Rests and Rest seats

Question 1

what are rests

Answer 1

• a rigid extension of a fixed or removable partial denture that transmits functional forces to the teeth and prevents movement toward the mucosa

Question 2

what is a rest seat

Answer 2

the portion of a natural tooth or cast restoration prepared to receive an occlusal, incisal, lingual, internal, or semi precision rest

Question 3

what are the functions of rests

Answer 3

• 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

Question 4

what are the functions of rests

Answer 4

• direct forces down the long axis of teeth

Question 5

what happens if cervical movement of the RPD is not prevented

Answer 5

the underlying hard and soft tissues may be damaged

Question 6

what should you evaluate in rest seat preparation

Answer 6

• interoclusal and interincisal relationships in static and excursive movements

Question 7

what are the types of rests

Answer 7

occlusal
- embrasure
- cingulum
- hooded
- incisal

Question 8

what is the location of occlusal rests

Answer 8

• mesial/distal pits of premolars and molars
• centered over the marginal ridge whenever possible

Question 9

what is the shape of occlusal rest seats

Answer 9

• concave, “saucer, “spoon shaped”
• ball and socket analogy
• triangular in shape - base of triangle over marginal ridge

Question 10

what is the width of occlusal rest seats

Answer 10

-1/3 the buccolingual width
- one half the width between the cusp tips

Question 11

what is the depth of occlusal rest seats

Answer 11

• 1-1.5mm reduction over marginal ridge
• 1.5-2mm reduction at deepest portion
• deepest portion located in pit area

Question 12

the floor of the occlusal rest seat should incline towards the axial center forming an angle less than _____

Answer 12

90 degrees

Question 13

describe the depth of occlusal rest seats

Answer 13

• 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

Question 14

what is the extended occlusal rest designed for

Answer 14

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

Question 15

what is the location of embrasure rest seats

Answer 15

• 2 adjacent posterior teeth
• often used when no posterior modification space is present

Question 16

describe the form of embrasure rest seats

Answer 16

• follows form of occlusal rest
• occlusal rests are placed on adjacent posterior teeth
• avoid eliminating contact point

Question 17

how should the tooth be prepared for embrasure rest seats

Answer 17

• about 1 mm
• prepare “sluiceway” around 2 mm
• U shaped trough to accomodate clasp assembly
• depth ~1mm
• width ~2 mm

Question 18

how is occlusal clearance in embrasure rest seats measured

Answer 18

by placing two pieces of 18 gauge wire across the prep

Question 19

what are the indications for lingual rest seats

Answer 19

• in canines having a gradual lingual shape
• in canines using composite resin
• most anterior teeth if placed in cast restorations

Question 20

what are the location and type of cingulum rest seats

Answer 20

• thick well defined cingulum enamel
• maxillary - cut into enamel
• mandibular - composite resin

Question 21

what is the proximal view of cingulum rest seats

Answer 21

• floor 1 mm wide
• extends from marginal ridge to marginal ridge

Question 22

what is the incisal view of cingulum rest seats

Answer 22

• crescent shaped
• widest portion at the center

Question 23

what is the lingual view of cingulum rest seats

Answer 23

inverted “V”, chevron shaped

Question 24

what does placement of rests on inclined surfaces do

Answer 24

displaces the tooth and destroys bone

Question 25

what should you avoid in maxillary cingulum rest seats

Answer 25

opposing occlusion

Question 26

describe the lingual cingulum rest

Answer 26

• 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

Question 27

why do a lingual resin bonded cingulum rest seat

Answer 27

mandibular canine does not have enough thickness of lingual enamel

Question 28

describe the lingual resin bonded cingulum rest seat

Answer 28

• 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

Question 29

what are the indications for hooded rests

Answer 29

• 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

Question 30

describe the shape of hooded rest seats

Answer 30

• 1.0-1.5mm reduction
• extends from marginal ridge to marginal ridge

Question 31

what are the indications for incisal rests

Answer 31

-poor esthetics
- occlusal interference
- increased torquing forces
- primarily on mandibular canines
- used as an indirect retainer and/or auxillary rest

Question 32

what should an incisal rest look like

Answer 32

small, V shaped notch located 1.5-2 mm from the proximal- incisal angle of the tooth

Question 33

what would cause a rest fracture

Answer 33

• underpreparation of rest seat- minimum of 1mm reduction is needed
• inadequate thickness of metal