Rest and Rest Seat in Removable partial denture

REST AND REST
SEAT IN
REMOVABLE
PARTIAL
DENTURE

• INTRODUCTION
• HISTORICAL REVIEW
• TYPES / CLASSIFICATION
• FUNCTIONS
• REQUIREMENTS
• BASIC CONSIDERATIONS
• OCCLUSAL RESTS AND REST SEATS
• LINGUAL REST AND REST SEATS
• INCISAL REST AND REST SEATS
• CONCLUSION
• REFERENCES
CONTENTS

• control the position of the prosthesis in relationship to
the teeth and supporting structures
• Provide vertical support for the partial denture by its
position on the abutment teeth.
• controls the direction of functional forces so that they
are in the long axis of the tooth, at right angles to the
correct occlusal plane
INTRODUCTION

• it must be positive and not allow the prosthesis to
slide off the tooth or allow the tooth to move out of
existing relationship to other teeth as increased
occlusal pressure is exerted.
• Rests preserve the remaining oral structures by
 Controlling the position of the prosthesis in relation to
the teeth, periodontium and mucosa. and
 Controlling the amount and direction of movement of
the abutment teeth.

• Rest should always be located on tooth surfaces
properly prepared to receive them.
• The rests must be rigid and must receive positive
support from the abutment teeth.

Rest :
A projection or attachment, usually on the side of an object –
(GPT 8).
The components of a removable partial denture that serve
primarily to transfer forces occurring against the prosthesis
down the long axis of the abutment teeth are called rests.
(- Stewart)
DEFINITION

• Any unit of a partial denture that rests on a tooth
surface to provide vertical support is called rest.
(McCracken’s)
• A rigid extension of an RPD, usually from a minor
connector that contacts a remaining tooth (or)
teeth to dissipate vertical or horizontal forces.
(Renner / Boucher)

Rest seat :
• The prepared recess in a tooth or restoration created
to receive the occlusal, incisal, cingulum or lingual
rests. (GPT 8)
• The prepared surface of a tooth or fixed restoration
into which a rest fits. (Stewart)
• The prepared surface of an abutment to receive the
rest is called the rest seat. (McCracken’s)

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. (depending on type of
material )
 Restores the occlusal plane.
 Provides reciprocation and stabilization (in lone
standing teeth).
 Contoured so that when increased force is applied to
the prosthesis the rest will engage more securely to
prevent separation.

• The primary purpose of the rest is to provide vertical
support for the partial denture and thus resist the
movement in a cervical direction . In doing so they
also serve the following functions:
 maintains components in their planned positions
 maintains established occlusal relationships by
preventing settling of the denture
FUNCTIONS

Slides down incline
Fractured abutment, no rest seat

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

• Maintain a clasp -tooth relationship
FUNCTIONS

• Provide reciprocation and stabilization.
• Provide rigid prosthetic support
• Prevent supra eruption.
• Restore occlusion
FUNCTIONS
• Act as indirect retainer-Prevents rotation (Class I
or II RPD’s only)

• Prevents impingement of soft tissues.
FUNCTIONS

• Deflection of food by bridging the gap between two
teeth
• Restores anatomy as required

• Provides positive seat by
extending over the incisal edge
• Splints mobile teeth.

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

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

FORM OF OCCLUSAL REST
AND REST SEAT
• Rounded triangle shape with apex
towards centre of occlusal surface
• Base resting on marginal ridge
• Deepest part of occlusal rest – inside
• lowered marginal ridge .
• Provides bulk & no occlusal
interference

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.
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.

Angle formed b/w occlusal rest & vertical minor
connector should be less than 900
.
Occlusal forces directed along long axis of
abutment tooth.

Greater than 900
– slippage of prosthesis away
from abutment – orthodontic forces to inclined
plane on abutment – tooth movement

“Positive” Rest Seats
 Explorer will not slide off rest seat

Rest seats not prepared opposing functional cusps.
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.
In tooth borne cases rest must be extended to the
center of the tooth.

Preparation of occlusal rest - Enamel
preparation of proximal tooth surfaces first - provide
proximal guiding planes & eliminate undesirable
undercuts.
When proximal preparation follows occlusal rest seat
preparation - marginal ridge is too low and too sharp -
is impossible to correct the rest preparation without
making it too deep - irreparable.
prepared with diamond points of approximately the
size of no.6 and 8 round burs or with carbide burs.

larger of the two diamonds is used first to lower
the marginal ridge and to establish the outline
form of the rest seat.
The smaller diamond point - deepens the floor of
the occlusal rest seat & forming the desired spoon
shape inside the lowered marginal ridge.

Avoid Undercuts
• Must draw along path of insertion

Occlusal rest seat prep. in existing
restoration.
same as in enamel. Proximal preparation first and
then rest seat should be placed.
an existing restoration may be perforated in the
process of making an ideal occlusal rest seat – filled
with gold foil.

Occlusal rest seats in new restoration.
They should be placed in the wax pattern.
location of the occlusal rest should be known 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.
made larger and deeper than that in sound enamel.
tooth borne dentures slightly deeper than a distal
extension base.

EXTENDED OCCLUSAL REST
 Indicated in Kennedy’s Class II, Mod I & Kennedy’s
Class III – most posterior abutment is mesially
tipped molar.
 Minimizes further tipping and directs forces along
long axis of abutment.

This rest should extend more than one-half the
mesiodistal width of the tooth, be approximately
one-third the buccolingual width of the tooth.
If abutment is severely tilted the extended occlusal
rest may be in the form of an onlay.

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.
Not to eliminate contact point of abutment teeth.
Sufficient tooth structure must be removed to allow for
adequate bulk of the component.

Internal Occlusal rests
(Intracoronal Rests)
A totally tooth supported partial denture supported
by means of cast retainer on all teeth – occlusal
support and horizontal stabilization.
Is not a retainer & should not be confused with an
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.

Advantages
Facilitates the elimination of visible clasp arm
buccally
Permits the location of the rest seat in a more
favorable position in relation to the “tipping” axis
of the abutment.
Retention provided by lingual clasp arm lying in a
natural or prepared infrabulge area on the
abutment tooth.

SUPPORT FOR RESTS :
 Rests may be placed on sound enamel, cast restorations, or silver
amalgam alloy restorations.
 The use of silver amalgam alloy restoration - least desirable
because of its tendency to flow under pressure and also because
of the comparative weakness of a marginal ridge made of this
alloy.
• Rests placed on sound enamel are not conducive to caries in a
mouth with a low caries index.
• There is always a possibility that an existing restoration may be
perforated in the process of preparing an ideal occlusal rest.
Although some compromise is permissible, the effectiveness of
the occlusal rest seat should not be jeopardized for fear of
perforating an existing inlay or crown.

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

Control position of
unopposed teeth
o when 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
o 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.

o Ring rests are usually best for isolated teeth
provided there is sufficient area of tooth above the
survey line to accommodate them.
o An additional advantage of ring rests is that they
provide a better distribution of load to the
abutment.
A Roy MacGregor/David M Watt: Designing Partial Dentures;Wright Co. P-209

DEFINITIONS
Lingual rest : according to GPT 9
- A metallic extension of a removable partial denture
framework that fits into a prepared depression within
an abutment tooth’s lingual surface.
Lingual rest seat
- The depression prepared on the lingual surface of an
abutment tooth to accept the metal rest of a partial
denture

LINGUAL RESTS (Cingulum rests)
 used as an indirect retainer or auxiliary
rest.
 Canine is preferred over an incisor -
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.

Preparation I
 Slightly rounded V on lingual surface at junction of
gingival & middle 1/3 of tooth – Apex of V directed
incisally.
 all line angles to be eliminated – must be prepared in
enamel – should be highly polished
 floor towards cingulum and no enamel undercut

Preparation II
Most satisfactory lingual rest (support) – on prepared
rest seat in cast restoration.
First it has to be planned in a wax pattern.
The contour of the framework can then restore the
lingual form of tooth.

 Lingual rest can be placed on lingual surface of cast veneer
crown/ a three quarter crown/ an inlay/ a laminate veneer/
a composite restoration or an etched metal restoration.
 an etched metal restoration is more conservative than a
three quarter crown and displays less metal.
 Ball type of rest can be given in conservative restorations.

Correct Preparation
2.5- 3 mm
1.5 mm
2mm

Preparation of Rest Seats
· Cingulum Rest Seats &
Guiding Planes
- Long, medium diameter
cylindrical diamond
- Inverted cone bur
8837K-014

Cingulum Rest Preparation
• Do not create an enamel undercut
• Cylindrical bur along the long axis of the tooth

Definition :GPT9
A rigid extension of an RPD that contacts a tooth at the
incisal edge.

The ideal incisal rest fulfills the following
requirements
1) It provides a positive seat by extending over the
incisal edge onto the labial surface of the tooth.
2) It restores anterior anatomy as required.
3) It stabilizes mobile teeth.
 Incisal rests are used predominantly as auxiliary
rests or as indirect retainers.

 Incisal rest is used primarily on mandibular canines
and in some instances, on the centrals and laterals.
 The incisal rest is rarely used on maxillary teeth
because of esthetic considerations and occlusal
interference with the mandibular anterior teeth.

This rest provides definite support with relatively little
loss of tooth structure and little display of metal.
An incisal rest is more likely to lead to some
orthodontic movement of the tooth because of
unfavourable leverage factors.

Preparation :
prepared in the from of a rounded notch at an incisal
angle (or) on an incisal edge, with the deepest
portion of the preparation apical to the incisal edge.
The notch should be beveled both labially and
lingually
2.5 mm wide and 1.5mm deep

Trouble shooting ill-fitting rests
• Occlusal rests must fit their respective rest seats
on the abutment teeth with precision if they are to
perform their functions.
• Poor impression, duplicating technique causes
castings not to fit.
• The under site of the rest seat is sandblasted to
give it a stain finish.
• Carbon is deposited on under side of the rest.

• In some instance rest does not contact its rest seat at
all.
• The rest may be refitted by burnishing a small piece
of 0.001 platinum foil to the rest seat in the tooth.
• The denture is then seated and the platinum foil is
sealed to the rest.
• Denture is invested burned out and solder is
deposited in its place.

COMPLICATIONS
• Dentinal exposure leading to hypersensitivity and
caries.
• If not planned properly it can lead to damage to
periodontium.
• In individuals with high caries index it can lead to
caries.
• If rest seat is not of adequate depth it can lead to
fracture of rest.

 Cecconi B.T. 1974 :
Conducted a laboratory study to determine the effects of
several types of partial denture rests on abutment tooth
movement.
And found that depth of the rest is a more significant factor, in
force transmission to abutment tooth, than the type of rests.
Also said that bilateral loading of a RPD causes significantly less
abutment tooth movement than does unilateral loading
.
 Richard W. Toth. Et al 1986 :
Conducted a study to determine the shear strength of lingual
rest seats prepared in bonded composite.
The average load at failure of lingual composite rests was 21-
95kg. The average shear strength was 2.37kg/mm2.
.
REVIEW OF LITERATURE

 Bradley G. Seto et al 1987 :
Compared the shear bond strength of custom-fabricated,
etched-metal, resin bonded cingulum rest seats with and
without horizontal cingulum grooves prepared on the surface
of the cingulum. The debonding loads were significantly higher
for the cingulum rest seats with horizontal grooves
 Byron P. Sansom et al 1987 :
Conducted a study on rest seat designs for inclined posterior
abutments of mandibular and found that Rest seat
placement on an abutment tooth primarily affects stress
concentration of that tooth. Inclined abutments demonstrate
greater resultant bending of interradicular bone than vertical
abutments, and the greater the inclination the greater the
bending. Mesial rests on inclined molar abutments
demonstrated maximum tipping forces. Channel rests on
inclined molar abutments demonstrate the least amount of
tipping force

 Cynthia G. Gapido et al 2003 :
Evaluated the fatigue resistance of cast occlusal rests using Co-Cr
and Ag-Pd-Cu-Au alloys of 0.8mm thick. Found that Co-Cr occlusal
rest is more rigid and resistant to fatigue than Ag-Pd-Cu-Au
occlusal rest. The use of 0.8mm thick and 2.0mm wide occlusal
rest may be adequate if fabricated with Co-Cr alloy.
Conclusion :
To ensure a successful prosthesis the principles governing the
design preparation and placement of rest and rest seats must
be followed.

References :
• Removable partial prosthodontics -Mc Crakens –
10th edition
• Clinical removable partial prosthodontics - Stewart
3rd edition
• Removable partial prosthodontics - Ernest. L. Miller
– 2nd edition
• Partial removable prosthodontics -F. James
Kratochvil
• JPD – 1974, 32 – 141 – 151
• JPD – 1986, 56 – 99 – 103.
• JPD – 1987, 58 – 458 – 462.
• JPD – 1987, 58 – 57 – 62.
• JPD – 2003, 90 – 261 – 269.

Rest and Rest Seat in Removable partial denture

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