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The all-on-four concept generally
requires that posterior teeth beyond
the first premolar are supported
by a cantilever. Efforts to reduce or
eliminate the cantilever by distal
orientation of terminal implants are
advocated. Bridge screws emerging
at the first premolar or molar position
can be achieved in this way. Malo
has reported that this approach is
associated with high implant and
prosthesis survival over 10 years. 26 In
2011, Malo reported on 245 patients in
whom 980 implants were placed and
immediately loaded. The cumulative
patient-related and implant-related
success rates were 94.8 percent and
98.1 percent, respectively, at five years,
and 93.8 percent and 94.9 percent,
respectively, at 10 years. 26
One other key factor contributing to
the overloading of these components
is prosthesis misfit. However, the accuracy
of contemporary impression
materials and the introduction of precision-milled
frameworks for the ISFP
reduce the prevalence of prosthetic
misfit and its impact on the therapeutic
outcome. The geometric distribution
of the implant-abutment and the abutment-prosthesis
interfaces remain the
significant features of ISFP therapy
that can be clinically managed to reduce
complications.
When considering the number of teeth
to be provided distal to the canine or
first premolar site, the minimal provision
of one additional premolar and
one molar is sufficient to meet the
esthetic and functional requirements
of most individuals. 27 These general
guidelines can be converted to linear
measurements. The average dimension
of a mandibular premolar is
approximately 6 mm, and that of the
mandibular first molar is approximately
10 mm. 23 Thus, a cantilever of
16 mm can suffice to provide function
and esthetics. The functional relationship
between the cantilever length
and the A-P spread has been debated
(Figs. 7–9). More than 30 years of opinion,
experimentation, and calculation
have generated an array of suggested
Figure 8: A-P spread in clinical situation. The parallel
placement of the implants resulted in approximately
4 mm to 5 mm of A-P spread.
solutions. To provide the broadest
range of success for the largest set of
patients, a conservative estimate of
this functional relationship should be
selected. For a mandible with no more
than four implants to be restored with
a rigid framework, utilizing a cantilever
length to A-P spread ratio of 1.5:1
has been advised. 28 Thus, for the ideal
situation of four implants placed in
the parasymphyseal mandible with
the distal-most implants located in the
first premolar region, a cantilever of
approximately 15 mm (one premolar
and molar tooth) requires 10 mm of
A-P spread. This represents the third
Rule of 10.
Applying the Rules of 10
Several key steps are required for
using the Rules of 10 in the treatment
of mandibular edentulism. The previously
mentioned reference points
(i.e., occlusal plane and osseous crest)
must be firmly established. To assure
accurate measurement, all treatment
should begin with the proper fabrication
of complete dentures and
verification of ideal tooth position
(Figs. 10, 11).
Rule No. 1 requires a volumetric
assessment of the edentulous mandible
with cone-beam computed tomography
(CBCT). However, other
important information can be found
in the radiographic process, and no
radiograph should be made for ISFP
treatment-planning purposes without
the presence of a radiographic stent.
Figure 9: A-P spread in clinical situation. The divergent
placement of the implants resulted in
approximately 10 mm of A-P spread measured at the
abutment/prosthesis interface.
Figure 10: Conventional dentures
Figure 11: Surgical guide
The resultant images should display
the location of the planned prosthesis
in relation to the mandible.
Rule No. 2 requires that the plane of
occlusion is properly located and the
appropriate vertical dimension of occlusion
is defined. If the patient is also
edentulous in the maxilla, this involves
the fabrication of ideal maxillary and
mandibular dentures. The dentures
will define the location of the occlusal
– “Rules of 10” — Guidelines for Successful Planning and Treatment of Mandibular Edentulism Using Dental Implants – 95