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RESTS AND REST SEATS

RESTS AND REST SEATS . Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics. Definitions: A rest : is a rigid extension of partial denture, which contacts a remaining tooth in a prepared rest seat to transmit vertical forces. A rest seat :

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RESTS AND REST SEATS

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  1. RESTS AND REST SEATS Dr. Waseem Bahjat MushtahaSpecialized in prosthodontics

  2. Definitions: A rest : is a rigid extension of partial denture, which contacts a remaining tooth in a prepared rest seat to transmit vertical forces. A rest seat : is the prepared surface of a tooth or a fixed restoration into which a rest fits. An onlay (overlay): is an enlarged occlusal rest covering the entire occlusal surface and extending buccally and lingually.

  3. Functions of rests: 1-Transmit the vertical forces to the abutment teeth . 2-Act as a vertical stop prevent injury of tissues under the denture. A denture without rests is called gum stripper. 3-It maintains the retentive clasp in position. 4-It may be used as indirect retainer. 5-Prevents food impaction, when it is placed on the proximal surface adjacent to the edentulous space. 6- Maintains occlusal contact with opposing teeth by preventing denture settling. 7-An on lay bas the added functions of: A-Establishing more acceptable occlusal plane. B-Increasing the reduced vertical dimension of occlusion. C-Alteration of cusp height and angles to improve occlusion.

  4. Requirements of rests: l-Should have sufficient thickness of metal to prevent fracture, especially at the junction of the rest and minor connector. 2-Should be placed only on surfaces that will direct forces along the long axes of teeth. Should not be placed on inclined tooth surface. 3-Should be extended as close to the center of the tooth mesiodistally, as much as possible, to help axial direction of force. 4-Should be laced in rest seats, which have smooth rounded line angles to improve adaptation of casting.

  5. Types of rests I-Occlusal rests; seated on the occlusal surfaces of a posterior tooth. 2-Cingulum or lingual rest; seated on the lingual surface of a tooth. 3-Incisal rest; seated on the incisal edge of a tooth.

  6. Occlusal Rests Types: 1- Conventional. II- Extended. III- Onlay (Overlay).

  7. I-conventional occlusal rest Design: I-The rest demonstrates a rounded triangular outline form when viewed from the occlusal. The base of the triangle resting on the marginal ridge and the rounded apex directed towards the center of the tooth . 2-The tissue surface of the rest should be smooth and rounded (spoon shape). All angles, walls and ledges should be avoided, to prevent the transmission of lateral forces to the abutment.

  8. 3-Its width varies from one third to one half the mesiodistal diameter of the tooth and should be one half the distance between the buccal and lingual cusp tips . 4-Floor of the rest seat should be inclined apically as it approaches the center of the tooth. The angle between the rest and the minor connector should be less than 90 degrees, to transmit the vertical forces along the long axis of the tooth .If this angle is more than 90 degrees an inclined plane effect will occurs, which results in slippage of the rest and orthodontic movement of the abutment.

  9. 5-Teeth marginal ridges should be reduced l.5mm for base metal alloy, and 2 mm for gold alloys, to give enough thickness of metal to avoid its fracture. 6- The occlusal rest is prepared in sound enamel or in crowns and inlay. It is prepared in enamel if there is law caries index and good oral hygiene. 7-An amalgam restoration is not suitable to support an occlusal rest due to its tendency to flow. So if amalgam is present it should be replaced by cast restoration

  10. II- Extended occlusal rest Indication: For tooth borne segments of partial dentures in which the most posterior abutment is mesially tipped molar. Advantage: I-It promotes axial force direction. 2-Minimize further tipping of the abutment. 3- Minimize rotation of the abutment by its buccal and lingual dovetail extension

  11. Design: An extended rest should be carried more than one-half way the mesiodistal width of the tooth, be approximately one-third the buccolingual width of the tooth, and at least of 1 mm thickness. The preparation should be round with no undercuts or sharp angles

  12. III- Onlay Indication: In severely tilted posterior abutment of a tooth borne segment. Advantages: l-Direct the forces along the long axis of the tooth. 2-Promote RPD stability by its buccal and lingual bevels and proximal guiding plane. 3- Minimize further tipping of the tooth. 4-Restore the occlusal plane.

  13. Design: 1-Cast restorations placed on tipped teeth may be fabricated with a flat occlusal surface perpendicular to the long axis. The tooth preparation for onlay must include removing or restoring pits, fissures, and grooves. A bevel of 1 to 2 mm on the facial and lingual surfaces and 2 to 3 mm guiding plane on the proximal . 2- The occlusion is restored with a base metal alloy or gold occlusal onlay as part of the partial denture framework

  14. Lingual (Cingulum) Rest Indications: 1- When there is no posterior tooth to place an occlusal rest. Occlusal rest is preferred than lingual rest because of its mechanical advantage and ease of seat preparation. 2- The lingual or Cingulum rest is used primarily on maxillary canine, because the morphology of the tooth permits seat preparation. 3 It is rarely used on incisors when the canine is missing. In this situation multiple rests must be used to distribute the force over a number of incisors. 4-To prepare a rest seat in enamel there should be: a- prominent cingulum. b- good oral hygiene. c- law caries index.

  15. Design: 1-The rest seat is half moon shaped when viewed from the lingual. The broadest portion is in the middle of the lingual surface and becomes less broad as it approaches the proximal surface .2-Rest is V shaped when viewed from the proximal, with rounded line angles. The V shaped preparation direct the force into apical direction along the long axis of he tooth. 3-Mesiodistal length of preparation should be a minimum of 2.5 mm, labiolingual width about 2mm, and incisal apical depth a minimum of 1.5 mm. 4-Often difficult to obtain a positive apically inclined rest seat due to tooth angulation or anatomy. The use of cast restoration may be required to establish a definite rest seat

  16. Incisal Rests Indications: 1-Incisal rests are used mostly on mandibular canines when the abutment is sound and when a cast restoration is not indicated. 2-lt may be used as an auxiliary rest for indirect retention. Disadvantages: 1-The display of metal may be objectionable. 2-Greater mechanical leverage than lingual rests, due to the longer minor connector required

  17. Design: 1-The incisal rest seat is prepared on the proximal line angle of the tooth, usually the distal' for esthetic reason. 2- When seen from the facial surface, its floor is concave and inclined towards the center of the tooth to direct the forces along the long axis of the tooth . 3-The outline form of the rest seat is saddle shaped (Convex). with buccal and lingual bevel, when viewed from the proximal. 4-All borders of rest seats are rounded, especially at the junction of the axial wall of the preparation and the floor of the rest seat. 5-It should be approximately 2.5mrn wide, and 1.5mm deep

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