الفهرس 
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Abstract
The aim of this study is to assess the cleaning and shaping efficiency of rotary RaCe Nickel-Titanium system using two different techniques regarding apical transportation, canal centering ability, and amount of debris extruded apically.
A total of forty eight human permanent mandibular molars were used in the study. Occlusal surfaces were reduced and flattened to standardize working length. Molars were hemi-sectioned getting rid from the distal root.Patency was insured in mesiobuccal canals and a glide path was obtained by using Scout-RaCe files before instrumentation.Samples were divided into two groups (24 each) according to the technique of preparation used. In group A crown-down instrumentation technique was used while in group B step-back technique was used.
Apical transportation was assessed by the method described by Cimis et al (34). Pre-instrumentation radiographs was taken by K-file #15 while post-instrumentation radiographs by the master apical file, radiographs were taken by constant 90-degrees angle of the central beam to the film using a paralleling device.
All radiographs were scanned and printed with the same enlargement and resolution. Canal centering ability was assessed by a technique developed by Gambill et al (1) which measures the distance from the edge of the un-instrumented canal to the periphery of the root (mesial and distal) on Pre- and Post-instrumentation radiographs obtained by computerized tomography. Apical extrusion of debris was assessed by placing samples in an assembly described by Myers and Montgomery (60) to receive the apically extruded debris during root canal enlargement. A microbalance will be used to weigh the extruded debris after total dryness.Results of this study showed that both groups caused apical transportation. Group A resulted in less apical transportation as compared to group B. However the difference between them wasn’t statistically significant. Group A showed better centering ability at the apical and middle sections of the canal while group B showed better centering in coronal section, however the differences between groups weren’t statistically significant. Group A extruded more debris apically as compared to group B. However the difference between them wasn’t statistically significant.