The effect of variations in hourglass specimen design on microtensile bond strength to dentin

Purpose: To correlate the microtensile bond strength (mu TBS) of dentin bonding agents to dentin for different hourglass designs with the stress distribution across the adhesive interface utilizing finite element stress analysis (FEA). Materials and Methods: Adper Scotchbond Multipurpose Plus dentin bonding agent was bonded to flat dentin surfaces in three experimental groups. A 5-to 6-mm layer of resin composite (Filtek Supreme, 3M ESPE) was built up incrementally. After 24 h water storage at 37 degrees C, the bonded teeth were sectioned into hourglass designs of circular, parabolic, and spline shape, with a ca 1 mm(2) cross-sectional area at the bonded interface (n = 30). The specimens were subjected to mu TBS testing at a crosshead speed of 0.5 mm/min. The fracture surfaces were examined under SEM to determine the modes of failure. FEA models were constructed using the TOMECH program. A load of 100 N was applied and the stresses at the bonded interfaces were viewed using the FEMSYS visualization software. Results: One-way ANOVA, Tukey's and Kaplan-Meier tests revealed significant differences in the mu TBS for the three designs: 36.7 +/- 8.4, 21.4 +/- 7.9 and 13.8 +/- 4.3 MPa, for the circular, parabolic, and spline hourglass, respectively (p < 0.05). FEA stress distribution data in the adhesive layer showed significant differences among the three designs, which is consistent with the bond strength values. The spline hourglass design showed a three-fold increase in the local stress concentration at its narrowest cross section. Conclusion: The specimen geometry greatly influences the measurement of the mu TBS. The results are consistent with the FEA predictions.