Prosthodontics & Occlusion · patient case question
What is the ideal total occlusal convergence (taper) for a full-coverage crown preparation?
Patient case: “I Want Teeth That Don't Come Out”
Patient: 64-year-old female
Chief Complaint: “I'm done with these plates. I want teeth that don't come out.”
Background: Wearing maxillary and mandibular removable partial dentures for eight years. Good general health, non-smoker, financially able to proceed.
Medications: Atorvastatin
Vital Signs: BP 126/78 mmHg · HR 72 bpm · RR 16 · T 98.4°F
Findings: Maxilla: first molars and canines present in both quadrants. Mandible: second molars and some premolars present. Remaining teeth are periodontally sound with good bone support. Adequate ridge volume in the edentulous spans on CBCT. No parafunction.
Show answer and explanation
Correct answer: B. About 6 degrees
- A. 20 degrees is a common real-world value but represents a substantial loss of retention compared with the ideal.
- B. CORRECT. Approximately 6 DEGREES of TOTAL convergence — that is, about 3 degrees per wall — is the classical ideal. Retention falls sharply as taper increases, because the resistance to displacement along the path of insertion depends on near-parallel opposing walls. Clinically achieved tapers are commonly 12–20 degrees; the accepted practical range is about 6–10 degrees.
- C. 2 degrees is too little. Walls that near-parallel risk UNDERCUTS, which prevent the restoration seating at all, and they trap hydraulic pressure during cementation.
- D. 30 degrees provides very poor retention and resistance form and would require auxiliary features such as grooves and boxes to compensate.
Note: Retention and resistance also depend on axial wall HEIGHT (more is better), the surface AREA of the preparation, surface roughness and the cement used. Where wall height is limited, add proximal grooves or boxes rather than accepting excessive taper.
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