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Management of stresses in rpd / orthodontic course by indian dental academy
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Management of stresses in rpd / orthodontic course by indian dental academy

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Indian Dental Academy: will be one of the most relevant and exciting training center with best faculty and flexible training programs for dental professionals who wish to advance in their dental practice,Offers certified courses in Dental implants,Orthodontics,Endodontics,Cosmetic Dentistry, Prosthetic Dentistry, Periodontics and General Dentistry.

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Management of stresses in rpd / orthodontic course by indian dental academy Management of stresses in rpd / orthodontic course by indian dental academy Document Transcript

  • 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. 1
  • 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 2
  • 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. 3
  • 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. 4
  • 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. 5
  • 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. 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. 6