Fracture toughness versus micro-tensile bond strength testing of adhesive-dentin interfaces

Objective. To assess interfacial fracture toughness of different adhesive approaches and compare to a standard micro-tensile bond-strength (mu TBS) test. Methods. Chevron-notched beam fracture toughness (CNB) was measured following a modified ISO 24370 standard. Composite bars with dimensions of 3.0 x 4.0 x 25 mm were prepared, with the adhesive-dentin interface in the middle. At the adhesive-dentin interface, a chevron notch was prepared using a 0.15 mm thin diamond blade mounted in a water-cooled diamond saw. Each specimen was loaded until failure in a 4-point bend test setup and the fracture toughness was calculated according to the ISO specifications. Similarly, adhesive-dentin micro-specimens (1.0 x 1.0 x 8-10 mm) were stressed in tensile until failure to determine the mu TBS. Results. A positive correlation (r(2) = 0.64) was observed between CNB and mu TBS, which however was only nearly statistically significant, mainly due to the dissimilar outcome of Scotchbond Universal (3M ESPE). While few mu TBS specimens failed at the adhesive-dentin interface, almost all CNB specimens failed interfacially at the notch tip. Weibull moduli for interfacial fracture toughness were much higher than for mu TBS (3.8-11.5 versus 2.7-4.8, respectively), especially relevant with regard to early failures. Significance. Although the ranking of the adhesives on their bonding effectiveness tested using CNB and mu TBS corresponded well, the outcome of CNB appeared more reliable and less variable. Fracture toughness measurement is however more laborious and requires specific equipment. The mu TBS nevertheless appeared to remain a valid method to assess bonding effectiveness in a versatile way. (C) 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.