In view of the wide variety of mechanized endodontic instrumentation systems, the ideal would be that the choose of this system based on the anatomical characteristics of the canal and the biological precepts. In teeth with lesions, contaminated dentin should be removed. However, in many systems, the manufacturer's approach disregards the fact that in wider root canal, instruments used in the apical third small diameter (less than 0.40 mm) may not touch the walls at the final apical. Therefore, the establishment of the initial apical file (LAI), the first instrument that measures the resistance of the walls in working length, could be a criterion in the choice of which system to use, and how many files work within the root canals after LAI determination. However, there is a lack of studies that demonstrate that LAI determination affects the degree of cleaning achieved, as well as the impact that LAI-based instrumentation may have on microcracks formation, canal transportation, postoperative pain and quality of obturation.In this way, the investigators propose a study in vivo performed, in patients from 8 to 18 years of age, therefore in large root canals. The protocol recommended by the manufacturer will be compared by means of a prospective randomized clinical trial, with the addition of the XP Endo finisher and with the protocol based on the initial apical file evaluating postoperative pain, exacerbation between the sessions, the quality of the obturation and periapical radiolucency repair. Considering that currently performing endodontic treatments using mechanized instruments is taught in undergraduate courses throughout the country, studies of this nature help the execution of a endodontics more critical and effective.