Prosthodontics & Occlusion · patient case question
Given her periodontal status, which design modification best reduces torquing forces on the abutment adjacent to the distal extension?
Patient case: Attachment-Retained Partial Denture
Patient: 47-year-old female
Chief Complaint: “I don’t want a metal clasp showing when I smile.”
Background: Missing #18, #19, and #20 on the left with a dentate right side — a mandibular Kennedy Class II arch. Generalized moderate chronic periodontitis, currently stable after non-surgical therapy: probing depths 3–5 mm, no bleeding on probing, grade I mobility on the abutments.
Medications: None
Vital Signs: BP 118/74 mmHg · HR 70 bpm · RR 16 · T 98.6°F
Findings: An attachment-retained removable partial denture is planned, using an intracoronal (precision) attachment within a splinted crown on #21 rather than an extracoronal clasp. A radiograph of an existing prosthesis on the contralateral side shows a radiopaque box-and-key interface between a retainer and an adjacent unit.
Show answer and explanation
Correct answer: A. Place the occlusal rest in the mesial fossa rather than the distal fossa
- A. Correct. A mesial rest shifts the fulcrum line away from the distal extension. When the base is loaded, the framework rotates about the mesial rest and the retentive arm moves into a non-retentive area rather than being levered against the abutment. This is the core stress-releasing principle behind the RPI and RPA systems.
- B. Incorrect. A distal rest places the fulcrum immediately adjacent to the extension base, so functional loading levers the abutment distally — exactly the destructive torque to be avoided in a periodontally involved tooth.
- C. Incorrect. A rigid connection transmits the full displacement of the extension base directly to the abutment, maximizing rather than minimizing torque.
- D. Incorrect. Removing the rest allows the framework to settle into the gingival tissues, causing soft tissue trauma and losing all vertical support and load direction.
Note: The RPI system — mesial Rest, distal Proximal plate, I-bar — is designed specifically for distal extension cases. On rotation, all three components disengage from the abutment rather than binding against it.
Practice Prosthodontics & Occlusion questions Prosthodontics & Occlusion study notes