The impact of hydrofluoric acid surface treatments on the performance of a porcelain laminate restorative material.

OBJECTIVES Hydrofluoric (HF) acid etching increases the bond strength between composite resin and porcelain surfaces and has been advocated as a pre-cementation technique for ceramic restorations. The internal surface flaw distribution which is implicated in the premature failure of ceramic restorations is modified by the etching process and little agreement exists amongst researchers as to the appropriate etching regime. The purpose of the current study was to examine the impact of HF acid concentration and etching time on the performance of a low fusing feldspathic porcelain. METHODS Sets of 30 Vitadur-Alpha dentin porcelain discs (15 mm diameter, 0.9 mm thickness) were etched with HF acid of three different concentrations (5, 10 and 20%) and for three different etching periods (45, 90 and 180s). Mean flexure strengths, standard deviations and the associated Weibull moduli (m) and characteristic stress (sigma(0)) were determined using bi-axial flexure (ball on ring). Contact profilometry was utilised to characterise the roughness of the etched porcelain surfaces. RESULTS A univariate general linear analysis of means revealed a significant reduction in the mean strength values of the as-fired control compared with groups subjected to HF acid etching. Further significance (P<0.05) was discovered with the impact of acid concentration. Altering etching time also resulted in changes in the reliability of the fracture strength data. Contact profilometry demonstrated an increase in surface roughness following HF acid etching and an increase in roughness associated with increasing HF concentration. CONCLUSIONS Etching of feldspathic porcelain is a dynamic process and the impact is dependent on substrate constitution, surface topography, acid concentration and etching time. A significant reduction of the flexural strength of a low fusing feldspathic porcelain has been demonstrated to result from etching and clear evidence exists that the nature of surface flaw modification is a function of etching time and HF acid concentration. Favourable combinations of HF acid concentration and etching time have been identified which enhance the reliability of the porcelain utilised although variability in clinical technique will result in the reduced reliability of porcelain laminate restorations in function.