Rest and rest seats

Rest and Rest seats
Dr. Kriti Trehan
MDS I

Contents
 Introduction
 Types/ Classification
 Functions
 Requirements
 Occlusal Rest and rest seats
 Lingual rest and rest seat
 Incisal rest and rest seat
 Review of Literature
 References

 Appropriate use of teeth requires consideration as
to how best to engage teeth for the supportive
qualities they provide.
 The most effective resistance can be provided if
the tooth is stressed along its long axis, the
prosthesis framework should engage the tooth in a
manner that encourages axial loading.
 The use of rest with removable partial denture
prostheses has been developed since rests were
first described by Bonwilll in 1899.
Introduction

Rest :
According to GPT 9
 A rigid extension of a removable partial denture
that contacts the occlusal, incisal, cingulum, or
lingual surface of a tooth or restoration, the surface
of which is commonly prepared to receive it.
According to McCracken’s
 The components of a removable partial denture
that transfer forces down the long axes
of the abutment teeth are called rests.

Rest seat
According to GPT 9
 The prepared recess in a tooth or restoration
created to receive the occlusal, incisal, cingulum or
lingual rest.
According to McCracken’s
 The prepared surface of an abutment to receive the
rest.

 According to McCracken, on the basis of tooth
surface prepared to receive them:
a) Occlusal
b) Cingulum
c) Incisal
d) Lingual
Classification

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

1. To provide resistance against occlusal load.
2. To direct forces of mastication parallel to the long
axis of the abutment tooth.
3. To maintain the retentive terminal of the clasp arm
in its intended position.
Functions

3. To prevent the denture base from seating past its
intended position.
4. To give lateral stability against undue horizontal
stresses to the soft tissue of the ridges.
5. To close a small space between teeth bv bridging
that space with occlusal rests.

 To maintain occlusal contact with opposing teeth.
 To prevent extrusion of abutment teeth.
 To help where tipping or rotation of a tooth has
impaired functional occlusion
 To provide indirect retention with auxillary rests.

Basic Consideration
 Forces that are applied to a removable partial
denture must be transferred to the supporting
teeth and tissues in an atraumatic fashion.
 Rest should be sturdily placed in properly
prepared rest seat.
 Rest seat should be within the confines of the
greatest tooth mass-more PDL fibres.

 Should not be placed on inclined tooth surface.
 No sharp angles, should be round in aspects.
 Minimum 1mm thick.
 No undercuts in the path of insertion

Primary rest
 A rest that is part of a retentive clasp assembly is
referred to as a primary rest.
 Prevent vertical movement of a prosthesis toward
the tissues and also help transmit applied forces
to the supporting teeth.
 Should be shallow and saucer shaped, and should
function as ball and-socket joint.

Auxiliary or secondary rests
 A rest that is responsible for additional support or
indirect retention is called an auxiliary rest or
secondary rest.
 Are used as indirect retainers in extension base
removable partial dentures (Class I, Class II, and
long-span Class IV applications).

A rigid extension of a removable dental prosthesis
that contacts the occlusal surface of a tooth or
restoration, the occlusal surface of which may have
been prepared to receive it. –GPT8

Form of the Occlusal Rest and
Rest Seat
Mesiodital diameter of rest.
Buccolingual width
Form

 Angle formed by the occlusal rest and the
vertical minor connector should be less than 90
degrees.

The floor of the occlusal rest seat
Reduction of the marginal ridge of 1.5 mm

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.
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.
o Most ideal way of getting positive support.
o Indicated in - rotated/inclined tooth.
- mandibular bicuspid with rudimentary cusp.
- abraded tooth.
o These are generally made larger and deeper than the
enamel.
o 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.
 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 “tipping” axis of
the abutment.

Long Or Continuous Rests
Splinting periodontally weakened teeth
 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.

Ring Rests
 For clinical situations with a buccally
inclined maxillary molar serving as a distal
terminal abutment for a removable partial
denture (RPD), the use of a ring rest with a
long guiding plane can be considered as a
cost-effective and efficient alternative
design option.

 The proposed rest seat design offers several advantages for
the existing situation:
1. Conservative approach in contrast to additional and
significantly higher reduction for a full-coverage tooth
preparation or extraction.
2. Ring rest seat design maintains the conventional RPD
principles in terms of stress transition of the occlusal loads,
despite the severe inclination of the abutment tooth.
3. Maintenance of already existing and functional, occlusal
contacting surfaces.
4. Tooth preparation lends itself well to prevent
overcontouring of the clasp.

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.

 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.
 This rest should extend more than one-
half the mesiodistal width of the tooth,
be approximately one-third the
buccolingual width of the tooth.
 Should allow for a minimum of 1-mm
thickness of the metal; the preparation
should be rounded with no undercuts or
sharp angles.

A metallic extension of a partial removable dental
prosthesis framework that fits into a prepared
depression within an abutment tooth’s lingual surface.

 Lingual or cingulum rests are used primarily
on maxillary canines.
 The thickness of enamel on the lingual
surface of a mandibular canine rarely allows a
lingual rest to be used.
 Lingual rest provides improved esthetics.

Outline form
 A slightly rounded V 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 proximal view
demonstrates the correct angulation of the
floor of the rest seat (< 90°)

 The mesiodistal length of the preparation should
be a minimum of 2.5 to 3 mm, labiolingual width
about 2 mm, incisal-apical depth a minimum of 1.5
mm.
 Located closer to the rotational center of the
supporting tooth and does not tend to tip the
tooth.

Round Lingual Rest Seat Form
 Mesial of the canine teeth when typical
cingulum rest contraindicated
 Large restoration
 Lack of clearance with the opposing
teeth
 Poor cingulum
 Spoon shaped, similar to occlusal rest
seat.
 More difficult due to the incline of the
lingual surface.

Lingual rest seats on Cast Restorations
 When a crown is to be placed on an anterior
tooth and a rest seat is required, the rest seat
should be placed in the wax pattern.
 The floor of the rest seat is readily carved to be
the most apical portion of the preparation.
 A saddle-like shape, which provides a positive
rest seat located favorably in relation to the long
axis of the tooth, is formed.

 The framework of the denture is made to fill out the
continuity of the lingual surface so that the tongue
contacts a smooth surface without the patient being
conscious of bulk or irregularities.
 The lingual rest may be placed on the lingual surface
of a cast veneer crown a three-quarter crown, an
inlay, a laminate veneer, a composite restoration.

 Incisal rests are placed at the incisal angles of
anterior teeth .
 Less desirable than lingual rests and rest seats for
anterior teeth. Nevertheless, they may be used
successfully if abutments are sound and cast
restorations are not indicated.

 The incisal rest may be used on a canine
abutment in either arch, it is more applicable to
the mandibular canine.
 This type of rest provides definite support with
relatively little loss of tooth structure and little
display of metal..

 The labial view demonstrates
inclination of the floor of the rest
seat, which allows forces to be
directed along the long axis of the
tooth as nearly as possible.
 The proximal edge of the rest
seat is rounded rather than
straight.
 The lingual view shows that all
borders of the rest seat are
rounded to avoid sharp line
angles.

 Should appear as a small , Vshaped notch
located approximately 1.5 to 2.0 mm from
the proximalincisal angle of the tooth.
 The deepest part of the preparation
should be toward the center of the tooth
mesiodistally.
 The notch should be rounded and should
extend slightly onto the facial surface to
provide a positive seat for the rest.

Alternative cingulum rest seat
1. John R. Ivanhoe, describes an alternative
cingulum rest seat design to that of the
conventional “inverted V” described for use with
teeth having a prominent cingulum.
2. Maxillary canines and incisors often lack an
adequate cingulum to allow for a conventional
cingulum rest preparation .

1. Cingulum rests on mandibular canines have often
not been used because of the perception that there
is not adequate thickness of enamel in the rest
region.
2. This has recently been refuted in research by Rossi,’
who found that if the rest seats are prepared on
mandibular canines “in the middle third of the
crown length, then the average depth of preparation
would be well within the confines of the enamel
thickness.”

 ADVANTAGES
1. The shape of the bur used in preparing this rest seat
virtually eliminates any possibility of undercutting the
preparation and minimizes the creation of sharp angles
or lines.
2. The rest seat created is deepest near the center of the
tooth and therefore helps direct the forces from the
removable partial denture along the long axis of the
tooth.
3. Because of its U shape, the rest seat conforms to the
cementoenamel junction of the tooth and may
therefore be placed as far gingivally as possible.

4. The rest seat created is large enough to prevent
gingival movement of the removable partial
denture.
5. Preparation of this rest seat is simple, easy, fast.
and requires a minimum of equipment.

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 many functions.
 Factors that produce ill-fitting rests in removable
partial dentures are:
1. Poor impression and duplicating techniques cause
castings not to fit.
2. Moisture and saliva on prepared abutment teeth will
cause defects in impressions made with hydrocolloid
or rubber base impression materials.

 Mishandling of the master and refractory casts will
cause castings not to fit.. Rubbing and chipping of
casts introduce avoidable inaccuracies.
 Excessive polishing of the occlusal rests on the
,metal framework will cause them not to fit.
 An interfering high spot on the underside of the
rest can cause poor seating.
 In some instances, a rest does not contact its rest
seat at all, even though the remainder of the partial
denture goes into place.

REFERENCES
 Mc Cracken’s Removable partial denture
prosthodontics. 13th edition, Pg No.55-66.
 Stewart, Rudd and Kuebker: Clinical Removable denture
prosthodontics. 4th edition, Pg No.100-14.
 BT Cecconi .Effect of rest design on transmission of
forces to abutment teeth. J Prosthet Dent. 1974
Aug;32(2):141-51.

 Ivanhoe JR. Alternative cingulum rest seat. J
Prosthet Dent. 1985 Sep;54(3):395-6.
 Khalaf Al-Rawi, Salah. Rest and Rest Seat. 2015.
 Alhelal A, Kattadiyil MT , Jekki B. Ring Rest Seat
Design for Severely Tilted Molar Abutment Tooth:
An Alternative Option. J Prosthodont. 2017
Jun;26(4):327-330.

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