Determining the temporal development of dentin-composite bond strength during curing

• A new method is presented for measuring the rate of tooth-composite bond formation.
• The peak rate of bond formation is ∼0.6 s−1.
• Rule of reciprocity does not hold; >2 J/cm2 is required to form adequate bonding.
• The adhesive-dentin interface is the weakest in the fully formed bond.
• The current results will help determine the likelihood of interfacial debonding.

ObjectivesAs composite restorations cure a competition develops between bond formation and shrinkage stress at the composite-dentin interface. Thus, understanding the temporal development of tooth-composite bond strength should enable better assessment of tooth-composite debonding.MethodsIn this study, bond strengths of composite-dentin specimens obtained from tensile test at different curing times were used to determine the bond formation rate. By varying the composite thickness and output from the curing light, their effects on the rate of bond formation for two different materials (a conventional and a bulk-fill composite) were also investigated. The proportions of cohesive and adhesive failure were determined by analysis of electron micrographs of the fractured surfaces.ResultsThe development of dentin-composite bond strength (S) with time (t) can be described by the equation: S = Smax(1 − exp(−αt)), where Smax is the final bond strength (∼12 MPa for both composites) and α the rate of bond formation. Using bulk-fill and thinner specimens gave faster bond formation. In fact, the higher the irradiance at the interface, the higher the rate of bond formation. However, α had a maximum value of ∼0.6 s−1 and the rule of reciprocity did not hold. A minimum dose of ∼2 J/cm2 was required to achieve adequate bond strength. The predominant failure mode changed from cohesive in the composite and adhesive to interfacial at the adhesive-dentin interface, indicating the latter to be the weakest link in the cured dentin-composite assemblies considered.SignificanceWhen combined with the temporal development of shrinkage stress, the current results will help determine the likelihood of tooth-composite debonding.

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