Assessment of the Separation Incidence of Mtwo Files Used with Preflaring: Prospective Clinical Study Iracema C. Ehrhardt, DDS, MSc,* M ario L. Zuolo, DDS, MSc, † Rodrigo S. Cunha, DDS, MSc, PhD, ‡ Alexandre S. De Martin, DDS, MSc, PhD, § Daniel Kherlakian, DDS, MSc, † Maria Cristina C. de Carvalho, DDS, MSc, † and Carlos Eduardo da S. Bueno, DDS, MSc, PhD § Abstract Introduction: The Mtwo rotary system was designed to be used at full length without performing previous cervical enlargement or creating a manual glide path. However, to our knowledge, no study has investigated whether preflaring influences instrument fatigue. The purpose of this clinical study was thus to evaluate the incidence of separation of Mtwo instruments during the preparation of mandibular and maxillary molars and bicuspids that was preceded by cervical preflaring. Methods: A total of 556 treatments were performed by 6 endodontists after calibration. Cervical enlarge- ment and exploration with hand files were undertaken before using the rotary files. The canals were negotiated with small K-type hand files. Cervical and middle thirds were prepared with hand files and Gates-Glidden burs. Irrigation was performed with 2 mL of 2.5% NaOCl after each instrument. The working length was established, and the apical third was prepared with Flexofiles until a #15 file could reach the working length, establishing a glide path. The Mtwo instruments were used following manufacturer’s recommendations and disposed of after being used in 5 teeth. Each file was examined under 8 magnification after use. In cases of file deformation or separation, the entire file kit was no longer used, except when using the 10.04 file. Because of its high failure rate, which is well-established in the related literature, this instrument could be replaced at any time. Results: Separation and deformation rates were 1.98% and 28.78%, respectively. Ten of the total separations observed occurred with the 10.04 file (90.01%) and one with the 15.05 file (9.09%). Conclusions: The use of Mtwo rotary instruments preceded by cervical preflaring with manual files and Gates-Glidden burs provides a low separation incidence. (J Endod 2012;38:1078–1081) Key Words Accident prevention, dental instruments, endodontics D espite the advantages of using nickel-titanium rotary instruments, instrument sepa- ration is the most frequent procedural accident observed during their clinical use. Unexpected separation of rotary instruments without apparent deformation is not an uncommon occurrence among professionals (1, 2). The instrumentation technique indicated for most of these instruments follows the crown-down concept, according to which higher-caliber files precede thinner files as they progress apically. This technique is essential to reducing instrument friction against the dentinal walls, thus minimizing separation risks (3–7). An instrument system that, unlike others, does not adopt the crown-down concept has recently been introduced in endodontics. Mtwo instruments (VDW, Munich, Germany) have an S-shaped cross section, with a double cutting blade, a noncutting safety tip, and positive cutting angle, rendering the instrument effective for cutting dentin walls. In addition, the instruments have a progressively widening space between blades from the tip toward the handle, thus preventing the instrument from locking in the canal walls, eliminating threading and binding in continuous rotation and reducing the trans- port of debris toward the apex (3, 4, 7–9). The basic series of the Mtwo system comprises 8 instruments with tapers ranging from .04–.07 and tips ranging from 10–40. According to the manufacturer, the first 4 instruments (10.04, 15.05, 20.06, and 25.06) should be used up to the working length, thus resulting in their more intensive use. The remaining 4 files (30.05, 35.04, 40.04, and 25.07) are used occasionally in cases requiring more extensive enlargement. Still according to the manufacturer, the instruments should be used following the simultaneous shaping technique, where all files reach the working length. According to Kuzekanani et al (10), the manufacturer claims that a crown-down instrumentation sequence is not necessary, because each instrument creates access to the apex for the following instrument and is used up to the working length to prepare the entire canal. It is believed that the protocol recommended by the manufacturer produces greater friction of the spirals against the canal walls, which clinically means greater instrument fatigue and operator concern (11). Preflaring with hand files and Gates- Glidden burs generally reduces the contact of the instrument with the root canal walls, rendering the canal patent to receive the rotary files and thus allowing a more effective and safer action (12, 13). Changing the protocol recommended by the manufacturer by preflaring with hand files and Gates-Glidden burs may have consequences that, until now, have not been sup- ported by clinical studies on the Mtwo rotary system. Furthermore, there are few studies in the related literature supporting the hybridism of the Mtwo rotary system technique with the manual technique, especially on the issue of separations (14). From the *Graduate Program in Endodontics, S~ ao Leopoldo Mandic Dental Research Center, Campinas, S~ ao Paulo, Brazil; † School of Professional Development, State of S~ ao Paulo Dental Association (EAP-APCD), S~ ao Paulo, Brazil; ‡ Department of Restorative Dentistry, Winnipeg, Manitoba, Canada; and § Department of Endodontics, S~ ao Leopoldo Mandic Dental Research Center, Campinas, S~ ao Paulo, Brazil. Address requests for reprints to Dr Iracema C. Ehrhardt, Av. Nove de Julho, 5483, cj 31, 01407-200 S~ ao Paulo, SP, Brazil. E-mail address: iracabral@yahoo.com.br 0099-2399/$ - see front matter Copyright ª 2012 American Association of Endodontists. doi:10.1016/j.joen.2012.05.001 Clinical Research 1078 Ehrhardt et al. JOE — Volume 38, Number 8, August 2012