Objectives: Polymerization shrinkage stress can impair bonding effectiveness, especially at the bottom of a high C-factor occlusal class-I cavity. The purpose of this study was 1) to evaluate the effect of a low-shrinking composite on the bonding effectiveness in a high polymerization shrinkage stress environment and 2) to evaluate the effect of thermo-cycling on these pre-stressed interfaces.
Methods: A new low-shrinking composite (els, Saremco) was bonded into standardized occlusal class-I cavities (4.5x4.5x2.5 mm) using a 3-step etch-and-rinse adhesive (cmf, Saremco). A 2-step etch-and-rinse adhesive (XP Bond, Dentsply) and a conventional composite (Z100, 3M ESPE) served as control. The restored teeth were subjected to 20,000 thermo-cycles or 3 weeks of water storage. Then, the teeth were sectioned to 1x1 mm sticks by an automated, water-cooled diamond saw (Accutom, Struers), of which the 4 central sticks were further processed for micro-tensile bond strength (μTBS) testing (5 teeth per group were used).
Results: 3-way ANOVA revealed a significant effect for the adhesive and composite (both p<0.0001), but no effect for thermo-cycling (p=0.994).
mean ± SD (ptf/n) | control | 20,000 thermo-cycles |
cmf/els | 26.2 ± 9.2 MPaa,b (0/20) | 27.5 ± 9.2 MPaa (0/20) |
cmf/Z100 | 16.3 ± 16.9 MPab,c,d (7/20) | 20.3 ± 13.6 MPaa,b,c (1/20) |
XP Bond/els | 13.7 ± 12.1 MPac,d (3/20) | 8.3± 10.5 MPad,e (9/20) |
XP Bond/Z100 | 0.0 ± 0.0 MPae (20/20) | 0.1± 0.4 MPae (19/20) |
ptf = pre-testing failure = 0 MPa. Means with the same superscript are not statistically different (Turkey multiple comparisons, p<0.05).
Conclusion: The low-shrinking composite in combination with the 3-step etch-and-rinse adhesive provided the best bonding performance, even after thermo-cycling. The 2-step etch-and-rinse adhesive suffered strongly from polymerization shrinkage stress, which could be partially restored by using the low-shrinking composite.