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Prosthodontics & Occlusion · patient case question

What is the most likely reason this prosthesis has loosened?

Patient case: Three-unit fixed partial denture that has become loose

Patient: Female, 61 years old

Chief Complaint: “My bridge feels loose when I bite.”

Background: Three-unit porcelain-fused-to-metal bridge from #3 to #5 placed eight years ago, replacing #4. Reports night-time clenching. No pain.

Vital Signs: BP 128/80 mmHg · HR 72 bpm

Findings: The prosthesis moves as a unit under digital pressure. Radiographs show short, markedly tapered abutment preparations with minimal occlusogingival height, and a flattened occlusal surface on the #3 retainer. No periapical radiolucency; both abutments respond within normal limits to percussion.

  1. Recurrent caries at both margins
  2. Periodontal attachment loss on the abutments
  3. Failure of the cement to bond chemically to porcelain
  4. Loss of RETENTION AND RESISTANCE FORM from over-tapered, short preparations, so functional stress exceeds what the preparation geometry can withstand
Show answer and explanation

Correct answer: D. Loss of RETENTION AND RESISTANCE FORM from over-tapered, short preparations, so functional stress exceeds what the preparation geometry can withstand

  • A. Recurrent caries would be visible radiographically or clinically and would usually loosen one retainer rather than the whole unit; nothing in the findings supports it.
  • B. The abutments are described as sound on percussion with no periapical or crestal changes. Periodontal failure would show mobility of the individual abutments, not a prosthesis moving as a unit on intact teeth.
  • C. Cement failure is a consequence, not a cause. Luting cements fail when the preparation geometry lets the restoration move — no cement compensates for inadequate form.
  • D. CORRECT. Retention resists removal ALONG the path of insertion and depends on near-parallel walls and adequate surface area; resistance resists tipping or rotation under OBLIQUE load and depends on preparation height and rigidity. Short, over-tapered preparations lose both. Under a clencher's lateral loading, the restoration micro-rotates, the cement seal fatigues, and the bridge loosens as a unit.

Note: What this case adds is the failure MECHANISM — how taper and preparation height translate into loss of retention and resistance under function.

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