Purpose: Reciproc nickel-titanium instruments are designed specifically to prepare root canals using one single instrument under reciprocating movement. This study aimed to determine the shaping ability of Reciproc instruments in comparison with twisted file (TF) instruments, with regard to the working time and post-instrumentation canal shape in simulated curved canals. The influence of prior glide path preparation on the shaping ability of Reciproc instruments was also investigated. Methods: Simulated canals with a 30-degree curvature in resin blocks were divided into 4 groups (n=8, each) and prepared using the following instruments: Group 1, Reciproc R25 (25/.08); Group 2, RaCe (10/.04; for glide path preparation) followed by Reciproc R25; Group 3, RaCe (10/.06) followed by Reciproc R25; and Group 4, TF (25/.08). The time required for instrumentation was recorded. After preparation of the canal, the increase in canal width at the inner and outer sides of the curvature was measured by digital image analysis at 1-, 2-, 3-, 4-, 5-, 7-, 9- and 12-mm levels from the apical foramen. Canal centering ratios were then calculated. Data were analyzed by one-way analysis of variance and Bonferroni-Dunn test at the significance level of 5 %. Results: Group 4 exhibited significantly smaller canal width increases at 1- to 4-mm levels of the inner side and significantly larger increases at 7- to 12-mm levels of the outer side, compared with the other groups. Group 4 also showed significantly smaller centering ratios at 1-, 2-, 7-, and 9-mm levels, compared with any of the other groups. Total instrumentation time in Group 1 was significantly longer than that in the other groups. The time taken for Reciproc instrumentation was significantly shorter in Groups 2 and 3 compared with that in Group 1. Conclusion: Under the present experimental conditions, Reciproc instruments, regardless of prior glide path preparation, produced less canal deviation in the apical and coronal regions compared with TF instruments. When the canal preparation was performed with one single file, Reciproc instrumentation required significantly longer time than TF instrumentation. Following prior glide path preparation, however, the time taken for Reciproc instrumentation was significantly reduced and the total instrumentation time was similar to that required for TF instrumentation.
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