Improved properties of amorphous calcium phosphate fillers in remineralizing resin composites

Abstract

Objectives. The rationale for this study was based on the hypothesis that the mechanical strength of methacrylate composites containing the bioactive filler, amorphous calcium phosphate, can be enhanced by synthesizing this filler in the presence of glass-forming agents. Specifically, this study was conducted to prepare composites with zirconia- and silica-modified amorphous calcium phosphate fillers, and to determine whether the remineralization potential from the release of calcium and phosphate ions and the mechanical properties of the corresponding methacrylate composites were enhanced. Methods. The modified amorphous calcium phosphates were synthesized at pH 10.5 by mixing 800 mmol/L Ca(NO 3) 2 solutions and either 250 mmol/L zirconyl chloride (ZrOCl 2) or 4.4 mol/L tetraethoxysilane (TEOS) solutions with solutions containing 525 mmol/L Na 2HPO 4 and 11 mmol/L Na 4P 2O 7. After washing and drying, the amorphous calcium phosphates were mixed with visible light-activated resins and photopolymerized to form composite disks that were then examined for their ability to release Ca 2+ and total ionic phosphate (PO 4 3− + HPO 4 2− + H 2PO 4 −, hereafter indicated as PO 4) by immersion in HEPES-buffered (pH 7.4) saline at 37°C. Solution ion concentrations were compared at regular intervals up to 265 h. Biaxial flexural strengths of the composites before and after immersion were compared, and significant differences were established by Student's test (p < 0.05). Results. Both ZrOCl 2- and TEOS-modified amorphous calcium phosphate composite disks released Ca 2+ and PO 4 ions at sustained levels requisite for remineralization to occur. The transformation of amorphous calcium phosphate into hydroxyapatite within the composites was also retarded, particularly in the case of amorphous calcium phosphate modified with ZrOCl 2. Biaxial flexure strength values of composite disks showed that TEOS- and ZrOCl 2-amorphous calcium phosphate-filled composites increased in strength by 33% and 21% before immersion and by 25% and 27% after immersion, respectively, compared to unmodified amorphous calcium phosphate composites (controls). All strength increases except TEOS after immersion were significant (p < 0.05). Significance. Properly modified amorphous calcium phosphate fillers can be used to prepare bioactive composites with enhanced mechanical properties for more demanding dental applications without compromising their remineralizing potential.