Ceramics: Material Selection, Bonding and Failure Modes
Core concepts
Dental ceramics split into glass-rich materials (feldspathic, leucite-reinforced, lithium disilicate) and polycrystalline materials (zirconia, alumina). Glass-rich ceramics can be etched with hydrofluoric acid, creating a microretentive surface that is then coupled with a silane primer and resin cement. Polycrystalline ceramics contain no glass phase, so etching is ineffective; they are conditioned by airborne-particle abrasion followed by an MDP-containing primer and adhesive resin cement. Know the numbers: fracture toughness and flexural strength rise from feldspathic through lithium disilicate to zirconia, while translucency generally falls. Understand coefficient of thermal expansion (CTE) mismatch in metal-ceramic restorations, layering versus monolithic design, minimum thickness, and the need to polish rather than leave adjusted ceramic surfaces rough.
Traps
The single most tested error is etching zirconia or alumina with hydrofluoric acid; the second is using silane alone on a polycrystalline surface. Others include applying phosphoric acid to any ceramic, choosing a non-adhesive cement for an etched glass-ceramic veneer, ignoring minimum thickness, and blaming "weak porcelain" for a fracture that began at a thin connector or an unpolished adjustment. CTE mismatch questions usually ask which layer is in tension.
Rule of thumb
Ask one question first: does the material contain a glass phase? If yes, etch and silane. If no, abrade and use MDP. Everything about bonding follows from that.
Worked example
A lithium disilicate veneer is planned at 0.5 mm thickness. The laboratory crystallizes the restoration, the clinic tries it in, then the intaglio is etched with 5% hydrofluoric acid for about 20 seconds, washed and dried, silanated, and cemented with a light-cured resin cement. By contrast, a monolithic zirconia crown is abraded with 50-micron alumina, primed with an MDP-containing primer, and seated with an adhesive resin or reinforced glass-ionomer cement - no HF etch at any stage.