rests and rest seats Flashcards
Rest
A rigid extension of a fixed or removable partial denture that transmits functional forces to
the teeth and prevents movement toward the mucosa.
Rest Seat
The portion of a natural tooth or cast restoration prepared to receive an occlusal, incisal,
lingual, internal, or semi-precision rest
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
failure to prevent cervical RPD movement
If this is not accomplished, the underlying hard and soft tissues may be damaged.
Guiding Principles of Rest Seat Preparation:
• Evaluate inter-occlusal/inter-incisal relationships in both static & excursive movements
Types of Rests:
Occlusal
Embrasure
Cingulum
Hooded
Incisal
Location of Occlusal Rests:
Location of Occlusal Rests:
Mesial/distal pits of premolars and molars
Centered over the marginal ridge whenever possible
deepest portion of posterior occlusal rest seat
pit area
shape of occlusal rest seats
• Concave, “saucer”, “spoon”-shaped
• “Ball and socket” analogy
• Less than half of a sphere
• Triangular in shape
• Base of triangle over marginal ridge
Width of Occlusal Rest Seats: undermine?
• One-third the buccolingual width
• One-half the width between the cusp tips
• Do not undermine the triangular ridges
Depth of Occlusal Rest Seats:
• 1.0-1.5 mm reduction over marginal ridge
• 1.5-2.0 mm reduction at deepest portion
• Deepest portion located in pit area
the floor of occlusal rest seat prep angle
The floor of rest seat should incline towards the axial center, forming an angle less than 90
occlusal line angles of rest preps shape
rounded
should occlusal rest seats be deep and narrow
no, shallow and wide
The angle formed by the occlusal rest and the vertical minor connector should be
less than 90 degrees.
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, and therefore the disto-occlusal rest would not have been needed
diverge walls for placement
Location of Embrasure Rest Seats:
used when?
• Two adjacent posterior teeth
• Often used when no posterior modification space is present
Form of Embrasure Rest Seats
• Follows form of occlusal rest
• Occlusal rests are placed on adjacent posterior teeth
• Avoid eliminating the contact point
• Allow adequate tooth reduction: approximately 1 mm
• Prepare “sluiceway” of around 2 mm
additional prep of embrasure occlusal rests
Requires additional marginal ridge preparation
U-shaped trough to accommodate clasp assembly adjacent marginal ridges.
Depth: ~1mm, Width: ~2mm
Embrasure Rest Seats Occlusal clearance evaluation
Embrasure Rest Seats
Occlusal clearance is
evaluated by placing two
pieces of 18-gauge wire
across the preparation.
Indications for Lingual
Rest Seats:
• Can be prepared in canines having a gradual lingual slope
• Can be prepared in canines using composite resin
• May be used on most anterior teeth if placed in cast restorations
Location & Type of Cingulum Rest
Seats:
• Thick well-defined cingulum enamel
• Maxillary - cut into enamel
• Mandibular - composite resin
Form of Cingulum Rest Seats: proximal, lingual and incisal views
• Proximal view
• Floor 1 mm wide
• Extends from marginal ridge to marginal
ridge
• Incisal view
• “Crescent”-shaped
• Widest portion at the center
• Lingual view
• Inverted “V”, chevron-shaped
placement of cingulum rest on an inclined surface? (ie no seat)
Placement of rests on inclined surfaces displaces the tooth and destroys bone.
what should be avoided with cingulum rest seats
the opposing occlusion
Lingual Cingulum Rest of Primarily maxillary canines form
Chevron “V” shaped
Rest seat slopes downward slightly– <90 degree angle with path of insertion
Width of floor: 0.75-2.0 mm
Iniciso-apical depth: 1.0-1.5 mm
Lingual Resin-bonded Cingulum Rest Seat when/why?
Mandibular canine does not
exhibit a sufficient thickness
of lingual enamel.
Lingual Resin-bonded Cingulum Rest Seat procedure
Lingual cingulum area roughened with diamond & acid-etch
Composite resin placed & shaped to resemble chevron rest seat
Clinical studies report long-term success
Indications for Hooded Rests:
when not used?
• 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
Shape of Hooded Rest
Seats:
• 1.0 - 1.5 mm reduction
• Extends from marginal ridge to marginal ridge
Indications for Incisal Rest and cons
desireable?
esthetics?
occlusion?
torque?
used on which teeth most commonly?
• Least desirable of all rests
• poor esthetics
• occlusal interference
• Increased torquing forces
• Primarily used on mandibular canines
Incisal Rests Primarily used as
an indirect retainer and/or auxiliary rest
incisal rest seat form
An incisal rest seat should appear as a small, V-shaped notch located 1.5 to 2.0 mm from the proximal-incisal angle of the tooth.
Rest Fracture
Under preparation of rest seat-Minimum of 1 mm reduction is needed.
Inadequate thickness of metal
