Variability of Slot Inclination in Conventional and Self-ligating Brackets

Objectives: The use of straight wire techniques requires reliable appliances in order to obtain proper results. The aim of this study was to evaluate the accuracy of the built-in torque information by comparing the inclination of bracket’s slots to the nominal values stated by the manufacturer.
Methods: In the present study, a total of 620 maxillary right central brackets manufactured by seven companies (Dentsply Gac, American Orthodontics, Rocky Mountain Orthodontics, GC Orthodontics, 3M Unitek and Dentaurum) were selected. The sample comprised 0.018x0.025 inch and 0.022x0.028 inch, metal and ceramic, conventional and self-ligating brackets. Images were obtained with a binocular magnifier. Slot inclination of gingival and occlusal walls was measured at both mesial and distal sides using ImageJ software. Statistical analyses (sign test and compliance test) were performed using XLStat (Addinsoft©) and Ellistat© softwares.
Results: Statistical analysis showed that the slot inclination of 43 to 70% of studied brackets was significantly different from nominal values. In general, gingival inclinations tended to be superior to the nominal value, meaning that incorporated torque was lower than expected. The comparison between gingival and occlusal inclinations displays a significant difference for 35 to 46% of the brackets. Overall, gingival inclination was superior to occlusal inclination, hence exhibiting the divergence of the slot walls.
Comparison between mesial and distal sides showed that up to 27% of the brackets were significantly asymmetrical.
Conclusions: This study shows that a great proportion of measured brackets display torque information inconsistent with the stated value. Such manufacturing inaccuracies can prevent the orthodontist from obtaining the outcomes desired when establishing the prescription. We would expect the producers to improve their manufacturing precision in order to enhance the reproducibility of incorporated information.