The effect of delayed light-activation of Variolink II and Calibra dual-cure cements on the shear bond strength of lithium disilicate bonded to dentin

Statement of Problem: Light-activation of some dual-cured cements appears to interfere with the self-cured mechanism and restricts the cement from achieving its maximum mechanical properties.;Purpose: To evaluate the shear bond strength of lithium disilicate bonded to dentin using modifications of the light-activation time of Variolink II and Calibra dual-cure resin cements.;Materials and methods: Two hundred caries-free and restoration-free dentin surfaces from extracted human molars were collected and randomly divided into one of two treatment groups and further divided into one of five subtreatment groups. The teeth were stored in distilled water at 37oC and 100% humidity and were sectioned mesio-distally with a low speed diamond saw (Buehler, USA). The sectioned teeth were embedded in acrylic resin (Caulk Orthodontic resin Dentsply, USA), leaving only the buccal or lingual surface exposed. These surfaces were ground to superficial dentin with 320 grit wet silicon carbide abrasive paper (Buehler, USA) and further polished with 600 grit paper to obtain a flat standardized bonding surface. Lithium disilicate rods (2 mm x 3 mm diameter) shade B1 (IPS e. Max Press HT, Ivoclar, USA) were fabricated from wax sprues, invested and heat pressed using the lost wax technique. The bonding surface of the lithium disilicate rods were microetched with aluminum oxide particles to removed excess investment material, treated with 5% hydrofluoric acid for 20 seconds (IPS Ceramic Etching Gel, Ivoclar), rinsed and dried with oil free air. Ceramic primers were applied (Group A: Monobond Plus, Ivoclar; Group B: Silane Coupling Agent, Denstply) and the surfaces conditioned according to the manufacturer's instructions.;Lithium disilicate rods were bonded to the dentin surfaces and modifications of the starting time for the light-activation were performed in each group. After cementation, the specimens were stored in water at 37 ° C for 24 hours. One group of the specimens was then subjected to the shearing test. A second group was subjected to 500 thermocycles from 5 °C and 55 °C according to ISO/TS 11405:2003 with a dwell time of 20 seconds and transfer time of 5 to10 seconds, (N= 200 total).;Results: The highest shear bond strength for dentin was recorded for Calibra dual-cred resin cement with a mean value of 12.411 MPa. The lowest dentin shear bond strength was recorded for Variolink II with a mean value of 9.292 MPa.;A 3-way variance of ANOVA pairwise comparison was performed at the 0.05 level of significance. There was a significant difference in the shear bond strength between Calibra and Variolink II with a significant p value of 0.001 for both dual-cured resin cements. There was a statistical significance on the shear bond strength within both cements when the time of the light-activations was modified, specifically between the manufacturer's recommendations and the delayed light-activation of ten minutes after beginning the mixing procedure, with mean values of 10.658 and 13.424 MPa respectively.;The thermocycling aging protocol had an effect on the shear bond strength of Calibra and Variolink II with a significance of p= 0.001 and a mean value for the thermocycling group of 12.767 MPa. The groups stored for 24 hours at 37 °C at 100% humidity showed a lower mean value of 9.136 MPa on shear bond strength for both cements.;The bond failures that occurred more frequently were a combination of adhesive/cohesive failure for both cements Calibra and Variolink II cements. Variolink II showed a 7% adhesive failure at the lithium disilicate/cement interface and the Calibra groups showed a 10% cohesive failure.;Conclusion: Calibra showed apparently stronger shear bond strength compared to Variolink II. Both cements showed significantly lower bond strength when cured without light activation compared to the bond strength achieved with light activation. A delay of 10 minutes after the start of the mixing procedure showed significant differences on the shear bond strength compared to manufacturer's recommendations.;Comparing cements, Calibra was the only cement that showed cohesive failures and Variolink II was the only one that showed an adhesive failure to the lithium disilicate ceramic material due to apparently better bonding to dentin structure by Variolink II. The thermocycling aging protocol simulated the intraoral conditions and apparently increased the degree of conversion of the dual-cured resin cements.;Clinical Significance: The use of dual-cure resin cements such as Calibra should be considered the cement of choice by clinicians when increased mechanical properties of the esthetic restorations are desired. When dual-cured cements are used it is recommended to delay the light curing procedures of the dual-cured cements to the maximum time clinically possible to obtain the best physical and mechanical properties of these materials. (Abstract shortened by UMI.).