The Effect of Rotation Angle of Stainless Steel Files in Step-back Preparation

Abstract

 Purpose: Although step-back preparation (SBP) has been widely used, its scientific basis has not been established. Recently, the outcomes of various root canal preparations have been evaluated by using micro CT, thus improving the scientific basis. The aims of this study were to build an experimental system to evaluate possible factors for optimally preparing root canals by using micro CT and to investigate the effect of rotation angle of stainless steel (SS) files in the SBP.

 Methods: Forty-nine transparent root canal models made with resin templated from a maxillary canine were used in this experiment. The samples were prepared with SBP with five different rotation angles (15°, 30°, 60°, 90° and 180°) by using SS files (n=7). Both the JH Endo system and Ni-Ti rotary system (Reciproc) were used as controls. The 3-D root canal morphology of the samples was photographed both before and after instrumentation with micro CT. Digital images of the root canals from the apical foramen to 4 mm coronal site were superimposed and then evaluated using computer software. The distance of transportation of the prepared root canal and the volume of prepared root canal inner walls were calculated from the data of pre- and post-instrumentation images.

 Results: The distance of transportation of prepared root canals from the apical foramen to 4 mm coronal site was significantly longer in the five SBP groups than the two control groups and tended to be positively correlated with the degree of rotation angle. The volume of prepared root canal inner walls was significantly higher in the five SBP groups than the two controls and tended to be positively correlated with the degree of rotation angle. From the horizontal images, the optimal rotation angles were both 30° and 60° to maintain the original shape of the canal. The preparation time was significantly longer in the five SBP groups than the two controls.

 Conclusions: This study shows the effectiveness of our experimental system to investigate root canal preparation scientifically and that the optimal rotation angle of SS files in SBP is around 30 to 60 degrees.