Mechanical behavior of external root resorption cavities restored with different materials: a 3D-FEA study

Abstract

Background

The aim was to evaluate the stresses in teeth, with external root resorption (ERR) restored with different materials using finite element analysis (FEA).

Methods

In this study, a Micro-CT scan was conducted on a prepared maxillary central tooth. DICOM-compatible images obtained from the sections were converted into stereolithography format using Ctan software. Utilizing the VRMesh Studio program, a solid model was generated based on the 3D image obtained through micro-CT scanning. External root resorption cavities were strategically designed at the apical, middle, and coronal thirds levels on the buccal surface of the tooth root. Subsequently, these created resorption cavities were restored using Biodentin, mineral trioxide aggregate (MTA), and glass ionomer cement (GIC). The models were devided into 12 groups according to their location,the type of restoration, material used and the null group was added. To allow measurement of Von Mises stress values, a simulated oblique force of 100 N, was applied directed towards the palatal region of the upper central tooth at a 45° angle to the occlusal plane.

Results

The highest von Mises stress value in the dentin was observed in the unrestored coronal cavity model (99.00 MPa). FEA results demonstrated that using a repair material significantly reduced the stress levels in the dentin. The lowest stress values were seen in cavities restored with Biodentine. The stress values in cavities treated with Biodentine and MTA were found to be similar.

Conclusions

Restoring the external resorption cavity in the tooth significantly reduced stress levels in the dentin. Biodentine and MTA absorbed more force, transmitting less stress to the dentin compared to GIC.

Clinical relevance

Biodentin and MTA can be used in the repair of external resorption cavities.

Objectives

Root resorption is the loss of dental hard tissue, which can occur either physiologically (in primary teeth) or pathologically. Pathological root resorption is classified into two types based on its location: internal, originating inside the root canal system, and external, occurring on the outer surface of the root [1].

External root resorptions can arise from various causes, including mechanical trauma, inflammation, orthodontic tooth movements, periodontal issues, neoplastic formations such as cysts and tumors, impacted teeth, systemic disorders, or even idiopathic factors. In addition, chemical irritation during bleaching procedures involving the use of 30% hydrogen peroxide or other irritating agents can also lead to root resorption [2].

Various treatment regimens have been suggested depending on the degree of resorption. The main goal of treatment is to remove the resorbed tissue and repair the defect. This process includes elevating the periodontal flap, performing curettage to remove granulation tissue, and restoring the defect with a suitable material such as mineral trioxide aggregate (MTA), resin-modified glass ionomer cement (GIC), calcium-enriched mixture, or Biodentine, followed by repositioning the flap to its original position [3].

It has been reported that in comparison to vital teeth, teeth with root fillings are more prone to fracture [4]. Destructive tissue losses are observed in external root resorptions, and therefore, these teeth require restoration with a material that can withstand occlusal loading. Repairs with mineral trioxide aggregate (MTA) (Angelus,Londrina,PR,Brazil) have shown excellent results because it has the capacity to provide adequate sealing. As MTA has good levels of biocompatibility and bioactivity, it has been shown to be tolerated well by periradicular tissues [5].

The recent introduction of Biodentine (Septodont, Saint-Maur-des-Fossés, France), a calcium silicate-based material, provides an alternative to MTA. In comparison to MTA, Biodentine offers several advantages over MTA, including improved color stability, a shorter setting time, and easier handling. However, in terms of biocompatibility, low cytotoxicity, and the ability to trigger pulp cell differentiation leading to mineralized tissue formation, Biodentine is similar to MTA [6]. The literature indicates that repair materials such as MTA and Biodentine are effective for treating resorption and can facilitate periodontal reattachment [7].

Glass-ionomer cements (GICs) are essential materials in clinical practice due to their versatility, self-adhesive properties to enamel and dentin, and excellent biocompatibility. The formation of water-based GICs occurs through an acid–base reaction between calcium-based fluoroaluminosilicate glass powder and polyalkenoic acid liquid [8].

Restorations made by clinicians must comply with the principles of oral rehabilitation. Therefore, it is of great importance to recognize and analyze the forces present in the mouth, taking into account the stress and deformation areas in the repair materials used [9]. Finite element analysis (FEA) includes a sequence of computational procedures which aim to predict specific outcomes, particularly the mechanical behavior, of complex geometric assemblies. This is achieved by the integration of results obtained from smaller elements defined by a specific mesh. FEA is extremely useful in the assessment of the mechanical properties of biomaterials and human tissue, the direct in vivo measurement of which can present challenges [10]. This analysis successfully simulates different anatomical structures and clinical conditions through a mathematical model [11]. Finite element analysis (FEA) has been widely applied in studies in many areas of dentistry, such as endodontics, orthodontics, prosthesis and surgery [12,13,14].

The loss of material observed in teeth with advanced external root resorption is likely to result in a reduction in the fracture resistance of teeth [15]. Studies evaluating the effect of restoring resorption defects using different repair materials on fracture resistance using FEA are limited in the literature. The aim of this study was to assess the effect of restoring resorption defects with different repair materials on fracture resistance.

Methods

Approval of the study protocol was obtained from the Ondokuz Mayıs University Clinical Research Ethics Committee(KAEK 2021/86).This study followed the principals of the Declaration of Helsinki. Patient ( and parents of patient under 18 years of age) signed a written informed consent from allowing their data and radiological findings to be used for future research.A maxillary central tooth without resorption, caries, immature apex, previous restoration, crack or fracture was selected for the procedure. The convantional access cavity was prepared with a high-speed diamond bur under water cooling. Root canal was prepared using ProTaper Next rotary instruments up to X5 (Dentsply Sirona, Ballaigues, Switzerland) in accordance with the manufacturers’ instructions. The root canal was irrigated with 5 mL of 5.25% NaOCI throughout the preparation. Final irrigation was performed 5 mL of 17% EDTA and 5 mL 5.25% NaOCI respectively. The prepared tooth was then scanned in a micro-CT device (Skyscan 1272, Bruker, Aartselaar, Belgium) at 10 μm voxel size, 80 kV X-ray tube voltages and 125 μA anode current.

The NRecon, version 1.6.3 dedicated software (Bruker, Kontich, Belgium) was used to configurate the BMP files, which were then converted into a single stereolithography (STL) file with CTAn software (Bruker, Belgium). The 3D surface model in STL format was converted into a 3D solid model using Rhinoceros 4.0 software (3670 Woodland Park Ave N, Seattle, WA 98103 USA). The resorption defects were simulated on the buccal aspect of the maxillary central incisor. Three different types of resorption cavities were then designed on the 3D solid model obtained from the micro-CT scan, using Algor Fempro software (ALGOR, Inc. 150 Beta Drive Pittsburgh, PA 15238–2932 USA). The resorption cavities was created in a semicircle positioned on the outer root surface and tangential to the root canal wall. The diameters of the external root resorption (ERR) cavities in the apical,middle and coronal root regions were 2.5, 4 and 4.5 mm respectively. Each structure (enamel, dentin, composite filling, alveolar bone, cortical bone, spongious bone, periodontal ligament, glass ionomer cement, MTA, gutta-percha, Biodentine) of the model was created separately. After the creation of defects, all models were meshed for finite element analysis with and without restorations. Three-dimensional meshes were used elements with the dimensions of highest 4,0 mm and lowest 0,5 mm were used to form finite elements model. In critical zones, element and nodal points were increased while modeling. This model provides the load transfer by establishing a linear connection with the nodal points in contact with one another. Models were created using approximately 112,301 nodes and 551,952 tetrahedral solid elements. Since the structures and materials in the dental models are monoblock structures and the materials and structures do not move, the ‘connected’ interface modeling type was prefered between different materials.This study used average anatomic dimensions of the surrounding tissues and the geometry of the tooth models with reference to previous literature [16,17,18,19]. All the materials used were determined to be homogeneous, isotropic and linearly elastic. The Young’s modulus and Poisson’s ratio were determined according to the literature. The material properties are summarised in Table 1.

Table 1 Material properties used in the finite element models

The ERR models were separated into 3 groups according to localization (coronal, middle, apical) (Fig. 1a, b, c). The resulting resorption cavities were divided into 4 subgroups according to the restoration materials, one of which was the control group. (Null, Biodentine, MTA, GIC). A total of 12 groups were modeled.

Fig. 1

Finite element models of this study. a Coronal resorption cavity b Middle resorption cavity c Apical resorption cavity

The model is fixed at the bottom and back of the jawbone in a way that it has 0 movement at each DOF (Degree of freedom). To simulate the bite force, an oblique force of 100 N at 45 angle to the occlusal plane was applied to the cingulum on the palatal surface of crown. The linear analyses were performed under the defined loading conditions (Fig. 2). Then, the stress distributions on each model were analyzed on dentin tissue and repair materials in the tooth regions adjacent to the resorption sites using Algor Fempro software (ALGOR, Inc. 150 Beta Drive Pittsburgh, PA 15238–2932 USA). The calculated numeric data were converted into colour graphics to be able to visualise the stress distributions in the FEA models more easily.

Fig. 2

Loading conditions

Results

In the present study, the stress distributions on the dentin surface, where resorption cavities of different sizes were created, and the stress absorption values of the repair materials used were evaluated by FEA analysis using Algor Fempro Software (ALGOR, Inc. 150 Beta Drive Pittsburgh, PA 15238–2932 USA).All the stress levels were measured in megapascals (MPa).

The von Mises stress values measured from the peaks of the cavities created on the dentin surface in all ERR models are presented in Table 2. In all models, the highest von mises values in dentin were obtained in models without repair. When the von misses values in dentin for each ERR model restored with different repair materials were examined, the lowest stress values in dentin were observed in Biodentin [coronal (10.56 MPa), middle (10.14 MPa), and apical (5.05 MPa),]. The stress values in dentin in the Biodentin and MTA applied models were similar (Fig. 3a, b, c).

Table 2 Von Misses stress values (MPa) at selected points on the top of cavity in the dentin surface of the root in ERR models

Fig. 3

a Stress values in dentin in the cavity formed in the coronal part of the root and repaired with MTA, Biodentin and GIC, respectively. b Stress values in dentin in the cavity formed in the middle part of the root and repaired with MTA, Biodentin, and GIC, respectively. c Stress values in dentin in the cavity formed in the apical part of the root and repaired with MTA, Biodentin, and GIC, respectively. The stress are defined with colors according to the scale from high (red) to low (blue)

When the stress values of different repair materials applied to each ERR model were evaluated, the material with the highest von misses stress was Biodentin. This result showed that Biodentin material absorbed more stress. This was followed by MTA and GIC respectively (Fig. 4) (Table 3).

Fig. 4

The maximum von Mises values (MPa) of the MTA, Biodentin, and GIC repair materials in the apical cavity. The stress are defined with colors according to the scale from high (red) to low (blue)

Table 3 Maximum von Mises stress values (MPa) in repair materials in FEA models

The highest von misses value in dentin was obtained in the coronal model without restoration (99.00). When each material was evaluated on its own, it was determined that the stress values ​​in dentin decreased from coronal to apical.

Discussion

External root resorptions are the most commonly encountered type of root resorption in clinical practice. The treatment planning for external root resorption is determined based on the lesion localization, size, and the vitality of the tooth. Incorrect diagnosis and inappropriate treatment can lead to tooth loss [20]. The repair material used in resorption treatment should have mechanical properties that strengthen the tooth structure [5]. In the present study, stress distributions resulting from the repair of maxillar central teeth with different sizes of external root resorptions using various restoration materials were evaluated through finite element analysis.

Various repair materials, including glass ionomer cements, light-curing resin composites, amalgam, mineral trioxide aggregate (MTA), and Biodentin, have been suggested for the restoration of resorptive defects [21]. MTA is a material known for its ability to set in the presence of moisture, its radiopacity, antimicrobial properties, biocompatibility, and excellent sealing capabilities [22]. Biodentin is a bioactive calcium silicate-based material with properties closely resembling those of dentin [23]. Due to these superior properties, in this study, the stress distributions of MTA and Biodentin in the restoration of resorptive defects were evaluated.

The resistance value of the tooth against forces in the mouth can be determined by the fracture strength test. The use of extracted teeth in the evaluation of fracture resistance presents a disadvantage in terms of standardization. Also, Stress analysis methods are considered superior to fracture resistance tests in determining long-term deformation [24]. Stress analysis in dental structures has been examined using various methods such as FEA, strain gauges, brittle coating analysis, holography, 2D and 3D photoelasticity, and digital moiré interferometric examination [25]. The ability to evaluate the stresses that may occur in any part of the model to be analyzed in the FEA method the ability to rotate three-dimensional models as desired and to evaluate them from different angles, and the ability to change the direction and amount of the applied force make this method advantageous compared to other stress analysis methods [24]. Finite element analysis (FEA) has been widely used to understand and predict biomechanical events for the past few decades [26]. Due to these characteristics, FEA was used in our study to evaluate stress distribution.

Fracture strength testing can be performed in various force directions, including axial, lateral, oblique, torsional, and bending, to comprehensively evaluate the material's mechanical properties. Oblique loads have been shown to pose a greater risk compared to vertical loading in teeth with endodontic restoration [27]. As the risk of fracture can increase due to high lateral forces created by oblique occlusal loads [28], this has been used most often in studies that have applied FEA [16, 29]. Therefore, in our study, oblique loading was preferred to evaluate the stress distributions of restorations at different levels of the root restored with different materials.

This study investigated the mechanical responses of endodontically treated teeth with external root resorption under functional loads, focusing on different repair materials. The use of three-dimensional finite element analysis (FEA) allowed us to evaluate anatomical locations that are virtually inaccessible in vivo. While FEA analysis results are typically expressed as stress distributions within the examined structures, von Mises stress reflects the entire stress field and has therefore been widely used as an indicator of the potential for damage development [30]. The results of this study are presented in accordance with the von Mises criteria.

The highest stress values in dentin in the ERR cavity were examined in this study. Jiang et al. [31] stated that the location and likelihood of failure development are indicated by stress concentrations with loading, which are thus linked to the resistance of the tooth to fracture. Therefore, the highest stress areas and values were examined in this study.

The current study results showed that the repair material type affected the distribution of stress in the external resorption cavities. The highest stress accumulation in the dentin of the resorption cavities was observed when glass ionomer cement was applied, and the least stress accumulation was observed when Biodentin was applied. It was also seen that Biodentin and MTA caused less stress accumulation in dentin than glass ionomer cement.

Rajawat and Kaushik [25], compared the stress distributions in teeth with simulated external cervical resorption defects restored with mineral trioxide aggregate, Biodentine, glass ionomer cement, and Bioaggregate. The lowest stress values were observed in the Bioaggregate group. While Bioaggregate, Biodentine and MTA showed similar results in dentin, they noted that as cavity size increased, Biodentine exhibited lower stress resistance values than MTA, suggesting that materials with elastic moduli similar to or greater than dentin could be preferred for restoring the cervical third of the tooth with resorptive lesions. In our study, von Mises stress values in dentin were measured at the deepest point of the resorption cavity (top of the cavity). In the analyses performed, Biodentin showed lower stress values ​​than MTA in the cavity in every third of the root. The results of our study were found to be consistent with the results of the study by Rajawat and Kaushik.

Çoban Öksüzer and Şanal Çıkman [24], evaluated the stress distributions on dentin and repair materials under static force applied to teeth with cervical external root resorption restoration using different filling materials, by utilizing finite element analysis. For the restoration of the cervical resorption cavity, they used MTA, Biodentine, BioAggregate, calcium-enriched cement [CEM], glass ionomer cement [GIC], and resin-modified glass ionomer cement [RMGIC]. They reported that the highest stress values ​​in the dentin occurred in the unrepaired models. Additionally, they found that the von Mises stress values ​​in dentin were higher in models restored with MTA, GIC, and RMGIC compared to those repaired with Biodentine, BioAggregate, and CEM. They also noted that Biodentine, BioAggregate, and CEM absorbed more force compared to MTA, resulting in less stress being transmitted to the dentin. In our study, the highest stress values ​​in the dentin were also observed in the unrepaired models. Although the von Mises values ​​of the repair materials were similar, the highest values ​​were observed in the Biodentine group in all three-thirds of the root canal. The results of the Öksüzer ve Şanal Çıkman’s study revealed results parallel to our results.

Aslan et al. [32], evaluated stress distributions in simulated mandibular molar teeth with various types of iatrogenic root perforations after repair using Biodentine or MTA. The study concluded that the use of MTA and Biodentine could potentially reduce the risk of harmful stress in the areas of root perforation. In the present study, it was observed that Biodentine and MTA caused less stress accumulation in dentin compared to glass ionomer cement (GIC). The findings of our study demonstrated that Biodentine and MTA absorbed more force than GIC and led to less stress accumulation in dentin across the apical, middle, and coronal thirds of the root canal. Our study's results were found to be consistent with those of Aslan et al.'s study.

Although most dental materials and tooth tissues are anisotropic and non-homogeneous, the materials used in the current study were considered to be linear, isotropic and homogeneous. This study is limited to testing the effect of a single load on the fracture resistance of the tooth. In a patient, complex functional forces occur during chewing, and this may influence the fracture resistance results. Therefore, future studies are needed to examine the effects of different force directions on the fracture resistance of the tooth [33]. However, a strong aspect of this study compared to several other FEA studies in the literature was that micro-CT scanning, which accurately reflects the anatomic structure of a real tooth, was used to obtain the models to be tested.

Conclusion

The presence of external root resorption (ERR) in the tooth markedly elevated stress levels in the dentin. FEA results revealed that the application of repair materials significantly lowered these stress values. Compared to GIC, both Biodentine and MTA demonstrated greater force absorption, resulting in reduced stress transmission to the dentin.