This retrospective study reported the incidence of separated hand and rotary instruments in primary endodontic treatments and retreatment cases in an endodontic post graduate program. The different management approaches following their separation was also reported to provide a full scope on the expected prognosis. While SI may not necessarily affect the treatment outcome [13], instrument separation early in the treatment can hinder adequate canal disinfection, which can retard the healing process [14]. Previous studies have reported either one or the other of the above parameters using older generation NiTi systems [11, 15–17]. With the continuous evolution in endodontic rotary systems, it is essential to maintain reporting the incidence of instrument separation and their management to better assess how improvements in instrument design and metallurgy have influenced the operators’ clinical experience, and the pattern of instrument separation in todays’ practice and in educational institutions.
The incidence of instrument separation overall was 4.1% in primary endodontic treatment and 6% in retreatment cases. There was a statistically significant difference in the distribution of separated hand and rotary instruments between the two treatments with an increased incidence of separated rotary NiTi files in retreatment cases (p = 0.023). This may stem from the complexity of retreatment cases and the inability to gauge the canals with hand files before operating rotary instruments to remove gutta purcha. Similar results were observed by Alfouzan & A. Jamleh [18] where the incidence of file separation in retreatment cases was 4 times higher than in initial treatments. There is little information on the incidence of SI in retreatment cases in the endodontic literature. Ruivo et al. [12] showed an incidence of only 0.68% during endodontic retreatment. In their study, however, all retreatments were performed with reciprocating files compared to continuous rotation rotary files that were used in this study. Reciprocating systems have been associated with a lower instrument separation rate, but also with a reduced efficiency in removal of gutta purcha in retreatment procedures [2]. In this study, the highest incidence of instrument separation was observed in mandibular molars, and the MB root of maxillary molars, with the apical area being the most prevalent location for file separation. These results confirm those of Iqbal et al (2006) [7] and are likely derived from the curvatures and anatomical complexities associated with molar teeth [19].
There is a wide variation in the incidence of separated rotary NiTi instruments in primary endodontic treatments, ranging between 0.3 and 23%, with an average of 5% [20]. Multiple factors can contribute to such wide range such as the level of complexity of the case, the frequency of using the instruments and the operators’ experience [20]. Studies with a very low instrument separation rate (less than 1%) were primarily performed by either dental students, where cases generally have a lower level of complexity (Shen et al. 2009) or by experienced endodontists (Alfouzan & Jamleh 2018). In this study, all cases were treated in the endodontic post-graduate program, where cases with a higher level of complexity are managed. Students, however, have not yet fully developed their clinical experience, which can explain the slightly higher incidence compared to the aforementioned studies. All students were instructed to use new files for each treatment and thus overuse of the instruments can be excluded as a potential contributor to instrument separation. In this study, the incidence of separated rotary NiTi files in primary endodontic treatments was 1.9%. These results were lower than other studies that utilized earlier generations of NiTi rotary systems (Cheung et al. 2005; Peng et al. 2005; Shen Cheung et al. 2006). Such reduced incidence can be attributed to the improved mechanical properties of the more modern rotary systems. Our results were similar to those of Ehrhardt et al. (2012), that utilized Mtwo rotary system (Dentsply-Sirona) and to Iqbal et al. (2006), where Profile series 29 (Dentsply-Sirona) was the most frequently separated rotary system. These systems require at least 4–6 rotary NiTi instruments to complete the mechanical objectives. In this study, XP shaper (FKG Dentaire) was the most separated rotary file system, which only requires one or two instruments to achieve the mechanical objective.
Our results showed around 2% incidence of instrument separation among SS hand files, with 70% being sizes 6, 8, and 10. These results were 8 times more than those reported by Iqbal et al (2006) [17] among post-graduate students. The results cannot be explained but it may stem from the limited experience endodontic residents may have today in using hand instruments compared to 15 years prior. While historically cases could be completely performed using hand instrumentation [7], hand instrumentation was only limited to achieving patency and minimal glidepath in this study. In today’s practice, rotary files are available in small tip sizes, and thus clinicians may often shift to operate with rotary files early in the treatment to minimize other procedural errors such as transportation or ledging, and to reduce the operation time. It has been proposed that the primary reason for NiTi rotary instrument separation is cyclic fatigue [20]. This, however, is unlikely the cause in our study given that the post-graduate students use new instruments in every case. Also, 70% of the SI had a small tip size with a narrow taper (0.01–0.04), which is directly proportional to increased resistance to cyclic fatigue [21]. Our results suggest that separated rotary NiTi files today are primarily caused by torsional stresses. This is likely due to the early introduction of rotary instruments during mechanical preparation, combined with the use of rotary systems with fewer instrument sequences to achieve the mechanical objectives. These results should be interpreted with care, given that no mechanical analysis was performed on any of the SI. Also, we did not record all the files used or the sequence they were used in for any treatments performed due to the complexity of the instrumentation protocols today. Students may often use different files, from different systems, in different sequences (crown down or full-length instrumentation) to address the same tooth/canal, and thus, correlating between the instrument type and its separation was not possible. This can be considered a limitation in this study.
In this study, we divided the SI instruments into three groups according to their mode of operation (hand or rotary) and core design (adaptive vs. solid core). XP Shaper has been classified as a file with an adaptive core [4], accordingly, the file can expand and enlarge the canal beyond its original size. On the other hand, all the other separated rotary files were solid core files [4] and are a part of a system that is used to enlarge the canal sequentially. In this study, XP shaper was the most frequently separated rotary instrument, primarily in retreatment cases. This may stem from the fact that XP Shaper has been the primary rotary instrument used by the post-graduate student during endodontic retreatment. Previous studies have shown that XP Shaper operated at higher speed allows faster instrumentation and more gutta purcha removal compared to other solid core rotary instruments [2]. While the instrument can be more efficient in endodontic retreatment, operating the instrument at such a high speed may require more clinical experience to avoid its separation.
The management of SI was evaluated twice to understand the effect of the type of separated instrument on its retrieval or successful management (retrieve + bypass). Overall, instruments were successfully managed in 53.6% of the cases either through removal (25.6%) or bypassing (28%). A significant association was observed between the file core design and its successful management, with XP shaper being the most successful managed separated instrument (73%). This can be attributed to their small core size (.01 taper) that can facilitate easier negotiation or removal compared to hand files (.02 taper) or other rotary files with larger taper (≥ .04 taper). There was also an association between successful retrieval and management, and the level of the SI in the canal (P < 0.001). When the SI was located coronally, it was successfully managed in 100% of the cases. This percentage dropped to 71% in the middle third and 46% in the apical third. These results appear similar to those of Hulsmann & Schinkel [22]. Instruments located at the apical third are less likely to be visible and can result in more treatment complications when attempting removal or bypassing, which may jeopardize the integrity of the tooth [17, 22–24].