LAUSR

Objectives The aim of the present study was to determine the effect of taper (.08, .06, and .04) of separated K3XF instruments on duration taken for the secondary fracture formation during ultrasonic activation. Materials and methods Ten 25/.08 K3XF (SybronEndo, Orange, CA, USA), ten 25/.06 K3XF, and ten 25/.04 K3XF instruments were used for the study. The apical 5 mm of the instruments was cut to simulate the fragments in root canals. Fragments of the instruments were sandwiched between two straight dentin blocks. An ultrasonic tip was used to cause a secondary fracture of the fragment. The time needed for the secondary fracture was recorded for each instrument. The data were statistically analyzed using the Kruskal-Wallis H test (alpha = 0.05). Results Secondary fractures occurred in all instruments. In the .08 taper group, secondary fractures took longer than in the case of the .06 and the .04 taper groups ( P  < 0.05). There were no significant differences between the .06 and the .04 taper groups in terms of the time required for the occurrence of a secondary fracture ( P  > 0.05). Conclusions In the .08 taper group, secondary fracture took longer time than in the case of the .06 and the .04 taper groups due to its larger cross-sectional area involved. Clinical relevance Typically, when removing separated instruments, a much lower power setting is chosen. The purpose of this in vitro study was to determine which tapered files were more resilient to secondary fracture, thus allowing a higher power setting to be chosen. Thus, the results of the present study cannot be used in clinical practice. If the clinician knows the taper of the broken file, the clinician should be very careful with regard to secondary fractures when using ultrasonics to remove the separated smaller tapered instruments.