This study found data about the technical quality of RCT and associated factors that might be used to support decisions about criteria for determining technical difficulty. The null hypothesis was rejected, and the degree of root canal curvature was the main factor associated with unsatisfactory technical quality of RCT, regardless of any other pre- or intraoperative variables. Therefore, a curvature degree cut-off point was set for the selections of cases for undergraduate students. Moreover, findings revealed that reciprocating and hand instrumentation have the same effect on the technical quality of RCT conducted by undergraduate students.
The criteria for technical quality in this study were built based on the 2006 ESE guidelines, which, although currently under review, are the most recent version available [29, 30]. According to the ESE criteria, a root canal treatment is technically adequate if working length is as close to the apex as possible, considering an apical constriction located at 0.5 to 2 mm from the apex, and if the canal is completely filled with a solid or semi-solid material and sealer. However, it is not clear if unintentional sealer extrusion into the periapical tissues should be inadequate. Therefore, this study classified technical quality according to two criteria: whether unintentional extrusion of sealer was accepted or not.
Since the 1950s, studies have claimed that overfilling decreases the success rate of RCT [31, 32]. The presence of extruded root canal fillings or any exogenous materials that may cause a foreign body reaction have been described as biological factors that may lead to the persistence of asymptomatic radiolucency after RCT. The widely accepted idea that gutta-percha is biocompatible is inconsistent with the clinical finding that extruded gutta-percha may be associated with delayed periapical healing [10, 31, 33–35].
The ASE criteria used here included sealer extrusion in the definition of adequate technical quality to differentiate unintentional extrusion from an overfilling that could impair periapical repair. According to Burklein et al. [28], unintentional extrusion may occur more frequently when there are large apical lesions and warm obturation techniques are used, because of the hydraulics generated by three-dimensional filling of the root canal system. Although not desired, small amounts of radiopaque material in the periapical region to the width of the periodontal ligament are highly unlikely to contribute to the perpetuation of apical periodontitis when the root canal system has been adequately cleaned and shaped [36]. A recent retrospective evaluation of the outcome of RCT with unintentional overfilling concluded that treatment prognosis was not associated with the presence of extruded, reabsorbed or persisting filling material in periapical tissues [8].
This study found that technical quality was satisfactory in 45.9% and 62.3% of the cases according to NSE and ASE criteria, respectively, which agrees with other findings in the literature [37–42]. The technical quality of RCT, associated with root canal disinfection, might affect healing outcomes [6]. An adequate quality of filling compaction [4, 35] and the absence of technical complications [43] impact healing positively, whereas underfilling is frequently associated with the inability to debride the apical segment of the root canal, which may harbour persistent intracanal infection [35, 44].
According to our data, sealer extrusion plays an important role on the rate of technical quality. Studies that did not accept sealer extrusion might have underestimated the technical outcomes of RCT conducted by students [12, 13, 37, 39–42, 45–53]. In addition, despite this vast literature reporting on the prevalence of an adequate technical quality of RCT in undergraduate schools, few studies have investigated factors that might affect technical difficulty [15].
According to our findings, the degree of canal curvature is the most important factor in the selection of cases for RCT by undergraduate students. Other studies have not discussed the importance of the degree of curvature; instead, they found that molar teeth are the dental group with the highest complexity and, therefore, play an important role in the technical quality of RCT [6, 11, 12, 15, 37–39, 42, 47, 49–51, 53].
Although there was an association of dental group with technical outcomes in bivariate analysis, the same effect was not found in multivariate analysis. Data demonstrated that the concept that the difficulty of an RCT might be primarily predicted by dental group should be reviewed, as about 84% of teeth have root canal curvatures [54, 55]. Studies have shown that curvatures are a risk factor for RCT outcomes [56–60], and their presence should, therefore, be included in decision making and case selection according to level of professional training.
Many methods to determine and classify root canal curvatures are available [61]. However, in clinical routine, simple measurement tools and periapical radiographs are used, which have limitations due to their two-dimensional quality. Cone-beam CT (CBCT) may show the actual anatomy of the canal and offer better diagnostic possibilities [61]. The reproducible method used in this study took into consideration that CBCT resources are not universally available. In fact, they are, unfortunately, scarce in the reality of many educational institutions, operators, and patients.
The complexity of root canal curvatures has been defined according to the system developed by Schneider [26], considering the degree of canal curvature, classified as straight (5º or less), moderate (10º to 20º), or severe (25º to 70º). Such classification does not include all degrees of curvature continuously. Studies that used Schneider’s method demonstrated that severe curvatures affect the technical quality of endodontic procedures [60–63]. The AAE classification (2019) [59] includes the degree of curvature using continuous measures to classify the difficulty of a RCT, from minimally difficult (< 10º), to moderately difficult (between 10º and 30º), and highly difficult (> 30º). However, operator skill remains out of this classification, and the technical capacity of less experienced operators may be overestimated. This study evaluated the degree of root canal curvature as a continuous variable, regardless of dental group. Technical quality was satisfactory for more than 50% of radiographic curvatures of up to 7º and 12º according to NSE and ASE criteria, respectively. Therefore, curvatures greater than 12º should be classified as highly complex for undergraduate students and should be cautiously indicated.
Apical resorption, not associated with technical quality in the bivariate analysis, was significant in multivariate analysis for the NSE criterion. The same was not observed in RCT when the ASE criterion was used, that is, apical resorption did not affect technical quality in these cases. Teeth with pulp necrosis always have apical inflammatory resorption, even if not visible radiographically [64]. Root resorption, which leads to a modification of the anatomy of the apical constriction, may favour sealer extrusion at the time of root canal filling [65]. In addition, straight canals with root resorption are more likely to have an unsatisfactory technical quality when sealer extrusion is not accepted [60]. For this reason, it may have ceased to be significant when the criteria for satisfactory technical quality included acceptance of unintentional sealer extrusion into the periapical tissues.
The sample evaluated in this study included a low proportion of endodontic retreatments, which may be considered a limitation. However, the cases included depict the endodontic treatments usually performed by undergraduate students [66]. Further studies with a greater number of secondary endodontic treatments conducted by undergraduate students should be performed to define technical quality predictors for this clinical condition and population. Moreover, most of the sample consisted of endodontic treatments conducted using hand instruments, which may be a study limitation, but reflects the reality of less economically developed countries. The use of engine driven instruments during undergraduate practice depends on its availability.
Type of instrumentation technique was not associated with the technical outcome of RCT by undergraduate students, regardless of the criteria used. Ex vivo studies found that preparations of curved canals using engine driven instruments are more centred [67, 68]. However, in the present study, technical outcomes were not affected by the type of instrumentation, not even in the supplementary analysis including only teeth with highly curved canals. In agreement with other findings [69], the main determinant of endodontic errors by undergraduate students was the difficulty, rather than the instrumentation method. However, advantages of the reciprocating technique, such as its less steep learning curve and increased self-efficacy of professionals in training [17–21], should be considered when recommending it for use in undergraduate clinics.
The technical quality of RCT plays a highly important role in healing outcomes and, consequently, in maintaining tooth functionality [4, 70, 71]. Evidence shows that technical quality is not the only important factor in periapical repair [36], but technical errors may favour the maintenance of microorganisms that cause periapical inflammation. Further clinical studies evaluating the impact of unintentional sealer extrusion on periapical outcome should be carried out.