Effect of preparation taper and sealer type on fracture resistance of mesiobuccal roots with MB2 canals in maxillary first molars: an in vitro study

Abstract

Background

The mesiobuccal roots of maxillary first molars frequently contain a second mesiobuccal (MB2) canal, where limited dentin thickness near the furcation may compromise root strength during instrumentation. This study evaluated the effects of MB2 preparation taper and sealer type on fracture resistance in these roots.

Methods

Sixty extracted maxillary first molars with an MB2 canal in the mesiobuccal root were identified using cone-beam computed tomography (CBCT). Teeth with two canals in the mesiobuccal root, either separate or merging within the apical 1–2 mm, were included. Mesiobuccal roots were sectioned at the furcation level and remaining root length from the cut surface to the apex was standardized to 9 ± 1 mm. The specimens were allocated into four experimental groups (n = 10 each) according to MB2 taper (25/0.04 or 25/0.06) and sealer type (bioceramic or epoxy resin–based). Two control groups (n = 10 each) were included: negative control, with no preparation or obturation, and positive control, with MB1 and MB2 canals prepared to 25/0.06 without obturation. Dentin thickness in the danger zone was measured before and after preparation at 8 mm from the apex using CBCT. Fracture resistance was evaluated by vertical loading using a universal testing machine.

Results

No significant differences were found among the groups in terms of initial dentin thickness. Dentin removal in the danger zone did not differ significantly between the 25/0.04 and 25/0.06 taper groups (p = 0.921). Preparation taper significantly affected fracture resistance (p < 0.001), whereas sealer type (p = 0.295) and the taper-sealer interaction (p = 0.559) were not significant. Mean fracture resistance values for the 0.04 taper groups ranged from 583.9 to 600.4 N, whereas those for the 0.06 taper groups ranged from 426.6 to 484.1 N. The negative control group exhibited the highest fracture resistance value (638.2 N), whereas the positive control group showed the lowest fracture resistance value (385.2 N).

Conclusions

Within the limitations of this study, MB2 preparation taper had a greater influence on fracture resistance than sealer type. Increasing taper reduced fracture resistance despite similar dentin removal at the danger-zone level, suggesting that overall root geometry and stress distribution may play an important role.

Background

Endodontically treated teeth may fail due to vertical root fractures (VRFs), which develop from cyclic fatigue and crack propagation within dentin [1, 2]. VRFs are a major cause of post-treatment tooth loss, with risk factors categorized into anatomical (predisposing) and iatrogenic (contributing) factors [1, 3]. Anatomical characteristics directly influence stress distribution, and stress concentration is frequently associated with features such as an isthmus, unfavorable geometry, and reduced radicular dentin volume [4–7].

In maxillary molars, vertical root fractures are most frequently observed in the mesiobuccal roots, which exhibit greater anatomical complexity [4]. These roots frequently contain a second mesiobuccal canal (MB2) and may also exhibit an isthmus between MB1 and MB2 [8–11]. The presence of an MB1-MB2 isthmus, combined with a narrow mesiodistal morphology and a furcation-facing “danger zone” on the distal wall, may further limit remaining dentin thickness after instrumentation and create a biomechanical environment prone to stress concentration [9, 12, 13].

Because the distal wall of the MB2 canal has been identified as a danger zone with limited dentin thickness, MB2 preparation should be performed cautiously to preserve radicular dentin and avoid excessive furcation-side dentin removal [14]. More extensive instrumentation of MB2 canals has been associated with reduced residual dentin thickness in the furcation-facing danger zone [15]. In addition, increasing preparation taper has been associated with lower fracture resistance [16, 17]. Lower taper preparations have been associated with higher fracture resistance [18], whereas in MB2 canals, even 0.04 taper preparation may increase the extent of critically thin dentin areas [19]. Nevertheless, evidence regarding the effect of preparation taper on fracture resistance remains inconsistent [20].

The biomechanical behavior of the root following preparation may depend not only on the amount of residual dentin but also on the physical properties of obturation materials [1]. Although root canal sealers have been suggested to influence fracture resistance through chemical and micromechanical interactions with dentin, the literature remains inconclusive regarding the effect of sealer type on fracture resistance [21]. Some studies have demonstrated higher fracture resistance with calcium silicate-based bioceramic sealers [22–24], whereas others have reported superior performance of epoxy resin-based sealers or no significant differences between groups [25–29].

Although MB2 anatomy, danger-zone dentin thickness, and the effects of preparation taper have been investigated separately, it remains unclear how MB2 preparation taper influences fracture resistance in the mesiobuccal roots of maxillary first molars. Moreover, whether sealer type modifies the biomechanical response to different MB2 preparation tapers has not been specifically evaluated. Therefore, the aim of this study was to evaluate the effects of MB2 preparation taper (0.04 and 0.06) on dentin thickness and fracture resistance and to determine whether different sealer types modify this biomechanical response. The null hypothesis was that MB2 preparation taper, sealer type, and their interaction would have no significant effect on fracture resistance.

Methods

Ethical approval for this study was obtained from the Non-Interventional Clinical Research Ethics Committee of Çanakkale Onsekiz Mart University (Approval No: 2024 − 192). The study was conducted in accordance with the principles of the Declaration of Helsinki. Informed consent was obtained for the use of extracted teeth for research purposes. The required sample size was calculated using G*Power 3.1 software (Heinrich-Heine University, Düsseldorf, Germany) based on data from a previous study [26]. Using the F-test family, an effect size of 0.672, an alpha level of 0.05, and a power of 0.95, the minimum required sample size was calculated as 54. To compensate for potential procedural errors and specimen loss, a total of 60 teeth (n = 10 per group) were included in the study.

Extracted maxillary first molars with fully developed roots, separate mesiobuccal roots, root curvature of less than 10°, and no caries, cracks, or fractures were selected. The teeth had been extracted for periodontal or prosthetic reasons and were stored in 0.1% thymol solution until use. Before experimental procedures, external tooth surfaces were cleaned of calculus, plaque, and soft tissue remnants using periodontal curettes. For CBCT scanning, the specimens were positioned in a silicone index to standardize their orientation. A total of 167 maxillary first molars were scanned using cone-beam computed tomography (CBCT) (NewTom 5G; QR Srl, Verona, Italy) with the following parameters: 110 kVp, 11.11 mA, and a 10 × 10 cm field of view, producing axial slices with a thickness of 0.100 mm. DICOM data were analyzed in a darkened room by an experienced radiologist using NNT software (version 11.5; NewNet Technologies, Naples, FL, USA). Teeth were considered eligible when an MB2 canal was present in the mesiobuccal root and the canal configuration corresponded to either Vertucci type IV, with two distinct canals extending to the apex, or Vertucci type II, in which the MB1 and MB2 canals merged within the apical 1–2 mm (Fig. 1A). Teeth with caries, cracks, fractures, internal or external resorption, previous endodontic treatment, canal calcification, severe curvature, or unsuitable MB2 canal configuration were excluded. Finally, 60 teeth met the inclusion criteria and were included in the study. (Fig. 2).

Fig. 1.

A Representative CBCT image of a maxillary first molar showing the mesiobuccal root. B Preoperative axial section at 8 mm from the apex showing dentin thickness measurements in the danger zone. C Postoperative section at the same level used to calculate dentin removal

Fig. 2.

Flow diagram showing specimen selection, group allocation, and fracture resistance testing

The mesiobuccal roots were sectioned at the furcation level using a precision cutting machine (Mecatome T215, Presi, Germany), and the remaining root length from the cut surface to the apex was standardized to 9 ± 1 mm. This approach was adopted to eliminate the influence of coronal tooth structure and to allow a more controlled evaluation of root-level biomechanical behavior. Working length was determined by inserting a size 10 K-file until it was visible through the apical foramen and subtracting 0.5 mm.

After sectioning, all specimens were examined under a stereomicroscope (SMZ745T, Nikon, Tokyo, Japan) at ×20 magnification to detect cracks or defects; specimens with defects were excluded. Buccolingual and mesiodistal dentin thicknesses were measured on the root cross-sections, and specimens were allocated to four experimental groups and two control groups (n = 10 each) to achieve comparable baseline dentin thicknesses.

The specimens were assigned to the following groups:

• Negative control: No root canal preparation or obturation was performed.

• Bioceramic 0.04 taper (BC 0.04): The MB1 canal was prepared to size 25/0.06, whereas the MB2 canal was prepared to size 25/0.04 using VDW Rotate instruments (VDW, Munich, Germany). Canals were obturated with a bioceramic sealer (TotalFill BC Sealer; FKG Dentaire SA, La Chaux-de-Fonds, Switzerland).

• Resin 0.04 taper (R 0.04): The MB1 was prepared to 25/0.06 and the MB2 to 25/0.04. Canals were obturated using an epoxy resin–based sealer (AH Plus; Dentsply DeTrey GmbH, Konstanz, Germany).

• Bioceramic 0.06 taper (BC 0.06): Both MB1 and MB2 canals were prepared to 25/0.06. Canals were obturated with TotalFill BC Sealer.

• Resin 0.06 taper (R 0.06): Both MB1 and MB2 canals were prepared to 25/0.06 and obturated with AH Plus.

• Positive control: Both MB1 and MB2 canals were prepared to 25/0.06 without obturation.

Canal preparation was performed using three VDW Rotate instruments in sequence. In all experimental groups, MB1 preparation was standardized at 25/0.06, whereas MB2 preparation was completed with either 25/0.04 or 25/0.06 according to group allocation; thus, taper variation was applied only to the MB2 canal to specifically evaluate its effect. All root canal preparations were performed by a single operator. Irrigation was carried out using a 30-gauge side-vented needle positioned 2 mm short of the working length. During instrumentation, each canal was irrigated with 2 mL of 2.5% sodium hypochlorite after each file, and a total of 6 mL of 2.5% sodium hypochlorite was used per canal during instrumentation. Final irrigation was performed using 2 mL of 17% EDTA, followed by 2 mL of 2.5% sodium hypochlorite and 2 mL of sterile saline. Post-preparation CBCT scans were obtained using the same imaging parameters.

After preparation, the canals were obturated with TotalFill BC Sealer or AH Plus according to the manufacturers’ instructions using a single-cone technique. VDW gutta-percha cones matching the final preparation size were used for obturation. Thus, a 25/0.06 cone was used in canals prepared to 25/0.06, whereas a 25/0.04 cone was used in MB2 canals prepared to 25/0.04. In the AH Plus groups, the sealer was coated onto the master cone and introduced into the canal together with the cone. In the TotalFill BC Sealer groups, the sealer was placed into the canal using a plastic intracanal tip, after which the master cone was inserted. The coronal canal entrances were sealed with a temporary filling material (Coltosol F, Coltene/Whaledent, Altstätten, Switzerland). All instrumentation and obturation procedures were performed by the same operator. Specimens were stored under identical conditions at 37 °C and 100% humidity for two weeks in an incubator (Nüve, Ankara, Türkiye) before fracture testing to allow complete setting of the sealers.

After removal of the temporary filling material, roots were wrapped with 0.2-mm-thick aluminum foil and embedded in acrylic resin. Before complete polymerization of the resin, roots were removed, the aluminum foil was replaced with a thin layer of silicone impression material (Zhermack, Rovigo, Italy) to simulate the periodontal ligament, and specimens were re-embedded up to 2 mm below the coronal level [15]. Fracture resistance testing was performed using a universal testing machine (Shimadzu, Kyoto, Japan). A vertical compressive load was applied along the long axis of the root using a 2-mm-diameter stainless-steel spherical tip positioned at the center of the coronal root surface. The load was applied at a crosshead speed of 0.5 mm/min. The maximum load at fracture was recorded in Newtons, and failure was defined as the point at which a sudden drop in the load-displacement curve was observed.

CBCT images obtained before and after preparation were evaluated by an experienced radiologist and an endodontist who were blinded to the group allocation of the specimens. To assess taper-related dentin removal at the coronal aspect of the danger zone, where the diameter of tapered instruments increases, dentin thickness was measured at a level of 8 mm from the apex on the furcation side of both MB1 and MB2 canals (Fig. 1B-C). The amount of dentin removed in the MB2 canal was calculated by comparing pre- and post-preparation measurements. For calibration, inter-examiner agreement was assessed on 15 randomly selected teeth measured independently by both the endodontist and the radiologist. The intraclass correlation coefficient (ICC; two-way mixed-effects model, absolute agreement) for inter-examiner agreement was 0.775 (95% CI: 0.618–0.867; p < 0.001). All measurements were then performed by an endodontist with 11 years of experience. Intra-examiner reliability was assessed by repeating the measurements on the same 15 teeth. As dentin thickness was measured at two time points and two predefined sites per tooth, a total of 60 paired measurements were included. The ICC for intra-examiner reliability was 0.805 (95% CI: 0.693–0.879; p < 0.001).

Statistical analysis

Statistical analyses were performed using IBM SPSS Statistics for Windows (Version 20.0; IBM Corp., Armonk, NY, USA). The normality of the data distribution was assessed using the Shapiro–Wilk test, and homogeneity of variances was evaluated with Levene’s test. Parametric tests were used when normality and homogeneity assumptions were met; otherwise, non-parametric tests were applied.

To confirm baseline anatomical standardization among all groups, initial dentin thicknesses in the danger zone were compared using the Kruskal–Wallis test while mesiodistal (MD) and buccolingual (BL) root widths were analyzed using one-way analysis of variance (one-way ANOVA). The effect of preparation taper (0.04 vs. 0.06) on the amount of dentin removed from the danger zone at the MB2 level was evaluated using the Mann–Whitney U test.

In the experimental groups, the effects of preparation taper (0.04 and 0.06) and sealer type (bioceramic and epoxy resin–based) on fracture resistance were analyzed using two-way ANOVA. To compare fracture resistance across all groups, including the control groups, one-way ANOVA was applied. When appropriate, pairwise comparisons were performed using Tukey’s post hoc test. The level of statistical significance was set at p < 0.05.

Results

To ensure anatomical standardization of the included roots, root widths in the mesiodistal (MD) and buccolingual (BL) directions, as well as baseline dentin thicknesses at the MB1 and MB2 levels in the danger zone, were measured (Table 1). One-way ANOVA showed no statistically significant differences among the groups in MD or BL root width. Likewise, Kruskal–Wallis analysis revealed no significant differences among the groups in baseline dentin thickness at the MB1 and MB2 levels in the danger zone. These findings confirmed that the groups were comparable in terms of baseline anatomical characteristics.

Table 1.

Initial anatomical features and dentin thicknesses (mm) of the study groups

	n	Root width (BL) Mean ± SD	Root width (MD) Mean ± SD	MB1 initial dentin thickness Mean ± SD Median (Q1-Q3)	MB2 initial dentin thickness Mean ± SD Median (Q1-Q3)
BC 0.04	10	6.42 ± 0.38	2.72 ± 0.46	1.10 ± 0.18 1.10 (0.90–1.30)	0.95 ± 0.29 0.90 (0.68–1.18)
R 0.04	10	6.41 ± 0.47	2.73 ± 0.36	1.08 ± 0.26 0.95 (0.90–1.25)	0.95 ± 0.21 0.90 (0.78–1.07)
BC 0.06	10	6.36 ± 0.60	2.54 ± 0.59	0.95 ± 0.50 0.9 (0.83–1.13)	0.83 ± 0.39 0.70 (0.58–1.03)
R 0.06	10	6.17 ± 0.43	2.37 ± 0.36	0.88 ± 0.24 0.85 (0.78–0.90)	0.66 ± 0.24 0.70 (0.58–0.83)
Positive Control	10	6.04 ± 0.43	2.57 ± 0.35	1.00 ± 0.09 1.00 (0.90–1.10)	0.77 ± 0.20 0.75 (0.60–0.93)
Negative Control	10	6.30 ± 0.60	2.60 ± 0.37	1.09 ± 0.20 1.10 (0.90–1.25)	0.87 ± 0.15 0.85 (0.78–0.93)
p value		0.463*	0.440*	0.056#	0.072#

*One-way ANOVA; #Kruskal–Wallis test

When the effect of MB2 preparation taper (0.04 and 0.06) on dentin removal in the danger zone was evaluated, the mean ± SD values were 0.155 ± 0.100 mm for the 0.04 taper group and 0.160 ± 0.099 mm for the 0.06 taper group. The corresponding median (25th–75th percentile) values were 0.20 (0.10–0.20) mm and 0.15 (0.10–0.20) mm, respectively. The difference between the groups was not statistically significant (U = 196.50, Z = − 0.100, p = 0.921, r = 0.016).

Root canal preparation taper had a statistically significant effect on fracture resistance (F(1,36) = 15.470, p < 0.001, partial η² = 0.301). Sealer type had no significant effect on fracture resistance (F(1,36) = 1.132, p = 0.295, partial η² = 0.030). The interaction between preparation taper and sealer type was not statistically significant (F(1,36) = 0.347, p = 0.559, partial η² = 0.010).

When all groups, including the control groups, were compared using one-way ANOVA, a statistically significant difference in fracture resistance was found among the groups (F(5,54) = 8.522, p < 0.001). The mean fracture resistance values and statistical comparisons are presented in Table 2. Tukey post hoc analysis showed that the negative control group showed significantly higher fracture resistance than the 0.06 taper bioceramic, 0.06 taper resin, and positive control groups (respectively p = 0.035, p = 0.001 and p < 0.001). Additionally, the positive control group showed significantly lower fracture resistance than the 0.04 taper bioceramic and 0.04 taper resin groups (respectively p = 0.001 and p = 0.003). However, no significant difference was found between the positive control group and the 0.06 taper bioceramic or 0.06 taper resin groups (p = 0.362 and p = 0.960).

Table 2.

Fracture resistance values of the experimental and control groups

	n	Mean ± SD
BC 0.04	10	600.40 ± 98.93 ab
R 0.04	10	583.90 ± 152.67 ab
BC 0.06	10	484.10 ± 107.88 bc
R 0.06	10	426.60 ± 60.42 c
Positive Control	10	385.20 ± 90.04 c
Negative Control	10	638.20 ± 133.09 a

The same superscript letters indicate no statistically significant difference between groups

Discussion

The main finding of the present study was that increasing MB2 preparation taper from 0.04 to 0.06 significantly reduced fracture resistance in the mesiobuccal roots of maxillary first molars, whereas sealer type and the taper–sealer interaction had no significant effect. Notably, this reduction occurred despite the absence of a significant difference in dentin removal at the measured danger-zone level between the taper groups. This finding is clinically relevant because it suggests that the biomechanical effect of taper enlargement in anatomically vulnerable MB2 canals may not be fully explained by localized dentin loss at a single cross-sectional level. Accordingly, the null hypothesis was partially rejected.

Increased preparation taper has been associated with reduced fracture resistance [13, 16, 17]. In decoronated mandibular premolars, specimens prepared with a 0.04 taper exhibited higher fracture resistance than those prepared with a 0.06 taper [18]. Similarly, studies evaluating middle mesial canal preparation have shown that fracture resistance values of non-prepared controls are comparable to those of 25/0.04 preparations, whereas more extensive preparation results in a significant reduction in fracture resistance [16]. In addition, the presence of a middle mesial canal itself does not appear to predispose to fracture; however, its preparation has been shown to reduce fracture resistance in the mesial roots of mandibular molars [17]. Consistent with these findings, in the present study, the 0.04 taper groups exhibited fracture resistance values comparable to those of the non-prepared control, whereas the 0.06 taper groups demonstrated lower fracture resistance. These findings suggest that more conservative preparation protocols may better preserve root biomechanics in additional canals such as MM and MB2, whereas increased taper may contribute to structural weakening.

However, not all studies have reported similar findings. In maxillary premolars, no significant difference in fracture resistance was observed between 4% and 6% taper preparations when the final apical size was standardized to 30 [20]. This discrepancy may be attributable to methodological differences, as the coronal structure was preserved in that study and the specimens were tested as whole teeth with coronal restorations. Such conditions may have influenced stress distribution and masked the root-level biomechanical effect of preparation taper. In contrast, the use of isolated mesiobuccal root segments in the present study may have allowed a more direct assessment of the effect of taper variation in the MB2 canal on root dentin biomechanics.

Although dentin removal at the measured danger-zone level did not differ significantly between the 0.04 and 0.06 taper groups and showed a negligible effect size, preparation taper had a substantial effect on fracture resistance. This supports the interpretation that fracture resistance does not depend solely on localized linear dentin loss at a single cross-sectional level. Rather, the biomechanical response of the root may be influenced by overall canal geometry, canal wall morphology, dentin thickness, and stress distribution along the root. Previous finite element studies have shown that canal shape, external root morphology, and dentin thickness affect tensile stress distribution, and that canal taper can influence root stress distribution depending on the loading condition [7]. In maxillary first molar models, the cervical and furcation regions have also been identified as critical stress concentration areas [30], although the effect of canal taper may be influenced by overall model conditions [31]. In addition, the MB2 canal of maxillary first molars has been reported to have limited dentin thickness in the danger zone, particularly compared with the MB1 canal, and further critical thinning may occur after preparation [12, 19]. Therefore, the lower fracture resistance observed in the 0.06 taper groups may be attributed to taper-related changes in overall root geometry and local stress distribution rather than to localized dentin removal alone. Furthermore, although the baseline MB2 dentin thickness did not differ significantly among the groups, it showed a tendency toward statistical significance. This finding should be considered when interpreting the fracture resistance results, because even small baseline anatomical variations in the MB2 canal may affect the remaining structural reserve and local stress distribution after preparation. Therefore, this baseline tendency may have contributed, at least in part, to the lower fracture resistance observed in the 0.06 taper groups. However, because the difference did not reach statistical significance, the observed reduction in fracture resistance cannot be attributed solely to pre-existing anatomical variation.

The negative control (non-prepared) group exhibited the highest fracture resistance, whereas the positive control (prepared-unfilled) group showed the lowest values. This pattern suggests that preparation weakens root structure and that obturation may help preserve fracture resistance under the present experimental conditions. Consistent with this observation, previous studies have reported that non-prepared controls demonstrate the highest, and prepared but unfilled groups the lowest, fracture resistance values [22, 26–29]. Taken together, these findings indicate that although canal preparation adversely affects root biomechanics, obturation may mitigate this effect to some extent.

In the present study, no significant difference in fracture resistance was observed between epoxy resin–based and calcium silicate–based sealers. This finding is consistent with previous studies reporting no significant difference in fracture resistance between epoxy resin–based and calcium silicate–based sealers [26, 27, 29]. However, other studies have reported higher fracture resistance with calcium silicate–based sealers [22–24], while formulation-dependent differences among calcium silicate–based sealers have also been described [28]. Taken together, these findings indicate that the effect of sealer type on fracture resistance is not determined solely by chemical classification, but may also be influenced by formulation-specific properties and characteristics of the experimental model.

The absence of a significant sealer effect in the present study may be related to the standardized obturation protocol and experimental time point. The single-cone technique was used to standardize obturation and avoid additional compaction forces. Under these conditions, the possible contribution of the sealer to fracture resistance may have been influenced by cone adaptation, sealer thickness, interfacial adaptation, setting characteristics, and the anatomical configuration of the mesiobuccal root with an MB2 canal. Moreover, the two-week storage period may not fully reflect potential longer-term changes in the mechanical or interfacial behavior of calcium silicate–based sealers. Therefore, the biomechanical effect of preparation taper may have outweighed any potential differences attributable to sealer type in the present study.

The present experimental model was designed to evaluate the biomechanical effects of preparation taper and sealer type on root dentin under standardized conditions. For this reason, the mesiobuccal roots of maxillary first molars with an MB2 canal were selected. These roots represent a structurally vulnerable anatomical region because of their limited intercanal dentin thickness and proximity to the furcation. Assessing dentin thickness in the danger zone allowed targeted evaluation of the effect of preparation in the most critical area. Limiting taper variation to the MB2 canal also made it possible to assess the effect of this variable independently of the other canals. In addition, the similarity of baseline dentin thickness values among the groups in the mesiodistal, buccolingual, and danger zone measurements reduced the likelihood that the observed differences were attributable to pre-existing anatomical variation.

Clinically, these findings suggest that taper enlargement in MB2 canals should be performed cautiously, particularly in roots with limited dentin thickness toward the furcation. When adequate cleaning and shaping can be achieved, a conservative preparation taper may help preserve radicular dentin and reduce biomechanical weakening.

However, this study has several limitations. First, fracture resistance testing was performed on isolated root segments. Although this design allowed a more controlled and direct assessment of root dentin biomechanics, it does not fully reproduce the clinical conditions of intact teeth with coronal structure, periodontal support, and restorative components. Second, fracture resistance was evaluated under static vertical loading, which does not simulate the cyclic, oblique, and multidirectional forces encountered in vivo. Third, dentin thickness was assessed at a single predefined level in the danger zone using CBCT. Although this approach enabled standardized pre- and post-preparation comparisons, it may not fully reflect cumulative dentin loss along the entire root, and the spatial resolution of CBCT may limit the detection of very small dimensional changes compared with micro-computed tomography. Fourth, the inclusion of mesiobuccal roots with different internal canal configurations may have introduced anatomical variability that could have influenced the fracture resistance outcomes. Finally, only a single-cone obturation technique was evaluated. Although this approach allowed standardization across sealer types, different obturation techniques may influence sealer adaptation and fracture resistance.

Conclusions

Within the limitations of this in vitro study (isolated roots, static loading and single‑level dentin measurement), increasing MB2 preparation taper from 0.04 to 0.06 significantly reduced fracture resistance, regardless of sealer type. Sealer type (bioceramic vs. epoxy resin-based) did not significantly affect fracture resistance. Therefore, a conservative 0.04 taper may be preferable for MB2 canals when root strength is a concern. However, clinical validation is needed.

Abbreviations

BC

Bioceramic

BL

Buccolingual

CBCT

Cone-beam computed tomography

CI

Confidence interval

DICOM

Digital Imaging and Communications in Medicine

EDTA

Ethylenediaminetetraacetic acid

ICC

Intraclass correlation coefficient

MB1

First mesiobuccal canal

MB2

Second mesiobuccal canal

MD

Mesiodistal

NaOCl

Sodium hypochlorite

R

Resin

SD

Standard deviation

VRF

Vertical root fracture

Authors' contributions

Conceptualization: Ö.S. Formal analysis: Ö.S., R.S. Investigation and Methodology: E.S., Ö.S., E.B. Resources: R.S., E.E. Supervision: E.E. Visualization: R.S. Writing - original draft: Ö.S. Writing – review & editing: E.S.

Funding

This work was supported by the Scientific Research Projects Coordination Unit of Çanakkale Onsekiz Mart University (Project ID: TSG-2025-5179).

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval for this study was obtained from the Non-Interventional Clinical Research Ethics Committee of Çanakkale Onsekiz Mart University (Approval No: 2024 − 192). All procedures involving extracted human teeth were performed in accordance with relevant guidelines and regulations and with the principles of the Declaration of Helsinki. Informed consent was obtained for the use of extracted teeth for research purposes.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.