Influence of particle abrasion or hydrofluoric acid etching on lithium disilicate flexural strength

Lithium disilicate is a translucent, glass-containing material used for ceramic restorations. Clinicians frequently use alumina abrading or hydrofluoric acid etching to create micromechanical retention in the intaglio surface before bonding a lithium disilicate restoration to the tooth. Few studies have investigated how the etching or abrasion processes affect the flexural strength of lithium disilicate ceramics.

The purpose of this study was to measure and compare the flexural strength of e.max CAD after alumina abrasion at differing pressures and acid etching at differing concentrations and times.

Bars of e.max CAD (9 groups of 10; 22×2.5×2.5 mm) were prepared, polished sequentially with 180, 320, and 600 abrasive paper, and sintered according to the manufacturer's instructions. Four groups were particle abraded (30-μm alumina particles from 10 mm at 55, 100, 200, or 300 kPa for 10 seconds). Four groups were etched with either 5% hydrofluoric acid (20 seconds or 120 seconds) or 9.5% hydrofluoric acid (20 seconds or 120 seconds). The control was polished and fired only (no treatment). Specimens were placed onto an Instron (1 mm/min crosshead speed) and loaded to failure in a 3-point flexural test. One-way ANOVA and the Dunnett t test determined intergroup differences (α=.05).

Compared with the control, the 100, 200, and 300 kPa alumina abraded groups produced significantly lower flexural strengths (P<.001); however, the flexural strength of the 55 kPa abraded group was not statistically different from the control (P=.080). The flexural strength of the 5% and the 9.5% hydrofluoric acid-etched groups also were not significantly different from the control (P>.050); however, the 9.5% hydrofluoric acid at 20 seconds group was nearly statistically significant (P=.051).

Alumina particle abrasion at pressures of 100 kPa and higher significantly reduced flexural strength by creating stress risers in e.max CAD and should not be used. Hydrofluoric acid etching should be used to increase micromechanical retention and clean the intaglio surface of the restoration before bonding.