Objectives: The aim of this study was to evaluate the microtensile bond strength of ten adhesives to dentine and enamel.
Methods: After grinding sixty freshly extracted caries free human molars an adhesive was applied, according to the manufacturer's instructions, to the dentin or the enamel surface. The adhesive layer was light cured and Z250 composite was applied in 3 successive layers of 2 mm, which were each light cured for 20 s. The specimens were stored in water at 37°C for 24 h until sectioning. Whereafter the specimens were cut into rectangular microbars (1 mm2). From the obtained microbars 10 specimens per tooth were randomly selected to evaluate their microtensile bond strength in a universal testing machine. The obtained strength values were analysed with ANOVA with random effects and Weibull survival analysis. The characteristic strength (h), the Weibull modulus (b), and L5, which refers to the bond strength at which 5% will fail, were calculated.
Results: A significant Spearman correlation for dentine between L5 (r=-0.782; P=0.005) and the type of bonding; (1) three-step total-etch, (2) two-step total-etch, (3) two-step self etch, and (4) one-step self etch, was found. The same significant correlation was found between L5 and the type of bonding for the enamel specimens (r =-0.625; P=0.048). In contrast, no significant correlation between the average microtensile bond strength and the type of bonding was found. Also pre-failures of specimens during manipulation coincided with the lowest L5 values, while the mean microtensile bond strength of these adhesives was not necessary the lowest value.
Conclusion: Understanding the laboratory measurements of the failure of adhesive systems is a prerequisite for eventual extrapolation of the obtained results to the clinical situation. Based on these data one should consider to include L5 as parameter in the description of the failure of dental adhesives.