The Effect of Preflaring on the Rates of Separation for 0.04 Taper Nickel Titanium Rotary Instruments David D. Roland, DDS, Wallis E. Andelin, DDS, David F. Browning, DDS, G-Hong Robert Hsu, DDS, and Mahmoud Torabinejad, DMD, MSD, PhD The purpose of this study was to compare the rates of separation of 0.04 taper nickel titanium (NiTi) rotary instruments using two different instrumen- tation techniques. Twenty sets of 0.04 taper Profile Series 29 rotary instruments, sizes 2 to 6 were used in the mesial (mandibular) or buccal (maxillary) ca- nals of extracted human molars with a 20 to 30 degree root curvature according to the Schneider classification. The rotary instruments were used up to 20 times either with the crown-down technique recommended by the manufacturer or with a com- bination of preflaring with hand files in a passive step-back technique followed by rotary instrumen- tation. Statistical analysis of the data showed that the combination technique allowed more uses be- fore separation compared with the crown-down technique recommended by the manufacturer p < 0.0001. The introduction of rotary nickel titanium (NiTi) instruments has significantly affected endodontic cleaning and shaping procedures. When NiTi instruments were first introduced to endodontics, they were recommended as an alternative to stainless steel files (1). They stay more centered in the canal, produce rounder prepara- tions, and reduce procedural accidents, such as transportation and ledging (2). Due to their cost, rotary NiTi instruments are com- monly reused, which can result in their separation. Endodontic instruments that fracture and remain in the canal before complete biomechanical preparation could pose a threat to the success of the endodontic treatment (3, 4). NiTi rotary instru- ments, although more elastic than stainless steel files, can separate at any time with little or no warning (5). The fracturing of NiTi instruments below their elastic limit, without visible or tactile warning, can be due to the formation of slip bands in the alloy (5, 6). Manufacturers have developed and marketed many rotary sys- tems; however, very little or no information has been provided on the number of safe uses of these instruments (7). Numerous in- vestigators have addressed the length of time that these instruments can be used (3, 8 –11), yet there is little agreement based on a clinical protocol to aid the clinician to determine how many times these instruments can safely be used (5). The purpose of this study was to compare the separation rates of 0.04 taper NiTi instruments, using the manufacturer’s recom- mended technique, with a combination of passive step-back tech- nique and rotary NiTi instrumentation. MATERIALS AND METHODS Extracted maxillary or mandibular permanent human molars between 18 and 25 mm in length with fully formed apices and no previous root canal treatments were used in this study. Experimen- tal canals were limited to mesial roots of mandibular molars and the buccal canals of maxillary molars. The roots had to have a curvature between 20 and 30 degrees according to the method of Schneider (12), and initial patency of the experimental canal no larger than a size #2 (0.129) Series 29, 0.02 taper, hand file (Dentsply Tulsa Dental Products, Tulsa, OK). Standard access preparations were made to allow straight-line access to the canals (13). Working length was visually established and recorded when the patency file was seen emerging through the apical foramen of each canal. Twenty sets of 0.04 taper Profile Series 29 sizes 2 to 6 rotary NiTi instruments (Dentsply Tulsa Dental Products) were used. Profile Series 29 instruments sizes 2 to 6 have tip diameters of 0.129, 0.167, 0.216, 0.279, and 0.360, respectively. The instrument sets were divided equally into two groups. Two operators were assigned to each group (group A and group B), and each was given five sets of instruments. Krupp et al. (14) demonstrated that these sample sizes were adequate for similar experimental tests. In group A, all canals were initially instrumented with Series 29 hand files [sizes #1 (0.10), #2 (0.129), and #3 (0.167)] until the size #3 (0.167) file was able to reach the working length. After initial preparation, the rotary instruments were used in the crown-down technique recommended by the manufacturer to a final apical preparation size #4 (0.216). In group B, all canals were initially negotiated with the Series 29 hand files in a passive technique (15, 16) until the size #2 (0.129) file freely reached the working length. After preflaring with the passive step-back technique, the final canal shape was achieved with rotary instruments in a step-back enlargement to an apical size #4 (0.216). An electric motor at a setting of 300 rpm was used with a “light touch” (9) for all instrumentation. Each JOURNAL OF ENDODONTICS Printed in U.S.A. Copyright © 2002 by The American Association of Endodontists VOL. 28, NO. 7, JULY 2002 543