CERAMIC BRACKET FRACTURE-RESISTANCE TO 2ND-ORDER ARCH WIRE ACTIVATIONS

The fracture resistance of ceramic brackets to orthodontic activations has been incorrectly estimated by previous investigations that have r eported second order loads in terms of force magnitudes rather than mo ments. Because force magnitudes alone do not reflect the influence of distance from the site of force application on total load, it is impos sible to apply previous results to actual clinical situations. The pur pose of this study was to determine the average moments (in gram-milli meters) necessary to fracture various ceramic brackets subjected to se cond order tipping activations and compare them with actual clinical l oads. Central and lateral incisor ceramic brackets from seven manufact urers 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 brac kets 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 in cisor brackets were no longer apparent. Second order activations requi red to fracture the ceramic brackets in this study were all much great er than measured clinical orthodontic loads. It is unlikely that secon d order arch wire activations are a significant cause of ceramic brack et failure.