Prosthodontics & Occlusion · patient case question
For how long should hydrofluoric acid be applied to the intaglio surface of a lithium disilicate crown?
Patient case: Ceramic Crown Delivery
demo: 38-year-old male
chief: Delivery of a lithium disilicate crown on tooth #30.
history: Bruxism. Existing crown on #6 has become discolored at the margin.
meds: None
findings: Lithium disilicate crown returned from the laboratory. Adjacent tooth #6 shows a grey marginal discoloration beneath a ceramo-metal crown.
Show answer and explanation
Correct answer: D. 20–60 seconds
- A. Five seconds is insufficient to create the retentive etch pattern.
- B. Ten minutes would severely degrade the restoration.
- C. Three minutes over-etches lithium disilicate and weakens the ceramic surface.
- D. Correct. Lithium disilicate is etched with roughly 5% hydrofluoric acid for about 20 seconds — the recall answer here is 60 seconds — which selectively dissolves the glassy matrix and creates micromechanical retention before silanation.
Note: Etching time is ceramic-specific: about 20 s for lithium disilicate, 60–120 s for feldspathic porcelain. Zirconia is not etched with HF at all — it is air-abraded and treated with an MDP primer.
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