Rest and Rest Seat

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Rest and Rest Seat:

Rest and Rest Seat

PowerPoint Presentation:

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, and a rest seat is the prepared surface of a tooth or fixed restoration into which a rest fits.

PowerPoint Presentation:

The rest that is a component part of a direct retainer unit is referred to as a primary rest, whereas additional rests that may be used for indirect retention or extra support are called auxiliary, or secondary rests. Auxiliary, or secondary, rests are used as indirect retainers in distal extension partial dentures.

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There are three forms of rests: The occlusal rest, so named because it is seated on the occlusal surface of a posterior tooth; The lingual, or cingulum rest, seated on the lingual surface of a tooth, usually a maxillary canine; The incisal rest, seated on the incisal edge of a tooth, usually a mandibular canine

Functions of a occlusal rest:

Functions of a occlusal rest Occlusal rest is a rigid extension of a removable partial denture that contacts the occlusal surface of a tooth or restoration, the occlusal surface of which may have been prepared to receive it.

PowerPoint Presentation:

Transmit stress along the long axis of the tooth. Maintains components in their planned positions Act as a vertical stop, preventing injury and over displacement of the soft tissues Assist in distributing the occlusal load among two teeth or more so that each can bear a proportionate share of the masticatory load in concert with the residual ridges.

PowerPoint Presentation:

Prevent extrusion of the abutment tooth. Prevent the ingress of food between the abutment tooth and the clasp, by deflecting it away from the immediate area. Provide resistance to lateral displacement of the prosthesis. Contribute indirect retention (in some cases). Improvement of occlusion Act as the reciprocal clasp arm.

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Force on a partial denture is transmitted to occlusal rest, and because of its design, further transmitted down long axis of abutment tooth. .

SUPPORT FOR RESTS :

SUPPORT FOR RESTS

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The nearest thing to an ideal surface on which to place an occlusal rest is gold alloy Natural tooth enamel is next in order of suitability followed by fused porcelain and silver amalgam in that order A rest should never be placed on a silicate cement restoration, nor on one of acrylic resin

PowerPoint Presentation:

Rest seat preparations should be made in sound enamel In most instances, preparation of proximal tooth surfaces is necessary to provide proximal guiding planes and to eliminate undesirable undercuts The preparation of occlusal rest seats always must follow proximal preparation, never precede it.

PowerPoint Presentation:

When proximal preparation follows occlusal rest seat preparation, the inevitable consequence is that the marginal ridge is too low and too sharp, with the center of the floor of the rest seat too close to the marginal ridge. Often impossible to correct the rest preparation without making it too deep - irreparable damage to the tooth.

The dimension of the occlusal rest:

The dimension of the occlusal rest The actual thickness of the rest depends on the alloy used. Since chrome cobalt has a higher modulus of elasticity than gold, occlusal rests made of the former may be thinner. Less deep rest seat preparation.

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Mesiodistally- extend upto the centre of mesiodistal fissure The width of the rest where it covers the marginal ridge should be as great as possible, so that the line of action of the occlusal force on the saddle lies within, or at the most only slightly outside, this dimension. Such a condition ensures stability of the denture and is particularly important in the case of a unilateral appliance.

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The outline form of an occlusal rest should be triangular, with the base of the triangle resting on the marginal ridge and the rounded apex directed toward the center of the tooth.

PowerPoint Presentation:

The shape of the rest should follow as closely as possible the outline of the mesial or distal fossa of the occlusal surface of the tooth in which the rest seat is prepared. The size of the rest varies from one-third to one-half the mesiodistal diameter and ap­proximately half the buccolingual width of the tooth measured from cusp tip to cusp tip.

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LINGUAL OR CINGULUM RESTS The lingual or cingulum rest is used primarily on maxillary canines. The main indication for their use is missing canines.

PowerPoint Presentation:

The lingual rest is nearer the center of rotation of the supporting tooth, so it does not tend to tip the tooth. Lingual rest is confined to the lingual surface of the anterior tooth, it is more acceptable esthetically. Because of the configuration of the lingual rest, it is also less subject to breakage and distortion.

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INCISAL REST AND REST SEATS Incisal rests are less desirable for anterior teeth than lingual rests. They may be used successfully , however ,if the abutment tooth is sound and a cast restoration is not indicated.

PowerPoint Presentation:

Most frequently used on mandibular canines but may be used on maxillary canines. Not generally indicated on incisor teeth. Incisal rests are positioned near the incisal angles of abutment teeth. Whether they are designed for the mesioincisal or distoincisal angle depends on the type of clasp planned for that tooth. If the tooth is not to be clasped, the rest is placed on the distoincisal surface for esthetic reasons.

Rests:

Rests Tooth Borne ( Cl III & IV) Immediately adjacent edent space Tooth & Tissue Borne ( Cl I & II) Mesial prefered Distal if: Tooth rotated Mesial centic stop or heavy occlusion Large mesial restoration

Rests:

Rests Ensure sufficient rest seat depth (1.5mm) Avoid incisal rests Esthetic Increased tilting Use bonded cingulum rests, if no prominent cingulum

Avoid Rests on Restorations:

Avoid Rests on Restorations Avoid when possible (not always) Amalgam High creep, low yield strength ensure adequate remaining bulk, adequate restoration If restoration fails, restoration will be more complex, expensive

Avoid Rests in Areas of Heavy Occlusal Contact:

Avoid Rests in Areas of Heavy Occlusal Contact Extruded, tilted teeth Check for clearance for maxillary cingulum rests ( intraorally , and on cast)

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