Rests and Rest seats Flashcards
what are rests
- a rigid extension of a fixed or removable partial denture that transmits functional forces to the teeth and prevents movement toward the mucosa
what is a rest seat
the portion of a natural tooth or cast restoration prepared to receive an occlusal, incisal, lingual, internal, or semi precision rest
what are the functions of rests
- direct forces down the long axis of teeth
- prevent cervical movement of the RPD
- limit lateral movement of the RPD
- maintain retentive arm in proper vertical relation
- improve the occlusal plane
what are the functions of rests
- direct forces down the long axis of teeth
what happens if cervical movement of the RPD is not prevented
the underlying hard and soft tissues may be damaged
what should you evaluate in rest seat preparation
- interoclusal and interincisal relationships in static and excursive movements
what are the types of rests
occlusal
- embrasure
- cingulum
- hooded
- incisal
what is the location of occlusal rests
- mesial/distal pits of premolars and molars
- centered over the marginal ridge whenever possible
what is the shape of occlusal rest seats
- concave, “saucer, “spoon shaped”
- ball and socket analogy
- triangular in shape - base of triangle over marginal ridge
what is the width of occlusal rest seats
-1/3 the buccolingual width
- one half the width between the cusp tips
what is the depth of occlusal rest seats
- 1-1.5mm reduction over marginal ridge
- 1.5-2mm reduction at deepest portion
- deepest portion located in pit area
the floor of the occlusal rest seat should incline towards the axial center forming an angle less than _____
90 degrees
describe the depth of occlusal rest seats
- round proximal occlusal line angle
- preparation should be wide and shallow(spoon shaped/rounded) rather than narrow and deep
- the angle formed by the occlusal rest and the vertical minor connector should be less than 90 degrees
what is the extended occlusal rest designed for
to ensure maximum bracing from the tooth
- the forces are directed down the long axis of the abutment and the disto occlusal rest would not be needed
what is the location of embrasure rest seats
- 2 adjacent posterior teeth
- often used when no posterior modification space is present
describe the form of embrasure rest seats
- follows form of occlusal rest
- occlusal rests are placed on adjacent posterior teeth
- avoid eliminating contact point
how should the tooth be prepared for embrasure rest seats
- about 1 mm
- prepare “sluiceway” around 2 mm
- U shaped trough to accomodate clasp assembly
- depth ~1mm
- width ~2 mm
how is occlusal clearance in embrasure rest seats measured
by placing two pieces of 18 gauge wire across the prep
what are the indications for lingual rest seats
- in canines having a gradual lingual shape
- in canines using composite resin
- most anterior teeth if placed in cast restorations
what are the location and type of cingulum rest seats
- thick well defined cingulum enamel
- maxillary - cut into enamel
- mandibular - composite resin
what is the proximal view of cingulum rest seats
- floor 1 mm wide
- extends from marginal ridge to marginal ridge
what is the incisal view of cingulum rest seats
- crescent shaped
- widest portion at the center
what is the lingual view of cingulum rest seats
inverted “V”, chevron shaped
what does placement of rests on inclined surfaces do
displaces the tooth and destroys bone
what should you avoid in maxillary cingulum rest seats
opposing occlusion
describe the lingual cingulum rest
- primarily maxillary canines
- chevron V shaped
- rest seat sloped downward slightly ( less than 90 degrees with path of insertion)
- width of floor: 0.75- 2mm
- inciso- apical depth: 1-1.5mm
why do a lingual resin bonded cingulum rest seat
mandibular canine does not have enough thickness of lingual enamel
describe the lingual resin bonded cingulum rest seat
- alternative mandibular canine lingual rest
- lingual cingulum area roughened with diamond and acid etch
- composite resin placed and shaped to resemble chevron rest seat
- clinical studies report long term success
what are the indications for hooded rests
- mandibular first premolars only
- to provide for increased occlusal support
- decreases torque by lowering the center of rotation
- not on distal abutment of distal extensions
describe the shape of hooded rest seats
- 1.0-1.5mm reduction
- extends from marginal ridge to marginal ridge
what are the indications for incisal rests
-poor esthetics
- occlusal interference
- increased torquing forces
- primarily on mandibular canines
- used as an indirect retainer and/or auxillary rest
what should an incisal rest look like
small, V shaped notch located 1.5-2 mm from the proximal- incisal angle of the tooth
what would cause a rest fracture
- underpreparation of rest seat- minimum of 1mm reduction is needed
- inadequate thickness of metal
