown over the crown margin and cannot prevent a reduced crown from sinking, so an oc-clusal stop is added for a reduced full-cast crown. This crown will not sag occlusally on loading.Fig 2-34 For veneer crowns, the tooth structure in the area of the veneer sur-faces should be prepared with a shoulder that turns into a chamfer approximally and incorporates the lingual surfaces. As a result, less tooth substance is removed lingually, and the tooth preparation is more stable. This mixed form of chamfer and shoulder preparation is the usual ap-proach for anterior and posterior teeth. 31Phases of Tooth PreparationThe mixed form of chamfer and shoulder prep-aration results from a shoulder preparation that follows a vestibular course and turns into a cham-fer approximally, which is then continued lingual-ly (Fig 2-34). This method results in less loss of substance from the tooth preparation than with shoulder preparation alone. Mixed preparation is mainly used for veneer crowns to sink the mate-rial into the tooth in the visible vestibular region.Phases of Tooth PreparationTooth preparation is not part of the work of a den-tal technician. The following description is for in-formational purposes only; it gives an overview of what a dentist does and should enhance un-derstanding of the collaborative relationship be-tween dentist and dental technician.The tooth is prepared under water cooling with specially designed rotary instruments, usually diamond-tipped burs of varying grit size. Stan-dardized preparation kits contain cylindric prepa-ration tools with working parts approximately 4 to 8 mm long and special shapes such as pointed conical, needle-shaped separator, rounded bud-shaped, and ball-shaped burs. The instrument shafts are designed for micromotor-driven contra-angle handpieces or for ball-bearing or air-bearing turbines.Preparation is done in the high-speed range (160,000 to 450,000 rpm), at which only minimal working pressure is required, no vibrations occur, and the treatment time can be very short. High-speed instruments require water-spray cooling. Use of a rotary instrument without water cooling would result in pulp damage due to friction heat. Even briey exceeding a temperature of 51.6°C causes protein coagulation. Following is a brief outline of the phases of tooth preparation.1. Preparation of the approximal surfacesA needle diamond (separator) is used to sepa-rate the approximal surfaces. The purpose of this phase is to separate or clear a space around the tooth being prepared in the dental arch so that the adjacent teeth are not accidentally damaged during subsequent preparation (Figs 2-35 to 2-37).2. Marking of depthIn order for the dentist to know how much den-tal substance to remove in subsequent phases of preparation without injuring the pulp, depth is marked with a groove-cutting or step bur. These instruments have a depth marking or a depth stop. They are used to create one or more 1-mm-deep grooves over the entire occlusal surface and onto the vestibular and lingual surfaces as far as the height of contour.Figs 2-35 and 2-36 Preparation is done under water cooling, and the buccal and lingual vertical surfaces are prepared rst.Fig 2-37 Separation: The approximal surfaces are prepared to obtain a gently conical tooth preparation. The adjacent teeth must not be damaged in the pro-cess. 32Coronal Restoration3. Preparation of the occlusal surfaceThe occlusal surface is removed as far as the depth marking without injuring the adjacent teeth (Figs 2-38 and 2-39). Depending on the design and the material used for the articial crown, adequate interocclusal rest space should be cre-ated. The occlusal relief should retain its basic morphology (eg, cusp size, ssure conguration). The dental arches are checked in working and bal-ancing positions to ensure that the preparation is adequate.4. Preparation of buccal and oral surfacesA cylindric bur that is angled or rounded at the tip is used for gentle conical reduction of the buccal and oral surfaces to just above the gingival crev-ice. This vertical circulatory preparation follows the curved path of the gingival attachment. 5. Preparation of the cusp bevel The cusp bevel at the transitional surfaces to the vertical surfaces is prepared with special tapered Fig 2-38 Occlusal preparation: The tran-sitions between the occlusal and vertical surfaces are ground, creating an occlusal bevel.Fig 2-39 The occlusal surface is ground down. Adequate interocclusal rest space should be created and the angle of cus-pal inclination maintained.Fig 2-40 Lowering and precise regrind-ing of the path of the preparation margin is done with a suitable abrasive tool, in this case a tapered torpedo.Fig 2-41 (left) The preparation target is checked with a probe. There must be no undercuts on the vertical surfaces. A conical preparation with a maximum preparation angle of 6 degrees is cre-ated. Fig 2-42 (right) Insertion groove parallel to the path of insertion for clear-cut xa-tion of the crown on eventual insertion. 33Impression of the Prepared Toothinstruments. This allows the cusp ridges, which have been displaced outward by the occlusal sur-face preparation, to be corrected into their proper position. Cusp ridges and cusp tips are moved to-ward the central ssure.6. Preparation of the nish line The nish line at the gingival margin is prepared with suitable instruments. For tangential prepara-tion, the nish line is relocated into the oor of the gingival crevice with a pointed tapered abra-sive (Fig 2-40). For chamfer preparation, a thicker conical tool (tapered torpedo) is used to create a clear nish line in the oor of the gingival crevice. The shoulder is mimicked with a rounded roller to create a chamfered transition to the vertical prep-aration surfaces and an almost horizontal step. In this phase, the gingival margin must be pushed back. Retraction cords have proved effective for this, or the pressure from the cooling water jet can also push back the gingival margin.7. Smoothing of surfaces The tooth preparation and transitions are smooth-ed using a cylindric nishing diamond with a very ne grit. The surface of the tooth preparation is checked for undercuts using a probe held cross-wise (Fig 2-41). For safety, an insertion groove can be cut onto a vertical surface, allowing clear-cut xing of the crown on insertion (Fig 2-42).Impression of the Prepared ToothAfter preparation is completed, a check impres-sion can be taken to produce a preparation check model. This allows the dentist to make an accu-rate check of the tooth preparation and identify any corrections needed.An accurate model of the prepared tooth is re-quired to fabricate articial crowns (Fig 2-43). This means that a specic impression needs to be tak-en using a ring-supported single-preparation im-pression or an integrated complete impression, which is subdivided into single-phase (mono-phase) and dual-phase impression-taking.Single-phase impression techniques use either one owable material or two owable materials simultaneously (double-mix technique). In dual-Gingival situationPath of preparation marginUnprepared dental archPrepared abutmentEdentulous jaw segmentPositional relationship of abutmentsFig 2-43 When a coronal restoration is produced, the impression needs to provide an accurate representation of the intraoral situation, and this must be reproduced in a working model. The impres-sion and model must accu-rately depict the prepared abutments with the path of the preparation margin and the gingival situation as well as the unprepared dental arch, the positional relation-ship of the abutments, and the edentulous segments of the jaw. 34Coronal Restorationphase impression-taking, an initial impression made of harder silicone is corrected with a low-viscosity second material (eg, correction impres-sion, double impression). The entire occlusal area of the jaw and the antagonists are included. A ring-supported impression is taken in two working phases:1. The actual impression of the tooth preparation is taken with a copper-ring impression made of thermoplastic impression material or silicone, where the ring acts as a carrier for the impres-sion material.2. An impression of the entire jaw is taken with the ring in place in a combined impression with a prefabricated impression tray.A matching ring is cut to t the path of the cer-vical margin. Enough space for the impression material is left between the tooth preparation and the ring. When the cervical edge of the ring is adapted to the gingival contour, it is drawn in toward the prepared tooth. After a dry eld is cre-ated and the prepared tooth is isolated, the ring is pushed as far as the preparation margin, and the heated plastic impression material is pressed occlusally as far as the edge of the ring. If low-viscosity silicone is used for the impression, the ring is sealed occlusally with wax, the impression material is poured in, and the ring is placed on the tooth preparation. A combined impression is usually taken with viscous silicone materials over the ring that is already in place. A ring-free im-pression is an integrated complete impression. It requires widening of the gingival crevice by suit-able methods; otherwise, the impression material will not penetrate the gingival crevice. The gingi-val margin can be displaced by placing a retrac-tion cord or elastic ring or by using a medication that reduces the tissue tension in the gingival margin.A dual-phase correction impression rst in-volves taking a primary impression out of viscous silicone; this takes on the function of a custom tray. The primary impression can be taken before preparation so that the pre-preparation space can be exploited to ensure tension-free reception of the low-viscosity correction material.If the primary impression is taken after prepa-ration in order to prevent compressive stresses from the primary material, the impression should be cut out slightly in the preparation area, and Fig 2-44 Taking an impression of a prepared tooth and the whole jaw is a precondition for an accurate working model. One method is a double-mix impression in which two materials are used to take an impression. Before taking the impression, a low-viscosity material is applied to the object from a disposable syringe. The gin-gival crevice is widened beforehand so that the impression material can reach the preparation margin.Fig 2-45 An impression tray is used to push the more solid second material over the low-viscosity material. Both materials set at the same time but have no effect on each other. The possibility of the impression materials warping is ruled out. This method is suitable for silicone and hydrocolloid materials. 35Impression of the Prepared Toothdrainage grooves should be created for the low-viscosity correction material. Only then is the low-viscosity correction material poured into the primary impression, which is briey pushed over the prepared tooth under pressure but held in place without pressure until it sets. Under pres-sure, the secondary material may deform the pri-mary material and distort the complete impres-sion because of the latter’s recovery properties.In the double-mix impression technique, two impression materials are mixed at the same time: a low-viscosity material from a special impres-sion syringe and a higher-viscosity second ma-terial for the impression tray (Fig 2-44). First the gingival crevice is lled throughout with the sy-ringed material, then the impression tray with the high-viscosity silicone is immediately placed over it (Fig 2-45). The two materials set simultaneously without displacing each other; no elastoplastic stresses arise between the two materials. This working method is also used for hydrocolloid im-pression material.A split model can be produced with the impres-sion: The dental arch is cast in dental stone, and dowel pins are placed in the position of the pre-pared tooth (Fig 2-46) with the aid of a light point indicator (Pindex, Coltene). The dental arch is iso-lated, and a model base is fabricated from stone (Fig 2-47). After the plaster has set, the dies must be cut out and the saw cuts taken down to the plaster base (Fig 2-48). The separated die can be lifted off the model and placed back into the mod-el by means of the dowel pin (Figs 2-49 and 2-50). Figures 2-51 to 2-56 illustrate the steps of taking a ring impression. Figures 2-57 and 2-58 provide helpful hints for marking the dies.Fig 2-46 During impression-taking, the dental arch and part of the alveolar ridge (approximately 1.5 cm high) are cast. A dowel pin is placed precisely in the pre-pared tooth.Fig 2-47 The other part of the dental arch is tted with retention rings, the plaster is isolated, and the dental arch is based.Fig 2-48 On the nished model, the tooth die is sawn out, and the prepara-tion margin is exposed.Fig 2-49 In a split model, a preparatory working step is to grind the preparation margin clear from the adjacent gingival area.Fig 2-50 The saw cut should run paral-lel so that the sawn die can be smoothly taken out; the nish line of the gingiva on the adjacent teeth must remain visible. 36Coronal RestorationFig 2-51 The principle of a ring impres-sion: A prefabricated ring that ts the preparation circumference is adapted to the path of the preparation margin.Fig 2-52 The ring is pushed slightly be-yond the preparation margin below the gingiva; it lies close to the widest circum-ference of the preparation.Fig 2-53 Thermoplastic impression ma-terial is pressed into the ring; the ring is removed, checked, reheated, and rmly put in place.Fig 2-55 A multipart working model can be produced with a ring-supported impression. In what is known as a die (preparation model), the prepared tooth can be removed while the adjacent tis-sues remain unharmed.Fig 2-54 A combined impression is taken over the ring impression with silicone im-pression material in a prefabricated impression tray.Fig 2-56 (left) In the ring impression, a tooth preparation with a conical root part and dowel pin is rst prepared in model-ing cement or dental stone. The prepa-ration is isolated, and the combined im-pression is cast.Fig 2-57 (right) Marking a die involves precisely depicting the path of the prepa-ration margin and the spatial relationship to the gingival margin, which must not be ground free. 37Crown Margin and Marginal PeriodontiumCrown Margin and Marginal PeriodontiumThe marginal periodontium, the typical cellular seal of the mucosa to the oral cavity, can be al-tered in its structure by the crown margin and preparation work as soon as the preparation mar-gin lies below the gingival crevice. This results in pathologic changes to the cellular structure in the marginal periodontium if the crown margin is pushed below the gingival margin (subgingival).In exceptional cases, a crown margin (prepara-tion margin) may be placed deep in the oor of the gingival crevice for caries prophylaxis (pre-vention) in a dentition highly prone to caries or for esthetic reasons. It is imperative that this mar-gin be respected.Foreign body irritation of the marginal peri-odontium may affect the success of a coronal res-toration. Possible sources of foreign body irrita-tion include the coronal restorative material, the general accuracy of t, and defective shaping of the crown margin.Irritation due to materialsAll the materials used (metal, ceramic, acrylic resin) should be tissue compatible. The substance itself or the surface texture may be the source of irritation leading to chronic inammation. Poorly polished metallic surfaces or roughly prepared ceramic surfaces lead to constant irritation if con-taminants and deposits accumulate in grooves, pits, and other rough areas, which then leads to inammatory changes.High-glaze red ceramic surfaces never cause tissue disorders, even when combined with metal frameworks. Acrylic resin surfaces occasionally display tissue incompatibility but also have con-densed surfaces with no deposits or contamina-tion. When processing acrylic resin, keep in mind that degradation of the acrylic resin is a continu-ous process; residual monomers can evaporate and subsequently cause irritation. Thus, acrylic resin veneers should always be kept clear of the periodontium.Light-cured composites exhibit adequate tissue compatibility. However, veneering material for metal carries a risk (however small) of gap for-mation between metal and acrylic resin in which deposits become lodged and lead to irritation.Fig 2-58 Placing a subgingival groove on a removable die and thereby marking the path of the prepara-tion margin is recommended. This ensures that the preparation margin will be clearly identiable, and the groove makes it possible to gently bevel the crown margin. The crown margin can be modeled to be slightly longer so that it can be subsequently polished on the model with a rubber polisher. For this reason, the die must be made of extremely hard modeling material. 38Coronal RestorationABCDEFig 2-59 The most common mistakes when shaping the crown margin for a tooth preparation with tangential and chamfer prepa-ration are the following: crown margin too long (A), crown margin too short (B), crown margin too thick (E), and crown margin sticking out (D). If a crown margin has several faults, such as an excessively thick, long, protruding, and frayed margin, extreme damage may ensue (C).CADBFig 2-60 Similar defects can occur with shoulder prepa-ration: The crown margin may overhang horizontally if the shoulder bevel is not accurately reproduced (A); the preparation margin may not be accurately corrected (B); the crown margin may protrude beyond the shoulder bevel (C); and the crown margin may be too short if the shoulder bevel is not accurately reproduced (D). 39Full CrownsIrritation due to inaccurate tIf the articial crown is sunk into the tooth as a re-sult of accurate preparation, a microgap between the crown and the prepared tooth may be found on insertion that corresponds to the thickness of the cement. An absolutely smooth transition is not achievable. This accuracy of t, however, can be kept within tolerances of 0.2 mm, which do not jeopardize the success of the coronal restoration. Inaccuracies above this level sooner or later lead to irritation because the gap can be washed clear of cement and provide an entry point for contami-nant deposits.Irritation due to defective shaping of the crown marginA common error when shaping the crown mar-gin is failure to respect the preparation margin. Although this rarely happens with shoulder and chamfer preparation, it is always encountered with tangential preparation (Fig 2-59). If the crown margin is too long, the marginal periodontium is torn and retracts.If the crown margin is too short, the risk of car-ies cannot be ruled out. The crown in a tangential preparation must taper very thinly, usually giving rise to a ragged, frayed crown margin that is un-stable and will damage the marginal periodonti-um as the margin bends and exposes some areas of the tooth preparation where caries will develop.The coronal restoration must not occupy any more space in the crevice than the natural tooth did previously. Thus, the crown margin must not overhang or be too narrow in the case of shoul-der or chamfer preparation (Fig 2-60). The trauma caused by pressure invariably leads to retraction of the periodontium and an excessively narrow margin that is prone to caries.The causes of defective shaping of the crown margin can only be identied in each individual case. In most cases, however, the main cause is unsatisfactory fabrication of dies without a clear-ly visible preparation margin, which was either damaged or poorly created to begin with.Full CrownsA full crown covers the prepared tooth complete-ly like a shell (full-coverage crown in metal) or a jacket (acrylic resin or ceramic jacket crown). Ve-neer crowns are full crowns made from different materials; these have a metal framework that is veneered with acrylic resin or ceramic. Every full crown exhibits the anatomical features and func-tional characteristics that are necessary to fulll its functions (Fig 2-61).A full crown is retained on the prepared tooth by static friction and gripping resistance; that is, adequate retention depends on the size and incli-nation of the contact surface: The steeper and tall-er the preparation, the more rmly a full crown will be seated.Fig 2-61 Full crowns completely cover the prepared tooth as far as the preparation margin. Full crowns can be made from one material or combinations of materials: (a) metal full-coverage crowns; (b) jacket crowns made of ceramic or acrylic resin; (c) veneer crowns made from a metal framework and a tooth-colored veneering layer in ceramic or acrylic resin.a b c 40Coronal RestorationMetal full-coverage crowns come in two funda-mentally different designs: banded crowns and full-cast crowns. The full-cast crown is the more advanced construction and was developed be-cause in banded crowns (collar crowns or prefab-ricated crowns) the margins of the crown shell damage the gingiva. In contrast, full-cast crowns have the following features:• They are sufciently stable against mechanical stresses.• They t completely to the prepared tooth with-out a thick layer of cement.• They have a marginal t that is clearly dened by the preparation margin.• They require the least reduction of substance from the tooth because chamfer or tangential preparation is sufcient.• They have almost unlimited durability.Full-cast crowns cannot satisfy esthetic de-mands, however, and are therefore reserved for posterior teeth. Another drawback may be their weight in the case of large molar crowns, espe-cially if large contact points need to be created or a positional correction becomes necessary be-cause of crooked teeth.To decrease the weight, a reduced full-cast crown can be fabricated by scraping out the up-per part of the carved wax crown from the inside or by building up (blocking out) the tooth prepara-tion before completing the wax-up. The scraping-out method is not recommended because of the high risk of error (scraping out may cause the crown to warp).Heavily damaged tooth preparations are built up on the model or in the mouth with plastic ll-ing material. It is important to ensure that occlu-sal support is created in the hollowed-out occlusal surface, which will prevent the crown from being inserted too deeply. To provide the necessary re-tention, the crown ts closely to the prepared tooth in the lower third, and the occlusal surface is carved thickly enough (a minimum of 0.5 mm) to prevent it from sagging or being damaged dur-ing mastication.Reduced full-cast crowns can also be produced with prefabricated crown matrices made of wax or acrylic resin. In this case, the matrix is adapted and, at the preparation margin, fused onto the prepared tooth to a width of about 2 to 3 mm; an occlusal stop is also included. The advantages of this method are the calculable crown weight and time savings if the antagonists are favorable.In adverse occlusal conditions, the matrix has to be reworked before casting because adjustment with a rotary instrument is not possible given the thickness of the material. There are fundamental drawbacks: The occlusal relief of prefabricated crowns will only be correct by chance in a few cases, including during articulation movements; as a result, the actual function of the articial crown is not fullled.The following steps should be followed to fab-ricate a full-coverage crown with seminished parts (Fig 2-62):1. Isolate the prepared tooth against wax.2. Adapt the crown matrix (matrices are available in different crown designs and sizes) and trim the margin to match the path of the preparation margin.3. Model the crown margin with casting wax pre-cisely to the preparation margin and the form of preparation. Flood approximately 2 to 3 mm of the wax onto the die to guarantee adequate retention of the crown to the tooth preparation.4. Flood the occlusal stop from the inside.5. Invest and cast the crown as would be done normally.6. Finish and polish the cast crown.Fabrication of a full-cast crownFor a full-cast crown, the tooth is reduced to be slightly conical and with a chamfer preparation. The preparation margin is exposed on the remov-able die by means of a subgingival groove that starts precisely at the preparation margin. The margin is marked and its path traced with a red pen without applying pressure or scraping the preparation. A black or blue pen is unsuitable because if the casting wax is blue or green, the margin will cease to be visible later. The full-cast crown is carved out of a single piece of wax. The prepared tooth must be isolated against wax for this purpose.The wax pattern is duplicated precisely by the investing and casting technique, so the nished cast can only be as good as the wax pattern. Er-rors such as faulty margins or parts of occlusal surfaces will reappear in the gold casting. Over- 41Full Crownssized parts or carelessly carved occlusal surfaces would then have to be reworked, which would in-volve considerable loss of gold and time. As wax is easier and, above all, less expensive to work with than gold, a few more minutes spent on waxing up will save hours of corrective reduction and soldering.Using specic wax-processing techniques to avoid stresses is advisable. It is important to en-sure that the wax pattern is always machined on the die. Single crowns should be invested im-mediately after waxing up. Preparation of an un-derlining foil is a suitable option for preventing a single crown from warping in wax (Fig 2-63). The Fig 2-62 Principles of fabricating a reduced full-cast crown from seminished parts: (a) The seminished part is selected from a collection of different crown matrices and trimmed at the crown margin. (b) The seminished part is xed onto the isolated tooth preparation with a drop of wax; this xation also acts as an occlusal stop. (c) The margin is marked to incorporate about a third of the preparation and clearly map the path of the preparation margin. (d) The waxed-up crown is tted with a sprue and a reservoir in the casting and is normally invested, cast, and nished. (e) The nished crown requires only a small amount of metal, which is why the occlusal contacts cannot be reworked.b cFig 2-63 Using a foil lining as a basic framework for the crown wax-up offers several advantages: First, it is a rigid base for the wax pattern, which can be deformed with relative ease, and second, the thinness of the foil lining guarantees minimal thickness of the crown framework. A suitable thermoplastic foil with spacer foil is heated; the die is pressed into a molding compound (a) to beyond the preparation margin, so that the thermoplastic foil lining and spacer foil are pressed around the die. The spacer foil is then removed (the thermoplastic foil lining shrinks after cooling by about the thickness of the spacer foil), and the foil lining is trimmed to about 2 mm above the preparation margin. (b) The foil lining, without spacer foil, is placed back on the die, and the crown margin is quickly ooded with casting wax so that no ow lines are formed on the inside; any slight step that may occur between coping and the wax margin is carefully smoothed. (c) The actual crown framework is carved on the coping. The foil lining is sufciently thick for the framework in the veneered area.ab cdeaSpacer foilLining foilMolding compound 42Coronal RestorationFig 2-64 A full-cast crown is reconstructed by the systematic wax-up technique. The rst step involves building up the oc-clusal surface outline by building up the vertical surfaces. The surfaces are convex above the gingiva; the overcontoured con-tact points lie on the approximal surfaces and are brought into precise contact by reworking and polishing.Fig 2-65 The occlusal surface can be built up with colored wax as far as the occlusal contacts. The nished wax pattern has all the morphologic features of the eventual metal crown. The more precise the wax-up, the less reworking will be need-ed later, which saves time and materials.Fig 2-66 The surface elevations of the dental crown are shaped so that esthetic demands are satised, no niches are created where deposits could accumulate, and the marginal periodontium is protected. The approximal surfaces are shaped so that the contact points lie directly occlusally, in the depth of the interdental embrasures. The contact points must not lie too high or too deep to spare the interdental papilla.Fig 2-67 The vertical curvatures of the vestibular and lingual surfaces must be reproduced so that food is deected away and the marginal periodontium is protected. These vertical cur-vatures are relatively slight; they should be reshaped in their natural thickness because excessively strong curvatures pro-duce the opposite of what is intended: they create niches in which deposits will form and chronic inammation can ensue. A die with an impression of the cervical margin will allow the dentist to check this.Fig 2-68 Active masticatory movements under tooth contact are sideways (lateral) or forward (protrusive). They can be pro-jected onto the occlusal surfaces of the teeth as a regular move-ment pattern. This gure plots mediotrusion or laterotrusion (blue), lateroprotrusion (green), and protrusion (gray) at the man-dibular incisal point. This movement pattern is often called the occlusal compass and can be transferred to each occlusal point. 43Full Crownsactual waxing up can be done on the underlining foil. This then produces a warp-free crown frame-work and a minimal crown thickness because it avoids the risk of carving too thinly in places.The occlusal surface is contoured according to the systematic wax-up technique (Figs 2-64 to 2-87). The highest elevations of the cusps are rst established, then the individual enamel segments Fig 2-69 Here the movement pattern (occlusal compass) of the mandibular incisal point has been transferred to the poste-rior teeth. The opposing cusp tips are represented as spheres in their contact elds. The protrusive movement is gray, the pure sideways movement (mediotrusion/laterotrusion) is blue, and the combined sideways/protrusive movements (lateropro-trusion) are shown in red and yellow. An appropriate clearance for these extrusive movements must be left when waxing up the occlusal surface.Fig 2-70 The movement paths of the opposing cusps on the maxillary posterior teeth show the opposite pattern. The op-posing cusp tips are again shown as spheres in their contact elds. Protrusion is shown in gray, mediotrusion/laterotrusion in blue, and lateroprotrusive movements in red and yellow. Freedom of movement must be incorporated into the wax-up of the occlusal surfaces.Fig 2-71 A perspective view of the maxillary pos-terior teeth shows the movement path of the op-posing cusps in the occlusal relief. The functional clearance can be seen in the supplemental grooves and central fossae, on the triangular ridges of the cusps, and in the interdental embrasures. Red and yellow indicate lateroprotrusion; gray, protrusion; and blue, mediotrusion/laterotrusion. 44Coronal RestorationFig 2-72 Projection of the movement pattern (occlusal com-pass) in a mandibular rst molar shows the freedom of move-ment for the opposing cusps. The crosshatched area denotes the border region for backward (retrusive) movements of the opposing cusp.Fig 2-73 A corresponding movement pattern is shown on the maxillary rst molar, radiating from the central fossa, to dem-onstrate freedom of movement. The movement pattern can also be projected onto the occluding cusp tips.Fig 2-74 The rst step in the systematic reconstruction of the tooth morphology is to create the outline of the occlusal surface. This is derived from the structure of the vertical tooth surfaces approximally, lingually, and buccally. The tooth propor-tions should be accurately produced during this working step.Fig 2-75 As a guide, the movement lines of the occlusal com-pass and the border region can be marked on the basic, at occlusal surface; the cusp segments are placed on these lines. These orientation lines make it easier to determine the propor-tions of the different cusp segments. 45Full CrownsFig 2-76 The cusp segments can be applied with different- colored wax. The cusp sphere for the mesiolingual cusp is placed on the curvature line of the border region.Fig 2-77 The mesiobuccal cusp sphere lies on the lateropro-trusion line toward the buccal region; the distobuccal cusp sphere lies behind the border region, slightly toward the mid-dle of the tooth.Fig 2-78 The mesiolingual cusp is built up until it comes into contact with the central fossa of its mandibular antagonist. The outer surface is adapted to the vertical curvature of the tooth surface. The rudimentary cusp shape extends to the middle of the tooth on the intersection of the movement lines.Fig 2-79 The distobuccal cusp sphere is applied in the same way. The vestibular surface curvature is matched to the tooth shape; the cusp extends as far as the intersection of the move-ment lines and broadens mesially to the laterotrusion line. 46Coronal RestorationFig 2-80 The sturdy mesiobuccal cusp sphere is placed be-tween the protrusion and the laterotrusion line. The vestibular tooth surface curvature is reproduced so that the proportion of the molar is preserved. The distolingual cusp sphere is added later.Fig 2-81 The next step is to shape the mesiolingual cusp. The central crest of the cusp is taken onto the mesiobuccal cusp, the cusp ridge is lengthened to the distobuccal cusp and forms the crista transversa, and the mesial approximal mar-ginal ridge is built up. Occlusal points must be formed at the places marked.Fig 2-82 The distobuccal cusp is the next to be shaped; it forms the cusp crest and approximal marginal ridge with the respective occlusal points. The approximal marginal ridge is separated from the distal triangular ridge by a distinct supple-mental groove.Fig 2-83 The mesiobuccal cusp has the following features: cusp crest, cusp ridge, supplemental groove, and triangular ridge. The occlusal point appears on the cusp crest facing the central fossa.Crista transversa 47Full CrownsFig 2-84 The distolingual and mesio-approximal marginal ridge is shaped and displays the marked occlusal points. The ssure paths and occlusal surface pits are traced, and if neces-sary, the occlusal contacts and approximal contact points are checked and reapplied.Fig 2-85 Carving of the occlusal surface is completed by clos-ing the vestibular, lingual, and approximal surface curvatures. The wax pattern is then smoothed and degreased. The crown is lifted off the die, and the crown margins are checked. The sprue is then attached.Figs 2-86 and 2-87 When attaching the sprue, the following applies: (1) The sprue attaches to the thickest part of the cast object. (2) The sprue has a minimum diameter of 1.5 mm and has a reservoir in the casting. A reservoir is not necessary for thicker sprues (3 mm and more). (3) The sprue should be long enough that the casting lies outside the heat center. (4) Molten metal should be injected without a change of ow direction and should ow from thick to thin cast parts. (5) The cervical opening of the crown should be upward when placed on the base socket mold former so that no undercuts are created that would allow bubbles to form during investment. 48Coronal Restorationand marginal ridges are placed, and the ssures are traced. The circular surface bulges are then shaped. Checking the model in its working posi-tions should reveal areas of occlusal relief that need to be corrected. The transitions of occlusal surface and vertical surfaces are then reworked. Finally, the crown margin is checked: Is the mar-gin too long, too short, too thick, uneven, or stick-ing out? The following points should also be kept in mind:• Overcontoured margins can break off.• If margins are too short, they are extended to the nish line.• The margin must be of even thickness but should be thinly tapered and entirely ll the preparation shape.• The crown-preparation interface should be ush without a step.• The surface of the crown is smoothed and cleaned with a wax stress-relieving spray. The use of spirits or alcohol is unsuitable because the resulting evaporative heat from these liq-uids is withdrawn from the wax and the crown warps.Sprueing is done quickly to prevent increased transfer of heat to the wax-up. The wax pattern should not be altered when the sprue is being t-ted.The cast object is invested using precision in-vestment material under vacuum. The ask is placed on the base socket mold former and lined with investment eece (wet slightly) so that un-impeded setting and expansion of the investment material can take place and no pseudocontraction occurs. The investment material is then stirred and poured in under vacuum. About 20 minutes after investment, wax burnout is mandatory, even if casting will not take place until later. The ask is preheated to the necessary casting tempera-ture, depending on the investment material. This is done according to the temperature guide for precise thorough heating of the ask. Casting in the specied equipment is performed without de-lay after the relevant process. Once the ask is cooled to room temperature in air, the cast ob-ject is divested. Airborne-particle abrasion is then performed with plastic beads.Finishing involves smoothing machining marks from the surface of the wax pattern, surface grind-ing the sprue attachment, and removing any bubbles. Fine abrasives, rubber polishers, and ne burs are used for nishing. Tungsten carbide burs and cutters are recommended for reworking the ssures, and the resulting surface can usually be polished directly with handpiece brushes.Banded CrownsFull crowns made of metal include banded crowns made of sheet metal (gold, silver-palladium, or steel); these crowns used to be fabricated by a variety of methods but are now outdated. Follow-ing are the different types of banded crowns:• Collar crowns: With this type of crown, the crown is soldered or cast onto a ring that is soldered to t or seamlessly drawn. The crown holds the oc-clusal relief and is cast or punched out of sheet metal.• Prefabricated crowns: These deep-drawn metal shells made of sheet metal (in different sizes and shapes) can be pushed over the tooth prepara-tion, for which the margin must be suitably cut out cervically to the gingival margin. In the past, these crowns were used as provisional (tempo-rary) crowns.• Deep-drawn crowns: These are deep-drawn out of gold sheet over the prepared die in the dental laboratory.These types of crown are outmoded because the benet of material savings—these crowns are extremely lightweight—is outweighed by signi-cant functional deciencies. In principle, tangen-tial preparation is used for these crowns. A closer look at collar crowns reveals the problems associ-ated with banded crowns.One of the advantages claimed for collar crowns is that the minimal (tangential) preparation need-ed for the tooth largely preserves the tooth sub-stance and that the tight-tting ground ring allows epithelial adhesion because rolled gold sheet de-livers clean surfaces without pits or pores, there-by preventing deposits and contaminants at the 49Banded Crownsmargin. However, a major disadvantage of these crowns is the lack of stability, particularly because no occlusal stop can be incorporated, except in the case of cast crowns. The very thick layer of ce-ment between the tooth preparation and the crown cannot handle the dynamic loading of masticatory pressure and thereby shatters. As a result, the crown slips in an apical direction and the crown margin bends so far that it lies on the alveolar bone. In addition to the traumatic chang-es to the tooth with the coronal restoration, the opposing tooth is also damaged as it follows the displacement of the replacement crown. In the process, irreparable periodontal damage is sus-tained.With this method, the crown ring is usually trimmed in the mouth. In addition to the variation between the prefabricated ring and the circumfer-ence of the tooth preparation, another drawback is the inaccuracy of the marginal path because the crown margin is shaped by cutting with plate shears. The marginal nish of banded crowns is therefore always poor and leads to serious mar-ginal damage.Techniques for fabricating collar crowns with a cast crown are summarized here as an example of how banded crowns used to be produced. Fol-lowing are the steps for fabricating a collar crown (Figs 2-88 to 2-95): 1. The sheet metal ring is prepared based on a specied ring size. 2. The ring is contoured according to the root cross section and cut according to the gingival margin. 3. The ring is anatomically contoured with con-touring pliers. 4. The ring is trimmed to just below the occlusal level. 5. The prepared die is built up to the antagonists with modeling wax, and the edge of the ring is entirely exposed. 6. The occlusal surface of the crown is carved out of casting wax and tted with a sprue. 7. Investing, casting, and divesting are done in the usual way. 8. The ring and crown are tted together and joined with a little solder. 9. The sprue attachment and soldered joint are smoothed.10. Polishing is done with felt wheels, short brushes, pastes, and high-luster buffs in the usual way. As the crowns are relatively thin, there is a risk of polishing through the crown.There are other possible approaches to fab-ricating the occlusal surface of the crown—for example, modeling the surface slightly beyond the ring and investing it with the ring in order to join both parts by the casting-on technique. The crown surface can also be punched out of sheet metal and soldered to the ring; this delivers ex-tremely light crowns that are even more unstable than normal collar crowns. In conclusion, banded crowns, including collar crowns, are entirely out-dated prosthetic solutions! 50Coronal RestorationFig 2-88 A collar crown is fabricated with a cast soldered-on occlusal surface following the working steps depicted here. First, the ring measurement is taken at the actual preparation margin.Fig 2-89 The ring is bent into an anatomical shape with suit-able contouring pliers. It must have contact with contact points and overhang the gingival crevice.Fig 2-90 The ring is shaped in keeping with the die cross section. The edge of the ring is cut out to follow the path of the cervical margin and pushed about 1 mm below the gingival margin.Fig 2-91 A (thermoplastic) stent im-pression is usually taken with the crown ring shaped in this way, in the form of a squash bite.Fig 2-92 This impression is used to pro-duce a cast xator that shows the few teeth involved. Only terminal occlusion can be reconstructed with this cast.Fig 2-93 The ring is prepared, and the die is built up with wax until it is a uni-form distance from the antagonist in or-der to achieve uniform material thickness and minimal weight.Fig 2-94 The modeled occlusal surface is removed. It is essential to ensure that the edge of the ring is accurately marked in the occlusal surface margin and con-tains xings that guarantee clear posi-tioning of the ring in relation to the cap.Fig 2-95 The cast occlusal surface is soldered to the ring. An occlusal stop is placed to avoid displacement of the crown. 51Full Crowns Made of Tooth-Colored MaterialFull Crowns Made of Tooth-Colored MaterialThese full crowns known as jacket crowns are made of ceramic or acrylic resin. The advantage of crowns without frameworks lies in their excellent esthetic effect. However, the materials require a minimum thickness of 1 mm, which means that shoulder preparation is always required (Figs 2-96 to 2-98).Coronal restorative material is intended to be laid fully in the tooth to prevent irritation of the marginal periodontium due to pressure. For this purpose, the tooth preparation is ground to be slightly conical; to maintain stability, the prepara-tion must not exceed three-quarters of the origi-nal length. For static support, an incisal plateau is prepared.A circular or rounded shoulder approximately 1 mm wide is placed up to the epithelial fusion line. The inclination of the shoulder to the preparation should at least be at right angles, but a shoulder sloping down to the preparation would be better. If the shoulder slopes outward, the ceramic mate-rial is widened and will break in response to shear loading. Acrylic resin swells when absorbing wa-ter and is pressed outward by the sloping shoul-der. A shoulder sloping inward compensates for shear forces in the ceramic crown and presses a swelling acrylic resin crown onto the tooth at the margin.If tangential preparation were used, the crown margin—as with metal crowns—would have to taper thinly, and the ceramic crowns would break during placement. Although acrylic resin crowns could be placed because the material is capable of elastic deformation, fatigue would cause plas-tic deformation to occur after a short time, and the material would lift off the tooth. In addition to the mechanical stress on the gin-giva involving inammatory processes, deposits and bacteria would become lodged under the margin, leading to traumatic changes to the en-tire periodontium. A chamfer preparation is inad-visable for precisely the same reasons.The indication for these full crowns is deter-mined by the material properties of the acrylic resin or the ceramic being used. For example, a porcelain crown (or an all-ceramic crown) has absolute color and dimensional stability and very good tissue compatibility. It is resistant to high compressive stress and abrasion but less resistant to shear forces. A material thickness of 0.8 mm is required, which means that shoulder preparation involves substantial reduction of the tooth; therefore, this type of crown is not an op-tion if the pulp cavity is large. Veneer crowns can be fabricated by the metal-ceramic technique us-ing chamfer preparation and offer similar esthetic advantages to porcelain crowns.Acrylic resin crowns have elastic properties and sufcient abrasion resistance if light-cured com-posites are used; they are therefore well suited to long-term restorations. Acrylic resin often shows discolorations, and swelling of the material may cause a gap between the tooth and the crown. However, long-term studies have shown that composites are subject to only minor changes and thus remain dimensionally stable and color-stable for prolonged periods.The advantage of acrylic resin crowns lies in their rapid, less complex, almost problem-free, and inexpensive fabrication. Today crowns can be produced from acrylic resins that are indis-tinguishable from a natural tooth or a porcelain crown in terms of shape and color. The elastic deformability and abrasion properties are other advantages because the tooth is not stressed as much during mastication as is the case with a rig-id metal or porcelain crown.Another advantage of an acrylic resin crown is its use as a temporary replacement for 1 to 2 years when a shoulderless preparation becomes neces-sary (in adolescents) because of the risk to the pulp. Once the pulp (possibly due to the irritation caused by grinding) has diminished, preparation for the denitive crown can then be carried out.In principle, full crowns made of ceramic or acrylic resin are used for maxillary anterior teeth for esthetic reasons. In the mandible, jacket crowns are more difcult to fabricate because the necessary loss of substance weakens the man-dibular anterior teeth too much, leaving insuf-cient retention.Less plaque is deposited on ceramic crowns than on acrylic resin jacket crowns. Owing to the risk of fracture, acrylic resin jacket crowns are not indicated for posterior teeth; by contrast, ceramic crowns can be used for premolars, given favor-able occlusal relationships. 52Coronal RestorationF0F0F0FNFNFVFVFHFHFHFHFig 2-97 To get a better overview of preparation depth for anterior teeth, a groove is removed with a groove cutter. The amount of dental tissue removed depends on the depth of the groove. Fig 2-98 A conical preparation is created with a cylindric bur. At the same time, a shoulder sloping slightly inward toward the preparation is ground. Jacket crowns always require shoulder preparation, which demands considerable loss of dental tissue, particularly on anterior teeth.Fig 2-96 Jacket crowns must be prepared with a circular shoulder approximately 1 mm wide. The tooth is prepared to be slightly conical with a shoulder sloping inward. A shoulder sloping down to the preparation is an important requirement for stability. In response to compressive stress, the crown restoration material slips toward the preparation; the acrylic resin jacket crown widens if the shoulder slopes outward, and a porcelain crown may break. The advantage of an inward-sloping shoulder can be seen from the simple division of forces at an inclined plane. 53Acrylic Resin Jacket Crown FabricationAcrylic Resin Jacket Crown FabricationThe working process for fabricating an acrylic resin jacket crown starts with the preparation of the die. Different acrylic resin materials require different processing methods, but there are two basic methods.The rst method involves carving the crown out of wax, then investing and pressing the acrylic resin into the resulting mold. This indirect mold-ing technique is also used for ceramic crowns that are fabricated by the casting process (Dicor, Dentsply) or press technique (IPS Empress, Ivo-clar). Fabricating acrylic resin crowns by the liq-uid resin press molding process, however, is an outdated technique and is only presented here to give a complete overview.The second method involves applying the acrylic resin dough to the die layer by layer, carv-ing it, and curing the individual layers in an ultra-violet light-curing device. This method resembles the classic fabrication of a porcelain crown in its working sequence and necessary manual skills. When processing acrylic resin, however, no plat-inum foil is drawn over the die. This is because material shrinkage does not need to be consid-ered to the same extent as it does in porcelain ring.The systematic waxing-up technique is relevant to the fabrication of jacket crowns and all-ceramic crowns. In addition to following the principles of wax processing to avoid stresses and warping of the wax crown, the following points should be noted when waxing up the crown:• The tooth shape must match the original in all anatomical details. Any discrepancies will be particularly noticeable in the visible area. The adjacent teeth offer the best clue to nding the right shape.• The length and width of the tooth must harmo-nize with those of the adjacent teeth.• The mesial inclination should be determined from the vestibular view. The mesial inclination can be corrected most effectively if the margin-al ridges of the vestibular surface are suitably emphasized: The mesial marginal ridge is more prominent, and the distal aspect is gently round-ed in keeping with the curvature characteristic. The angle characteristic also reinforces the im-pression of the mesial inclination.• The approximal inclination can be checked by comparing the position of the incisal area with the adjacent teeth and the position of the neck of the tooth; a recessed neck has a very unfavor-able effect. The tooth must not protrude out of the arch or tip inward.• The vertical curvature must match the anatomi-cal pattern; hence, cervically the stronger curva-ture is needed to protect the marginal area.• The crown must lie within the dental arch and not be twisted in a vestibular or inward direction in the dental arch.• Occlusal contacts must be avoided in the ter-minal occlusion (Fig 2-99). The sagittal distance between the anterior teeth must be maintained.• The approximal contacts should t closely in the right place, that is, in the upper third of the crown. They can be slightly thickened before in-vesting to make up for the loss of material dur-ing nishing.• The precise surface form of the approximal sur-face must be respected. The contact point is not a bulge that is specially added, but the tooth widens steadily from the cervical margin to the Fig 2-99 The occlusal contacts on jacket crowns are problem-atic. If contact on an anterior tooth in terminal occlusion lies too far incisally and is too pronounced, the articial crown may fracture. Therefore, it is advisable to omit occlusal contacts on anterior crowns; even slight gliding contacts should be avoided. 54Coronal Restorationapproximal contact; the interdental papilla must not be squashed.• The transition from crown to preparation should be ush (Figs 2-100 and 2-101). The crown must neither protrude beyond the shoulder width nor be narrower.• The shoulder must be marked accurately in the wax crown.• The inside of the crown must be smooth and re-produce the precise contour of the tooth prepa-ration.Again, it should be noted that the more precise the wax pattern, the fewer corrections will be nec-essary at the nishing stage. Once the wax crown has been relaxed and corrected, the crown can be invested.Fig 2-101 On maxillary incisors, the mesial contact point is higher than the distal contact point. If an anterior tooth appears too wide because of shaping of the contact, the tooth can be worked to look narrower by emphasizing the mesial and distal marginal ridges, which are pushed slightly toward the middle of the tooth.Fig 2-100 The preparation-crown transition and marginal t of the shoulder must be accurately shaped for jacket crowns. The coronal restorative material must nish with the edge of the shoulder; it must not protrude, and the crown margin must not be too narrow. It is a signicant error to make the crown margin too short so that it can only be lled by cement that is later rinsed clear. 55Ceramic Crown FabricationCeramic Crown FabricationThe same indications for acrylic resin jacket crowns apply to full crowns made from ceramic. Optimal color matches can be made with suitable layer thicknesses (at least 1 mm). Because tissue incompatibility is not an issue with mineral mate-rials, ceramic crowns are the best solution for this situation. Figure 2-102 outlines the different types of dental ceramic systems.The classic approach to fabricating a porcelain crown allows for free layering of the ceramic ma-terials. The individual layers are applied and red separately. For this purpose, a folded platinum cone is produced as a coping over the tooth prep-aration. The platinum foil is placed closely over the preparation and is intended to act as a carrier for the ceramics during ring and to compensate for material shrinkage as a spacer (Fig 2-103).Application of materials and the ring se-quence, including any corrections, take place in a succession of working steps. Ceramic powders dyed with metallic oxides are mixed with distilled water to a paste that can be modeled.Shrinkage (drying shrinkage and sintering) of the mineral material on ring is approximately 20% to 25% and is compensated for by enlarging the shape during modeling. The ceramic is red in a furnace under vacuum. Only then is the core material applied and red (core ring). The dentin and incisal materials are then built up and red. In the process, the relatively squarely applied ma-terials merge into rounded surface contours as a result of sintering.The ceramic materials should be mixed and ap-plied without bubbles; excess uid must be re-moved by suction before ring, and the materi-als must be applied in even layers before drying. On ring, the ceramic always shrinks down to the thickest part so that thinner parts are under stress and may tear. The nal shaping is done by corrective grind-ing and application of correction materials in a separate ring. The nal ring process is the glaze ring, during which the surface of the crown is glazed. Finally, the platinum foil is removed.Note that the described process of fabricating a jacket crown is an outdated technique, which is All-ceramicSinteringDental ceramic systemsCastingPressing MillingMetal- ceramicFired onto cast or galvanized metal frame-worksComputer- milled ceramic bodyFired onto folded carrier folds (Vitadur)Fired onto refractory casts (Duceram)Al2O3 glass inltration ceramic (In-Ceram)Glass-ceramic (Dicor)Pressed ceramic (IPS Empress)Fig 2-102 Diagram of dental ceramic systems. 56Coronal Restorationwhy no error analysis is presented. A number of all-ceramic systems are available, such as glass inltration ceramic (In-Ceram, Vita), pressed ce-ramic (IPS Empress), and cast glass (Dicor), for which crystal-reinforced ceramics are used. These materials are usually strengthened with leucite or aluminum oxide. In the Duceram (Vita) technique, the ceramic is applied layer by layer onto a plati-num carrier foil (or onto a refractory die) in the classic form and red layer by layer.Vita’s In-Ceram and Hi-Ceram methods are hard-core systems for all-ceramic crowns and xed partial dentures. These methods involve working with glass-inltrated aluminum oxide ceramic that is reinforced with zirconium oxide. The po-rous Al2O3 hard-core crown sintered at 1,120°C is inltrated with lanthanum glass melted in a spe-cial glass inltration ring at 1,100°C. The hard core then forms the frame to carry the actual ce-ramic layering. This technique does not result in any marginal defects, such as those found where the platinum foil is folded in the conventional technique.Interspace varnish (approximately 45 µm) is ap-plied to the tooth preparation of the master cast before the die is duplicated with special stone. A slip of ne-grain Al2O3 and mixing uid are ap-plied to the plaster die to form a thin framework crown that is dry-sintered for 2 hours in a sinter ring (wet sintering should not be used). This results in minimal shrinkage, which is aided by expansion of the stone die. A thin suspension of special glass powder and distilled water (glass-inltration material) is applied to the extremely brittle coping and red at 1,100°C for 4 hours. In the process, the coping is inltrated by the melt-ed glass and hardens.The nished coping can be reworked with di-amond burs and coated with sintered ceramic (mainly Vitadur N, Vita). The inside of the nished ceramic crown is silanized by tribochemical coat-ing for adhesive xation in the mouth and is in-serted with composite.Cerestore (Coors Biomedical) is a castable, shrinkage-free ceramic in which the core (mixed with silicone) is cast into a mold in the injection molding process at 180°C. After a 12-hour ring process (1,300°C), the material sinters and forms spinel crystals.The IPS Empress technique involves carving the crown (inlay) in wax and investing it in phosphate-bound investment material (Fig 2-104). The ask is heated to 900°C so that a precerammed glass material inltrated with leucite crystals, heated in 1 2 34 5Fig 2-103 The foil is squeezed around the die. Notches the width of the die are made at the incisal edge, and the foil is bent lin-gually. The rst cut is made level with the shoulder. (1) Below the shoulder, the foil is trimmed to 2 mm on one side. (2) The second cut is made level with the tubercle. (3) The third cut shortens the points of the foil up to the incisal edge. The long part of the foil below the shoulder is attened and pressed on. (4) The foil is cut off parallel to the die from the tubercle to the shoulder. Above the tubercle, part of the foil is trimmed to 2 mm, then attened. (5) The long part of the foil above the tubercle is bent into a U shape and pressed onto the die. The same is done with the portion of foil below the shoulder. The foil is then trimmed to about 2 mm below the preparation margin. The foil must lie rmly in place and must not be crinkled. The platinum cone must be relatively easy to remove. 57Ceramic Crown Fabricationa pressing furnace at 1,100°C, is pressed into the mold (Figs 2-105 and 2-106). For a 15- to 30-minute pressing process, the pressed ceramic has a con-sistency like glass during glass-blowing. Leucite crystals will grow during the pressing process. The glass ingot is carefully divested, the sprue is separated, and shape corrections are reground. Color matching is done by staining with mineral stain or by subsequent layering with the system’s own sinter ceramic (Fig 2-107).Pneumatic systemPress pistonFurnace chamberAluminum oxide rodCeramic ingotFlaskFig 2-104 In the IPS Empress system, the wax objects are placed on a special base socket mold former with sturdy sprues. (Courtesy of Ivoclar Vivadent.)Fig 2-105 In the IPS Empress press furnace, the precerammed glass mass with leucite crystals is pressed into the mold at 1,100°C with the aid of an aluminum piston. Leucite crystals grow during the approximately 15 to 30 minutes allowed for the pressing process. (Courtesy of Ivoclar Vivadent.)Fig 2-106 The preheated ask is loaded with the pre-heated ceramic ingot and the aluminum oxide rod and placed in the press furnace; the furnace chamber is closed, and the press piston is hydraulically pulled out. The pressing process lasts up to half an hour, depending on the size of the object. (Courtesy of Ivoclar Vivadent.)Fig 2-107 The pressed ceramic object can be stained with mineral stains, then red. Color matching can also be done by subsequent lay-ering. Stains are red in additional ring runs, during which more leu-cite crystals are formed. (Courtesy of Ivoclar Vivadent.) 58Coronal RestorationIn the case of cast glass (Dicor technique), the crown is also carved in wax and invested in phosphate-bound material. The ask is heated to 900°C, and the glass is poured in the centrifu-gal casting technique. In the casting centrifuge, the ask is heated to the casting temperature of 1,110°C. In a disposable casting crucible made of zirconium oxide, the glass mass is melted and be-comes more viscous than molten metal. For this purpose, the centrifuge must spin for 4 to 5 min-utes. The cast glass object is carefully divested and abraded with Al2O3, then coated with special ceramming investment and cerammed at 1,075°C according to a set program, which results in crys-tallization of mica glass. Color matching is done by staining with mineral stains that are baked in at 950°C without vacuum. The color layer can be abraded and reapplied. Corrections, such as con-tact points, can be subsequently applied with a ceramic from the same product range and red on.A foil crown is a metal-ceramic crown with a metallic carrier framework made of multilayered rolled precious metal foil approximately 0.1 mm thick. In terms of construction, the foil crown lies between all-ceramic crowns and metal-ceramic crowns. To exploit the esthetic depth of the ce-ramic, the die of the prepared tooth is encased in a very thin prefabricated part made of precious metal foil, without creasing, and the ceramic is red on. This means that the entire preparation space is used for the ceramic crown. The same depth of color is achieved as with an all-ceramic crown, and the stability is similar to that of a metal- ceramic crown.The foil technique involves adapting a ve-layer pure precious metal foil (Ultralite foil, Ulbrich) to the die and pressing it in a pressure chamber lled with plasticine (die stamp) in a cold plastic state. The cavities caused by folding are then closed over an open ame by diffusion melting. Ceramic bonding is supported by a 30-mm-thick, red-on pure gold mesh that is applied with a brush in the form of pure gold granules and fused at 900°C. The ceramic modeling is done by the usual layer-ing technique.The different precious metal layers of the foil have excellent thermal constancy because of the bimetal effect, so that remarkably good crown margin accuracy is achieved even after several ceramic rings. For esthetic reasons, however, and to avoid any tissue contact between the metal and the gingiva, the foil can be trimmed to about 1 mm above the preparation margin and the crown margin red in ceramic.Fabrication of a galvano (electroformed) gold coping is another way of layering and ring the ceramic tooth mold onto a very thin carrier cop-ing. The metal carrier framework is produced by a galvanoplastic method: The surface of the tooth preparation is rendered electroconductive, and galvanic metal plating takes place with the aid of direct current out of a saline solution contain-ing the plating metal. The tooth preparation acts as the cathode (negative pole), and a plate of the metal being deposited serves as the anode. The electrolysis conditions are selected so that a smooth, uniform deposit is produced on the mold.Computer-aided design/computer-assisted man-ufacturing (CAD/CAM) technology offers comput-erized fabrication of ceramic inlays, onlays, and half-shells in which the molded part is fabricated directly with the aid of computer-controlled mill-ing machines. This is done after an optical impres-sion has been taken with a measuring camera and after the preparation margins have been dened on a monitor. Using a construction program, the computer builds the molded part out of the available dental data and the plotted construc-tion lines. With minimal loss of dental tissue, this method provides metal-free restorations made from tooth-colored ceramic that have properties similar to those of dental enamel. The molded parts are adhesively cemented with composite.Veneer CrownsVeneer crowns are full crowns that combine the mechanical stability of full-cast crowns with the esthetic benets of acrylic resin or ceramic crowns. With this type of crown, a stable metal framework forms the functional parts, such as oc-clusal and approximal contacts, preparation cov-erage, and the crown margin, and tooth-colored veneering material forms the visible covering and matches the anatomical surface curvatures (Fig 2-108).Veneer crowns are indicated for all tooth shapes in which the preparations can be adequately pre-pared; this usually applies to all maxillary ante- 59Veneer Crownsrior and posterior teeth and mandibular canines and posterior teeth. Use of the metal-ceramic technique can also be extended to molars with occlusal surface veneering. The parts of a veneer crown are the crown coping, margin or shoulder, veneer retention, shoulder retention, and ceramic or acrylic resin veneering material.The crown framework should have a minimum thickness of 0.35 to 0.5 mm for ceramic and 0.25 to 0.4 mm for acrylic resin (Fig 2-109). In addition, the veneering material should have a minimal thickness of 0.8 to 1.2 mm. Thinner layers of metal impair stability, and excessively thin veneers re-sult in color discrepancies. Stability is a primary concern in veneer crown frameworks for xed partial denture abutments.The frameworks of crowns veneered with acryl-ic resin must be designed as a retention surface in order to create a proper bond between acrylic resin and metal (Figs 2-110 and 2-111). The acrylic resin must not come into contact with the mu-cosa; the aim is to extend the veneer into the Rim retentionMarginal or shoulder rimRetention for veneerVeneering materialCrown frameworkIncisal rimMax 1.2 mmMin 0.25 mmMin 0.8 mmMax 0.5 mmShoulder max 1.3 mmFig 2-108 The veneer crown comprises a metal framework and a tooth-colored veneer.Fig 2-109 The minimum material thick-nesses in a veneer crown are as follows: (a) An acrylic resin veneer crown should have a crown framework 0.25 to 0.4 mm thick and veneering acrylic resin at least 0.8 mm thick. (b) A ceramic veneer crown should have a crown framework 0.35 to 0.5 mm thick and veneering ceramic at least 0.8 mm thick. The crown frameworks must be worked to be more stable if the veneer crowns are being used as xed partial den-ture abutments. Splinted veneer crowns should also have more stable crown frame-works.a b 60Coronal Restorationapproximal area so that no metal can be seen. When creating a metal-acrylic resin bond, the metal framework is fabricated with box retention and additional retentions. The chemical metal-to-acrylic resin bond is stable enough with a chemi-cal bonding layer (silane layer) (Fig 2-112), but box retention and (small) additional retentions have also proved effective. Preparation for veneer crowns requires sub-stantial reduction of dental tissue wherever two materials overlap each other. A shoulder is man-datory in the vestibular visual area and must measure between 0.95 and 1.3 mm.Acrylic resin veneers are mainly used with tele-scopic crowns, less commonly for stand-alone crowns or xed partial denture abutments. The Fig 2-110 The metal framework is made to match the form of the preparation margin and creates the undercut “watch-glass notch” to the veneer surface that holds the acrylic resin. If the veneer surface is worked without a rim, the veneer acrylic resin will taper thinly and may lift off due to swelling and form a crack for plaque to become lodged. Even when chemical bonds are involved, the acrylic resin may lift off in the marginal area so that a rim retention also needs to be shaped here.ABCDFig 2-111 The retention surface for an acrylic resin veneer crown is worked as a rim reten-tion, which is undercut all the way around. For esthetic reasons, the transition can be moved lingually in the area of the incisal edge (B), al-though stability suffers as a result. The transi-tion shown in A is extremely stable but not esthetically attractive. A good compromise is to place the transition incisally (C). The trans-parency of the cutting edge does suffer some-what, but the rim is stable enough. Overlapping the acrylic resin over the rim should be avoided because the acrylic resin will lift off (D). 61Veneer Crownsrst acrylic resin veneers were characterized by low wear resistance, thermal expansion in the bond to metal, and faster aging. The development of microlled, abrasion-resistant acrylic resins (composites) and adhesive bonding to silanized metal surfaces has largely overcome these draw-backs. The latest veneer crowns have adequate mechanical quality and durability, have the best color effect and color-fastness, and measure up to ceramic veneers in esthetics.The metal framework is the supporting part in acrylic resin veneer crowns. It absorbs mastica-tory pressure and holds and supports the veneer, so it is essential to achieve absolutely closed con-tainment of the tooth preparation. Incisal edge protection may be created for stability reasons. In principle, the veneer is rimmed by metal, but the veneer metal must have no contact with the mucosa.The rim itself denes the size of the veneer. The aim is to produce an extensive veneer with approximal spread and veneering of the incisal edge (or the occlusal edges in the case of pre-molars). A full veneer, such as those produced with porcelain, can also be created using modern composites.The retention areas must offer good enough mechanical anchorage, which is achieved with a roughened, wettable surface. Box retention and microretention beads on the veneer surface work well. As noted, the gingival margins should be formed out of metal because the mucosa should not have any contact with the acrylic resin.An undercut box retention that surrounds the veneer should also be formed in the case of an adhesive bond (silanization) between metal and composite. Any overlapping of the acrylic resin from the box retention to the crown framework should be avoided because polymerization shrink-age will cause the edges to feather and lift off, and cracks will form in which deposits may become lodged.Figure 2-113 shows a multiple-unit metal frame-work for acrylic resin veneer crowns supported by two preparations. Veneering materialOpaque layerSilane borderSiO2 layerAbraded surfaceFig 2-112 Schematic diagram of the layer buildup of a crown veneered by the silanization method. With a chemical adhesive bond between metal framework and acrylic resin, the acrylic can be cured onto an organic intermediate layer of silane. The metal surface is roughened and enlarged by airborne-particle abrasion, the SiO2 layer (silane) is applied by tribochemical coating, and a silane bonder is applied to provide surface conditioning to allow polymerization on the veneering acrylic resin. The comparison of mechanical and chemical bonding of the veneer material shows the space required by mechanical retentions. 62Coronal RestorationFig 2-113 There have been repeated attempts to optimize acrylic resin veneer crowns by special framework designs. In the fol-lowing system, a delicate open-metal framework was formed for the veneer material, whereby the acrylic resin lay directly on the tooth preparation. The metal framework undertook all the functional tasks expected from a veneer crown framework: transfer of masticatory forces to the tooth preparation, marginal t at the shoulder, approximal contacts, and occlusal contacts. Prefabricated masticatory bars in the form of mesio-occlusodistal inlays were used, which were supported by an occlusal stop on the tooth. The crown margin was shaped like a “watch-glass notch” to secure the acrylic resin and was completed by the dental technician. The masticatory bar of the system could also be used as a partial denture pontic, for which a reinforcing wire had to be inserted in the lower third to create the necessary stability. The prefabricated blanks were calculated for maximum masticatory force. (a) Three-unit metal framework supported by two preparations, with a reinforcing bar in the middle. (b) Framework in cross section. (c) Occlusal view. (d) The framework for a veneer crown can be closed lingually. (e) The framework has a “watch-glass notch” that holds the acrylic resin. a bcd e 63Ceramic VeneerCeramic VeneerIn terms of mechanical strength and esthetic ad-vantages, a ceramic (or porcelain) veneer is the safest solution for providing a xed replacement. The term metal-ceramic refers to metal frame-works covered with red-on ceramic material as well as to the technique in which ceramic mate-rials are red on directly. The composition and properties of the alloys and materials used in this technique have been specically developed for bonding upon ring; therefore, other types of materials may not be substituted.The ceramic materials for the metal-ceramic technique differ chemically from porcelain, which is made up of kaolin, feldspar, and quartz and forms what are known as multicrystals. Ceram-ics contain no kaolin, but their main constituent is feldspar, and they form leucite crystals (Figs 2-114 and 2-115). Firing produces a feldspar glass that can be classied between hard porcelain and ordinary glass based on its properties and com-position.In its melted state (at a minimum of 1,160°C), feldspar disperses large amounts of quartz. If the resulting feldspar glass is heated again, it will melt at a lower temperature (approximately 1,000°C). This is why potash feldspar, which has a high degree of purity, is used as a constituent of the ceramic materials. For the metal-ceramic technique, additives (ux) of potassium phosphate, potassium carbonate, and sodium carbonate are used to reduce the softening temperature.Colored metal oxides are added to create tooth shades (Fig 2-116). Organic dyes help to distin-guish layers before ring. For plastic formability during layering, the powder (ground prefritted ce-ramics) contains added organic substances (eg, sugar, starch). When mixed with distilled water, these provide cohesion for layering. During r-Fig 2-114 Dental ceramic materials consist of 70% to 80% feldspar (potash/sodium feldspar) and 10% to 20% quartz; they contain only minute quantities of kaolin. Bonding oxides, bind-ers, expansive additives, and ux (2% to 4%; potassium phos-phate, potassium carbonate, sodium carbonate, borax, lead oxide, potassium oxide, and manganese oxide) are also added. The dyes are made of heat-resistant metal oxides and salts. The percentages shown here indicate the maximum levels.Fig 2-115 Magnication shows leucite crystals in the ne- particle ceramic Omega 2000 from Vita. The leucite crystals are cubic tetrahedron–shaped crystals that are formed in pot-ash feldspar at 1,170°C. The leucite crystals will not soften and give the ceramic stability during ring. 64Coronal Restoration Iron oxide Red/yellow Chromium oxide Green Cobalt Blue Iridium Black Silver Orange Nickel Gray Gold Purple Tin White Titanium Yellowish brown Manganese VioletFluorescence enhancers Cerium Samarium UraniumBlue/white Reddish Green/yellowFig 2-116 Inorganic dyes for dental ceramics are heat-resistant metallic oxides and salts that produce a variety of color shades.Fig 2-117 Sintering gives rise to ring shrinkage. On ring, the surface energy of the ceramic par-ticles decreases, and without melting, the sur-faces of the particles react with each other; they fuse and the spaces between them decrease. The ring temperature is well below the melting temperature of the ceramic particles; only the components with the lowest melting point ow around the other materials.Holding timeFig 2-118 Firing is performed in three phases: predrying, ring, and cooling. Predrying evapo-rates the mixing uid and oxidizes the dyes and binding agents. During ring, the material is placed in a furnace, and a vacuum is drawn. The temperature is increased over a period of about 4 minutes, after which the ring temperature remains constant throughout the holding time. Cooling is then performed slowly and gently. 65Ceramic Veneering, these constituents gasify; thus, vacuum ring has proved advantageous.On ring, the volume changes from drying shrink age and sintering of the ceramic materials (Fig 2-117). On drying, the liquid evaporates from the mix, and the organic dyes and binding agents gasify (Fig 2-118). During ring, the powder grains on the surface melt and sinter (Fig 2-119).The volume change is between 25% and 35% and always occurs in the direction of the great-est mass because the cohesive forces between the powder parts tend to reduce surface tension. For this reason, practitioners should follow these processing principles:• Before layering, the mass must be intensely con-densed. The more it is condensed, the smaller the volume change will be.• Firing temperature and time must be adhered to. If they are exceeded, the internal friction of the vitried material decreases, the thickest part of the mass shrinks, and surface contours are lost.• Layer thickness should be consistent over the entire veneer. Thin areas will shrink the most and will be drawn to the thick parts.The alloys for metal-ceramic crowns are matched to the strength and thermal behavior of the ce-ramic materials. The properties required of the gold-platinum group for ring purposes are the following:• A high melting range is required in which the solidus point of the alloy is higher than the tem-perature at which the ceramics are red on. The ring temperature of the ceramics is between 950°C and 1,000°C, and the melting range of the alloys is a maximum of 1,300°C.• The heat resistance ensures that the alloys re-main rm on ring and are not deformed by in-trinsic weight.• The thermal expansion of the metal-ceramic al-loy and the ceramics must be virtually the same so that the ceramic does not come under ten-sion when cooling after ring.• The alloys must harden so that their mechanical values can be increased by the annealing pro-cess.• High-yield strength and a high modulus of elas-ticity account for the high exural strength of the alloy so that the framework will not be de-formed by masticatory forces in the mouth.• Alloys for metal-ceramic crowns must tolerate intraoral conditions as effectively as previous metals. TransparentIncisalDentinMore opaqueNeckShade effectFig 2-119 The schematic structure of a metal-ceramic veneer crown shows the distribution of the individual materials, which cre-ate the subtle coloring of the veneer. 66Coronal RestorationRetention of ceramics onto the metal frame-work can be attributed to three mechanisms:1. Mechanical surface interlocking occurs when the ceramic material is wet and shrinks on the surface so that compressive forces ensue. Roughened surfaces offer good microreten-tion.2. Intermolecular forces (van der Waals) form the second retention mechanism; the molecules of the interlayer become dipoles and therefore attract. Overlapping of the molecular orbitals then occurs.3. The chemical bond strength of bonding oxides is attributed to the increased concentration of nonprecious metal atoms at the boundary layer between ceramic and metal. During oxi-dation annealing, the nonprecious metal at-oms migrate along the grain boundaries of the structure at the alloy surface and oxidize there. During the ring process, the bonding oxides diffuse into the ceramic mass and bond to the silicone oxides.Bonding via the oxygen bridges of the bonding oxides and the silicone requires sufcient metal oxides in the boundary layer. Bonding oxides should therefore be formed to a moderate extent during correct oxidation annealing.Framework Design for Ceramic-Fused-to-Metal CrownsThe design of the metal framework for ceramic-fused-to-metal crowns is extremely important. The function and structural balance of the frame-work determine the success of the restoration.The minimum thickness of the crown wall after nishing should be at least 0.3 mm for a single crown and 0.4 to 0.5 mm for a xed partial den-ture, depending on its span length.The modeled crown and xed partial denture frameworks should allow for a consistent veneer layer thickness (Figs 2-120 and 2-121). Relative-ly thin ceramic layers, red onto a rigid metal framework, are the most stable. The veneer layer should be no less than 0.8 mm thick to ensure color fastness (Fig 2-122). The best layer thickness (for color and stability) is 1.0 to 1.2 mm.A consistent layer thickness is best achieved if the framework compensates for deformations of the tooth preparation and the surfaces being veneered are given the anatomical shape of the tooth on a reduced scale (ie, contoured 1 mm smaller). Varying layer thicknesses cause unwant-ed color shifts and stresses because the thinner parts are pulled away from their base toward the thicker layers as a result of shrinkage.If a full veneer is to be constructed for esthetic reasons, the veneer must be seated on the frame-work like a crown. The transitions between metal and ceramic are not placed in the area of an oc-clusal contact because the ductile metal may ow as a result of masticatory forces, leading to crack-ing and fracturing of the veneer. The functional area is therefore shaped entirely in the metal or entirely in the ceramic. The incisal area should in any case be worked without an incisal wrapover for esthetic considerations.Occlusal contacts can be made of metal for sta-bility reasons because the ceramic is protected against harmful shear stresses and because natu-ral antagonists abrade less than is the case with ceramic occlusal surfaces. The crown margin for the preparation border is fully fashioned in metal. In principle, therefore, ceramic veneered crowns can be fabricated for any form of preparation.Shoulder preparation is a good approach be-cause it provides excellent support to the metal framework and elastic deformation cannot occur. The preparation is fully encased in metal as far as the shoulder. The veneer surface is hollowed out toward the crown margin, leaving a very thin metal margin visible. However, where there is a shoulder, this metal is concealed by the gingival margin in the oor of the gingival crevice.The mixed form of shoulder and chamfer prep-aration is ideal in terms of stability and reduction of tooth substance (Fig 2-123). The tooth prepa-ration obviously needs to be wrapped as far as the preparation margin. On the vestibular side, the shoulder is covered by a metal chamfer that merges lingually into a wider metallic edge. It is stable enough to prevent exural stresses on the ceramic. 67Framework Design for Ceramic-Fused-to-Metal CrownsChamfer preparation requires a slightly thicker metal edge than a circular shoulder for reasons of stability (Figs 2-124 and 2-125). This metallic border might become visible if the chamfer is not placed low enough (Fig 2-126). If the metal-lic crown margin tapers thinly, the ceramic mate-rial may ake off due to elastic deformation. It is also inadvisable to let the metal framework end before the chamfer or to shape the preparation margin with ceramic. The crown margin cannot Fig 2-121 In the case of xed partial dentures, a consistent veneer thickness must be obtained. The ceramic veneer must not entirely surround (wrap around) the xed partial denture because the ve-neering material shrinks so much during ring that it would rupture. It is possible, however, to bring the veneering material in contact with the mucosa because ce-ramic is compatible with the mucosa.Fig 2-120 Fixed partial dentures for ce-ramic veneers have to be shaped to en-sure that the veneers have a consistent material thickness so that color shifts and stresses do not occur in the veneering material. The crown framework must therefore compensate for an uneven preparation.0.5 mm0.8 mm1.6 mmFig 2-122 The minimum layer thick-nesses of the metal framework and the veneer are given. The incisal area should have no metal wrapover and can be up to 1.6 mm thick. 68Coronal RestorationFig 2-123 The combination of shoulder and chamfer preparation is suitable for ceramic veneers because it allows ve-neering of the lingual surface.Fig 2-124 Chamfer preparation is well suited to ceramic veneered crowns. The metal framework can be thinly tapered at the preparation margin so that no metal border is visible in the mouth.Fig 2-125 Even if normal thickness of the crown margin is maintained at the preparation margin for stability reasons, the metallic border may be concealed by the gingiva.Fig 2-126 If a ceramic veneer is to be applied to a tooth with tangential prepa-ration, a wide cervical metallic edge will become visible.Fig 2-127 The transitions from metal to ceramic must never have sharp-edged grooves, recesses, or undercuts. The tran-sitional areas have convex contours, and the outer join ends ush at right angles. This is necessary because ceramic under-goes severe shrinkage during ring and cannot compensate for tensile and shear stresses. These kinds of stresses occur in recesses, grooves, and undercut areas.Fig 2-128 The metal-to-ceramic transitions must be rounded in the approximal area, and the outer transition should be right-angled and ush. Undercut areas, as used for framework re-tention, should be avoided because the veneer will rupture. Sharp-edged corners in the retention surface lead to stresses and tears. 69Design of Lingual Surfaces on Anterior Teethbe shaped as accurately with fused-on ceramic as with metal. Furthermore, a crown margin made of ceramic is unstable and can easily break off.It is imperative to avoid sharp edges, grooves, recesses, or undercuts—such as those required for retention of acrylic resin veneers—on ceramic veneer surfaces (Figs 2-127 and 2-128). The tran-sitional areas between metal and ceramic must be convex, and the joint must always run perpen-dicular so that:• An adequate layer thickness is created in this area, just as it is on the entire veneer surface.• No stresses can occur as a result of shrinkage of the ceramic at the enclosed areas.• No air is trapped when the materials are applied.• No contaminants (grease or particle-abrasion dust) become lodged, because these gasify dur-ing ring and, because they are so deeply em-bedded, cannot be fully removed by suction even during vacuum ring.Design of Lingual Surfaces on Anterior TeethThe more stable the metal framework, the more resistant the overall structure. The success of the overall construction must not be jeopardized and the framework weakened for the benet of es-thetics. In eugnathic bite relationships, when an-tagonist contact is relieved via canine guidance, the entire crown surface can be veneered.In dysgnathic bite relationships (deep overbite, overbite less than 2 mm) or in the case of canine guidance, it is essential to fashion the functional surfaces on canines out of metal. The transitions between metal and ceramic lie at least 2.5 mm away from centric contacts (Fig 2-129).If the vertical overbite of the maxillary anterior teeth is small, the transition must be placed cervi-cally. Incisal edge protection is inappropriate be-cause it impairs the transparency of the cutting edge if the incisal layer of ceramic becomes too thin and too unstable and the thin metal edge of-fers no support. To address severe additional an-tagonist loading, the incisal layer thickness must be 1.2 mm and placed out of contact in terminal occlusion.The shape of the veneer surface can be inu-enced by a property of ceramic that causes it to shrink in the direction of gravity. This has no inu-ence on partial vestibular veneers, but in the case of full veneers, the position of the object during ring can have an impact. The inuence of gravity is minimal, however, if the thickness is uniform.Design of the framework for posterior teeth In principle, crowns on the posterior teeth can al-ways be fully veneered with ceramics, provided the following:• The patient’s occlusal relationships and move-ment are clinically normal.• An adequate interocclusal rest space of at least 1.4 mm has been created in the preparation stage.• The framework can be placed without any dif-culty and ts closely. (Note: In this case, a frame-work try-in is essential because stresses can lead to fracture even at the cementation stage; furthermore, stresses caused by t inaccuracies may add to those caused by chewing.)• The occlusal contacts can be accurately recon-structed. (This condition is difcult to satisfy, however, so occlusal veneering becomes a deci-sion of general principle.)A very smooth, polished occlusal surface will not abrade the antagonists any more than a natu-ral tooth, but a ceramic veneer may have inaccu-rate contacts with the antagonist in the occlusal area. This can lead to excessive abrasion where there are faulty contacts with the antagonist; trau-matic stresses may also arise, thereby affecting the periodontal tissue surrounding the antagonist and the crown tooth.To avoid this situation, a full veneer on the oc-clusal surfaces needs to be ground back in a fully adjustable articulator, then a glaze ring should be performed (Fig 2-130). Yet even if performed with total concentration and accuracy, the end re-sult will always be second class when measured against the accuracy of sculpted metallic occlusal surfaces. From a functional point of view, how-ever, veneering of occlusal surfaces should not be sanctioned uncritically. The esthetic quality is 70Coronal Restorationoptimal, provided the color match is successful. If metallic occlusal surfaces are preferred for func-tional reasons, it is important to ensure that the functional contacts are at least 2 mm away from the metal-to-ceramic transition.Fig 2-129 In teeth with occlusal surfaces, the transi-tions between metal and ceramic should be at least 2.5 mm away from the antagonist contact. One pos-sible method is to veneer the entire occlusal surface as far as the lingual surface. The ceramic is loaded un-der pressure if all hindrances to gliding movements are eliminated.Fig 2-130 A functional occlusal surface has punctate occlusal contacts that lie correctly in relation to the temporomandibular joints. Grinding to create punctate occlusal contacts and smoothing them with a glaze ring will produce good results but is technically de-manding.Fig 2-131 The approximal surfaces can be fully veneered. In the case of veneer crowns that are to be soldered, an ap-proximal surface must be fabricated from metal. The soldering area must be at least 3 mm2. From the vestibular as-pect, the veneer will conceal the solder-ing surface that lies interdentally.Fig 2-132 A complete vestibular veneer is the goal. Depending on the form of preparation margin (eg, chamfer), it may be necessary to fashion a crown margin out of metal that is wide enough to stand out above the crevice and be visible.Fig 2-133 In dysgnathic bite relation-ships (eg, a deep overbite), the functional surfaces should be made from metal. If the metal-to-ceramic transition lies in the area of the antagonist contact, the ceramic will be subject to bending and will break because of the ductility of the metal. 71Partial CrownsDesign of approximal contactsAntagonist contacts are made entirely of metal or entirely of ceramic to protect the ceramic against bending stress. If the approximal contacts are fashioned from ceramic, they can be corrected at any time by grinding and rering. To enhance the esthetic impression, the contacts on the anterior teeth and mesial contacts on the posterior teeth are shaped out of ceramic veneering material. All the basic principles governing the contouring of approximal surfaces apply here. The contact point lies in the occlusal third and must not displace the interdental papilla.If individual teeth are being soldered for splint-ing, the contact areas are obviously fabricated in metal (Figs 2-131 to 2-133). They need to be large enough—an area of 3 mm2 is sufcient—and the veneer should conceal the soldered surface inter-dentally. In such cases, an oral metal wall helps to secure the stability of the unit.Partial CrownsPartial crowns are crowns cast from metal that do not enclose the tooth on all sides but leave a sub-stantial part of the natural tooth visible from the vestibular aspect. The other surfaces are ground back, which means the crown can be classied according to the amount of ground surface (Fig 2-134): • A half crown (open-face crown) for the anterior region covers the tooth lingually as far as the incisal edge and up to half of the approximal surface (Fig 2-135).• A three-quarter crown for the posterior region covers the tooth occlusally up to the buccal cusp and approximally entirely; only the buccal sur-face remains uncovered (Fig 2-136).• A four-fths crown for the posterior region en-closes the tooth with a pronounced retention and covers the buccal surface at the margins.• A seven-eighths crown for the maxillary molar covers the distal portion of the buccal surface. The mesiobuccal part of the vestibular surface forms the veneer (Fig 2-137).The preparation surfaces are covered with met-al and form an open ring that gains its support and retention from ground-in channels and pins as well as cervical shoulders. The basic idea be-hind using a partial crown is to preserve the natu-ral veneering of the tooth.In patients with extensive caries lesions, a mesio-occlusodistal lling is not provided if it would mean the narrow lingual tooth wall would have to be ground back too thinly. The cusps would need to be enclosed to prevent a fracture. Restoration with a full crown requires loss of more tooth sub-stance and covers the intact vestibular surface. An extensive mesio-occlusodistal lling, includ-ing treatment of approximal defects, calls for a relatively thick, central masticatory bar, which means the tooth would be greatly weakened from the occlusal.Partial crowns offer the advantages of multiple-surface llings, reduce the risk of fracture of weakened dental tissue, and provide consider-able strengthening of the residual tooth. Follow-ing are the advantages of a partial crown:• A good esthetic effect is provided by the natu-ral tooth color because the vestibular part of the crown is spared.• There is minimal loss of tooth substance.• Considerable reinforcement is provided for the restored tooth. • The tooth remains accessible to subsequent pulp testing.• The crown margins are easy to check and keep clean.• Occlusal and approximal contacts can be built up in the same way as for a full crown.• They are suitable as xed partial denture abut-ments or to receive attachment components for a partial denture.A partial crown is contraindicated in the follow-ing circumstances:• The tooth is susceptible to caries.• The tooth is short, thereby offering insufcient retention.• The tooth is nonvital and discolored. 72Coronal RestorationFig 2-134 Partial crowns can be classied according to the amount of tooth enclosed. They are divided into (a) half crowns, used primarily for anterior teeth; (b) three-quarter crowns, used primarily for premolars; (c) four-fths crowns; and (d) seven-eighths crowns for molars.a b c dFig 2-135 Retention of partial crowns is dependent on how far the tooth prepara-tion is enclosed. Half crowns for anterior teeth encase the lingual surface, small parts of the approximal surfaces, and the approximal stabilization channels.Fig 2-136 Retention of three-quarter crowns for premolars is achieved by parallel channels created in the reten-tive surfaces. The stability of a half-ring, as represented by a partial crown, is in-creased by extending the partial crown and continuing the channels onto the oc-clusal surface.Fig 2-137 The greatest coverage of a tooth is provided by a seven-eighths crown for molars, in which only the me-siobuccal part of the vestibular surface remains uncovered. The marginal area to the vestibular window is again stabilized by preparation of channels. Fig 2-138 A channel-and-pin prepara-tion on an anterior tooth displays paral-lel walls approximally and lingually that contain the parallel channels and the occlusal shoulder channel. The cervical shoulder is often omitted because the tooth would have to be weakened too much in the area of the tubercle. Incisal edge protection—that is, beveling of the incisal areas—must not be omitted on anterior teeth, even though it creates an unfavorable esthetic impression. 73Statics of Channel-Shoulder-Pin RetentionPartial crowns can be classied according to the nature of their retention:• Window crowns are sheet-metal sleeves that are pushed over the tooth and leave the labial surface clear and the cervical margin enclosed. Window crowns are obsolete and are no longer made because they do not meet modern de-mands in terms of functional quality.• Carmichael crowns are cast partial crowns that gain their mechanical retention from lateral en-closure of the prepared tooth and approximal grooves.• The obsolete Vest’s or claw crown is similar to the Carmichael crown; it has a clawlike enclo-sure down toward the root but has no approxi-mal ridges or shoulders.• Partial crowns with channel-shoulder-pin prepa-ration are cast partial crowns in which parallel channels approximally and crosswise channels occlusally are prepared on the normal prepara-tion surface, as is a cervical shoulder (Fig 2-138). In addition, they may also incorporate parapulp-al pin xation.All partial crowns are prepared according to this principle today. They offer the strongest re-tention to the tooth preparation and good inher-ent stability, so they can also be used as xed par-tial denture abutments.After an impression of the prepared tooth has been taken, a die is made from dental stone and the partial crown is waxed up. Waxing up of the occlusal surfaces and vertical surfaces is done according to well-known principles. The crown margin should be molded to precisely follow the preparation margin so that it can be nished in-traorally.Statics of Channel-Shoulder-Pin RetentionA partial crown is not as stable as a full crown against pull-off forces from the transverse and oc-clusal directions (Fig 2-139). Additional retentive structures therefore have to be created to counter forces from various directions. The same problem occurs with milled attachment ttings, which are pushed over a partial crown in the form of a half-ring.Retention against occlusal pull-off forcesThe ideal basic preparation for a full crown is con-sidered to be a cylindric preparation, which offers the greatest static friction because of its parallel surfaces but is difcult to produce and displays a piston effect upon placement. This effect does not apply with a partial crown. The pull-off forces in an occlusal direction can therefore be compen-sated for by parallel retentive surfaces with high static friction.The vestibular surface is left clear and cannot be used for retention. This must be offset by en-larging the retentive surfaces with channels and pins. The channels and pins parallel the retentive surfaces.Poor retention would be provided by under-cut cribs in the cervical area (after Vest) because these would bend apart on placement and would have to spring back by means of elastic restor-ing force. It would be necessary to determine the spring constant and spring travel, but this is too inaccurate owing to processing errors.Retention against lingual pull-off forcesIf a partial crown, such as a half-ring, is pushed occlusally over the tooth preparation, this half-ring can be pulled off with application of mini-mal force. Apart from the adhesive forces of the cement, there is no mechanical retention acting in this direction. Channels and pins, which are created approximally in the retentive surfaces and run parallel to them, secure the crown’s posi-tion against this direction of force (Fig 2-140).The half-ring can extend beyond half the cir-cumference like a ferrule and thus be braced against lingual pull-off forces. For this structure to be pulled off lingually, the half-ring would have to bend open so that it could slip over the fer-rule. This is why the half-ring needs to be braced against being bent open. 74Coronal RestorationRetention against bending openIf a partial crown is used as a xed partial den-ture abutment or to hold an anchor for a remov-able restoration, forces acting approximally and lingually as well as torsional forces can bend the half-ring open.Rigidity to prevent its being bent open cannot be achieved by vertical channels. The channel prole must continue at the crown margin, both cervically as a shoulder and occlusally, in order to produce a rigid frame (Fig 2-141). This frame in the form of channels is easy to create and ex-tremely stable because a round prole in every direction shows the same amount of resistance against possible bending.A circular shoulder resembles shoulder prepa-ration and provides material reinforcement of the half-ring, static support against occlusal forces, and retention against bending open. Further-more, a shoulder preparation accurately estab-lishes the crown margin.Parapulpal pin retentions are prepared parallel to channels and offer additional bracing against bending open and against occlusal and lingual pull-off forces (Fig 2-142).Preparation with a special retentive structure comprising parallel surfaces, channels running in the same direction, parapulpal pins, and shoulder preparation, as well as channels that merge oc-clusally, constitutes the typical channel-shoulder-pin retention with a dened path of insertion. The partial crown is thus secured against these forces but particularly against twisting (torsional forces).Fig 2-139 Retention of a par-tial crown must be effective against occlusal and lingual pull-off forces. A half-ring on a tooth preparation gains its ad-hesive force from the parallel walls by means of static fric-tion, but static friction of the parallel walls is insufcient to withstand transversally di-rected forces.Fig 2-140 Channels milled parallel to the path of inser-tion offer the best reten-tion against lingually directly forces. Some bracing against bending open is necessary because only slight bending open will loosen the crown. A special design is therefore needed.Fig 2-141 Vertical channels are joined by one channel run-ning occlusally, giving rise to a frame that resists bending in the cervical area.Fig 2-142 Flexural rigidity is increased by placing addi-tional parallel pins alongside the channels. Additional re-tention against pull-off forces and bending open is achieved with the channels.Fig 2-143 Channel-and-pin anchorage secures the crown against all forces that may arise. In this design, the half-ring can be prevented from bending open by a cervical step or shoulder. This step can be shaped sloping down to the tooth. In the incisal area, edge protection is cre-ated to protect against masti-catory forces. 75Post CrownsRetention against masticatory forcesThough the described preparation protects the partial crown against pull-off forces and offers similar stability to a full crown, both the tooth and the crown are inadequately secured against dy-namic masticatory forces. The incisal or vestibu-lar transitions from crown material to tooth are particularly at risk from masticatory forces. A thin metallic base is usually insufcient because the ductile metal can undergo plastic deformation by masticatory forces. As a result, the vestibular enamel surfaces may break away. Therefore, inci-sal and/or cuspal enclosure become necessary, to which a grinding edge is added, this being bev-eled in the vestibular direction and giving rise to a metal edge about 0.5 mm thick (Fig 2-143).Post CrownsIn the case of devitalized teeth, the coronal restora-tion can be retained with a post sunk into the pre-pared root canal (Fig 2-144). Unlike full-coverage crowns, which are pushed over a dentin core, post crowns involve placing a cast or prefabricat-ed post in the root canal and xing the coronal restoration to the upper end of the post.Devitalized prepared teeth tend to have brittle dentin, and the preparation can break. An accu-rately tting metal post is inserted to provide sta-bility. Accuracy of t is crucial to the stability of this root and post unit.Post crowns are mainly indicated in the maxil-lary anterior and premolar area where the roots are sturdy enough. Mandibular anterior teeth can-not generally be restored with pivot teeth because their roots are too short and often stunted. In prin-ciple, multiple-rooted molars can also be treated with pivot teeth provided the root canals follow the same direction. As this is rarely the case, how-ever, other designs are more appropriate.A distinction should be made between post crowns, in which the post and the crown are fabri-cated in combination, and post and core crowns, in which an accurately tting endodontic post and a metal buildup are inserted as an articial tooth preparation (Fig 2-145). This core buildup has the same dimensions as a prepared tooth and can be tted with any possible form of crown.The post and crown unit is more stable than a post and core crown; with the post and core, the post and crown can be separated again and the coronal restoration can be replaced or integrat-Fig 2-144 Devitalized teeth can be restored with technical constructions, provided the periodontium is intact. A tooth that is al-most entirely destroyed by caries is ground back as far as the cervical margin, and the pulp is removed. If necessary, the root apex may be resected (apicectomy), and the tooth is sealed at the root apex with a lling. A metal post can be placed in the root canal as a form of retention for a replacement tooth. 76Coronal Restorationed into a xed partial denture unit. In very rare cases, the root canal coincides with other abut-ment teeth in its path of insertion. Furthermore, the post is difcult to remove without damaging the root. Therefore, it is better to construct a post and core that does not differ in shape from other prepared teeth.Statics of post retentionThe root of the prepared tooth must be prepared to allow for absorption of forces and the special stresses on the post. As a rule, the devitalized tooth is ground down to a 2-mm-tall preparation; the root canal is then cleaned, widened slightly, and closed apically with a root lling.In terms of absorption of forces, the post is stressed via the crown in three different ways: bending (exion), compression, and twisting (tor-sion) (Fig 2-146). The vertical compressive forces must be transferred vertically to the surface of the root preparation, which requires a suitably large surface contact between the post and the prepa-ration (Fig 2-147). If this contact is absent or in-adequate, the post is pressed into the root canal, and the wedge effect will fracture the root. The horizontal bending forces stress the root on one side from the vestibular direction via the aforementioned surface contact; as a result, the post may bend out of shape (Fig 2-148). In ex-treme cases, wedge-shaped chipping may occur at the root on the vestibular side. The root prepa-ration is therefore given a vestibular, roof-shaped bevel, and the post is worked appropriately (ie, its thickness) to provide stability. The root prepara-tion is then given a chamfer preparation.In the case of post crowns, torsional stresses may loosen and separate the post. The rooike bevel of the preparation provides minimal resis-tance to such torsion; complete torsional rigidity is achieved by creating an auxiliary cavity in the form of a second post arranged in parallel, eccen-tric widening of the post, and a lateral groove on the preparation.Torsional stresses are compensated for if a nonround root preparation is tted with a crown or ring enclosure. There are three types of posts: conical, cylindric, and threaded (Fig 2-149).A conical post can be easily and clearly pre-pared in the root canal because it follows the natural shape of the root canal. The post can be easily tted but, owing to its shape, produces a wedge effect and may fracture the root.Fig 2-145 There are two possible forms of restoration with post crowns: in a post and core, the root preparation is tted with a post to which an articial core is xed; a post and core can be tted with any type of crown, including one within a xed partial denture. In a classic post crown, the root preparation is tted with a post to which the replacement tooth is xed. 77Post CrownsA cylindric post requires a parallel-walled root canal, which is difcult to create. If the accuracy of t is good, it provides excellent retention because of the static frictional resistance of the parallel walls.A threaded post requires a thread in the root ca-nal, which has to be cut with a thread cutter. This can easily lead to fractures. A prefabricated root post is screwed in place and tted with a form of retention for the coronal restoration.FVFHFig 2-146 A post crown is loaded by masticatory forces just like a natural tooth. In the process, the post retention is stressed in three different ways: the masticatory forces can be divided by a forces parallelogram into vertical (FV) and horizontal (FH) masticatory force components. The third stress arises from masticatory forces that act laterally on the tooth and twist it.a b cFig 2-147 The vertical masticatory force component applies compression to the post retention, but the post cannot transfer this force to the tooth because it would fracture the root. For the vertical force component, post crowns must have a hori-zontal, at support on the root preparation. The contact area is not for retention but serves to absorb forces.Fig 2-148 The horizontal masticatory force component bends the post. The horizontal force can pull the post retention like a nail out of a wall, where the labial wall of the root prepara-tion breaks off. If the labial part of the root preparation is given a rooike bevel, the horizontal force component is well ab-sorbed. The ring-shaped enclosure of the root preparation also anchors the post against bending forces.Fig 2-149 Three different forms of retention post are identi-ed: (a) A conical post is easy to fabricate but has a tendency to a wedge effect, and hence fracture of the root is possible. (b) A cylindric post is difcult to fabricate and has excellent re-tention because of the parallel walls. (c) A threaded post must be prepared precisely, can easily result in fractures, and is not secured against twisting. 78Coronal RestorationPost Crown DesignsPost and core crownA root post is prepared—cast or as a prefabri-cated threaded post—and tted with a metallic pseudopreparation. This post can be divided into a power arm and a work arm according to its loading: The part that extends into the root is the power arm, and the part that extends into the re-placement crown is the work arm. Hence the lon-ger the power arm and the shorter the work arm, the more rigid the retention (Fig 2-150).The length of the coronal restoration is taken to be the minimum length of the post anchored in the root. However, the retention post (power arm) should be at least two-thirds the length of the root canal.Stability is further inuenced by the thickness of the retention post and the size and position of the auxiliary cavities. The post should have a diam-eter of 1.3 to 1.8 mm. When taking the impression, special emphasis should be placed on ensuring that a precise impression is taken of the root ca-nal and that the surface of the root preparation with auxiliary cavities and gingival crevice is pre-cisely reproduced. A ring-retained impression is most suitable, where the root canal impression is taken with a post surrounded by thermoplastic material.In a cast post and core crown, the post is tted with a core buildup that exactly overlaps the root preparation and lls the auxiliary cavity but that leaves the chamfer preparation clear. The post is cemented in place so that a root preparation treated in this way forms the basis for fabrication of any type of coronal restoration.Fabrication of a custom-made postOn the die, the root canal is isolated and lled with casting wax. A castable metal post is then heated and carefully pushed as far as the oor of the canal. The root preparation cover and the aux-iliary cavity are lled. The core buildup is mod-eled in the same way as a prepared tooth: slightly conical on the vestibular and lingual sides and al-most parallel approximally (Figs 2-151 and 2-152). The coronal restoration should have a consistent minimum thickness.To ensure that the castable post adheres in the investment material, it can be cleaned of wax to about a 1-mm height at the root apex. Investing, casting, and nishing are performed following the usual procedure. Casting is typically done in gold, but other materials are permissible if color problems (eg, with acrylic resin jacket crowns) are likely. For better retention of the replacement crown, the core buildup is not polished.According to Richmond, dowel crowns were produced as a unit comprising root post, core cover, and crown framework, then veneered. The classic Richmond crown encases the root prepa-WP = 1.5 WFig 2-150 The stability of the post anchorage depends on the length of the post in the root canal. The aim is a minimum post length that corresponds to crown height. The crown height is the work arm (W), and the post length in the root canal is the power arm (P). If the power arm is too short, the root will break on loading. A power arm length 1.5 times the work arm length is ideal. The post should therefore be longer than the crown length. 79Post Crown Designsration with a sheet-metal crown to which a pre-fabricated post is soldered. The veneer consists of a long pivot tooth made of porcelain, which is riveted into a back protection plate that is cast and soldered onto the crown. This type of crown is now obsolete.In the case of an occlusal crown, the root post, the core cover, and the crown framework are cast in a single piece. The core cover (post cap) is accurately seated on and encompasses the root preparation. The occlusal margin is also the crown margin and ends with the preparation bor-der. Owing to the good placement accuracy and precise path of the crown margin, the occlusal crown provides excellent protection against car-ies, the necessary stabilization of the root prepa-ration against fractures, and good protection of the marginal tissue.If vestibular coverage of the preparation is very short, the lingual coverage is kept very high to compensate; this leaves a good esthetic impres-sion because only a narrow metallic edge re-mains, and this is often covered by the gingival margin. In occlusal crowns, the oral surface made of metal can be fashioned as a back protection without incisal coverage and fully veneered with acrylic resin or ceramic in the vestibular area.The choice of veneering material and the ma-terial for the palatal occlusal surface depends on the stability of the remaining root preparation. It is preferable to use acrylic resin as the veneer-ing and occlusal material because of its favorable abrasion characteristics in preventing periodontal damage.Simple post crowns are suitable as temporary solutions to the long treatment phases for xed partial dentures. For this purpose, prefabricated conical root posts are tted into a root canal, and a solid acrylic resin crown is polymerized onto the prepared retentions. This means the acrylic resin encases the root preparation on all sides like an acrylic resin jacket crown.Fig 2-151 Post cores in multiple-rooted teeth are necessary if the natural coronal dentin core has been lost due to massive caries lesions but the external enamel wall appears mechani-cally strong. A buildup with plastic lling material is only strong enough if posts are let into the root canals and embraced as a unit by the buildup. Two-rooted teeth can be tted with a cen-tral post in the longest and most readily accessible root canal, and a short safety post to counter twisting forces can be in-serted into another canal parallel to the rst.Fig 2-152 Three-rooted maxillary molars with diverging root canal axes can be restored with divided post cores. For this purpose, the buccal root canals are made parallel, and shorter pins with a primary core are inserted. The longer secondary post is placed in the palatal root and pushed through the pri-mary core like a sliding attachment. In terms of preparation, as much hard dental tissue as possible should be preserved, and the subsequent nish of the crown margin should be moved into the dental tissue to prevent corrosive processes that oc-cur when different metals come into constant contact in the oral cavity.

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