Nominal shear or fracture mechanics in the assessment of composite-dentin adhesion?

This study addresses the anticipated problem of discriminating among high-performing dentin adhesives. The simplicity of the nominal shear bond test, despite being heavily criticized, has made it a routine procedure for the determination of bonding efficacy. A fracture mechanics approach has been suggested as a better assessment of bonding efficacy (Versluis et al., 1997). However, experimental complexity is a major limitation. It is hypothesized that a new, simplified interfacial fracture toughness test (Lin, 1994) will evaluate bonding agents differently if compared with the traditional shear bond test. Therefore, the objective of this study was to compare the performances of six dentin bonding agents subjected to the interfacial fracture toughness test (critical plane strain energy release rate) or to the nominal shear bond test (shear bond strength). Their performances were also characterized by scanning electron micrography of the fracture surfaces for evidence of dentin cohesive failure. Statistical analyses showed only marginal differences between these determinants of the two tests. However, when the analysis was applied only to the materials that had 100% frequency of dentin cohesive failure in shear testing, which also had high bonding efficacy, the difference in adhesive strengths between the two tests became significant. The reliability of the nominal shear test is questioned when dentin cohesive failure occurs, which usually is associated with high bonding efficacy. Since it is expected that bonding efficacy will increase further, the interfacial fracture toughness test is the preferred methodology to distinguish among high-performing dentin adhesives.