Does single versus stepped curing of composite resins affect their shear bond strength?

BACKGROUND Shrinkage occurs during polymerization of all resin composites, affecting not only the physical properties of the composites but also the marginal integrity of the restoration. Stepped photo-polymerization, in contrast to single-cure photo-polymerization, is said to minimize this shrinkage. The thickness of the increments of resin composite being cured may also be a factor in final shear bond strength. This study compared the shear bond strength resulting from these two curing methods with resin composites of various thicknesses. METHODS Resin composite in increments of 1.5, 3 and 4.5 mm thickness, cured by stepped or single-cure photo-polymerization, was bonded to human third molar dentin with either the Scotchbond Multipurpose adhesive system or the Singlebond adhesive system. Each experimental group consisted of 12 specimens. After 7 days of storage in water, shear bond strength was tested to failure, and the mode of failure was recorded. RESULTS The thickness of the resin composite and the method of curing had no significant effect on shear bond strength for bonds mediated by Scotchbond Multipurpose adhesive, but significantly lower shear bond strength was recorded for bonds mediated by Singlebond adhesive for resin composite 4.5 mm in thickness. With thicker resin composite, there was a tendency toward a greater proportion of adhesive cohesive bond failures. CLINICAL SIGNIFICANCE The stepped photo-polymerization system of curing appears to offer no advantages over single-cure photo-polymerization, except that the former reportedly improves marginal adaptation and reduces marginal leakage. These results suggest that increments of resin composite to be cured by either method should be no thicker than 2 mm, particularly when the bond is mediated by a single-bottle adhesive.