Conservative preparation of curved root canals using novel nickel-titanium instruments: influence of operator experience

Abstract

Background

Nickel-titanium (NiTi) instrumentation systems allow predictable dental root canal treatment. Novel systems promoting conservative preparation are constantly introduced into the market. Therefore, this study aimed to explore how operator experience would influence the performance using a new system – Race Evo- in preparation of curved root canals.

Methods

Three groups of participants, with no previous experience with Race Evo, were compared: Third-year dental students, general dental practitioners (GPs), and endodontists. Each participant was asked to prepare 3 simulated resin blocks. Preparation time, the occurrence of procedural errors, and canal deviation were measured.

Results

Endodontists prepared the canals with significantly fewer procedural errors (p = 0.027), and in less time than students (p < 0.001) and GPs (p = 0.019). However, there was no significant difference in performance between undergraduate students and GPs. In fact, endodontists and students alike prepared the canal with less deviation compared to GPs (p = 0.006 and p = 0.002).

Conclusion

Proficiency in instrumenting curved canals with a new conservative NiTi system was not influenced by experience related to that of the general dental practitioner, but required more specialized experience in root canal treatment. Regardless of experience level, a learning period was still necessary.

Endodontics; Experience; Nickel-titanium; Practitioner; Skills.

1. Introduction

Dental root canal treatment is perceived as complex and challenging by both general dental practitioners (GPs) and undergraduate students [1, 2]. Cleaning and shaping curved root canals is difficult due to the tendency of endodontic instruments to deviate the canal from its original axis. This results in improper dentin removal and canal straightening cascading into procedural errors that reduce treatment success [3, 4].

Nickel-titanium (NiTi) instruments revolutionized root canal treatment through their unique mechanical properties such as super-elasticity and shape memory [5]. They have gained increasing popularity among endodontists and general dental practitioners (GPs) alike [6]. Furthermore, the majority – though not all – undergraduate dental schools are training their students on using these instruments [7, 8].

Several studies have examined the influence of practitioner experience on canal preparation using rotary NiTi instruments and reported its association with faster and safer canal preparation [9, 10, 11, 12, 13]. Most of these studies were conducted with instrument systems that prepare the canal using multiple instruments that significantly enlarge the root canal’s taper. Shen et al. (2009) have reported that instrumentation errors associated with NiTi instruments were not only influenced by the operator, but also by preparation technique and instrument design [14].

Recently, new heat-treated NiTi instruments were introduced along with the concept of conservative root canal preparation. One such instrumentation system is Race Evo (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland). It is used in a continuous rotary motion of 800 rpm, which is higher than what most manufacturers of NiTi instruments advocate. It has a triangular cross-section and alternating cutting edges, which reduces the screw effect, allowing better control of the instrument’s passage within the canal [15]. A recent study found that these instruments had superior cyclic fatigue compared to conventional NiTi Race instruments in spite of rotating at high speeds, irrespective of numbers of autoclaving cycles [16]. This system advocates a conservative canal preparation to a taper of 0.04 and an apical diameter of 0.30 mm using only three instruments. Limited information is available in the literature regarding the performance of this new conservative instrumentation system, operated at speeds higher than other NiTi files [17], especially when it comes to operator experience.

Therefore, the aim of the present study was to compare the performance of inexperienced third-year dental students, general dental practitioners, and experienced endodontists in using Race Evo to prepare simulated curved canals. Operator performance was assessed by measuring the time required for preparation, the occurrence of procedural errors, and quantitively calculating root canal deviation before and after instrumentation. The null hypothesis was that there would be no significant difference in performance between operators of different levels of experience.

2. Material and methods

This randomized cross-sectional in-vitro study was approved by Princess Nourah bint Abdulrahman University (PNU) Institutional Review Board, Riyadh, Saudi Arabia (approval no. 21-0136).

2.1. Resin blocks

To calculate sample size, an a priori power analysis using G∗Power 3.1 statistical software was done [18]. Type I error α was set at 0.05, with an effect size of 0.65. The calculated sample size of 40 was increased to 45 to account for any exclusions due to procedural errors.

Forty-five simulated J-shaped root canals made from clear resin canal blocks (Root Canal Endo Training Block Model; 18.5 mm length; Jining Xingxing Medical Instrument Co., Ltd. China) were used. Each canal has a 50° curvature angle with a 4 mm radius. The canal walls were covered with an orange resin coating.

2.2. Subject selection

Fifteen subjects were invited to participate in this study, conducted in the College of Dentistry simulation laboratory at PNU. Participants were told that the aim was to see how well they are able to perform root canal preparation using a novel Niti instrumentation system.

The participants were divided into three groups based on their level of experience using NiTi instruments: Group (1) comprised five randomly selected third-year dental students, familiar with manual root canal preparation, but with no experience in NiTi rotary instrumentation; Group (2) comprised five randomly selected GPs working at PNU from different educational backgrounds; and Group (3) comprised five randomly selected endodontists practicing at PNU with 3–7 years of professional experience, from diverse educational backgrounds. All participants were not previously exposed to using the Race Evo rotary instrumentation system (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland). Each participant was asked to prepare three simulated canals using Race Evo.

2.3. Instruments and preparation techniques

Participants began by watching an instructional video explaining how the Race Evo system is used. They were also given the manufacturer’s brochure, which indicated the instruments' sequence: First, a hand #15 K file is taken to full working length, then Race Evo rotary instrument size 15/taper 0.04, followed by size 25/taper 0.04, and size 30/taper 0.04 [19]. The Race Evo rotary instruments were used on an electric motor (X-smart, Dentsply–Sirona, Charlotte NC; USA) according to manufacturer’s instructions. Each rotary instrument was used with gentle 2–3 mm strokes concomitant with light apical pressure that allows the instrument to advance passively within the canal, until working length is reached. After three strokes the file is removed, cleaned and canal irrigation and recapitulation were performed.

All instruments were lubricated with EDTA gel during canal preparation, and warm water was used for rinsing after each file. Fractured instruments were replaced with new instruments, otherwise the same set of instruments was used to instrument the three canals. The time required for complete instrumentation of each canal (including irrigation and instrument change) was recorded.

2.4. Assessment of the occurrence of procedural errors

Procedural errors prevent complete preparation of the canals to full working length. They include instrument fracture, canal blockage and transportation which involves apical zipping, canal perforation and ledge formation. After preparation, radiographs were taken of all blocks to record file fracture. Canals in which instrument fracture occurred were excluded from the quantitative assessment of canal preparation. The blocks were also examined by two endodontists with more than 7 years of experience under the magnification of dental loupes (JTL Gobiz, Korea ×3) to detect the presence of procedural errors. Inter-rater agreement was examined using Cohen’s Kappa coefficient, and found to be (k) = 1.000, indicating perfect agreement between the evaluators.

2.5. Quantitative assessment of canal preparation

To standardize the experimental setup, a specific mold was made for each of the blocks, so that pre- and post-instrumentation images could be taken in an identical predefined position. All images were taken with a professional digital camera (Nikon D5100), and then exported to a photo editor, Pixlr E (Pixlr E; Online Photo Editor & Designer Maker). Images were overlaid using the “Auto Align Layers” function, which automatically aligns layered images based on similarities of layer content.

Canal deviation after preparation was measured on the overlaid images on 7 different planes: The first starts at the apex (D0) and then progresses every 1 mm until 5 mm from the apex (D5). The remaining two planes were defined as D18 at the orifice and D10 halfway between the orifice and the beginning of the apical curve. Measurements were made using a micrometer scale and the difference between the outer and inner aspects of the original and prepared canal walls was calculated using ImageJ, an open-source scientific image processing program (version 5.2, LOCI, University of Wisconsin) (Figure 1). Negative values indicated that the preparation was inclined at the expense of the outer canal wall and vice-versa.

Figure 1.

Measurements of the canal deviation from the overlayed original and prepared canal walls using ImageJ software.

2.6. Statistical analysis

Descriptive analysis of the data was conducted using SPSS (IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp Version 22). Data distribution was normal according to the Shapiro–Wilk test, so Chi Square tests, multi-factorial analysis of variance, one-way ANOVA followed by Post-hoc Tukey tests. Statistical significance was set at p < 0.05.

3. Results

The mean time endodontists needed to prepare the canal was significantly less time (4.15 ± 1.6 min) compared to students (13.31 ± 6.05 min) and general practitioners (9.34 ± 2.91 min) (p = 0.001 and p = 0.019). However, operator time did not statistically differ between trials (p = 0.754). Figure 2 shows the three trials performed by operators from the three groups.

Figure 2.

Preparation of the simulated resin canals performed by undergraduate dental students, GPs and Endodontists.

Procedural errors were either instrument fracture and/or canal transportation (ledge – canal perforation). The frequency of procedural errors in the three operator groups can be seen in Table 1. Instruments fractured in nine blocks (20%). Canal transportation was a common error, especially among GPs (73.3%) and students (46.7%). The Chi-square test revealed that the occurrence of total procedural errors was significantly related to the operator (p = 0.027), with endodontists making the fewest procedural errors, followed by students; the GPs group was associated with the greatest number of errors.

Table 1.

Frequency of different procedural errors according to different operators.

Operator	N	Instrument Fracture		Canal Transportation		Total Procedural Errors
		N	%	N	%	N	%
Student	15	4	26.7%	7	46.7%	10	66.7%
GP	15	3	20%	11	73.3%	14	93.3%
Endodontist	15	2	13.3%	5	33.3%	7	46.7%

A significant difference between operators was found in deviation measurements in the apical third (1 mm–3 mm from the apical foramen). Endodontists and students alike prepared the canal with significantly less deviation towards the outer canal wall compared to GPs (p = 0.006 and p = 0.002) (Table 2).

Table 2.

Post hoc Tukey test for canal deviation measurements according to operator groups.

Canal Level	Operator Groups		Deviation Mean Difference	Sig.
D1	Student	Endodontist	-0.1077	0.710
		GP	0.3101	0.088
	Endodontist	Student	0.1077	0.710
		GP	0.4178	0.012∗
	GP	Student	-0.3101	0.088
		Endodontist	-0.4178	0.012∗
D2	Student	Endodontist	0.0512	0.833
		GP	0.3408	0.002∗
	Endodontist	Student	-0.0512	0.833
		GP	0.2896	0.006∗
	GP	Student	-0.3408	0.002∗
		Endodontist	-0.2896	0.006∗
D3	Student	Endodontist	0.0929	0.441
		GP	0.2651	0.004∗
	Endodontist	Student	-0.0929	0.441
		GP	0.1722	0.063
	GP	Student	-0.2651	0.004∗
		Endodontist	-0.1722	0.063

^∗The mean difference is significant at the 0.05 level.

4. Discussion

In spite of the innovative instrument design, conservative instrumentation sequence and alloy treatment of Race Evo, it was only operators of advanced experience level (endodontists) who prepared the canals with fewer procedural errors and in less time, compared to those of less or inexperience. This is consistent with previous studies that maintained a positive role for operator experience [12, 13]. Several studies have found that rotary NiTi instruments fracture more frequently in inexperienced hands [9, 10, 11]. In the present study, although more instruments fractured in the student group, there was no significant difference between the three operator groups (p = 0.854). This is in accordance with a study by Alencar et al. (2010) that found no significant difference in instrument fracture between students and endodontists during NiTi rotary preparation of moderately curved canals [20].

At first glance, the incidence of fracture was relatively high (20%) compared to other studies (which found less than 10%) [21, 22]. However, it was less than the incidence of Profile fracture (29%) during “the learning period” for that NiTi system [23]. In contrast to Madarati and Habib (2018) [6], who reported frequent separation of larger diameter or taper instruments, such as the Profile GT size 20 taper.06 and ProTaper F3 (size 30 taper.09), the 15/0.04 file most frequently fractured in the present study. A study comparing the Scout Race (a size 15/0.02 rotary file) with other glide path instruments, found that the Scout Race was the least resistant to cyclic fatigue. This was attributed to its alternating pitch design, which is similar to that of the Race Evo [24]. It could be argued that the degree of canal curvature subjected the 15/0.04 instruments to significant fatigue after the first use, which resulted in their fracture during subsequent uses. However, no significant difference was found in occurrence of instrument fracture between the trials.

Many studies found that the Race and iRace rotary instrumentation systems have good shaping ability and prepare the apical third of root canals with little transportation [15, 25]. Although the new Race Evo boasted the same design, with a superior heat-treated NiTi alloy, canal transportation occurred frequently in this study. A reason for this may have been the severity and model of canal curvature (small radius of curvature) [26]. It has been suggested that canals with high degrees of curvatures and small radii cannot be instrumented without some degree of transportation, regardless of the instrument’s alloy [26]. A recent study conducted on natural primary molars found that there was no significant difference between conventional Race NiTi files and stainless-steel hand files in terms of surfaces cleaned during canal preparation [27]. In the present study, the negative deviation values recorded at 1–3 mm in the apical third were consistent with the literature, which indicates that transportation occurred in the most apical part of the canal [25]. This observation, however, may be related to the micro-hardness of the resin blocks used in the study. Due to resin’s lower micro-hardness compared to dentin, occurrence of an irregular enlargement of the canal apical third, without transportation of the foramen (outer widening) was exclusively reported in simulated resin canal preparation [28].

The Race Evo instruments' heat treatment allows them to undergo the phase transition between martensite and austenite at a temperature of 32–35 °C [29]; perhaps this was not adequately simulated during preparation of the resin blocks in this study. At lower room temperature, the file will exhibit a stress-induced martensitic state, which is unstable. So, an immediate retransformation from the martensitic to the austenite phase occurs, which would lead to a spring-back effect from the instrument [30].

Only 31% of the canals were successfully instrumented without the occurrence of any procedural errors. A possible explanation for the high incidence of procedural errors is the simulated resin blocks, which differ from human dentin in both hardness and thermal properties [31]. In addition to the size of the resin shavings created during preparation, which when compounded with difficulty of removing with irrigation, increases the incidence of canal blockage and subsequent canal transportation [28]. Studies reporting a lower incidence of procedural errors were performed on natural teeth with moderate curvature (around 45° with radii of 5–8 mm) [20, 22]. Another explanation may be the fact that none of the participants had any prior experience using the Race Evo instrumentation system, nor previous versions of the Race system. While other studies evaluating NiTi systems' instrumentation of severely curved canals, were performed by operators experienced with the instrumentation system being evaluated [32].

Operator experience in this study was defined as experience with the use of NiTi rotary instrumentation per se, and not experience of the particular system -Race Evo- being studied. As a result, the three blocks distributed to each participant during the study corresponded to the learning phase described by Mandel et al. (1999) [23]. Even for endodontists, three blocks do not appear to provide enough of a learning period, as performance did not improve significantly between the first and third trials. Competence in using certain NiTi instrumentation systems does not automatically translate into using others.

An interesting finding in this study was that third-year dental students prepared canals with fewer procedure errors compared to GPs. Baumann and Roth (1999) discovered no such distinction between GPs and students [22]. An explanation could be that the participating students had been attending the preclinical endodontic course and were engaged in active learning about root canal treatment, hence approached the presented task with more caution. Their prudence would explain the longer time that they took to instrument the canals compared to the GPs [33]. The GPs, on the other hand, may have approached NiTi instrumentation more aggressively [22], or perhaps were unprepared to deal with curved canals, since they referred difficult cases in their practices to endodontic specialists. This highlights the importance of capitalizing on the undergraduate period to expose dental students to NiTi rotary instrumentation in order to build their competence in using more than one instrumentation system.

More research is recommended on how to best utilize undergraduate dental education to produce more proficient dentists in root canal treatment. The skills necessary are not guaranteed to develop with more experience as a general dental practitioner.

Although the use of simulated resin-blocks allowed for standardization of canal anatomy in terms of diameter and taper, angle and radius of a curvature. The results rendered cannot be readily translated to the clinical situation due to the differences between resin and dentin [28]. Another limitation of the current study is the small number of operators and the convenient sampling that was used to recruit the participants which may have influenced the results. Despite the fact that sample size calculations attempted to account for possible exclusions due to instrument fracture, the distribution of these exclusions was unequal among the operator groups and may have been a source of bias when comparing canal deviation among the different operators.

NiTi rotary systems have become an essential part of providing adequate root canal treatment. Dental school programs should focus on their undergraduate students' proficiency in using these systems. However, with the ever-evolving market of NiTi instruments, it is up to continuing education training to address this issue and provide scientific development for the unique set of psychomotor skills required.

Endodontists and general dental practitioners should be aware that regardless of previous experience using NiTi rotary systems, a learning period will be required for new systems. It is recommended that they invest in some form of focused training before employing the system for the first time on patients.

5. Conclusion

Within the limitations of this study, proficiency in instrumenting curved canals with a new conservative NiTi system was not influenced by experience related to that of the general dental practitioner, but required more specialized experience in root canal treatment. While there was no significant difference between the performance of students and GPs, endodontists prepared curved canals using Race Evo in significantly less time, and with fewer procedural errors. Regardless of experience level, a “learning period” before using such rotary systems is still needed. Attending extensive hands-on workshops may prove beneficial.

Declarations

Author contribution statement

Reem Barakar and Rahaf Almohareb: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R162), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia (PNURSP2022R162).

Data availability statement

Data will be made available on request.

Declaration of interest’s statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.