Effect of fracture strength of primer–adhesive mixture on bonding effectiveness

The purpose of this study was to measure the micro-tensile bond strength (μTBS) of two adhesives to dentin and to correlate this μTBS with the micro-tensile fracture strength (μTFS) of primer–adhesive mixtures of the two adhesives. The three-step etch&rinse adhesive OptiBond FL (Kerr) and the two-step self-etch adhesive Clearfil SE Bond (Kuraray) were used to bond Filtek Z100 (3M ESPE) to twenty mid-coronal dentin surfaces. The μTBS was determined at 1 and 24 h after light-curing. In addition, the μTFS of primer–adhesive mixtures (primer:adhesive=1:3 by weight) and of the pure adhesives (controls) were measured after the same time intervals. The μTBS to dentin of OptiBond FL at 24 h was significantly higher than at 1 h and than the μTBS of Clearfil SE Bond at 24 h. The mean μTFS of the primer–adhesive mixtures was always lower than the μTFS of the respective adhesives. The highest reduction in μTFS due to mixing primer with the adhesive resin was measured for Clearfil SE Bond. Both the μTFS of the primer–adhesive mixtures and the pure adhesives of OptiBond FL and Clearfil SE Bond increased with time. The early 1 h μTBS to dentin was lower than the 24 h μTBS for OptiBond FL, but not for Clearfil SE Bond. Differences in polymerization efficiency as well as in the resulting interfacial strength may explain such varying early bond strengths to dentin.