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“RULES OF 10”
Figure 5: Accounting for a minimal restorative
dimension. A fixed or removable mandibular prosthesis
must allow for placement of: (i.) the transmucosal
abutment; (ii.) adequate room and access for periimplant
mucosal hygiene; (iii.) restorative components,
abutment, and bridge screws; and (iv.) an
esthetic and phonetically accepted veneer.
Figure 6: Defining the depth of implant placement.
Implant placement may be at the osseous crest if
there is sufficient buccolingual width at that location
and sufficient restorative dimension (from the crest
to the occlusal plane). However, if these requirements
are not met, implant placement is planned in
a subcrestal location with a need for accompanying
alveolectomy.
Figure 7: A-P spread in clinical situation. Providing
a single premolar and a single molar (16.5 mm
in length) in the distal cantilever requires approximately
10 mm A-P spread (X) to maintain a 1.5:1
relationship.
average-size mandibular prosthetic
teeth precisely at the soft tissue crest
with only a minimal dimension for the
prosthetic components.
It becomes evident that the planning
of an implant-supported or implantretained
prosthesis for the edentulous
mandible begins with defining a superior-inferior
reference, namely, the
occlusal plane. Space accommodation
for the dimension and location of
teeth, frameworks, attachments, retaining
abutments (balls, bars, etc.), and
biologic width will direct planning of
implant position.
Finally, the location of the osseous
crest in relationship to the planned
implant position dictates the extent
of the alveolectomy required (Fig. 6).
Jensen and colleagues provide an
excellent review of the surgical and
prosthetic considerations for the proposed
alveolectomy and describe it
as the creation of a mandibular
“shelf.” In addition to establishing
restorative space and alveolar width,
the shelf design facilitates visualization
of the inferior alveolar nerve,
inspection of any lingual concavities,
and collection of bone stock for any
secondary grafting. 24
This approach differs from the evaluation
of bone as a primary step in the
planning of mandibular implant prostheses.
This second rule is essential for
providing a robust and lasting fixed
or removable prosthesis supported or
retained by dental implants.
Rule No. 3: Anterior/posterior
distribution of implants must be
at least 10 mm for the ISFP
The ISFP was originally envisioned for
treatment of mandibular edentulism by
using the abundant bone of the mandibular
parasymphysis. A cantilever
design of the ISFP was inherent to the
solution, using multiple anterior implants.
The implants must be able to
support functional loads at the posterior
occlusal contacts via the cantilever.
These loads, however, are magnified
within the framework and components,
and potentially at the implant-bone interface.
In the early conceptualization
of this therapy, the anterior-posterior
distribution of dental implants was
recognized as a key factor affecting
the incidence of complications in the
cantilevered mandibular ISFP. To counteract
the imposed bending moments
of the loaded cantilever, maximum
distribution of implants was recommended.
This anterior-posterior distribution
of implants is referred to as the
“A-P spread.” Clinicians were quick to
point out that there were anatomic constraints
for implant placement in the
parasymphyseal mandible. Com pared
to curved or V-shaped mandibles,
square-shaped mandibles often provide
little anterior-posterior dimension
anterior to the inferior alveolar nerve
(Figs. 7–9). Additionally, anatomic variations
in the inferior alveolar nerve (e.g.,
anterior loop) are not uncommon 25 and
can reduce the available A-P spread.
A number of different models have
been used to estimate the proper
cantilever length in relationship to
the A-P spread. These approaches
include the use of photoelastic models,
piezoelectric strain sensors, and finite
element models. The results are diverse
and the majority examined the stresses
that accumulate at the implant-bone
interface. Interestingly, the focus on
the implant, per se, does not match
the clinical situation where implant
failures are infrequent and prosthesis
complications are more prevalent.
Any discussion of cantilever length
requires that: 1) the position of the
distal-most implant be anticipated; and
2) the number of teeth to be provided
distal to that implant be defined.
For the purposes of establishing a
concept that meets the needs of most
patients, the goal is to have the distal
implant in the distal-most location
that does not impose on the inferior
alveolar nerve, which is generally
located in the canine or first premolar
region. Further, distal inclination of
the posterior implants may place the
prosthetic interface even more distal
in the first premolar region. 26
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