Introduction

The complex root canal system anatomy plays a crucial role in the success of endodontic treatment. Residual necrotic tissue and bacteria or their metabolic products may cause recurrent periapical inflammation and pain after treatment.1 Failures in endodontic treatment result from inadequate biomechanical preparation, untreated root canals, iatrogenic complications, or reinfection.2 The primary objective of non-surgical retreatment is the removal of gutta-percha and sealer with biomechanical preparation, eliminating persistent microorganisms and restoring hermetic apical obturation.3

It has long been known that restorative and endodontic treatments are precipitating factors for tooth fractures. Access cavity preparation, biomechanical preparation, root filling strains, post placement and retreatment procedures have been associated with weakened mechanical properties of the tooth.4,5,6 Increasing the preparation size of the apical third - a critical region for infection - increases the volume of irrigation solution delivered to the apical region and mechanical debridement.7,8,9 Although rotary file systems enhance root canal preparation and offer improved debridement, using files with larger tapers may be associated with decreased fracture resistance (FR),10 potentially leading to vertical root fractures, a significant cause of failure in root canal treatments.11,12 Therefore, investigators have suggested that root canal preparation should be completed with an instrument more than one file size larger before the filling.13,14

Root canal sealers, which adhere to the dentine, may reinforce the root dentine structure and increase FR.6 AH Plus (Dentsply DeTrey, Konstanz, Germany) is an epoxy resin-based sealer that has been the gold standard in comparison with other endodontic sealers due to its ease of use, good mechanical properties, wettability and ideal sealing properties.15 MTA Bioseal (ITENA Clinical, Paris, France), a new mineral trioxide aggregate (MTA)-containing root canal filling paste, offers properties such as high alkalinity (pH >10), limited expansion, low solubility and optimum flowability.16 In recent years, calcium silicate-based sealers (CSBS), such as NeoSealer Flo (Avalon Biomed, Texas, USA), have also been introduced to the market, offering favourable physicochemical properties, including osteoinductive effect, ability to set in the presence of tissue fluids, biocompatibility, antibacterial activity and lack of shrinkage upon setting.17,18 It has been reported that a CSBS can penetrate the dentinal tubules due to its nanoparticles.19,20

Most studies to date have focused on the effect of root canal sealer on tooth roots after short-term storage (ranging from 24 hours to six months).21,22,23,24,25,26,27,28,29,30 While numerous studies have examined the FR of MTA Fillapex (Angelus, Londrina, Brazil),22,24,29 investigations involving MTA Bioseal have primarily concentrated on its dentine tubule penetration,31 cytotoxic effect32 and physicochemical properties.16 To our knowledge, no studies in the endodontic literature compare the effect of MTA Bioseal with other filling materials on FR. Additionally, our literature review found no research exploring the effects of various canal sealers and apical preparation sizes on the FR of 12-month-aged retreated roots. Therefore, the present study aimed to assess the impact of different apical preparation sizes and root canal sealers on the FR of retreated roots. The null hypothesis was that no significant difference in FR would be observed between the groups.

Materials and methods

Ethics committee approval dated 11.10.2023 and numbered 2023.10.09 of the Non-interventional Research Ethics Committee of Kirikkale University was obtained. An informed consent form was obtained from patients whose teeth had been extracted in the Department of Oral and Maxillofacial Surgery for use in an in vitro study. The sample size was calculated at the significance level of 0.05, the effect size of 0.40, and the power of 95% using G* Power v3.1 (Heinrich Heine, Universität Düsseldorf, Germany). In total, 100 human mandibular premolars, extracted for periodontal reasons, were used in the study. The teeth were rinsed with phosphate-buffered saline solution (Gunduz Kimya, Istanbul, Turkey). Immediately after extraction, the periodontal tissues were removed from the root surface with periodontal curettes and the teeth were kept in 0.1% thymol solution at 4°C until used.

Buccolingual and mesiodistal digital radiographs were taken to determine that each root had a single canal, root formation of Stage 5, no prior root canal treatment and no anatomical malformations. The teeth were decoronated by measuring 13 mm from the root apex. The root segments were stored in distilled water until used. The buccolingual and mesiodistal internal diameters of the roots were measured with a coronal tip digital calliper and one-way analysis of variance (ANOVA) evaluated whether there was a significant difference between the groups.9 The root segments were randomly divided into three main groups (AH Plus, NeoSealer Flo and MTA Bioseal) according to the root canal sealer used, with each main group having two subgroups according to apical preparation sizes (30.04, 40.04) after retreatment procedures. Table 1 shows the ingredients and lot numbers of the root canal sealers used in the study.

Table 1 Sealers, composition and batch
Full size table

The teeth were randomly divided into ten groups of ten teeth as follows:

  • Control Group 1 (intact tooth group) - root segments without instrumentation and obturation

  • Control Group 2 - root segments instrumented with retreatment files and then instrumented up to 25/0.04 without obturation

  • Control Group 3 - root segments instrumented with retreatment files and then instrumented up to 30/0.04 without obturation

  • Control Group 4 - root segments instrumented with retreatment files and then instrumented up to 40/0.04 without obturation

  • Group 5A (AH Plus) - root segments instrumented up to 30/0.04 and obturated with AH Plus and a single gutta-percha cone

  • Group 5B (AH Plus) - root segments instrumented up to 40/0.04 and obturated with AH Plus and a single gutta-percha cone

  • Group 6A (NeoSealer Flo) - root segments instrumented up to 30/0.04 and obturated with NeoSealer Flo and a single gutta-percha cone

  • Group 6B (NeoSealer Flo) - root segments instrumented up to 40/0.04 and obturated with NeoSealer Flo and a single gutta-percha cone

  • Group 7A (MTA Bioseal) - root segments instrumented up to 30/0.04 and obturated with MTA Bioseal and a single gutta-percha cone

  • Group 7B (MTA Bioseal) - root segments instrumented up to 40/0.04 and obturated with MTA Bioseal and a single gutta-percha cone.

Preparation of root segments

A size 10 K file (VDW, Munich, Germany) was placed into the canal until it was visible at the apical foramen. The working length (WL) was determined as 1 mm short of the measurement. A VDW R25 file (VDW, Munich, Germany) was used in the VDW Silver Reciproc electric motor's ‘all' mode with up and down motion. During the instrumentation, 2.5% sodium hypochlorite (NaOCl) was used between every three pecking motions until the WL was reached. Final irrigation was performed with 10 ml of 5% NaOCl, 10 ml of distilled water, 10 ml of 17% ethylenediaminetetraacetic acid (EDTA) for one minute, and 10 ml of distilled water, respectively. All root segments were dried with sterile paper points and obturated with AH Plus and a single gutta-percha cone R25 (VDW). The access openings were sealed with 3M Universal Single Bond (3M ESPE, St Paul, MN, USA) and covered with Filtek Z250 Universal Restorative (3M ESPE, St Paul, MN, USA) composite resin and stored at 37 °C in 100% humidity for one month.

After one month, ProTaper Universal instruments (PTU, Dentsply Tulsa Dental, Tulsa, OK, USA) were used in a handpiece with adjustable torque according to the manufacturer's instructions. D1 (9% taper, size 30), D2 (8% taper, size 25), and D3 (7% taper, size 20) files were sequentially used in a crown-down technique to negotiate the pre-established WL and were manipulated with a brushing motion. Also, 2.5% NaOCl was used between retreatment files. Root canal lengths were confirmed using a size 15 K-file (VDW). The ‘K3XF' rotary nickel-titanium files (Kerr Endodontics, Orange, CA, USA) (25/0.04, 30/0.04, 35/0.04, 40/0.04, respectively, according to the experimental groups) were used in the WL with an electric endodontic motor (Elements Motor, Kerr Endodontics) to complete canal preparation and 2.5% NaOCl was used between files. The final irrigation was reapplied as previously described and obturated with root canal sealer and a single gutta-percha cone K3XF (Kerr Endodontics) per the experimental groups. One endodontist performed the instrumentation and obturation to simulate the clinical scenario.

The restoration of all root segments was reapplied as described before, and the segments were incubated at 37 °C for 12 months in 100% humidity.33 The root segments were coated with silicone paste 2 mm apical to the cementoenamel junction to mimic a periodontal ligament.9 Subsequently, the teeth were embedded in resin blocks and positioned vertically. A cone-shaped rod with a diameter of 3.5 mm was used on a universal testing machine (Instron, Canton, MA, USA) directly over the root segments (Fig. 1). A load at a speed of 1 mm/min was driven downward vertically until a fracture occurred. The fracture pattern was recorded in Newtons.

Fig. 1
figure 1

Illustration of the fracture resistance test with the root embedded in resin blocks to mimic periodontal ligaments

Full size image

Data analysis

SPSS (version 24.0, IBM, Armonk, NY, USA) was used for statistical analysis. The normality of the variables was analysed with the Shapiro-Wilk test (p >0.05). As the data were normally distributed in all groups, ANOVA and post-hoc Tukey tests were performed for all data, with p <0.05 deemed significant.

Results

Table 2 shows the mean FR values of experimental groups. The result of one-way ANOVA revealed a significant difference between Control Group 1 (intact teeth) and Control Groups 2, 3 and 4 (preparation, no obturation) (p <0.05). There were significant differences between 25/0.04 and 40/0.04 and 30/0.04 and 40/0.04 in the control groups (p <0.05). There were no statistically significant differences between any of the tested sealers or apical preparation sizes (30/0.04 or 40/0.04) (p >0.05).

Table 2 Mean fracture resistance and standard deviation of the experimental groups
Full size table

Discussion

Chemomechanical preparation, which combines mechanical instrumentation with files and irrigation with chemical solutions during root canal treatment, is known to reduce the volume of root dentine and induce dentine dehydration.This process reduces the modulus of elasticity and the flexural strength of the dentine, thereby compromising the structural integrity of the tooth.34,35 This situation is more severe in retreatment cases for various reasons, such as unnecessary tissue loss, removal of old root canal fillings, reuse of chemical solutions for disinfection and larger apical preparation sizes. Many studies have been conducted to improve the FR of endodontically treated teeth due to the ability of root canal fillings to bond to the root canal dentine surface. Recently, it has been shown that tricalcium silicate sealers yield better clinical outcomes in strengthening the tooth structure due to their micromechanically and chemical bonding to the root dentine surface.21,28

To minimise the risk of bias in the present study, the buccolingual and mesiodistal dimensions of 100 mandibular premolars were measured with a digital calliper and teeth with no statistical differences were included in the experiment (p >0.05).9 In the irrigation protocol, NaOCl was chosen for its superior proteolytic properties concerning endodontic treatment, its antimicrobial effects and its ability to dissolve organic tissue, which may reduce elastic modulus and flexural strength.36 Furthermore, EDTA, with its low surface tension, removes the smear layer in dentinal tubules up to 2.5-4.0 μm deep while simultaneously having a slight weakening effect on dentine.37 In this study, distilled water was used to neutralise the negative effects of the irrigation solutions. Since many studies indicate spreaders related to vertical root fracture, the single cone gutta-percha technique was used instead of the laterally condensed gutta-percha technique.38,39,40

FR values were tested with a universal testing machine (Instron, Canton, MA, USA) as it is an easy and commonly used method.9,22,41,42

In the present study, as in previous in vitro studies, the negative control group (no preparation, no obturation) had the highest FR, whereas the FR of all the positive control groups (preparation, no obturation) was lower than that in all the experimental groups.21,22,29 These results can be attributed to biomechanical preparation and tissue loss. The lack of a statistically significant difference between the negative control groups, which had 25/0.04 and 30/0.04 preparations, may be due to the similarity of the tapers.9

Epoxy resin and CSBS have gained popularity in current applications among various commercially available root canal filling sealers.43 CSBS forms a mineral layer by releasing calcium, phosphate and silicon ions during setting and induces a chemical bond with the dentine walls, which increases sealing ability.44 It is reported that CSBS exhibits a slight hygroscopic expansion of up to 0.2%, albeit not in all brands, which directly affects dimensional stability.45 In this study, the FR of the newly introduced NeoSealer Flo and MTA Bioseal were compared with AH Plus with a different experimental technique.

Studies indicate that AH Plus root canal sealer increases the FR of endodontically treated teeth, possibly due to its high adhesion to root dentine, which is enabled by its deep penetration into microvoids, creep capacity and long setting duration, which gives it adequate time to improve the mechanical bond between the dentine and the sealer.46,47,48 In the present study, NeoSealer Flo, MTA Bioseal and AH Plus were found to reinforce retreated roots more effectively across all negative control groups (prepared, without obturation), in agreement with previous reports,21,23 whereas Ersoy et al.,29 Grande et al.49 and Kim et al.50 reported no clear effects of the use of root canal sealers in improving the roots' FR.

In line with Topçuoğlu et al.23 and Sağsen et al.,22 the present study found no significant difference in FR between CSBS sealers and AH Plus, whereas other studies have reported higher FR with CSBS than AH Plus.27,28 This discrepancy may be attributable to variations in product brands and the absence of bioceramic-coated cones, which could influence chemical adhesion.

This study found no statistically significant differences between endodontically retreated and intact teeth in terms of fracture load, which aligns with a previous study.22 Studies have demonstrated that biomechanical preparation diminishes FR but subsequent obturation with root canal sealers and gutta-percha can restore mechanical properties comparable to those of intact teeth. These findings are consistent with the results observed in the present study.51,52 On the other hand, our results contradict those of Ganesh et al.53 and Khalap et al.,54 who report that retreatment procedures decreased the mechanical properties of retreated teeth. Both researchers used a laterally condensed gutta-percha technique, Coltosol as the final restoration, and Endosolv R as a solvent, which differs from our experimental design. It is crucial to note that temporary filling materials harden through water absorption and their hygroscopic expansion, ranging from 17-20%, can reduce value accordingly.55 Additionally, those researchers employed larger tapers for the retreatment procedures using ProTaper instruments up to F3 (size 30, 0.09 taper). As previously discussed, using a laterally condensed gutta-percha technique and larger tapers has been associated with decreased FR. These methodological differences may account for the variation in results across studies.

Ma'aita et al.30 reported that MTA (ProRoot MTA, Dentsply, Tulsa, OK, USA) enhanced FR after one and six months compared to AH Plus and gutta-percha. ProRoot MTA is composed of tricalcium silicate and dicalcium silicate naturally composed, bismuth oxide, tricalcium aluminate, and tetra-calcium aluminoferrite, which differs from the composition of MTA Bioseal.56 The observed differences in outcomes may be attributed to the presence of salicylic resin in MTA Bioseal and its use in combination with gutta-percha. It is important to consider that, similar to sealers, the mechanical properties of gutta-percha can be influenced by factors such as storage time, humidity and temperature.57,58

Due to the lack of investigations on aged root segments and applied different apical preparation sizes in retreatment procedures, the results of the present study could not be directly compared to previous research. Overall, the findings suggest that root canal sealers can effectively reinforce root segments against fracture, with no significant difference observed in FR between the 30/0.04 and 40/0.04 apical preparation sizes when obturated with sealers. This result could be attributed to using a similar taper despite varying apical preparation sizes.

This study may be limited by the fact that the ageing of root segments was performed only in a 100% humidity environment and they were not exposed to chewing forces. Also, to standardise the study, the teeth were used as root segments by removing their clinical crowns before FR evaluation, which differs from the clinical scenario. It is crucial to admit that factors such as the integrity of the coronal tooth structure, the quality and type of coronal restoration, occlusal relationships, and the presence of parafunctional habits, significantly influence the FR of teeth. These factors play a key role in modulating the mechanical properties of dental tissues and their contributions should be carefully noted when assessing the long-term durability and functional outcomes of retreated teeth.

Conclusion

Within the limitations of this study, it could be concluded that obturation with root canal sealers could strengthen the retreated teeth, with no significant difference in FR observed between AH Plus, NeoSealer Flo and MTA Bioseal. Additionally, the clinical selection of apical sizes 30.04 and 40.04 may present a comparable risk of FR in retreated teeth.