The evaluation of the technical quality of root canal fillings performed by preclinical students on extracted teeth
Authors: Mehmet Aksan, Ozgur Genc Sen, Elif Bastug Guven
Published: 2026

ABSTRACT
BackgroundThe aim of this study was to evaluate the technical quality of root canal fillings performed by undergraduate preclinical students and to interpret the findings according to tooth group and canal configuration.MethodsThis retrospective observational study included 1,050 root canal fillings of extracted teeth performed by 120 third-year dental students between 2019 and 2021. Canal preparation was performed using a standardized manual step-back instrumentation protocol, and obturation was performed using cold lateral condensation with gutta-percha cones and resin-based sealer. Standardized digital periapical radiographs were used to assess treatment quality based on length, homogeneity, and taper criteria. A scoring system (0, 1, or 2) was used for statistical analysis, and inter-examiner agreement was assessed using the Kappa test.ResultsOverall, 692 of 1,050 root canal fillings (65.9%) were technically adequate. Adequacy was highest in mandibular premolars (82.6%) and lowest in mandibular molars (44.8%). The rate of adequate root canal fillings was significantly higher in single-rooted teeth than in multi-rooted teeth (p < 0.05). Among the evaluated criteria, adequacy rates were 82.3% for length, 82.4% for taper, and 77.5% for homogeneity. The lowest adequacy rates for all three criteria were observed in mandibular molars.ConclusionsMost root canal fillings performed by preclinical students met the predefined technical adequacy criteria. Lower adequacy rates were observed in multi-rooted teeth, particularly mandibular molars. These findings should be interpreted within the limitations of a single-institution retrospective preclinical study.

Background

In dental education, the preclinical practice course is fundamentally designed to enhance students’ manual dexterity and technical skills [1]. Students need to undergo comprehensive, hands-on training in a laboratory setting to establish a solid foundation before transitioning to patient care [2]. In many dental schools worldwide, endodontic preclinical training is conducted using extracted human teeth [3]. Throughout this process, instructors and assistants guide students through every stage of root canal treatment, striving to equip them with the necessary competence to perform high-quality root canal fillings [4].

Radiographic techniques are the most commonly employed methods to evaluate the technical quality of root canal fillings [5]. According to the guidelines established by the European Society of Endodontology (ESE), an adequate root canal filling should exhibit a conical shape extending continuously from the coronal to the apical portion. In addition, the filling material should be placed homogeneously without voids between the filling and the canal walls, and the obturation should ideally terminate within 0.5–2 mm of the radiographic apex [6]. Achieving these technical standards consistently can be challenging for students during their initial training phase. Although preclinical courses provide essential foundational knowledge for future clinical practice, studies focusing specifically on standardized radiographic evaluation of endodontic preclinical training outcomes across different tooth groups remain limited [7, 8]. Recent dental education studies have also emphasized that students’ perceived difficulty, confidence, and procedural concerns may influence learning in endodontics [9, 10]. However, the technical quality of root canal fillings performed on extracted teeth remains insufficiently characterized in relation to tooth group and canal configuration in the preclinical setting. We hypothesized that technical adequacy would be lower in multi-canal tooth groups than in teeth with simpler single-canal anatomy. Therefore, the purpose of this study was to evaluate the technical quality of root canal fillings performed by preclinical students at the Yuzuncu Yil University Faculty of Dentistry and to interpret the findings according to tooth group and canal configuration.

Methods

Ethical approval and study design

This retrospective observational study was approved by the Non-Interventional Clinical Research Ethics Committee of Yuzuncu Yil University (Decision number: 2021/08 − 03). The study was conducted in accordance with the ethical principles of the Declaration of Helsinki. The study was based on archived preclinical training samples obtained during the undergraduate endodontic preclinical course. For standardization, new digital periapical radiographs of all completed samples were obtained by a single researcher under the same radiographic conditions before evaluation.

Study sample and eligibility criteria

No prospective sample size calculation was performed because this was a retrospective observational study based on archived preclinical training samples. All eligible completed samples available in the assignment archive between 2019 and 2021 were included. The final sample consisted of 1,050 teeth, which was considered adequate for descriptive evaluation of root canal filling quality across the predefined tooth groups. Extracted human teeth were eligible for inclusion if they had been used by third-year undergraduate dental students during the preclinical endodontic training course, had completed root canal treatment and obturation, and could be classified into one of the predefined tooth groups. The predefined tooth groups were incisors, canines, mandibular premolars, maxillary first premolars, mandibular molars, and maxillary molars. Mandibular premolars included in the dataset were evaluated as single-rooted/single-canal teeth, whereas maxillary first premolars were evaluated separately because they were treated as two-canal teeth in the preclinical protocol and in the radiographic assessment. For standardization, new digital periapical radiographs of all completed samples were obtained by a single researcher under the same radiographic conditions before evaluation. No samples were excluded on the basis of previously available or poor-quality radiographs. Teeth were excluded only if the root canal filling procedure had not been completed, if the tooth type could not be clearly identified, or if severe structural defects, root fracture, extensive resorption, open apices, or marked canal calcification prevented standardized assessment of filling length, taper, or homogeneity.

Preclinical application and data collection

The study included 1,050 root canal fillings performed on extracted teeth by 120 third-year undergraduate preclinical students at Yuzuncu Yil University, Faculty of Dentistry, between 2019 and 2021. (Fig. 1). Before each session, the responsible assistants provided anatomical training regarding the specific tooth group (incisors, canines, premolars, or molars) and demonstrated the root canal treatment procedure on extracted teeth. Biomechanical preparation was performed manually; no rotary or reciprocating instrumentation system was used in the preclinical protocol evaluated in this study. Root canal preparation was performed using the step-back technique with 0.02 taper stainless steel K-files (VDW GmbH, Munich, Germany), and coronal flaring was performed using Gates-Glidden drills (VDW GmbH, Munich, Germany). A 2% sodium hypochlorite solution (Microvem, Turkey) was used for irrigation. The canals were obturated with 0.02 taper gutta-percha cones (VDW GmbH, Munich, Germany) and a resin-based root canal sealer (ADSEAL, Meta Biomed Co., Ltd., Cheongju, Republic of Korea) using the cold lateral condensation technique. The sealer was used in accordance with the standardized preclinical course protocol and was introduced into the canal with the gutta-percha cones during obturation under instructor supervision. The same preclinical protocol was applied to all samples during the relevant training sessions. Students practiced on extracted teeth embedded in plaster or acrylic blocks during three-hour laboratory sessions. Endodontic treatment for each sample was intended to be completed in a single scheduled laboratory session; students who could not complete the procedure within class time were granted an additional three hours of supervised practice under the same course protocol. Completed samples were collected at the end of the training sessions and stored in the assignment archive room until radiographic evaluation.

Fig. 1Distribution of teeth included in the study according to type and location

Distribution of teeth included in the study according to type and location

Radiographic procedure

To ensure standardization, digital periapical radiographs of all completed samples were retaken by a single researcher under standardized conditions before evaluation. The procedure was performed at 70 kVp, 10 mA, and 0.25 s exposure time using a dental X-ray unit (Gendex Expert DC, KaVo, Germany) and a digital sensor (Planmeca Oy, Helsinki, Finland). Radiographs were taken in the buccolingual direction; for multi-rooted teeth, a 20-degree mesial or distal angulation was applied to reduce root superposition. The distance between the X-ray tube and the tooth was set to 2 cm, and the tooth-sensor distance was 0.5 cm (Fig. 2).

Fig. 2X-ray image of the tooth embedded in the plaster

X-ray image of the tooth embedded in the plaster

Radiographic evaluation

Planmeca Romexis® (Version 3.2.0, Helsinki, Finland) was used for radiographic measurements, contrast adjustment, and magnification.

Before the evaluation, a calibration session was conducted in which the two examiners independently reviewed a sample set of 50 radiographs. Discrepancies were discussed with a senior endodontist until consensus on the interpretation of the ESE criteria was achieved. Treatment quality was evaluated by two postgraduate endodontic students from the Department of Endodontics who had received training in radiographic assessment of endodontic treatment quality and were calibrated under the supervision of a senior endodontist. During evaluation, the examiners were blinded to student identity; however, blinding to tooth group was not feasible because tooth morphology was visible on the radiographs. Treatment quality was assessed based on root canal filling length, homogeneity, and taper (Table 1). Each criterion was scored as 0, 1, or 2. If the criterion was met, it was scored as 1; if not met, it was scored as 0. For the length parameter only, a score of 2 was used to indicate overfilling. Overall filling quality was scored as adequate only when all criteria were scored as 1. In multi-rooted teeth, the filling was considered adequate only if all canals in all roots met the length, homogeneity, and taper criteria.

Table 1Criteria used in evaluating the technical quality of root canal fillingsCriteriaDefinitionScoreLengthRoot canal filling ending > 2 mm short of the radiographic apex (Fig. 3b)0The apical end of the root canal filling ends 0–2 mm from the radiographic apex (Fig. 3e)1Root canal filling beyond the radiographic apex (Fig. 3a)2TaperNon-consistent taper (Fig. 3d)0Consistent and uniform taper from the root canal orifice to the apex (Fig. 3e)1HomogeneityThere are voids between the root canal filling and the walls (Fig. 3c)0There are no voids present between the root canal filling and the walls (Fig. 3e)1Root Canal Filling QualityInadequate technical quality (Fig. 3a, b, c and d)(Failure to meet one or more of the length, taper, and homogeneity criteria)0Adequate technical quality (Fig. 3e)(All criteria are met)1

Table 1Criteria used in evaluating the technical quality of root canal fillingsCriteriaDefinitionScoreLengthRoot canal filling ending > 2 mm short of the radiographic apex (Fig. 3b)0The apical end of the root canal filling ends 0–2 mm from the radiographic apex (Fig. 3e)1Root canal filling beyond the radiographic apex (Fig. 3a)2TaperNon-consistent taper (Fig. 3d)0Consistent and uniform taper from the root canal orifice to the apex (Fig. 3e)1HomogeneityThere are voids between the root canal filling and the walls (Fig. 3c)0There are no voids present between the root canal filling and the walls (Fig. 3e)1Root Canal Filling QualityInadequate technical quality (Fig. 3a, b, c and d)(Failure to meet one or more of the length, taper, and homogeneity criteria)0Adequate technical quality (Fig. 3e)(All criteria are met)1

Criteria used in evaluating the technical quality of root canal fillings

Inadequate technical quality (Fig. 3a, b, c and d)

(Failure to meet one or more of the length, taper, and homogeneity criteria)

Adequate technical quality (Fig. 3e)

(All criteria are met)

Fig. 3a.Lower molar with overfilling (score 2). b.Canine tooth with underfilling (score 0). c. Incisor tooth root canal filling with inadequate homogeneity (score 0). d. Lower premolar tooth root canal filling with inadequate taper (score 0). e. Canine tooth root canal filling with adequate length, taper, and homogeneity (score 1)

a.Lower molar with overfilling (score 2). b.Canine tooth with underfilling (score 0). c. Incisor tooth root canal filling with inadequate homogeneity (score 0). d. Lower premolar tooth root canal filling with inadequate taper (score 0). e. Canine tooth root canal filling with adequate length, taper, and homogeneity (score 1)

Statistical analysis

Descriptive statistics were expressed as mean, standard deviation, minimum, and maximum for continuous variables, and as frequencies and percentages for categorical variables. Adequacy rates between predefined independent tooth groups were compared using the Z-test for proportions, as the primary comparisons involved proportional outcomes across independent groups. No correction for multiple pairwise comparisons, such as Bonferroni adjustment, was applied; therefore, the pairwise comparison results were interpreted cautiously together with the corresponding adequacy rates to reduce the risk of overstatement. Inter-examiner agreement was determined by calculating the Kappa coefficient and interpreted according to the Landis and Koch classification [11]. The statistical significance level was set at 5%, and calculations were performed using SPSS version 20.0.

Results

The statistical evaluation to determine inter-examiner agreement yielded a Kappa value of 0.797, indicating substantial agreement and supporting the reliability of the radiographic assessments.

In total, 1,050 teeth were evaluated, and 692 root canal fillings (65.9%) were classified as technically adequate. Adequacy rates varied among tooth groups, with the highest rate observed in mandibular premolars (82.6%) and the lowest rate observed in mandibular molars (44.8%). Detailed adequacy rates and pairwise comparisons between tooth groups are presented in Tables 2 and 3. Overall, adequate filling rates were significantly higher in single-rooted teeth than in multi-rooted teeth (p < 0.05), and mandibular molars showed the lowest adequacy rate among all tooth groups.

Table 2Evaluation of the quality of root canal fillings in teethAdequate Number of Root Canal FillingsN (Total Number of Teeth)Incisors173(%82.4)210Canines120(%76.9)156Mandibular Premolars90(%82.6)109Maxillary First Premolars102(%64.6)158Mandibular Molars94(%44.8)210Maxillary Molars113(%54.6)207Total692(%65.9)1050

Table 2Evaluation of the quality of root canal fillings in teethAdequate Number of Root Canal FillingsN (Total Number of Teeth)Incisors173(%82.4)210Canines120(%76.9)156Mandibular Premolars90(%82.6)109Maxillary First Premolars102(%64.6)158Mandibular Molars94(%44.8)210Maxillary Molars113(%54.6)207Total692(%65.9)1050

Evaluation of the quality of root canal fillings in teeth

120

(%76.9)

90

(%82.6)

102

(%64.6)

94

(%44.8)

113

(%54.6)

692

(%65.9)

Table 3Root canal filling quality rates and intergroup comparisonsDental Group (Quality filling rate) P Incisors (%82.4) - Canines (%76.9)0,202Incisors (%82.4) - Mandibular Premolars (%82.6)0,967Incisors (%82.4) – Maxillary First Premolars (%64.6)0,001*Incisors (%82.4) – Mandibular Molars (%44.8)0,001*Incisors (%82.4) – Maxillary Molars (%54.6)0,001*Canines (%76.9) - Mandibular Premolars (%82.6)0,255Canines (%76.9) – Maxillary First Premolars (%64.6)0,015*Canines (%76.9) – Mandibular Molars (%44.8)0,001*Canines (%76.9) – Maxillary Molars (%54.6)0,001*Mandibular Premolars (%82.6) – Maxillary First Premolars (%64.6)0,001*Mandibular Premolars (%82.6) –Mandibular Molars (%44.8)0,001*Mandibular Premolars (%82.6) – Maxillary Molars (%54.6)0,001*Maxillary First Premolars (%64.6) - Mandibular Molars (%44.8)0,001*Maxillary First Premolars (%64.6) - Maxillary Molars (%54.6)0,053Mandibular Molars (%44.8) – Maxillary Molars (%54.6)0,044** Indicates significant difference

Table 3Root canal filling quality rates and intergroup comparisonsDental Group (Quality filling rate) P Incisors (%82.4) - Canines (%76.9)0,202Incisors (%82.4) - Mandibular Premolars (%82.6)0,967Incisors (%82.4) – Maxillary First Premolars (%64.6)0,001*Incisors (%82.4) – Mandibular Molars (%44.8)0,001*Incisors (%82.4) – Maxillary Molars (%54.6)0,001*Canines (%76.9) - Mandibular Premolars (%82.6)0,255Canines (%76.9) – Maxillary First Premolars (%64.6)0,015*Canines (%76.9) – Mandibular Molars (%44.8)0,001*Canines (%76.9) – Maxillary Molars (%54.6)0,001*Mandibular Premolars (%82.6) – Maxillary First Premolars (%64.6)0,001*Mandibular Premolars (%82.6) –Mandibular Molars (%44.8)0,001*Mandibular Premolars (%82.6) – Maxillary Molars (%54.6)0,001*Maxillary First Premolars (%64.6) - Mandibular Molars (%44.8)0,001*Maxillary First Premolars (%64.6) - Maxillary Molars (%54.6)0,053Mandibular Molars (%44.8) – Maxillary Molars (%54.6)0,044** Indicates significant difference

Root canal filling quality rates and intergroup comparisons

* Indicates significant difference

The distribution of root canal fillings according to length, taper, and homogeneity is presented in Table 4. When each criterion was evaluated independently, adequacy rates were 82.3% for length, 82.4% for taper, and 77.5% for homogeneity. The lowest adequacy rates for all three criteria were observed in mandibular molars.

Table 4Findings on “length”, “taper”, and “homogeneity” according to tooth type and locationNumber of teethNumber of root canalLengthTaperHomogeneityAdequateUnderfillingOverfillingAdequateInadequateAdequateInadequateIncisors210210180(%85.7)18(%8.6)12(%5.7)190(%90.5)20(%9.5)186(%88.6)24(%11.4)Canines156156131(%84)16(%10.3)9(%5.7)138(%88.5)18(%11.5)134(%85.9)22(%14.1)Mandibular Premolars10910995(%87.2)7(%6.4)7(%6.4)95(%87.2)14(%12.8)95(%87.2)14(%12.8)Maxillary First Premolars158316258(%81.6)40(%12.7)18(%5.7)268(%84.8)48(%15.2)238(%75.3)78(%24.7)Mandibular Molars210630495(%78.6)94(%14.9)41(%6.5)470(%74.6)160(%25.4)433(%68.7)197(%31.3)Maxillary Molars207621522(%84)55(%8.9)44(%7.1)521(%83.9)100(%16.1)497(%80)124(%20)Total105020421681(%82.3)230(%11.3)131(%6.4)1682(%82.4)360(%17.6)1583(%77.5)459(%22.5)

Table 4Findings on “length”, “taper”, and “homogeneity” according to tooth type and locationNumber of teethNumber of root canalLengthTaperHomogeneityAdequateUnderfillingOverfillingAdequateInadequateAdequateInadequateIncisors210210180(%85.7)18(%8.6)12(%5.7)190(%90.5)20(%9.5)186(%88.6)24(%11.4)Canines156156131(%84)16(%10.3)9(%5.7)138(%88.5)18(%11.5)134(%85.9)22(%14.1)Mandibular Premolars10910995(%87.2)7(%6.4)7(%6.4)95(%87.2)14(%12.8)95(%87.2)14(%12.8)Maxillary First Premolars158316258(%81.6)40(%12.7)18(%5.7)268(%84.8)48(%15.2)238(%75.3)78(%24.7)Mandibular Molars210630495(%78.6)94(%14.9)41(%6.5)470(%74.6)160(%25.4)433(%68.7)197(%31.3)Maxillary Molars207621522(%84)55(%8.9)44(%7.1)521(%83.9)100(%16.1)497(%80)124(%20)Total105020421681(%82.3)230(%11.3)131(%6.4)1682(%82.4)360(%17.6)1583(%77.5)459(%22.5)

Findings on “length”, “taper”, and “homogeneity” according to tooth type and location

180

(%85.7)

18

(%8.6)

12

(%5.7)

190

(%90.5)

20

(%9.5)

186

(%88.6)

24

(%11.4)

131

(%84)

16

(%10.3)

9

(%5.7)

138

(%88.5)

18

(%11.5)

134

(%85.9)

22

(%14.1)

95

(%87.2)

7

(%6.4)

7

(%6.4)

95

(%87.2)

14

(%12.8)

95

(%87.2)

14

(%12.8)

258

(%81.6)

40

(%12.7)

18

(%5.7)

268

(%84.8)

48

(%15.2)

238

(%75.3)

78

(%24.7)

495

(%78.6)

94

(%14.9)

41

(%6.5)

470

(%74.6)

160

(%25.4)

433

(%68.7)

197

(%31.3)

522

(%84)

55

(%8.9)

44

(%7.1)

521

(%83.9)

100

(%16.1)

497

(%80)

124

(%20)

1681

(%82.3)

230

(%11.3)

131

(%6.4)

1682

(%82.4)

360

(%17.6)

1583

(%77.5)

459

(%22.5)

Discussion

The technical quality of root canal fillings is among the factors that directly affect the prognosis of root canal treatment [12]. It is vital that root canal treatment be taught in dental preclinical education at a level that ensures competence, as this is critical for both students’ preparation before transitioning to clinical cases and for ensuring professional standards and patient safety after graduation [1]. The aim of this study is to evaluate the quality of root canal fillings performed on extracted teeth by preclinical (third-year undergraduate) students at the Faculty of Dentistry, Yuzuncu Yil University. While there are many clinical studies in the literature evaluating root canal filling quality [13], few studies have focused specifically on preclinical applications [7]. Therefore, our study contributes to filling this gap in the literature.

Some studies investigating root canal filling quality have evaluated fillings in multi-rooted teeth separately for each canal [14].However, this approach does not reflect clinical reality [7]. In multi-rooted teeth, inadequate filling in even a single canal can lead to the failure of the root canal treatment for the entire tooth [15]. For this reason, in this study, as in many others, all root canal fillings within a tooth had to meet the adequacy criteria for the treatment to be considered successful [16].

In our study, radiographic images of root canal fillings were evaluated based on the criteria of length, taper, and homogeneity [17]. Different evaluation criteria can be used in studies of root canal filling quality [18]. While some studies radiographically evaluate only the length and homogeneity of root canal fillings [19], others, such as ours, included taper among these criteria [20]. These criteria were recommended by the American Association of Endodontists (2009) for evaluating root canal filling quality and have been used in many previous studies [21, 22].Many studies have reported that a root canal filling 0–2 mm short of the radiographic apex with adequate density provides faster healing in periradicular tissues [23]. Therefore, in the evaluation of the length criterion in our study, a distance of 0–2 mm from the radiographic apex was considered “adequate”, a distance of more than 2 mm was considered “short”, and filling beyond the radiographic apex was considered “overfilled”.

In our study, the rate of root canal fillings with adequate length was 82.3%. These results are similar to the 84% rates of adequate length found in studies by Gadamer (2018) [24], Unal et al. (2011) [5], and Kelbauskas et al. (2009) [13]. In other research in the literature, these rates were lower [17]. For instance, Barrieshi-Nusair et al. (2004) [17] reported 61.3%, Eleftheriadis and Lambrianidis (2005) [25] 62.7%, and Lupi-Pegurier et al. (2002) [26] 39%. In their study in Serbia, Vukadinov et al. (2014) found the rate of adequate root canal filling length to be 89.73%. This result, which represents one of the highest success rates in the literature, may be due to the authors considering a distance of 0–3 mm between the root canal filling and the radiographic apex as adequate [27].

The rates of short and overfilled root canal filling vary significantly in the literature [28]. Barrieshi-Nusair et al. (2004) [17] found a short filling rate of 34.5% and an overfilling rate of 4.2%, while Kelbauskas et al. (2009) [13] found a short filling rate of 10.5% and an overfilling rate of 5.42%. In our study, the short filling rate was 11.3%, and the overfilling rate was 6.4%. Compared to other studies, these rates can be considered at a successful level [29]. Adams et al. (2008) [30], Er et al. (2006) [16], and Barrieshi-Nusair et al. (2004) [17] found the highest short filling rate in mandibular molars. In our study, the group with the highest short filling rate was also mandibular molars (14.9%). This may be related to the complex anatomy of mandibular molars, the presence of multiple canals, and the generally higher incidence of curved roots [31].

These anatomical features may create clinical challenges; specifically, mandibular molars often present with narrow, constricted canals and significant mesial root curvatures, which pose a substantial challenge for students using the step-back technique with stainless steel files [32]. Such complex morphologies may increase the risk of procedural errors, such as ledging or apical transportation, particularly for inexperienced operators [25]. Furthermore, the presence of isthmuses and lateral canals in the molar regions may have contributed to the lower homogeneity scores recorded in these tooth groups; such structures are difficult to clean and effectively obturate using the cold lateral condensation technique [33]. These findings suggest that conventional hand-instrumentation protocols may be more challenging for students in complex molar systems; however, whether alternative instrumentation or irrigation training would improve outcomes requires further investigation [34].

The density and homogeneity of the root canal filling are important for the success of endodontic treatment [35]. Studies evaluating root canal fillings have reported that homogeneously filled canals show more consistent healing and function over longer periods [33]. Similarly, teeth with nonhomogeneous root canal fillings of inadequate density may have poor prognoses due to microleakage [36]. Many researchers have reported that nonhomogeneous and inadequately condensed root canal fillings significantly increase the prevalence of apical periodontitis [6]. Therefore, homogeneity was used as a criterion in the evaluation of root canal filling quality in our study.

Creating a dense and homogeneous filling may be more difficult in multi-rooted teeth than in single-rooted teeth. Román-Richon et al. (2013) also stated that density problems increase as the number of canals increases [37]. In our study, the rate of adequate homogeneity and density in multi-canal teeth, especially in mandibular molars, was lower than in single-rooted teeth. Among the criteria we evaluated (taper, length, homogeneity), homogeneity had the lowest adequacy rate at 77.5%. This result is similar to the homogeneity rates of 79.42% reported by Akbar et al. (2017) [38] and 75.8% by Elemam et al. (2015) [39]. In studies on root canal filling quality in the literature, rates of adequate homogeneity range from 35% to 92.6% [18].These differences in success rates may be related to factors such as the type of tooth studied, student skill, preparation and filling techniques, and the canal instruments used.

Although some studies do not include taper as a criterion because they consider it subjective [25], most studies, similar to ours, state that taper should be considered so that the filling follows the original shape of the canal [20].In our study, the rate of root canal fillings with adequate taper was 82.4%. This result is quite similar to the 82% rate of adequate taper found by Fonseka et al.(2013) [40]. However, our rate was lower than the 97% adequate taper rate found by Fong et al. (2018) [41]. The use of rotary instruments may have resulted in the high rate of adequate taper in that study [41]. Additionally, some studies in the literature have reported adequate taper rates of 60% (Balto et al., 2010) [7], 44.9% (Eskandarloo et al., 2017) [20], and 68.8% (Elemam et al., 2015) [39]. These rates are lower than the results of our study. These differences in taper values may be influenced by many factors, such as the use of rotary instruments, Gates–Glidden drills, the tooth groups studied, and flaring techniques [40].The use of Gates–Glidden drills by the students in our study may have allowed them to create a better taper through effective flaring in the coronal and middle thirds.

Studies of root canal filling quality in the literature show widely varying results [28]. In our study, the rate of adequate quality root canal filling was 65.9%. This result is very similar to the rates of 66% found by Fong et al. (2018) [41] and 65.7% by Rapo et al. (2017) [42]. However, Hayes et al. (2001) [1] reported 13%, Elsayed et al. (2011) [29] 24.2%, Adams et al. (2008) [30] 32.5%, and Habib et al. (2018) [43] 32.6%, all lower than our study. The low percentage of adequate root canal fillings may be influenced by many factors, such as study design, criteria followed, techniques used for root canal treatment, academic staff-to-student ratio, and whether the training is provided by specialist endodontists [34].

Barrieshi-Nusair et al. (2004) determined an adequate root canal filling rate of 47.4% in their clinical study [17]. Our success rate was higher than those reported in clinical studies, likely because working on extracted teeth eliminated intraoral obstacles, such as the tongue and cheek, as well as moisture from saliva. Additionally, the field of view decreases as one moves toward the posterior regions of the mouth [26]. In our study, the students worked with a direct field of view on teeth embedded in plaster or acrylic. Even though they were less experienced than clinical students, these factors may have enabled them to perform more successful treatments.

Studies by Vukadinov et al. (2014) and Unal et al. (2011), which found higher rates than our study, reported adequate root canal filling rates of 74.2% and 79%, respectively. In these studies, length and homogeneity were evaluated as criteria, but taper was not [5, 27].

Regarding tooth group comparisons, the similar adequacy rates observed for incisors and mandibular premolars may be related to their relatively simpler canal configurations in the present sample. This interpretation is supported by previous studies reporting no significant difference between anterior and premolar teeth in terms of adequate root canal filling quality [33, 44]. By contrast, maxillary first premolars and molars showed lower adequacy rates, which may reflect the greater procedural difficulty associated with multi-canal anatomy. Several studies have similarly reported that technical quality tends to decrease in posterior teeth and in teeth with more complex root canal morphology, and that premolars may show higher adequacy rates than molars [25, 33, 37, 39, 45].

Baaij and Özok (2018) [4] compared students supervised by an endodontist (88% good quality) versus a general dentist (59% good quality) in uncomplicated root canal treatments and observed significantly more quality fillings in those supervised by an endodontist. In our faculty, endodontics education is provided by specialists in this department and by residents continuing their specialty training. The importance of the academic staff/student ratio in student education has been emphasized by ESE, although no minimum required ratio has been specified [46]. In our faculty’s preclinical education during the years covered by the study, the average academic staff/student ratio was 1:16. In a recent study, Sacha et al. (2021) reported this ratio as 1:4 for the best and 1:38 for the worst (1:15 mean) among German-speaking countries [3]. In 2022, Mergoni et al. reported academic staff/student ratios ranging from 1:4 to 1:20 across 28 Italian Dental schools [47]. While our ratio is reasonable compared to ratios reported in the literature, increasing the number of academic staff could enable easier communication with students and improve the quality of education.

This study has several limitations. First, it was conducted at a single institution and included only third-year preclinical students; therefore, the findings may not be generalizable to students from different institutions or with different levels of clinical experience. A more comprehensive study involving multiple universities would likely provide findings with greater generalizability. However, the single-center design may also have enhanced the internal consistency and reliability of the findings by minimizing variability arising from differences in educational and clinical training backgrounds. Second, because the study was retrospective and based on archived preclinical samples, no prospective sample size calculation was performed, and the analysis was limited to the available records. Third, although radiographic procedures and evaluation criteria were standardized, two-dimensional periapical radiographs may not fully reflect the three-dimensional quality of root canal fillings. Fourth, student-related variables such as individual manual skill, prior experience, motivation, practice time, and perceived difficulty of each tooth group could not be controlled or evaluated. Fifth, the students worked with a direct field of view on teeth fixed in plaster or acrylic blocks rather than on phantom manikins or patients, which may limit direct translation to clinical conditions. Finally, the materials and educational protocols reflected the preclinical curriculum used between 2019 and 2021, including conventional hand instrumentation rather than rotary or reciprocating systems; therefore, the results may not fully reflect outcomes achievable with contemporary instrumentation protocols and should be interpreted in this context. Future studies should evaluate students’ perceptions of the difficulty of each tooth group and should include students from different institutions and training levels to better determine how perceived difficulty relates to technical performance.

Conclusion

This retrospective observational study found that 65.9% of root canal fillings performed by third-year preclinical students on extracted teeth met the predefined technical adequacy criteria. Adequacy was higher in teeth with simpler canal configurations and lower in multi-canal tooth groups, particularly mandibular molars. Within the limitations of a single-institution retrospective design, extracted-tooth setting, and two-dimensional radiographic assessment, these findings suggest that complex tooth groups may require additional attention during preclinical endodontic training. However, the results should not be directly generalized to clinical performance or patient outcomes. Further multicenter studies involving different student levels and contemporary training protocols are needed to confirm these observations.