Influence of Different Calcium Phosphates on an Experimental Adhesive Resin

Purpose: To formulate adhesive resins with 2 wt% of hydroxyapatite (HAp), α-tricalcium phosphate (α-TCP), or octacalcium phosphate (OCP) and to compare these groups with an unfilled adhesive regarding the degree of conversion, polymerization rate, microshear bond strength and mineral deposition. Materials and Methods: The experimental adhesive resin was formulated mixing 66.6 wt% bisphenol A glycol dimethacrylate (bis-GMA), 33.3 wt% 2-hydroxyethyl methacrylate (HEMA), and a photoinitiator system. OCP, α-TCP, or HAp were added in concentrations of 2 wt% to the adhesive, and a group without additional filler was used as the control. Calcium-phosphate particle sizes were determined using a laser-diffraction particle-size analyzer. The degree of conversion (DC) of the adhesives was determined with FTIR-ATR. The polymerization rate (Rp) was determined using differential scanning calorimetry equipped with a photocalorimetric accessory. Adhesive bonding was evaluated using the microshear bond strength test in sound bovine mandibular incisors. Mineral deposition in human third molars affected by caries was evaluated using micro-Raman spectroscopy after selective removal of carious dentin. Data were analyzed by one-way ANOVA and Tukey's test (α = 0.05). Results: HAp presented the highest mean particle size (26.7 nm), while that of α-TCP was 6.03 nm and OCP was 4.94 nm. The DC of all groups was above 50%. The OCP group showed the fastest Rp, with no difference from the control group (p > 0.05). The α-TCP group presented the highest microshear bond strength (p = 0.005) and mineral deposition at the interface. Conclusion: Incorporation of α-TCP nanofiller into adhesive resins can improve bond strengths and may be a promising strategy to achieve therapeutic remineralization at the composite-dentin interface. Keywords: tooth remineralization, dentin bonding agents, dental caries, hydroxyapatite