| Abstract|| |
Context: Noncarious cervical lesions may penetrate the pulp and require root canal treatment followed by crown placement. Such teeth may be susceptible to fracture, especially at the cervical area.
Aims: To estimate which combination of restorative material and crown resulted in homogenous stress–strain distribution of endodontically treated abfracted mandibular premolar using three-dimensional finite element model (FEM).
Settings and Design: A three-dimensional model of mandibular single-rooted premolar along with alveolar bone was created in finite element analysis (FEA) software preprocessor ANSYS rel 14.5 FEM software (ANSYS Inc., Houston, USA, 1994). Cervical lesion was created in the model with specific dimensions, 3 mm mesiodistally and 2 mm gingivoocclusally with enamel occlusal margin and dentin gingival margin.
Materials and Methods: Tooth was simulated to be root canal treated and restored with different types of cements and crowns followed by placing a static load of 300 N at an angle of 135°. Analysis was run and stress distribution pattern was studied.
Results: Cervical region of an endodontically treated tooth is subjected to stresses, irrespective of restorative material used. Porcelain fused to metal (PFM) crowns showed least strain values with different postendodontic, restorative materials.
Conclusions: FEA is a predictable and reproducible model to predict stress–strain behavior. PFM crowns with different postendodontic restorative materials showed least strain values in the cervical area of abfracted, endodontically treated premolars.
Keywords: Noncarious cervical lesions; porcelain fused to metal restorations; postendodontic restorations; three-dimensional finite element analysis
|How to cite this article:|
Kaushik M, Kumar U, Sharma R, Mehra N, Rathi A. Stress distribution in endodontically treated abfracted mandibular premolar restored with different cements and crowns: A three-dimensional finite element analysis. J Conserv Dent 2018;21:557-61
|How to cite this URL:|
Kaushik M, Kumar U, Sharma R, Mehra N, Rathi A. Stress distribution in endodontically treated abfracted mandibular premolar restored with different cements and crowns: A three-dimensional finite element analysis. J Conserv Dent [serial online] 2018 [cited 2019 Aug 21];21:557-61. Available from: http://www.jcd.org.in/text.asp?2018/21/5/557/241191
| Introduction|| |
Noncarious cervical lesions (NCCLs) are characterized by loss of coronal and radicular tooth substance at the cementoenamel junction in the absence of caries.
The development and activity of muscles of mastication, emotional status of an individual, occlusal trauma, wear/toothbrush abrasion, chemical erosion, and poor quality of cervical enamel are the factors that contribute to this process.
Deep cervical lesions may approximate the pulp, necessitating root canal treatment.
Endodontically treated teeth are more likely to fracture than vital teeth because of the loss of tooth structure due to caries, access cavity preparation, and instrumentation of root canal. Loss of moisture from dentin translates to desiccation and decreased resilience.
The material used to restore NCCLs and access cavities is a critical determinant of endodontic success. The most commonly used materials for both are composites, glass ionomers, resin-modified glass ionomer cement (RMGIC), and compomers.,
An extensively damaged endodontically treated posterior tooth needs full-coverage restorations to prevent tooth fracture. All-ceramic and porcelain fused to metal (PFM) crowns protect the tooth from fracture and offer good esthetics.
Finite element analysis (FEA) can provide detailed quantitative data at any location with a mathematical model. Three-dimensional finite element approach consists of dividing a geometric model into a finite number of elements in which the variables of interest are approximated with mathematical functions. The solid model is then transferred into a finite element model (FEM) program. During this three-dimensional analysis, the element mode is specified and connected with each other at their nodes. The force–displacement relation written in terms of nodal variables helps form the element stiffness matrix. These element matrices are combined to give the global stiffness matrix. The bone with all its displacements is then fixed, thus preventing rigid body displacements in the direction of the three coordinate axes. Assumptions imposed on the FEA regarding model geometry, load magnitude, load direction, and use of a fine mesh influence its relative accuracy. The FEA depicts the internal stresses of a system, and on that basis, predictions about fracture susceptibility can be made.
The aim of this study was:
- To determine the stress and strain distribution of endodontically treated abfracted mandibular premolar restored with different materials and different types of crowns using three-dimensional FEM
- To estimate which restorative material resulted in the most homogenous stress- strain distribution.
| Materials and Methods|| |
A three-dimensional model of mandibular single-rooted second premolar along with alveolar bone was generated using FEA software preprocessor ANSYS rel 1505 FEM software (Ansys Inc., Houston, USA, 1994).
A cervical lesion was created in the model, 3 mm mesiodistally, 2 mm gingivoocclusally, with the occlusal margin in the enamel and gingival margin in the dentin. The lesion was assumed to be in contact with the pulp. The internal and external line angles were maintained sharp. Cementum was not replicated as it is a very thin layer and has the same physical properties as dentin. Bone was created as a cylindrical block around the tooth. The root portion with periodontal ligament 0.2 mm thick was inserted inside the bone. An access cavity from cementoenamel junction to the external surface of the enamel and a pulpal space mirroring the dimension of F3 ProTaper Universal (Dentsply Maillefer, Ballaigues, Switzerland) were simulated. The cervical lesion and access cavities were restored with different materials followed by simulation with different crowns.
The models were grouped into three groups based on the restorative material – glass ionomer cement (GIC), composite, and compomer. Each group had three subgroups, without crown being the control and PFM and all-ceramic crowns being the experimental groups. In the present study, the following assumptions were made in terms of the properties of the materials concerned.
- Tooth under consideration was composed of enamel, dentin, and restorative material
- Pulp space was filled with gutta-percha
- Access cavity and cervical lesions were filled and the teeth were crowned.
The materials considered were assumed to be homogenous, isotopic, and linearly elastic.
To calculate stress distribution, a 300 N static load was applied to the contact area, at the lingual slope of the buccal cusp  of the crown with a 135° angle to the longitudinal axis of the tooth. The data were recorded and analyzed based on the color patterns where warm color denoted higher stresses; red – maximum stress and blue – least stress [Figure 1].
|Figure 1: The color patterns denoting stress distribution, with red denoting maximum stress and blue denoting least stress. (a) Control GIC, (b) control composite, (c) control compomer (d) PFM GIC, (e) PFM composite, (f) PFM compomer, (g) all-ceramic GIC, (h) all-ceramic composite, (i) all-ceramic compomer. GIC: Glass ionomer cement, PFM: Porcelain fused to metal|
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| Results|| |
A structural linear static analysis was performed to evaluate the stress distribution at the cervical area of dentin restoration surface.
Abfractions restored with GIC and metal-ceramic crown demonstrated the maximum Von Mises stress values of 1157.2 MPa. This indicates high yield strength, that is, fracture will occur only at these high stresses.
Abfractions restored with glass ionomer and all-ceramic crowns show the least Von Mises stress values of 692.3 MPa.
PFM crown and noncrowned tooth showed more fracture resistance than all-ceramic crowns [Table 1].
|Table 1: Maximum Von Mises stress values obtained after the application of load from the three-dimensional tooth models prepared with different restorative materials and crowns|
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| Discussion|| |
The prevalance of NCCL has been estimated to be between 31% to 56%.
Mandibular premolars were selected in the present study because they are most susceptible to NCCLs, due to their anatomical location, lingual orientation in the arch, and anatomically small cervical cross section which cannot sustain tensile forces.
GICs, RMGICs, a GIC/RMGIC liner base laminated with a resin composite, and resin composite in combination with a dentin-bonding agent are some of the restorative options for both NCCL and post endodontic restorations.
In general, endodontically treated teeth are considered to have increased susceptibility to fracture than vital teeth. The structural durability of the tooth following endodontic therapy is directly proportional to the remaining tooth structure present.
Restorations providing full occlusal coverage, such as crowns, are most suitable to restore a posterior tooth after root canal therapy. All-ceramic and PFM crowns are popular esthetic choices and were simulated in the present study.
An analysis of the results reveals that all the tooth models restored with different materials showed high resultant stresses concentrated at the cervical margin. Hence, this area is the potential failure site. When the load, loading angle, and restoration size were kept fixed, the resulting Von Mises stress in the immediate vicinity of the restored area was found to be inversely proportional to the Young's modulus value of the restorative material.
As the Young's moduli of the enamel and the restorative material are different, the continuity of the structure is disrupted which gives rise to stress concentration. If the restorative material has a high Young's modulus, the destruction becomes less prominent. Therefore, strictly from a mechanical point of view, it can be said that the best approach is to apply restorative materials which have as large as possible, Young's moduli. The Young's modulus of composite, 21 GPa closely matches that of dentin, 19.8 GPa. This could explain the positive results obtained with composite in our study. In particular, microfilled composites showed less stress concentration in dentin. With this type of resin, much of the transferred energy is absorbed by the restoration rather than transmitted to the dentin–restoration interface. GIC, which has a low modulus of elasticity, 10.8 GPa will allow the restoration to flex, absorbing induced stresses with reduced resultant stress on the tooth.
Loading angles of 30° and above create a bending moment causing flexure of the tooth, resulting in maximal tensile stresses on the buccal surface of root dentin, close to the alveolar bone  and in deep dentin next to pulp.
NCCLs with acute angles have more stress accumulation in the lesion's center due to the acute angles promoting smaller areas for stress dissipation. Lesions with a rounded internal angle showed less stress concentration than acute-angled lesions.
In the present study, tooth restored with PFM crowns showed maximum resistance to fracture probably attributable to the high modulus of elasticity. High modulus of elasticity is expected to enable better stress distribution.
Tooth restored with all-ceramic crowns showed the least resistance to fracture. This may be probably due to the greater amount of tooth reduction which reduces its fracture resistance. Furthermore, even if some of the surface layer of porcelain in PFM crown fractures, the metal substructure underneath will characteristically stay intact, thus maintaining the crown's seal over, and reinforcement of, the tooth. In comparison, the full thickness of an all-ceramic may fracture, thus compromising both functions.
When an external force is applied to an object, it is observed that complex stress is produced within it. The implantation axis of a tooth and its shape dictate how the loading is transferred. When stress is present, there is also deformation or strain.
Many brittle materials such as porcelain have a tensile strength that is markedly lower than the corresponding compressive strength which may be a possible reason for lower fracture resistance of all-ceramic crowns. Hence, restoring tooth with PFM crowns yielded best results.
This study was conducted on a mandibular premolar replicating ideal conditions and ideal values of all elements involved. The cervical cavity created was also a specific width and depth. These conditions will vary in a clinical scenario where the lesion could be larger and the properties of enamel and dentin may vary due to age changes. In a clinical scenario, anisotropic properties of enamel, dentin, and oral cavity may give rise to nonlinearities and stresses which were not simulated in the present study.
| Conclusions|| |
Within the limitations of the present study:
- The cervical region of the restored tooth is subjected to highest stress concentration, irrespective of the restorative material used
- The mechanical properties of crown and restorative material influenced the stress concentration and strain in dentin
- In terms of materials used, microfilled composite caused the least strain in dentin
- PFM crowns and noncrowned tooth showed more fracture resistance than all-ceramic crowns.
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Conflicts of interest
There are no conflicts of interest.
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Prof. Mamta Kaushik
Department of Conservative Dentistry and Endodontics, Army College of Dental Sciences, Secunderabad, Telangana
Source of Support: None, Conflict of Interest: None