Comparison of shear and microtensile failure stresses in the repair of dental composite restorations

Failure of dental composite restorations frequently occurs. To preserve tooth structure, repair of existing restorations can be done by relayering (composite-to-composite-bonding). This study compares shear and microtensile failure in composite-to-composite-bonding. Disk-shaped and rectangular-blocks of a nanohybrid and nanofilled composite were prepared for shear and microtensile measurements, respectively. Half of all specimens were aged using thermocycling. Non-aged and thermocycled specimens were conditioned by intermediate-adhesive-resin-application (IAR-application) or silica-coating and silanization followed by IAR-application (SC-application). Resin composites, of the same kind as the substrate, were adhered onto the substrates and shear or microtensile forces applied to the interface. Significant differences between shear and microtensile failure stresses, effects of conditioning, aging and composite types were observed. Mean failure shear stresses (7 - 28 MPa) were significantly lower than microtensile ones (32 - 53 MPa), regardless of aging or conditioning, with average standard deviations approaching 50%. The dependability of the bonds, indicated by its Weibull modulus, was similarly low in shear and microtensile modes. Failures after shear were more frequently cohesive than after application of a tensile force, except in thermocycled composites after IAR-application. Clinically, restorations are mainly exposed to shear and shear evaluations are to be preferred over microtensile ones, also since Weibull moduli are similar for both modes. Hence, considering the high percentages of cohesive failure in shear combined with the low Weibull moduli, this study indicates that the least dependable link in composite-to-composite-bonding is the composite itself and not the adhesive interface, with the exception of bonding created after thermocycling by IAR-application.