The development of novel orthodontic adhesives containing monocalcium phosphate monohydrate and nisin

Objectives: White spot lesion is the most prevalent aesthetic complication that occurred due toinitial caries around brackets during orthodontic treatment. The aim of the current study was to prepare resin composite-based orthodontic adhesives that contain monocalcium phosphatemonohydrate (MCPM) and nisin as ion releasing and antibacterial agents. The effect of rising MCPM and nisin levels on the degree of monomer conversion (DC), biaxial flexural strength (BFS), water sorption (Wsp), water solubility (Wsl), enamel shear bond strength (SBS), mineral precipitation, ion release, and the growth of S. mutans was examined.Materials and methods: Resin composite-based orthodontic adhesives were prepared. Theadhesives were incorporated with MCPM (5 or 10 wt%) and nisin (5 or 10 wt%). DC was determined using FTIR-ATR (n = 5). BFS of materials after immersion in water for 24 h was assessed using a universal testing machine (n = 8). Wsp and Wsl of the materials after immersionin water for 4 weeks were determined using gravimetric studies. SBS of the materials to enamelwas tested (n = 5). The debonded surface was determined using the adhesive remnant index (ARI).Apatite formation on the debonded surface of orthodontic adhesive was determined using SEMEDX.Ion release (Ca,P) at 4 weeks was determined with ICP-OES (n = 3). The inhibition of S.mutans growth (Log CFU/mL) was assessed using the broth dilution method (n = 3). The commercial orthodontic adhesive (Transbond XT, Trans) was used as the comparison.Results: The additives reduced DC, but the values (39 – 42 %) were comparable to that of Trans (38 %). DC of the experimental materials was reduced by 4% by increasing MCPM level. The experimental adhesives containing MCPM and nisin showed BFS (109 – 145 MPa) significantlylower than Trans (202 MPa). The increase of nisin reduced BFS by 22%. The Wsp and Wsl ofexperimental materials were increased by 13 – 17 % and 23 – 48 % due to the incorporation of the additives. The experimental adhesives showed lower SBS (19 – 29 MPa) than Trans (31 MPa).The addition of MCPM promoted the release of Ca and P ions and the precipitation of mineral apatite on the adhesive interface. The addition of nisin showed no effect on the growth of S.mutans.Conclusions: The experimental adhesives with added MCPM and nisin showed comparable monomer conversion but lower strength than the commercial materials. The addition of MCPM reduced SBS but promoted apatite formation and Ca, P ions release. The increase of nisin showed no inhibition effect on the growth of S. mutans. Although the materials showed no antibacterial effects, the ability to promote ion release and calcium phosphate apatite precipitation wasexpected to enhance remineralization during orthodontic treatment. This could potentially help toprevent the formation of white spot lesions.