RESUMEN
INTRODUCTION: This ex vivo study used cone beam computed tomography to evaluate the amount of dentin removal from the distal wall of the mesial canal of human mandibular first molars caused by 4 instruments used to flare the cervical third. METHODS: Thirty-two mesial roots were divided into 4 groups prepared by using ProTaper, K3, Gates-Glidden, or LA Axxess. The dentin thickness of the distal cervical wall of mesial canals was measured before and after the preparation by using computed tomography and Adobe Photoshop software. RESULTS: There was no statistically significant difference between the study groups (P > 05). CONCLUSIONS: All the instruments used for cervical preparation seemed to be safe and did not damage the dentin structure of the distal wall of mesial root canals of mandibular molars.