Rest Seats Flashcards
What is a rest?
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 can happen if rests do not prevent cervical movement of the RPD?
the underlying hard and soft tissues may be damaged
What is the guiding principle of rest seat preparation?
Evaluate inter-occlusal/inter-incisal relationships in both static & excursive movements
What are the different types of rests?
Occlusal
Embrasure
Cingulum
Hooded
Incisal
Ball
Where are the occlusal rests located?
- Mesial/distal pits of premolars and molars
- Centered over the marginal ridge whenever possible
What are the different shapes of the occlusal rest seats?
- concave/”spoon” shaped
- “ball and socket”
- triangular
If an occlusal rest seat is a “ball and socket” type what type of sphere is it?
less than half of a sphere
If an occlusal rest seat is triangular in shape, the base of the triangle is…
over the marginal ridge
What should the width be of an occlusal rest seat?
- 1/3 of the buccolingual width
- 1/2 the width between cusp tips
What should the depth be of an occlusal rest seat over the marginal ridge?
1.0-1.5 mm
What should the dpeth be of an occlusal rest seat at the deepest portion?
1.5-2.0 mm
Where is the deepest portion of an occlusal rest seat?
pit area of the tooth
The floor of rest seat should incline towards the axial center, forming an angle less than…
90 degrees
Preparation on the tooth for an occlusal rest seat should be…
wide and shallow
or
deep and narrow
wide and shallow
What is the purpose of an extended occlusal rest?
- Designed to ensure maximum bracing from the tooth.
- The extended rest would ensure that the forces are directed down the long axis of the abutment
Where are embrasure rest seats located?
- two adjacent posterior teeth
(used when no posterior modification space is present)
What is the form of an embrasure rest seat?
- Allow adequate tooth
reduction: approximately 1 mm - Prepare “sluiceway” of around 2 mm
What does an embrasure occlusal rest require for additional marginal ridge preparation?
U-shaped trough to accommodate clasp assembly adjacent marginal ridges
What is the depth and width of the marginal ridge preparation for an embrasure occlusal rest?
Depth: ~1mm
Width: ~2mm
How do you determine occlusal clearance for an embrasure rest seat?
placing two pieces of 18-gauge wire across the preparation
What are the indications for lingual rest seats?
- canines having a gradual lingual slope
- canines using composite resin
- most anterior teeth if placed in cast restorations
Where is a cingulum rest seat located?
Thick well-defined cingulum enamel
* Maxillary - cut into enamel
* Mandibular - composite resin
What is the form of cingulum rest seat from the proximal view?
- Floor 1 mm wide
- Extends from marginal ridge to marginal ridge
What is the form of cingulum rest seat from the incisal view?
- “Crescent”-shaped
- Widest portion at the center
What is the form of cingulum rest seat from the lingual view?
- Inverted “V”, chevron-shaped
What happens if you place cingulum rests on inclined surfaces?
displaces tooth and destroys bone
What is the shape of a lingual cingulum rest?
Chevron “V” shaped
Where are lingual cingulum rests usually located?
mostly maxillary canines
What is the angle of a lingual cingulum rest seat?
slopes downward slightly
– <90 degree angle with path of insertion
What is the width of a lingual cingulum rest?
0.75-2.0 mm
What is the iniciso-apical depth of a lingual cingulum rest?
1.0-1.5 mm
Why do you sometimes do a lingual resin-bonded cingulum rest seat?
Mandibular canine does not exhibit a sufficient thickness of lingual enamel.
What is the process of making a lingual resin-bonded cingulum rest seat?
- Lingual cingulum area roughened with diamond & acid-etch
- Composite resin placed & shaped to resemble chevron rest seat
Why are incisal rests not used very often?
- poor esthetics
- occlusal interference
- Increased torquing forces
Where are incisal rests usually used?
Primarily used on mandibular canines
An incisal rest seat should appear as a small, V-shaped notch located __________ from the proximal-incisal angle of the tooth
1.5 to 2.0 mm
Minimum of ______ reduction is needed to prevent rest fracture
1 mm
