Compositional boundaries for functional dental composites containing calcium orthophosphate particles

Objectives: To investigate the interrelationships among handling, degree of conversion (DC), mechanical behavior and Ca²⁺ release of composites containing dicalcium phosphate dihydrate (DCPD, CaHPO₄.2H₂O), as a function of total inorganic content and DCPD: glass ratio. Methods: Twenty-one formulations (1 BisGMA: 1 TEGDMA, in mols) with inorganic fractions ranging from zero to 50 vol% and different DCPD: glass ratios were evaluated for viscosity (parallel plate rheometer, n = 3), DC (near-FTIR spectroscopy, n = 3), fracture toughness/K_1C (single-edge notched beam, n = 7–11) and 14-day Ca²⁺ release (inductively coupled plasma optical emission spectroscopy, n = 3). Data were analyzed by ANOVA/Tukey test (except viscosity, where Kruskal-Wallis/Dunn tests were used, α: 0.05). Results: Viscosity and DC increased with DCPD: glass ratio among composites with the same inorganic content (p < 0.001). At inorganic fractions of 40 vol% and 50 vol%, keeping DCPD content at a maximum of 30 vol% did not compromise K_1C. Ca²⁺ release showed an exponential relationship with DCPD mass fraction in the formulation (R² = 0.986). After 14 days, a maximum of 3.8% of the Ca²⁺ mass in the specimen was released. Conclusion: Formulations containing 30 vol% DCPD and 10–20 vol% glass represent the best compromise between viscosity, K_1C and Ca²⁺ release. Materials with 40 vol% DCPD should not be disregarded, bearing in mind that Ca²⁺ release will be maximized at the expense of K_1C.