Rests and Rest Seats- prosthodontics. Autosaved pptx

presentation on rests , different types , use, advantages

1. Rests and Rest Seats

2.  Introduction  Types/ Classification  Functions  Requirements  Occlusal Rest and rest seats  Lingual rest and rest seat  Incisal rest and rest seat  References

3. Forces that are applied to a removable partial denture must be transferred to the supporting teeth and tissues in an atraumatic fashion. In the case of a tooth-supported removable partial denture, all the stresses are transferred to the abutment teeth. In a tooth-tissue-supported removable partial denture, only a portion of the stresses are transferred to the teeth, while the edentulous ridge must absorb the remainder of the load.

4. The components of a removable partial denture that transfer forces down the long axes of the abutment teeth are called rests. The prepared surfaces of the teeth into which rests fit are called rest seats. The relationship between a rest and a rest seat must be such that forces transmitted from the prosthesis to an abutment are directed apically down the long axis of the tooth. In this manner, stress can be absorbed by the fibers of the periodontal ligament without damaging the ligament or the supporting bone.

5. In addition to transferring forces, each rest should serve as a vertical stop for the prosthesis. Firm, positive contact between a rest and rest seat minimizes vertical displacement of the prosthesis and prevents injury to the soft tissues. A rest also must maintain the retentive clasp in its proper position. If the clasp is not supported, it will lose its ability to retain the prosthesis in its intended position.

6. Rest According to GPT 9  A rigid extension of a removable partial denture that contacts the occlusal, incisal , cingulum , or lingual surface of a tooth or restoration, the surface of which is commonly prepared to receive it. According to McCracken’s  The components of a removable partial denture that transfer forces down the long axes of the abutment teeth are called rests.

7. Rest seat According to GPT 9  The prepared recess in a tooth or restoration created to receive the occlusal, incisal , cingulum or lingual rest. According to McCracken’s  The prepared surface of an abutment to receive the rest.

8. CLASSIFICATION According to McCracken, on the basis of tooth surface prepared to receive them: Occlusal Cingulum Incisal Lingual

9.  According to Stewart on the basis of function they serve: Primary rests Secondary or auxiliary rests  According to Kratochvil , on the basis of their location in the arch, rests can be : a) Anterior rests b) Posterior rests

10. Primary rests A rest that is part of a retentive clasp assembly is referred to as a primary rest. Primary rests prevent vertical movement of a prosthesis toward the tissues and also help transmit applied forces to the supporting teeth. The transmission of lateral forces may be increased by deepening the accompanying rest seats, but this should be done only for entirely tooth supported prostheses. In all other applications, rest seats should be shallow saucer shaped and should function as ball-and-socket joints. This geometry permits dissipation of potentially harmful lateral forces.

11. Auxiliary or secondary rests A rest that is responsible for additional support or indirect retention is called an auxiliary rest or secondary rest. Are used as indirect retainers in extension base removable partial dentures (Class I, Class II, and long-span Class IV applications). These rests are placed anterior or posterior to the axis of rotation to prevent the extension bases from lifting away from the underlying ridges.

12. Types of rests are: Occlusal rests—so named because they are seated on the occlusal surfaces of posterior teeth. Lingual or cingulum rests—those seated on the lingual surfaces of anterior teeth, usually maxillary canines. Incisal rests—those seated on the incisal edges of anterior teeth.

13. Occlusal rests and rest seats It is essential that a rest seat be prepared in the occlusal surface of each tooth that is to receive an occlusal rest. A rest should never be placed on a tooth that has not been adequately prepared. The outline form of an occlusal rest seat should be roughly triangular, with the base of the triangle located at the marginal ridge and the rounded apex directed toward the center of the tooth.

14. If there is one descriptive phrase that can be applied to occlusal rest seats, it is “smooth gentle curves.” Sharp angles, walls, and ledges must be avoided. Any portion of the rest seat that restricts movement of the rest may transmit undesirable horizontal forces to the tooth.

15. The shape of the rest seat should follow the outline of the mesial or distal fossa for the chosen abutment. The rest seat should occupy one-third to one-half the mesiodistal diameter of the tooth and approximately one half the buccolingual width of the tooth measured from cusp tip to cusp tip.

16. The floor of the occlusal rest seat must be inclined slightly toward the center of the tooth. The enclosed angle formed by a line dropped down the proximal surface of the tooth parallel to the long axis of the tooth and the floor of the rest seat must be less than 90 degrees so that the transmitted occlusal forces can be directed along the vertical axis of the tooth.

17. An angle greater than 90 degrees will not yield the desired axial loading and will produce an inclined plane effect. This inclined plane effect can produce slippage of the prosthesis away from the abutment teeth. In addition, it can cause orthodontic movement of abutment teeth, with concurrent pain and bone loss.

18. When viewed in cross section, the deepest part of an occlusal rest seat should be located near the center of the mesial or distal fossa . From its depth, the floor of the rest seat should rise gently toward the marginal ridge. The most common mistake in occlusal rest seat preparation is insufficient reduction of the marginal ridge. This leads to construction of a rest that is extremely thin and subject to fracture.

19. An occlusal rest must be at least 0.5 mm thick at its thinnest point and should be between 1.0 and 1.5 mm thick where it crosses the marginal ridge.

20. Internal Occlusal rests  A totally tooth supported partial denture may use internal occlusal rests for tooth occlusal support and horizontal stabilization.  Occlusal support is derived from the floor of the rest seat and from an additional occlusal bevel if provided.  Horizontal stabilization is from the near-vertical walls of this type of rest seat.  Should be parallel to the path of placement slightly tapered occlusally and dovetailed to prevent dislodgement proximally.

21. Advantage:  Facilitates the elimination of visible clasp arm buccally .  Permits the location of the rest seat in a more favorable position in relation to the “tipping” axis of the abutment.

22.   Long Or Continuous Rests  Splinting periodontally weakened teeth  The rest can be designed to extend entirely across the occlusal surface of two or more teeth and, in some instances, across the entire arch.  When occlusal force is delivered in one area, all the remaining teeth act in unison to provide support. With planning, this type of rest can restore the occlusal plane, provide support, and splint the arch.

23. Ring Rests  For clinical situations with a buccally inclined maxillary molar serving as a distal terminal abutment for a removable partial denture (RPD Ring rest seat design for severely tilted molar abutment tooth: an alternative option . Abdulaziz alhelal , BDS, MS,1,2 mathew T. Kattadiyil , BDS, MDS, MS,3 & balsam J

24. Inter proximal occlusal rests (embrasure rest)  The design of a direct retainer assembly may require that interproximal occlusal rests be used.  Interproximal occlusal rest seats are prepared as individual adjoining occlusal rest seats.  Preparations must be extended farther lingually .  Adjacent rests rather than a single rest are used to avoid inter proximal wedging by the framework.  Also to shunt the food away from contact points.  Care must be exercised to avoid eliminating contact point of abutment teeth.  Sufficient tooth structure must be removed to allow for adequate bulk of the component to be so shaped that occlusion will not be altered.

25. Inter proximal occlusal rests (embrasure rest)

26. Extended occlusal rest  Indicated - in Kennedy class II, modification 1 and Kennedy class III situations when the most posterior abutment is a mesially tipped molar.

27. Lingual or cingulum rests and rest seats Lingual or cingulum rests are used primarily on maxillary canines. The normal morphology of a maxillary canine permits preparation of a satisfactory rest seat with minimal tooth reduction. The thickness of enamel on the lingual surface of a mandibular canine rarely allows a lingual rest to be used.

28. Lingual rests on incisors are also rare. The main indication for their use is missing canines. In this instance, multiple incisors should receive lingual rests to distribute the stresses over a number of teeth because a single incisor seldom offers adequate support.

29. However, a lingual rest is preferred to an incisal rest. A lingual rest is located closer to the rotational center of the supporting tooth and does not tend to tip the tooth. The longer lever arm associated with an incisal rest magnifies the rotational movement of the prosthesis and may result in significant tooth movement. Since a lingual rest is confined to the lingual surface of an anterior tooth, it permits improved esthetics. Because of the configuration of the lingual rest, it is also less subject to breakage and distortion.

30. The form of a lingual rest seat should be V shaped when viewed in cross section. The preparation should display a relatively upright lingual wall that originates at the level of the cingulum and extends incisally . The remaining wall should originate at the crest of the cingulum and incline labiogingivally toward the center of the tooth. This wall must provide a definite seat for the rest and must prevent the migration of the abutment away from the framework.

31. When viewed from the lingual aspect, the outline form of a lingual rest seat should be crescent shaped. This geometry provides some freedom of movement in a mesiodistal direction. Hence, the rest and rest seat may function as a ball-and socket assembly. Sharp line angles and corners must be avoided because they complicate the fit of the cast framework and may serve as stress concentrators.

32. Incisal rests and rest seats Incisal rests and rest seats are less desirable than lingual rests and rest seats for anterior teeth. Nevertheless, they may be used successfully if abutments are sound and cast restorations are not indicated. If a cast restoration is planned for an anterior abutment, an incisal rest is never indicated. A lingual rest should be incorporated into the restoration instead.

33. Incisal rests are most frequently used on mandibular canines. They are not indicated on incisors except under unusual circumstances. If stabilization of incisors is indicated and placement of a fixed restoration is not warranted, incisal rests may be incorporated into a lingual plate to support these teeth.

34. Incisal rests should be positioned near the incisal angles of abutment teeth. Whether they are designed for mesioincisal or distoincisal angles depends on the prescribed clasping system. If an individual tooth is not to be clasped, yet requires an incisal rest, the rest should be placed where it will exert the least impact upon esthetics. In most instances, this requires that the rest be placed at the distoincisal surface of the abutment

35. An incisal rest seat should appear as a small, V-shaped notch located approximately 1.5 to 2.0 mm from the proximal- incisal angle of the tooth. The deepest part of the preparation should be toward the center of the tooth mesiodistally . The notch should be rounded and should extend slightly onto the facial surface to provide a positive seat for the rest.

36. function 1. Support (the main function): Resist displacement towards denture supporting structures preventing impingement of gingival tissues adjacent to the abutment. 2. Maintains components in their planned positions: 3. Distributing the Occlusal Load 4. May provide Indirect Retention: A rest can act as an indirect retainer preventing the rotation of distal extension around the fulcrum axis 5. Direct food away from tooth contact and embrasure area 6. Prevent extrusion of abutments.

37. Composite Buildups for Cingulum Rests: When a cingulum is poorly developed, with insufficient bulk for preparation for a cingulum rest seat, a rest seat can be made using composite resin. Research has demonstrated that these "bonded rest seats" can provide acceptable strength and longevity. The cervical portion of the buildup should have a flat emergence profile (not over contoured) with bulk increasing toward the incisal .

38. Enamel should be pumiced, rinsed, etched, bonded, in a relatively dry, isolated environment. Care must be taken to ensure the cervical composite is well adapted and that most of the form is finalized prior to curing. The bonded rest seat should be smooth and well polished, with no sharp line angles

39. conclusion Proper understanding of the biomechanical design, function and placement of the rest is necessary. Topography of the rest should be such that it restores the topography of the tooth before rest seat preparation as It plays crucial role in maintaining the health of the tissues.

40. reference STEWART’S CLINICAL REMOVABLE PARTIAL PROSTHODONTICS Fourth Edition McCracken's removable partial prosthodontics 12th Ed – Carr Removable Partial Denture Manual . Robert W. Loney , DMD, MS 2011 Ring rest seat design for severely tilted molar abutment tooth: an alternative option . abdulaziz alhelal , BDS, MS,1,2 mathew T. Kattadiyil , BDS, MDS, MS,3 & balsam J

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