Rest seats, guide planes and undercuts

Rest seats provide direct dental support for a partial denture. They need to be large enough to provide support and deep enough for the rest on the partial denture to fit into the cavity or rest seat in the crown (or tooth) without interfering with the occlusion. The size of the rest seats needs to be sufficient to provide support and normally about one-third of the proximal width of the tooth is sufficient. The shape of the cavity should avoid sharp internal line angles and follow the general shape as shown in Figure 16.1. There should also be sufficient occlusal clearance to allow a minor connection between the base of the partial denture and any rest seats and clasps around the same tooth (Figure 16.2).

Figure 16.1 The fit surface of this partial removable cobalt chromium prosthesis shows the general outline of the rests (arrowed) having a rounded character in all dimensions. Note the obturator bottom left which seals a maxillary defect created following surgery to remove a tumour.

Figure 16.2 Sufficient occlusal clearance is required not only for the rest seat, but also the connector between the rest seat (and clasps if present) and base of denture.

Knowing where the rest seat and connectors will ultimately be placed will therefore influence tooth preparation when a crown is planned. Occlusal reduction in the region of the rest seat and connector has to be greater than elsewhere on the occlusal surface as sufficient interocclusal space has to be provided for both the metal substructure to the crown and the thickness of the rest itself (Figure 16.3). Where crowns are concerned it is always important to place rest seats into metal as placement into ceramic will lead to shear fractures (Figure 16.4).

Figure 16.3 During crown preparation, the occlusal reduction in the region where a rest seat is planned has to be greater to allow for the thickness of the rest on the denture and the metal of the crown.

Figure 16.4 Inappropriate placement of rest seats onto ceramic in the metal?ceramic crowns on the lower molar teeth has led to ceramic fracture.

The retention of metal-based partial dentures relies heavily upon clasps. To create the necessary resistance to displacement of the clasps, an undercut, normally on the buccal surface of the crown (tooth), needs to be present. When designing crowns and partial dentures the depth of undercut required for cast cobalt chromium clasps is generally 0.25 mm; to a degree, however, this will depend on the length of the clasp arm (longer clasp arms can engage deeper undercuts). For wrought clasps a deeper undercut can be engaged (0.5 mm) as these have greater inherent flexibility. These undercuts have to be prescribed to the technician so that they can be incorporated into the crown contour.

Guide planes designed on the proximal surfaces of crowns increase the surface area in contact with the partial denture saddle area and increase the resistance to displacement and so retention. The guide planes, when present, need to be parallel to each other to maximize the retention. There is little value in having a single crown with a guide plane without reciprocating action from another crown or tooth.

This section emphasizes the importance of planning the denture design before any tooth preparation takes place. Figures 16.5 and 16.6 illustrate this point clearly.

Figure 16.5 Having a denture design prior to preparation of the lower molar teeth and lower right premolar tooth for crowns has enabled appropriate prescription of rest seats, undercuts and guide planes in the indirect restorations.

Figure 16.6 The crowns for the patient seen in Figure 16.5 cemented (left) and the new cobalt chromium denture (right).

Precision attachments

There are situations where precision attachments are still indicated when fixed and removable prosthodontics are considered together. The routine use of implants has, to some extent, reduced the need for precision attachments on teeth because the cost of the attachment and laboratory work can approach that of implant retained prostheses. Precision attachments linked to fixed laboratory-made prostheses can be used to retain and stabilize removable prostheses. There are a number of designs that can be considered, ranging from extracoronal stud or seeker (ball and socket) attachments to intracoronal movable joints similar to those used in conjunction with fixed–movable bridge designs (see Chapter 19). To illustrate how such precision attachments can be used, consider the following three clinical scenarios.

Patient 1

The patient seen in Figure 16.7 has missing lower posterior teeth with severe resorption of the edentulous ridge, making it difficult to tolerate a lower removable prosthesis due to the lack of retention and stability. To address this, the lower canine teeth have been prepared for metal–ceramic crowns which have distal cantilevered bars and stud attachments incorporated into the metal substructure (Figure 16.8). The lower removable cobalt chromium prosthesis has a corresponding female attachment (socket) embedded into the acrylic of the denture base (Figure 16.9). This ‘ball and socket’ arrangement provides sufficient retention and stability without the need for clasping anterior teeth (Figure 16.8).

Figure 16.7 Patient with lower anterior teeth only. Due to the severe resorption of the lower edentulous ridge the patient is unable to tolerate their existing denture due to poor retention and stability.

Figure 16.8 The patient seen in Figure 16.7 with crowns on the canine teeth that incorporate cingulum rest seats and distal extension bars with a stud attachment. The denture (inset) therefore avoids unsightly clasps in the anterior region.

Figure 16.9 Diagrammatic representation of stud attachment on the distal cantilever arm. The metal housing for the socket joint is embedded into the denture base. The socket joint is provided by plastic colour-coded inserts of varying stiffness and hence retentiveness.

Patient 2

Precision attachments can be used in conjunction with other conventional means of retention for removable prostheses. Consider the patient seen in Figure 16.10; gold crowns have been placed on the molar teeth incorporating rest seats, guide planes and undercuts to achieve support, stability and conventional retention; however, retention is not so easily achieved anteriorly without the presence of unsightly metal clasps. One way to overcome this is to use a precision attachment on the upper left lateral incisor root. This tooth has been root treated and prepared for a cast post and diaphragm onto which is soldered the male component of a Rothermann type precision attachment; the female ‘clip’ attachment is embedded into the fit surface of the denture (Figure 16.11). The male component consists of a disc with a concavity around its periphery; the female clip seats into this concavity on insertion of the denture, giving good retention.

Figure 16.10 Patient for which an upper cobalt chromium prosthesis is planned. Gold crowns have been placed on the two molar teeth incorporating rest seats, guide planes and appropriate undercuts. Additional retention is gained anteriorly with a Rothermann-type precision attachment incorporated into a cast post and diaphragm on the left lateral incisor.

Figure 16.11 Female component of Rothermann-type attachment embedded in the denture base (left), the male component attached to the cast post and diaphragm (bottom right) and diagrammatic representation of how the female component clips into the recess on the sides of the male component (top right).

Patient 3

Figure 16.12 shows the die stone model of a patient who presented with multiple retained roots in the upper arch; the only teeth with natural crowns were the central incisors and the upper left second molar tooth. He was unable to wear a conventional denture due to a marked retch reflex. As for the previous patient the roots were root filled and prepared for cast post, diaphragm and Rothermann type attachments (Figure 16.13). The precision attachments provide excellent retention and allow a horseshoe cobalt chromium denture to be made with minimal palatal coverage.

Figure 16.12 Model of patient whose retained roots have been root treated, and prepared for cast posts and diaphragms onto which precision attachments can be attached.

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