Root canal instrumentation is a decisive aspect of the so-called chemo-mechanical root canal debridement, and therefore an important step that decides on the successful outcome of root canal treatment (Ng et al. 2008). This procedure was performed using stainless-steel hand files for almost a century. Advances in metallurgy, especially since the first use of nickel-titanium rotary instruments in the late 1980s (Walia et al. 1988), have sparked a surge in instrument development over the recent decades (Haapasalo & Shen 2013). These developments have mainly aimed at simplifying the procedure for the clinician by creating instrument designs, sequences, and motorized movements that made root canal treatments more efficient (as opposed to effective). However, as has recently been demonstrated, newer systems, at least when introduced to licensed dentists, do not necessarily result in higher treatment quality or better outcomes (Dahlström et al. 2015, Jordal et al. 2022).
From a didactic point of view, the implementation of modern root canal instrumentation systems in dental student courses is also not without possible draw-backs. To prepare the students for their later profession, the focus is still on hand instrumentation, as motorized instrumenting systems are not ubiquitously available in dental offices (Thiessen et al. 2020). A further concern is that, while early-generation rotary files were standardized in taper and diameter and thus easy to combine with the ISO-normed hand instruments, newer-generation files feature variable tapers and individualized tip diameters (Haapasalo & Shen 2013). This is the reason why at our school, a first-generation rotary nickel-titanium file system was used until very recently. However, a comparison of that system with newer-generation files performed in our pre-clinical course changed our view (Marending et al. 2016). That study was performed in 3D-printed molar replicas, and showed similar shaping outcomes for two newer-generation rotary systems compared to the first-generation system that the students knew and were taught. However, the students much preferred the newer systems over the first-generation counterpart, and the time to full canal instrumentation was almost halved (Marending et al. 2016). This prompted us to change from a first-generation to a so-called “fifth-generation” (Haapasalo & Shen 2013) rotary system in the 2019/2020 third year pre-clinical course. That system, which is marketed under the trade name ProTaper Next (Dentsply, Ballaigues, Switzerland) features an off-centered rotation to minimize the engagement between the rotary instrument and the root canal wall (Hashem et al. 2012) and thermally processed nickel-titanium with a martensite phase component (Shim et al. 2017). Students attained better results with a fifth-generation rotary system as compared to a conventional austenitic rotary system in resin training blocks (Çelik et al. 2019). However, clinical data regarding the quality of the root canal shaping are missing.
The change in instrument systems performed in our clinical student course offered a unique opportunity to compare treatment quality attained by non-biased operators (i.e. the students) between a more traditional and a contemporary rotary system in clinics. Students received the corresponding lectures and pre-clinical training in their third year, and performed their first root canal treatments in their first clinical course in their fourth year of study. This controlled retrospective study aimed to assess the radiological quality of root canal treatments performed by fourth-year dental students using a fifth-generation rotary system (ProTaper Next, Dentsply) versus that obtained using a first-generation system (ProFile, Dentsply). The primary outcome that was evaluated was the radiographic quality of the root canal fillings as assessed by two calibrated observers using a standardized scoring system (Molander et al. 2007, Dahlström et al. 2015). The secondary outcome was the number of separated and non-retrieved instruments per group that could be identified on the root-filling periapical radiographs.
Materials and methods
Ethics and regulatory issues
This project complied with the Declaration of Helsinki, regulatory demands, and local law (HFG and the HFV, Swiss Federal Council). It was approved by the Ethics Commission of the Canton of Zurich (BASEC-Nr. 2021-00894). All patients treated at the University of Zurich Center of Dental Medicine are asked to sign an informed consent sheet that their censored data could be used for scientific purposes. In addition, patients were contacted by telephone or via e-mail whether they gave their informed consent to the current protocol, especially to the use of their personal data as described below. Patients that could not be reached or did not give their informed consent were excluded from the analysis (Fig. 1). All the data used for this investigation were analyzed and stored under strict observation of data protection laws.
The materials and techniques the students used for root canal treatments were similar, as were their teachers and their overall clinical set-up. The only difference was the use of the ProTaper Next (Dentsply) in 2020/21 as opposed to the ProFile (Dentsply) system used for root canal instrumentation in 2019/20. Before clinics, the students had received 13 lectures in endodontology, and a total of 55 hours of supervised pre-clinical training (in their third year and the beginning of the fourth year), as described elsewhere (Marending et al. 2016). In both academic years, 20/21 and 19/20, the step-down procedure was performed using One-Flare instruments (Coltène Micro-Mega, Besançon, France), and the glide path was prepared using either rotary instruments (One G, Coltène Micro-Mega) or small hand files (Ready Steel K-Files, Dentsply) up to size 15 (Fig. 2). Canals were instrumented to a minimal size of 35/.04 (ProFile) or 30/.07 (ProTaper Next X3). The exception to that rule was the second mesiobuccal canal (mb2) in maxillary molars, which was instrumented to a minimum size of 30/.04 (ProFile) or 25/.06 (ProTaper Next X2). The root canals were irrigated with 1% NaOCl (Hedinger, Stuttgart, Germany) containing 9% HEDP (Dual Rinse HEDP, Weinfelden, Switzerland). Canals were dried with paper points before root-filling. An epoxy resin sealer (AH Plus, Dentsply) was administered on the master point. The latter was a 4 %-tapered (Coltène, Altstätten, Switzerland) gutta-percha point, or a proprietary gutta-percha cone (Dentsply), corresponding to the final ProFile or ProTaper Next instrument that was used to working length, respectively. Subsequently, lateral compaction was applied using finger spreaders of size A and B and the corresponding auxiliary gutta-percha points (Dentsply) to increase the amount of gutta-percha in the root fillings. A final radiograph was taken after root filling to get the respective credit for the root canal treatment by the student. Each of these working steps that was done by the students was supervised/controlled by a postgraduate student/resident in conservative dentistry or endodontics: caries excavation/pre-endodontic buildup, rubber dam placement, access cavity, step-down, working length determination, master apical rotary, fitting of master cones, master cone radiograph, root filling radiograph, temporization. Steps were signed by the responsible supervisors in a certification booklet, which is used as the main document for the students to fulfill their clinical requirements and get the credit points for the clinical course in Conservative Dentistry.
Treated teeth and inclusion criteria
All the certification booklets of the students in the respective years were collected by the doctoral candidate/first author (L.M.). He checked the entries and identified all teeth and respective patients with apparently finalized root canal treatments (note: some patients received more than one root canal treatment by different students). Subsequently, he contacted all these patients to ask whether their censored radiographs could be used for this study. Teeth of patients who did not give informed consent that their censored data could be used for retrospective analysis were excluded, as were counterparts with radiographs that were missing or of insufficient quality (Fig. 1). Chart entries were used to verify that the respective systems had indeed been applied.
Assessment of root filling quality
Two independent observers, an endodontist and teacher (M.M.) as well as an endodontic resident (K.H.), were trained and calibrated to a five-scale score (Molander et al. 2007). Inter-observer agreement was assessed by judging the filling quality in 229 roots. To that end, pre-selected radiographs depicting roots with root canal fillings of varying quality were used. These training radiographs were not related to this study. One week prior to the assessment observer agreement, the two observers read and discussed the concepts suggested by the original authors (Molander et al. 2007, Dahlström et al. 2015). Based on these discussions, slight modifications and specifications were made to the published scoring system: A sealer "puff" over the apex (overfilling) was judged as “correct length”. Over-extension of the master cone by more than 0.5 mm was judged as defective length. When there were apparent gaps between a root canal post and the root filling, the seal was judged as defective (Table I). Roots containing fractured instrument segments were included in this analysis.
The individual root was used as a unit of observation. The outcome measure was the modified Molander score per root (Table I). In cases of two canals per root, the lesser outcome (i.e. higher score value) was tabulated. Radiographic length measurements and/or masterpoint radiographs were included for the assessment when necessary. As a secondary outcome, the number of separated files was counted per year/instrument system. To compare the root canal filling quality scores between the two instruments, Chi-squared test was applied. To compare the frequency of separated instruments (expected value below 5 per total treatments), Fisher’s exact was used.
Identification of cases
Because of the retrospective nature and the search mode of this study, teeth treated in the fourth-year student course at the Clinic of Conservative and Preventive Dentistry, University of Zurich, were the initial unit of observation (Fig. 1). There were 44 fourth-year students enrolled in the academic year 2020/21, and 40 in 2019/20. According to the search criteria delineated above, 66 teeth received a complete endodontic treatment in the academic year 2020/21 using ProTaper Next instruments, versus 70 in the year 2019/20 using ProFile instruments. A total of 16 and 20 teeth had to be excluded from these groups, respectively, because informed consent by the patients could not be obtained, or because the final radiographs were of insufficient quality for proper analysis. This resulted in 50 teeth per group that were included in the analysis (Fig. 1, Table II).
The Kappa coefficient regarding the degree of agreement between the two observers when judging the 229 reference roots was 0.62, suggesting "substantial agreement" (Landis & Koch 1977). When judging the study roots and their root fillings, they had to exclude a total of three roots because of anatomical blurring (1) or apparently non-prepared canals (2). This resulted in 97 roots prepared with ProTaper Next instruments and 81 prepared with ProFile counterparts that could be properly assessed and were thus analyzed for the study (Table II). When comparing the shaping outcomes between the two systems as judged by the quality of the root fillings on the final radiographs, which was the primary outcome assessed in this study, then no statistically significant difference was found between the two rotary systems under investigation (Table III). The overall quality of the root fillings was good, with 81% of the roots instrumented using ProTaper Next and 79% of the counterparts instrumented using ProFile achieving a modified Molander (2007) score of I or II. In the studied roots, only one fractured ProTaper Next and no ProFile fragment was identified. Consequently, there was no statistically significant difference between the two rotary systems under investigation in the secondary outcome of this study either (Fisher's exact test, p = 1.00).
This study, performed in a controlled clinical student course setting, revealed no apparent difference in the technical quality of root fillings obtained after canal instrumentation using a fifth-generation rotary system (ProTaper Next) versus a first-generation counterpart (ProFile). Moreover, there was no significant difference in the number of fractured instruments between the two systems.
This retrospective study used the unique opportunity that, despite a retrospective design, it was possible to single out the impact of using a fifth-generation rotary system over a first-generation counterpart on clinical treatment outcomes using a stringent study design (Schulz & Grimes 2002). However, there were some minor differences in tooth types and patient age between groups (Table II). It is unlikely that these differences could have led to a systematic error in the current investigation. Observers were calibrated, which was not done in the studies performed by the developers of the system (Molander et al. 2007, Dahlström et al. 2015). The system proved to be relatively robust, with little disagreement between observers.
The radiographs assessed in this study all stem from student course patients. As Switzerland is a fee-for-service country in dentistry, i.e. there is no state-covered dental plan for the general population, the patients in these courses tend to be pre-selected in that they are individuals with either a lot of free time, little money, or both. Student course fees are roughly one quarter of the counterparts charged in private practice. However, the treatments patients receive in these student courses are performed under supervision by experienced dentists. The data used for this study are deriving from final periapical radiographs, which are taken as a matter of course after a root canal treatment, and are requested by the national quality guidelines in dentistry.
A recent randomized trial showed that the preparation sizes and tapers described in this communication should suffice to result in a good treatment outcome (Fatima et al. 2021). However, it is an inherent limitation of this study that only root filling quality was compared, and not true patient-related treatment outcomes. While it has been shown in a multitude of clinical studies that the quality of the root filling as assessed on the final radiograph does correlate with clinical outcomes (Strindberg 1956, Ng et al. 2008), this correlation is not always straight-forward. As an example, healing can be obtained even when instruments are fractured, if basic treatment principles are followed and root canals are sufficiently decontaminated (Spili et al. 2005).
The quality of root fillings was high in this study, with "excellent" Molander scores of I reached in 59 % to 60 % of the roots under investigation (Table III). This is in line with the quality of root fillings obtained by general practitioners using ProFile instruments after respective lectures and training, who also reached 59 % of "excellent" (score I; Molander et al., 2007) root fillings (Dahlström et al. 2015). However, and despite the purported improvements in rotary instrument quality and design (Haapasalo & Shen 2013), there was no difference in root filling quality or fractured instruments that apparently could not be retrieved between a first- and a fifth-generation rotary system under current conditions. This is in line with a more recent study on molar replicas: whilst the students preferred the newer rotary systems for their ease of use and controllability, the objective shaping outcomes were similar between ProFile and the newer systems (BioRace, FKG Dentaire, La-Chaux-de-Fonds, Switzerland, and HyFlex, Coltène, Altstätten, Switzerland) (Marending et al. 2016). Other in vitro studies showed that different rotary and reciprocating systems were similar regarding their preservation of root canal anatomy (Rubio et al. 2017). Different cross sections and geometrical file designs all maintained root canal curvature, were safe to use (Bürklein et al. 2015), and had similar transportation and centering abilities in the apical part of the canal (Kabil et al. 2021). Consequently, there may be a subjective benefit to the care provider from choosing a more modern instrumenting system, but not necessarily an objective one for the patient other than that the treatment session may be shorter. On the other hand, as shown in a most recent study, instrumenting systems with minimized instrument numbers may actually reduce root filling quality and treatment outcomes (Jordal et al. 2022). However, and as delineated above, there may be other reasons for dentists to prefer more modern root canal systems over older ones, which were not investigated in this study. As shown in a recent survey (Thiessen et al. 2020), more than half of the recent graduates from Swiss dental schools work in practices that use reciprocating single-file systems for endodontic treatments. Therefore, and despite any objectifiable benefits on treatment outcomes, it still does appear important from a didactical standpoint to teach more modern systems with reduced file numbers to dental students. The likelihood is high that after their graduation the former dental students will be working in a clinical environment, in which such systems are being used.
This project will be supported by institutional funds. M.Z. declares a conflict of interest in that he is involved with one product mentioned in this text (Dual Rinse HEDP) through a patent (EP3284456A1; US10434038B2). The other authors deny any conflict of interest related to this study.