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DR. Luma Nassrat Assistant Lecturer, Department of Prosthodontics

College of Dentistry Tikrit University

Rests and Rest Seats:

Forces that are applied to a removable partial denture must be transferred to the supporting teeth and tissues in

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RPD LEC.8 | PROSTHODONTICS | 31-12-2018

Rests and Rest Seats3RD GRADE

LEC. 9

an atraumatic fashion.

The components of a removable partial denture that transfer forces down the long axes of the abutment teeth are called rests.

The prepared surfaces of the teeth into which rests fit are called rest seats.

The relationship between a rest and a rest seat must be such that forces transmitted from the prosthesis to an abutment are directed apically down the long axis of the tooth. In this manner, stress can be absorbed by the fibers of the periodontal ligament without damaging the ligament or the supporting bone.

In the case of a tooth-supported removable partial denture, all the stresses are transferred to the abutment teeth. In a tooth-tissue-supported removable partial denture, only a portion of the stresses are transferred to the teeth, while the edentulous ridge must absorb the remainder of the load.

In addition to transferring forces, each rest should serve as a vertical stop for the prosthesis. Firm, positive contact between a rest and rest seat minimizes vertical displacement of the prosthesis and prevents injury to the soft tissues.

A rest also must maintain the retentive clasp in its proper position. If the clasp is not supported, it will lose its ability to retain the prosthesis in its intended position.

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A rest that is part of a retentive clasp assembly is referred to as a primary rest. A rest that is responsible for additional support or indirect retention is called an auxiliary rest or secondary rest.

Primary rests prevent vertical movement of a prosthesis toward the tissues and also help transmit applied forces to the supporting teeth. The transmission of

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lateral forces may be increased by deepening the accompanying rest seats, but this should be done only for entirely tooth supported prostheses. In all other applications, rest seats should be shallow saucer shaped and should function as ball-and-socket joints. This geometry permits dissipation of potentially harmful lateral forces.

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Auxiliary or secondary rests are used as indirect retainers in extension base removable partial dentures (Class I, Class II, and long-span Class IV applications). These rests are placed anterior or posterior to the axis of rotation to prevent the extension bases from lifting away from the underlying ridges.

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Primary rest secondary rest

Types of rests are:

1. Occlusal rests—so named because they are seated on the occlusal surfaces of posterior teeth.

2. Lingual or cingulum rests—those seated on the lingual surfaces of anterior teeth, usually maxillary canines.

3. Incisal rests—those seated on the incisal edges of anterior teeth.

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Occlusal rests and rest seats It is essential that a rest seat be prepared in the occlusal surface of each

tooth that is to receive an occlusal rest. A rest should never be placed on a tooth

that has not been adequately prepared.

The outline form of an occlusal rest seat should be roughly triangular, with

the base of the triangle located at the marginal ridge and the rounded apex directed

toward the center of the tooth. If there is one descriptive phrase that can be

applied to occlusal rest seats, it is “smooth gentle curves.” Sharp angles, walls, and

ledges must be avoided. Any portion of the rest seat that restricts movement of the

rest may transmit undesirable horizontal forces to the tooth.

! The shape of the rest seat should follow the outline of the mesial or distal

fossa for the chosen abutment. The rest seat should occupy one-third to one-half

the mesiodistal diameter of the tooth and approximately one half the buccolingual

width of the tooth measured from cusp tip to cusp tip.

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The floor of the occlusal rest seat must be inclined slightly toward the center

of the tooth. The enclosed angle formed by a line dropped down the proximal

surface of the tooth parallel to the long axis of the tooth and the floor of the rest

seat must be less than 90 degrees so that the transmitted occlusal forces can be

directed along the vertical axis of the tooth. An angle greater than 90 degrees will

not yield the desired axial loading and will produce an inclined plane effect. This

inclined plane effect can produce slippage of the prosthesis away from the

abutment teeth. In addition, it can cause orthodontic movement of abutment teeth,

with concurrent pain and bone loss.

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When viewed in cross section, the deepest part of an occlusal rest seat should

be located near the center of the mesial or distal fossa. From its depth, the floor of

the rest seat should rise gently toward the marginal ridge. The most common

mistake in occlusal rest seat preparation is insufficient reduction of the marginal

ridge. This leads to construction of a rest that is extremely thin and subject to

fracture. An occlusal rest must be at least 0.5 mm thick at its thinnest point and

should be between 1.0 and 1.5 mm thick where it crosses the marginal ridge.

Occlusal rests on amalgam restorations:

Placing occlusal rests on large amalgam restorations is hazardous at best. The

primary reason for attempting this is economics, since an amalgam restoration costs

less than a complete coverage restoration (ie, crown).

✓ The loads from the rest increases the amalgam’s tendency to flow leading to

failure of restoration.

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Occlusal rests on cast restorations

When a cast restoration is planned for an abutment tooth, the wax pattern

should display ideal contours. An appropriate rest seat should be carved into the

wax pattern. Upon completion of the casting process, restoration contours should

be refined in preparation for delivery.

Lingual or cingulum rests and rest seats

Lingual or cingulum rests are used primarily on maxillary canines. The normal

morphology of a maxillary canine permits preparation of a satisfactory rest seat with

minimal tooth reduction. The thickness of enamel on the lingual surface of a

mandibular canine rarely allows a lingual rest to be used. Lingual rests on incisors

are also rare. The main indication for their use is missing canines. In this instance,

multiple incisors should receive lingual rests to distribute the stresses over a

number of teeth because a single incisor seldom offers adequate support.

Although a lingual rest may be used successfully, an occlusal rest in the mesial

fossa of the first premolar is preferred if the occlusion permits. An occlusal rest is

located in a more favorable orientation (nearly horizontal), and the accompanying

rest seat is easier to prepare.

However, a lingual rest is preferred to an incisal rest. A lingual rest is located

closer to the rotational center of the supporting tooth and does not tend to tip the

tooth. The longer lever arm associated with an incisal rest magnifies the rotational

movement of the prosthesis and may result in significant tooth movement. Since a

lingual rest is confined to the lingual surface of an anterior tooth, it permits

improved esthetics. Because of the configuration of the lingual rest, it is also less

subject to breakage and distortion.

The form of a lingual rest seat should be V shaped when viewed in cross

section. The preparation should display a relatively upright lingual wall that

originates at the level of the cingulum and extends incisally. The remaining wall

should originate at the crest of the cingulum and incline labiogingivally toward the

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center of the tooth. This wall must provide a definite seat for the rest and must

prevent the migration of the abutment away from the framework.

When viewed from the lingual aspect, the outline form of a lingual rest seat

should be crescent shaped. This geometry provides some freedom of movement in

a mesiodistal direction. Hence, the rest and rest seat may function as a ball-and-

socket assembly. Sharp line angles and corners must be avoided because they

complicate the fit of the cast framework and may serve as stress concentrators.

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Cigulum rest in a cast crown

Incisal rests and rest seats

Incisal rests and rest seats are 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. If a cast restoration is planned for

an anterior abutment, an incisal rest is never indicated. A lingual rest should be

incorporated into the restoration instead. Incisal rests are most frequently used on

mandibular canines but may be used on maxillary canines. They are not indicated

on incisors except under unusual circumstances. If stabilization of incisors is

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indicated and placement of a fixed restoration is not warranted, incisal rests may be

incorporated into a lingual plate to support these teeth.

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Incisal rests should be positioned near the incisal angles of abutment teeth.

Whether they are designed for mesioincisal or distoincisal angles depends on the

prescribed clasping system. If an individual tooth is not to be clasped, yet requires

an incisal rest, the rest should be placed where it will exert the least impact upon

esthetics. In most instances, this requires that the rest be placed at the

distoincisal surface of the abutment.

An incisal rest seat should appear as a small, V-shaped notch located

approximately 1.5 to 2.0 mm from the proximal-incisal 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. The enamel on the lingual

surface should be prepared as a shallow depression to accommodate the minor

connector and avoid annoying the tongue. Because of the greater length of this

minor connector, care must be taken to ensure its rigidity.

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Summary of Functions of the Rests:

1. Support (the main function): Resist displacement towards denture

supporting structures preventing impingement of gingival tissues adjacent to

the abutment.

2. Maintains components in their planned positions: Maintain the

components of the dentures in their positions and prevents spreading of clasp

arm.

3. Distributing the Occlusal Load: When forces are exerted on t partial

denture it will be transmitted to teeth and clasps through the rests.

4. May provide Indirect Retention: A rest can act as an indirect retainer

preventing the rotation of distal extension around the fulcrum axis.

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5. Direct food away from tooth contact and embrasure area: When

teeth are extracted and drifted apart food will accumulate and be trapped in

embrasure areas, but the presence of a rest closing this space will prevent

food entrapment and maintain the health of the supporting structures. 6. Improve the existing occlusion: Maintaining the relationship between

natural and artificial teeth.

7. Prevent extrusion of abutments.

8. Provide reference for relines or impressions. Ex: Altered cast technique

(3-finger technique)

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9. Direct forces along long axis of abutment. 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.

REFERENCES: Stewart's Clinical Removable Partial Prosthodontics. Ch.2

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