Antibacterial and remineralizing nanocomposite inhibit root caries biofilms and protect root dentin hardness at the margins

Objectives: Senior patients have a high incidence of tooth root caries. The objectives of this study were to: (1) develop a bioactive composite with calcium (Ca) and phosphate (P) ion-release and antibacterial capabilities via nanoparticles of amorphous calcium phosphate (NACP) and dimethylaminohexadecyl methacrylate (DMAHDM); (2) inhibit root biofilms of Streptococcus mutans, Lactobacillus acidophilus and Candida albicans in a biofilm-based recurrent root caries model to protect root dentin hardness under biofilms for the first time. Methods: Five groups were tested: (1) Heliomolar nanocomposite (Commercial control); (2) Experimental composite control (0% NACP, 0% DMAHDM); (3) Remineralizing composite (30% NACP); (4) Antibacterial composite (3% DMAHDM); (5) Remineralizing and antibacterial composite (NACP + DMAHDM). Colony-forming units (CFU), lactic acid and polysaccharide of biofilms were evaluated. Demineralization of bovine root dentin with restorations was induced via multi-species biofilms, and root dentin hardness was measured. Results: Adding NACP and DMAHDM into composite did not compromise the mechanical properties (p > 0.05). Biofilm lactic acid, polysaccharides and CFU were greatly reduced via DMAHDM (p < 0.05). Ca and P ion releases were substantially increased at cariogenic low pH. With multi-species biofilm acid attack, root dentin hardness (GPa) decreased to 0.12 ± 0.03 for Commercial control, and 0.11 ± 0.03 for Experimental control. Root dentin hardness was 0.20 ± 0.04 for NACP group, 0.21 ± 0.04 for DMAHDM group, and 0.30 ± 0.03 for NACP + DMAHDM group which was more than 2-fold that of control groups (p < 0.05). Conclusions: The novel NACP + DMAHDM nanocomposite had strong antibacterial effects and Ca and P ion release. When tested in a multi-species recurrent root caries model, NACP + DMAHDM nanocomposite substantially reduced root dentin demineralization and protected dentin hardness around the restorations under biofilms. Therefore, this novel bioactive composite is promising to inhibit root caries and protect tooth structures.