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1. MANAGEMENT OF STRESSES IN RPD
1) A properly constructed FPD is superior to a RPD. The reason for
this is that, the FPD does not move in function.
2) Main consideration in designing a cast partial denture seems to
be related to stress on supporting tissues and also its dissipation
and distribution. Biomechanical principles are helpful to
understand and plan out different designs of RPD.
3) A removable partial denture in the mouth can perform the action
of 2 simple machines, the Lever and the inclined plane.
4) Lever is a rigid bar supported at some point along its length. If
the lever rests against its support and a weight is applied at
another point, rotation or movement will occur around the
support.
5) Partially edentulous situations can be broadly divided into 2
types. Tooth supported and distal extension base. Class I Lever
situation is applicable to distal extension type.
6) The all tooth supported partial denture is rarely subjected to
stresses, because leverage type of forces are not involved and
there are no fulcrums around which the partial denture may
rotate, inclined plane forces are also not a factor here.
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2. Forces acting on partial denture
Distal extension base partial denture rotates in 3 cranial planes
because of differences in support characteristics of the abutment teeth
and soft tissue covering the residual ridge.
Factors influencing magnitude of stresses transmitted to
abutment teeth
1. Length of span
Longer the edentulous span, longer will be the denture base and
greater will be the force transmitted to the abutment teeth.
2. Quantity of supporting ridge
Bone: Large, well formed ridges with parallel sides help to stabilize the
denture against lateral forces.
Soft tissue mucosa: Soft, flabby, displaceable tissue contributes little to
the vertical support of denture and nothing to lateral stability allowing
excessive movement with resultant transmission of stresses to adjacent
abutment tooth.
3. Qualities of clasp
More the flexibility  Less stress transmission to abutment
teeth but increased lateral and vertical stress transmission to residual
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3. ridge. So a decision has to be made whether the ridge or the abutment
tooth requires most protection.
4. Clasp design:
Retentive arm should be designed so that it is passive when
completely seated. A clasp should be designed so that during
placement or removal, the reciprocal arm contacts the tooth before the
retentive GP passes over the greatest-bulge. This will neutralize the
stress to which the abutment tooth is subjected to.
5. Length of clasp
Flexibility increases with increase in length of clasp doubling
the length will increase flexibility by 5 times.
6. Material used in clasp construction
Clasp of Cr-Co exerts greater stress than of gold because of its
more rigidity, keeping all other factors equal. So use smaller diameter
of Cr-Co to accomplish the same purpose.
7. Abutment tooth surface
Surface of gold crown / restoration offers more frictional
resistance to clasp arm movement than does the enamel surfaces of a
tooth. So greater stress is exerted on a tooth restored with gold than on
a tooth with intact enamel.
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4. 8. Occlusal Harmony
i. Role of opposing occlusion
If there is CD opposing, less stress transmitted to RPD. If
natural teeth are present, heavy stresses transmitted to RPD.
ii. Paste of denture base agent which load is applied
E.g.: If load is applied at far end of distal extension base,
stresses will be more due to more movement.
Controlling stress by design considerations
i) Major Connector
In mandibular arch, lingual plate major connector with rests can
aid in distribution of functional stresses to remaining teeth. It also
contribute to cross arch stabilization.
In maxillary arch, use of broad palatal coverage can distribute
stress over a large area. For this, it must be rigid and must receive
vertical support-through rests from several teeth.
ii) Minor Connector
It offers horizontal stability to P.D. against lateral forces on the
prosthesis. Through contact of minor connector tooth abutment, the
prosthesis receives stabilization against lateral stresses.
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5. iii) Direct retention
For Class III situations, quadrilateral distribution of clasp
assembly is easy to accomplish and is considered ideal especially when
modification area exists.
Tripod clasping is used primarily for Class II modified arches. If
modification space is not present one clasp on the dentulous side of the
arch should be positioned as far anteriorly as possible and other as far
posteriorly as possible. Unfortunately for Class I single retentive clasp
on each side of the arch is possible, so indirect retention is very crucial
here.
iv) Rests:
It helps force transmission parallel to long axis of tooth, floor of
the preparation must form less than 90° with the perpendicular line
dripped down the long axis of the tooth, if it is greater than 90°,
inclined plane action is setup and stress is magnified.
Its preparation should be saucer shaped without sharp angles or
ledges. Rest must be free to move within the rest seat to release
stresses that would otherwise be transferred to the tooth, this should be
similar to the action of ball and socket joint.
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6. v) Indirect Retention
It resists rotational movement around the fulcrum line. It should
be located as anteriorly as possible in case of Class I and II situations.
It must be positioned in a definite rest seat to transmit forces through
long axis of abutment tooth.
vi) Denture base
If it is broader, well adapted, it can resist displacing stresses
more efficiently due to optimum advantage of factors of adhesion and
cohesion.
vii) Occlusion
Eliminating teeth contact in eccentric excursions decreases
stresses.
Reduction in size of occlusal table also reduces inadvertent
stresses.
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7. v) Indirect Retention
It resists rotational movement around the fulcrum line. It should
be located as anteriorly as possible in case of Class I and II situations.
It must be positioned in a definite rest seat to transmit forces through
long axis of abutment tooth.
vi) Denture base
If it is broader, well adapted, it can resist displacing stresses
more efficiently due to optimum advantage of factors of adhesion and
cohesion.
vii) Occlusion
Eliminating teeth contact in eccentric excursions decreases
stresses.
Reduction in size of occlusal table also reduces inadvertent
stresses.
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