Effect Of Repeated Firings On Physical Properties And Resin Bonding Of Lithium Disilicate Glass-ceramic

Objectives:Lithium disilicate is a popular glass-ceramic with adequate aesthetic properties and excellent mechanical properties.But,prior to final cementation of the restoration on tooth abutment,several adjustments to improve the esthetic properties or deficient proximal contact are needed.These corrections make additional firings inevitable.Whether microstructure,mechanical properties and resin bonding would be affected by the alternations of firing cycles is unclear.Hence,the aim of this study was to evaluate the effects of different firing cycles on microstructure,surface hardness and fracture toughness of lithium disilicate glass-ceramic,as well as their bond strength to resin.Methods:Three hundred and twenty lithium disilicate glass-ceramic discs were made and assigned to different groups(n=60)to receive one,two,three or four firings.Composite resin cylinders were bonded to the ceramic specimens which were pre-treated with hydrofluoric acid followed by silanization,using conventional or self-adhesive resin cement.Shear bond strength(SBS)tests were carried out to evaluate the bond between ceramic and resin after water storage for 24 h or 3 mo at37°C.The plates ceramic undergone different firings after polishing or hydrofluoric acid etching was observed using a scanning electron microscope(SEM),and their surface roughness were tested.The surface Vickers hardness,fracture toughness and related Weibull analysis results of the polished ceramics received different firings were also compared.Results:The microstructure of lithium disilicate glass ceramics undergone one to four firing cycles shown no visual difference.HF etching increased surface roughness,and the roughened surface improved the resin bonding of lithium disilicate glass-ceramic.Surface roughness of the ceramics were not significantly influenced by repeated firing cycles.Specimens undergone no firing obtained the highest surface hardness.Surface hardness of the specimens received repeated firings showed no significant difference.One firing cycle increased fracture toughness of lithium disilicate glass-ceramic significantly;however,repeated firings failed to increase it further.Weibull analysis indicated that the structural reliability of the specimens received 0-4 firings was significantly different.Furthermore,repeated firing cycles and 3-mo water storage had no significant effect on the SBS.Conclusions:1.Repeated firings had no significant effect on microstructure and surface roughness of glass-ceramic.2.Glass-ceramic undergone one firing cycle obtained the highest fracture toughness,but repeated firing cycles significantly decreased the fracture toughness.3.Repeated firings had no significant effect on resin bonding of glass-ceramic.