Introduction Success of any endodontic treatment depends on strict adherence to

Introduction Success of any endodontic treatment depends on strict adherence to endodontic triad. up to Root Canal Size (RCL) & (RCL C1 mm) respectively; sub group C & D were instrumented using ProTaper Common (PTU) up to RCL and (RCL -1mm) respectively; subgroup E & F were instrumented using ProTaper Next (PTN) up to RCL & (RCL -1 mm) respectively. Stereomicroscopic images of the instrumentation sequence were compared for each tooth. The data was analyzed statistically using descriptive analysis by Phi and Cramers test to find out statistical significance 1233533-04-4 manufacture between the groups. The level of significance was arranged at p< 0.05 using SPSS software. Results Stainless steel hand file group showed most splits followed by ProTaper Common & ProTaper Next though statistically not significant. Samples instrumented up to 1mm in short supply of operating length (RCL-1mm) showed lesser quantity of splits. Summary All organizations showed splits formation, the stainless steel group being the highest. Working 1mm in short supply of apex reduces the incidence of crack formation. Keywords: HES7 Apical root crack, Instrumentation size, Protaper next, Protaper universal, Root canal instrumentation, Stainless steel K file Intro Success of any endodontic treatment depends on stringent adherence to endodontic triad. Preparation of root canal system is recognized as being probably one of the most important stages in the root canal treatment [1]. Therefore, successful endodontic treatment requires predictably formed root canals to facilitate three dimensional obturation. At times we inevitably find yourself damaging the root dentin which becomes Gateway for infections like perforation, zipping, dentinal splits and minute complex fractures, therefore resulting in failure of treatment. The development 1233533-04-4 manufacture of nickelCtitanium alloys for the manufacture of manual tools initially and then rotary endodontic tools, has brought about a total revolution in endodontics over the last few years. These systems differ from each other in the design of trimming blades, body taper, metallurgy, tip configuration. Despite the obvious advantages of these instruments over hand instrumentation, these NiTi have tendency to fracture without any warning [2]. A new NiTi rotary instrument ProTaper Next (PTN) is machined from a wire (termed M-wire) by subjecting it to a proprietary novel thermo- mechanical processing. It is said that this new M-wire instrument has considerably improved flexibility compared to conventional rotary instruments that are machined from super elastic (SE) austenitic NiTi wire C ProTaper Universal (PTU) [3]. Also, till date, not many studies have reported the relationship between instrumentation length and development of apical root 1233533-04-4 manufacture cracks. Aim The aim of the study was to evaluate and compare the incidence of apical root crack formation after root canal preparation with hand and newer rotary files and to evaluate the potential effect of various endodontic instrumentation lengths on crack formation in apical root dentin. Materials and Methods The present in vitro study was carried out in the Department of Conservative Dentistry and Endodontics at JSS Dental College, Mysuru from April 2015 to September 2015 after the approval by the Ethical Committee of the institution. A) Selection of teeth A total of 70 freshly extracted premolars with complete apices and single, straight root and root canal were selected and stored in 0.1% thymol. Pre operative radiographs using RVG was taken to exclude teeth with curved roots and anatomic irregularities. B) Tooth.