Comparison of the static frictional resistance and surface topography of ceramic orthodontic brackets: an in vitro study

Objectives: The aim of this study was to investigate the frictional resistance (FR) and surface topography of newly available polycrystalline alumina (PCA) ceramic brackets characterised by a yttria-stabilised zirconia (YSZ) coating of the slots, compared with monocrystalline alumina (MCA) ceramic brackets and stainless steel (SS) brackets. Methods: The FR was investigated using a universal testing machine. The test groups included PCA (Clarity Advanced, 3M Unitek, CA, USA) and MCA (Inspire Ice, Ormco, CA, USA). The control group included SS brackets. A sliding test was performed for each bracket type with three bracket-wire angulations (0 degrees, 5 degrees, 1.0 degrees). A total of 225 sliding tests were performed in a dry environment, and 225 tests were performed in a wet environment of artificial saliva. A scanning electron microscope was used for qualitative assessments. The surface topography of the bracket slots was quantitatively assessed using an optical profilometer. Results: In the dry environment, the overall FR values were significantly lower for PCA and SS brackets compared with MCA brackets (p < 0.001), but no significant difference was found between PCA and SS brackets. In the wet environment, there were no significant differences between the bracket groups and their overall FR values. There was a significant correlation between the overall FR and the bracket-wire angulation values (p < 0.001). The bracket slot surface topography revealed that the PCA bracket slots had the highest roughness values, followed by SS and MCA brackets (p < 0.001). There was no significant correlation between the roughness values of the bracket slots and the FR in a passive configuration for all bracket types. Conclusion: A yttria-stabilised zirconium coating of the PCA ceramic bracket slots might be a positive approach to apply for the reduction of FR.