Resilient Anchoring and Supporting Elements










Resilient Anchoring
and Supporting
Elements
181
Clasps as Anchoring and Supporting Elements
The simplest, cheapest, and most commonly used anchoring and supporting element for
xing a removable partial denture to the residual dentition is a clasp. There are two types
of clasps: wrought-wire clasps and cast clasps.
Clasps are  exible rings that are open on one side and that embrace the bulbous tooth
and gain retention in the undercut area of the tooth. This implies the actual principle of a
clasp: The parts of the clasp engaged in the undercut area have to be bent apart when the
clasp is being inserted and withdrawn and must therefore be  exibly malleable.
The anchoring function takes place because the lower clasp arms positioned below the
most bulbous part of the anatomically shaped tooth bend apart, are  exibly malleable,
and develop spring forces on insertion and withdrawal of the clasp. The path of the clasp
arms and the position of the clasp tip are designed so that the arms can be  exibly bent
open on insertion and withdrawal, without any permanent deformation and while apply-
ing only de ned clasp forces.

182
Resilient Anchoring and Supporting Elements
Spring deection is the amount by which a clasp
has to be bent open when it is being withdrawn
from or placed on the tooth. The spring deection
of a clasp on a tooth is equivalent to the horizontal
undercut width of the curved tooth surface in the
infrabulge area, relative to the prosthetic equator.
The prosthetic equator or clasp survey line is the
widest circumference of the tooth relative to the
denture’s path of insertion. This prosthetic equa-
tor divides the crown of the tooth into two areas,
where the occlusal area denotes the suprabulge
and the cervical area the infrabulge (Fig 6-1). The
cervical area, relative to the prosthetic equator, is
undercut and suitable for retention of clasps.
The basic clasp design can be demonstrated us-
ing the example of a double-arm clasp with rest.
The double-arm clasp engages in the undercuts
with the two clasp arms on the vestibular and
lingual aspect. Both clasp arms have to be bent
apart on insertion and withdrawal and thereby of-
fer retentive forces.
Following are the functioning segments of a
double-arm clasp (Figs 6-2 to 6-5):
The clasp body or encircling part is the central
segment from which all the other clasp compo-
nents originate.
The clasp rest is a tongue-shaped occlusal rest
on the clasped tooth that is always arranged
horizontally and secures the vertical position
(periodontal support).
The clasp shoulder is part of the encircling part
and forms the transition between the clasp body
and upper arm.
The upper arm (or proximal arm) surrounds the
tooth from the occlusal in the suprabulge area
and widens the enclosure of the tooth.
The enclosure parts are rigid and secure the hor-
izontal position (shear distribution).
The lower arm (or distal arm) is the resilient re-
taining arm that engages in the retention area
and takes on the actual retentive function.
The clasp tip forms the outer end of the taper-
ing clasp arm. The clasp tip lies deepest in the
infrabulge area and is bent open throughout the
spring deection.
The clasp appendix (clasp anchor, retention) or
minor connector extends from the clasp body
to the denture framework as a partial enclosure
(see Fig 6-5). The minor connector is rigidly con-
nected to the denture framework.
The lingual clasp arm can be constructed as a
so-called guide arm (reciprocal/bracing arm) whose
entire length runs along or above the equator. It
is intended to counterbalance the retaining arm
and prevent the tooth from tipping lingually if the
retaining arm is pulled above the equator. The
tooth surface of the guide arm must be prepared
parallel to the path of insertion so that the (rigid)
guide arm can lie against the tooth throughout
the entire movement of the retaining arm out of
the undercut area to above the equator.
It is possible to design a clasp with a functional
separation into guide and retaining arms, but the
vestibular clasp arm has to be placed far into the
infrabulge area for esthetic reasons. Otherwise,
this design should be rejected, because the teeth
Fig 6-1 The widest circumference of
the tooth can be dened with a parallel-
guided graphite point. Depending on the
incline of the tooth, the following emerge:
(1) the anatomical equator relative to the
tooth axis and (2) the prosthetic equator
relative to the path of insertion of a den-
ture. This equator divides the tooth into
the suprabulge and infrabulge areas. A
clasp is placed so that the resilient lower
arm lies in the infrabulge position and
all the other parts of the clasp are in the
supra bulge position.
Suprabulge: occlusal portion,
rest area
Infrabulge: cervical portion,
retentive area

183
Wrought-Wire Clasps
have to be prepared and the best retention areas
usually lie on the lingual aspect.
Wrought-Wire Clasps
Wrought-wire clasps are pieces of wire that are
bent out of spring-hard, orally compatible steel
wire or a wire made of precious metal alloy. The
diameter of the wire is generally 0.8 mm for steel
and 1.0 mm for precious metal. Seminished parts
(eg, clasp crosses) are also used.
The range of uses for wrought-wire clasps is
limited because they do not fulll the require-
ments of anchoring and supporting elements. In
particular, they lend themselves to interim den-
tures because hardly any damage can occur due
to the shorter wearing time. A wrought-wire clasp
has the advantage of low cost.
Wrought-wire clasps cannot be bent so precise-
ly that they lie entirely pressure free in the resting
position and evenly around the tooth. They do not
ensure adequate positional stability against hori-
zontal thrusts because of the excessive elasticity
of the wrought wire, which, despite strain harden-
ing, exes too much in response to bending.
Clasp shoulder
Upper arm
Clasp body
Clasp appendix
Clasp rest
Clasp shoulder
Upper arm
Lower arm
Clasp tip
Fig 6-3 The clasp body with clasp shoul-
der is referred to as the enclosure and
acts to brace against horizontal shear
forces. The clasp rest is an occlusal rest
that transfers masticatory forces to the
clasped tooth as it secures the vertical
position.
Fig 6-4 The clasp arm divides into an up-
per (proximal) and lower (distal) arm as
well as the clasp tip. The resilient lower
arm is placed in the infrabulge area and
takes on the retentive function. The clasp
arms taper down toward the clasp tip.
Fig 6-5 The minor connector to the
denture framework starts from the clasp
body; it is also known as the clasp ap-
pendix or retention. It connects the clasp
structure to the denture framework or
the denture saddle.
Fig 6-2 A clasp can be
divided into several func-
tional segments, which are
marked here in different
colors.

184
Resilient Anchoring and Supporting Elements
A C-clasp is a single-arm wire
clasp in which the counterbearing
is the denture ange; used as an
interim solution.
A double-crescent (buttery) clasp
is guided around a denture tooth
into the saddle.
A J-clasp is a single-arm clasp
with a long retaining arm.
The appendix of a T-clasp cross
can be laid over the occlusal
surface (Elbrecht clasp).
A double-arm clasp is bent out of
a T-clasp cross and engages in the
undercuts on both sides.
A double-arm clasp can also be
shaped out of two single-arm
clasps.
A double-arm clasp with occlusal
rest is also called a three-arm
clasp and is bent out of a clasp
cross.
In a G-clasp, the lingual clasp arm
is guided occlusally to a rest that
is distant from the saddle.
A double-arm clasp embraces two
teeth from the buccal/labial and
lingual aspect.
A ball-head clasp or drop clasp is
made from a prefabricated part
and engages in the interdental
niche.
A Jackson clasp is a closed cir-
cumferential clasp that is guided
approximally over sections of the
closed dentition.
Single-arm wire claspsDouble-arm wire claspsDouble-arm clasps with restWire clasp modications
A single-arm clasp made of a
prepared ball-head prefabricated
part is guided into the infrabulge
position from the buccal/labial
direction.
Fig 6-6 Common forms of wrought-wire clasps.

185
Cast Clasps
There is no bodily enclosure of a tooth with a
wrought-wire clasp; that is, the clasp does not
protect the clasped tooth from twisting. A wrought-
wire clasp can be bent out of shape by stress
caused by denture movements. Correction by ac-
tivation (rebending) ultimately means that, in the
resting position, the clasp transfers uncontrollable
forces to the clasped tooth and tips or twists it.
Many wrought-wire clasp designs do not in-
clude an occlusal rest, and if they do, it is not
stable enough and warps. The denture will be dis-
placed under masticatory pressure, and the wire
clasp may sink into the marginal periodontium. A
sunken clasp loses retentive function because the
tooth tapers in a cervical direction and the clasp
arm no longer touches the tooth. When it is then
activated, the clasp bends open again on inser-
tion over the wide tooth circumference.
Greatly reduced residual dentitions in which the
static relationships do not permit rigid support of-
fer a specic indication for this type of clasp. In
such cases, the wrought-wire clasp only takes on
a retentive function and no periodontal support
is possible.
Wrought-wire clasp constructions are impor-
tant in orthodontics, where, owing to their high
elasticity, they are used as active spring compo-
nents for regulating malpositions of teeth.
Following are possible wrought-wire clasp de-
signs for use as retentive elements for interim
dentures (Fig 6-6):
Single-arm clasps (C-clasps) only touch the
tooth on the vestibular (labial/buccal) surface,
which is why the denture base must be guided
to the tooth as the reciprocal. The base is made
hollow to avoid stresses on tissues. The C-clasp
lies with the clasp shoulder on the prosthetic
equator and runs from there into the retention
area; it can be guided over two teeth to form a
double crescent shape.
A J-clasp (Bonyhard clasp) is a single-arm clasp
made from a prefabricated part. It lies close to
the cervical margin and is placed with the long
retaining arm inserted into the denture body.
A clasp with an angled arm in a double cres-
cent shape is placed in the retention area of the
clasped tooth; then, with one bend around the
articial tooth on the denture, it is tted into the
denture material. As a result, the retaining arm
and spring deection are enlarged, which is why
the clasp can be very deeply positioned.
Double-arm clasps are bent out of seminished
parts (clasp crosses) with and without occlusal
rests. Double-arm clasps embrace the tooth, ap-
proaching from the approximal aspect as well as
the vestibular and lingual aspect; the clasp arms
are usually guided into the retention area of the
teeth just behind the clasp shoulder. A double-
arm clasp can also be placed in the form of two
double crescent shapes around two premolars
simultaneously.
A G-clasp is a double-arm clasp where the ex-
tended lingual clasp arm with the clasp tip is
bent in an occlusal direction to form a rest. The
lingual arm does not run in the retention area of
the clasped tooth.
Loop-type clasps are pieces of wire that are scal-
loped around the tooth as the vestibular part
of the clasp engages in the retention area. Ex-
amples of this type of clasp include the Jackson
clasp and the O-clasp.
Ball-head clasps are made from prefabricated
parts. They are guided in the interdental embra-
sure over the teeth and placed in the interden-
tal niche of two teeth; in orthodontics, these are
known as drop clasps.
Cast Clasps
Cast clasps are generally waxed up as a unit with
the denture framework using the model casting
technique and then cast from chromium-nickel
alloy. The high modulus of elasticity of this al-
loy gives cast clasps relatively high rigidity. The
spring deections of such clasps must be precise-
ly xed in order to achieve a dened withdrawal
force, and these deections must be adhered to.
Even slight variations (± 0.1 mm) can cause the
clasp force to be too strong or not strong enough.
Fixed rules govern the design, that is, the shape
and path of the clasp arms (Figs 6-7 and 6-8).
Shortening, lengthening, strengthening, or weak-
ening the clasp arm has a direct inuence on
clasp force. It is also necessary to use specially
preformed (wax) proles and only modify these
while closely monitoring the clasp forces that
arise.

186
Resilient Anchoring and Supporting Elements
When used properly and fabricated correctly, a
cast clasp is a reliable, technically simple struc-
ture and the most affordable retentive and sup-
porting element. A cast-clasp design is preferable
to any wrought-wire clasp. A cast clasp consists
of the aforementioned functional parts.
Advantages of cast clasps
Cast clasps have high accuracy of t, apart from
variations caused by processing errors. This ac-
curacy of t means only a conditionally rigid con-
nection between the residual dentition and the
denture, but it prevents twisting of the clasped
tooth because of its great stiffness and bodily en-
closure of the tooth. The residual dentition can be
adequately braced with groups of cast clasps and
a stable cast denture framework (Fig 6-9).
Roughly axial loading of the clasped tooth can
be achieved because of the occlusal rest, which is
part of every cast clasp. The low elasticity of the
clasp material allows clasp structures to have a
slim design.
Disadvantages of cast clasps
Mechanical wear of the enamel occurs during
insertion and removal of the denture and in re-
sponse to relative movements of the denture
while functioning (Fig 6-10). Deposits, and conse-
quently caries, can develop under the clasps; this
risk is increased if the occlusal rest or a lingual
guide plane is prepared. A clasped tooth should
therefore be protected against mechanical de-
struction with a coronal restoration, especially
because occlusal rest surfaces and adequate re-
tention areas relative to the path of insertion can
be created in the process. If, however, an abut-
ment tooth has a coronal restoration, a different
anchoring element should be used, such as a pre-
fabricated attachment instead of a clasp.
Cast clasps do not provide a rigid connection
between the residual dentition and the denture.
In response to stresses, denture movement al-
ways affects the anchoring tooth. Denture move-
ments put an eccentric load on the clasped tooth
because, in the case of a clasp, forces are always
transmitted in a punctiform fashion via the dif-
ferent clasp parts: Vertical forces are transferred
eccentrically to the approximal marginal ridge by
the occlusal rests. Transverse stresses are trans-
mitted by alternating parts of the enclosure or by
the resilient lower arms of the clasp. The clasp
and the clasped tooth do not form a mechanical
unit but only a loose, unstable join.
Fig 6-7 Fixed rules govern the position and path of cast clasps.
The rigid clasp parts (clasp body, shoulder, and upper arm) lie
in the rest area (suprabulge position), and resilient lower clasp
arms are guided into the undercut retentive area (infrabulge po-
sition) so that the clasp tips lie at the deepest point and have to
be bent open as wide as possible on insertion and withdrawal
of the clasp.
Fig 6-8 The position of the clasp tip relative to the prosthetic
equator can be determined in the vertical and horizontal direc-
tion: The vertical distance of the clasp tip from the prosthetic
equator is called the retention depth (R); this is how far the
clasp has to be raised out of the resting position. In the pro-
cess, it opens by the spring deection (s), which is the hori-
zontal distance; this distance is measured to determine the
retentive force of a clasp.
s s
R
R

187
Requirements for Cast Clasps
Activation of cast clasps is not possible because
it leads to mechanical weakening of the cast ma-
terial. Strain hardening of the metal arises and the
clasp becomes harder; however, the permanent
bending strength decreases rapidly, microcracks
appear, and the clasp breaks. If cast clasps are ac-
tivated, the retentive force increases for the time
being, but the actual functioning of the clasp is
lost. The clasp arms no longer t without pressure
but are now in a prestressed state, even in the
resting position, and they transfer forces to the
tooth. The occlusal rest normally holds the tooth
while the activated clasp arms lift and thereby tip
the tooth in the opposing region. The tooth loos-
ens and is lost.
In addition, clasps are always cosmetically un-
favorable.
Requirements for
Cast Clasps
Cast-clasp components must meet the require-
ments that apply to anchoring and supporting el-
ements (Fig 6-11):
They must secure the horizontal position by
means of rigid enclosing parts.
They must brace the residual dentition by physi-
cally engaging the abutment tooth.
They must ensure periodontal hygiene due to
precise t.
They must provide pressure-free close tting in
the resting position to avoid orthodontic forces.
They must secure the vertical position by means
of occlusal rests.
They must ensure the retentive function by
means of dened spring forces.
In terms of securing horizontal position, the
clasp body, clasp shoulders, clasp upper arms,
the projection to the minor connector, and the oc-
clusal rest form the rigid enclosing parts. They lie
above the surveyed prosthetic equator in the su-
prabulge area. They brace the denture against
horizontal thrusts and stabilize it. Horizontal forc-
es from a specic direction are not transferred by
the whole enclosure, only by certain, mainly
punctiform parts, and this results in eccentric
tooth loading. The horizontal positional stability
of the clasp also applies to the clasped tooth,
which is secured against twisting, tipping, and
displacement.
Bracing the residual dentition by means of
splinting is possible because the aforementioned
parts are relatively rigid and embrace the tooth
bodily (ie, extensively and with accuracy of t).
These anchoring parts achieve the effect of splint-
Fig 6-9 The advantages of cast clasps over wrought-wire
clasps lie in their higher accuracy of t, their higher rigidity,
and their bodily enclosure of the tooth. As a result, the residual
dentition is splinted and stabilized. All the forces acting on the
denture are transferred to the remaining teeth via the group of
clasps. All of the teeth enclosed in the group of clasps are able
to support each other in this grouping.
Fig 6-10 The enclosure of the clasped tooth also brings disad-
vantages: (1) Deposits and caries lesions can form under the
relatively wide clasp arms. (2) Mechanical wear of the enamel
results every time the clasp is inserted and withdrawn. (3)
Horizontal thrusts are only transferred to the clasped tooth in a
punctiform or linear fashion, and the tooth is tipped.

188
Resilient Anchoring and Supporting Elements
Secure the horizontal position by
means of rigid enclosing parts
Brace the residual dentition by bodily
engaging the abutment tooth
Secure the vertical position
using occlusal rests
Ensure periodontal hygiene by means of
precise t and minimal distance from the
marginal periodontium
Ensure the retentive function
with dened spring forces
Fig 6-11 Requirements for cast clasps.

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Resilient Anchoring and Supporting Elements181Clasps as Anchoring and Supporting ElementsThe simplest, cheapest, and most commonly used anchoring and supporting element for  xing a removable partial denture to the residual dentition is a clasp. There are two types of clasps: wrought-wire clasps and cast clasps.Clasps are  exible rings that are open on one side and that embrace the bulbous tooth and gain retention in the undercut area of the tooth. This implies the actual principle of a clasp: The parts of the clasp engaged in the undercut area have to be bent apart when the clasp is being inserted and withdrawn and must therefore be  exibly malleable.The anchoring function takes place because the lower clasp arms positioned below the most bulbous part of the anatomically shaped tooth bend apart, are  exibly malleable, and develop spring forces on insertion and withdrawal of the clasp. The path of the clasp arms and the position of the clasp tip are designed so that the arms can be  exibly bent open on insertion and withdrawal, without any permanent deformation and while apply-ing only de ned clasp forces. 182Resilient Anchoring and Supporting ElementsSpring deection is the amount by which a clasp has to be bent open when it is being withdrawn from or placed on the tooth. The spring deection of a clasp on a tooth is equivalent to the horizontal undercut width of the curved tooth surface in the infrabulge area, relative to the prosthetic equator. The prosthetic equator or clasp survey line is the widest circumference of the tooth relative to the denture’s path of insertion. This prosthetic equa-tor divides the crown of the tooth into two areas, where the occlusal area denotes the suprabulge and the cervical area the infrabulge (Fig 6-1). The cervical area, relative to the prosthetic equator, is undercut and suitable for retention of clasps.The basic clasp design can be demonstrated us-ing the example of a double-arm clasp with rest. The double-arm clasp engages in the undercuts with the two clasp arms on the vestibular and lingual aspect. Both clasp arms have to be bent apart on insertion and withdrawal and thereby of-fer retentive forces.Following are the functioning segments of a double-arm clasp (Figs 6-2 to 6-5):• The clasp body or encircling part is the central segment from which all the other clasp compo-nents originate.• The clasp rest is a tongue-shaped occlusal rest on the clasped tooth that is always arranged horizontally and secures the vertical position (periodontal support).• The clasp shoulder is part of the encircling part and forms the transition between the clasp body and upper arm.• The upper arm (or proximal arm) surrounds the tooth from the occlusal in the suprabulge area and widens the enclosure of the tooth.• The enclosure parts are rigid and secure the hor-izontal position (shear distribution).• The lower arm (or distal arm) is the resilient re-taining arm that engages in the retention area and takes on the actual retentive function.• The clasp tip forms the outer end of the taper-ing clasp arm. The clasp tip lies deepest in the infrabulge area and is bent open throughout the spring deection.• The clasp appendix (clasp anchor, retention) or minor connector extends from the clasp body to the denture framework as a partial enclosure (see Fig 6-5). The minor connector is rigidly con-nected to the denture framework.The lingual clasp arm can be constructed as a so-called guide arm (reciprocal/bracing arm) whose entire length runs along or above the equator. It is intended to counterbalance the retaining arm and prevent the tooth from tipping lingually if the retaining arm is pulled above the equator. The tooth surface of the guide arm must be prepared parallel to the path of insertion so that the (rigid) guide arm can lie against the tooth throughout the entire movement of the retaining arm out of the undercut area to above the equator.It is possible to design a clasp with a functional separation into guide and retaining arms, but the vestibular clasp arm has to be placed far into the infrabulge area for esthetic reasons. Otherwise, this design should be rejected, because the teeth Fig 6-1 The widest circumference of the tooth can be dened with a parallel-guided graphite point. Depending on the incline of the tooth, the following emerge: (1) the anatomical equator relative to the tooth axis and (2) the prosthetic equator relative to the path of insertion of a den-ture. This equator divides the tooth into the suprabulge and infrabulge areas. A clasp is placed so that the resilient lower arm lies in the infrabulge position and all the other parts of the clasp are in the supra bulge position.Suprabulge: occlusal portion, rest areaInfrabulge: cervical portion, retentive area 183Wrought-Wire Claspshave to be prepared and the best retention areas usually lie on the lingual aspect.Wrought-Wire ClaspsWrought-wire clasps are pieces of wire that are bent out of spring-hard, orally compatible steel wire or a wire made of precious metal alloy. The diameter of the wire is generally 0.8 mm for steel and 1.0 mm for precious metal. Seminished parts (eg, clasp crosses) are also used.The range of uses for wrought-wire clasps is limited because they do not fulll the require-ments of anchoring and supporting elements. In particular, they lend themselves to interim den-tures because hardly any damage can occur due to the shorter wearing time. A wrought-wire clasp has the advantage of low cost.Wrought-wire clasps cannot be bent so precise-ly that they lie entirely pressure free in the resting position and evenly around the tooth. They do not ensure adequate positional stability against hori-zontal thrusts because of the excessive elasticity of the wrought wire, which, despite strain harden-ing, exes too much in response to bending.Clasp shoulderUpper armClasp bodyClasp appendixClasp restClasp shoulderUpper armLower armClasp tipFig 6-3 The clasp body with clasp shoul-der is referred to as the enclosure and acts to brace against horizontal shear forces. The clasp rest is an occlusal rest that transfers masticatory forces to the clasped tooth as it secures the vertical position.Fig 6-4 The clasp arm divides into an up-per (proximal) and lower (distal) arm as well as the clasp tip. The resilient lower arm is placed in the infrabulge area and takes on the retentive function. The clasp arms taper down toward the clasp tip.Fig 6-5 The minor connector to the denture framework starts from the clasp body; it is also known as the clasp ap-pendix or retention. It connects the clasp structure to the denture framework or the denture saddle.Fig 6-2 A clasp can be divided into several func-tional segments, which are marked here in different colors. 184Resilient Anchoring and Supporting ElementsA C-clasp is a single-arm wire clasp in which the counterbearing is the denture ange; used as an interim solution.A double-crescent (buttery) clasp is guided around a denture tooth into the saddle.A J-clasp is a single-arm clasp with a long retaining arm.The appendix of a T-clasp cross can be laid over the occlusal surface (Elbrecht clasp).A double-arm clasp is bent out of a T-clasp cross and engages in the undercuts on both sides.A double-arm clasp can also be shaped out of two single-arm clasps.A double-arm clasp with occlusal rest is also called a three-arm clasp and is bent out of a clasp cross.In a G-clasp, the lingual clasp arm is guided occlusally to a rest that is distant from the saddle.A double-arm clasp embraces two teeth from the buccal/labial and lingual aspect.A ball-head clasp or drop clasp is made from a prefabricated part and engages in the interdental niche.A Jackson clasp is a closed cir-cumferential clasp that is guided approximally over sections of the closed dentition.Single-arm wire claspsDouble-arm wire claspsDouble-arm clasps with restWire clasp modicationsA single-arm clasp made of a prepared ball-head prefabricated part is guided into the infrabulge position from the buccal/labial direction.Fig 6-6 Common forms of wrought-wire clasps. 185Cast ClaspsThere is no bodily enclosure of a tooth with a wrought-wire clasp; that is, the clasp does not protect the clasped tooth from twisting. A wrought-wire clasp can be bent out of shape by stress caused by denture movements. Correction by ac-tivation (rebending) ultimately means that, in the resting position, the clasp transfers uncontrollable forces to the clasped tooth and tips or twists it.Many wrought-wire clasp designs do not in-clude an occlusal rest, and if they do, it is not stable enough and warps. The denture will be dis-placed under masticatory pressure, and the wire clasp may sink into the marginal periodontium. A sunken clasp loses retentive function because the tooth tapers in a cervical direction and the clasp arm no longer touches the tooth. When it is then activated, the clasp bends open again on inser-tion over the wide tooth circumference.Greatly reduced residual dentitions in which the static relationships do not permit rigid support of-fer a specic indication for this type of clasp. In such cases, the wrought-wire clasp only takes on a retentive function and no periodontal support is possible. Wrought-wire clasp constructions are impor-tant in orthodontics, where, owing to their high elasticity, they are used as active spring compo-nents for regulating malpositions of teeth.Following are possible wrought-wire clasp de-signs for use as retentive elements for interim dentures (Fig 6-6):• Single-arm clasps (C-clasps) only touch the tooth on the vestibular (labial/buccal) surface, which is why the denture base must be guided to the tooth as the reciprocal. The base is made hollow to avoid stresses on tissues. The C-clasp lies with the clasp shoulder on the prosthetic equator and runs from there into the retention area; it can be guided over two teeth to form a double crescent shape.• A J-clasp (Bonyhard clasp) is a single-arm clasp made from a prefabricated part. It lies close to the cervical margin and is placed with the long retaining arm inserted into the denture body.• A clasp with an angled arm in a double cres-cent shape is placed in the retention area of the clasped tooth; then, with one bend around the articial tooth on the denture, it is tted into the denture material. As a result, the retaining arm and spring deection are enlarged, which is why the clasp can be very deeply positioned.• Double-arm clasps are bent out of seminished parts (clasp crosses) with and without occlusal rests. Double-arm clasps embrace the tooth, ap-proaching from the approximal aspect as well as the vestibular and lingual aspect; the clasp arms are usually guided into the retention area of the teeth just behind the clasp shoulder. A double-arm clasp can also be placed in the form of two double crescent shapes around two premolars simultaneously.• A G-clasp is a double-arm clasp where the ex-tended lingual clasp arm with the clasp tip is bent in an occlusal direction to form a rest. The lingual arm does not run in the retention area of the clasped tooth.• Loop-type clasps are pieces of wire that are scal-loped around the tooth as the vestibular part of the clasp engages in the retention area. Ex-amples of this type of clasp include the Jackson clasp and the O-clasp.• Ball-head clasps are made from prefabricated parts. They are guided in the interdental embra-sure over the teeth and placed in the interden-tal niche of two teeth; in orthodontics, these are known as drop clasps.Cast ClaspsCast clasps are generally waxed up as a unit with the denture framework using the model casting technique and then cast from chromium-nickel alloy. The high modulus of elasticity of this al-loy gives cast clasps relatively high rigidity. The spring deections of such clasps must be precise-ly xed in order to achieve a dened withdrawal force, and these deections must be adhered to. Even slight variations (± 0.1 mm) can cause the clasp force to be too strong or not strong enough.Fixed rules govern the design, that is, the shape and path of the clasp arms (Figs 6-7 and 6-8). Shortening, lengthening, strengthening, or weak-ening the clasp arm has a direct inuence on clasp force. It is also necessary to use specially preformed (wax) proles and only modify these while closely monitoring the clasp forces that arise. 186Resilient Anchoring and Supporting ElementsWhen used properly and fabricated correctly, a cast clasp is a reliable, technically simple struc-ture and the most affordable retentive and sup-porting element. A cast-clasp design is preferable to any wrought-wire clasp. A cast clasp consists of the aforementioned functional parts.Advantages of cast claspsCast clasps have high accuracy of t, apart from variations caused by processing errors. This ac-curacy of t means only a conditionally rigid con-nection between the residual dentition and the denture, but it prevents twisting of the clasped tooth because of its great stiffness and bodily en-closure of the tooth. The residual dentition can be adequately braced with groups of cast clasps and a stable cast denture framework (Fig 6-9).Roughly axial loading of the clasped tooth can be achieved because of the occlusal rest, which is part of every cast clasp. The low elasticity of the clasp material allows clasp structures to have a slim design.Disadvantages of cast claspsMechanical wear of the enamel occurs during insertion and removal of the denture and in re-sponse to relative movements of the denture while functioning (Fig 6-10). Deposits, and conse-quently caries, can develop under the clasps; this risk is increased if the occlusal rest or a lingual guide plane is prepared. A clasped tooth should therefore be protected against mechanical de-struction with a coronal restoration, especially because occlusal rest surfaces and adequate re-tention areas relative to the path of insertion can be created in the process. If, however, an abut-ment tooth has a coronal restoration, a different anchoring element should be used, such as a pre-fabricated attachment instead of a clasp.Cast clasps do not provide a rigid connection between the residual dentition and the denture. In response to stresses, denture movement al-ways affects the anchoring tooth. Denture move-ments put an eccentric load on the clasped tooth because, in the case of a clasp, forces are always transmitted in a punctiform fashion via the dif-ferent clasp parts: Vertical forces are transferred eccentrically to the approximal marginal ridge by the occlusal rests. Transverse stresses are trans-mitted by alternating parts of the enclosure or by the resilient lower arms of the clasp. The clasp and the clasped tooth do not form a mechanical unit but only a loose, unstable join. Fig 6-7 Fixed rules govern the position and path of cast clasps. The rigid clasp parts (clasp body, shoulder, and upper arm) lie in the rest area (suprabulge position), and resilient lower clasp arms are guided into the undercut retentive area (infrabulge po-sition) so that the clasp tips lie at the deepest point and have to be bent open as wide as possible on insertion and withdrawal of the clasp.Fig 6-8 The position of the clasp tip relative to the prosthetic equator can be determined in the vertical and horizontal direc-tion: The vertical distance of the clasp tip from the prosthetic equator is called the retention depth (R); this is how far the clasp has to be raised out of the resting position. In the pro-cess, it opens by the spring deection (s), which is the hori-zontal distance; this distance is measured to determine the retentive force of a clasp.s sRR 187Requirements for Cast ClaspsActivation of cast clasps is not possible because it leads to mechanical weakening of the cast ma-terial. Strain hardening of the metal arises and the clasp becomes harder; however, the permanent bending strength decreases rapidly, microcracks appear, and the clasp breaks. If cast clasps are ac-tivated, the retentive force increases for the time being, but the actual functioning of the clasp is lost. The clasp arms no longer t without pressure but are now in a prestressed state, even in the resting position, and they transfer forces to the tooth. The occlusal rest normally holds the tooth while the activated clasp arms lift and thereby tip the tooth in the opposing region. The tooth loos-ens and is lost.In addition, clasps are always cosmetically un-favorable.Requirements for Cast ClaspsCast-clasp components must meet the require-ments that apply to anchoring and supporting el-ements (Fig 6-11): • They must secure the horizontal position by means of rigid enclosing parts.• They must brace the residual dentition by physi-cally engaging the abutment tooth.• They must ensure periodontal hygiene due to precise t.• They must provide pressure-free close tting in the resting position to avoid orthodontic forces.• They must secure the vertical position by means of occlusal rests.• They must ensure the retentive function by means of dened spring forces.In terms of securing horizontal position, the clasp body, clasp shoulders, clasp upper arms, the projection to the minor connector, and the oc-clusal rest form the rigid enclosing parts. They lie above the surveyed prosthetic equator in the su-prabulge area. They brace the denture against horizontal thrusts and stabilize it. Horizontal forc-es from a specic direction are not transferred by the whole enclosure, only by certain, mainly punctiform parts, and this results in eccentric tooth loading. The horizontal positional stability of the clasp also applies to the clasped tooth, which is secured against twisting, tipping, and displacement.Bracing the residual dentition by means of splinting is possible because the aforementioned parts are relatively rigid and embrace the tooth bodily (ie, extensively and with accuracy of t). These anchoring parts achieve the effect of splint-Fig 6-9 The advantages of cast clasps over wrought-wire clasps lie in their higher accuracy of t, their higher rigidity, and their bodily enclosure of the tooth. As a result, the residual dentition is splinted and stabilized. All the forces acting on the denture are transferred to the remaining teeth via the group of clasps. All of the teeth enclosed in the group of clasps are able to support each other in this grouping.Fig 6-10 The enclosure of the clasped tooth also brings disad-vantages: (1) Deposits and caries lesions can form under the relatively wide clasp arms. (2) Mechanical wear of the enamel results every time the clasp is inserted and withdrawn. (3) Horizontal thrusts are only transferred to the clasped tooth in a punctiform or linear fashion, and the tooth is tipped. 188Resilient Anchoring and Supporting ElementsSecure the horizontal position by means of rigid enclosing partsBrace the residual dentition by bodily engaging the abutment toothSecure the vertical position using occlusal restsEnsure periodontal hygiene by means of precise t and minimal distance from the marginal periodontiumEnsure the retentive function with dened spring forcesFig 6-11 Requirements for cast clasps. 189Securing the Vertical Position with Cast Claspsing the residual dentition because the denture framework acts as a large, relatively rigid con-nector between the individual clasps. In addition, the rigid clasp parts on the lingual tooth surfaces can be guided over one quadrant or the whole dental arch as continuous enclosures or splinting elements.Achieving periodontal hygiene is problematic with clasp structures. Fabricating clasp compo-nents by the model casting process ensures very good accuracy of t. The enclosure provided by the clasping parts lies at on the tooth surface and thereby provides a place for deposits and car-ies to accumulate, and this is accentuated if the insides of the clasps are rough and unpolished.Problem areas with clasps, apart from the ex-tensive embracing parts, are the minor connec-tors and their junctions with the clasp body. The minor connectors should always be guided clear of the periodontium, especially if they have to be directed to the clasp interdentally within closed parts of the dental arch (eg, Bonwill clasp). In the case of closed-saddle shaping of the minor con-nector, acrylic coverage is avoided on the muco-sal side, and the metal is polished to a smooth nish.Pressure-free close tting in the resting posi-tion means the clasp arms must lie entirely ten-sion free; that is, they absolutely must not be ac-tivated. It is only on insertion, on withdrawal, and during functioning that clasps develop their pre-cisely dened retentive forces and transfer them to the clasped tooth.Any prestressing of the clasp arms by activa-tion (rebending) can exert forces on the tooth in the resting position, which produce tipping or twisting on the tooth. The periodontal tissues are rst loaded in the nal position of the denture, then additionally as they are functioning. Even if the precise direction and the extent of these force effects cannot be determined, the clasp in the resting position acts as a form of regulation, and when functioning, overloading of the periodon-tium ensues.The requirement for a tension-free resting po-sition of the clasp indicates that uncontrolled forces must not be transferred to the tooth. Every cast clasp leads to calculable and controllable in-appropriate stresses on the clasped tooth; these must not be increased, however, by rebending the clasp arms, because this means the structure will denitely fail.Activation of cast clasps should basically be re-jected for the reasons outlined earlier (ie, strain hardening, microcracks, prestressing). Bending cast clasps open to better t the model cast den-ture onto the model is wrong! The need to rebend is always a sign of planning and surveying errors; in most cases, these are attributable to the den-tal technician. Securing the vertical position by means of occlusal rests and the retentive function by means of dened spring forces is discussed in the sections that follow.Securing the Vertical Position with Cast ClaspsThe partial denture is supported on the residual teeth, which is why every clasp is provided with an occlusal rest. The functions of the occlusal rest are the following:• To absorb masticatory loads that strike the re-placement teeth and distribute them to the peri-odontium of the clasped teeth by means of axial pressure.• To divert food particles away from the interface between the denture and the residual dentition, thus taking on a similar function to the approxi-mal contact points.• To stabilize the position of the clasp relative to the tooth.• To prevent the denture from tipping sideways onto the mucosa.• To prevent the clasp arms from slipping down in a cervical direction. Failure to do this would damage the marginal periodontium, the reten-tive function would no longer be fullled be-cause the clasped tooth tapers cervically, and in this case the clasp arms would stand out. In the process, the nonresilient enclosure by the upper clasp arms would be displaced, and the actual splinting effect would be lost. The denture might become embedded, thereby overloading the mucosal areas and causing the marginal peri-odontium to be squeezed and destroyed in the area bordering the gap. 190Resilient Anchoring and Supporting ElementsThe shape and position of the occlusal rest have a decisive inuence on the functioning of this im-portant part of the clasp. Positioning of the rest in the dental arch is guided by static considerations. The teeth with the largest root surface area and hence the greatest periodontal loading capacity should ideally be chosen: rst molars, then pre-molars, then canines and incisors.The bottom of the rest lies perpendicular to the tooth axis because it is essential to ensure that largely axial forces are transferred. If the bottom of the rest inclines outward, the rest will act like an inclined plane so that the tooth is tipped and the clasp slips away from the tooth and down-ward (Fig 6-12). The disadvantages of a clasp that has slipped off have been mentioned; in this case, the tooth will suffer additional damage to its peri-odontal tissue as a result of tipping.The rest area is prepared into the tooth as a cav-ity so that no malocclusions will occur (Fig 6-13). A spoon-shaped, not a box-shaped, recess should be created. This shape avoids notch effects and al-lows rotational movements of the occlusal rest, which can occur as the denture is functioning. The Fig 6-12 The bottom of the rest should always be placed hori-zontally so that axial loading on the teeth is roughly achieved. If the bottom of the rest slopes away, it acts like an inclined plane; the rest will slip off, and the tooth will be tipped. If the bottom of the rest slopes down to the middle of the tooth, the tooth will tip toward the rest, which will have no effect in the case of a bounded saddle but will lead to tooth loosening in a free-end saddle because the tooth is pulled in a distal direction.Fig 6-13 For an occlusal rest, the surface must be prepared so that no malocclusions can arise. The recess should be spoon shaped rather than box shaped in order to avoid a notch effect and thus fracture of the rest as well as to ensure that slight movement of the rest is still possible.Fig 6-14 The rest must not interfere with the opposing oc-clusion and is therefore recessed in the tooth. To ensure resis-tance to fracture, the rest should be 2.5 to 3.0 mm long and wide and 1.5 mm thick. The tooth should be protected with a lling in the rest area or a coronal restoration. 191Dened Retentive Force with Cast Claspsdimensions of the recess allow a break-proof rest to be created: The width and length of a rest is approximately 2.5 to 3.0 mm, and the thickness must not be less than 1.5 mm (Fig 6-14). The tran-sition from the bottom of the rest to the clasp body should be rounded off so that no notch ef-fect occurs at this point and the rest does not frac-ture. The tooth should be protected, at least in the rest area, by a precise (gold) lling that is reason-ably substantial if coronal restoration is not being considered. The occlusal rests are prepared in the same way as crowns.An occlusal rest produces unfavorable static re-lationships on a tooth. Because it will always lie on the approximal marginal ridge, any rest will eccentrically load the abutment tooth and tip it slightly (Fig 6-15). An upright tooth with a healthy periodontium will bear such loading to a limited extent without being damaged. Tipped teeth—and tipping is usually the case—should not be loaded by the rest but instead should be provided with support on the opposite side to their tilt. Bev-eling the bottom of the occlusal rest at the middle of the tooth increases the tipping because of the effect of the inclined plane.Double rests are suitable for achieving axial loading; if a free-end saddle sinks, however, one of the rests becomes the fulcrum and the other rest lifts off. For the same reason, letting the oc-clusal rest extend over the entire occlusal surface is not a solution.Claw-like rests on canines and incisors embrace the incisal edges over to the labial aspect. For this purpose, a cavity for the rest must always be pre-pared in order to produce a horizontal surface for the vertical forces.The drawback of occlusal rests is indicated by the very name: the rest merely lies on and is not rigidly connected to the clasped tooth; it offers a statically indeterminate system with all of its dis-advantages. A cast clasp with occlusal rest is nev-ertheless a far better solution than a single-arm wrought-wire clasp.Dened Retentive Force with Cast ClaspsThe main function of a cast clasp is to hold the denture on the residual dentition. The retentive force of a clasp is mainly based on the elasticity of the clasp arms: On withdrawal from the tooth, the lower clasp arms have to be widened over the prosthetic equator. The clasp force on withdrawal is in the region of 5 to 10 N. If every clasp on a denture achieves this clasp force, a denture can-not be lifted off, even by sticky foods, by its own weight, or by tongue pressure.Fig 6-15 Every clasp loads a tooth eccentrically; that is, the tooth is tipped by a clasp rest. An upright tooth with a healthy periodontium can tolerate such stresses. In the case of an in-clined tooth, a rest will further increase the tipping if the rest is placed on the side of the tilt. The more inclined a tooth is, the more it will be tipped by the rest. The rest therefore should be positioned on the opposite side.http://dentalebooks.com 192Resilient Anchoring and Supporting ElementsFirst requirementThe same retentive forces should be assigned to all of the clasps in a group in order to achieve a uniformly rm seating of the denture. The re-tentive force of a clasp depends on the extent to which the resilient clasp arms are bent open. This means a clasp arm must be pulled off the tooth so that it is bent open by precisely the dened spring deection (Fig 6-16). The spring deection must be accurately dened and measured relative to the dimensions of the clasp arms.Second requirementA clasp must not tip or twist a clasped tooth on withdrawal. Therefore, both clasp arms (lingual and labial/buccal) extend into the retention area and develop the same spring forces on withdraw-al. If the undercut widths differ, this can be off-set by changing the length of the clasp arms or changing the prole thickness.Third requirementBoth clasp arms have the same retention depth below the prosthetic equator. The retention depth is the amount by which the clasp arms have to be raised until they lie on the prosthetic equator. It is no use if both clasp arms are able to develop the same spring force but one clasp arm lies on the prosthetic equator after the clasp has briey been lifted, while the other arm still lies below the equator; the rst clasp arm will exert its full spring force, and the second may only exert half its spring force (Fig 6-17). As a result, the clasped tooth is tipped in the direction of the lower-lying clasp arm. If the clasp is lifted higher, the rst arm loses contact with the tooth; now it lies in the su-prabulge position, and the second clasp arm acts alone. The clasped tooth tips in the direction of the higher-lying arm. The clasped tooth is subject to tensile loading and is vigorously shaken in the process.Alternative requirementThe cast clasp is constructed with a vestibular re-taining arm and a lingual guide arm; that is, the vestibular clasp arm lies with its last third in the retention area, while the lingual clasp arm runs above or on the prosthetic equator and becomes the reciprocal of the retaining arm. These clasp paths are universally proposed. If the clasp is pulled off, the lingual reciprocal clasp immediate-ly loses contact with the tooth because it is locat-ed in the suprabulge area (Fig 6-18). The further the clasp is lifted off, the more the vestibular resil-ient arm will load the clasped tooth transversally (ie, tip it lingually). On insertion or withdrawal, the clasped tooth is tipped with the spring force, leading to damage to the periodontium.To avoid tipping of teeth, the guide arm must be guided parallel to the path of insertion, which is ensured by corrective reduction in the tooth surface (Fig 6-19). The advantage of the structural subdivision into retaining arm and guide arm is that even sections of dental arches in which there are no lingual undercuts on the teeth can be treated; this can happen with maxillary teeth that have a labial/buccal inclination, especially ante-rior teeth. The vestibular clasp arm can be placed more deeply and thus is more esthetically accept-able. Insertion and withdrawal is easier in a group Fig 6-16 Ideally both clasp arms bend open evenly on insertion and withdrawal of the clasp, provided that they have the same retention depth and simultaneous-ly reach the prosthetic equator; then they are widened by the maximum spring deection and produce maximum spring force. The tooth is loaded axially and not transversally.http://dentalebooks.com 193Dened Retentive Force with Cast Claspsof clasps in which only the vestibular retaining arms are active. It is assumed that horizontal po-sitional stability is more effective if a rigid guide arm runs over the entire lingual surface of the teeth.The disadvantage is that a guide plane has to be prepared in the tooth, which impairs caries prevention. The active retaining arm acting with full spring force produces distinct signs of wear on the tooth surface. Undercuts lying lingually enough can always be found on posterior teeth, especially if the clasp tips are guided to the ap-proximal interdental area. Therefore, there is lim-ited justication for constructing a guide arm for anterior teeth only.Fig 6-17 Both clasp arms belong in the reten-tion area and should have the same retention depth, which means they both have to be lifted by the same amount in order to travel through their full spring deection. If a clasp arm moves lower below the equator to achieve the same spring deection as the other clasp arm, the tooth will be subject to tipping. In fact, if one clasp arm has reached its full spring force because it lies on the equator, the other arm might have only developed half its spring force. The tooth will be vigorously shaken on with-drawal of the clasp, or the clasp will jump off the tooth before that clasp arm has developed its full spring force.Fig 6-18 If one clasp arm lies properly in the retention area but the other arm is left on the prosthetic equator as a guide arm, the guide arm will immediately lose its tooth contact on lifting of the denture and will be ineffective as a reciprocal. The clasped tooth tips toward the guide arm. Such clasp structures use the reten-tion area on one side and allow the clasp arm to be positioned more deeply. In the case of ante-rior teeth, the vestibular clasp arm is placed as low as possible in a cervical direction.Fig 6-19 If the lingual clasp arm is designed as a guide arm, it must be guided in parallel over the entire retention depth of the retaining arm and maintain contact with the tooth. To do this, the bulge of the lingual surface must be attened (ie, prepared parallel to the path of in-sertion). In addition, the guide arm is reinforced so that it remains rigid.RRRR1R1R2R2http://dentalebooks.com 194Resilient Anchoring and Supporting ElementsDeterminants of Spring ForceThe underlying principle of clasps as resilient an-choring elements is a technical spring-clip tting: A resilient ring open on one side can be bent apart and springs back into its original position when the force of bending open subsides. If such a ring is pushed over a conical shaft, it widens with the gradient of the cone. More and more force is needed to push the ring further because the re-storing force—the force that tries to restore the ring to its original shape—also becomes greater.The necessary retentive forces of a clasp should lie in the region of 5 to 10 N so that the tensile loading on withdrawal of the clasp cannot dam-age the periodontal tissue. Therefore, an appro-priate spring deection is set for every clasp, par-ticularly to ensure that the same withdrawal forces are achieved for each clasp within a group. The necessary spring deections have to be mea-sured for this purpose.The spring deection is the amount by which such a ring is bent apart. The force with which the ring is bent apart or will spring back is referred to as spring force. The relationship between spring force and spring deection is linear:Spring force = Spring deection × Spring constantorF = s × cThe spring constant is a guide value that com-bines several values relating to a clasp (indicating the property of elasticity). The elastic behavior of a body is described by Hooke’s law, which states that a solid body (eg, a bar) can be deformed by external inuences up to a certain degree and will spring back into its original state when these ex-ternal forces are no longer effective. Permanent deformation occurs when the body is deformed beyond a certain limit. The limit of loading capac-ity above which permanent deformation remains is known as the elastic limit.The deformation of a body depends on its ma-terial; a wooden bar is easier to bend than a bar made of steel. To describe the elastic behavior of a material, there is a measure known as the mod-ulus of elasticity (Young’s modulus [E]). This is a material specication; similar to the value for the hardness of a substance, it is a specic value for that material.A rmly anchored bar that is subject to a force at its free end will bend in the direction of the ex-erted force (Fig 6-20). The greater the force, the greater the bending (deection). If the force is too great, the bar will bend out of shape or even break. The deection of the bar for a xed load will vary in size as a result of the following chang-es in its shape:•The longer the bar is, the more it will deect in response to the same load.•The thicker the bar is, the smaller the deection in response to the same load; a short, thick bar can absorb considerable force.•A round bar will not deect as much as a at board with the same surface area of the cross section. •If the board is placed on its edge, it will not bend as much as the round bar with the same cross-sectional surface area.•A bar with a T-profile and the same cross- sectional surface area deects the least.•A bar tapered down to the tip will bend most at its tip; its stiffness is greatest at the thick end.Fig 6-20 A rmly anchored bar can bend when it is loaded with a force at one end. The deection is dependent on the force acting on the bar, the shape of the bar, and the material the bar is made from. The illustration assumes a dened force, which acts on bars made from the same material but with dif-ferent proles. A board will deect more than a round bar with the same cross-sectional surface area. A board placed on its edge, however, will deect more than a beam with a T-prole.http://dentalebooks.com 195Determinants of Spring ForceA semi-ellipsoid prole that tapers to its tip is used for cast clasps (Fig 6-21). The clasp arms are placed so far into the infrabulge area of the teeth that the arms bend open by a certain spring deection. The spring deection is precisely mea-sured and must not exceed a specic value to en-sure that the clasp does not deform permanently. The relationships outlined previously also apply to the clasp arm:•The longer a clasp arm is, the less spring force arises in the clasp when the same spring deec-tion is chosen as for a short clasp arm.•A shorter clasp arm has greater spring forces for the same spring deection.•The smaller the modulus of elasticity, the small-er the spring force.•Tapering of the prole makes the clasp tip more elastic.When constructing a clasp, a certain retentive force is required at which the exact spring deec-tion is determined with reference to clasp arm length, prole thickness (Fig 6-22), and material properties (modulus of elasticity). Figure 6-23 out-lines the determinants of clasp force.Fig 6-21 The prole for a clasp arm is semi-ellipsoid and ta-pers down to the clasp tip. The variables used for calculating the spring force of such a prole bar are the length of the re-taining arm (clasp arm length, L), prole width, prole height, taper factor, and modulus of elasticity as a material constant. These make up the spring constant. Once the spring deec-tion (s) is measured, the spring force can be determined from the equation F = s × c. The clasp force should lie in the region of 5 to 10 N, which means a range of 2.5 to 5 N for the arm of a double-arm clasp. For this retentive force, with reference to clasp arm length and prole thickness, a spring deection is determined, and then the path of the clasp arm around the tooth is dened.Fig 6-22 A at clasp-arm prole will easily bend open on with-drawal from the clasped tooth without bending vertically; a high clasp-arm prole develops considerable clasp forces but will deect in the vertical direction.sLLLVariables affecting clasp forceEffect on spring forceThe longer the clasp arm, the smaller the clasp force.The thicker the clasp prole, the greater the clasp force.The more pronounced the taper, the more elastic the clasp arm.The larger the spring deection, the greater the clasp force.The higher the modulus of elasticity, the greater the clasp force.Length of clasp arm Thickness of clasp proleTaper of clasp arm Size of spring deectionModulus of elasticity.Fig 6-23 Determinants of clasp force.http://dentalebooks.com 196Resilient Anchoring and Supporting ElementsDetermining Retentive ForceThe retentive forces of a clasp are generally de-termined experimentally because the conditions for calculating the forces are complex and include variables that change at every point of the clasp arm’s movement on the tooth surface. A distinc-tion must be made between spring force and re-tentive force: spring force is the force with which the clasp arm is bent open, while retentive force arises as a reaction force on the tooth surface.All of the variables included in the spring con-stant as a guide value—such as bar length, cross-section geometry, and modulus of elasticity—are used to determine spring force. For a semi-ellipsoid bar (the shape of a clasp arm) that tapers down to the bar tip (clasp tip), the following formula has been developed; spring force (F) is equivalent to spring deection (s) times spring constant (c):F = s × cThe formula for the spring constant is as follows: C = 3 × E × J0 / L3where E is the modulus of elasticity, J0 is the axial moment of inertia, and L is the clasp arm length or bar length.The axial moment of inertia indicates the spe-cic inherent stability of a semi-ellipsoid prole:J0 = π × B × H3/12where B is the prole width and H is the prole height.The ratio of nal thickness to initial thickness is given to describe the taper; a variable ϕ is derived from this. The complete formula for calculating spring force is therefore: F = If real values for a clasp arm are inserted into this formula, the spring force is obtained. A clasp arm with the following dimensions is used as an example: clasp arm length = 12 mm, prole width = 1 mm, prole height = 0.8 mm, spring deection = 0.5 mm, taper factor (ratio 8:10) = 1.054, modu-lus of elasticity (for steel) = 2.2 × 105 N/mm2. A spring force of 6.1 N is calculated from these val-ues.The spring force acts in the plane of the spring clip. On withdrawal, the clasp is pulled over the tooth perpendicular to this direction of spring force in the path of insertion. In the process, the clasp arms bend apart on the inclined surfaces of the tooth. The force relationships at the tooth surface can be represented by the physical model of an inclined plane.The physical model breaks down the forces on the body lying on the inclined plane into weight force, slope force, and normal force (Figs 6-24 and 6-25). In this case, weight force can be equated to spring force. The slope of the inclined plane is ex-pressed as the angle α and coincides with the an-gle of inclination of the tooth surface (relative to the path of insertion), the undercut angle. Normal force is the vertical force on the inclined plane (tooth surface) and is determined by the expres-sion: spring force × cos α. Slope force is the force that pulls the body downward. When applied to clasps, it is the force that tries to pull the clasp arm back into its resting position; it is calculated on the inclined plane from the expression: spring force × sin α (see Fig 6-24). Withdrawal force runs in the path of insertion and is related to the slope force in the angle α. The withdrawal force can be calculated from the expression: slope force × cos α. Thus, withdrawal force equals spring force × tan α (Fig 6-26).One variable of withdrawal force is the under-cut angle, which is 10 degrees on average (Figs 6-27 and 6-28). In the clasp resting position, the undercut angle is very large (up to 30 degrees) and the spring force is zero. If the clasp arm lies on the prosthetic equator, the spring force is at its maximum and the undercut angle is 0 degrees with tan α = 0; the withdrawal force would also be zero in this position.Spring force also produces friction on the tooth surface, which runs parallel to the slope force and becomes effective on withdrawal. It is calculated as the normal force multiplied by the friction coef-cient µ and is added to the withdrawal force. This results in the retentive force of the clasp as the sum of the withdrawal and friction forces (Fig 6-29):3 · B · H3 · π 12 14 · ϕEL3×s× ×http://dentalebooks.com 197Determining Retentive ForceFig 6-24 A body with weight W lying on the inclined plane is pressed vertically onto the inclined plane with a normal force (FW = W cos α). The slope force (FS = W sin α) pulls the body downward onto the inclined plane depending on the ex-isting friction force (FF = FWµ).Fig 6-25 The relationships of the in-clined plane, when applied to the tooth surface, give this picture: Weight force W is equivalent to the spring force.Fig 6-26 (right) The retentive force due to bending open along the tooth surface as a function of the angle of inclination of an inclined plane.45° 90°FRFR = FS tan αFig 6-27 The inclines of the tooth sur-faces to the path of insertion differ in size on the vestibular and lingual aspects. The undercut angle decreases toward the equator. On the vestibular side, the undercut angle is very large at the rest-ing point of the clasp, so that clasp force is initially very high despite low spring force. On the lingual side, the undercut angle and clasp force are smaller.Fig 6-28 Where the undercut angle is smaller, the retention depth is very large (lingually) and the retentive force slowly increases. Conversely, a large undercut inclination means the retention depth remains small (vestibular aspect); the re-tentive force starts immediately and can become larger than on the lingual side. This correlation can be used as an argu-ment for using a design with a stable guide arm if the undercut angle can be determined individually relative to the path of insertion.Fig 6-29 Retentive force is calculated from the slope and friction forces: Re-tentive force = (Spring force × tan α) + (Spring force × cos α) × µ.http://dentalebooks.com 198Resilient Anchoring and Supporting ElementsRetentive force = (Spring force × tan α) + (Spring force × cos α) × µThe friction coefcient as a rst approximation is taken to be tan α, which gives the following for-mula for calculating the retentive force of a clasp:Retentive force = Spring force × tan α × (1 + cos α)This gives a retentive force of 5.4 N if an under-cut angle of 25 degrees is assumed and the previ-ously calculated spring force of 6.1 N is applied. This value is very close to the value ascertained experimentally by a surveying system.Surveying CastsAs detailed previously, the variables determining the retentive force of a clasp are spring deection (undercut width), undercut angle, elastic material behavior (modulus of elasticity), spring length (clasp arm length), and thickness and shape of the spring prole (clasp prole). One or more of these variables are determined by means of mea-suring equipment and used for clasp fabrication.When surveying casts, an attempt is made to establish the prosthetic equator for all of the teeth that will hold clasps and, relative to that equator, to determine the spring deection for each clasp arm as a variable of retentive force. The prosthet-ic equator is the clasp survey line with reference to a common path of insertion. The path of inser-tion is the direction in which a denture is inserted and lifted out. With reference to the path of inser-tion, the cast is centered in a parallelometer. The position of the prosthetic equators and hence the undercuts on the clasped teeth can be altered by tipping the cast. The aim is to nd the most favor-able position of the clasp survey line for all of the teeth, thereby ensuring large enough retention areas and a sufcient safety distance (approxi-mately 1 mm) from the marginal periodontium.There are three cast positions (Fig 6-30):• Neutral position: The cast is located in the hori-zontal plane relative to the occlusal plane; the undercuts are balanced.• Mesial tipping: The cast is mesially lowered rela-tive to the occlusal plane. The mesial undercuts become larger; on the anterior teeth, the clasp survey line is displaced in the incisal direction. The denture has to be moved backward into the mouth to be removed.• Distal tipping: The cast is distally inclined rela-tive to the occlusal plane. The distal undercuts become larger; on the anterior teeth, the clasp survey line is displaced cervically in the oral ves-tibule. The denture has to be lifted forward out of the mouth. A parallelometer (surveyor) is needed to estab-lish the tipping of the cast, the clasp survey line, and spring deection. A favorable inclination of the cast is rst determined with this device, and the teeth are checked with a gauge to ensure that adequate undercuts are available and the mini-mum distance from the marginal periodontium can be maintained. The clasp survey line is then marked, and the spring deection is established with a suitable analyzing rod or gauge.The undercut width or spring deection is the horizontal distance to the clasp survey line. There-fore, the gauge for checking undercut width must be a parallel rod that enables a horizontal dis-tance to be measured at its lower end. The under-cut gauges in the Ney system (Dentsply) are avail-able for measuring this distance (Fig 6-31).Ney undercut gauges are parallel rods that wid-en into a bead shape at the end (Fig 6-32). The edge of the bead protrudes beyond the parallel rod by a certain amount. The width of the bead edge cor-responds to the necessary spring deection. The parallel rod can then be guided around the tooth being surveyed so that the edge of the bead and the rod touch the tooth at the same time—the rod at the prosthetic equator and the bead in the retention area. If different spring deections are planned, the dentist can choose from three dif-ferent bead sizes with different edge thicknesses: 0.25, 0.5, or 0.75 mm in the Ney system. The Ney surveying system uses only spring deection in this rough three-way division for a clasp material based on chromium-nickel: (1) short clasp arm = small gauge, (2) medium clasp arm = medium gauge, and (3) long clasp arm = large gauge. For this purpose, a standardized clasp arm prole ta-pering toward its tip and made from wax or ex-ible plastic patterns is supplied, and this is moved in line with the marking on the cast.http://dentalebooks.com 199Surveying CastsThe maximum spring deection denotes the position of the clasp tip, from where the arm ris-es continuously out of the undercut as far as the clasp survey line. It is important to ensure that no part of the clasp arm is placed in an undercut depth larger than that of the tip, because other-wise, on withdrawal of the clasp, the arm will be widened above this point up to the equator, pro-ducing considerably greater spring force. Keep the following in mind as simple rules:• The rst third of the clasp arm, starting with the clasp shoulder, should lie above the equator.• One third should lie on the prosthetic equator.• The last third should lie in the retention area.Figures 6-33 to 6-36 describe the components of a parallelometer.Fig 6-30 As a result of tooth tipping, the position of the prosthetic equator and hence the size and position of the undercuts will change. Three cast positions in relation to the parallelometer axis are distinguished: (a) Neutral position, in which the cast and hence the occlusal plane are located in the horizontal plane. (b) Mesial tipping, in which the occlusal plane tilts in a mesial direction. (c) Distal tipping, in which the occlusal plane, represented by the cast, drops distally.a b cDistal tippingNeutral positionMesial tippingFig 6-31 The gauges in the Ney system can be used to de-termine the spring deection (s) or the retention depth (R). The shank of the gauge lies against the tooth at the widest circumference and is pushed up until the edge of the bead also touches the tooth in the undercut. The contact point of the edge of the bead indicates the position of the clasp tip.Fig 6-32 The Ney undercut gauges are parallel rods in which the ends have been widened to form a bead; the size is mea-sured from the shank to the edge of the bead. Three edge widths make up the basic set (0.25 mm, 0.5 mm, and 0.75 mm). The small bead is for short clasp arms, the medium bead for medium clasp arms, and the large bead for long clasp arms.0.25 mm 0.5 mm 0.75 mmhttp://dentalebooks.com 200Resilient Anchoring and Supporting ElementsFig 6-33 The surveyor (parallelometer) consists of a horizon-tal base (platform) with a vertical column to which a rigid or movable horizontal arm is xed. On the surveying platform, a surveying table can be moved horizontally. The parallelometer rod is xed onto the horizontal arm with a clamping device to receive carbon markers, analyzing rods, or undercut gauges.Fig 6-34 The surveying table has a clamping device for the cast and can be tipped in all directions via a ball joint.Fig 6-35 The prosthetic equator can be established with the parallelometer. The perpendicular parallelometer rod can be moved at will in the horizontal plane. The vertical parallelom-eter rod indicates the insertion path of the denture. The cast being surveyed is tipped relative to the insertion path. The cast holder can be moved as desired in relation to the insertion path so that the cast can be placed in the appropriate inclination to the insertion path. The cast holder can be moved on the bottom part of the parallelometer entirely without interference.Fig 6-36 A microanalyzer is a surveying instrument for deter-mining the spring deection in the infrabulge area of a clasped tooth when surveying for cast clasps. The device can be clamped into the parallelometer rod and moved in parallel. The surveying head has a removable probe that can be extended up to 0.8 mm and can be continuously pressed in. The amount the probe is extended can be read off the instrument scale.http://dentalebooks.com 201Measuring Spring DeectionMeasuring Spring DeectionSpring deection is not measured with the Ney system, but three variables to measure the bend-ing open of a spring are offered for short, me-dium, and long clasp arms. The requirement for dened retentive force cannot be satised with this surveying system—even as a rough approxi-mation—because only one variable of retentive force is used in the aforementioned three-way grading. Modern surveying systems, such as the Rapid Flex system (DeguDent), interrelate four variables: modulus of elasticity, clasp arm length, prole thicknesses, and spring deection. The clasp arm length or the free retaining arm length is measured for each clasp from the clasp tip to the clasp shoulder in millimeter graduations (Fig 6-37). The undercut widths (ie, the spring deec-tion with reference to the anticipated retentive force) can be read off suitable tables. In the pro-cess, clasp arm thickness (prole thickness) is also included as a variable. This is because a lon-ger spring deection pertains more to a long, thin clasp than to a short, thick clasp.The semi-ellipsoid wax prole has a constant height-to-width ratio and tapers toward the tip (Fig 6-38). The appropriate clasp arm lengths can be cut from 30-mm-long wax proles. If an 18-mm-long clasp is required, a thick clasp arm can be cut out if the prole is shortened starting from the apex; a thin clasp arm is produced if the thick end of the prole is cut off. This demon-strates the principle of variability of prole thick-ness (Fig 6-39). This variability, however, is not unlimited with the Rapid Flex system because the wax prole can only be changed by trimming the Fig 6-37 Clasp arm length, which is determined by the shape of the clasp, is measured rst. The length of the free retaining arm is measured from clasp tip to clasp body or to the rigid clamping device.LLLFig 6-38 The correct part can be cut out of a uniform clasp prole with semi-ellipsoid cross section and constant taper to-ward the tip. If a clasp arm of a specic length is required, the prole can be adapted to the clasped tooth starting from the tip, without changing the prole; the excess is cut off. If, based on data from the tables, the prole is trimmed from the tip by 5 mm, for instance, and the clasp prole is then adapted, the result is a markedly thicker clasp arm.Fig 6-39 A comparison of the two parts shows that they have the same clasp arm length, prole shape, and taper but differ-ent prole thicknesses. If the same spring deection is created in both clasp arms, the thicker prole will be more rigid and produce more spring force. However, because the same spring force is meant to be generated with both clasp arms, the thin prole will require a larger spring deection.h1b1b2Lh2http://dentalebooks.com 202Resilient Anchoring and Supporting Elementstip in millimeter steps (Fig 6-40). The maximum shortening allowed is only 5 mm. This range of variation is enough to allow for any clasp arm length between 5 and 30 mm with the necessary spring forces between 0 and 10 N. The permitted tolerance of spring force is +0.01 N, which is less than the variation caused by process and system errors in casting and nishing.The tables (data template) indicate the exact spring deection for each clasp arm in the different thicknesses and lengths and thereby identify the expected spring force. For an 18-mm-long clasp arm, six clasp arm thicknesses can be matched in the tables with spring deections between 0.1 and 1.0 mm for achievable spring forces between 1.0 and 11 N. Thus, differentiation and accuracy in terms of retentive force can be achieved with the Rapid Flex system that are not possible using the Ney surveyor.Practical procedureThe path of the clasp arm is temporarily marked with reference to the surveyed clasp survey line (prosthetic equator) so that the location of the clasp tip is dened. A surveying wheel (micro-mini) is used to measure the clasp arm length ac-curately from the clasp tip to the rigid clamping device (clasp body).If a clasp is being constructed in which both arms engage in the retention area—which hap-pens in most cases—the retentive force of the two arms must be equivalent to the total force of the clasp; for example, if the assumed retentive force is 10 N, the value of 5 N per clasp arm is sought in the table, and the spring deection as well as the prole thickness are selected accordingly.The data template is a set of tables for two moduli of elasticity (chromium-nickel and gold-platinum alloys). These indicate the spring forc-es that may be expected, arranged according to clasp arm lengths, prole thicknesses (or in 1-mm trimming steps), and spring deections (undercut widths).The value from the table indicating maximum spring deection is transferred to the cast with an electromechanical measuring device known as the Scribtometer. The Scribtometer allows for continuous measurement of distance in the hori-zontal plane. A movable measuring needle is t-ted into the parallel guide shaft of this instrument, and the horizontal excursion of this needle can be read off a scale. The shaft touches the tooth at the clasp survey line, and the measuring needle touches in the retention area; the undercut width is therefore readable. Once the Scribtometer shows the selected undercut width, the measur-ing needle marks the point where the clasp tip should lie by applying an electrical impulse to the lacquer-coated tooth surface (Fig 6-41).The path of the clasp arm is precisely plotted; the clasp prole is trimmed (or not), starting from the tip and based on the information from the data template, and then moved. The rest of the clasp prole is trimmed at the clasp shoulder and removed (Fig 6-42).Fig 6-40 The spring deection related to clasp arm length can be found from a table. By reference to that, the clasp prole is trimmed from the tip. The Rapid Flex system allows a maxi-mum of 5 mm of trimming in millimeter steps.http://dentalebooks.com 203Cast-Clasp DesignsCast-Clasp DesignsAll forms of cast clasps must meet the require-ments previously outlined:• Secure the horizontal and vertical position• Enclose the body of the clasped tooth• Present a t that aids periodontal hygiene• Meet the dened retentive forceOnly the double-arm clasp with occlusal rest meets all of these requirements. Any modica-tion of the double-arm clasp and any other form of clasp will fall short of the basic form to some extent when it comes to fullling these functions. However, 90% of all prosthetic cases can be treat-ed with double-arm clasps.The classic Ney clasp system with its ve clasp types offers calculable retentive forces identical for all clasps if surveying is done properly. The differences lie in their functional quality, depend-ing on which of the aforementioned requirements have priority or are to be ignored.The double-arm clasp with occlusal rest is the standard design of cast clasp. It is also called an E-clasp, Ney No. 1, or Akers clasp (Fig 6-43). It is the simplest and most practicable form of clasp because it ensures adequate positional stability Fig 6-41 The surveying head is positioned in the undercut area of the tooth, where the clasp tip is expected to lie, and at the same time is guided with the parallel shaft against the prosthetic equator. The surveying head is guided vertically along the tooth until the horizontal undercut value (spring de-ection) found in the data tables is displayed on an instrument scale. This point is color-marked in contact paint on the tooth surface via an electrical impulse.Fig 6-42 The rst surveying step involves locating the pro-visional clasp path and checking whether there are sufcient undercuts without the clasp lying too close to the marginal periodontium. The clasp arm lengths are then measured, and the individual spring deections are established for all the clasp arms. This means consulting the data tables to nd out how much the clasp prole needs to be shortened. An intermediate step involves checking whether the chosen spring deections are available on the actual clasped tooth; if necessary, a differ-ent shortening factor and different spring deection will have to be selected. This process seems laborious, but therein lies its advantage: Several variables (spring deection, prole thick-ness, and clasp arm length) for determining the precise clasp force and clasp length are combined. This means a level of ac-curacy is achieved that no other method can provide.http://dentalebooks.com 204Resilient Anchoring and Supporting Elementsand offers rigid occlusal support. The design is also conducive to periodontal hygiene. Double-arm clasps fulll all the tasks required of them. They are universally indicated; they can be used in small, bilateral undercut widths as well as for bounded or free-end dentures and in alternating partially edentulous dental arches.Modications of the double-arm claspFollowing are a number of modications of the double-arm clasp. Note that none of the modi-cations offers any advantages over double-arm clasps.1. Bonwill clasp A Bonwill clasp is a common modication in the form of two double-arm clasps that are connect-ed to the denture framework at the clasp shoulder via a shared minor connector (Figs 6-44 and 6-45). The Bonwill clasp is used within closed segments of the dental arch. For this purpose, the interden-tal embrasure between the teeth and the two rest surfaces has to be prepared. The joint clasp shoul-der then runs lingually to buccally/labially.From the standpoint of statics, the Bonwill clasp is an excellent solution. The minor connector is unfavorable in terms of periodontal hygiene, how-ever, because it covers the interdental papilla. The shared clasp shoulder can be esthetically disad-vantageous with premolars; if the necessary prep-aration is not protected with llings, there is a strong susceptibility to caries.2. Split or Bonyhard clasp A split or Bonyhard clasp is a double-arm clasp with two rests that is xed to the framework by two minor connectors; each connector bears an occlusal rest and one clasp arm, one lingual and the other buccal/labial (Figs 6-46 and 6-47).The periodontal support is absolutely secured; horizontal thrusts can be absorbed well enough. The minor connectors are questionable in terms of periodontal hygiene, while the abutment tooth is always subject to rotation during placement and removal of the clasp.3. Back-action claspA back-action clasp is a single-arm clasp in which the lingual clasp arm runs from the minor con-nector over the lingual tooth surface and around the tooth in a vestibular direction and engages in the retention area on one side; the occlusal rest sits rigidly on the minor connector (Fig 6-48). The clasp arm can start directly from the denture saddle or be directed to the clasped tooth by its own minor connector remote from the saddle. This clasp modication offers sufcient periodon-tal support but not complete horizontal positional stability. Only unilateral retention areas are ex-ploited, and the undercut widths have to be large enough with these elastic clasps. If a clasp arm is guided through an interdental embrasure over the row of teeth, the clasp bed must be prepared. This modication is therefore unfavorable in terms of esthetics, caries prevention (interdental clasp bed), and periodontal hygiene (minor con-nector).Fig 6-43 The simplest double-arm clasp with occlusal rest is the Ney No. 1, also known as an E-clasp, which is the most practicable and most commonly used clasp. The E-clasp engages in the reten-tion areas on both sides and can also be used on teeth that have minimal under-cuts.http://dentalebooks.com 205Fig 6-45 The combination of a double-arm clasp with a back-action clasp is a possible modication of the Bonwill clasp.Fig 6-46 The split double-arm clasp with two rests and two minor connectors to the framework offers no advantages.Fig 6-47 Bonyhard clasps (clasps with stems) are split clasps with four rests and three minor connectors to the den-ture framework.Cast-Clasp Designs4. Circumferential clasp A circumferential clasp has the same design as the back-action clasp and has a second rest on half of the clasp arm distance. The modication uses unilateral retention areas and offers good bodily enclosure; yet it is also esthetically and functionally unfavorable. Back-action and circum-ferential clasps are indicated if precise positional bracing can be omitted or if positional bracing is guaranteed by other clasps in a group of clasps.Figures 6-49 to 6-57 show various cast-clasp de-signs and their effects.Fig 6-44 A Bonwill clasp has two double-arm clasps that are joined together at the clasp shoulders. They have a shared mi-nor connector to the denture framework that covers the in-terdental papilla, which is hygienically disadvantageous. The Bonwill clasp is used on teeth that stand close together. Inter-dental embrasures have to be prepared for these clasps.Fig 6-48 The back-action clasp is a single-arm clasp in which one rest starts from the minor connector. The long clasp arm is very elastic and can be directed into the undercut area. One modication involves placing a second rest on the clasp arm that is remote from the saddle; this clasp is known as a circumferential or ring clasp.http://dentalebooks.com 206Resilient Anchoring and Supporting ElementsFig 6-49 The Ney No. 2 clasp is a split double clasp with a rigid rest on the clasp body. The clasp arms are borne by arch-shaped denture saddles made of guided spring stems. Retention areas close to the saddle with large undercut widths can be used. This clasp does not offer horizontal stability and is questionable in terms of periodontal hygiene. In free-end dentures, the lower clasp arms directed toward the saddle press the free-end saddle onto the dental arch.Fig 6-50 The Ney No. 3 is a combination of the Ney No. 1 and Ney No. 2 clasps. The occlusal rest is rigidly connected to the clasp body. The clasp does not secure horizon-tal position and does not offer bodily enclosure; the vestibular clasp arm can be placed far down in a cervical position and is disadvantageous for periodontal hygiene. It is used for teeth with large undercut depths that are tipped in the vestibular direction.Fig 6-51 The Ney No. 4 clasp is a ring-shaped single-arm clasp that bears an occlusal rest on half of the clasp ring. It is an elastic clasp without adequate horizontal and vertical positional stability. This clasp is used on teeth with unilateral retention surfaces. In the case of free-end dentures, this clasp, open distally, can press the free-end saddle onto the dental arch.Fig 6-52 The use of a Ney No. 4 clasp as a corresponding system for free-end dentures produces a pivot about which the denture moves. (a) Rest close to the saddle. (b) Rest remote from the saddle; this design can function as a secondary or indirect retainer.a bhttp://dentalebooks.com 207Cast-Clasp DesignsFig 6-53 The Ney No. 5 clasp is a double-arm clasp in which the lingual part is xed to the denture framework and supported by a minor connector on half of the arm. This framework clasp has two rests. The elastic vestibular clasp arm extends into the retention area on one side. This clasp offers precise positional stability as a splinting element for terminal molars. It is unfavorable in terms of periodontal hy-giene.Fig 6-54 The rigid parts of the Ney No. 5 clasp are the two occlusal rests and the lingual clasp arm, which is joined to the clasp appendix by a rigid clasp tail. It is used as a rigid splint-ing element. If this clasp is used with a bounded denture, the double rest ensures exact axial loading of the clasped tooth.Fig 6-55 The good splinting effect of the Ney No. 5 clasp can be exploited by tting this clasp symmetrically onto both halves of the jaw, making sure that the braced inner clasp arm does not lie in the retention area. It is still necessary to check that the good splinting effect is not undermined by unilateral loading of the tooth on clasp withdrawal.Fig 6-56 For alternating bounded dentures, full clasping with double-arm clasps is adequate for periodontal hygiene pur-poses.Fig 6-57 Full clasping of the residual dentition, in which a Bonwill clasp has been placed, is not satisfactory in terms of periodontal hygiene.http://dentalebooks.com 208Resilient Anchoring and Supporting ElementsCast Clasps on Coronal RestorationsIf a tooth has a restoration to protect it as a clasped tooth, the retention areas are prepared during the wax-up procedure. It is generally enough to give the replacement tooth the appropriate bulging anatomical shape because the clasp systems are adapted to the anatomically shaped teeth (Figs 6-58 and 6-59). If a single crown is integrated into a group of clasps, the direction of inclination and retention surfaces of a common path of inser-tion can be adapted to those of the other clasped teeth.When the clasped teeth of a clasp unit have coronal restorations, the path of insertion for the teeth is xed, even at the wax-up stage. Slight distal tipping is always the least advantageous inclination as a path of insertion for the patient and for the anterior clasp position. Once the cross section of the restoration has been roughly waxed up, the undercut areas are ascertained with a par-allelometer, and the vertical surface curvature is suitably thickened or reduced. In this situation, it is appropriate for every clasp structure, and es-pecially for loading of the tooth, to wax up the undercuts, which guarantees a uniform retention depth below the prosthetic equator for all the teeth affected by a clasp.The natural tooth shape has the following dis-tinctive feature: At its widest circumference (pros-thetic equator), there are different degrees of cur-vature of the buccal and oral surfaces depending on the inclination of the tooth. Thus, in a man-dibular premolar, in keeping with the crown incli-nation, the oral undercut runs very steeply, while the buccal undercut is poorly developed despite the pronounced curvature. Where the maxillary teeth have a vestibular inclination, the vestibular undercuts are often steeper.If a clasp is placed in which both clasp arms have to be bent open by the same spring deec-tion, different buccal and lingual retention depths arise. This inuences clasp forces and the trans-verse loading of the tooth. It is assumed that the vestibular retention depth is smaller than the lin-gual. During withdrawal, the vestibular clasp arm is bent open by its spring deection, while the lin-gual arm is only partly bent open. In the process, the following phenomena occur: The tooth is jolt-ed to and fro on withdrawal of the clasp, and the transverse thrusts that arise are uncontrolled and harmful. On coronal restorations, the vestibular and lingual retention depth for the clasp arms can be shaped to the same size during wax-up by cor-recting the vertical curvatures. If a lingual guide arm is constructed to the ves-tibular retention depth, a parallel guide is waxed up in the form of a circular notch (Fig 6-60). The circular notch is created by reference to the path of insertion and should be milled according to the same principles as when combined with prefabri-cated components. The lingual clasp arm is sunk into the replacement crown material and secures the vertical and horizontal position.In this context, it becomes clear that it is ques-tionable to design cast clasps with guide arms and retaining arms for teeth that do not have res-torations. All the guide arms of a group of clasps must run parallel to each other and to the path of insertion. Milling the lingual surfaces in the mouth in the same way in parallel is very time-consuming and is not justied by the outcome. If an acrylic resin veneer crown is tted with a clasp construction, the functional clasp arm may abrade the veneer, and the retentive force of the clasp may be lost. Therefore, the veneer should be shaped so that the lower clasp arm in the rest-ing position does not lie on the acrylic resin but on a metal guide (Fig 6-61). In the mandible, such a design is usually possible without loss of es-thetics. Because a removable partial denture has to be protected periodontally against masticatory forces, at least one occlusal rest is necessary on each cast clasp. This rest is already prepared on the replacement crown.The shape of the occlusal rest also determines the shape of the oor of the rest in the replace-ment crown. Thus, the oor of the rest is modeled perpendicular to the tooth axis in a spoon-shaped recess; the rest is modeled so broadly (approxi-mately 2.5 to 3 mm) and deeply (1.5 mm) that the eventual rest is fracture resistant and does not interfere with occlusion. If a guide arm is con-structed, the oor of the rest can be shaped with its walls parallel to the parallel milling. Where a circular notch is shaped with a cervical shoulder, an occlusal rest can be dispensed with.http://dentalebooks.com 209Cast Clasps on Coronal RestorationsFig 6-58 The surface bulbosities of natural mandibular teeth produce more pronounced undercuts on the buccal side; as a result of the lingual inclination of the posterior teeth, under-cuts for clasps are found in the lingual area. Where coronal restoration is done, reproducing the natural surface bulbosity is enough to obtain adequate undercuts for clasps.Fig 6-59 In the case of maxillary posterior teeth, the under-cuts relative to the tooth axis mainly lie lingually, which is put into perspective by the vestibular inclination within the posi-tion in the dental arch. For cast clasps, these undercuts are generally adequate, which requires reproduction of the surface bulbosity when making coronal restorations.Fig 6-60 If the lingual clasp arm is to be shaped as the guide arm, it must be directed parallel to the path of insertion throughout the length of the retention depth of the active clasp arm. A guide plane is milled into the crown so that the clasp arm acts as a shear distributor. This method lends itself to an-terior teeth because the lingual undercuts are very poor. Fitting a canine with a veneer crown and then placing a cast clasp is extremely questionable from an esthetic viewpoint.Fig 6-61 If a clasped tooth will undergo coronal restoration, a full-metal crown is best suited for bearing the mechanical stress. If two different metals are combined in the mouth, galvanic processes may arise, which argues against such an indication. In the case of a full crown with an acrylic resin veneer, the acrylic resin is subject to mechanical stress, which is why a metallic sliding surface must be created for the clasp arm. This reduces the esthetic advantage of the veneer crown, apart from the fact that clasps are always esthetically disadvantageous.http://dentalebooks.com http://dentalebooks.com

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