Introduction Root canal preparation is a critical step in endodontic success, with rotary file systems playing a key role in shaping and cleaning the root canal system. This study aimed to compare the effects of ProTaper Next (Dentsply Maillefer, Ballaigues, Switzerland) and Mtwo nickel-titanium (Ni-Ti) (VDW, Munich, Germany) rotary file systems on dentinal crack formation using a scanning electron microscope (SEM) (Zeiss Evo 18
Carl Zeiss AG, Oberkochen, Germany). Methods Fifteen patients requiring bilateral extraction of single-rooted mandibular premolars for orthodontic reasons were selected. Extracted teeth were divided into two groups: group 1 (ProTaper Next) and group 2 (Mtwo). After standardized root canal preparation, the specimens were sectioned at 3, 6, and 9 mm from the apex and examined under SEM for crack formation. Results Both file systems induced dentinal cracks post-instrumentation. ProTaper Next files resulted in fewer cracks than Mtwo, but the difference was not statistically significant (p >
0.05). Cracks were more prevalent in the coronal third for both groups. The Mtwo system caused more cracks in the coronal and middle thirds, though this difference remained non-significant (p >
0.05). A statistically significant difference (p <
0.05) was observed in the apical third, where the Mtwo files induced more cracks than ProTaper Next. Conclusion Both ProTaper Next and Mtwo rotary files contribute to dentinal crack formation, with ProTaper Next causing fewer cracks, particularly in the apical third. These findings underscore the clinical importance of file selection in minimizing dentinal damage and reducing the risk of root fractures during endodontic treatment.