CHAPTER 6 Rests and Rest Seats
The capacity for teeth to resist functional forces and remain stable over time is provided through their sophisticated support mechanisms. Studies have shown that displacement and recovery following loading are far better for natural teeth than for oral mucosa. Consequently, appropriate use of the teeth to help resist functional forces in removable prostheses is a critical strategy to control prosthesis movement and achieve functional stability.
Appropriate use of teeth requires consideration as to how best to engage teeth for the supportive qualities they provide. Because 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. The various forms of rests have as a main goal a form that allows for axial loading. It is important to realize that this goal can be achieved only through some form of tooth modification.
Vertical support must be provided for a removable partial denture. Any component of a partial denture on a tooth surface that provides vertical support is called a rest (Figure 6-1). Rests should always be located on properly prepared tooth surfaces. The prepared surface of an abutment to receive the rest is called the rest seat. Rests are designated by the surface of the tooth prepared to receive them (occlusal rest, lingual rest, and incisal rest). The topography of any rest should restore the topography of the tooth that existed before the rest seat was prepared.
Figure 6-1 A, Occlusal rest seats have been prepared on molar and premolar teeth to provide vertical support for the removable partial denture. B, Framework for tooth-supported removable partial denture. Rests on patient’s right provide vertical support to the replacement dentition; rests on patient’s left provide cross-arch support and stabilization. C, Tooth support for this prosthesis is provided by rests that occupy definite, prepared, rest seats on the canine and occlusal surfaces of selected posterior teeth. D, Kennedy Class III, modification 1 maxillary arch with rest seats prepared on the lingual surfaces of the canine and lateral incisor, and on the occlusal surfaces of the premolar and molar.
Thus rests serve to support the position of a partial denture and to resist movement toward the tissue. They serve to transmit vertical forces to the abutment teeth and to direct those forces along the long axes of the teeth. In this respect, tooth-supported removable partial denture rests function in a manner similar to fixed abutment retainers. It is obvious that for this degree of stability to exist, the rests must be rigid and must receive positive support from the abutment teeth, which means that under occlusal loading, the rest and the tooth remain in stable contact and no independent movement or slippage occurs.
In a removable partial denture that has one or more distal extension bases, the denture becomes increasingly tissue supported as the distance from the abutment increases. Closer to the abutment, however, more of the occlusal load is transmitted to the abutment tooth by means of the rest. The load is thereby distributed between the abutment and the supporting residual ridge tissue.
When rests prevent movement of the denture in an apical direction, the position of the retentive portion of the clasp arms can be maintained in designated relation to the tooth undercuts. Although passive when it is in its terminal position, the retentive portion of the clasp arm should remain in contact with the tooth, ready to resist a vertical dislodging force. Then, when a dislodging force is applied, the clasp arm should immediately become actively engaged to resist vertical displacement. If settling of the denture results in clasp arms that stand away from the tooth, some vertical displacement is possible before the retainer can become functional. The rest serves to prevent such settling and thereby helps to maintain the vertical stability of the partial denture.
The use of an implant as a rest can also be considered. In this application, the implant eliminates compression of supporting soft tissues, controls vertical movement of the denture base, eliminates or alters fulcrum lines, and serves to increase support and stability of the prosthesis.
Figure 6-2 The deepest part of an occlusal rest preparation should be inside the lowered marginal ridge at X. The marginal ridge is lowered to provide bulk and to accommodate the origin of the occlusal rest with the least occlusal interference.
Figure 6-4 The occlusal rest should be spoon shaped and slightly inclined apically from the marginal ridge. The rest should restore the occlusal morphology of the tooth that existed before preparation of the rest seat.
Figure 6-5 The floor of the occlusal rest seat should be inclined apically from the lowered marginal ridge. Any angle less than 90 degrees is acceptable as long as the preparation of the proximal surface and lowering and rounding of the marginal ridge precede completion of the rest seat itself.
Figure 6-6 The result of force applied to an inclined plane when the floor of the occlusal rest preparation inclines apically toward the marginal ridge of the abutment tooth. F, Occlusal force applied to the abutment tooth. AB, Relationship of the occlusal rest to the abutment tooth when the angle is greater than 90 degrees. ABC, Removable partial denture framework. ABD, Abutment tooth.
When an existing occlusal rest preparation is inclined apically toward the reduced marginal ridge and cannot be modified or deepened because of fear of perforation of the enamel or restoration, then a secondary occlusal rest must be employed to prevent slippage of the primary rest and orthodontic movement of the abutment tooth (Figure 6-7). Such a rest should pass over the lowered marginal ridge on the side of the tooth opposite the primary rest and should, if possible, be inclined slightly apically from the marginal ridge. However, two opposing occlusal rests on diverging tooth inclines will function to prevent unfavorable forces if all related connectors are sufficiently rigid. In any tooth-tissue–supported partial denture, the relation of the occlusal rest to the abutment should be that of a shallow ball-and-socket joint, to prevent a possible transfer of horizontal stresses to the abutment tooth. The occlusal rest should provide only occlusal support. Stabilization against horizontal movement of the prosthesis must be provided by other components of the partial denture rather than by any locking effect of the occlusal rest, which will cause the application of leverages to the abutment tooth.
Figure 6-7 Diagnostic cast evaluation of mesially tipped molar abutment. Anterior tilt of molar precludes preparation of acceptable rest seat on the mesio-occlusal surface. The patient could not afford a crown to improve axial alignment or orthodontic treatment to upright the molar. Occlusal rests will be used on mesio-occlusal and disto-occlusal surfaces to support restoration and direct forces over the greatest root mass of the abutment. The proposed ring clasp design is outlined.
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, should be approximately one-third the buccolingual width of the tooth, and should allow for a minimum of 1-mm thickness of the metal; the preparation should be rounded with no undercuts or sharp angles (Figure 6-8).
Figure 6-8 Cast shows extended occlusal rest on the mandibular first molar, designed to ensure maximum bracing from the tooth. If placed on a mesially inclined molar next to a modification space (as in Figure 6-7), the extended rest would ensure that the forces are directed down the long axis of the abutment, and therefore the disto-occlusal rest would not have been needed.
In situations in which the abutment is severely tilted, the extended occlusal rest may take the form of an onlay to restore the occlusal plane (Figure 6-9). The tooth preparation for this type of extended rest must include removing or restoring pits, fissures, and grooves; placing a 1- to 2-mm bevel on the buccal and lingual occlusal surfaces to allow the extended rest (onlay) to provide stabilization; allowing the rest to restore the contour and occlusion of the natural tooth; and ensuring that the rest directs the forces down the long axis of the tooth. Tooth preparation must also include a 1- to 2-mm guiding plane on the mesial surface of the abutment.