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Some added concern is focused on
the IRO. One evaluation revealed a
93.9 percent success rate for implants
supporting an IRO, and the authors
concluded that implant-retained overdentures
are an established treatment
modality, with implant success rates
that are very similar to the results obtained
with implant-supported fixed
prostheses. 16 A long-term examination
of a two-implant mandibular IRO
specifically considered the impact of
implant length as one variable affecting
implant survival. A high survival
rate (95.5 percent) was recorded after
20 years of loading. Although 21 percent
of implants were 8.5 mm or shorter,
implant length and bone quality
did not affect implant survival. 17 The
conclusions that may be drawn are
that implants of approximately 10 mm
have equally high survival in the parasymphyseal
mandible for both IRO
and ISFP, and that increasing implant
length beyond 10 mm does not improve
biologic outcomes in the ISFP
with an appropriately designed cantilever.
Thus, a mandible of 10 mm
height, or inferior-to-superior dimension,
is sufficient for an IRO or ISFP.
Conventional concepts for planning
implant therapy have focused on bone
quality and quantity. 18 However, when
considering the parasymphyseal mandible,
rarely is type III and type IV bone
encountered. Further, ridge resorption
frequently results in a tall mandible
that displays narrow buccolingual
dimension (5 mm) and ensures that the osseous
crest is at least 10 mm to 12 mm
inferior to the planned occlusal plane
(Rule No. 2). Paradoxically, ISFP or
IRO treatment is facilitated by marked
alveolar resorption. Thus, more favorable
prosthetic scenarios involve residual
mandibles of 10 mm to 15 mm
in height, while the more challenging
prosthetic scenarios are associated with
large residual alveolar ridges (e.g., after
extraction). Infrequently, mandibles
of less than 10 mm superior-to-inferior
dimension are encountered. When four
implants of 10 mm cannot be placed
in a severely resorbed mandible, additional
implants of shorter dimension
may be considered. For example, in an
8 mm mandible, the use of 8 mm or
9 mm implants might be considered
if additional implants are included.
Mandibular fracture is not common,
but is recognized as a serious potential
complication among high-risk individuals.
19
Rule No. 2: Interocclusal
(restorative) dimension measured
from ridge crest to occlusal plane
must be ≥10 mm
The interocclusal dimension directly
impacts the quality and integrity
of both an IRO and an ISFP. Both
overdentures and fixed prostheses
require a minimal dimension to provide
structural integrity and to permit
the establishment of proper contours
in support of comfort, mastication,
and speech.
When planning for implant placement,
it is essential to first understand the
planned position of the prosthetic
teeth. In other words, plan down from
the occlusal plane and not up from
the osseous crest. This assures better
control of the restorative dimension.
The location of the occlusal plane
is defined by proper denture construction
at the appropriate vertical
dimension of occlusion. While it is
beyond the scope of this discussion,
widely accepted anthropomorphic
averages suggest that the distance
from the mandibular incisal edge to
the reflection of the buccal vestibule
is approximately 18 mm. 20 Therefore,
if an existing denture measures less
than 15 mm to 16 mm from the incisal
edge to the buccal flange, there may
be cause to reconsider the vertical
dimension of occlusion and/or the
placement of the occlusal plane.
This concept of restorative dimension
was initially addressed by Phillips and
Wong 21 and reiterated by Lee and Agar 22 ;
however, there is little data in support
Figure 3: Conceptualization of stresses and strains
encountered for a mandibular prosthesis supported
by dental implants. High magnitude masticatory forces
(i.) are enacted through long lever arms (ii.), creating
bending moments and force magnification in the
components (iii.). The forces cause deformation in the
prosthesis and challenge the integrity of the implantabutment
interface. The transmitted forces are further
encountered at the implant-bone interface (iv.).
Figure 4: The mandible must be at least 10 mm in
superior-inferior dimension. Rarely are mandibles of
less than 10 mm observed clinically.
of this inferior-superior dimension for
planning of a mandibular IRO or ISFP.
Practically, the restorative dimension
for any implant prosthesis includes
four key components, each with
its own minimum dimension. They
are: 1) the transmucosal dimension
(biologic width) of approximately
2 mm; 2) a supramucosal abutment
height (0 mm to 2 mm) that permits
hygiene; 3) a framework or attachment
height between 3 mm and 5 mm; and
4) acrylic veneer thickness greater
than 2 mm (Fig. 5). It must also be
acknowledged that the replacement
mandibular teeth should accommodate
their full contours. The average
height of mandibular anterior teeth is
approximately 10 mm. 23 A minimum
10 mm of restorative space places
– “Rules of 10” — Guidelines for Successful Planning and Treatment of Mandibular Edentulism Using Dental Implants – 93