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:
(5)
• 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
Prevent cervical movement of the RPD
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:
(5)
Occlusal
Embrasure
Cingulum
Hooded
Incisal
Location of Occlusal Rests:
(2)
Mesial/distal pits of premolars and
molars
Centered over the marginal ridge
whenever possible
hape of Occlusal Rest Seats:
(3)
• Concave, “saucer”, “spoon”-shaped
• “Ball and socket” analogy
• Triangular in shape
• “Ball and socket” analogy
• Less than half of a sphere
• Triangular in shape
• Base of triangle over
marginal ridge
Width of Occlusal Rest Seats:
(3)
• One-third the
buccolingual width
• One-half the width
between the cusp
tips
• Do not undermine
the triangular
ridges
Depth of Occlusal Rest Seats:
(3)
• 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 rest seat
should incline towards
the axial center, forming
an angle less than —
90
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.
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
Location of Embrasure
Rest Seats:
(2)
• Two adjacent posterior teeth
• Often used when no posterior
modification space is present
Form of Embrasure Rest Seats
(5)
• 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
Embrasure Occlusal Rest
Requires
additional marginal ridge preparation
U-shaped trough to accommodate clasp
assembly
adjacent marginal ridges.
Depth: ~1mm, Width: ~2mm
Embrasure Rest Seats
Occlusal clearance is
evaluated by
placing two
pieces of 18-gauge wire
across the preparation.
Indications for Lingual
Rest Seats:
(3)
• 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:
(3)
• Thick well-defined cingulum enamel
• Maxillary - cut into enamel
• Mandibular - composite resin
Form of Cingulum Rest Seats:
• Proximal view
(2)
• Floor 1 mm wide
• Extends from marginal ridge to marginal
ridge
Form of Cingulum Rest Seats:
Incisal view
(2)
• “Crescent”-shaped
• Widest portion at the center
Form of Cingulum Rest Seats:
Lingual view
(1)
• Inverted “V”, chevron-shaped
Placement of rests on
inclined surfaces
displaces the —
and destroys —.
tooth
bone
maxillary cingulum rest seats
avoid
opposing occlusion
Lingual Cingulum Rest
Primarily
maxillary canines
Lingual Cingulum Rest
shape
chevron V shaped
Lingual Cingulum Rest
Rest seat slopes — slightly
downward
– <90 degree angle with path of insertion
Lingual Cingulum Rest
Width of floor:
0.75-2.0 mm
Lingual Cingulum Rest
Iniciso-apical depth:
1.0-1.5 mm
— does not
exhibit a sufficient thickness
of lingual enamel.
Mandibular canine
Lingual Resin-bonded Cingulum
Rest Seat
(4)
Alternative Mandibular Canine Lingual
Rest
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:
(3)
• Mandibular first premolars only
• To provide for increased occlusal support
• Decreases torque by lowering the center
of rotation
Hooded Rests:
Not on
distal abutment of
distal extensions
Shape of Hooded Rest
Seats:
(2)
• 1.0 - 1.5 mm
reduction
• Extends from
marginal ridge to
marginal ridge
Shape of Hooded Rest
Seats:
(2)
• 1.0 - 1.5 mm
reduction
• Extends from
marginal ridge to
marginal ridge
Least desirable of all rests
Incisal Rest
Indications for Incisal Rest:
• Least desirable of all rests
(3)
• poor esthetics
• occlusal interference
• Increased torquing forces
Indications for Incisal Rest:
Primarily used on
mandibular canines
Incisal Rests
•Primarily used as an
indirect retainer
and/or auxiliary rest
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 (2)
Under preparation of rest seat-
Minimum of 1 mm reduction is
needed.
Inadequate thickness of metal
