Ceramic bracket fracture resistance to second order arch wire activations

The fracture resistance of ceramic brackets to orthodontic activations has been incorrectly estimated by previous investigations that have reported second order loads in terms of force magnitudes rather than moments. Because force magnitudes alone do not reflect the influence of distance from the site of force application on total load, it is impossible to apply previous results to actual clinical situations. The purpose of this study was to determine the average moments (in gram-millimeters) necessary to fracture various ceramic brackets subjected to second order tipping activations and compare them with actual clinical loads. Central and lateral incisor ceramic brackets from seven manufacturers were subjected to mesial-distal tipping arch wire activations at two speeds of load application with a testing apparatus designed for that purpose. Significant differences in fracturability among the brackets of various manufacturers and between central and lateral incisor brackets were found. There were no differences related to the speed of load application. Once the influence of bracket width was considered, the differences in fracture resistance between central and lateral incisor brackets were no longer apparent. Second order activations required to fracture the ceramic brackets in this study were all much greater than measured clinical orthodontic loads. It is unlikely that second order arch wire activations are a significant cause of ceramic bracket failure.