The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment Planning










567
24
The Edentulous Mandible:
Fixed Versus Removable
Prosthesis Treatment
Planning
RANDOLPH R. RESNIK AND CARL E. MISCH
H
istorically, the edentulous mandibular patient has been
one of the most common patients to be treated with dental
implants. e placement of dental implants in this area has
been shown to be very successful in obtaining the support, reten-
tion, and stability of a mandibular prosthesis, whether it is xed
or removable. From a bone volume conservation standpoint, com-
plete edentulous patients should be treated with sucient implants
to support a prosthesis in the maxilla or mandible. e continued
bone loss after tooth loss and associated compromises in esthetics,
function, and health make all edentulous patients possible implant
candidates. e bone loss that occurs during the rst year after
tooth loss is 10 times greater than in the following years. In the case
of multiple extractions, this often means an approximate 4-mm ver-
tical bone loss within the rst 6 months. As the bony ridge resorbs,
the muscle attachments become level with the edentulous ridge,
thereby compromising the t of a mandibular prosthesis. Rather
than waiting until the patient has lost most of the residual bone, the
clinician should inform and emphasize to the patient the benets of
implants and why they should be inserted before the bone is lost.
erefore the profession should treat bone loss from extractions in
a similar fashion as bone loss from periodontal disease. Rather than
waiting until the bone is resorbed or the patient complains, the
dental professional should educate the patient about the bone loss
process caused not only by periodontal disease but also by the lack
of stimulation and its consequences of bone resorption, and explain
how implants are available to treat the condition. erefore most
completely edentulous patients should be informed of the necessity
of dental implants to maintain bone volume, function, masticatory
muscle activity, esthetics, and psychological health. Ideally patients
who have non-restorable teeth should be given the option to include
implants to support the future prosthesis. e traditional complete
denture may be presented as a temporary measure to provide cos-
metic and oral function during implant treatment. For an edentu-
lous patient, two treatment options exist: (1) xed (FP-1, FP-2, or
FP-3) or (2) removable (RP-4 or RP-5) prosthesis (Fig. 24.1).
Mandibular Treatment Planning Principles
Anteroposterior Spread
e distance from the center of the most anterior implant to a line
joining the distal aspect of the two most distal implants on each
side is called the anteroposterior (A-P) distance or the A-P spread
1
(Fig. 24.2). In theory, the greater the A-P spread, the farther the
distal cantilever may be extended to replace the missing poste-
rior teeth. As a general rule, when ve to six anterior implants are
placed in the anterior mandible between the foramina to support
a xed prosthesis, the cantilever should not exceed two times the
A-P spread, with all other stress factors being low.
e range of implant and prosthesis survival may be because of
the broad application of the same implant position, regardless of
crown height, opposing dentition, implant length, A-P position
of implants, and parafunction. e arch form, the position of the
mental foramina, force factors, and bone density are important
criteria when four to six implants are placed only in the anterior
segment to replace the entire mandibular arch. e anterior arch
form and foramina position aect the position of the distalmost
implants. erefore a cantilever distance is variable for dierent
patients.
e A-P distance is aected by the arch form. e types of arch
forms may be separated into square, ovoid, and tapering. A square
arch form in the anterior mandible has a 0- to 6-mm A-P spread
between the most distal and most anterior implants (Fig. 24.3).
An ovoid arch form has an A-P distance of 7 to 9 mm and is the
most common type (Fig. 24.4). A tapering arch form has an A-P
distance greater than 9 mm (Fig. 24.5).
Hence whereas a tapering arch form may support a 20-mm can-
tilever, a square arch form requires the cantilever to be reduced to
12 mm or less, however is directly dependent upon force factors.
e position of the mental foramen can aect the A-P spread. e
mental foramen is most often found between the root apices of the
premolars. However, it may be located as far anterior as just distal
to the canine and as far distal as the mesial of the rst molar apex.
2
e farther forward the foramen, the shorter the cantilever length
Deceased

568
PART V Edentulous Site Treatment Planning
because the A-P spread is reduced. e A-P spread is only one of
the force factors to be considered for the extent of the distal canti-
lever. If the stress factors are high (e.g., parafunction, crown height,
masticatory musculature dynamics, opposing arch), the cantilever
length of a prosthesis should be reduced and may even be con-
traindicated. e density of bone is also an important criterion.
e softest bone types (D3 and D4) should not have as great of a
cantilever than the denser types (D1 and D2).
erefore, the length of the posterior cantilever depends on
the specic force factors of the patient, of which A-P spread is
only one. e number and size of implants may also aect the
cantilever length. Stress equals force divided by the area over
which force is applied. e area over which the forces are applied
from the prosthesis to the implants can be modied through the
number, size, and design of the implants. A cantilever rarely is
indicated on three implants even with a similar A-P spread as ve
implants.
A
B
C
Fig. . Mandibular Edentulous Prostheses. (A) Fixed prothesis (FP-
3), which is commonly fabricated from zirconia, porcelain fused to metal,
or acrylic/denture teeth. (B) Removable prosthesis (RP-4), which is totally
implant supported; note the flangeless nature of the prosthesis. (C) Remov-
able prosthesis (RP-5), which is primarily supported by the soft tissue.
A-P
Fig. . The anteroposterior (A-P) distance is determined by a line
drawn from the distal portion of the distalmost implant on each side of the
arch and another parallel line drawn through the center of the anteriormost
implant from the cantilever.
A
B
C
D
E
A-P
Cantilever
Fig. . A mandibular square arch form has an anteroposterior (A-P)
distance of 0 to 6 mm. As a result a cantilever is limited. (From Misch CE.
The completely edentulous mandible: treatment plans for fixed restora-
tions. In: Dental Implant Prosthetics. 2nd ed. St. Louis, MO: Mosby; 2015.)
A
B
C
D
E
A-P
Cantilever
Fig. . A mandibular ovoid arch form has an anteroposterior (A-P)
distance of 7 to 9 mm and is the most common type. A cantilever may
extend to 18 mm with the ovoid-type arch. (From Misch CE. The com-
pletely edentulous mandible: treatment plans for fixed restorations. In:
Dental Implant Prosthetics. 2nded. St. Louis, MO: Mosby; 2015.)

569
CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment Planning
A
B
C
D
E
A-P
Cantilever
Fig. . A mandibular tapered arch form has an anteroposterior (A-P)
distance of greater than 9 mm and is the type least observed. A cantilever
is least at risk for this arch form. (From Misch CE. The completely eden-
tulous mandible: treatment plans for fixed restorations. In: Dental Implant
Prosthetics. 2nd ed. St. Louis, MO: Mosby; 2015.)
Mandibular Flexure
Medial Movement
Many reports have addressed the dimensional changes of the man-
dible during jaw activity as a result of masticatory muscle action.
3-6
Five dierent movements have been postulated.
Medial convergence is the one most commonly addressed.
7
e mandible between the mental foramina is stable relative to
exure and torsion. However, distal to the foramina, the mandible
exhibits considerable movement toward the midline on open-
ing.
8,9
is movement is caused primarily by the attachment of
internal pterygoid muscles on the medial ramus of the mandible.
e distortion of the mandible occurs early in the opening cycle,
and the maximum changes may occur with as little as 28% open-
ing (or about 12 mm). is exure has also been observed during
protrusive jaw movements.
10
e greater the active opening and
protrusive movements, the greater the amplitude of mandibular
exion. e amount of movement varies among individuals and
depends on the density and volume of bone, and the location of the
site in question. In general the more distal the sites, the more medial
exure. e amplitude of the mandibular body exure toward the
midline has been measured to be as much as 800 μm in the rst
molar-to-rst molar region to as much as 1500 μm in the ramus-to-
ramus sites (Fig. 24.6). In a study by Hobkirk and Havthoulas
11
on
deformation of the mandible in subjects with xed dental implant
prostheses, medial convergence up to 41mm was observed.
Torsion
Torsion of the mandibular body distal to the foramina has also been
documented in both animal and human studies.
12,13
Hylander
14
evaluated larger members of the rhesus monkey family (macaque)
and found the mandible twisted on the working side and bent
in the parasagittal plane on the balancing side during the power
stroke of mastication and unilateral molar biting (Fig. 24.7). Para-
sagittal bending of the human jaw during unilateral biting was
conrmed by Marx,
15
who measured localized mandibular distor-
tion invivo in humans by using strain gauges on screws attached
to cortical bone in the symphyseal and gonial regions. Abdel-Latif
et al.
12
conrmed that the mandibles of patients with implant
prostheses measured up to 19 degrees of dorsoventral shear. e
torsion during parafunction is caused primarily by forceful con-
traction of the masseter muscle attachments (Fig. 24.8). erefore
parafunctional bruxism and clenching may cause torsion-related
problems in the implant support system and prosthesis when the
mandibular teeth are splinted from the molar-to-molar regions.
e posterior bone gain in edentulous patients restored with
cantilevered prostheses from anterior implants may be a conse-
quence of the mandibular exure and torsion, which stimulate the
bone cells in the region. Because the bite force may increase 300%
with an implant prosthesis compared with a denture, the increased
torsion may stimulate the posterior mandibular body to increase
in size, as reported by Wright etal.
16
and Reddy etal.
17
Misch
18
has observed the increase in exure in the posterior man-
dible is a result of the mental foramen weakening of the facial cortical
plate. As such, the mandible exes and has torsion distal to the fora-
men. e most common position of the mental foramen is between
the rst and second premolar teeth. erefore, when bilaterally
0.8 mm
Fig. . The flexure of the mandible during opening and protrusive
movements occurs distal to the mental foramina. The amount of flexure
depends on the amount of the bone volume and the sites in question.
The medial movement from the first molar to the first molar region may
be 800 mm.
Contraction Expansion
Fig. . Unilateral molar biting causes the mandible to undergo torsion,
with the bottom of the mandible expanding outward and the crest of the
mandible rotating medially. (From Misch CE. The completely edentulous
mandible: treatment plans for fixed restorations. In: Dental Implant Pros-
thetics. 2nd ed. St. Louis, MO: Mosby; 2015.)

570
PART V Edentulous Site Treatment Planning
splinting teeth distal to the premolar positions, mandibular dynam-
ics should be considered. Posterior rigid, xated implants splinted
to each other in a full-arch restoration are subject to a considerable
buccolingual force on opening and during parafunction.
19,20
A study by Miyamoto etal.
21
identied jaw exure as the primary
cause of posterior implant loss in full-arch splinted mandibular pros-
theses. e more distal the rigid splint from one side to the other, the
greater the risk that mandibular dynamics may inuence the implants
or prosthesis prognosis. In addition, the body of the mandible exes
more when the size of the bone decreases. As a result the division C
minus height (C−h) or division D mandible exes or exhibits torsion
more than the division A mandible, all other factors being similar.
e dierence in movement between an implant and a tooth has
been addressed as a concern for dentists when splinting these objects
together. e natural tooth movement ranges from 28 μm apically
and 56 to 108 μm laterally. In contrast, the rigid implant has move-
ment up to 5 μm apically and 10 to 60 μm laterally. Yet the man-
dibular exure and torsion may be more than 10 to 20 times the
movement of a healthy tooth. erefore the exure and torsion of the
mandibular body are more critical in the patient evaluation compared
with whether an implant should be joined to the natural dentition.
Some authors have suggested four implants in the mandible with
a full-arch splinted xed restoration—two in the rst molars and
two in the canine regions (Fig. 24.9).
22
Additional implants have
been used with this full-arch splinted restorative option, with up
to four other implants in the premolar and the incisor regions.
23
However, complete cross-arch splinting of posterior molar implants
with a rigid, xated prosthesis should be reconsidered in the man-
dible. e exure of the mandible is thwarted by the prosthesis, but
this introduces lateral stresses to the implant system (cement, screw,
crestal bone, and implant–bone interface).
24-26
ese lateral stresses
place the molar implants, screws, and bone at increased risk because
of the mandibular exure and torsion previously addressed.
In complete mandibular subperiosteal implants, pain on opening
was noted in 25% of the patients at the suture removal appointment
when a rigid bar connected molar-to-molar regions. When the con-
necting bar was cut into two sections between the foramina, the pain
on opening was immediately eliminated. is clinical observation
does not mean that the other 75% of patients did not have exure of
the mandibular arch on opening. e observation does demonstrate,
however, that exure may be relevant to postoperative complications.
Implant Overdentures Advantages
(RP-4 and RP-5)
For an implant-supported overdenture the implants ideally
should be placed in planned, specic sites, and their number
should be predetermined by the clinician and patient. e pri-
mary indications for a mandibular implant overdenture (IOD)
are problems often found with lower dentures, such as lack
of retention or stability, decrease in function, diculties in
speech, tissue sensitivity, and soft tissue abrasions. If an eden-
tulous patient is willing to remain with a removable prosthesis,
an overdenture is often the treatment of choice. In addition,
if cost is a concern for the patient, the overdenture may serve
as a transitional prosthesis until additional implants may be
inserted and restored. When evaluating mandibular IODs,
many advantages exist (Box 24.1).
Enhanced Soft Tissue Support
Bone loss dictates the appearance of the inferior third of the
face. A maxillary overdenture often provides improved sup-
port for the lips and soft tissues of the face compared with a
xed prosthesis because the prosthesis contour does not have to
accommodate daily hygiene requirements. Denture teeth also
provide an esthetic replacement for the natural dentition, which
is more challenging for the technician to re-create with porcelain
fused to metal restorations. For the laboratory to create pink
Fig. . The mandible flexes toward the midline on opening or during
protrusive movements as a result of the internal pterygoid muscle attach-
ments on the ramus (blue arrows). The mandible also torques, with the
inferior border rotating out and up, and the crestal region rotating lingually.
The movement is caused by the masseter muscles during forceful biting
or parafunction (red arrows).
Fig. . Some authors propose that the ideal implant positions to support
a mandibular full-arch prosthesis are the bilateral molars and bilateral canines,
splinted together with a rigid structure. These positions are not ideal because
of the mandibular dynamics (flexure and torsion) during opening and function.
1. Enhanced soft tissue support
2. Increased chewing efficiency compared with conventional dentures
3. Less expense/implants
4. Esthetics
5. Ease of hygiene
6. Parafunctional habits
7. Less food impaction
BOX
24.1
Advantages of Mandibular Implant
Overdentures

571
CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment Planning
interdental papilla, as well as replace the soft tissue drape, is
easier with an overdenture compared with porcelain-metal xed
restorations or zirconia prosthesis. In addition, the teeth can be
positioned in the most esthetic position, without any restriction
as to the relationship to the atrophied crest, because stability
now is provided by the implant and does not depend on tooth
position on the crest of the ridge (Fig. 24.10).
Increased Chewing Efficiency Compared with
Conventional Dentures
A study of chewing eciency compared wearers of complete den-
tures with patients with implant-supported overdentures. e
complete-denture group required 1.5 to 3.6 times the number
of chewing strokes compared with the overdenture group.
27
e
chewing eciency with an IOD is improved by 20% compared
with a traditional complete denture.
28,29
Less Expense/Implants
When cost is a factor, two implant-retained IODs may improve
the patient’s condition at a signicantly lower overall treat-
ment cost than a xed implant–supported prosthesis. A survey
by Carlsson etal.
30
in 10 countries indicated a wide range of
treatment options. e proportion of IODs selected versus
xed implant dentures was highest in the Netherlands (93%)
and lowest in Sweden and Greece (12%). Cost was cited as the
number one determining factor in the choice. However, in gen-
eral, overdenture treatment is less expensive than a xed implant
prosthesis, mainly because of the decreased number of implants
required.
Esthetics
e esthetics for many edentulous patients with moderate-to-
advanced bone loss is improved with an overdenture compared
with a xed restoration. Soft tissue support for facial appearance
often is required for an implant patient because of advanced bone
loss, especially in the maxilla. Interdental papilla and tooth size
are easier to reproduce or control with an overdenture. Denture
teeth easily reproduce contours and esthetics compared with time-
consuming and technician-sensitive porcelain-metal or zirconia
xed restorations. e labial ange may be designed for optimal
appearance, not daily hygiene.
Ease of Hygiene
Hygiene conditions and home and professional care are
improved with an overdenture compared with a fixed pros-
thesis. Peri-implant probing is diagnostic and easier around
a bar than a fixed prosthesis because the crown often pre-
vents straight-line access along the abutment to the crest of
the bone. The overdenture may be extended over the abut-
ments to prevent food entrapment during function. Speech is
not compromised because the denture may extend onto the
soft tissues in the mandible and prevent air and saliva from
escaping (Fig. 24.11).
Parafunctional Habits
An overdenture may be removed at bedtime to reduce the nox-
ious eect of nocturnal parafunction, which increases stresses on
the implant support system. e overdenture also may provide
stress relief between the superstructure and prosthesis, and the
soft tissue may share a portion of the occlusal load. e prosthesis
A
B
Fig. . (A and B) Soft tissue support: because of the ability to
modify the flange of the prosthesis, ideal soft tissue support can be
obtained.
Fig. . Hygiene: because of the removable nature of the overdenture,
hygiene access is much easier to complete in comparison with a fixed
prosthesis.

572
PART V Edentulous Site Treatment Planning
is usually easier to repair than a xed restoration. In most cases,
there exists a reduced cost of overdenture treatment in compari-
son to a xed prosthesis. In addition, long-term denture patients
do not appear to have a psychological problem associated with
the ability to remove their implant prostheses.
31,32
erefore,
denture patients usually adapt very well into an overdenture
treatment.
Less Food Impaction
Especially with an RP-5 prosthesis, there is less food impaction
with an overdenture in comparison with a xed prosthesis. e
anges of the prosthesis (RP-5) usually extend to form a periph-
eral seal that minimizes food impaction. Because of the nature
of the xed mandibular prosthesis, it often is overextended for
esthetic reasons. In comparison with a conventional denture,
food particles migrate and become impacted under the pros-
thesis during swallowing. Because a lower denture “oats” and
moves during function, the food more readily goes under and
out, whereas the IOD traps the food debris against the implants,
bars, and attachments.
Review of the Overdenture Literature
In 1986 a multicenter study reported on 1739 implants placed
in the mandibular symphysis of 484 patients. e implants were
loaded immediately and restored with bars and overdentures with
clips as retention. e overall success rate was 94%.
33
Engquist
et al.
34
reported a 6% to 7% implant failure for mandibular
implant–supported overdentures and a 19% to 35% failure rate
for maxillary IODs. Hyperplasia below the bar occurred in 25%
of the patients. Jemt etal.
35
reported on a 5-year prospective, mul-
ticenter study on 30 maxillae (117 Brånemark implants) and 103
mandibles with 393 implants. Survival rates in the mandible were
94.5% for implants and 100% for prostheses; in the maxilla the
survival rates were 72.4% for implants and 77.9% for prostheses.
Higher failure rates in the maxilla were related directly to poor
density and quantity of bone with a characteristic cluster failure
pattern.
35
Wismeijer etal.
36
reported on 64 patients with 218 titanium
plasma-sprayed implants, with a 97% survival rate with overden-
tures in a 6.5-year evaluation. Naert etal. found 100% implant
success at 5 years for overdentures with dierent anchorage sys-
tems. In Belgium, Naert at al. reported on 207 consecutively
treated patients with 449 Brånemark implants and Dolder-bar
overdentures. In this report the cumulative implant failure rate
was 3% at the 10-year benchmark.
37
Misch
38
reported a less than 1% implant failure rate and
no prosthesis failure over a 7-year period with 147 patients
when using the organized treatment options and prosthetic
guidelines presented in this chapter. Kline etal.
39
reported on
266 implant-supported overdentures for 51 patients, with an
implant survival rate of 99.6% and a prosthesis survival rate of
100%. Mericke-Stern
40
reported a 95% implant survival rate
with two IODs.
In a randomized clinical report Awad etal. compared satisfac-
tion and function in complete dentures (48 patients) versus two
IODs in 56 patients. ere was signicantly higher satisfaction,
comfort, and chewing ability in the IOD group.
41
omason etal.,
42
in the United Kingdom, reported a 36%
higher satisfaction for the IOD patients than the complete den-
ture wearers in the criteria of comfort, stability, and chewing.
In a 10-year study of IODs in Israel, with 285 implants
and 69 IODs, Schwartz-Arad etal.
43
reported the implant sur-
vival rate was 96.1% with higher success rates in the mandible.
Many reports have been published over the last two decades
that conclude that implant-supported overdentures represent a
valid benecial option for denture wearers. It should be noted
that the majority of reports are for IODs supported by only two
implants.
44,45
Mandibular Overdenture Treatment
Planning (RP-4 and RP-5)
Anatomy of the Mandible
In treatment planning the mandible for a xed or removable pros-
thesis, the mandible is divided into three regions: (1) anterior
mandible, (2) posterior right, and (3) posterior left. e available
bone in the anterior mandible is divided into ve equal columns
of bone serving as potential implant sites, labeled A, B, C, D,
and E, starting from the patient’s right side.
46,47
Regardless of the
treatment option being used, all ve implant sites are mapped out
at the time of treatment planning and surgery (Fig. 24.12).
Mandibular Implant Site Selection
Anterior retention and stability for an overdenture oer sev-
eral advantages. e greatest available height of bone is located
in the anterior mandible, between the mental foramina. is
A B C D E
?
Fig. . (A) The mandible is divided into three regions for treatment planning. (B) The anterior mandible
is positionally documented via A, B, C, D, and E positions.

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56724The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningRANDOLPH R. RESNIK AND CARL E. MISCH†Historically, the edentulous mandibular patient has been one of the most common patients to be treated with dental implants. e placement of dental implants in this area has been shown to be very successful in obtaining the support, reten-tion, and stability of a mandibular prosthesis, whether it is xed or removable. From a bone volume conservation standpoint, com-plete edentulous patients should be treated with sucient implants to support a prosthesis in the maxilla or mandible. e continued bone loss after tooth loss and associated compromises in esthetics, function, and health make all edentulous patients possible implant candidates. e bone loss that occurs during the rst year after tooth loss is 10 times greater than in the following years. In the case of multiple extractions, this often means an approximate 4-mm ver-tical bone loss within the rst 6 months. As the bony ridge resorbs, the muscle attachments become level with the edentulous ridge, thereby compromising the t of a mandibular prosthesis. Rather than waiting until the patient has lost most of the residual bone, the clinician should inform and emphasize to the patient the benets of implants and why they should be inserted before the bone is lost. erefore the profession should treat bone loss from extractions in a similar fashion as bone loss from periodontal disease. Rather than waiting until the bone is resorbed or the patient complains, the dental professional should educate the patient about the bone loss process caused not only by periodontal disease but also by the lack of stimulation and its consequences of bone resorption, and explain how implants are available to treat the condition. erefore most completely edentulous patients should be informed of the necessity of dental implants to maintain bone volume, function, masticatory muscle activity, esthetics, and psychological health. Ideally patients who have non-restorable teeth should be given the option to include implants to support the future prosthesis. e traditional complete denture may be presented as a temporary measure to provide cos-metic and oral function during implant treatment. For an edentu-lous patient, two treatment options exist: (1) xed (FP-1, FP-2, or FP-3) or (2) removable (RP-4 or RP-5) prosthesis (Fig. 24.1).Mandibular Treatment Planning PrinciplesAnteroposterior Spreade distance from the center of the most anterior implant to a line joining the distal aspect of the two most distal implants on each side is called the anteroposterior (A-P) distance or the A-P spread1 (Fig. 24.2). In theory, the greater the A-P spread, the farther the distal cantilever may be extended to replace the missing poste-rior teeth. As a general rule, when ve to six anterior implants are placed in the anterior mandible between the foramina to support a xed prosthesis, the cantilever should not exceed two times the A-P spread, with all other stress factors being low.e range of implant and prosthesis survival may be because of the broad application of the same implant position, regardless of crown height, opposing dentition, implant length, A-P position of implants, and parafunction. e arch form, the position of the mental foramina, force factors, and bone density are important criteria when four to six implants are placed only in the anterior segment to replace the entire mandibular arch. e anterior arch form and foramina position aect the position of the distalmost implants. erefore a cantilever distance is variable for dierent patients.e A-P distance is aected by the arch form. e types of arch forms may be separated into square, ovoid, and tapering. A square arch form in the anterior mandible has a 0- to 6-mm A-P spread between the most distal and most anterior implants (Fig. 24.3). An ovoid arch form has an A-P distance of 7 to 9 mm and is the most common type (Fig. 24.4). A tapering arch form has an A-P distance greater than 9 mm (Fig. 24.5).Hence whereas a tapering arch form may support a 20-mm can-tilever, a square arch form requires the cantilever to be reduced to 12 mm or less, however is directly dependent upon force factors. e position of the mental foramen can aect the A-P spread. e mental foramen is most often found between the root apices of the premolars. However, it may be located as far anterior as just distal to the canine and as far distal as the mesial of the rst molar apex.2 e farther forward the foramen, the shorter the cantilever length † Deceased 568PART V Edentulous Site Treatment Planningbecause the A-P spread is reduced. e A-P spread is only one of the force factors to be considered for the extent of the distal canti-lever. If the stress factors are high (e.g., parafunction, crown height, masticatory musculature dynamics, opposing arch), the cantilever length of a prosthesis should be reduced and may even be con-traindicated. e density of bone is also an important criterion. e softest bone types (D3 and D4) should not have as great of a cantilever than the denser types (D1 and D2).erefore, the length of the posterior cantilever depends on the specic force factors of the patient, of which A-P spread is only one. e number and size of implants may also aect the cantilever length. Stress equals force divided by the area over which force is applied. e area over which the forces are applied from the prosthesis to the implants can be modied through the number, size, and design of the implants. A cantilever rarely is indicated on three implants even with a similar A-P spread as ve implants. ABC• Fig. . Mandibular Edentulous Prostheses. (A) Fixed prothesis (FP-3), which is commonly fabricated from zirconia, porcelain fused to metal, or acrylic/denture teeth. (B) Removable prosthesis (RP-4), which is totally implant supported; note the flangeless nature of the prosthesis. (C) Remov-able prosthesis (RP-5), which is primarily supported by the soft tissue.A-P• Fig. . The anteroposterior (A-P) distance is determined by a line drawn from the distal portion of the distalmost implant on each side of the arch and another parallel line drawn through the center of the anteriormost implant from the cantilever.ABCDEA-PCantilever• Fig. . A mandibular square arch form has an anteroposterior (A-P) distance of 0 to 6 mm. As a result a cantilever is limited. (From Misch CE. The completely edentulous mandible: treatment plans for fixed restora-tions. In: Dental Implant Prosthetics. 2nd ed. St. Louis, MO: Mosby; 2015.)ABCDEA-PCantilever• Fig. . A mandibular ovoid arch form has an anteroposterior (A-P) distance of 7 to 9 mm and is the most common type. A cantilever may extend to 18 mm with the ovoid-type arch. (From Misch CE. The com-pletely edentulous mandible: treatment plans for fixed restorations. In: Dental Implant Prosthetics. 2nded. St. Louis, MO: Mosby; 2015.) 569CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningABCDEA-PCantilever• Fig. . A mandibular tapered arch form has an anteroposterior (A-P) distance of greater than 9 mm and is the type least observed. A cantilever is least at risk for this arch form. (From Misch CE. The completely eden-tulous mandible: treatment plans for fixed restorations. In: Dental Implant Prosthetics. 2nd ed. St. Louis, MO: Mosby; 2015.)Mandibular FlexureMedial MovementMany reports have addressed the dimensional changes of the man-dible during jaw activity as a result of masticatory muscle action.3-6 Five dierent movements have been postulated.Medial convergence is the one most commonly addressed.7 e mandible between the mental foramina is stable relative to exure and torsion. However, distal to the foramina, the mandible exhibits considerable movement toward the midline on open-ing.8,9 is movement is caused primarily by the attachment of internal pterygoid muscles on the medial ramus of the mandible.e distortion of the mandible occurs early in the opening cycle, and the maximum changes may occur with as little as 28% open-ing (or about 12 mm). is exure has also been observed during protrusive jaw movements.10 e greater the active opening and protrusive movements, the greater the amplitude of mandibular exion. e amount of movement varies among individuals and depends on the density and volume of bone, and the location of the site in question. In general the more distal the sites, the more medial exure. e amplitude of the mandibular body exure toward the midline has been measured to be as much as 800 μm in the rst molar-to-rst molar region to as much as 1500 μm in the ramus-to-ramus sites (Fig. 24.6). In a study by Hobkirk and Havthoulas11 on deformation of the mandible in subjects with xed dental implant prostheses, medial convergence up to 41mm was observed. TorsionTorsion of the mandibular body distal to the foramina has also been documented in both animal and human studies.12,13 Hylander14 evaluated larger members of the rhesus monkey family (macaque) and found the mandible twisted on the working side and bent in the parasagittal plane on the balancing side during the power stroke of mastication and unilateral molar biting (Fig. 24.7). Para-sagittal bending of the human jaw during unilateral biting was conrmed by Marx,15 who measured localized mandibular distor-tion invivo in humans by using strain gauges on screws attached to cortical bone in the symphyseal and gonial regions. Abdel-Latif et al.12 conrmed that the mandibles of patients with implant prostheses measured up to 19 degrees of dorsoventral shear. e torsion during parafunction is caused primarily by forceful con-traction of the masseter muscle attachments (Fig. 24.8). erefore parafunctional bruxism and clenching may cause torsion-related problems in the implant support system and prosthesis when the mandibular teeth are splinted from the molar-to-molar regions.e posterior bone gain in edentulous patients restored with cantilevered prostheses from anterior implants may be a conse-quence of the mandibular exure and torsion, which stimulate the bone cells in the region. Because the bite force may increase 300% with an implant prosthesis compared with a denture, the increased torsion may stimulate the posterior mandibular body to increase in size, as reported by Wright etal.16 and Reddy etal.17Misch18 has observed the increase in exure in the posterior man-dible is a result of the mental foramen weakening of the facial cortical plate. As such, the mandible exes and has torsion distal to the fora-men. e most common position of the mental foramen is between the rst and second premolar teeth. erefore, when bilaterally 0.8 mm• Fig. . The flexure of the mandible during opening and protrusive movements occurs distal to the mental foramina. The amount of flexure depends on the amount of the bone volume and the sites in question. The medial movement from the first molar to the first molar region may be 800 mm.Contraction Expansion• Fig. . Unilateral molar biting causes the mandible to undergo torsion, with the bottom of the mandible expanding outward and the crest of the mandible rotating medially. (From Misch CE. The completely edentulous mandible: treatment plans for fixed restorations. In: Dental Implant Pros-thetics. 2nd ed. St. Louis, MO: Mosby; 2015.) 570PART V Edentulous Site Treatment Planningsplinting teeth distal to the premolar positions, mandibular dynam-ics should be considered. Posterior rigid, xated implants splinted to each other in a full-arch restoration are subject to a considerable buccolingual force on opening and during parafunction.19,20A study by Miyamoto etal.21 identied jaw exure as the primary cause of posterior implant loss in full-arch splinted mandibular pros-theses. e more distal the rigid splint from one side to the other, the greater the risk that mandibular dynamics may inuence the implants or prosthesis prognosis. In addition, the body of the mandible exes more when the size of the bone decreases. As a result the division C minus height (C−h) or division D mandible exes or exhibits torsion more than the division A mandible, all other factors being similar.e dierence in movement between an implant and a tooth has been addressed as a concern for dentists when splinting these objects together. e natural tooth movement ranges from 28 μm apically and 56 to 108 μm laterally. In contrast, the rigid implant has move-ment up to 5 μm apically and 10 to 60 μm laterally. Yet the man-dibular exure and torsion may be more than 10 to 20 times the movement of a healthy tooth. erefore the exure and torsion of the mandibular body are more critical in the patient evaluation compared with whether an implant should be joined to the natural dentition.Some authors have suggested four implants in the mandible with a full-arch splinted xed restoration—two in the rst molars and two in the canine regions (Fig. 24.9).22 Additional implants have been used with this full-arch splinted restorative option, with up to four other implants in the premolar and the incisor regions.23 However, complete cross-arch splinting of posterior molar implants with a rigid, xated prosthesis should be reconsidered in the man-dible. e exure of the mandible is thwarted by the prosthesis, but this introduces lateral stresses to the implant system (cement, screw, crestal bone, and implant–bone interface).24-26 ese lateral stresses place the molar implants, screws, and bone at increased risk because of the mandibular exure and torsion previously addressed.In complete mandibular subperiosteal implants, pain on opening was noted in 25% of the patients at the suture removal appointment when a rigid bar connected molar-to-molar regions. When the con-necting bar was cut into two sections between the foramina, the pain on opening was immediately eliminated. is clinical observation does not mean that the other 75% of patients did not have exure of the mandibular arch on opening. e observation does demonstrate, however, that exure may be relevant to postoperative complications. Implant Overdentures Advantages (RP-4 and RP-5)For an implant-supported overdenture the implants ideally should be placed in planned, specic sites, and their number should be predetermined by the clinician and patient. e pri-mary indications for a mandibular implant overdenture (IOD) are problems often found with lower dentures, such as lack of retention or stability, decrease in function, diculties in speech, tissue sensitivity, and soft tissue abrasions. If an eden-tulous patient is willing to remain with a removable prosthesis, an overdenture is often the treatment of choice. In addition, if cost is a concern for the patient, the overdenture may serve as a transitional prosthesis until additional implants may be inserted and restored. When evaluating mandibular IODs, many advantages exist (Box 24.1).Enhanced Soft Tissue SupportBone loss dictates the appearance of the inferior third of the face. A maxillary overdenture often provides improved sup-port for the lips and soft tissues of the face compared with a xed prosthesis because the prosthesis contour does not have to accommodate daily hygiene requirements. Denture teeth also provide an esthetic replacement for the natural dentition, which is more challenging for the technician to re-create with porcelain fused to metal restorations. For the laboratory to create pink • Fig. . The mandible flexes toward the midline on opening or during protrusive movements as a result of the internal pterygoid muscle attach-ments on the ramus (blue arrows). The mandible also torques, with the inferior border rotating out and up, and the crestal region rotating lingually. The movement is caused by the masseter muscles during forceful biting or parafunction (red arrows).• Fig. . Some authors propose that the ideal implant positions to support a mandibular full-arch prosthesis are the bilateral molars and bilateral canines, splinted together with a rigid structure. These positions are not ideal because of the mandibular dynamics (flexure and torsion) during opening and function. 1. Enhanced soft tissue support 2. Increased chewing efficiency compared with conventional dentures 3. Less expense/implants 4. Esthetics 5. Ease of hygiene 6. Parafunctional habits 7. Less food impaction • BOX 24.1 Advantages of Mandibular Implant Overdentures 571CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment Planninginterdental papilla, as well as replace the soft tissue drape, is easier with an overdenture compared with porcelain-metal xed restorations or zirconia prosthesis. In addition, the teeth can be positioned in the most esthetic position, without any restriction as to the relationship to the atrophied crest, because stability now is provided by the implant and does not depend on tooth position on the crest of the ridge (Fig. 24.10). Increased Chewing Efficiency Compared with Conventional DenturesA study of chewing eciency compared wearers of complete den-tures with patients with implant-supported overdentures. e complete-denture group required 1.5 to 3.6 times the number of chewing strokes compared with the overdenture group.27 e chewing eciency with an IOD is improved by 20% compared with a traditional complete denture.28,29 Less Expense/ImplantsWhen cost is a factor, two implant-retained IODs may improve the patient’s condition at a signicantly lower overall treat-ment cost than a xed implant–supported prosthesis. A survey by Carlsson etal.30 in 10 countries indicated a wide range of treatment options. e proportion of IODs selected versus xed implant dentures was highest in the Netherlands (93%) and lowest in Sweden and Greece (12%). Cost was cited as the number one determining factor in the choice. However, in gen-eral, overdenture treatment is less expensive than a xed implant prosthesis, mainly because of the decreased number of implants required. Estheticse esthetics for many edentulous patients with moderate-to-advanced bone loss is improved with an overdenture compared with a xed restoration. Soft tissue support for facial appearance often is required for an implant patient because of advanced bone loss, especially in the maxilla. Interdental papilla and tooth size are easier to reproduce or control with an overdenture. Denture teeth easily reproduce contours and esthetics compared with time-consuming and technician-sensitive porcelain-metal or zirconia xed restorations. e labial ange may be designed for optimal appearance, not daily hygiene. Ease of HygieneHygiene conditions and home and professional care are improved with an overdenture compared with a fixed pros-thesis. Peri-implant probing is diagnostic and easier around a bar than a fixed prosthesis because the crown often pre-vents straight-line access along the abutment to the crest of the bone. The overdenture may be extended over the abut-ments to prevent food entrapment during function. Speech is not compromised because the denture may extend onto the soft tissues in the mandible and prevent air and saliva from escaping (Fig. 24.11). Parafunctional HabitsAn overdenture may be removed at bedtime to reduce the nox-ious eect of nocturnal parafunction, which increases stresses on the implant support system. e overdenture also may provide stress relief between the superstructure and prosthesis, and the soft tissue may share a portion of the occlusal load. e prosthesis AB• Fig. . (A and B) Soft tissue support: because of the ability to modify the flange of the prosthesis, ideal soft tissue support can be obtained.• Fig. . Hygiene: because of the removable nature of the overdenture, hygiene access is much easier to complete in comparison with a fixed prosthesis. 572PART V Edentulous Site Treatment Planningis usually easier to repair than a xed restoration. In most cases, there exists a reduced cost of overdenture treatment in compari-son to a xed prosthesis. In addition, long-term denture patients do not appear to have a psychological problem associated with the ability to remove their implant prostheses.31,32 erefore, denture patients usually adapt very well into an overdenture treatment. Less Food ImpactionEspecially with an RP-5 prosthesis, there is less food impaction with an overdenture in comparison with a xed prosthesis. e anges of the prosthesis (RP-5) usually extend to form a periph-eral seal that minimizes food impaction. Because of the nature of the xed mandibular prosthesis, it often is overextended for esthetic reasons. In comparison with a conventional denture, food particles migrate and become impacted under the pros-thesis during swallowing. Because a lower denture “oats” and moves during function, the food more readily goes under and out, whereas the IOD traps the food debris against the implants, bars, and attachments. Review of the Overdenture LiteratureIn 1986 a multicenter study reported on 1739 implants placed in the mandibular symphysis of 484 patients. e implants were loaded immediately and restored with bars and overdentures with clips as retention. e overall success rate was 94%.33 Engquist et al.34 reported a 6% to 7% implant failure for mandibular implant–supported overdentures and a 19% to 35% failure rate for maxillary IODs. Hyperplasia below the bar occurred in 25% of the patients. Jemt etal.35 reported on a 5-year prospective, mul-ticenter study on 30 maxillae (117 Brånemark implants) and 103 mandibles with 393 implants. Survival rates in the mandible were 94.5% for implants and 100% for prostheses; in the maxilla the survival rates were 72.4% for implants and 77.9% for prostheses. Higher failure rates in the maxilla were related directly to poor density and quantity of bone with a characteristic cluster failure pattern.35Wismeijer etal.36 reported on 64 patients with 218 titanium plasma-sprayed implants, with a 97% survival rate with overden-tures in a 6.5-year evaluation. Naert etal. found 100% implant success at 5 years for overdentures with dierent anchorage sys-tems. In Belgium, Naert at al. reported on 207 consecutively treated patients with 449 Brånemark implants and Dolder-bar overdentures. In this report the cumulative implant failure rate was 3% at the 10-year benchmark.37Misch38 reported a less than 1% implant failure rate and no prosthesis failure over a 7-year period with 147 patients when using the organized treatment options and prosthetic guidelines presented in this chapter. Kline etal.39 reported on 266 implant-supported overdentures for 51 patients, with an implant survival rate of 99.6% and a prosthesis survival rate of 100%. Mericke-Stern40 reported a 95% implant survival rate with two IODs.In a randomized clinical report Awad etal. compared satisfac-tion and function in complete dentures (48 patients) versus two IODs in 56 patients. ere was signicantly higher satisfaction, comfort, and chewing ability in the IOD group.41omason etal.,42 in the United Kingdom, reported a 36% higher satisfaction for the IOD patients than the complete den-ture wearers in the criteria of comfort, stability, and chewing.In a 10-year study of IODs in Israel, with 285 implants and 69 IODs, Schwartz-Arad etal.43 reported the implant sur-vival rate was 96.1% with higher success rates in the mandible. Many reports have been published over the last two decades that conclude that implant-supported overdentures represent a valid benecial option for denture wearers. It should be noted that the majority of reports are for IODs supported by only two implants.44,45 Mandibular Overdenture Treatment Planning (RP-4 and RP-5)Anatomy of the MandibleIn treatment planning the mandible for a xed or removable pros-thesis, the mandible is divided into three regions: (1) anterior mandible, (2) posterior right, and (3) posterior left. e available bone in the anterior mandible is divided into ve equal columns of bone serving as potential implant sites, labeled A, B, C, D, and E, starting from the patient’s right side.46,47 Regardless of the treatment option being used, all ve implant sites are mapped out at the time of treatment planning and surgery (Fig. 24.12). Mandibular Implant Site SelectionAnterior retention and stability for an overdenture oer sev-eral advantages. e greatest available height of bone is located in the anterior mandible, between the mental foramina. is A B C D E?• Fig. . (A) The mandible is divided into three regions for treatment planning. (B) The anterior mandible is positionally documented via A, B, C, D, and E positions. 573CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment Planningregion also usually presents a favorable bone density (e.g., D2) for implant support. In addition, overdentures with posterior movement (RP-5) gain better acceptance than removable pros-theses with anterior movement. An axiom in removable par-tial denture design is to gain rigid prosthetic support in the anterior region. When the prosthesis has poor anterior and poor posterior support, it rocks back and forth. is rocking action applies torque to the abutments and increases stresses on the overdenture components and bone–implant interface. erefore anterior forces should be resisted by implants or bars, whereas posterior forces may be directed on a soft tissue area such as the mandibular buccal shelf (i.e., primary stress-bear-ing area). In addition, the IOD treatment options presented are designed for anterior implant placement with adequate bone quantity.In this way the patient always has the option to obtain addi-tional implant support in the future. For example, a patient may receive adequate support for an IOD with four implants. How-ever, if the patient desires a xed prosthesis in the future, these four implants may fall short of the new requirements. If the clini-cian did not plan an additional implant site at the initial surgery but instead placed the four implants an equal distance apart, the additional space may not be available without removing one of the present implants. In addition, a patient may desire a completely implant-supported restoration as an RP-4 or xed prosthesis but cannot aord the treatment all at once. ree implants in the A, C, and E positions and an overdenture may be provided currently, two implants may be added in the B and D locations later, and a completely implant-supported overdenture or xed restoration may be fabricated.In addition, if an implant complication occurs that results in an implant failure, corrective procedures may be completed. If implants were placed in the A, B, D, and E positions, and an implant fails to achieve rigid xation, the failed implant may be removed and an additional implant placed in the C position at the same time. is saves time as additional surgery is not required which eliminates the additional bone healing time before another implant can be reinserted. Overdenture Treatment OptionsIn 1985 Misch39,48 presented organized treatment options for implant-supported mandibular overdentures in the completely edentulous patient. e treatment options range from primarily soft tissue support with secondary implant retention (RP-5) to a completely implant-supported prosthesis (RP-4) with rigid sta-bility (i.e., no soft tissue support) (Table 24.1). e initial treat-ment options are presented for completely edentulous patients with Division A (abundant) or B (sucient) anterior bone, treated with Division A anterior root form implants of 4 mm or greater in diameter.When evaluating the patient for an overdenture, the clini-cian should evaluate the patient’s existing dentures concerning support, retention, and stability. Support is related to the resis-tance to occlusal load. Retention describes the resistance of the prosthesis to movement away from the tissues. Stability is the lateral resistance criterion. e patient’s complaints, anatomy, desires, and nancial commitment determine the amount of implant support, retention, and stability required to address these conditions predictably. e amount of resistance pro-vided in IODs is related to the number and position of the implants.Overdenture Option 1e rst treatment option for mandibular overdentures (OD-1) is indicated primarily when cost is the most signicant patient factor and minimal retention is required. e patient should be educated about the amount of retention that may be obtained. Most two implant overdentures can be correlated with a two-legged chair (i.e. rotation and hinging will occur). Bone volume should be abundant (Division A or B) in the anterior, and the pos-terior ridge form should be an inverted U shape, with high parallel walls for good-to-excellent anatomic conditions for conventional denture, support, and stability. e buccal shelf (primary stress bearing area) should be prominent to withstand the forces. Under these conditions, two implants may be inserted in the B and D positions. e implants usually remain independent of each other and are not connected with a superstructure. e most common type of attachment used in OD-1 is a Locator or an O-ring design, because there will be associated prosthesis movement.Positioning of the implants in the B and D positions is a much better prosthetic option in OD-1 than positioning in the A and E regions (Fig. 24.13). Independent implants in the A and E positions allow a greater amplitude of rocking of the prosthesis compared with implants in the B and D regions (Fig. 24.14). When using B and D implants, the anterior movement of the prosthesis is reduced, and the prosthesis even may act as a splint for the two implants during anterior biting forces, thereby decreasing thestress to each implant. However, most situations do not allow the prosthesis to act as a true splint because a stress relief attachment permits movement in any plane. As a result, only one implant is loaded at a time in most situa-tions. e stability and support of the prosthesis are gained primar-ily from the anatomy of the mandible and prosthesis design, which is similar to a complete denture. e implant support mechanism is poor because stress relief is permitted in any plane. Mandibular Overdenture Treatment OptionsOption Description Removable Prosthesis TypeOD-1 2 implants (B and D positions) independent of each otherRP-5Ideal posterior ridge formIdeal anterior and posterior ridge formCost is a major factor (two-legged chair)OD-2 3 implants (A, C, and E positions)RP-5Ideal posterior ridge form (three-legged chair)OD-3 4 implants (A, B, D, and E positions)RP-4(favorable force factors)RP-5(unfavorable force factors)Patient desires greater reten-tion, major stability, and support (four-legged chair)OD-4 5 implants (A, B, C, D, and E positions)Patient has high demands or desiresRetention, stability, and support (four-legged chair)OD, Overdenture option. TABLE 24.1 574PART V Edentulous Site Treatment Planninge primary patient advantage with OD-1 is cost. e exist-ing restoration often may be adapted with an intraoral rebase and pickup procedure around the implants and attachments. Addi-tional indications are when arch shape is considerably tapered such that a connecting bar would be cantilevered too far to the facial or would interfere with speech and mastication if too lin-gual. Hygiene procedures also are facilitated with independent attachments.e disadvantages of the OD-1 relate to its relatively poor implant support and stability, compared with the other options, because of the independent nature of the two implants. e other disadvantages of OD-1 relate to an increase in prosthetic maintenance appointments. For the prosthesis to be inserted and function ideally, the two implants should be parallel to each other, perpendicular to the occlusal plane, at the same horizontal height (parallel to the occlusal plane), and equal distance o the midline. If one implant is not parallel to the other, the prosthe-sis will wear one attachment faster because of the greater dis-placement during insertion and removal than the other. If the angulation dierence is severe, the prosthesis may not engage one attachment at all. e implants also should be perpendicular to the occlusal plane. Because the goal is to allow the posterior regions of the overdenture to rock downward and load the soft tissue over the mandibular buccal shelves for support, the hinge rotation should be at 90 degrees to the rotation path. In addi-tion, because only two implants sustain the occlusal load dur-ing function or parafunction, minimization of the forces to the implant components and crestal bone by placing them in the long axis of the implant body and perpendicular to the occlusal plane is ideal (Fig. 24.15).e two independent implants should be positioned at the same occlusal height, parallel to the occlusal plane. If one implant is higher than the other, the prosthesis will disengage from the lower implant during function and rotate primarily on the higher implant. is situation will accelerate the wear of the attachment on the lower implant. In addition, because the higher implant receives the majority of the occlusal load, an increased risk for complications may occur, including abutment screw loosening, crestal bone loss, and implant failure.e implants should be equal distance o the midline. If one implant is more distal (farther from the midline), it will serve as the primary rotation point or fulcrum when the patient occludes in the posterior segments. As such, the more medial implant attach-ment will wear faster, and the more distal implant will receive a greater occlusal load (Box 24.2 and Fig. 24.16). AEBDAB• Fig. . Overdenture option 1 consists of two independent implants. These are best placed in the B and D positions (A) to limit the forward rock-ing of the restoration during function. Independent implants in the A and E positions (B) allow a greater rocking of the restoration and place greater leverage forces against the implants.AB• Fig. . Two-Implant Overdenture. (A) A and E position. (B) This position often results in an anterior and posterior “rocking” of the prosthesis. In addition, the attachment may impinge on the next phase tongue space requiring overcontouring of acrylic. 575CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningOverdenture Option 2ree root form implants are placed in the A, C, and E posi-tions for the second overdenture treatment option (OD-2). e advantages of splinting A, C, and E implants compared with implants in the B and D positions are many. e additional implant provides a sixfold reduction in superstructure exure (i.e. if splinted bar is used) and limits the consequences previ-ously discussed.49 In addition, screw loosening occurs less fre-quently because three coping screws retain the superstructure rather than two. Implant reaction forces are reduced with a third implant compared with two implants. e greater surface area of implant to bone allows better distribution of forces. e risk for abutment or coping screw loosening is reduced further because force factors are decreased. ree permucosal sites distribute stresses more eciently and minimize crestal bone loss. Because the crestal bone is the rst region of the bone to be aected, this represents a major advantage. e reduction in the maximum moment of force is twofold with a three-implant system com-pared with two implants in the A and E regions (Fig. 24.17).50 A three implant overdenture (RP-5) can be correlated to a 3-legged chair for patient educational purposes.BDBDBDABC• Fig. . (A) Treatment option 2 has implants in the B and D positions, and a bar joins the implants. The bar should not be cantilevered off the dis-tal side of the implants. The prosthesis movement will be reduced, and too much force on the bar and implants will increase complications. Attach-ments such as an O-ring (B) or a Hader clip (C), which allow movement of the prosthesis, can be added to the bar. The attachments are placed at the same height at equal distance off the midline and parallel to each other.Advantages• Decreasedcost• Greaterprosthesissupportthanconventionaldenture• Lessinvasivesurgicalandprostheticprocedures Disadvantages• Maynotmeetpatientexpectations• Increasedmaintenanceappointments• Continuouscostassociatedwithattachmentreplacement• Prosthesisrelinemustbecompletedmoreoften• Reliesonsofttissueforprimarysupport Indications• Lowestcost• Less-complicatedsurgery• Patientwhoneedsminimalincreasedretention(RP-5) • BOX 24.2 Treatment Option 1: Two Implants (B-D)AB• Fig. . (A) When two implants are used in the treatment plan, the implants should be as parallel as possible. (B) Two implants that are not parallel and at different heights, which most likely will lead to prosthetic complications. 576PART V Edentulous Site Treatment PlanningIdeally, the implants in the A, C, and E positions should not form a straight line. The C implant is anterior to the more distal A and E implants and directly under the cingulum posi-tion of the denture teeth. The prosthesis benefits from direct occlusal load to the implant support in the anterior arch. When more than two implants are in the anterior mandible, a tripod support system may be established. The greater the A-P spread of the A, C, and E implants, the greater the bio-mechanical advantage to reduce stress on the implant and the better the lateral stability of the implant bar and overdenture system. Rotation of the prosthesis may also be more limited compared with OD-1. Therefore the third implant for OD-2 is a considerable advantage for the mandibular edentulous patient. This is usually the first treatment option for a patient with minimal complaints who is concerned primarily with retention and anterior stability when cost is a moderate fac-tor. The posterior ridge form determines the posterior lingual flange extension of the denture, which limits lateral move-ment of the restoration. If the anterior and posterior ridge form is favorable (Divisions A or B), the implants are placed in the A, C, and E areas, and a wide range of attachments is available (Fig. 24.18).50If the posterior ridge form is poor (Division C−h), the lack of lateral stability places additional forces on the anterior implants. Implants then are best placed in the B-C-D position to allow greater freedom of movement of the prosthesis. e greater the stress to the system, the greater prosthesis movement/stress relief indicated. is increases the posterior movement of the prosthesis but decreases the amount of stress placed on the implants and screw-retained bar.e prosthesis movement for three implants with C−h posterior bone should be greater to minimize forces on the implants and bar or individual attachment system. If the patient with poor poste-rior ridge form requires more stability, more than three implants are indicated. In Division D posterior mandibles, ve anterior implants are indicated to support the prosthesis (Box 24.3). AB• Fig. . Three-Implant Overdenture. (A) A-C-E positions. (B) B-C-D positions.ACEACEAB• Fig. . Overdenture option 3 corresponds to implants in the A, C, and E positions connected with a bar. The attachments should be positioned to allow movement of the distal section of the prosthesis (A). Two non-aligned Hader clips will not allow movement (B). 577CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningOverdenture Option 3In the third mandibular overdenture option (OD-3), four implants are placed in the A, B, D, and E positions. ese implants usually provide sucient support to include a distal cantilever up to 10 mm on each side if the stress factors are low. e cantilevered superstructure is a feature of the four or more implant treatment option for three reasons: e rst relates to the increase in implant support compared with OD-1 to OD-3. e second is that the biomechanical position of the implants is improved in an ovoid or tapering arch form compared with OD-1 or OD-2. e third is related to the additional retention provided for the superstructure bar, which limits the risk for screw loosening and other related complications of cantilevered restorations (Fig. 24.19).In considering a distal cantilever for a mandibular overdenture bar, the implant position is the primary local determinant. Can-tilevers may be compared with a class 1 lever in mechanics. e distalmost implant on each side acts as a fulcrum when occlusal forces are applied to the distal cantilever. erefore the amount of the occlusal force is magnied by the length of the cantilever, which acts as a lever. For example, a 25-lb load to a 10-mm canti-lever results in a 250-lb moment force.is moment force is resisted by the length of the bar anterior to the fulcrum. erefore if the two anterior implants are 10 mm from the fulcrum (distal implants), the eect of the posterior can-tilever is countered. If the implants are 5 mm apart, the mechani-cal advantage of the lever is the 10-mm cantilever divided by the 5-mm A-P spread, which equals 2. A 25-lb distal force is magni-ed to 50 lb to the anterior implant and 75 lb (50 + 25 = 75) to the distal (fulcrum) implant.e mandibular arch form may be square, tapering, or ovoid. Square arch forms limit the A-P spread between implants and may not be able to counter the eect of a distal cantilever. erefore rarely are distal cantilevers designed for square arch forms. In a tapering arch form the A-P spread between implants in the A-E and B-D positions is greater and therefore permits a longer distal cantilever. is A-P spread is often at least 10 mm, and therefore often permits a cantilever up to 10 mm from the A and E positions. In an ovoid arch, which is most common, the A-P spread between AE and BD is usually 8 mm. erefore the cantilever may be up to 8 mm long distally from the A and E implants.e A-P spread is only one factor to determine the length of the cantilever. When stress factors such as occluding forces are greater, the cantilever is decreased. When the crown height is doubled, the moment forces are doubled. erefore under ideal, low-force conditions (crown height less than 15 mm, no parafunction, older females, opposing maxillary denture), the cantilever may be up to 1.5 times the A-P spread for OD-3 overdentures.e patient’s indications for this OD-3 include moderate-to-poor posterior anatomy that causes a lack of retention and stability, soft tissue abrasions, and diculty with speech. e edentulous posterior mandible resorbs four times faster than the anterior mandible. In the C−h posterior mandible the external oblique and mylohyoid ridges are high and often correspond to the crest of the residual ridge. e muscle attachments there-fore are at the crest of the ridge. e patient’s complaints and desires are more demanding than for the previous treatment options.The OD-3 prosthesis is indicated to obtain greater stabil-ity and a more limited range of prosthesis motion. The over-denture attachments often are placed in the distal cantilevers. The prosthesis is still RP-5, but with the least soft tissue sup-port of all RP-5 designs. The anterior attachment must allow vertical movement for the distal aspect of the prosthesis to Advantages• Increasedretention• Lessinvasivesurgeryandprosthetics• IncreasedA-Pspreadfromoptions1 Disadvantages• Maynotmeetpatientexpectations(RP-5prosthesis)• Increasedmaintenanceappointments• Continuouscostassociatedwithattachmentreplacement• Prosthesisrelinemustbecompletedmoreoften• Reliesonsofttissueforprimarysupport Indications• Relativelylowcost• Less-complicatedsurgeryandprosthetics• Patientwhoneedsminimalincreasedretention(RP-5) • BOX 24.3 Overdenture Treatment Option 2: Three Implants (A-C-E)AEBDA B• Fig. . (A and B) Four-implant overdenture in the A, B, D, and E position. 578PART V Edentulous Site Treatment Planningrotate toward the tissue. Clips, which permit rotation, are dif-ficult to use on cantilevered superstructures. The clip must be placed perpendicular to the path of rotation to allow move-ment, not along the cantilevered bar, where its only function then is retention.e patient benets from the three implants because of greater occlusal load support and lateral prosthesis stability. e prosthesis loads the soft tissue over only the rst and second molars and retro-molar pad regions. erefore the amount of occlusal force is reduced because the bar does not extend to the molar position, where the forces are greater. e amount of distal cantilever is related primar-ily to the force factors and to the arch form, which corresponds to the A-P spread from the center of the most anterior implants to the distal portions of the A and E implants. However, without the “C” implant position, the A-P spread is not as great (Box 24.4). Overdenture Option 4e fourth mandibular overdenture option (OD-4) is designed for three types of patients. is is a minimum treatment option for patients with moderate-to-severe problems related to a traditional restoration. e needs and desires of the patient are often most demanding and may include limiting the bulk or amount of the prosthesis, major concerns regarding function or stability, posterior sore spots, and the inability to wear a mandibular denture.e second patient condition is for the treatment of con-tinued bone loss in the posterior mandible. If no prosthetic load is on the posterior bone, the resorption process is delayed considerably and usually is reversed. e third patient condi-tion is a patient who suers from severe soft tissue sore spots or a history of xerostomia. Because of the completely supported nature of this implant treatment plan, no resultant force will be applied to the soft tissue.erefore even when no posterior implants are inserted, the attachments, cantilevered bar, and overdenture avoid load to the residual ridge and often halt its resorption process. Studies have shown that completely implant-supported prostheses may increase the amount of posterior bone height, even when no posterior implants are inserted.17,51 A better option to prevent this bone loss is the insertion of posterior implants before bone atrophy begins. is treatment option is more likely when the patient desires a xed restoration or the arch form is square (Fig. 24.20 and Box 24.5). Division C−h Anterior Mandiblese four treatment options proposed for mandibular implant–supported overdentures provide an organized approach to solv-ing a patient’s complaints or anatomic limitations. e prosthesis support and range of motion should be part of the initial diag-nosis. e treatment options initially proposed are designed for completely edentulous patients with Division A anterior bone in desire of an overdenture. ese options are modied if the anterior bone is Division C−h. e increase in crown/implant ratio and decrease in implant surface area mandate modication of these initial options.In the C−h anterior bone volume patient, one more implant is added to each option and OD-1 is eliminated completely. Ideally an RP-5 with good buccal shelf support is recommended. Advantages• Increasedanteroposteriorspreadfromoptions1and2• Maycantileverwithbar• MaybeusedasanRP-4orRP-5accordingtoforcefactors• Possiblenosofttissuesupport(RP-4) Disadvantages• Moreimplantsrequired• Moreexpensivetreatment• Surgicalandprostheticproceduresmorecomplicated Indications• Increasedretention• Decreasedprosthesismovement• Morerangeofprostheticoptions • BOX 24.4 Treatment Option 3: Four Implants (A-B-D-E)Hader clipO-ringAEBCDBA• Fig. . (A and B) Five-implant overdenture and a removable RP-4 prosthesis with implants in the A, B, C, D, and E positions. 579CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningPatient Education on Various Mandibular Overdenture Optionse clinician and sta can explain the amount of support each treatment option can provide by comparing them with the sup-port system of a chair. Treatment option OD-1 is related to a two-legged chair. e prosthesis provides some vertical support but can rock back and forth. Option OD-2 with three implants is com-pared with a three-legged chair. is system provides further sup-port but can be rocked one way or the other under lateral forces. A four-legged chair provides the greatest support and is similar to OD-3 and OD-4, which are stable, retentive prostheses. Fixed ProsthesisFixed Prosthesis AdvantagesPsychologicalA xed prosthesis provides the psychological advantage of act-ing and feeling similar to natural teeth, whereas an overdenture, even if fully implant supported, remains a removable prosthe-sis. In today’s society, most patients do not want to be able to remove the prosthesis. A xed prosthesis often is perceived as an actual body part of the patient, and if a patient’s primary request is not to remove the prosthesis, an implant-supported overdenture would not satisfy the psychological need of this patient (Box 24.6). Improved Speeche complete mandibular denture, and overdentures to an extent, often move during mandibular jaw movements during function and speech. e contraction of the mentalis, buccinator, or mylo-hyoid muscles may lift the prosthesis o the soft tissue. As a con-sequence the teeth may touch during speech and elicit clicking noises. e retentive nature of a xed prosthesis allows it to remain in place during mandibular movement. e tongue and perioral musculature may resume a more normal position because they are not required to limit the denture or overdenture movement. Decreased Soft Tissue IrritationSoft tissue abrasions and accelerated bone loss are more symptom-atic of horizontal movement of the prosthesis under lateral forces. A mandibular denture may move up to 10 mm during function. An implant-supported overdenture may limit lateral movements and direct more longitudinal forces. Under these conditions, spe-cic occlusal contacts and the control of masticatory forces are nearly impossible. An implant xed prosthesis provides ideal stability of the prosthesis, and the patient is able consistently to reproduce a determined centric occlusion.52 A xed prosthesis is especially benecial in patients with xerostomia (i.e., dry mouth) because there is no tissue contact, which minimizes any possible soft tissue irritation (Fig. 24.21). Increased Biting ForceHigher bite forces have been documented for mandibular xed prosthesis on implants. e maximum occlusal force of a patient with dentures may improve 300% with an implant-supported pros-thesis.53 Jemt etal.54 showed a decrease in occlusal force when the bar connecting implants was removed, which they attributed to the loss of support, stability, and retention. If enough implant support is provided, the resulting prosthesis may be completely supported, retained, and stabilized by the implant prosthesis (i.e., RP-4). Mül-ler etal.55 reported a greater masseter thickness, chewing eciency, and bite force in edentulous patients with xed implant–supported prostheses in a cross-sectional multicenter study. Less Bone ResorptionWhen implants are placed in the anterior mandible, the resorption of bone is decreased. Studies have conrmed that after the extrac-tion of mandibular teeth, an average of 4-mm vertical bone loss occurs during the rst year after treatment. is bone loss will con-tinue indenitely, with the mandible experiencing a fourfold greater vertical bone loss than the maxilla.56 e bone underneath an over-denture may resorb as little as 0.6 mm vertically over 5 years, and long-term resorption may remain at less than 0.05 mm per year.57,58A more recent clinical study by Wright etal.17 has evaluated the posterior mandibular bone loss in IODs (RP-5) compared with cantilevered xed prostheses from anterior implants. e annual bone loss index observed in the RP-5 overdentures ranged from +0.02 to +0.05, with 14 of 20 patients losing bone in the posterior regions. On the other hand, the xed prostheses group had a range from +0.07 to +0.015, with 18 of 22 patients gaining posterior bone area. Reddy etal.18 also found a similar clinical observation in 60 consecutively treated cantilevered xed prostheses supported by ve to six implants placed between the foramina. e mandibular body height was measured 5, 10, 15, and 20 mm distal to the last implant. e baseline measurements up to 4 years after function increased from 7.25 ± 0.25 to 8.18 ± 0.18 mm. Nearly all of the bone growth occurred during the rst year of function. erefore Advantages• Increasedanteroposteriorspreadfromoptions1,2,and3• Usuallybar-retainedcantilevercanbeused• RP-4prosthesis• Nosofttissuesupport Disadvantages• Moreimplantsrequired• Moreexpensivetreatment• Surgicalandprostheticproceduresmorecomplicated Indications• Highestamountofretentionforanoverdenture• Decreasedprosthesismovement• Morerangeofprostheticoptions • BOX 24.5 Treatment Option 4: Five Implants (A-B-C-D-E) 1. Psychological 2. Improved speech 3. Decreased soft tissue irritation 4. Increased biting force 5. Less bone resorption 6. Less soft tissue extension 7. Less long-term expenses 8. Less interocclusal space requirement 9. Patients with limited dexterity 10. Increased chewing efficiency • BOX 24.6 Advantages of Mandibular Fixed Implant Prosthesis 580PART V Edentulous Site Treatment Planningan important advantage for a complete implant-supported pros-thesis is the maintenance and possible regeneration of posterior bone in the mandible. is is especially important because poste-rior bone loss in this region may lead to neurosensory changes and even mandibular body fracture (Figs. 24.22 and 24.23). Less Soft Tissue Extensione implant xed prosthesis reduces the soft tissue coverage and extension of the prosthesis. is is especially important for new denture wearers, patients with tori or exostoses, or patients with low gagging thresholds. Also, the existence of a labial ange in a conventional denture may result in exaggerated facial contours for the patient with recent extractions. Implant-supported prostheses do not require labial extensions or extended soft tissue coverage. Fixed prosthesis should ideally be convex on the intaglio surface, not concave which leads to hygiene diculty. Less Long-Term ExpensesMandibular overdenture wearers often incur greater long-term expenses than those with xed restorations. Attachments such as Locator, O-rings, or clips wear and must be replaced regularly. Replacements appear more frequent during the rst year, but remain a necessary maintenance step. Denture teeth wear faster on an IOD than with a traditional denture because bite force and masticatory dynamics are improved.Walton and McEntee59 noted that there were three times more maintenance and adjustments for removable prostheses compared with xed restorations. IODs often require attachments to be changed on a regular basis, and denture teeth often wear, requiring a new pros-thesis to be fabricated more often. In a review of literature by Good-acre etal., IODs have retention and adjustment problems 30% of the time, relines 19%, clip or attachment fracture 17%, and fracture of the prosthesis 12%. A xed prosthesis (FP-3) requires less repair and less maintenance. Patient education of the long-term maintenance requirement should be outlined at the onset of implant therapy.60 Less Interocclusal Space Requiremente mandibular overdenture treatment plan may require up to 15 mm of space between crestal bone and the occlusal plane. When sucient crown height space is lacking, the prosthesis is more prone to compo-nent fatigue and fracture. e required crown height space (i.e., 15 mm for a bar-retained overdenture and 9 mm for independent attach-ments) provides adequate bulk of acrylic to resist fracture, space to set denture teeth without modication, and room for attachments, bars, soft tissue, and hygiene. However, with a xed prosthesis, only 8mm is required for a zirconia prosthesis and 10 mm for a porcelain fused to metal prosthesis. An osteoplasty to increase crown height space before implant placement or a xed restoration is often indicated when abundant bone height and width are present (Fig. 24.24). • Fig. . Soft Tissue Irritation. Because there is no contact with the soft tissue for a fixed prosthesis, patients who have a history of soft tissue irritation or xerostomia benefit greatly from a fixed prosthesis.• Fig. . Fixed Implant Prosthesis. A net increase in bone is seen with an FP-3 prosthesis (red arrows). With a removable overdenture prosthesis, bone loss is seen and is continuous.OverdenturesPosterior area indexFixed prostheses0.080.020.040.060.060.040.020• Fig. . Implant overdentures with posterior soft tissue support lose bone in the posterior regions almost 75% of the time. Fixed prostheses cantilevered from anterior implants gain bone in the posterior regions more than 80% of the time (right side of graph).• Fig. . Crown/Implant Ratio. It is imperative that sufficient space is present to fabricate a fixed prosthesis, which is approximately 10 mm for a porcelain fused-to-metal prosthesis (i.e., 8 mm for a zirconia prosthesis). 581CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningPatients with Limited DexterityA xed implant prosthesis is ideal for patients with limited dexter-ity, such as patients with autoimmune disorders (e.g., rheumatoid arthritis, scleroderma). With a removable prosthesis, it may be dif-cult or impossible to remove a prosthesis because of the attach-ment xation. Today, numerous hygiene devices are available to assist patients with daily hygiene. Increased Chewing EfficiencyGonçalves etal.61 showed in studies that a xed implant prosthesis results in signicantly increased chewing eciency in comparison with an IOD. In the xed implant prosthesis group the masseter and temporalis muscle showed greater thickness and patient satis-faction was much greater than with a removable prosthesis.61 Mandibular Fixed Prosthesis (FP-1, FP-2, and FP-3)In the past the functional and esthetic rehabilitation of edentulous patients have always been areas of frustration and challenge in the den-tal profession. However, with the use of dental implants, patients are now able to obtain clinically successful rehabilitation through the use of a xed prosthesis. Currently, edentulous patients have a full array of treatment options for a xed prosthesis in the mandibular arch. Implant Treatment Options for Fixed RestorationsTreatment Option 1: The Brånemark ApproachAmong the xed implant–supported options, the prosthesis fol-lowing the Brånemark protocol has been shown to have excel-lent longevity and clinical ecacy.62 is classical treatment plan involves four to six implants between the mental foramina, and bilateral distal cantilevers replace the mandibular posterior teeth, usually to the rst molar region. e mandible does not ex or exhibit signicant torsion between the mental foramina. erefore anterior implants may be splinted together without risk or com-promise. e placement of four to six anterior root forms between the mental foramina and a distal cantilever posterior of the most distal implant to replace the posterior teeth was the treatment of choice for clinical reports from 1967 to 1981 with the Brånemark system.62 is treatment approach resulted in an 80% to 90% implant survival rate for 5 to 12 years after the rst year of loading. In a long-term, 18- to 23-year study, Attard and Zarb63 reported an 84% success rate using this treatment option (Fig.24.25).Treatment option 1 depends greatly on patient force factors; arch form; and implant number, size, and design. As a result, this option should be reserved for patients with low force factors, opposing a removable prosthesis, lower biting force, favorable bone density, and available bone for ideal implant dimensions (Box 24.7 and Fig. 24.26). Treatment Option 2: Modified Brånemark TechniqueA second mandibular xed treatment plan involves a modied Brånemark technique. Bidez and Misch19 have evaluated dentate and edentulous mandibles, and developed a three-dimensional bone-strain model of exure and torsion. Studies were performed to evaluate dierent splinted implant options that would not compromise the prosthetic foundation. As a consequence a num-ber of implant site options have become available.64A slight variation of the ad modum Brånemark protocol is to place additional implants above the mental foramina because the mandible exes distal to the foramen. An implant above one or both foram-ina presents several advantages. First, the number of implants may be increased to as many as seven, which increases the implant surface area. Second, the A-P spread for implant placement is greatly increased. e more distal implant positions will reduce the class 1 lever forces generated from the distal cantilever prosthesis (Fig. 24.27). ird, the Ø 4.0-mm implantOptional implant Mental foramenA-POption 15 implants12-unit FPD• Fig. . The most common number of implants between the foramina for option 1 is five. These implants provide as great an anteroposterior (A-P) distance as possible between the foramina, with suf-ficient interimplant spacing for treatment of complications. FPD, Fixed partial denture.Advantages• Usuallysufcientbonebetweentheforaminaforimplantplacement• Relativelysafeareaforimplantplacement• Posteriorbonequantityforimplantisnotrelevant Disadvantages• Bilateralposteriorcantilever• Susceptibletoexcessiveforcefactors Indications• Lowforcefactors• Positiveanteroposteriorspread(ovoidortapering) • BOX 24.7 Fixed Treatment Option 1: Four to Six Implants Between the Foramina 582PART V Edentulous Site Treatment Planninglength of the cantilever is reduced dramatically because the distalmost implants are positioned at least one tooth more posterior.A prerequisite for treatment option 2 is the presence of available bone in height and width over one or both foramina. When avail-able bone is present, the foramen often requires implants of reduced height compared with the anterior implants. e most distal implant bears the greatest load when loads are placed on the cantilever (acts as fulcrum); therefore the greatest forces are generated on the short-est implants. A minimum recommended implant height of 8 mm and a greater diameter or an enhanced surface area design are recom-mended to compensate for the reduced implant length.e key implant positions in treatment option 2 are the second premolar positions, the canine positions, and the central incisor or midline position. e two optional implant sites are the rst premolar sites and are more often indicated when the patient force factors are greater than usual (Box 24.8 and Fig. 24.28). Option 25 to 7 implants12-unit FPD• Fig. . Treatment option 2 has five key implant positions: two implants placed over the mental foramina, two implants in the canine positions, and one implant in the midline. Secondary implants may be positioned in the first premolar sites. FPD, Fixed partial denture.Advantages• Decreasedcantilever• Increasedanteroposteriorspread Disadvantages• Musthaveadequateposteriorbone• Moreimplantsrequired Indications• Higherforcefactors• Squarearchforms • BOX 24.8 Fixed Treatment Option 2: Implants Anterior and Over ForaminaAB• Fig. . Treatment Option 2. (A) Cone beam computed tomography treatment plan for implants over the foramen area. (B) Clinical image of add-ing an implant over the right foramen to increase the anteroposterior spread.ABC• Fig. . Treatment Option 1. (A) Clinical image of traditional Bråne-mark mandibular technique. (B) Five-implant FP-3 prosthesis. (C) Hybrid Brånemark fixed prosthesis. 583CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment PlanningTreatment Option 3: Anterior Implants and Unilateral Posterior Implante third xed treatment option is used when inadequate bone is present over the foramina and support is required more pos-teriorly. e Bidez and Misch65 strain model of an edentulous mandible indicated that implants in one posterior section may be splinted to anterior implants without compromise. Misch evalu-ated full-arch xed prostheses on implants with one posterior seg-ment connected to the anterior region over 20 years and found no additional complications experienced during this time frame compared with those with independent segments.65erefore an improved treatment plan option to support a xed mandibular prosthesis consists of additional implants in the rst molar or second premolar position (or both) connected to four or ve implants between the mental foramina. Hence ve to seven implants usually are placed in this treatment option (Fig. 24.29).e key implant positions for treatment option 3 are the rst molar (on one side only), the bilateral rst premolar positions, and the bilateral canine sites. e secondary implant positions include the second premolar position on the same side as the molar implant and the central incisor (midline) position. On occasion an additional site may include the position over the mental foramen on the side of the cantilever. Although mandibular movement during function occurs, it has not been observed to cause complications, because the side opposite to the molar implant has no splinted implant(s).Treatment option 3 is a better option than anterior implants with bilateral cantilevers (option 1 or 2) for several reasons. When one or two implants are placed distal to the foramina on one side and are joined to anterior implants between the foramina, a con-siderable biomechanical advantage is gained. Although the num-ber of implants may be the same as option 1 or 2, the A-P spread is 1.5 to 2 times greater because, on one side, the distal aspect of the last implant now corresponds to the distal aspect of the rst molar. In addition, only one cantilever is present rather than bilateral cantilevers. When force factors are greater, six to seven implants may be used for this option. Five implants between the foramina and one or two implants distal on one side encompass the most common placement.66 is approach is superior to treat-ment option 1 or 2 with bilateral cantilevers because: (1) the A-P spread is dramatically increased, (2) more implants may be used if desired, and (3) only one side has a cantilever. However, this option requires available bone in at least one posterior region of the mandible (Fig. 24.30 and Box 24.9). Treatment Option 4: Anterior Implants and Bilateral Posterior ImplantsTreatment plan options for xed full-arch prostheses also may include bilateral posterior implants as long as they are not splinted together in one prosthesis. is option is selected when force fac-tors are great or the bone density is poor. Poor bone quality most often is observed in the posterior maxilla, but on occasion it is also found in the mandible. Several options for xed restorations are available when bilateral posterior implants are included; however, the prosthesis needs to be in more than one piece.67In treatment option 4, implants are placed in all three segments of the mandible. Key implant positions for this treatment option include the two rst molars, two rst premolars, and two canine sites. Secondary implants may be added in the second premolars or the incisor (midline) position (or both) (Fig. 24.31).Option 35 to 7 implants12-unit FPDA-P• Fig. . Treatment option 3 has key implant positions in one first molar site, bilateral first premolar positions, and two canine sites. Secondary implants may be used in the bilateral second premolar and midline posi-tion. The anteroposterior (A-P) distance is measured from the two distal-most implants to the anteriormost implant from the cantilever. FPD, Fixed partial denture.ABC• Fig. . Treatment Plan 3 Options. (A) Two unilateral posterior implants. (B and C) Clinical images of one unilateral implant that increases the anteroposterior spread and reduces the bilateral cantilever effect. 584PART V Edentulous Site Treatment PlanningProsthetically, all implants in the anterior and one posterior side may be splinted together for a xed prosthesis. e other posterior segment is restored independently with an independent three-unit, xed prosthesis supported by implants in the rst pre-molar and rst molar region.ree implants (rst premolar, second premolar, and molar) are used most often for the smaller segment to compensate for force factors and the alignment of the implants (because they are almost in a straight line). At least six implants typically are used in this option, but seven are more often used, so the smaller segment has three implants (Fig. 24.32).e primary advantage of this treatment option is the elimina-tion of cantilevers. As a result, risks for occlusal overload are reduced. Another advantage is that the prosthesis has two segments rather than one. e larger segment (molar to contralateral canine) has an improved advantage because it has implants in three to four dier-ent horizontal planes. Because no cantilever is present, less damag-ing forces are applied to the prosthesis. If the prosthesis requires repair, the aected segment may be removed more easily because only the segment requiring repair needs to be removed. e pros-thesis should exhibit posterior disclusion in excursions to limit lat-eral loads, especially to the prosthesis supported by fewer implants.Disadvantages for treatment option 4 include the need for abundant bone in both mandibular posterior regions and the additional costs incurred for one to four additional implants.Another modication for the completely edentulous man-dible is to fabricate three independent prostheses rather than two. e anterior region of the mandible may have four to ve implants. e key implants are in the two rst molar sites, the two rst premolar sites, and two canine regions. Secondary posi-tions are the two second premolar and central incisor (midline) sites. With this protocol, the posterior restorations extend from rst molar to rst premolar, and an anterior restoration replaces the six anterior teeth.e advantages of this option are smaller segments for indi-vidual restorations in case one should fracture or become unce-mented. In addition, if greater mandibular body movement is expected because of parafunction or a decrease in size of the body of the mandible, the independent restorations allow the most ex-ibility and torsion of the mandible.e primary disadvantage of option 4 is the greater number of implants required. In addition, the available bone needs are great-est with this treatment option. Nine implants are rarely required to replace the lower teeth, regardless of the bone density or force factors present. Option 4 is the treatment of choice when force factors are severe (Box 24.10 and Fig. 24.33). Treatment Option 5: All-on-Four Protocole treatment option 5 includes the “all-on-four” concept, which was developed to avoid regenerative procedures that potentially increase the treatment costs and patient morbidity. is protocol, developed Advantages• Unilateralcantilever• Increasesanteroposteriorspread Disadvantages• Musthaveadequateposteriorbone• Moreimplantsrequired Indications• Higherforcefactors• Squarearchforms • BOX 24.9 Fixed Treatment Option 3: Implant in Anterior and Unilateral PosteriorOption 46 to 9 implantsand two FPDs• Fig. . Treatment option 4 has implants in both molar sites. Other key positions include the two first premolar positions and the two canine sites. Secondary implants may also be positioned into second premolar locations and the midline. FPD, Fixed partial denture.Option 58 to 9 implants8-unit FPDplus two 2-unit FPDs• Fig. . Alternative prosthetic design is for three separate prostheses, first premolar to first premolar supported by four or five implants, and two posterior segments. FPD, Fixed partial denture.Advantages• Nocantilever• Increasesanteroposteriorspread• Highestsupport Disadvantages• Moreimplantsrequired• Bilateralposteriorbone Indications• Higherforcefactors• Pooranteroposteriorspread• Poorbonedensity • BOX 24.10 Fixed Treatment Option 4: Implants in All Three Regions 585CHAPTER 24 The Edentulous Mandible: Fixed Versus Removable Prosthesis Treatment Planningby Malo, uses four implants in the anterior part of a completely eden-tulous jaw to support a provisional, xed, and immediately loaded prosthesis. Most commonly, the two most anterior implants are placed axially, whereas the two posterior implants are placed at an angle (i.e., usually at an approximately 45-degree angle) to increase A-P spread along with decreasing the cantilever length (Fig. 24.34).68,69Tilted implants have been shown to generate favorable biome-chanical outcomes70 and in a metaanalysis, there was no signi-cant dierence in either failure rate compared with axially placed implants71 or marginal bone loss.72 e tilted implants oer sev-eral advantages, which include the use of longer implants (i.e., greater surface area and primary stability), reduced or eliminated cantilever length, and avoidance of vital structures such as the infe-rior alveolar canal.73 is procedure has become popular among clinicians and patients because of the decreased treatment costs and treatment duration.74 e literature has shown high survival rates and a low incidence of complications with this procedure (Fig. 24.35).75,76 Most dental implant and prosthetic survival rates approach 98% (Box 24.11).47,48,77,78 However, all-on-four proto-cols require the clinician to have additional surgical and prosthetic skills. Because of the increased skill level required, clinicians early on their learning curve should exercise caution in these cases. AB• Fig. . All-on-Four Protocol. (A) Two implants are positioned in the anterior and two implants in the posterior at an angle to increase the anteroposterior spread and avoid the mental foramen. (B) Clinical image of mandibular all-on-four.BAC• Fig. . Treatment Plan 4. (A and B) Clinical image of eight implant man-dibular implants with two independent prostheses. (C) An additional prosthetic option is three independent prostheses with one anterior and two posterior.AB• Fig. . All-on-Four Complication. (A and B) If one or more of the implants fail, the prosthesis is lost, and new implants and bone augmenta-tion may need to be completed to redo the prosthesis. 586PART V Edentulous Site Treatment PlanningAdvantages• Fixedimmediateprotocol• Acceptedsurgicalandprostheticprotocol• Fewerimplants,lowercosts• Fastertreatment Disadvantages• Technicallydifcult(surgicalandprosthetic)• Complicationsaredifculttoremedy Indications• Immediateplacementimplants• Immediateloading • BOX 24.11 Fixed Treatment Option 5: All-on-FourSummarye treatment of the edentulous mandible is a common procedure that implant clinicians see on a regular basis. ere exists a full array of options for patients which include 5 xed treatment plans and 4 removable overdenture treatment options. IODs borrow several principles from tooth-supported overdentures. e advan-tages of IODs relate to the ability to place rigid, healthy abutments in the positions of choice. e number, location, superstructure design, and prosthetic range of motion can be predetermined and based on a patient’s expressed needs and desires. Two implants placed just anterior to the mental foramina rarely should be used. e overdenture should be designed to satisfy the patient’s desires and anatomic limitations predictably.Many completely edentulous patients desire a xed restora-tion rather than a removable prosthesis. e nancial cost for a xed implant prosthesis often have been a deterrent but should be more similar to a completely implant-supported overdenture. e number and position of implants should be related to the amount of stress transmitted to the bone during occlusion and parafunction, and the density of the bone. Other considerations include mandibular exure and torsion. 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