In Vitro Effect of Gallium-Doped Bioactive Glass on Enamel Anti-Demineralization and Bond Strength of Orthodontic Resins

White spot lesions (WSL) that occur on teeth after orthodontic appliances have been attached are caused by bacterial demineralization of the enamel surface. This study investigated the anti-demineralization effect of orthodontic resins containing mesoporous bioactive glass nanoparticles (MBN) doped with gallium, which has antibacterial activity, as well as MBN with increased calcium and phosphate contents as these ions can remineralize enamel. Resins (CF, CharmFill Flow, Dentkist, Seoul, South Korea) containing 1%, 3%, and 5% Ga-doped MBN (GaMBN) were characterized using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and isothermal tests, and their physical properties were measured in terms of Vickers microhardness, bracket retention force, and adhesive remnant index (ARI). Cell viability in the resins was confirmed by testing human dental pulp stem cells (hDPSCs), and ion release tests were performed after 1, 7, and 14 days to determine whether the resins released Ga3+, Ca2+, and PO4. After 14 days, antibacterial activity was determined using Streptococcus mutans (S. mutans)—the bacteria that causes tooth decay—and the chemical remineralization effect was investigated using a cycle of acid–base solutions. The microhardness of the resins increased with GaMBN concentration whereas their bracket retention force, ARI, and cell viability remained unchanged. The bacterial activity of the 5%-GaMBN resin decreased after 24 and 48 h; however, the change in activity was not statistically significant. Anti-demineralization testing demonstrated that the degree of enamel demineralization decreased as the GaMBN concentration increased, which indicates that resins containing 5%-GaMBN may be viable orthodontic adhesives for preventing WSLs.