The cantilever fixed dental prosthesis is a restoration with one or more abutments at one end and unsupported at the other end. Forces transmitted through these cantilevered pontics results in tilting and rotational movements of the abutments. The greatest strain in the distal cantilever fixed partial denture is recorded mesial to the most distal retainer because most fractures occur in this region. There are various criteria and factors necessary for planning a cantilever fixed partial denture (FPD). This paper discusses briefly various factors related to cantilever fixed dental prosthesis.
Every dentist emphasizes on the correlations that
exist between biology and mechanics in treating
patients with either fixed or removable partial
dentures. Distribution of stress within physiologic
limitations of supporting structures in both types
of restorations plays a vital role resulting in a
successful outcome.
The cantilever fixed partial denture is a restoration
with one or more abutments at one end and
unsupported at the other end1. A class I lever
system is created if vertical and oblique forces
directed to the pontic result in forces on the
abutment teeth greater than the applied stress2.
The following factors by Ewing when using the
cantilever principle are a good periodontal
attachment (covering maximum root surface, good
alveolar bone support, favourable root length,
shape, and crown length, arch-to-arch relationship,
favourable tooth-to-tooth relationship.
Ante’s Law states that while selecting the number
of abutments for a fixed restoration, “the total
periodontal membrane area of the abutment teeth
should equal or exceed that of the teeth to be
replaced.”
Varied clinical experience also becomes an
important factor in treatment planning.
Cantilevered fixed dental prosthesis shows more
success in anterior than posterior because the
forces are less in the anterior region than posterior
quadrants A cantilever fixed dental prosthesis
requires at least two abutment teeth. The only
documented indication for a single abutment is
the replacement of a maxillary lateral incisor with
the canine as an abutment3. Thus, the anterior
cantilevered fixed dental prosthesis can be
the ideal indication for cantilever fixed dental
prosthesis.
In case of posterior region when a cantilever pontic
is used to replace a missing tooth, forces applied
to the pontic have an entirely different effect on
the abutment teeth. The pontic acts as a lever that
tends to be depressed under the forces with strong
occlusal vector. It also places maximum demands
on the retentive capacity of retainer. Even though
the use of cantilevered restorations in these regions
appears to be conservative the effect that it has
on the abutment teeth is detrimental.
The cross-arch unilateral two-unit cantilever fixed
dental prosthesis were analysed by Lundgren and
Laurell4 to register occlusal forces throughout light
tooth-tapping,chewing, swallowing, and maximal
biting.
They stated that in spite of functions the distal
cantilevered fixed dental prosthesis was subjected
to less stress than the contralateral posterior
abutment with equal or smaller than local anterior
forces.
The diminished forces on the cantilever units fixed
dental prosthesis attributed to a deflection of the
cantilever units and to not the intrusion of the
connected abutments.
Forces applied to the cantilevered pontic are
resisted through rotational and tilting movements
by the abutment teeth rather than those along
the long axes. Single cantilevered pontics with at
least two abutments are recommended, although
this may vary depending on the existing clinical
conditions and the location of the pontic in the
dental arch. The muscles of mastication exert the
strongest forces in the posterior arch. Placement
of cantilever pontic posteriorly requires additional abutments to withstand the forces.
Henderson et al. used a practical model and a
laboratory model of a three-abutment posterior
fixed dental prosthesis with strain gauges.
All the models, forces to the abutments through the
cantilevered pontics were resisted by rotational
and tilting movements of the abutments.
More than 5 hundredths of the force applied to the
cantilever pontic were absorbed by the abutment
nearest the cantilever pontic, but the addition of
abutment teeth lessened the force on the distal abutment. It was all over that the
abutment nearest the pontic of a cantilever style
of the mounted partial denture can assume over
fifty per cent of the load placed against the pontic.
However, a three-abutment cantilever FDP can
reduce the “combined total resultant” forces to
the distal abutment compared to a two-abutment
cantilever restoration.
Patients restored with FDPs with bilateral terminal
abutments, an average of 26% of the muscular
capacity was activated during chewing compared
with 37% in the bilateral terminal abutment group
in study5.
The variations were explained by the shortage
of terminal abutments inflicting lateral bending
forces that activate peripheral inhibitory feedback
reactions from the TMJ mechanoreceptors.
Antonoff6
declared that cantilever FPDs were a
lot of often indicated once reduced stress was
inherent, like a whole denture in the opposing
dentition.
However, Randow et al.7 reported no major
clinical significances between technical failures
of cantilevered FPDs and also the kind of opposing
dentitions.
They instructed that a well-supported, stable
complete denture might additionally generate
high functional loading.
Studies8,9 concludes that dentition can be
rehabilitated by use of FDPs with cantilever
pontic on specific, isolated abutments that are
periodontally compromised. Stable FDPs were
successful despite individual hypermobile
abutment teeth. Prolonged stability was achieved
by periodontal treatment and the development of
a stable, nontraumatizing occlusion. Balancing
contacts were established to prevent migration,
tilting, and increasing mobility when there was
a possibility of FDP mobility during mandibular
movements.
The forces of mastication declines with
periodontally compromised teeth in dentitions with
cross-arch unilateral posterior two-unit cantilever
FDPs. The quadrants with the cantilevers were
never assigned as the preferred chewing side10.
If the occlusion is stable and the cantilever is free
from premature contacts, the cantilever would be
only subjected to large forces.
Axially directed force of mastication is influenced by
the periodontal support with cross-arch extension
fixed dental prosthesis with unilateral cantilevers10.
The periodontal tissues has less affect on the local
forces on the distal unit of the cantilever because of
the deflection of the cantilever. Randow and Glantzs
stressed on the importance of mechanoreceptor
mechanism of periodontal membrane11. The vital
teeth with bone support had a more efficient form
of mechanoreceptor function at lower degrees of
bending than nonvital teeth.
The guidelines for key implant positions for fixed prostheses appears with the first rule of
not designing prosthetic cantilevers in the fixed
prosthesis for partially edentulous patients of fullarch maxillary fixed restorations. This is because
of the fact that cantilevers are significant force
magnifiers to the cement or prosthesis screws,
prosthesis superstructure, abutment screws,
implant bone interface and the implants.12,13
Cantilever options in esthetic zone, when two
adjacent teeth are missing anterior to canine and
the intra tooth space is less than 12mm which
usually occurs in the mandibular arch a cantilever
may be an acceptable option. In the mandibular
arch when a central incisor and a lateral incisor
are missing, a larger diameter implant in the
central incisor position and a cantilevered pontic
to replace the lateral incisor is indicated. In case
of a maxillary arch the intratooth distance will be
mostly greater than 12mm and hence two implants
can be inserted. In a completely edentulous
mandibles, a cantilever is often the most prudent
treatment option. Pontics are cantilevered from
anterior implants. When this option is considered,
the force factors of parafunction,crown height
space, masticatory dynamics,implant location and
opposing arch are closely scrutinized.14
It can be concluded from that the optimal treatment
for replacing missing teeth is a fixed dental
prosthesis secured at both ends. The cantilever is
considered a compromised solution especially for
unilateral edentulous dentitions. Abutments should
have suitable periodontal support, researchers
have demonstrated that extensive cross-arch
fixed dental prosthesis with cantilevers can be
inserted with a minimal periodontal ligament if the
occlusion is stable and harmonious. The deflective
capacity of the cantilever with the stimulation of the
mechanoreceptors in the periodontium reduces the
stress on the restoration aiding the compromised
periodontal ligament.
Technical failures are more common when nonvital teeth are abutments, because deterioration of tooth
structure can be insidious. More occlusal force can
also be inadvertently extended to nonvital teeth
because their pain threshold is more tolerant.
With the rapid advancement of osseointegrated
implants, the cantilever fixed dental prosthesis
are used in sparse.