Prosthodontics & Occlusion · patient case question
For a gold crown on a tooth with little remaining coronal structure, which feature most improves retention?
Patient case: Restoring a broken-down tooth
demo: 45-year-old female
chief: Wants to restore a heavily broken lower molar and to improve the appearance of her upper front teeth.
history: Nothing relevant medically. She has had a crown placed on tooth 12 elsewhere.
meds: None.
findings: Tooth 30 has minimal remaining coronal structure with 0.5 mm of sound dentin above the crown margin circumferentially. An impression and a working cast from the first attempt show voids at the margin and a torn finish line. The crown on tooth 12 is markedly brighter than the adjacent very yellow natural teeth.
Show answer and explanation
Correct answer: C. Proximal grooves and boxes placed in sound axial walls
- A. Deep subgingival margins violate the biologic width, cause chronic inflammation and make impressions and finishing harder. They do not add retention.
- B. Greater taper reduces retention. The ideal convergence is about 6 degrees total, and retention falls sharply as taper widens.
- C. Correct. When axial wall height is limited, auxiliary retentive features restore resistance form. Grooves and boxes in sound dentin resist rotational and lateral displacement, and they work by limiting the paths along which the restoration can be dislodged.
- D. Retention increases with axial wall height and surface area, so shortening the walls makes matters worse.
Note: The recall recorded the option as lingual retention grooves. Grooves are conventionally placed on the proximal walls, which resist rotation most effectively, though additional lingual features can be used. The concept being tested is auxiliary retention.
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