Evaluation of the Effect of Nanoparticles Coating (ZnO) on the Frictional Resistance of Self-ligating and Conventional Ceramic Brackets: An In Vitro Study

Objective This study investigated the influence of zinc oxide (ZnO) nanoparticle coating on the frictional resistance of conventional and self-ligating ceramic orthodontic brackets. Methods A total of 120 ceramic brackets (60 conventional, 60 self-ligating) were allocated equally into six groups (n = 20). Groups included ZnO-coated and uncoated brackets for both bracket types, with and without artificial saliva. ZnO coating was verified using scanning electron microscopy. Brackets were bonded to acrylic blocks with standardized curvature, and 0.019 × 0.025-inch stainless steel archwires were engaged. Frictional resistance was measured using a universal testing machine, and data were statistically analyzed at a significance level of p ≤ 0.05. Results A significant interaction was found between bracket type and coating condition regarding maximum load (F = 67.722, p < 0.001). Conventional brackets demonstrated greater maximum load than self-ligating brackets in both the uncoated (mean difference = 1.36, p < 0.001) and coated groups (mean difference = 0.46, p < 0.001). No significant difference was observed between bracket types in artificial saliva (p = 0.427). Among self-ligating brackets, ZnO coating significantly increased frictional resistance compared to uncoated (mean difference = 0.353, p < 0.001) and saliva conditions (mean difference = 0.488, p < 0.001). In contrast, ZnO coating reduced frictional resistance in conventional brackets. Conclusion ZnO nanoparticle coating exerts opposite effects on bracket systems: it increases friction in self-ligating ceramic brackets but reduces friction in conventional ceramic brackets. The presence of artificial saliva mitigates frictional forces in both bracket designs.