Effect of cement film thickness on the bond strength of all-ceramic systems

Cementation of all-ceramic restoration is critical. In spite of developments on adhesive techniques, it is thought that one of the common reasons of failure occurred on fixed partial dentures is improper cementation. Excessive cement film thickness is still a fault, ignored by practitioners. The aim of this study is to investigate the effect of cement film thickness changes on bond strength of all-ceramic restorations.One hundred and thirty five dentin discs, derrived from extracted human molars were embedded acrylic resin. On exposed dentin surface, a specimen was prepared in order to be subjected to shear bond strength testing. 135 all-ceramic discs (7 mm in diameter and 2 mm thickness) were fabricated from three different ceramic systems ( Rainbow, Lava and ICE Zirconia). Then both dentin and ceramic surfaces were treated with self-etching, silica coating and application of silane according to manufacturers? recommendations for cementation. Following the bonding of the dentin and the ceramic specimens with a resin cement (Panavia F2.0) at different cement film thicknesses, predetermined thicknesses (50, 100 and 200 µm) were established using a plexyglass mold. Nine main groups (n= 15) were formed by the three different ceramic systems combined with the three different cement thicknesses. All specimens were stored in distilled water at 37ºC for 24 hours and thermal cycled 500 times between 5 ºC and 55 ºC prior to testing. Shear-bond strength tests were performed using a universal testing machine at a constant crosshead speed of 1 mm/min. Statistical analysis was performed by multifactorial analysis of variance taking interactions between effects into account. For multiple paired comparisons, the Tukey HSD method was used (?= .05).In a 2-ways ANOVA model, the main factors were ceramic systems and cement film thickness. There were statistically significant differences between the mean bond strengths of the groups at the p<.05. Analysis of variance showed that bond strength was significantly affected by the adhesive thickness (p < .05). There were statistically significant difference between groups with 50 and 200 µm film thicknesses. However there were not statistically significant differences between the ceramic systems. Resin cement-dentin interface failure was predominant for all groups but mixed debonding combined with cohesive failure within cement increased with decreased cement film thickness in shear testing.Within the limitations of this study it can be concluded that no matter what the ceramic system is, the cement film thickness has a prominent effect on shear bond strength.Keywords: Cement film thickness, Zirconium oxide, Shear bond strength, Silica coating