FRACTURE STRENGTH OF CERAMIC BRACKETS DURING ARCH WIRE TORSION

This study evaluated the fracture strengths of eight new vintage ceram ic brackets with application of torsional forces. Palatal root torque was applied at the distal side of right maxillary central incisor brac kets with 0.022-inch dots by means of a 0.0215 x 0.027-inch rounded ed ge stainless steel arch wire. A specially designed apparatus that atta ched to an Instron machine was used to test the ceramic brackets. The amount of torque, degress of torsion at failure, and fracture location s were measured. The monocrystalline bracket did not break when the to rquing test was applied; the portion of the wire outside the slot of t he bracket twisted on itself. The mean torquing forces at failure rang ed from 5755.2 gm-mm to 9316.5 gm-mm and could be separated into three statistically different groups. The mean torsional rotation at fractu re ranged from 32.7 degrees to 68.1 degrees for the polycrystalline br ackets. The results suggested that all the brackets studied were suffi ciently strong to withstand the commonly accepted magnitudes of arch w ire torquing forces. The present investigation showed higher angulatio n values for all the brackets than those reported by Holt(2) who used the same apparatus with older style brackets.