Prosthodontics & Occlusion · patient case question
The replacement crown for #9 will not seat fully despite an apparently clean internal surface. What is the most likely cause?
Patient case: Crown that repeatedly debonds, and a new crown that will not seat
Patient: Female, 32 years old
Chief Complaint: “The crown on my front tooth keeps coming off, and it does not look right.”
Background: Crown on tooth #8 recemented three times in two years. No systemic disease. No reported grinding.
Vital Signs: BP 116/72 mmHg · HR 70 bpm
Findings: DEEP anterior overbite with heavy contact on the lingual surface of #8 in centric and on protrusive excursion. The abutment preparation is adequate in height and taper. Periodontium healthy: 2 mm probing depths, 4 mm of attached gingiva, no recession. At try-in of a replacement crown for #9, the restoration will not seat fully and a visible open margin remains, though the internal surface shows no interference on inspection.
Show answer and explanation
Correct answer: A. Excessively tight proximal contacts
- A. CORRECT. An over-tight PROXIMAL CONTACT is the commonest reason a casting will not seat, and it is the one most easily missed because the internal surface looks perfect. The crown binds against the adjacent tooth before it reaches the finish line, holding it high and leaving an open margin. The test is direct: check the contacts with unwaxed floss and shimstock, and adjust them until the crown drops fully before ever assessing the margin or the occlusion.
- B. Excess cement causes incomplete seating at CEMENTATION, but this is a dry try-in — no cement has been used.
- C. Insufficient occlusal reduction produces a HIGH restoration in occlusion but does not prevent it from seating on the preparation.
- D. An over-tapered preparation reduces RETENTION. It makes a crown come off, not fail to go on — the opposite problem.
Note: The seating checklist in order: proximal contacts first, then internal interferences using a disclosing medium such as fit-checker or an aerosol indicator spray, then any residue or blebs from the casting, then the margin, and only then the occlusion. Verify seating with a RADIOGRAPH before cementation on posterior teeth — an open margin discovered after cementation costs the restoration.
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