REST AND REST SEATS

  1. Rest = part of RPD = rigid extension of the RPD – provides support. 
  2. Rest Seat = Prepared on abutment teeth
  3. Functions: 
    1. Directs and distributes occlusal force apically to abutment teeth 
    2. Acts as vertical stop = doesn’t allow RPD to move in the tissue-ward direction
    3. Maintains retentive clasp in position = SUPPORT in RPD
    4. Rest do allow slight movement = dissipates horizontal forces
  4. TYPES
    1. PRIMARY = part of the clasp assembly 
    2. SECONDARY/AUXILLARY = indirect retention

OCCLUSAL 

  1. Shape = ROUNDED TRIANGULAR
  2. Base = Margin of tooth 
  3. Apex = center of tooth
  4. Floor = spoon-shaped
  5. Dimensions = 1/3rd to half of M-D diameter of tooth and ½ of B-L diameter of the entire tooth 
  6. Measured from cusp tip of buccal side to cusp tip of lingual side. (draw)
  7. ACUTE angle = floor is inclined to towards center of tooth and angle is less than 90* = hence, Increases the support (draw)
  8. If an obtuse angle is present, it causes Slippage of the prosthesis = therefore, avoiding the inclined plane effect 
  9. Occlusal rest = 0.5mm thick at the thinnest point 
  10. 1-1.5 mm = when it crosses the marginal ridge
  11. We use diamond or carbide round burs with water and light pressure 
  12. Preparation is done entirely on enamel only
  13. If the rest seat needs to be placed away from the edentulous area→it is not flared to the facial line angle and should be more flared lingually ( to provide space for minor connector)  
  14. Interproximally extended occlusal rest seat:
    1.  Flared more buccally and lingually without affecting the contact area 
    2. U shaped trough that is 1mm wide and 1 mm deep 
    3. Without breaking the contact = Use a shoulder bur to create a 1 mm Buccolingual clearance from the proximal side then use a round bur to create the seat 
  15. In RPA with intracoronal occlusal rest seat
    1. A partial denture that is totally tooth supported by means of cast retainers on all abutment teeth may use intracoronal rests for both occlusal support and horizontal stabilization
    2. Open the margins more and do slight lingual flaring for the minor connector
  16. Extended occlusal seat rest
    1. In Kennedy Class II, modification 1, and Class III situations in which the most posterior abutment is a mesially tipped molar
    2. an extended occlusal rest should be designed and prepared to minimize further tipping of the abutment and to ensure that the forces are directed down the long axis of the abutment.
    3. This rest should extend more than ½  the MD width of the tooth and 1/3 rd the B L width of the tooth
    4. 1mm thickness of metal = the preparation should be rounded with no undercuts or sharp angles 

LINGUAL REST/CINGULUM REST

  1. Only for maxillary canine 
  2. The mandibular canine is avoided as it lacks the bulk of enamel. 
  3. V-shaped = boomerang shape – Cross section view 
  4. Cresant/Half moon shape – Lingual view
  5. Dimensions = 2.5mm M-D to 2mm B-L 
  6. Depth = 1.5mm = as average thickness of enamel is 2 mm

INCISAL REST 

  1. Mandibular canines and rarely incisors 
  2. V-shaped notch = 1.5 – 2mm away from the proximal incisal angle
  3. Purpose = functioning as secondary rest for indirect retention
  4. Dimensions = 1.5mm wide, 1.5 mm deep, and 1.5mm away from the proximal incisal angle
  5. If multiple incisal rests are needed = rests are connected with linguloplate i.e Choice of major connector
  6. Lingual rest are more commonly preferred than incisal= as lingual rest will be very close to the rotational center of tooth = Bear loads in a much better manner than incisal rest
  7. Major problem = Anterior zone are aesthetic zones. Hence, lingual is chosen over incisal. 

EMBRASSURE REST 

  1. B-L = 3-3.5mm wide
  2. Depth = 1.5mm – 2mm deep
  3. If not met = definite chances of fracture of RPD components. It is very difficult to fix it with the same strength of RPD design 

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