Occlusal Rests and Rest Seats Guide

RESTS AND
REST SEATS
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

CONTENTS
INTRODUCTION
HISTORICAL REVIEW
TYPES / CLASSIFICATION
FUNCTIONS
REQUIREMENTS
BASIC CONSIDERATIONS
OCCLUSAL RESTS AND REST SEATS
INTERNAL OCCLUSAL RESTS
LONG/CONTINUOUS RESTS
RING RESTS
INTERPROXIMAL OCCLUSAL RESTS
EXTENDED OCCLUSAL RESTS
LINGUAL REST AND REST SEATS
INCISAL REST AND REST SEATS
SUMMARY AND CONCLUSION
REFERENCES

INTRODUCTION
The forces acting on the occlusal surface of a partial denture must
ultimately be absorbed by the alveolar bone, through underlying soft
tissues and teeth supporting the partial denture.
controlling factor in the triad of prosthesis-tooth- periodontium.
Since Bonwill (1899) first introduced and recommended the use of rests in
RPD, the use of rests have been inviolate and has gone unchallenged.

More important in distal extension cases.
Design and placement of rests are major factors in
planning for the control of forces.

DEFINITION
According to glossary of Prosthodontic
terms -
1. Rest - is a projection or attachment, usually on the side of
an object .
2. Rest seat - is the prepared recess in a tooth or restoration
created to receive the occlusal, incisal, cingulum or lingual
rest.

According to Ernest L. Miller -
1. Rest - is a projection of the clasp which lies in a prepared
recess of the abutment tooth and acts to support and
stabilize the removable partial denture
2. Rest seat - The prepared recess in a tooth created to
receive the occlusal, incisal or lingual rest.

According to McCracken’s -
1. Rest - Any unit of a partial denture that rests on a
tooth surface to provide vertical support.
2. Rest seat - The prepared surface of an abutment to
receive the rest

HISTORICAL REVIEW
Russell states that rest is a rigid extension of a partial
denture that contacts the remaining tooth structure to
dissipate functional forces.
According to Grant A.A. and Johnson W. a rest is an
extension from partial denture which is positioned on the
surface of a standing tooth capable of providing resistance
to displacement of the denture in tissueward direction.

Kratochvil stated that rest must be positive and should not
allow the prosthesis to slide off the tooth. They must provide
a positive connection between prosthesis and tooth.
McGregor and Stewart et al stated that rests maintain clasps
in their correct position, prevent food impaction and prevent
the denture from sinking into or causing overdisplacement
of the soft tissues.

CLASSIFICATION/TYPES
According to McCracken, on the basis of tooth surface
prepared to receive them:
Occlusal
Cingulum
Incisal
Lingual

According to Stewart,on the basis of function they serve :
Primary rests
Secondary or auxiliary rests
According to Kratochvil, on the basis of their location in the
arch, rests can be :
Anterior rests
Posterior rests

FUNCTIONS OF THE RESTS
The primary purpose of the rest is to provide vertical
support for the partial denture and thus resist the
movement in a cervical direction .
 -maintains components in
their planned positions
- Provide reciprocation
and stabilization.

 It transmits vertical load as well as the horizontal
forces to the tooth.

Directs and distributes occlusal loads to the
abutment teeth and directs forces in the long axis
of the teeth

Provide rigid prosthetic support.
 Maintains established occlusal relationships by
preventing settling of the denture.
Prevent supra eruption.
Restore occlusion.

Deflection of food by bridging the gap
between two teeth

Prevents impingement of soft tissues.
Protects the denture/abutment tooth junction.

Restore anterior guidance- anterior rests.
Restores anterior anatomy as required.
Provides positive seat by extending over the
incisal edge.

Splints mobile
teeth.

BASIC CONSIDERATIONS FOR RESTS AND REST
SEATS
Rests should be sturdy placed
in properly engineered
recesses, in the surfaces of the
teeth.
The recess should be prepared
within the confines of the
greatest tooth mass- more PDL
fibers.
Vertical stress will be resisted
by all of the fibers.

Floor of rest seat
1. Floor of the recess should be
less than 90° to the long axis
of the tooth- to direct stresses
axially.
2. Apical to the marginal ridge.

Rest should not be
placed on inclined tooth
surfaces-

Anterior rests should be
as close to the center of
the tooth as possible.
Positioned in line with
the residual ridge.

Rounded in all aspects (no
sharp line angles).
Minimal preparation in
dentin.

Center is deeper than
the surrounding rest
surface.

Anterior positioning of
the rest and the rotation
axis to favor
biomechanics:
1. Forward and downward
movement, disengagement
of DR.
2. More vertical direction of
forces.
3. More advantageous vertical
support from denture base
area.

Placed as close to the
gingiva and bone as
possible to reduce
leverage.

Placement of rest on tooth
surface away from
edentulous area-
Tipping force in opposite direction
Maintain contact with adjacent teeth
results
Multiple tooth support
Favorable direction of force

Basic requirements of rests and rest seats
Provides rigid support.
Extends to center of the tooth in tooth supported
situations.
Rounded, with no sharp angles (for ease of cleaning,
making impressions, prevent tooth fracture).

No undercuts in the path of insertion.
Minimum of 1 mm thick.
Restores the occlusal plane.
Provides reciprocation.
Contoured so that when increased force is applied to the
prosthesis the rest will engage more securely to prevent
separation.


OCCLUSAL REST AND REST SEAT
Outline form
• Rounded triangle with the base resting on the
marginal ridge and apex toward the center of the
occlusal surface.
• Should follow outline of mesial or distal fossa.
• Dimensions- ½ the buccolingual width from cusp
tip to cusp tip and 1/3 to ½ the mesiodistal width.
Is as long as it is wide, should be at least 2.5 mm
for both molars and premolars.

Reduction of the marginal ridge of
approximately 1.5 mm is usually
necessary.
Junction of occlusal rest to the abutment
should be shallow ball and socket joint in
distal extension cases.
The floor of the occlusal rest seat should
be apical to the marginal ridge and the
occlusal surface.
- Can be concave, or spoon shaped- for
distal extension.
- Can be box shaped- for tooth supported.

Rest seats not prepared opposing functional cusps.
In tooth borne cases rest must be extended to the center of
the tooth.
Minimum metal thickness is 0.5mm at thinnest point and 1-
1.5mm at marginal ridge.
proximo-occlusal line angle of the preparation should not be
sharp.

Preparation of occlusal rest.
Armamentarium for
rest preparation
Preparation in enamel

Method of obtaining positive support
Rest seat preparations in sound enamel.
Preparation of occlusal rest seats always must follow proximal preparation, never precede it.
Occlusal rest seats in sound enamel may be prepared with diamond points of approximately the
size of nos. 6 and 8 round burs or with carbide burs.
Occlusal rest seat prep. in existing restoration.
It is same as in enamel. Proximal preparation first and then rest seat should be
placed.
Rest seat preparation in amalgam should be avoided because of creep.
Though some compromise is permissible, the basic principles of rest seat preparation
should not be violated. When perforation occurs it may be filled with gold foil.

Occlusal rest seats in new restoration.
- They should be placed in the wax pattern.
- The location of the occlusal rest should be shown when the tooth is prepared for a crown or an
inlay so that sufficient clearance may be provided in the preparation for the rest.
Occlusal rest seats in crown/inlays/onlays.
- Most ideal way of getting positive support.
- Indicated in - rotated/inclined tooth.
- mandibular bicuspid with rudimentary cusp.
- abraded tooth.
- These are generally made larger and deeper than the enamel.
- Those made in abutment crowns supporting tooth borne dentures maybe slightly deeper than
those in abutments supporting a distal extension base.

Types of occlusal rests.
Internal Occlusal rests
A totally tooth supported partial denture may use
internal occlusal rests for tooth occlusal support
and horizontal stabilization.
An internal occlusal rest is not an internal
attachment .
Occlusal support is derived from the floor of the rest
seat and from an additional occlusal bevel if
provided.

Horizontal stabilization is from the near-vertical walls of this type of rest seat.
Should be parallel to the path of placement slightly tapered occlusally and
dovetailed to prevent dislodgement proximally.
Advantage:
 Facilitates the elimination of visible clasp arm buccally
 Permits the location of the rest seat in a more favorable position in relation to
the “tjpping” axis of the abutment. Retention provided by lingual clasp arm
lying in a natural or prepared infrabulge area on the abutment tooth.

Long Or Continuous Rests
Splinting periodontally weakened
teeth
The long or continuous rest can serve as
an effective stabilizing or unifying device.
The rest can be designed to extend entirely
across the occlusal surface of two or more
teeth and, in some instances, across the
entire arch.
When occlusal force is delivered in one
area, all the remaining teeth act in unison
to provide support. With planning, this
type of rest can restore the occlusal plane,
provide support, and splint the arch.

Control position of unopposed teeth
Many times a situation exists in which a tooth
has lost its antagonist in the opposing arch
but does not need a replacement for
masticating functions. Extending the rest in
the partial denture planning and design not
only gains support from that tooth but also
holds it in position, preventing elongation
and eliminating the necessity of a second
prosthesis in the opposing arch.

Ring Rests
Occlusal rests on isolated posterior teeth present a problem. Many of these
teeth are in firm occlusion with their opponents and any modification of their
occlusal surfaces allows them to change position either by tilting or by erupting
further.
Ring rests are usually best for isolated teeth provided there is sufficient area
of tooth above the survey line to accommodate them.
An additional advantage of ring rests is that they provide a better distribution
of load to the abutment.

Inter Proximal Occlusal Rests ( embrasure rest)
The design of a direct retainer assembly
may require that interproximal occlusal rests
be used.
Interproximal occlusal rest seats are
prepared as individual adjoining occlusal
rest seats.
Preparations must be extended farther
lingually.
The lingual interproximal area requires only
minor preparation.

Adjacent rests rather than a single rest are
used to
- Avoid inter proximal wedging by the
framework.
- Also to shunt the food away from
contact points.
Care must be exercised to avoid
eliminating contact point of abutment
teeth.
Sufficient tooth structure must be
removed to allow for adequate bulk of the
component to be so shaped that occlusion
will not be altered.

EXTENDED OCCLUSAL REST
Indicated - in Kennedy class II, modification 1 and Kennedy class III situations
when the most posterior abutment is a mesially tipped molar
1. Minimize further tipping of the abutment forces are directed down the
long axis of the abutment.
2. This rest should extend more than one-half the mesiodistal width of the
tooth, be approximately one-third the buccolingual width of the tooth.
3. If abutment is severely tilted the extended occlusal rest may be in the form
of an onlay.

LINGUAL RESTS(Cingulum rests)
Anterior teeth may be used to support an indirect
retainer or auxiliary rest.
Canine is preferred over an incisor as an indirect
retainer or an auxiliary rest. Normal morphology
requires minimal tooth preparation.
When a canine is not present, multiple rests that are
spread over several incisor teeth are preferable to the
use of a single incisor.
Lingual rest should be kept near the center of
rotation.

Preferable to an incisal rest
- It is placed nearer the horizontal axis of rotation (tipping axis) of the
abutment , less tendency to tip the tooth.
- More esthetically acceptable.
A lingual rest may sometimes be placed in an enamel seat at the cingulum
or just incisally to the cingulum.
Lingual rest seat preparations in enamel are rarely satisfactory on
mandibular anterior teeth because of a lack of thickness of enamel in
which to prepare a seat of adequate form to be truly supportive.

Outline form
A slightly rounded V(half moon shaped) is prepared on the
lingual surface at the junction of the gingival and the middle
-one third of the tooth. The apex of the V is directed incisally.
The floor of the rest seat should be toward the cingulum
rather than the axial wall. Care must be taken not to create an
enamel undercut, which interferes with placement of the
denture.
prep. Is broadest at lingual aspect..
Dimensions – mesiodistal width = 2.5-3mm,
labiolingual width = 2mm,
incisoapical depth = 1.5mm

Preparation
Preparation may be started by using an inverted
cone-shaped diamond stone and progressing to
smaller, tapered stones with round ends to
complete the preparation.
All line angles must be eliminated, and the rest seat
must be prepared within the enamel and must be
highly polished.
A predetermined path of placement for the
denture must be kept in mind in preparing
the rest seat.

Method of obtaining support
In a cast restoration
Plan and execute a rest seat in the wax pattern
Contour of the framework restores the lingual form
of the tooth.
Cast Co-Cr rest seats attached to lingual surface of
anterior teeth
Composite may be added to lingual surface

Inlay with pins
Crowns / onlays

INCISAL RESTS AND REST SEATS
Less desirable - more unfavorable leverage than lingual rest - orthodontic movement of
the tooth
Incisal rests are placed on prepared rest seats at the incisal angles of anterior teeth
Used predominantly as auxiliary rests or as indirect retainers.
May be used on a canine abutment in either arch, but more commonly on the
mandibular canine.
Provides definite support , relatively little loss of tooth structure and little display of
metal.

Outline form
Small ‘V’- shaped rounded notch is prepared at the incisal
angle of a canine or on the incisal edge of an incisor, with the
deepest portion of the preparation apical to the incisal edge.
Dimensions - 2.5 mm wide and 1.5 mm deep.
The notch is beveled both labially and lingually, and the
lingual enamel is shaped to accommodate the rigid minor
connector connecting the rest to the framework.
The floor of rest seat is extended slightly onto the labial
aspect of tooth.

Incisal rest is placed either at mesioincisal or distoincisal angle.
It can be incorporated into a lingual plate – for additional stabilization.
Multiple incisal rests can be placed for additional support.

Summary and conclusion
Rests and rest seats deserve special consideration in removable
partial denture construction. Proper understanding of the
functions, biomechanical design and placement of the rests is
necessary in partial denture treatment.
Rests play a crucial role in maintaining health of supporting
structures.
The topography of any rest should be such that it restores the
topography of the tooth existing before the rest seat is prepared.

References
A.A. Grant, W. Johnson: Removable denture prosthodontics. 2nd edition.
Albert Seidin. Occlusal rests and rest seats. J Prosthet Dent. 1958; 8:431-
440
Bert T Cecooni. Effect of rest design on transmission of forces to abutment
teeth. J Prosthet Dent. 1974; 32:141-151
Davenport, Basker and Heath :A color atlas of removable partial dentures,
1st
edition, 1980.
Ernest L. Miller, Joseph E. Grasso : Removable denture prosthodontics.
2nd
edition, 1986.

Frank J. Kratochvil. Influence of occlusal rest position and clasp design on movement
of abutment teeth. J Prosthet Dent. 1961; 13:114-121
Kratochvil Partial removable prosthodontics. Ist edition, 1988, W.B.Saunders.
Mc Cracken’s Removable partial denture prosthodontics. 11th edition, 2004.
Stewart, Rudd and Kuebker: Clinical Removable denture prosthodontics. 2nd
edition,
2001.

THANK YOU

Occlusal Rests and Rest Seats Guide

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