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°, 45°, and 60° 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° and 45° 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.