Objective : RMGIs have been recognized as self-adhesive materials with long-term clinical adhesion even though their measured bond strengths are low. The objective of this paper was to study the role of the dual curing mechanism of RMGI liners on bond formation. Spectroscopic studies, rate of modulus build-up and stress development were investigated.
Materials and methods : Polymerization shrinkage strain was measured via deflecting disc method for 2 mm thickness of 3M™ ESPE™ Filtek™ Supreme Plus composite alone, and 2 mm of composite over 0.5 mm of each test liner ( n = 5); percent strain reduction was calculated. Flexural modulus (FM) was measured 1 h post-cure per ISO 9917-2. Shear bond strength (SBS) was measured on bovine dentin immediately after bonding, 1d, and 6mo ( n = 5). After etching the cured liner with H3PO4 3M™ Single Bond™ Plus was applied and cured followed by a cylinder of composite. XPS spectra were collected on hydroxyapatite (HAP) disks before and after treatment with VBP-liquid, and on the liquid itself, from which elemental concentrations were calculated. FTIR spectra were collected for VBP-liquid, HAP powder, a liquid-HAP mixture and for mixed VBP liner, periodically during 24 h after light-curing. Data were analyzed via one-way ANOVA and Tukey’s t -test ( p < 0.05). Pearson’s correlation coefficient (PCC) was calculated for FM versus % stress reduction ( p < 0.05).
Results : SBS, FM and strain reduction are shown in Table 1 . A PCC of −0.820 ( p = 0.046) shows that strain reduction strongly correlated with post-cure FM. FTIR showed immediate decrease in methacrylate (1295, 1325 cm −1 ) and progressive decrease of carboxylic acid (∼1570 cm −1 ) with concomitant increase in carboxylate (∼1720 cm −1 ). XPS spectra of treated HAP showed broadening of the O–C O (289 eV) peak of the liquid and slight shift for the treated HAP, indicating an ionic bond. Elevated carbon and nitrogen on treated HAP indicate that the methacrylate-modified polyalkenoic acid in the VBP-liquid adhered to the HAP.