Amorphous Calcium Phosphate-Resin Nanocomposite: Thermal-Cycling, Three-Body Wear, and Two-Year Water-Aging

Half of all dental restorations fail within 10 years, with secondary caries and restoration fracture being the main reasons.  Calcium phosphate (CaP) composites can release Ca and PO₄ ions and remineralize tooth lesions.  However, there has been no report on their long-term mechanical durability. 

Objectives: To investigate the wear, thermal-cycling, and water-aging of composites containing amorphous calcium phosphate nanoparticles (NACP). 

Methods: NACP of 112-nm and glass particles were used to fabricate four composites with fillers of: (1) 0% NACP+75% glass; (2) 10% NACP+65% glass; (3) 15% NACP+60% glass; and (4) 20% NACP+50% glass.  Thermal-cycling was performed at 5ºC and 60ºC water baths for 10⁵ cycles.  Wear was performed for 4x10⁵ cycles.  Water-aging lasted for two years. 

Results: After thermal-cycling, flexural strength (mean±sd; n=6) of nanocomposite with 20% NACP was (89±13)MPa, slightly higher than (80±14)MPa of a commercial composite control (p>0.1), and much higher than (32±2)MPa of resin-modified glass ionomer (p<0.05).  Wear depth increased with increasing NACP filler level.  Wear depths of NACP nanocomposites were within the range for commercial controls.  After 2 years of water-aging, flexural strengths were (64±10)MPa for composite with 0% NACP, (61±8)MPa for 10% NACP, (50±8)MPa for 15% NACP, (46±9)MPa for 20% NACP, and (36±12)MPa for the commercial composite control.  They were all much higher than (15±2)MPa for resin-modified glass ionomer (p<0.05).  The mechanism of strength loss for resin-modified glass ionomer was identified as microcracking and air-bubbles.  NACP nanocomposites and control composite were generally free of microcracks and air-bubbles.

Conclusions: Combining NACP nanoparticles with reinforcement glass-particles resulted in novel nanocomposites with long-term mechanical properties higher than those of commercial controls, and wear within the range of commercial controls.  These strong long-term properties, plus the Ca-PO₄ release and acid-neutralization capability reported earlier, suggest that the new NACP nanocomposites may be promising for stress-bearing and caries-inhibiting restorations.