Comparison of the time required to create secondary fracture of separated file fragments by using ultrasonic vibration under various canal conditions.

INTRODUCTION: Nickel-titanium files often separate because of mechanical fatigue. The purpose of this study was to determine safe preparation techniques for separated file removal by using ultrasonics. METHODS: Fifty nickel-titanium file fragments were divided into 5 groups. An ultrasonic tip was activated on a file fragment positioned between dentin blocks simulating several canal conditions: Group 1 consisted of the fragment protruding from a pair of straight dentin blocks. For group 2, the fragment was also positioned between 2 straight dentin blocks except one block was positioned 1 mm more apically than the other block, simulating a troughed area that is often created during file removal attempts. For groups 3-5, the fragment was positioned similarly as group 2 but between blocks with 30 degrees , 45 degrees , and 60 degrees curvatures, respectively. The time it took for secondary fracture to occur was recorded, and the data were statistically analyzed. RESULTS: Fragments with dentin wall supporting on the opposite side of ultrasonic activation site resisted fracture significantly longer than those without it. Fragments in 30 degrees and 45 degrees curved blocks took significantly longer to fracture than the other groups (Fisher protected least significant difference, P < .05). CONCLUSIONS: Secondary fracture of separated files appeared to be reduced when the ultrasonic tip was applied to the inner curvature of the canal.