RPI & RPA. RPA (rest, proximal plate, Akers clasp) clasp design.
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Discussion: The mesial rest and proximal plate are designed identically to those of the RPI clasp. The difference is in the retentive arm. An Akers, or circumferential clasp arm is used. If a conventional Akers clasp is used, with the retentive arm coming off the proximal plate above the survey line and crossing the survey line in the middle of the tooth to engage the undercut, the releasing capability will be lost. With the RPA clasp system the circumferential clasp is placed so that the rigid portion of the clasp arm will contact the tooth only along its superior border at the level of the survey line.
Conclusion: 1. The circumferential-type retentive arm is easier to grasp for removal of the prosthesis.2. Simple in design. 3. Avoids problems associated with large tissue undercuts, excess tissue surrounding the abutment tooth and high frenum attachments.
Mesial Rest - point of rotation which exerts a mesial force on the tooth
Proximal plate - superior edge at bottom of guide plane to disengage during loading. Slightly lingual for reciprocationI Bar - 2.5 mm from gingival margin, crosses at right angles in a .01" undercut at the greatest M-D prominence to permit it to disengage during function
Advantages of the R.P.I.1. Proximal plate and I bar move away from the tooth during function to reduce torque2. Mesial minor connector and proximal plate provide reciprocation and eliminate need for a lingual arm3. I bar more esthetic4. Minimal contact alters contour less, advantageous on caries prone individual 5. Mesial rest eliminates Class I lever