Polymerization kinetics of dental adhesives cured with LED: Correlation between extent of conversion and permeability

Objectives The aim of this study was to analyze the polymerization kinetics of different adhesive films in relation to their permeability after exposure to different LED curing units. Methods One adhesive from each class was analyzed: a three-step etch-and-rinse (OptiBond FL; Sybron-Kerr), a two-step etch-and-rinse (One-Step, Bisco), a two-step self-etch (Clearfil Protect Bond, Kuraray) and a one-step self-etch adhesive (Xeno III; Dentsply DeTrey). Adhesive films were prepared and cured with SmartLite IQ (Dentsply) or L.E.Demetron I (Demetron Kerr) up to complete curing. Polymerization kinetic curves of the tested adhesives were obtained with differential scanning calorimetry (DSC). In particular, total reaction time and extent of polymerization (Ep) at 20, 40 or 60 s were compared. Permeability of the adhesive films was evaluated on flat dentin surfaces of human extracted teeth connected to a permeability device and statistically analyzed. Results Total reaction time differed among the adhesives tested: OptiBond FL < Clearfil Protect Bond < One-Step < Xeno III with both curing units (p < 0.05). At 20 s OptiBond FL showed the highest Ep, while the lowest values were obtained with One-Step and Xeno III (p < 0.05). Ep increased when curing time was prolonged (40 and 60 s) for all adhesives tested (p < 0.05), however, on simplified adhesives, incomplete polymerization took place even after prolonged exposure intervals. An inverse correlation was found between Ep of the adhesives and their permeability using LED curing units. Significance This study supports the hypothesis that, longer curing times than those recommended by the respective manufacturer decrease permeability of the bonded interfaces.