The purpose of root canal treatment is to remove all the pulp tissues, bacteria and their products, as well as to attain proper sealing of the root canal system.12 To achieve this goal, effective cleaning and shaping is important. Conventional stainless-steel (SS) hand instruments have some limitations, especially for their rigidity, which can result in procedural errors such as transportation, ledges and/or perforations. Since the first report introducing the nickel-titanium (NiTi) alloy for endodontics in 1988 by Walia et al.,3 NiTi rotary instruments have been marketed to overcome the disadvantage of SS files. The benefit and advantages of shaping with NiTi rotary files have been widely reported.4567 With the super-elasticity and low elastic modulus of NiTi rotary instruments, there is a low risk of transforming the original canal shape. In addition, the increased efficiency allows clinicians to shape the canal more predictably and expeditiously. NiTi files are likely to result in less debris extrusion and reduced chance of postoperative sensitivity. Rotary instrumentation has also been shown to produce a higher rate of clinical success than a hand filing technique.4

Few years ago, reciprocating systems such as Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland) were introduced, both of which adopt an asymmetric back-and-forth rotational movement, a motion similar to the balanced force technique.8 The manufacturers claim that only one file is needed, selected according to the uninstrumented canal's size, to shape the canal with the reciprocating motion.9 By reducing the steps/changes of instruments in conventional NiTi rotary systems, the root canal shaping may be achieved in a shorter time.1011 Therefore, it is plausible that novices may be able to master the reciprocating system more readily than the conventional rotary instrumentation that often involves the use of multiple files.

There is a growing trend that NiTi rotary techniques are taught at the undergraduate level in dental schools, with reports of the clinical performance of NiTi instruments operated by undergraduate students appearing nowadays.1213 Sonntag et al.14 observed that undergraduate dental students could achieve better canal preparations with NiTi, compared to SS files, although they noted more fractures with NiTi files. File separations may well be the main concern why beginners avoid using NiTi rotary systems. Yet, there is little information on perception of novices or, even, undergraduate students for their first experience with engine-driven NiTi instruments. Data related to reciprocating systems is lacking. Such information would be helpful to identify the 'most accepted' system that beginners or students may be most comfortable (or confident) with. Therefore, the purpose of this study was to compare two NiTi systems (rotary vs. reciprocating system) for their acceptance by undergraduate students who experienced NiTi instruments for the first time.

The protocol of this study was approved by the Pusan National University Dental Hospital Institutional Review Board (PNUDH-2015-029). The Institutional Review Board approved to obtain the consent in a verbal form. This study was conducted in full accordance with the World Medical Association Declaration of Helsinki.

Eighty-one sophomore undergraduate students of a university's School of Dentistry were asked to complete a questionnaire after using a rotary (ProTaper Universal [PTU], Dentsply Maillefer) and after a reciprocating system (WaveOne [WO], Dentsply Maillefer). These students did not have any prior experience of NiTi instrumentation. They were first introduced to the basic procedures of root canal therapy and practiced on 4 artificial teeth (1 incisor, 1 premolar, and 2 molars) with artificial root canals inside (B22X Series, Nissin, Kyoto, Japan), followed by 4 extracted teeth (1 incisor, 1 premolar, and 2 molars) with manual instrumentation using SS files for 10 to 12 hours. The extracted teeth were selected by the supervisors so that the teeth do not have moderate to severe root canal curvature or sclerotic canals. After that and in another session, the students were given an approximately 1 hour instruction on the manufacturer's recommended technique for using the rotary PTU instrument (see brief description of procedure below), before practicing with the system on 2 molars under supervision. Then they were asked to work on their own to prepare the buccal canals of another maxillary first or the mesial canals of a mandibular first molar. When using the NiTi files, working length was first determined and a glide path prepared to a size 15 SS K-file. The canals were shaped in the order of S1, S2, F1, and F2 with the PTU system. The irrigation procedure and patency filing were done as recommended. For the WO system, working length and glide path were similarly obtained before the use of the WO Primary file (notice that for very fine canals the WO Small file was used, according to the manufacturer's recommendation).

After completing the canal preparation, all students were asked to fill in a questionnaire for their experience when using the SS instruments and the NiTi rotary systems (Figure 1). The same questionnaire was completed to compare SS instruments and the NiTi reciprocating system for the last session. The questions concerned students' perception on the 'Ease of use', 'Flexibility', 'Cutting efficiency', 'Screwing effect', 'Feeling safety', and 'Instrumentation time' of the respective instrument, compared with SS files, on a 5-point Likert-type scale (Score 1, Much Worse; 2, Worse; 3, Neutral; 4, Better; 5, Much Better). The questionnaire also recorded the time taken for the shaping the last canal (that students did on their own) and for any file fracture. Lastly, students were requested to select one preferred NiTi system between PTU and WO.

The scores of each perception category between two NiTi systems were compared using t-test. Then, Pearson Chi-square test was used to examine any correlation, or association between the preferred choice of system and each perception categories. The significance level was set at p = 0.05.

All the questionnaires were returned. Excluding those that were not completed in full, there were 79 for PTU and 81 for WO system available for analysis; the response rate was 97.5% and 100%, respectively. The distribution of scores for each evaluation category was expressed graphically for the two systems (Figure 2). Of all who responded, 77 indicated their preference, with the single file WO system being more often preferred than PTU (n = 55 vs. 22). Fractures were noted for 2 PTU (each for S1 and F1) and 2 WO instruments (each for the Small and Primary file).

Comparing with SS hand instrumentation a value of greater than 4 (i.e. subjectively better) was accorded to the 2 NiTi instruments in the great majority of questions, except for PTU in the category of 'Screwing effect' (Table 1). When comparing the two NiTi systems, PTU gave a significantly higher score in flexibility (p < 0.05) and safety feeling (p < 0.05), while WO scored better in 'Screwing effect' (p < 0.05). A significant association between the 'Screwing effect' and students' preference for WO was observed (p < 0.05).

The mean time taken for instrumenting the canal by the students on their own was 285 ± 114 seconds for PTU and 135 ± 95 seconds for WO, for which the difference was statistically significant (p < 0.05).

This questionnaire survey was during the semester time of the undergraduate curriculum and, hence, a high response rate was obtained (as expected). Instead of artificial canals in resin blocks, extracted teeth were used for practice by students. Although simulated resin block can offer standardized canal shape, size, and curvature for all canals, the resin material is no substitute for the dental hard tissues. Furthermore, the extracted tooth is a much better representation of real clinical situation. PTU and WO were used as examples of a multi-sequence system with continuous rotary motion and a single-file reciprocating system, respectively. All students had never experienced NiTi instruments before, so the present results would reflect the perception of novices or new users who first attempt the use of engine-driven NiTi files. This perception may well be the experience of general practitioners who begin with these instruments.

The great majority of students felt that the two NiTi systems provided a better clinical result than manual SS files. The greatly reduced operation time, the easiness of use (much less tiring to the operator) and well-looking final canal shape would contribute to this perception. Various studies comparing NiTi rotary and SS files gave similar conclusions.1415 Others also reported that NiTi instruments produced more predictable canal shapes and were able to maintain the original canal configuration with little chance of procedural errors.161718 A clinical cohort study comparing manual SS instrumentation and a NiTi rotary technique performed by dental students has concluded that teeth prepared with SS files showed a higher incidence of procedural errors and a lower success rate.4 They further observed that the increased success rate was dependent on operator experience, with the treatment outcome and clinical success achieved by undergraduate students (i.e. novices) being significantly improved when NiTi files were used. A similar quantum improvement in treatment outcome was not observed for endodontic postgraduate students.4 Hence, engine-driven NiTi instruments should benefit the general practitioners and the patients at large.

File separation has been a worrying consideration for many practitioners who may want to incorporate NiTi instruments into their practice. It is undeniable that instruments will break if used beyond their physical strength or due to accumulation of fatigue damages. Wolcott et al.19 reported that the incidence of breakage of ProTaper (first-time use) was 17.7%, with the F3 file showing the greatest amount of separation.19 In contrast, only 4 files in total had separated, giving an incidence of 1.0% for all files used in the present study. Careless usage of the NiTi files and inherent or induced defects in the instrument would be the main reason of fracture here. While one may think that experienced users may be able to limit the amount of instrument breakage, Generali et al.20 reported that there was no significant difference in the amount of separation for WO Primary file between experienced practitioners and beginners; one reason might be related to the amount of stresses caused to the instrument for such single-file system.

In this study, 71% students preferred the WO after using the two NiTi systems for the first time. The reciprocating motion was associated with a significant reduction in the amount of 'screw-in' sensation experienced by the students. The reduced tendency of reciprocating files to 'screw in' to the canal was due to the periodic change in rotation direction. The fatigue resistance of the instrument operated in this motion is also enhanced, compared with the continuous rotation mode.2122 Despite being the preferred system, WO was inferior to PTU in the score for 'Feeling of Safety'. The multi-step, serial enlargement of the canal might have imparted a feeling of being safer and under control, than the use of one single file to complete the canal enlargement. The subjective feeling of PTU being more flexible than WO (see result in Table 1) might have also contributed to the 'feeling of safety' with the use of PTU instrument. Considering PTU is made of conventional NiTi alloy and WO made of M-wire (Dentsply Maillefer) with slightly different cross sectional configuration, there should be little difference in flexibility or, arguably, more flexible for WO.23 The reciprocating movement of WO files might have masked the tactile sensation of beginner users. The S1 and S2 files of PTU system have a smaller diameter and apical taper than WO Primary file, which could have given the students such a perception that PTU system is more flexible than WO.

One would expect that preparation using one single file should be quicker than the multi-step rotary instrumentation with multiple files. This is one selling point for reciprocating files. The trend was indeed observed from the times recorded by students in this present study. Interestingly, however, there was little difference in the subjective perception of reduction in 'Instrumentation Time', compared with SS files, for the two systems examined. Considering the experience being the students' first encounter with NiTi engine-files, the reliability of their judgments and of the understanding of criteria may be low and, hence, more data would be preferable. Nonetheless, the present results indicated a significantly higher preference for reciprocating files by beginners and novice operators.