Amorphous calcium phosphate composites in restorative dentistry: Physical, chemical and mechanical properties

Through its incorporation and release from polymeric matrices in the oral environment, amorphous calcium phosphate (ACP) has shown the potential to support the remineralization of tooth enamel. However, the highly agglomerative nature of as-made ACP filler particles prevents a simple optimization of the physical, chemical and mechanical properties of composites in which they are incorporated. In this study, as-made and milled ACP composites were compared in a number of physicochemical and mechanical categories. In addition, a test fixture was developed that allows the cyclic loading of specimens previously tested only monotonically, providing a more clinically relevant method of characterizing the composite-adhesive-dentin bond. A preliminary investigation was performed to characterize the fatigue response of bonded TPH composites.;Elimination of the large agglomerates in as-made ACP allowed for a 50% higher fill level in milled ACP composites, with a corresponding 50% reduction in shrinkage stress over as-made ACP composites. The mean biaxial flexure strength of milled ACP did not significantly deteriorate after 1 month of saline immersion (p<0.001), and were significantly stronger than as-made ACP composites after 3 months (p<0.001). As-made ACP composites absorbed significantly more water (p<0.001), and released significantly higher amounts of calcium and phosphate than milled ACP composites after 3 months immersion (p≤0.025). In an evaluation of shear bond strength, milled ACP composites produced a higher incidence of the failure mode consistent with stronger adhesion, and exhibited a mechanical response within aqueous immersion more akin to glass-reinforced composites than to those containing as-made ACP. The bonded TPH composites tested in fatigue did not exhibit an obvious endurance strength. Instead, the shear bond fatigue strength test method was seen to provide an indirect measure of the flaws present in the adhesive layer and the tooth structure.