Degree of conversion of luting resins around ceramic inlays in natural deep cavities: a micro-Raman spectroscopy analysis

This study evaluated the degree of conversion (%DC) of luting agents around ceramic inlays placed in deep natural cavities. Thirty-six cylindrical Class I cavities (diameter = 4 mm, depth = 4 mm) were prepared in freshly extracted human teeth and randomly divided according to the luting materials used for luting CAD/CAM fabricated inlays (Empress CAD). The dual-cure resin cements Clearfil Esthetic Cement and Variolink II Low and the light-cure composites Grandio Flow and Grandio were luted using the total-etch technique. The self-adhesive dual-cure cements RelyX Unicem and Maxcem Elite were used as recommended by the manufacturer. All of the restorations were photo-activated using a quartz halogen unit (Elipar TriLight; 750 mW/cm2) for 40 seconds. After 24 hour dry-storage in the dark, all the teeth were vertically sectioned into two halves (n = 12 per group) using a slow-speed diamond-saw in the bucco-vestibular direction under constant water lubrication to avoid specimen heating. The DC of the luting materials was measured by vibrational spectroscopy using a micro-Raman spectrometer at depths of 1, 3 and 4 mm on each side of the tooth halves (n = 24). Disc-shaped samples were produced for measurement of the maximum %DC of each material. Two-way ANOVA and the Student-Newman-Keuls post-hoc test were used in the statistical analysis (alpha = 0.05). All the materials showed no statistical differences in degree of conversion at all tested depths, except for Grandio Flow and Maxcem Elite. Dual-cure and light-cure luting materials showed polymerization homogeneity around ceramic inlays, although dual-cure conventional resin cements tended to show an overall higher conversion.