Classification:A- Extracoronal resta. Occlusal rest – embrasure and extended occlusalrests .these are placed on the occlusal surface of a bicuspid or molar teeth. b. Lingual (cingulum) rest - A rest placed on the cingulum of an anterior tooth.c. Incisal rest - A rest placed on an anterior tooth at the incisal edge.d. lingual ball rest.
A rest which is the component of direct retainer is the primary rest Auxillary rest or secondary rest is the additional rest that may be used for indirect retention
B- Intracoronal (precision) rest - A rest consisting of precision manufactured attachments that are placed within the a crown or retainer
SUPPORT -The primary purpose of the rest is to provide vertical support for the partial denture, prevents denture base from moving cervically & impinging gingiva
Slides down incline
Fractured abutment, no rest seat
Maintain a clasp -tooth relationship
Maintains established occlusal relationships by preventing settling of the dentureAct as indirect retainerPrevents rotation (Class I or II RPD’s only)
Preparation of Rest SeatsLight pressure High-speed handpiece +/- Waterspray Minimal heat is generated
Rest Seat Form(Conventional form)
Smooth flowing outline form (i.e. no sharp line angles)Rest not flared to facial line angle Lingual flared more - space for minor connector
Occlusal Rest Seat
The base of the triangular shape is at the marginal ridge and should be approximately one third the bucco-lingual width of the tooth . It should be as long as it is wide, and the base of the triangular shape (at the marginal ridge) should be at least 2.5 mm for both molars and premolars.Occlusal Rest Seat
The marginal ridge must be lowered and rounded to permit a sufficient bulk of metal to prevent fracture of the rest from the minor connector (1 to 1.5 mm)Occlusal Rest Seat
“Positive” Rest Seats An explorer tip will not slide off the rest seatDouble Embrasure or inter proximal Rest Seat
These rest seats are prepared as individual occlusal rest seats, with the exception that the preparations must be extended farther lingually than a single rest, Provides space for the retentive & bracing arms adjacent rests are used to prevent interproximal wedging by the framework. Additionally the joined rests are designed to shunt food away from contact point. In preparing such rest seats, care must be exercised to avoid reducing or eliminating contact points of abutment teeth, However, sufficient tooth structure must be removed to allow for adequate bulk of the component for strength and to permit the component to be so shaped that occlusion will not be altered.In Kennedy Class II, modification 1, and Kennedy Class III situations in which the most posterior abutment is a mesially tipped molar, an extended occlusal rest should be designed and prepared to minimize further tipping of the abutment and to ensure that the forces are directed down the long axis of the 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, and allow for a minimum of 1-mm thickness of the metal, and the preparation should be rounded with no undercut or sharp angles .
EXTENDED OCCLUSAL REST
Cingulum or Lingual Rest Seats
Indicated on lingual surface of canines or incisors . Lingual or cingulum rests on anterior teeth are often utilized when no posterior teeth are present or when indirect retention is necessary. Assessment of incisal and lingual contact areas where rests are to be placed. Sufficient space must be present or created to avoid interference with placement of rests.The canine is much preferred over an incisor due to its well-developed cingulum.. Root form, root length, inclination of the tooth, and ratio of the length of the clinical crown to the alveolar support must be considered in determining the site and form of rests placed on incisors. A lingual rest is preferable to an incisal rest, because it is placed nearer the horizontal axis of rotation (tipping axis) of the abutment and therefore will have less tendency to tip the tooth (mechanically). In addition ,lingual rests are more esthetically acceptable than are incisal rests.
Cingulum Rest Seat Form
The rest seat, from the lingual aspect is a broad inverted "V' ,often seen in the canine cingulum at the junction of the gingival and the middle one third of the tooth. The apex of the V is directed incisally. From the incisal view the rest seat is broadest at the central aspect of the canine (approximately( 1mm)..
The borders of the rest seat are slightly rounded to avoid sharp line angles in its preparation.
‘positive’ if an explorer tip does not slip off the rest seat when pulled lingually from the base of the rest seat..
Preparation Too High(unnecessary tooth reduction
If the preparation is started too high above the cingulum proper, much of the lingual surface of the tooth above the cingulum will need to be reduced, in order to obtain sufficient width for support. On maxillary anteriors, this may also cause the rest to interfere with the opposing tooth .
Preparation Too Low (excessive tooth reduction)
much of the cingulum will need to be reduced, in order to obtain sufficient width for support. Enamel is thinner in this area, and preparation could result in dentinal exposure, resulting in sensitivity. If correction of the outline form or depth is required, there will be little tooth structure remaining to make such changes.Maxillary Cingulum Rest Seats
. When a deep vertical overlap exists, care must be taken to ensure that the mandibular tooth does not prematurely contact the area of the planned metal framework.Maxillary Cingulum Rest Seats
The interocclusal relationship of a maxillary tooth with the incisal edge of the opposing mandibular tooth is manditory. . Mounted diagnostic casts should be used to assess this relationship, by drawing a line on the lingual surface of the maxillary abutment, where the mandibular tooth touches when the models are in contact with each other. The cingulum rest seat preparation should be 1.5-2.0 mm below this line to allow for adequate framework strength.Round Lingual Rest Seat
Rounded rest, prepared on mesial of the canine teeth when typical cingulum rest is contraindicated A-Large restoration B-Lack of clearance with the opposing teeth C-Poorly developed cingulumSimilar to an occlusal rest seat, with reduction of the mesial marginal ridge. However, preparation is more difficult due to the incline of the lingual surface of the canine and more tooth structure must often be removed.