[Effect of hydrofluoric acid etching time and resin bonding on the flexural strength of lithium disilicate glass ceramic].

OBJECTIVE: To analyze the effect of hydrofluoric acid(HFA) etching time and resin bonding on the flexural strength of IPS e.max(R) Press glass ceramic, and evaluate the efficacy of resin cements to seal the cracks of the etched ceramic. METHODS: Two hundred and twenty-five bars (25.0 mmx3.0 mmx2.0 mm) were made from IPS e.max(R) Press ingots using lost-wax, hot-pressed ceramic fabrication technology and randomly divided into five groups, forty-five each.In each group, the surfaces of ceramic bars were etched by 9.5% HFA gel for 0, 20, 40, 60 and 120 s respectively. Three specimens from each group were selected to observe the microstructure by the field emission scanning electron microscope (FE-SEM). Then each group were randomly subdivided into two subgroups (n = 20).One subgroup were coverd with a thin (approximately 0.1 mm) layer of resin cement (Variolink N), whereas the other subgroup remained unaltered.Half of the specimens were stored in 37 degrees C water bath for 24 h and the other half went through thermocycle 10 000 times before 3-point bending test to determine their flexural strength.Interfaces between resin cement and etched ceramic were examined with FE-SEM. RESULTS: FE-SEM results showed that etching with HFA resulted in preferential dissolution of glass ceramic, and partially supported crystals within the glass matrix were lost with the increasing of etching time.FE-SEM indicated that resin cement sealed the cracks and defects and bonded tightly to etched ceramic surface. The mean flexural strength values of group 0, 20, 40, 60 and 120 s were (384 +/- 33), (347 +/- 43), (330 +/- 53), (327 +/- 67) , and (317 +/- 41) MPa respectively. The mean flexural strength of each group except group 0 s increased significantly to (420 +/- 31), (435 +/- 50), (400 +/- 39), and (412 +/- 58) MPa respectively after the application of resin cement. CONCLUSIONS: Overtime HFA etching could have a wakening effect on IPS e.max(R) Press glass-ceramic. The application of dual-curing resin cement can compensate the strength loss of the etched glass ceramic.