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 PG. 0 RPD LEC.8 | PROSTHODONTICS | 31-12-2018 Rests and Rest Seats 3RD GRADE LEC. 9
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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
PG. !0
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
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.