RPI, RPA, and Bar Clasp

Clasp assemblies that accommodate functional prosthesis movement are designed to address the concern of a Class I lever. The concern is that the distal extension acts as a long “effort arm” across the distal rest “fulcrum” to cause the clasp tip “resistance arm” to engage the tooth undercut. This results in harmful tipping or torquing of the tooth, which is greater with stiff clasps and increased denture base movement. Two strategies may be adopted to change the fulcrum location and subsequently the “resistance arm” engaging effect (mesial rest concept clasp assemblies), or to minimize the effect of the lever through the use of a flexible arm (wrought-wire retentive arm).

Mesial rest concept clasps have been proposed to accomplish movement accommodation by changing the fulcrum location. This concept includes the RPI and RPA [rest, proximal plate, Akers] clasps. The RPI is a current concept of bar clasp design that refers to the rest, proximal plate, and I-bar component parts of the clasp assembly. Basically, this clasp assembly consists of a mesio-occlusal rest with the minor connector placed into the mesiolingual embrasure, but not contacting the adjacent tooth (Figure 7-11, A). A distal guiding plane, extending from the marginal ridge to the junction of the middle and gingival thirds of the abutment tooth, is prepared to receive a proximal plate (Figure 7-11, B). The buccolingual width of the guiding plane is determined by the proximal contour of the tooth (Figure 7-11, A and C). The proximal plate, in conjunction with the minor connector supporting the rest, provides the stabilizing and reciprocal aspects of the clasp assembly. The I-bar should be located in the gingival third of the buccal or labial surface of the abutment in a 0.01-inch undercut (Figure 7-11, D). The whole arm of the I-bar should be tapered to its terminus, with no more than 2 mm of its tip contacting the abutment. The retentive tip contacts the tooth from the undercut to the height of contour (Figure 7-11, E). This area of contact, along with the rest and proximal plate contact, provides stabilization through encirclement (see Figure 7-11, C). The horizontal portion of the approach arm must be located at least 4 mm from the gingival margin and even farther if possible.

Figure 7-11 Bar-type clasp assembly. A, Occlusal view. Component parts (proximal plate minor connector, rest with minor connector, and retentive arm) tripod the abutment to prevent its migration. B, The proximal plate minor connector extends just far enough lingually so that it combines with the mesial minor connector to prevent lingual migration of the abutment. C, On narrow or tapered abutments (mandibular first premolars), the proximal plate should be designed to be as narrow as possible but still sufficiently wide to prevent lingual migration. D, I-bar retainer located at greatest prominence of tooth in the gingival third. E, Mesial view of I-bar illustrating the retentive tip relationship to the undercut and a region superior to the height of contour, which serves a stabilization function in encirclement.

Three basic approaches to the application of the RPI system may be used. The location of the rest, the design of the minor connector (proximal plate) as it relates to the guiding plane, and the location of the retentive arm are factors that influence how this clasp system functions. Variations in these factors provide the basis for differences among these approaches. All advocate the use of a rest located mesially on the primary abutment tooth adjacent to the extension base area. One approach recommends that the guiding plane and the corresponding proximal plate minor connector should extend the entire length of the proximal tooth surface, with physiologic tissue relief eliminating impingement of the free gingival margin (Figure 7-12). A second approach suggests that the guiding plane and the corresponding proximal plate minor connector should extend from the marginal ridge to the junction of the middle and gingival thirds of the proximal tooth surface (Figure 7-13). Both approaches recommend that the retaining clasp arm should be located in the gingival third of the buccal or labial surface of the abutment in a 0.01-inch undercut. Placement of the retaining clasp arm generally occurs in an undercut located at the greatest mesiodistal prominence of the tooth or adjacent to the extension base area (Figure 7-14, A and B). The third approach favors a proximal plate minor connector that contacts approximately 1 mm of the gingival portion of the guiding plane (Figure 7-15, A) and a retentive clasp arm located in a 0.01-inch undercut in the gingival third of the tooth at the greatest prominence or toward the mesial away from the edentulous area (Figure 7-14, C). If the abutment teeth demonstrate contraindications for a bar-type clasp (i.e., exaggerated buccal or lingual tilts, severe tissue undercut, or a shallow buccal vestibule) and the desirable undercut is located in the gingival third of the tooth away from the extension base area, a modification should be considered for the RPI system (the RPA clasp) (Figure 7-15, B). Application of each approach is predicated on the distribution of load to be applied to the tooth and edentulous ridge.

Figure 7-12 Bar clasp assembly in which the guiding plane (GP) and the corresponding proximal plate (PP) extend the entire length of the proximal tooth surface. Physiologic relief is required to prevent impingement of the gingival tissues during function. Extending the proximal plate to contact a greater surface area of the guide plane directs functional forces in a horizontal direction, thus the teeth are loaded to a greater extent than the edentulous ridge.

Figure 7-13 Bar clasp assembly in which the guiding plane (GP) and the corresponding proximal plate (PP) extend from the marginal ridge to the junction of the middle and gingival thirds of the proximal tooth surface. This decrease (compared with Figure 7-23) in amount of surface area contact of the proximal plate on the guiding plane more evenly distributes functional force between the tooth and the edentulous ridge.

Figure 7-14 Occlusal view of the RPI (rest, proximal plate, and I-bar component parts) bar clasp assembly. Placement of the I-bar in a 0.01-inch undercut (A) on the distobuccal surface; (B) at the greatest mesiodistal prominence; and (C) on the mesiobuccal surface.

Figure 7-15 A, Bar clasp assembly in which the proximal plate (PP) contacts approximately 1 mm of the gingival portion of the guiding plane (GP). During function, the proximal plate and the I-bar clasp arm are designed to move in a mesiogingival direction, disengaging the tooth. Lack of sustained contact between the proximal plate and the guiding plane distributes increased functional force to the edentulous ridge. Asterisk (*) indicates center of rotation. B, Modification of the RPI (rest, proximal plate, and I-bar component parts) system (rest, proximal plate, Akers or RPA clasp) is indicated when a bar-type clasp is contraindicated and a desirable undercut is located in the gingival third of the tooth away from the extension base area.

The bar clasp, which gave rise to the RPI, is discussed here because of this association. It may not be configured to allow functional movement, but it can be. The term bar clasp is generally preferred over the less descriptive term Roach clasp arm. Reduced to its simplest term, the bar clasp arm arises from the denture framework or a metal base and approaches the retentive undercut from a gingival direction (see Figure 7-11). The bar clasp arm has been classified by the shape of the retentive terminal. Thus it has been identified as a T, modified T, I, or Y. The form the terminal takes is of little significance as long as it is mechanically and functionally effective, covers as little tooth surface as possible, and displays as little metal as possible.

In most situations, the bar clasp arm can be used with tooth-supported partial dentures, with tooth-supported modification areas, or when an undercut that can be logically approached with a bar clasp arm lies on the side of an abutment tooth adjacent to a distal extension base (Figure 7-16). If a tissue undercut prevents the use of a bar clasp arm, a mesially originating ring clasp, a cast, or a wrought-wire clasp or reverse-action clasp may be used. Preparation of adjacent abutments (natural teeth) to receive any type of interproximal direct retainer, traversing from lingual to buccal surfaces, is most difficult to adequately accomplish. Inevitably the relative size of the occlusal table is increased, contributing to undesirable and additional functional loading.

Figure 7-16 Bar clasp arm properly used on the terminal abutment. The combination of rest, proximal plate, and bar clasp contacting the abutment tooth provides more than 180 degrees encirclement. A uniform taper to the bar ensures proper flexibility and internal stress distribution. Taper can originate from the minor connector junction or at a finishing line indicating the anterior extent of the denture base. Encirclement does not require that retentive tip modification (in the form of a T) be provided, and such a modification adds little to the clasp assembly.

Specific indications for use of a bar clasp arm include (1) when a small degree of undercut (0.01 inch) exists in the cervical third of the abutment tooth, which may be approached from a gingival direction; (2) on abutment teeth for tooth-supported partial dentures or tooth-supported modification areas (Figure 7-17); (3) in distal extension base situations; and (4) in situations in which esthetic considerations must be accommodated and a cast clasp is indicated. Thus use of the bar clasp arm is contraindicated when a deep cervical undercut exists or when a severe tooth and/or tissue undercut exists, either of which must be bridged by excessive blockout. When severe tooth and ti/>