Prosthodontics & Occlusion · single question
For a Kennedy Class I RPD (missing teeth 17–19 and 30–31), where should rest seats be placed?
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Correct answer: B. Mesial of teeth 20 and 29 (RPI/RPA design)
For Kennedy Class I (bilateral free-end saddles), mesial rests on the most posterior abutment teeth (20 and 29) are preferred. Mesial rests: (1) direct forces toward the anterior denture, (2) minimize torquing of the abutment tooth, (3) reduce lever arm effect on the free-end extension. This is the basis of the RPI (Rest-Proximal plate-I bar) and RPA clasp systems.
- A. ❌ Distal rests on 20/29 increase the lever arm and torque on abutment teeth — not preferred for Class I.
- B. ✅ Mesial rests of teeth 20 and 29 — the standard for Kennedy Class I; reduces torque and lever arm on abutments.
- C. ❌ Cingulum rests on canines are too far from the extension base — insufficient support.
- D. ❌ Premolar rests at 21/28 are farther from the extension base, increasing leverage on abutments.
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