The effect of polymerization mode on monomer conversion, free radical entrapment, and interaction with hydroxyapatite of commercial self-adhesive cements

• We evaluated the effect of polymerization mode on monomer conversion of self-adhesive cements.
• Spin density and interaction of self-adhesive cements with hydroxyapatite were also evaluated.
• Choice of polymerization protocols affects the conversion of self-adhesive cements.
• Free radical density varies as a function of the materials and of the activation.
• One tested product provided more optimal characteristic than did the others.

ObjectivesThis study evaluated the degree of conversion, the free radical entrapment, and the chemical interaction of self-adhesive resin cements mixed with pure hydroxyapatite, as a function of the polymerization activation mode among a variety of commercial self-adhesive cements.Materials and methodsFour cements (Embrace WetBond, MaxCem Elite, Bifix SE, and RelyX U200) were mixed, combined with hydroxyapatite, dispensed into molds, and distributed into three groups, according to polymerization protocols: IP (photoactivation for 40 s); DP (delayed photoactivation, 10 min self-curing plus 40 s light-activated); and CA (chemical activation, no light exposure). Infrared (IR) spectra were obtained and monomer conversion (%) was calculated by comparing the aliphatic-to-aromatic IR absorption peak ratio before and after polymerization (n=10). The free radical entrapment values of the resin cements were characterized using Electron Paramagnetic Resonance (EPR) and the concentration of spins (number of spins/mass) calculated (n=3). Values were compared using two-way ANOVA and Tukey’s post-hoc test (α=5%). X-ray diffraction (XRD) characterized the crystallinity of hydroxyapatite as a function of the chemical interactions with the resin cements.ResultsThe tested parameters varied as a function of resin cement and polymerization protocol. Embrace WetBond and RelyX U200 demonstrated dependence on photoactivation (immediate or delayed), whereas MaxCem Elite exhibited dependence on the chemical activation mode. Bifix SE presented the best balance based on the parameters analyzed, irrespective of the activation protocol.ConclusionsChoice of polymerization protocol affects the degree of conversion, free radical entrapment, and the chemical interaction between hydroxyapatite and self-adhesive resin cement mixtures.