The development of novel orthodontic adhesives containing strontium/calcium phosphate and andrographolide

Objective: The progression of white spot lesions around orthodontic adhesives orbrackets is the common esthetic concern during orthodontic treatment. This could be due to the lack of ion releasing and antibacterial actions of the current resin composite orthodontic adhesives to prevent dental caries. The aim of the current study was to prepare orthodontic resin composite adhesives containing strontium-bioactive glass nanoparticles (SrBGNPs), monocalcium phosphate monohydrate (MCPM), and andrographolide (Andro) to promote ion releasing and inhibitory effect on growth of S. mutans. Effects ofincreasing concentration of Sr/CaP (SrBGNPs + MCPM) and Andro on the physical,mechanical, and the growth of S. mutans of experimental adhesives were examined.Materials and methods: The liquid phase contained light-activated methacrylate monomers. The powder phase contained silanated boroaluminosilicate glass, Sr/CaP (SrBGNPs + MCPM)(5 or 10 wt%), and Andro (5 or 10 wt%). The control formulationcontained 0 wt% of additives. The powder and liquid phases were mixed with a powderto liquid mass ratio of 4:1. The degree of monomer conversion was assessed using FTIRATR (n = 5). Water sorption / solubility after immersion in deionized water for 4 weeks was determined (n = 6). Biaxial flexural strength (BFS) / modulus (BFM) at 24 h was assessed using a mechanical testing frame (n = 7). Enamel shear bond strength (SBS) andadhesive remnant index (ARI) were determined using the SBS testing jig (n = 5). The mineral precipitation in the materials was examined under SEM-EDX (n = 2). Ion release was assessed using ICP-OES (n = 3). The inhibition of S. mutans growth was determinedby assessing the colony-forming unit (Log CFU/mL, n = 3). The commercial material (Transbond XT, Trans) was used as a comparison.Results: The degree of monomer conversions of experimental adhesives (46 – 62 %)were higher than that of Trans (38 %). The additives reduced degree of monomerconversion, but the effect of increasing levels of Sr/CaP and Andro on the conversion was negligible. Rising the level of Sr/CaP increased water sorption whilst rising the level ofAndro increased the water solubility of the materials. The additives reduced the BFS andBFM of the materials. The increase of Sr/CaP level showed minimal effect on the strengthof materials. The addition of Sr/CaP enabled the precipitation of calcium phosphate apatite and the release of Ca, P, and Sr ions. Rising Sr/CaP level reduced log CFU/mL ofS. mutans whilst the rising of Andro did not reduce the growth of S. mutans.Conclusions: The additives reduced the mechanical properties of the experimental orthodontic adhesives. The increase of Sr/CaP level promoted desirable anti-caries actions such as ion release and inhibition of S. mutans growth. The increase of Androlevel, however, reduced mechanical properties of the materials with no inhibition effect on the growth of S. mutans.