Journal of Conservative Dentistry

ORIGINAL ARTICLE
Year
: 2023  |  Volume : 26  |  Issue : 1  |  Page : 20--25

Stress distribution of endodontically treated mandibular molars with varying amounts of tooth structure restored with direct composite resin with or without cuspal coverage: A 3D finite element analysis


Ashtha Arya1, Mandeep S Grewal1, Vishal Arya2, Ekta Choudhary3, Jigyasa Duhan4,  
1 Department of Conservative Dentistry and Endodontics, Faculty of Dental Sciences, SGT University, Gurgaon, Haryana, India
2 Department of Preventive and Paediatric Dentistry, Faculty of Dental Sciences, SGT University, Gurgaon, Haryana, India
3 Department of Conservative Dentistry and Endodontics, School of Dental Sciences, Sharda University, Greater Noida, Uttar Pradesh, India
4 Department of Conservative Dentistry and Endodontics, PGIDS, Rohtak, Haryana, India

Correspondence Address:
Dr. Ashtha Arya
Department of Conservative Dentistry and Endodontics, Faculty of Dental Sciences, SGT University, Gurgaon, Haryana
India

Abstract

Background: Decision-making regarding whether cuspal coverage is required or not for the restoration of root canal-treated posterior teeth is still a matter of challenge for the dentist. Methodology: Four models of endodontically treated mandibular molars with mesio-occlusal (MO) cavity were designed and simulated with direct composite resin restorations. Group 1A – cavity width <½ the intercuspal distance restored without cuspal coverage, Group 1B – same as Group 1A but with cuspal coverage, Group 2A – MO cavity width >½ but <2/3rd the intercuspal distance restored without cuspal coverage, and Group 2B – same as Group 2A but with cuspal coverage. The models received occlusal load to simulate a mastication load. Static finite element analysis (FEA) was adopted for predicting the stress distribution generated in the restored tooth by the loading condition. Results: FEA of the models have shown that the variations in stress values were significant in bulk-fill material compared to enamel and other structures. Comparing the maximum and minimum principal stress values in the overall region demonstrated that 2A was safer, whereas 2B was found to be the worst case. Conclusions: The results indicate that restoration of endodontically treated mandibular molar with loss of one marginal ridge with composite resin without cuspal coverage revealed minimal internal stress values and showed the best performance overall.



How to cite this article:
Arya A, Grewal MS, Arya V, Choudhary E, Duhan J. Stress distribution of endodontically treated mandibular molars with varying amounts of tooth structure restored with direct composite resin with or without cuspal coverage: A 3D finite element analysis.J Conserv Dent 2023;26:20-25


How to cite this URL:
Arya A, Grewal MS, Arya V, Choudhary E, Duhan J. Stress distribution of endodontically treated mandibular molars with varying amounts of tooth structure restored with direct composite resin with or without cuspal coverage: A 3D finite element analysis. J Conserv Dent [serial online] 2023 [cited 2023 Feb 5 ];26:20-25
Available from: https://www.jcd.org.in/text.asp?2023/26/1/20/362912


Full Text

 Introduction



Endodontically treated teeth (ETT) have already lost considerable tooth structure due to caries and access cavity preparation, and tooth preparation for full-crown coverage would negatively affect the biomechanical characteristics of the restored tooth. Preservation of sound tooth structure has been advocated as the most crucial factor in increasing the longevity of root canal-treated teeth.[1],[2] With the advancements in adhesive dentistry, it has been endorsed that postendodontic restorations with direct composite resin preserve the sound tooth structure and favor reinforcement of the remaining dental tissue.[3],[4] The amount of tooth structure loss and the width of the cavity and cavity preparation design influence the stress values and fracture toughness of a tooth.[5] Many studies have reported that cuspal coverage with composite resin lowers the stress values in the dental tissues and increases the fracture resistance of teeth.[6],[7] On the other hand, some investigators have concluded that cuspal coverage along with adhesive composite resins has no significant influence on the fracture toughness of ETT.[8],[9] Due to these conflicting reports, it is challenging for the dentist to decide whether cuspal coverage is required or not in a clinical situation. There is still no consensus regarding the most appropriate restorative modality that would provide minimal internal stresses in root-filled posterior teeth with the loss of one marginal ridge. This finite element study aims to analyze the effect of cavity design preparation on stress values in endodontically treated mandibular molars with varying amounts of the remaining tooth structures restored with direct composite resin.

 Methodology



An intact, noncarious mandibular first molar was scanned with a cone-beam computed tomography (CBCT) machine (Planmeca CBCT, Helsinki, Finland). The DICOM file was then processed using materialized mimics that provide output in Tri-mesh which was converted to a solid model. The design changes of the model were generated using the SOLIDWORKS software.

In the 3D solid model of the intact mandibular first molar, root canal treatment was simulated, representing various structures, i.e., gutta-percha, flowable composite, and bulk-fill composite restoration. In this model, the mesio-occlusal (MO) access cavity preparations were developed; In Group I, the width of the access cavity was <½ the intercuspal distance, and in Group II, the width of the access cavity was >½ the intercuspal distance but <2/3rd the intercuspal distance. Two millimeter of gutta-percha was removed from the coronal portion of each canal, and Tetric N-Flow (Ivoclar Vivadent) was used for coronaradicular extensions and as a base in the pulp chamber. Each group was then subdivided into two subgroups: A and B. In subgroup A, the restoration of the access cavity was simulated as direct composite resin, Tetric N-Ceram Bulk Fill (Ivoclar Vivadent) restoration without cuspal coverage, and in subgroup B, the cavity preparation was the same as for subgroup A; however, the buccal cusps were reduced by 2 mm and the restoration was simulated as direct composite resin, with cuspal coverage.

The CAD model was imported into ANSYS Mechanical to generate a volumetric mesh. Except for the cortical part, all the parts were meshed with quadratic elements, with the number of nodes being 3975195 and the number of elements being 3745913. Two mechanical properties, the Poisson's ratio and the elastic modulus were attributed to the dental structures and the materials simulated, which were determined from a literature review [Table 1].[10],[11],[12],[13] All the material properties were considered linear elastic material. Static finite element analysis (FEA) was adopted to anticipate the stress distribution generated in the restored tooth by the loading condition. Load is applied by selection at the region of the central fossa, distal marginal ridge, mesiobuccal cusp tip, and distobuccal cusp tip. Hence, for each selected region, an oblique loading of 50N was applied, which indicates the total force of 200N to the tooth structure, simulating the mastication load in complete intercuspation.[14]{Table 1}

 Results



Using structural FEA, maximum von Mises stress, maximum principal stress, and minimum principal stress values in the enamel, dentin, resin composite, and other restorative materials were calculated [Table 2].{Table 2}

Minimum principal stress

Minimum principal stress in the mesiobuccal cuspal region was found to be maximum in Group 2B (321 MPa) [[Figure 1]a (iv)], which was followed by 1A [[Figure 2]a (i)] and 1B (252 Mpa) [[Figure 1]a (iii)], and Group 2A showed the least stress values (230 MPa) [[Figure 2]a (ii)]. It is to be taken into consideration that in models 1A and 2A, mesiobuccal cusps are made up of enamel; but in models 1B and 2B, mesiobuccal cusps are restored with Tetric N Ceram bulk fill material, hence the corresponding values.{Figure 1}{Figure 2}

Minimum principal stress in the bulk-fill material in the overall region was found to be highest in 2B (321), which was higher than Group 1A (270), followed by 1B (252), and the least stresses in 2A (210) [Figure 1]a.

The highest values for minimum principal stress in enamel in the overall region were exhibited by 1A at 252 MPa, followed by 2A at 230 MPa, and 2B at 205 MPa, and the least stresses were indicated by 1B at 191 Mpa [Figure 2]a.

It was found that the stress variations in both enamel and bulk-fill material are significant. Since the stress values in the bulk-fill material are very dominant, the comparisons are made only with the bulk-fill minimum principal stress in the overall region. Furthermore, from the results, it is found that the 2A model is comparatively having lower stress values in Tetric N-Ceram Bulk Fill when compared to the other models. In contrast to this, the stress produced in enamel for the 2A model is higher when compared to the 1B and 2B models. However, this is due to the different materials in cuspal regions.

Hence, when comparing the overall structure based on minimum principal stress, 2A is safer than 1B which is safer than 1A and 1A is safer than 2B.

Maximum principal stress

Maximum principal stress in the mesiobuccal cuspal region was found to be foremost in Group 1B (104 MPa) [Figure 1]b (iii)], which was followed by 1A (93 MPa) [[Figure 2]b (i)] and 2B (90 Mpa) [[Figure 1]b (iv)], and Group 2A showed the least stress values (63 MPa) [[Figure 2]b (ii)].

Maximum principal stress in the bulk-fill material in the overall region was found to be highest in 1B (104), which was higher than Group 2B (90), followed by 1A (57) and least stresses in 2A (56) [Figure 1]b.

The highest values for maximum principal stress in enamel in the overall region were exhibited by 1B and 1A at 108 MPa, followed by 2B at 103 MPa, and 2A demonstrated the least stresses at 102 Mpa [Figure 2]b.

It is evident that the differences are found only in the enamel and Tetric N-Ceram Bulk Fill because these are the parts which bear the loads directly.

The comparisons are made only with the bulk-fill maximum principal stress in the overall region as the stress variations in the enamel part are significantly less. Hence, when comparing the overall structure based on maximum principal stress, 2A is safer than 1A, which is safer than 2B, and 2B is safer than 1B. Comparing both the maximum and minimum principal stresses, 2B is the worst case, but 2A is safer.

 Discussion



In our study, direct composite restorations were simulated to restore endodontically treated mandibular molars, as studies suggest that composite resins provide 87% of the initial stiffness of endodontically treated teeth with an average survival period of 13.4 years.[15] They also concluded that cavities with up to three surfaces could be restored successfully with adhesive composite resins.[15]

FEA is a valuable noninvasive method for analyzing stress distribution and mechanical behavior of tooth structure and biomaterials, which can hardly be measured in clinical studies.[16]

Since this is a qualitative study, we considered the peak stresses in every plot. Furthermore, it is evident that the differences are found only in the enamel and Tetric N-Ceram Bulk Fill because these are the parts which bear the loads directly. Furthermore, we observed that peak stresses are always found in the cuspal regions in all the models. This concept confirms with Saint-Venant's Principle which states that loads applied on a surface give rise to a stress concentration near the point of application and stress concentration at points far away from the load is independent of the application of load.[17]

Since the nature of the load is compressive, the compressive stress values are very high.

J.E. Gordan in his scientific communication has explained that many structures tend to break down in tension. This is true as, in brittle materials, tensile strength is usually less than compressive strength.[18] Literature suggests that this postulation holds for both enamel and dentin.[19],[20]

Hence, we compared the models using both minimum principal stress and maximum principal stress.

Since the 2B model contains more bulk-fill material, i.e., the material with low stiffness, the local deformation is more when compared to the other models. Similarly, the 1B model deformation is more when compared to the 1A and 2A models.

Minimum principal stress in enamel parts in the 1B (191MPa) and 2B (205 MPa) is low when compared to the 1A (251 MPa) and 2A models (230 MPa), even though it has the least amount of enamel material [Figure 2]a. This is happening due to the more amount of low elastic modulus bulk-fill material that distributes the stresses to all the parts. This result is in accordance with the study by Jiang et al. who noticed that materials with high elastic modulus could not disseminate the stresses into the cavity, thus leading to more significant stress on the tooth structure.[10] When compared with the 1B and 2B models, the minimum principal stress of the 2B model seems slightly higher than the 1B model. This is due to a slight increase in maximum shear stress. Since the 1A model has more enamel regions, this model is stiffer when compared to the other models. Hence, the minimum principal stress in the 1A model is more when compared with all the other models. This concept also holds for the von Mises stress in enamel parts.

Although von Mises stress concentration cannot anticipate the pattern of fracture in a FE model, increased stress build up under load indicates failure mode of teeth. Nevertheless, the FE model in our study was not intended to assess the failure mode of the endodontically treated mandibular molar; however, the amount of the load would alter the stress values but not the pattern of stress transmission.[10]

In Tetric N-Ceram Bulk Fill, in model 2A, von Mises stress is 122 MPa, and minimum principal stress is 210 MPa [Figure 1]a which is significantly less when compared to the other models, and this stress is found in the central fossa region. However, the stresses are very high in the 2B model, with von Mises stress being 272 MPa and minimum principal stresses being 321MPa [Figure 1]a in the mesiobuccal regions. Hence, adding the low elastic modulus material in the central fossa will reduce the stress in the central fossa, but adding the low elastic modulus material in the mesiobuccal region will increase the stress.

In Tetric N-Ceram Bulk Fill, maximum principal stress values in 2A and 1A are similar (56 MPa), but there is a change in the stress pattern. Furthermore, the maximum principal stress in the mesiobuccal region of 1B (103MPa) and 2B (90MPa) is higher. This shows that adding the low elastic modulus material in the mesiobuccal part will increase the stress values [Figure 1]b.

The FE analysis has shown marked variation in stress in the bulk-fill material so the comparisons are made only with the bulk-fill stress values in the overall region.

On comparing both, the minimum principal stress and maximum principal stress values, it is found that 2B, i.e., the model with cavity width >½ but <2/3rd of the intercuspal distance and restored with cuspal coverage, is found to be the worst case because it has more materials with low elastic modulus. This confirms the conclusion of the study by Yamanel et al., which stated that low elastic moduli materials transmit more functional stress to the tooth structure.[14] However, 2A, i.e., the model with cavity width <½ the intercuspal distance and restored without cuspal coverage, is safer in both the comparisons; this shows that the amount of low elastic modulus materials in the tooth structure affects the tooth behavior in a significant manner. This finding was supported by other studies in the literature, which concluded that ETT with MO/DO cavities can be successfully restored with adhesive composite resin restorations without cuspal coverage.[21],[22] Mannocci et al. also in their prospective study have reported good survival rates of root-filled teeth with three axial surfaces restored with composite resins without the need for cuspal coverage.[23] Our results are contradictory to some studies which have suggested that cuspal coverage improves the survival rates in ETT,[24],[25] but these studies have not taken into consideration the fact that root canal treated teeth with a Mesio-occluso-distal cavity and that with two surface cavities would not have the same risk of fracture.[26] Therefore practicing cuspal coverage in all cases would lead to unnecessary removal of sound tooth structure that would negatively affect the fracture toughness of the teeth.[23] Furthermore, in 2A, the low elastic material is slightly higher when compared to 1A. This reduces the stress in the tooth parts by adding a little flexibility to the structure. Moreover, based on this study, it is found the location of low elastic modulus material will also affect the tooth strength.

Since the linear isotropic properties considered in the 3D finite element study will vary from the actual physical properties of the materials, the results of this study are for numerical analysis comparative purposes only, not for the validation of the tooth in the clinical behavior. Further clinical research is recommended before drawing any inference.

 Conclusion



Our study suggested that endodontically treated molar with loss of one marginal ridge restored with direct composite resin without cuspal coverage would reveal lower stress values in the tooth structure and restorative material based on the finite element analysis. Preserving the cuspal structure would enhance the integrity of the dental tissue and improve the clinical performance of the root-filled teeth.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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