Degradation of experimental composites containing calcium orthophosphate particles in different immersion media

The aim of the present study was to evaluate the effect of prolonged immersion in different media on the mechanical properties of composites containing dicalcium phosphate dihydrate particles (DCPD, CaHPO₄.2H₂O). Three formulations with the same resin phase (BisGMA/TEGMA, 1:1 in moles) were prepared. The total inorganic content was 50 vol%, consisting of barium glass (BG) or a mixture of BG and DCPD (15:35 or 35:15 vol%). The degree of conversion (DC) of the composites was determined using FTIR spectroscopy (n = 3). Biaxial flexural strength (BFS) and flexural modulus (FM) were determined using the "piston-on-three-spheres" method (n = 13) after 24 h and 6 months in deionized water (DW), citric acid (CA, pH 5) or 75 % ethanol solution (EtOH). Knoop microhardness (KHN, n = 6) was determined in fragments of the flexural test specimens. Data were analysed using ANOVA/Tukey's test (alpha: 5 %). DC was statistically higher for the 35 % DCPD composite (p < 0.001). Overall, mechanical properties were inversely related to DCPD fraction in all three media and both storage times. The 15 % DCPD composite had BFS statistically similar to the control in most of the tested conditions, and similar FM after 24h in all media. However, CA had a severe negative effect on the KHN of composites containing DCPD both after 24h and 6 months, while EtOH caused the highest reduction in KHN for the control, in comparison to DW. Based on these results, it is concluded that composites containing DCPD presented higher surface degradation than the control, which may limit their use as bulk restoratives.