Kinetics of polymerization and contraction stress development in self-adhesive resin cements.

Objectives: The aim of the study was to evaluate the contraction stress, microhardness and polymerization kinetics of three self-adhesive cements vs. conventional dual-cure resin cement. Methods: Cements tested were: RelyX Unicem (3M ESPE, St. Paul, MN, USA), MaxCem Elite (Kerr, Orange, CA, USA), Clearfil SA Cement (Kuraray, Tokyo, Japan) and Duolink (Bisco Inc., Schaumburg, IL, USA). Cements were irradiated with a LED-curing unit (bluephase, IvoclarVivadent) for 20 or 40s and the contraction forces (N) generated during polymerization were continuously recorded for 6h with a universal testing machine. Polymerization kinetics were monitored using micro-Raman spectroscopy and degree of conversion was calculated. Vickers microhardness was also recorded. All measurements were performed at 10min and 6h. Data were statistically analyzed by three-way ANOVA with repeated measures and Tukey's post-hoc test (α=0.05). Results: Irrespective of exposure time, stress analysis ranked in the following order: Clearfil SA Cement < MaxCem < RelyX Unicem ≤ Duolink (p<0.05). Stress was correlated with microhardness values (p<0.05). Kinetic curves showed that maximum degree of conversion was attained more quickly than maximum stress after light activation. Significance: The conventional resin-based cement showed higher stress values than the self-adhesive cements. The results were material-dependent and probably correlated to the composition of each material.