Occlusal Rest Seat and Cingulum Rest Preparation PDF

Occlusal Rest Seat and Cingulum Rest Preparation Role of Rests in Controlling Prosthesis Movement Rest seat: The prepared surface of the abutment tooth to receive a rest Rest: Any component of the RPD on tooth surface that provides vertical support o Rests are designated based on the tooth surface t...

Occlusal Rest Seat and Cingulum Rest Preparation Role of Rests in Controlling Prosthesis Movement Rest seat: The prepared surface of the abutment tooth to receive a rest Rest: Any component of the RPD on tooth surface that provides vertical support o Rests are designated based on the tooth surface that receives (occlusal rest, lingual rest, and incisal rest) o Remaining natural teeth must resist functional forces, this is critical to control movement and achieve functional stability o Vertical support for RPD must be provided *** Rests PRIMARY purpose: provide vertical support for RPD, while also: 1. Maintain components in the planned prosthesis 2. Prevent settling of RPD by maintaining established occlusal relationship 3. Prevent impingement of the soft tissue 4. Directs and distributes occlusal loads to abutment teeth. Support the position of the RPD and resist movement toward the tissue. Transmit vertical forces to the abutment teeth Direct those forces along the long axis of the tooth Tooth supported RPD rests function similar to fixed abutment retainers, allowing this degree of stability to exist. Rests must be rigid and must receive positive support from the abutment teeth. Under occlusal loading, the rest and tooth remain in stable contact so no independent movement or slippage occurs. In a RPD that has one of more distal extension bases, the denture becomes increasingly tissue supported. As the distance for the abutment increases, the denture becomes more tissue supported. However, occlusal load closer to abutment will transmit the load by means of rest The load is thereby distributed between the abutment tooth and the supporting residual ridge tissue. When rests prevent movement of RPD in apical direction, the position of the retentive portion of clasp arms can be maintained in designated relation to the tooth undercuts. Although passive when in its terminal position, the retentive portion of the clasp arm should remain in contact with the tooth, ready to resist a vertical dislodging force. Then , when a dislodging force is applied, the clasp arm should become immediately engaged to resist vertical displacement. If RPD settling causes clasp arms to stand away from tooth, then vertical displacement is possible before retainer becomes functional. Rests PREVENTS settling and thereby helps maintain vertical stability of RPD Implants can be used as rests Eliminates compression of supporting soft tissue Controls vertical movement of denture base Eliminates or alters fulcrum lines Serves to increase support and stability Posterior Rests Occlusal Rest function: o Transfers stress to the abutment tooth o Resists movement of prosthesis in cervical direction o Stabilizes the retentive arm in its proper position. o When placing it on restoration: AVOID glass ionomer and choose composite over amalgam in a Class II situation*** Design: Occlusal Rest and Extended Occlusal Rest o Occlusal Rest o Extended Occlusal Rests: 2 types § Elongated Occlusal Rest § Continuous Occlusal Rest § ONLY used occasionally to evenly distribute load over a molar tooth § Stabilize a lone standing tooth (to prevent it from drifting) Occlusal Rest and Rest Seat Form and Preparation: o Outline form: rounded triangle shape with apex towards the center of occlusal surface o Base of triangle: at marginal ridge, measures at least 2.5 mm for molars and premolars. o Small dimension rest seats do not provide adequate bulk to metal for rests, especially if rest is contoured to restore the occlusal morphology of the abutment tooth. o The marginal ridge of abutment tooth at the site of rest seat must be lowered to permit a sufficient bulk of metal for strength and rigidity of the rest and minor connector. (Reduction of marginal ridge 1.5 mm is necessary) o Occlusal rest seat floor should be apical to the marginal ridge and the occlusal surface and should be concave or spoon shaped. o Do not create sharp edges or line angles in the preparation o < 90 degrees angle formed by occlusal rest and vertical minor connector from which it originates. § Only in this way the occlusal forces will be directed along long axis of abutment tooth. o > 90 degrees fails to transmit occlusal forces along the vertical axis of the abutment tooth. § Permits slippage of prosthesis away from abutment tooth § Resulting in orthodontic –like forces applied to inclined plane of abutment with possible tooth movement. o Rest Seat Preparation and Dimensions § LabioLingually 2.5 mm at marginal Ridge § MesioDistally = 2.5 mm § Occlusal-Apically 1.5 mm § Floor of rest seat < 90 degrees Extended Occlusal Rest Seat Preparations (Elongated and Continuous) o Used to minimize futher tipping of abutment o In a Kennedy Class II Mod 1 and Kennedy Class III: (most posterior abutment is a mesially tipped molar) § An EXTENDED occlusal rest should be designed to minimize further tipping of abutment and to § Ensures forces are directed down the long axis of the abutment § Rest extend > ½ of mesiodistal width of tooth § Rest width buccolingually should be 1/3 tooth § Allow a minimum of 1 mm of thickness for metal. § The preparation should be rounded with no undercuts or sharp angles. o Severely tilted abutment: ONLAY form is created in the extended occlusal rest. Anterior Rests Maxillary: o Cingulum Ledge (groove): A groove or notch placed on the lingual surface o Rest seats need to be deep to provide a positive stop for the rest. o If dentin is exposed and sensitive, a composite resin restoration may need to be placed. Mandibular o Raised Cingulum: small semilunar shaped addition to lingual of an anterior tooth that is part of a crown or formed from composite restorative material. § BEST rest type for mandibular anterior tooth that is the direct retainer abutment*** § Easily made and provides better bracing potential than any other anterior rest seat *** o Incisal Notch – used infrequently § Unesthetic § Force placed on tooth from a distance from alveolar bone level § Used ONLY when a raised Cingulum or Ledge type rest is NOT feasible***