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CLINICALFEATURE
Main Problems in the RPD
(Kennedy Class 1& 2)
Instability
The mandibular free-end distal extension RPD rests on both
fixed and soft tissues.
Consequently the RPD tends to
rotate around a single place and
move down on the soft
underlying tissues. In a distal
extension RPD case with one or
more anterior teeth missing, the
anterior part goes down as the
posterior part goes up and vieversa:
this is the “teeter-totter
phenomenon. (Figure 1)
Figure 1 (top & bottom) Lateral
and horizontal movements of the
frame
Rocking can not only occur in a
vertical plane but often also in a
horizontal plane causing the wellknown
internal posterior soars
(Figure 2). RPD instability will
develop unbalanced forces that
will have consequence to slowly
mobilizing the supporting teeth
until they have to be pulled out. 4
Usually the new missing teeth is
already replaced and the clasp
displaced to the next proximal
teeth; increasing in this way the
distal extension span, “a domino
effect”.
Carious Destruction of the Abutments
Only a few of the most carious resistant teeth can survive the
environmental changes brought by the presence of a clasp-type
ackers and the constant food impaction between the saddle –
guiding plane and the distal aspect of the teeth. (Figure 3)
Destruction of Abutments by
Periodontal Diseases
A well known fact is the destructive potential of a RPD as
consequence of the amplitude of the denture movement and
the clasp mechanism which transfer these movements toward
the abutment teeth. This suggests our analysis about the
denture basis and its relationship to the denture instability. 5
1. The Soft Tissue Support.
Especially in the large distal extension cases, a very large
mucosal support is recommended in order o provides the
widest support under the saddle. The inaccuracy of the cast by
a simple alginate impression, or an inadequate functional
impression would produce negative consequences on the
fitness of the final acrylic basis. (Figure 4)
2. Altrough with good
impression both anatomic
and functional of the basis
by the “altered cast
technique “they are slight
movement in function
before that the basis
reaches the rock-bottom
of incompressibility of the
soft underlying structures.
Such kinds of movement
in a simple clasp RPD
drops the basis onto the
soft tissue not in a
simultaneous vertical
drop, BUT act as a hinging
mechanism or fulcrum
around which the denture
tips down in distal part and
up in the mesial part.
Stress breakers,
Stress Equalizers,
Trauma Absorbing
Figure 2 (top & bottom) Internal
Posterior lingual soars area
These devices are promoted on the fact that they should allow
to the denture basis to move slightly without straining on the
abutment. There are mainly two types of stress-breakers:
A. The most commons are the hinge type stress-breakers
(Figures 5a & 5b). This concept provide a disarticulation
between the basis and the retentive apparatus (clasp and
precision attachment), but act only on vertical occlusal forces,
they have no incidence in the lateral occlusal movements. 6
B. Other stress breakers like the ball-and-socket type allow
essentially the same tipping action and add to it lateral mobility
which tends to allow lateral or rolling mobility and also some
bucco-lingual rolling. (Figures 6a & 6b)
In the two cases, the problem remain in terms that the stressbreaking
leaves the denture basis in much the same situation
with a fulcrum acting a few millimeters distally and much freer
in its movement because the absence of retentive clasps. In
addition this type of device allows an excessive space for the
vertical drop and causes infra-occlusion on the posterior teeth
and consequently an overloading of the anterior remaining
teeth. Taking into the consideration the severe bone resorption
in senior patients, this will be a major weakness of the system.
(Combination Syndrome) (Figure 7a)
The sequel of this action in particularly when bone resorption
frequently occurs, is the continuing rocking of the denture when
loaded occlusally. The rests of the last abutment are in this
case acting as fulcrums. This phenomena establishes a selfperpetuating
or rather a self-worsening situation where more
36 Dental Asia • May / June 2008