Novel dental nanocomposites and bonding agents with antibacterial and remineralizing capabilities

By Hockin H. K. XuExtensive studies have been undertaken to improve dental composites with advances in filler compositions and resin chemistry. The property enhancements have enabled composites to be increasingly used as esthetic filling materials. Indeed, recent data showed that composite restorations represented the highest percentage among three categories: 77.3 million restorations (46.6%) were composites, 52.5 million (31.6%) were amalgams, and 36.2 million (21.8%) were crowns, totaling 166 million restorations placed in the USA in 2005. However, a major shortcoming of composites is that they accumulate more biofilms and plaques in vivo, compared to other restorative materials. The acid production by biofilms can lead to secondary caries especially at the tooth-restoration margins. Recurrent caries is the main reason for restoration failure, and replacement of failed restorations accounts for 50% to 70% of all restorative dentistry. Replacement dentistry costs $5 billion in the United States annually. To address this problem, the Biomaterials and Tissue Engineering Division at the Department of Endodontics, Prosthodontics and Operative Dentistry of the University of Maryland School of Dentistry has been developing novel nanocomposites. The nanocomposites possess antibacterial and remineralizing capabilities. Such a combination of capabilities is highly beneficial to inhibit caries, but is unavailable in any current restorative materials. Nanocomposite-containing nanoparticles of amorphous calcium phosphate (NACP), nanoparticles of silver (NAg), and quaternary ammonium dimethacrylate (QADM) had strong antibacterial capabilities that were maintained in a 180-day water-aging experiment. Mechanical strength and elastic modulus of nanocomposite after 180-day water-immersion matched those of commercial control composites without antibacterial properties. Incorporation of QADM into NACP nanocomposite greatly reduced biofilm viability, metabolic activity, colony-forming unit counts (CFU), and lactic acid production. The antibacterial results were not significantly different after water-aging for 1, 30, 90, and 180 days. The durable antibacterial properties, plus the calcium (Ca) and phosphate (P) ion release and acid neutralization properties, indicate that the novel nanocomposite may be useful in restorations to inhibit secondary caries.Besides composites, it is also highly desirable for bonding agents to be antibacterial to inhibit recurrent caries at the tooth-composite margins. The bonding agent adheres the composite restoration to the tooth structure. Residual bacterial in the tooth cavity can lead to caries and pulp damage. Dentin primer has direct contact with tooth structure, hence it would be useful to use antibacterial primers to kill residual bacteria in the dentinal tubules in the prepared tooth cavity. We have developed new primer containing QADM and nanoparticles of silver (NAg). The new primer in the uncured state increased the dental plaque microcosm bacterial inhibition zone ninefold, compared to commercial control primer. The cured QADM-NAg-containing primer reduced biofilm CFU and lactic acid production by an order of magnitude, compared to control. These strong antibacterial properties were achieved without compromising dentin bond strength. Furthermore, the method of combining dual antibacterial agents (QADM and NAg) in the same primer yielded the strongest antibacterial properties. Our results showed that: (1) QADM-NAg-containing primer did not decrease dentin bond strength; (2) QADM-NAg-containing primer had much larger bacteria inhibition zones, and greatly reduced biofilm viability, CFU and lactic acid, compared to control; (3) Adding QADM and NAg together in primer produced the strongest antibacterial effects. QADM and NAg could have wide applications in other bonding systems. The novel antibacterial QADM-NAg-containing primer may be promising to inhibit biofilms and secondary caries.Besides primer, we have also incorporated NACP and NAg into adhesive resin for the first time, yielding a potent antibacterial activity while maintaining a strong dentin bond strength. The rationale for adding NACP and NAg was to combine the Ca-P ions and remineralization capability with the antibacterial activity from NAg. NACP could have high levels of Ca and P ion release to remineralize tooth lesions. The antibacterial effect was provided by NAg, which reduced the CFU counts for total microorganisms, total Streptococci, and mutans Streptococci by an order of magnitude. Metabolic activity and lactic acid production of oral biofilms were also greatly reduced. The antibacterial and NACP-containing adhesive may help inhibit residual bacteria in the tooth cavity, hinder the invading bacteria along the tooth-restoration margins, and remineralize tooth lesions. The novel method of incorporating a remineralizing agent (NACP) and an antibacterial agent (NAg) together in the same adhesive may have wide applicability to other bonding systems.Please contact the author for references and additional information.