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 Table of Contents  
Year : 2022  |  Volume : 34  |  Issue : 2  |  Page : 115-120

Biomechanical stress analysis of ceramic and indirect hybrid composite endocrowns: A three-dimensional finite element analysis

Department of Conservative Dentistry and Endodontics, Sri Ramachandra Faculty of Dental Sciences, Sri Ramachandra Institute of Higher Education and Research (Deemed to be University), Chennai, Tamil Nadu, India

Date of Submission23-Aug-2021
Date of Decision26-Nov-2021
Date of Acceptance12-Mar-2022
Date of Web Publication01-Jul-2022

Correspondence Address:
Dr. Lakshmi Balaji
Department of Conservative Dentistry and Endodontics, Sri Ramachandra Faculty of Dental Sciences, Sri Ramachandra Institute of Higher Education and Research (Deemed to be University), No. 1, Ramachandra Nagar, Porur, Chennai - 600 116, Tamil Nadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/endo.endo_156_21

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Aim: The purpose of this study was to compare equivalent stresses in mandibular first molar restored with endocrowns made of ceramic and indirect composites using three-dimensional (3D)-finite element analysis (FEA).
Materials and Methods: Two 3D finite element models of mandibular first molar were designed. One model of intact tooth without any restoration was uses as a control. Another mandibular molar model with endocrown as postendodontic restoration was generated where the tooth was simulated with extensive coronal loss and divided into five groups with different materials such as lithium disilicate, zirconia, feldspathic, resin nanoceramic, and polymer infiltrated ceramic network (PICN). An axial loading force of 300 N at five diverse contacts areas was applied on the occlusal surface. Stress distribution at various regions of tooth, concentrating mainly on root dentin and alveolar bone, was measured using the Von Mises stress criteria by FEA software.
Results: The lowest stress concentration in root dentin was seen in the cervical root region of resin nanoceramic endocrown group followed by PICN endocrown, feldspathic ceramic endocrown, intact tooth, zirconia endocrown, and lithium disilicate endocrown. In alveolar bone, the result showed that the lowest stress value was seen in resin nanoceramic endocrown, followed by PICN endocrown, zirconia endocrown, feldspathic endocrown, intact tooth, and lithium disilicate endocrown.
Conclusion: Endocrown materials with the modulus of elasticity similar to that of dentin showed better stress distribution than the materials with high modulus of elasticity.

Keywords: Ceramic restoration, endocrown, finite element analysis, indirect composite restoration, stress analysis

How to cite this article:
Joshna BV, Kuzhanchinathan M, Balaji L. Biomechanical stress analysis of ceramic and indirect hybrid composite endocrowns: A three-dimensional finite element analysis. Endodontology 2022;34:115-20

How to cite this URL:
Joshna BV, Kuzhanchinathan M, Balaji L. Biomechanical stress analysis of ceramic and indirect hybrid composite endocrowns: A three-dimensional finite element analysis. Endodontology [serial online] 2022 [cited 2022 Aug 8];34:115-20. Available from: https://www.endodontologyonweb.org/text.asp?2022/34/2/115/349570

  Introduction Top

Endodontically treated teeth are more brittle and are more prone to fracture than vital teeth[1] due to the lower dentin moisture content in pulpless teeth.[2] The significant factors which are responsible for the fracture susceptibility of endodontically treated teeth are the amount of loss of tooth structure, root canal preparations, and effect of various irrigating solutions on dentin.[3],[4],[5] Conventionally, post and core retained crowns have been the choice of restoration for teeth with extensive coronal loss.[6] However, additional removal of tooth structure is required to place post in the root canal which can compromise the pericervical dentin may result in root fracture along with coronal tooth preparation when receiving full coverage restoration which further compromises the tooth structure.[7]

Endocrown restorations are currently recommended as alternative, conservative approach when half or more than half of the residual tooth structure is available with pulpal walls providing the macromechanical retention and adhesive system produce the micromechanical retention.[8],[9]

Endocrown preparation should have wide, stable surface parallel to the occlusal plane to resist the compressive stresses.[10] The pulp chambers that are trapezoidal shape in mandibular molars and triangular shape in maxillary molars provide retention and the saddle form of the pulpal floor provides stability.[10] Endocrown is not indicated when isolation cannot be achieved, depth of the pulp chamber is <3 mm, or when most of the circumference the cervical margin is <2 mm wide and also most commonly used only in molars.[11]

Endocrown restorations are less time-consuming and less expensive than the postretained crowns.[6],[8],[12] Furthermore, the preparation for endocrown and impression making is less technique sensitive[8] and protects the periodontium as the margin of the endocrown restoration is mostly supragingival.[10] Earlier, many studies have advocated the use of endocrown as postendodontic restoration.[8],[9],[10] In spite of the clinical advantages of endocrown over postretained crown, endocrowns are not widely practiced as a postendodontic restoration.[10] This could be due to the lack of awareness of the procedure and lack of guidelines in this regard.

Finite element analysis (FEA) is a best method to simulate the oral environment in vitro conditions and is widely used to assess the stress distribution on tooth and surrounding structures and provides information on how the design of restorations and materials with different properties can affect the teeth.[13],[14],[15] Cone beam computed tomography (CBCT) provides the high accuracy image and three-dimensional (3D) digital models of the samples for FEA that can be created out of Digital Imaging and Communications in Medicine (DICOM) data by software volume rendering.[16]

The purpose of this study is to compare equivalent stresses in mandibular first molar restored with endocrowns made of ceramic and indirect composites during functional masticatory loading using 3D-FEA.

  Materials and Methods Top

Cone-beam computed tomography data

Existing CBCT image of the mandibular first molars for reasons not related to this study was obtained and 3D models were generated by utilizing this data.

Generation of finite element models

Image segmentation and model generation

To generate 3D computer models of mandibular first molar, CBCT image of the mandible was extracted and using DICOM TO PRINT (D2P) software; 3D solid models with enamel dentin and pulp were generated. The model was edited using Geomagic Freeform Software with Haptic Device and converted to STL format to make the model compatible with the FEA computer-aided mechanical software.

Model A: Normal intact molar

The crown length was 7.5 mm and root length was 14 mm.[17] The enamel thickness was approximately 2 mm in occlusally and the thickness reduced cervically. Periodontal ligament of 0.2 mm thickness and alveolar bone of approximately 2 mm thickness were simulated around the roots.[18] The mesiobuccal, mesiolingual and distal root canals were designed as Vertucci type I configuration.

Model B: Endodontically treated molar restored with endocrown

In order to simulate molar tooth of extensive coronal loss, the tooth was designed in such a way to leave 3 mm remaining tooth structure cervivo-occlusally from the cementoenamel junction.[11] A cervical sidewalk of 2 mm was prepared on the tooth.[10] The pulpal walls were designed with the occlusal divergence of 6 degrees.[18] The mesiobuccal, mesiolingual, and distal root canals were designed as Vertucci Type I configuration and considered to be obturated with gutta-percha to simulate endodontically treated tooth. The endocrown was designed with 3 mm pulpal extension.[11] A layer of resin luting agent (70 μm thick) was created between prepared tooth surface and intaglio surface of the endocrown.[12] Periodontal ligament of 0.2 mm thickness and alveolar bone of approximately 2 mm thickness were simulated around the roots [Figure 1].[18],[19]
Figure 1: Model B – Endodontically treated molar restored with endocrown

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Mesh generation and boundary conditions

The models were meshed with the adaptive meshing with element size of 0.1 mm tetrahedrally. Model A had 75,000 elements and 91,000 nodes and Model B had 81,000 elements and 110,000 nodes. All the components were considered to be isotropic, homogenous, and linearly elastic.[18]

Load application

The vertical axial load of 300 N was applied over 1 mm2 area on five diverse contact points each on the occlusal surface of models to simulate the bite force.[12],[18]

Material properties and analysis

Ceramic materials used for the endocrowns were lithium disilicate, zirconia, and feldspathic. Hybrid resin-based materials used were resin nanoceramic and polymer infiltrated ceramic network (PICN). The endocrowns were considered to be cemented using a luting agent (Multilink automix).[20] The modulus of elasticity and poisons ratio of tissues and material components of the models were assigned accordingly and the models were imported to FEA software Ansys® Academic Research Mechanical, Release 18.1 [Table 1].
Table 1: Mechanical characteristics of the materials

Click here to view

  Results Top

The maximum von misses values at the restoration and tooth structure are visually analysed using shade images to demonstrate stress distribution. The red zone indicates the highest stress levels, whereas the dark blue zone indicates the lowest stress levels. Maximum von Mises (mvM) stress levels were calculated. The main concentration was on the mvM stress levels of root dentin and alveolar bone [Table 2].
Table 2: Maximum von Mises stress levels

Click here to view

Stress levels on enamel

  • PICN showed the least mvM stress (1.2 MPa)
  • Zirconia endocrown group showed the highest mvM stress (2 MPa).

Stress levels on crown dentin

  • Feldspathic ceramic endocrown showed the least mvM stress (0.10 Mpa).
  • Lithium disilicate endocrown showed the highest mvM stress (0.21 Mpa).

Stress levels on root dentin

  • Resin nanoceramic showed the least mvM stress (0.18 Mpa)
  • Lithium disilicate showed the highest mvM stress (0.81 Mpa).

Stress levels on alveolar bone

  • Resin nanoceramic showed the least mvM stress (0.09 MPa)
  • Lithium disilicate showed highest mvM stress (0.2 MPa).

Stress levels on endocrown

  • Feldspathic ceramic endocrown showed least vmM stress (1.09 MPa)
  • Resin nanoceramic endocrown showed highest vmM stress (1.54 Mpa).

Stress levels on luting agent

  • PICN showed least mvM stress (0.04 Mpa)
  • Lithium disilicate endocrown showed the highest mvM stress (1.08 MPa).

  Discussion Top

In order to overcome the limitations of conventional post and core retained crown restoration, endocrowns have been introduced and are considered as an alternative restoration for severely damaged endodontically treated posterior teeth.[21]

Endocrowns are commonly fabricated using lithium disilicate ceramic.[22] The presence of crystalline particles in lithium disilicate increases the fracture strength. The disadvantage of lithium disilicate ceramic are that they can potentially wear away the opposing natural tooth.[23]

Recently, newer Computer-aided design and Computer-aided manufacturing (CAD/CAM) indirect resin-based materials have been suggested as an alternative to ceramics, since they have more biomimetic properties with a similar elastic modulus to tooth structure.[24] Furthermore, these composite resins are repairable and are not as abrasive to opposing tooth structures as ceramic restorations.[25] CAD/CAM composite full coverage crowns have shown to offer superior marginal adaptation than all-ceramic crowns.[26] These new hybrid materials may be mainly divided into: those that consist mainly of resin matrix referred to as CAD/CAM composite resins (e.g.,) resin nanoceramic, and materials that are predominantly ceramic based called as hybrid ceramics (e.g.,) PICN.[27]

In this study, attempt has been made to compare the stress distribution pattern of various materials such as lithium disilicate, zirconia, feldspathic ceramic, zirconia, resin nano ceramic and PICN used for fabrication of endocrown using 3D FEA, as it allows a highly controlled analysis of several specific parameters on a single model.[28]

von Mises stress criterion was used in this study since it is the most commonly used criterion in dentistry.[28] It is used to study stress distribution of ductile materials but are frequently used for brittle materials such as ceramics and resin composites and it can identify the areas of highest stress concentration where possible fatigue failure is more likely to occur.[29] Mandibular first molar has been chosen in this study as they are liable to various directions and values of both functional and para-functional loads[18] and it has been demonstrated that a ceramic endocrown restoration in molar teeth is more beneficial than in premolar teeth, because of the large surface inside the pulp chamber of molars compared to premolars, which will improve the micromechanical retention given by the adhesive system.[30]

Studies have shown that when periodontal ligament (PDL) was not simulated in the finite element model, a large amount of applied force was transferred to the alveolar cortex, therefore 0.2 mm of PDL is simulated around the roots of the molar tooth which behaves as a stress absorber.[31]

Intracoronal extension also has been shown to influence the stress distribution pattern of endocrown. In order to mimic the clinical scenario of extensive coronal loss of tooth structure and as minimum of 3 mm depth is suggested by Fages and Bennasar,[11] a standardized intra coronal depth of 3 mm has been chosen to be used in the endocrown and post and core models in this study. Volume of the material also has shown to influence the stress distribution.[13] Hence, in order to standardize the volume of material, 4.5 mm of endocrown material is used from the cervical sidewalk (butt joint) of the tooth.

Axial loading was simulated in this study as it evaluates more directly the effect of inherent characteristics such as elasticity modulus of materials on their mechanical behavior rather than lateral loading, which would be more associated with the adhesion effect of the restoration on the bonding outcome. In this study, axial load of 300 N was applied in five contact points to simulate the maximum masticatory load.[18]

According to the result, resin nanoceramic endocrown and PICN exhibited lesser stress on root dentin and alveolar bone than other experimental groups. Lithium disilicate endocrown showed the highest von Mises stress on both root dentin and alveolar bone. This could be because resin-based materials absorb relatively more of the occlusal stress. They have shown a greater capacity to absorb compressive loading forces and reduce the impact forces by 57% more than porcelain.[32] The elasticity modulus of composite material is similar to that of dentin, so the composite tend to bend under loading, distribute stresses more evenly and have stress absorbing properties.[24]

There are various schools of thought regarding the association between the elastic modulus of restorative material and stress distribution on tooth structure. Materials with high elastic modulus are capable of protecting sound enamel tissue by transmitting less stress and materials with low elastic moduli transferred more stress to dental tissues.[13] However, Soares et al. stated that rigid materials with high elastic moduli, produce stress concentrations at critical areas that might cause catastrophic failures,[31] whereas, when materials of low modulus of elasticity when submitted to loading, absorb the forces better, thus reducing the probability of fracture, and even if fracture occurs, they are mostly repairable. The result of the current study also shows that materials with modulus of elasticity similar to dentin exhibit less stress concentrations on tooth structure and material with high modulus of elasticity exhibited more stress concentration on tooth structure.

  Conclusion Top

Within the limitations of this study, the following conclusions are drawn:

  1. Newer hybrid resin-based endocrowns (resin nanoceramic and PICN) exhibited lesser mvM stress than ceramic endocrowns (feldspathic, zirconia, and lithium disilicate)
  2. Resin nanoceramic endocrown presented the lowest mvM stress on root dentin and alveolar bone among the various endocrown materials tested
  3. Lithium disilicate endocrown presented highest mvM stress on root dentin and alveolar bone.

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Conflicts of interest

There are no conflicts of interest.

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