Restorative & Materials · patient case with 2 questions
Cementing a glass-ceramic crown
Patient: Female, 41 years old
Chief Complaint: “I am here to have the permanent crown fitted on my front tooth.”
Background: Healthy. Tooth #9 was root treated and restored with a fibre post and composite core. The definitive restoration is a pressed glass-ceramic crown.
Findings: The preparation has a 2 mm ferrule and adequate retention form. A feldspathic veneer for the adjacent tooth #8 is being processed at the same appointment. The crown try-in is satisfactory.
Question 1
For approximately how long is hydrofluoric acid applied to the intaglio surface of the FELDSPATHIC ceramic veneer before cementation?
Show answer and explanation
Correct answer: D. About 60 seconds
- A. Fifteen to twenty seconds is the PHOSPHORIC acid etch time for enamel, and it is also close to the correct figure for LITHIUM DISILICATE, which is etched with 5% hydrofluoric acid for roughly 20 seconds. Reading which ceramic the stem specifies is what decides this item.
- B. Over-etching is counterproductive. Excessive dissolution of the glassy phase weakens the surface layer and REDUCES rather than increases bond strength, leaving a friable surface that fails cohesively.
- C. Application for minutes has no basis in any protocol, and hydrofluoric acid is highly caustic — it requires rubber dam or a try-in outside the mouth, and skin contact causes deep, delayed, painful burns.
- D. CORRECT. FELDSPATHIC and leucite-reinforced glass ceramics are etched with approximately 9.5% HYDROFLUORIC ACID for about 60 SECONDS. Hydrofluoric acid selectively dissolves the GLASSY MATRIX, exposing the crystalline phase and creating a micro-retentive porous surface. The etch is then thoroughly rinsed and a SILANE coupling agent applied — silane is what chemically links the silica in the ceramic to the resin, and etching without silaning loses most of the achievable bond strength.
Note: Hold both figures: 5% HF for about 20 seconds on LITHIUM DISILICATE, 9.5% HF for about 60 seconds on FELDSPATHIC and leucite-reinforced ceramic. ZIRCONIA is the exception to the whole protocol — it has no glassy phase, so hydrofluoric acid does nothing. Zirconia is air-abraded with alumina and treated with an MDP-containing primer instead.
Question 2
Which cement is indicated for the lithium disilicate crown?
Show answer and explanation
Correct answer: B. Adhesive resin cement
- A. Zinc phosphate is a conventional luting cement with no adhesive capability whatsoever. It works purely by mechanical interlocking and depends entirely on the retention form of the preparation. It remains acceptable under full metal and zirconia crowns, but it does nothing for a glass ceramic.
- B. CORRECT. Glass ceramics derive a substantial part of their clinical strength from being ADHESIVELY BONDED to tooth structure: the resin supports the relatively brittle ceramic from within and limits crack propagation. The full protocol is hydrofluoric etch, rinse, silane, then an adhesive resin cement. Resin-modified glass ionomer is a defensible alternative for a lithium disilicate CROWN with adequate thickness and good retention form, but resin cement is mandatory for thin restorations such as veneers and for preparations with limited retention.
- C. Zinc oxide eugenol is a temporary and sedative cement. Critically, EUGENOL INHIBITS RESIN POLYMERISATION — so it must not even be used provisionally on a preparation that will later receive a resin-bonded restoration, or the definitive bond is compromised.
- D. Polycarboxylate bonds chemically to calcium in tooth structure and is notably kind to the pulp, but it is weak, relatively soluble, and it does not bond to ceramic.
Note: The contrast worth carrying: ZIRCONIA needs no hydrofluoric acid and no silane — it is air-abraded and primed with an MDP-containing agent, then luted with self-adhesive resin or resin-modified glass ionomer. Metal-ceramic and gold can be cemented conventionally. Match the cement to the substrate, not to habit.