Variety of calcium phosphates (CaPs), such as HAP [16–18], amorphous calcium
phosphates (ACP) [19–24], tetracalcium phosphate (TTCP) [25] and dicalcium phosphate anhydrous (DCPA) [26–28] have been studied as fillers to make mineral releasing dental composites. Skrtic et al. conducted pioneering research to investigate the physicochemical properties of dental composites containing unhybridized and hybridized amorphous calcium phosphates (ACP) [19–24]. Their research demonstrated that hybridization of ACP fillers using agents, such as tetraethoxysilane (TEOS) or ZrOCl2 solution, improved the mechanical properties, e.g. biaxial flexural strength, of the composites containing ACP fillers. However, the addition of both hybridized and unhybridized ACP fillers generally
degraded the biaxial flexural strength of the resin materials [22,23,29]. It was hypothesized that the strength degradation compared to unfilled resin is attributed to poor dispersion and insufficient interaction between ACP and resin. Such hypothesis has been supported by mechanical testing of dental composites containing particles with different sizes [29]. Both nanosized and microsized HAP particles were also studied as dental fillers and the mechanical tests indicated that microsized instead of nanosized HAP was favored in terms of mechanical properties [16–18]. Xu et al. reported that 20% DCPA (dicalcium phosphate
anhydrous) nanoparticles with silicon carbide (SiC) whiskers of average 0.9 μm in diameterand 14 μm in length improved the flexural strength of associated dental composites from 103 ± 32 MPa to 167 ± 23 MPa [27]. In summary, except for the positive results from Xu’s work where the reinforcement is most likely attributed to silicon carbide whiskers, most of other dental composites using CaP based nanoparticles did not have improved mechanical properties.