Effect of Repeated Firing on Porcelain-Composite Bond

Studies have shown that bond strength of composite resin to porcelain is affected by leucite volume %, etching time, and types and concentration of etchant. Repeated firing of porcelain reportedly modifies the leucite volume % of some porcelains. Objectives: This study examines the effect of repeated firing of porcelain with different contents of leucite on porcelain-composite bond strength. Methods: Square-shaped porcelain specimens (12mmx12mmx2mm) were fabricated from two medium-leucite-content porcelain; (A), Ceramco II (Ceramco) and (B), Creation (Willigeller) as well as (C) a high-leucite-content porcelain (Fortress, Chameleon Dental). Specimens of each porcelain were placed in three groups and were subjected to one, five, and 10 repeated firings. They were sanded with Al2O3 at 30 PSI on one side and etched for 90 seconds with 9.5% HF gel (Porcelain Etchant, Bisco). Etched specimens were steam cleaned, bonded and light-cured to a high moduli, hybrid composite resin (Z100, 3 MESPE) using a bonding jig (Ultradent Jig). Shear bond strength was performed with a testing machine (Sentran, KB3 Digital Indicator) at a cross head speed of 5mm/min. Data were analyzed using ANOVA (P<0.05). Results: Shear bond strength and SD (Mpa) for three porcelains (A,B,C) and three repeated firings (1, 5, and 10) were A1=24.2±4.0; A5=23.9±3.0; A10=20.4±3.9; B1=16.9±2.4; B5=17.7±1.1; B10=16.6±2.8; C1=23.7±3.4; C5=21.0±3.4; C10=22.7±3.7. No significant difference was recorded between groups of each porcelain. The mode of fracture was predominantly adhesive for the A group and all cohesive for groups B and C. Conclusion: Repeated firing of three porcelains in this study did not significantly affect the porcelain-composite bond strength. Thus it may not have resulted in volumetric changes of leucite crystals.