4.Rests and rest seats-1.pdf

RESTS AND REST SEATS  Rest A rigid extension of the partial denture resting in rest seats prepared on the occlusal, lingual or incisal surface. Provides vertical support. Support The Quality of the Prosthesis to resist tissue-ward movement.  Rest Seat P...

RESTS AND REST SEATS  Rest A rigid extension of the partial denture resting in rest seats prepared on the occlusal, lingual or incisal surface. Provides vertical support. Support The Quality of the Prosthesis to resist tissue-ward movement.  Rest Seat Portion of a tooth prepared to receive a rest  I. Occlusal Rest A Rigid Extension of a RPD That Contacts the Occlusal Surface of a Posterior Tooth or Restoration, in a Rest Seat Specially Prepared to Receive it. Forms and Requirements of Rest Seat Preparation Should Be Rounded Triangular in Shape the Base of the Triangle at the Marginal Ridge About 2.5 mm in Width, and Its Rounded Apex Is Directed Towards the Center of the Tooth.  Occlusal Rest Seat Form Base of triangle should be one third the bucco- lingual width of the tooth Width varies from one third to one half the mesiodistal diameter of the tooth Smooth flowing outline form (i.e. no sharp line angles)  REST SEAT Apex Is Directed Towards the Center Rounded Triangular m 2.5 mm  Rest Seat 1 m m The Marginal Ridge Is Lowered Approximately 1 to 1.5 Mm of Teeth in Relation to a Vertical Line (permit sufficient bulk )  Occlusal Rest Seat Form Marginal ridge must be lowered and rounded 1-1.5mm (2 mm for gold alloys) ✓Sufficient bulk of metal to prevent fracture  Occlusal Rest Seat Form Floor should be concave or spoon shaped ball & socket joint (saucer shaped) Prevents horizontal stresses & torque  Occlusal Rest Seat Form Floor inclined towards the center Angle formed by rest and minor connector should be less than 900 Deepest portion is central By this way: a. Directs the occlusal forces along the long axis of the tooth b. Prevents Slippage of the Prosthesis and orthodontic movement ✓Rest seat is prepared in enamel if there is low caries index and good oral hygiene. ✓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 Double Embrasure Rest Seat Form Interproximal Occlusal Rest Seats Adjacent teeth, the form is also modified Flared more dramatically on facial and lingual line angles to provide space for the retentive & bracing arms Ensure all line angles are smoothed & round. Preparations For The Occlusal Rest Must GUIDING PLANES Precede Making Master Cast And Follow Proximal s. Preparation (Guiding Planes And Elimination Of Undesirable Undercuts)  Requirements of the Occlusal Rest I. O.R. must fit the tooth ( minimize food collection and preserve their location in relation to the tooth) 2. It must be strong enough to withstand the loads without deform 3. It must not raise the vertical dimension of occlusion Not Raise the Vertical Dimension Functions Of The Occlusal Rest 1-Support (prevent settling) 2-Transmits Vertical Stress Along the Long Axis of the Tooth 3-Maintain the Clasp in Its Proper Position 4- Distributing the Occlusal Load 5- Resistance to Lateral Displacement of the Prosthesis (bracing) 6- It May Act As Indirect Retention 7- It prevents extrusion of abutment teeth 8- It prevents ingress of food between abutment teeth and clasp. 1-Support 2. Transmits Vertical Stress Along The Long Axis Of The Tooth 3. Secure the Clasp in Its Proper Position Prevents denture base from moving cervically & impinge gingiva Slides down incline Fractured abutment, no rest seat Major connector imbeds into tissue Gum stripper 4. Distribute the Occlusal Load 5. Resistance to Lateral Displacement of the Prosthesis but applying damaging horizontal forces on abutment teeth  6. Acts as indirect retainer Prevents rotation (Class I or II RPD’s only) 7. Prevents extrusion of the tooth. 8. Prevents food impaction Close the gap between two adjacent teeth  Special Considerations 1- Boxed shaped occlusal rest These rest seats should be restricted to tooth- supported dentures where the periodontal health of the abutment teeth is good. Provide Guide-surfaces Employed Only on a Perfectly Periodontally Healthy Tooth Helps in Preventing Lateral Movement of the Denture It Provides Increased Denture Retention  2-Tipped Molar (Mesially Inclined Mandibular Molar) A- An Additional Occlusal Rest (usually in the distal)  B. Extended occlusal rest An extended rest should be carried more than one-half way the mesiodistal width of the tooth, It should be approximately one-third the buccolingual width of the tooth, It should be at least of 1mm thickness.  C- A Casting Is Required, Such As Full Veneer Crown or 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. Design: 1- Teeth may be prepared with a flat occlusal surface perpendicular to the long axis. 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.  Rests I- Occlusal Rest II- Lingual Rests III. Incisal Rest IV-Embrasure Hooks  II-Lingual Rests A- Cingulum Rest (inverted V Rest) B. Ball Rest C. Canine Ledge A- Cingulum Rest (inverted V Rest)  Indications No posterior tooth On maxillary canine Multiple rests on incisors (in absence of max. canine).  Lingual Rest Seats Usually the canine, due to its well- developed cingulum When canine is not available, an incisor may be used < 900 ▪ To prepare a rest seat in enamel there should be: a- prominent cingulum. b- good oral hygiene. c- low caries index.  Cingulum Rest Seat Form Slightly rounded to avoid sharp line angles Test as ‘positive’ with explorer tip 1mm depth  Cingulum Rest Preparation Do not create an enamel undercut Cylindrical bur along the long axis of the tooth  Form of cingulum rest MD length of 2.5 mm Labiolingual width 2mm Incisal apical depth of 1.5 mm. Half -Moon Shaped V- Shaped M D Ling Lab 2 mm 1- 1.5 mm V shaped from proximal  II. Lingual Rests A- Cingulum Rest (inverted V Rest) B. Ball Rest C. Canine Ledge  B. Ball Rest 1.5 Mm Deep - 2.5 Mm Wide No sharp line angles  Round Lingual Rest Seat Form Spoon shaped, similar to occlusal rest seat More difficult due to the incline of the lingual surface Easily incorporated into crowns  II-Lingual Rests A- Cingulum Rest (inverted V Rest) B. Ball Rest C. Canine Ledge  C. Canine Ledge A Step-like Preparation 1.5 mm Deep No Sharp Line Angles  III- Incisal Rest Rigid Extension More Applicable on Mandibular Teeth Used Predominantly As Auxiliary Rests or As Indirect Retainers  It should be 2.5mm wide 1.5mm deep. From facial concave and inclined towards the center of the tooth Lingual view  The Lingual Rest Is Preferable to an Incisal Rest Placed Closer to the Center of Rotation of the Abutment Tooth More Esthetic Less Bothersome to a Curious Tongue  Disadvantages Greater mechanical leverage than lingual rests. Incisal Rest Lingual Rest Post Is More Readily Removed by Application of Force Near Its Top Than by Applying Same Force Lingual rest will Nearer Ground Level exert less leverage and reducing its tendency to tipping The display of metal  Rests I- Occlusal Rest II- Lingual Rests III. Incisal Rest IV-Embrasure Hooks  Embrasure Hooks Support Splinting of Natural Teeth Resistance to Lateral and Anteroposterior Movement Act As Indirect Retainer  Embrasure Hooks Poor Esthetics and Wedging Action on Teeth