Journal of Interdisciplinary Dentistry

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 9  |  Issue : 3  |  Page : 108--113

A comparative study of stress distribution between glass fiber post and cast post in an endodontically treated central incisor – A finite element analysis


N Vignesh1, S Lakshmi2, H Annapoorani2, Padmashini Gnanam1,  
1 Department of Prosthodontics, SRM Dental College, Chennai, Tamil Nadu, India
2 Department of Prosthodontics, Meenakshi Ammal Dental College, Chennai, Tamil Nadu, India

Correspondence Address:
Padmashini Gnanam
No. 10, 54th Street, Ashok Nagar, Chennai - 600 083, Tamil Nadu
India

Abstract

Background: Endodontically treated teeth with severe loss of tooth structure require post and core restorations for retention purposes. The post system includes components of different rigidity because the rigid component is able to withstand forces without distortion and the stress is transferred to less rigid substrate. The difference between the elastic modulus of dentin and the post material may be a source of stress for root structures. The use of post systems that have an elastic modulus similar to that of dentin result in the creation of a mechanically homogenous unit with better biomechanical performance. Thus the post and core material affects stress distribution in endodontically treated teeth.Aims and Objectives: In this study, FEA is used to evaluate the fracture resistance in an endodontically treated maxillary central incisor that was restored with two different post systems – 1) prefabricated glass fiber post and 2) custom made cast post. The qualitative stress distribution analyses were recorded by von Mises criteria. Results: von Mises stress in an intact tooth was concentrated in the crown near the cingulum under oblique load and on the incisal edge with vertical load. With horizontal load- In the crown region, maximum von Mises stress was observed in both the cast post and fiber post. Within the post region, maximum von Mises stress was observed in cast post when compared to fiber post. In the root dentin, more von Mises stress was observed in fiber post when compared to cast post. Conclusion: 1)Stress distribution is similar between cast post and fiber post during vertical load. 2)Cast metal post dissipates less stress to the dentin and gives better fracture resistance to the tooth when compared to glass fiber post during oblique loading. When considering the stress pattern, maximum stress was observed in the cervical region of the crown and in the area of load application during oblique and vertical loading.



How to cite this article:
Vignesh N, Lakshmi S, Annapoorani H, Gnanam P. A comparative study of stress distribution between glass fiber post and cast post in an endodontically treated central incisor – A finite element analysis.J Interdiscip Dentistry 2019;9:108-113


How to cite this URL:
Vignesh N, Lakshmi S, Annapoorani H, Gnanam P. A comparative study of stress distribution between glass fiber post and cast post in an endodontically treated central incisor – A finite element analysis. J Interdiscip Dentistry [serial online] 2019 [cited 2020 Jan 19 ];9:108-113
Available from: http://www.jidonline.com/text.asp?2019/9/3/108/273659


Full Text



 Clinical Relevance to Interdisciplinary Dentistry



This study deals with the stress distribution on the tooth when a post is placed and the tooth is restored clinically. Hence, it is useful for a dentist during the selection of an optimum post system for management of endodontically treated teeth.

 Introduction



Endodontically treated teeth with severe loss of tooth structure require post and core restorations for retention purposes.[1] The cast post core system is relatively time-consuming and involves an intermediate laboratory phase and an elaborate retention system, which makes the procedure relatively expensive. Prefabricated posts do not require a laboratory phase and allow the whole restoration to be performed in one visit, which makes it an easier and less expensive technique.[2]

The post system includes components of different rigidity because the rigid component is able to withstand forces without distortion and the stress is transferred to less rigid substrate. The difference between the elastic modulus of dentin and the post material may be a source of stress for root structures.[3] The use of post systems that have an elastic modulus similar to that of dentin results in the creation of a mechanically homogenous unit with better biomechanical performance. Thus, the post and core material affects stress distribution in endodontically treated teeth.

Placement of direct post and core is often necessary to provide a foundation and replace dentin and also to provide the necessary retention for subsequent prosthetic rehabilitation.[4] The serrations along the fiber post significantly increase the retention of resin composite core material.[5] However, very little information is available about the influence of these retentive areas in the coronary portion of the post on the stress distribution pattern in dentin post complex. It has been difficult to create a valid index of stress distribution at root structures based solely on experimental and clinical observation.

Finite element analysis (FEA) has recently become a powerful technique in dental biomechanics. In this study, the FEA is used to evaluate fracture resistance in an endodontically treated maxillary central incisor that was restored with two different post systems – (1) prefabricated glass fiber post and (2) custom-made cast post. The null hypothesis was that post design and composition do not affect the stress distribution.

 Materials and Methods



FEA was used to perform the stress analysis of the tooth. This analysis used anatomy-based geometric structures for dentin pulp feldspathic metal ceramic crown, composite resin core made with microhybrid composite resin, cortical bone, periodontal ligament, gutta percha, and each one of the post systems. In this study, two models were simulated, first model – endodontically treated central incisor tooth restored with glass fiber post and composite resin core and second model – endodontically treated central incisor tooth restored with custom-made cast metal post and core. The effect of the design of these systems on the stress distribution in coronal and radicular portions of an endodontically treated tooth was evaluated.

Two three-dimensional FEA models of maxillary central incisors were designed for analysis of stress distribution induced by applied loads. The stress distribution was analyzed using ANSYS 11. These models were generated from the digital image of the anatomical plate and an intact, endodontically treated maxillary central incisor in CAD software (SOLIDWORKS 2012, Dassault Systèmes, USA) to obtain the geometry and contour.

Methodology

Tooth

The model of tooth simulated the maxillary central incisor. The root was 13.0 mm in length, and the crown was 8.0 mm in length. The periodontal ligament was 0.25 mm in width. A layer of cortical bone with 0.4 mm in thickness was added between periodontal ligament and spongy bone. The material for prosthesis used was nickel-chrome (Ni-Cr) alloy (0.5 mm thickness) veneered with porcelain (1.5 mm thickness).

Two models were simulated.

Model 1 – A FEA model – endodontically treated central incisor restored with custom-made cast post with a metal ceramic superstructure which was simulated [Figure 1]. The maxillary central incisor was modelled according to Wheelers Atlas of Tooth Form 6, wherein the root was 13.0 mm length and crown was 8.00 mm length. Length of the post used was 10 mm. Conventional preparation techniques were applied for root canal treatment, and the preparation of core and creation of metal ceramic restorations which had Ni-Cr alloy were used as a metal substructure material and layered with feldspathic ceramic. Cement layer was ignored.Model 2 – A FEA model – endodontically treated central incisor restored with prefabricated glass fiber post with a metal ceramic superstructure which was simulated [Figure 2]. The maxillary central incisor was modelled according to Wheelers Atlas of Tooth Form 6, wherein the root was 13.0 mm length and crown was 8.00 mm length. Post length was 10 mm. Conventional preparation techniques were applied for the root canal treatment, and the preparation of core and creation of metal ceramic restorations which had Ni-Cr alloy were used as a metal substructure material and layered with feldspathic ceramic. Cement layer was ignored.

The instruments used to simulate the model were optical comparator and computer with SOLIDWORKS 2012 (Dassault Systemes, USA), ANSYS 11 (ANSYS, PA, USA) and HYPERMESH 10 software (Altair HyperWorks, MI, USA) for meshing the models. The computer used had INTEL Pentium dual core processor with 2GB RAM and a hard disk memory of 160 GB for the use of SOLID WORKS, ANSYS 11, and HYPERMESH software.{Figure 1}{Figure 2}

Preprocessing and modeling

This involves the collection of data for designing and simulating the model. The first phase is the collection of data for designing the structural geometry of the tooth and prosthesis. The structure, design, and material properties of bone tooth and post for maxillary central incisor were collected.

Endodontically treated maxillary central incisor was first observed for dimensions and structural formation through the optical comparator. A maxillary D-2 (Misch classification) bone model was simulated. The shape of the bone was simplified to a cuboidal block. A bone block measured 24 mm in height, 12 mm in mesiodistal length, and 12 mm in buccolingual width. It consisted of a spongy center surrounded by 2 mm of cortical bone.

The model of single tooth simulated the endodontically treated maxillary central incisor. The root was 13.0 mm length, and the crown was 8.0 mm length. The periodontal ligament was 0.25 mm in width. A layer of cortical bone 0.4 mm in thickness was added between periodontal ligament and spongy bone. The materials used in the models were considered to be isotropic, homogenous, and linearly elastic. The post was selected according to the size of the Piezo reamer used.

Processing

In this stage, all relevant information obtained in the preprocessing stage is put into control data. These control data form the basic unit to be analyzed. The ANSYS 11 software now employs the inbuilt graphic facilities over the geometric data. These geometric data are put into meshing. Meshing is done using HYPERMESH by giving a meshing command to the software. The structure and design of tooth, post, bone, and superstructures were processed using ANSYS11 software.

The mesh model was generated using a mapping approach with eight-node iso-parametric brick elements (solid 45). Loads were applied using ANSYS11 at the control points [Table 1] and displacement was seen at nodes in the analysis. The properties of all the materials are incorporated in the processing stage. Once the data are incorporated, the model simulates the clinical situation on which the analysis can be performed to visualize the stress analysis.{Table 1}

The qualitative stress distribution analyses were recorded by von Mises criteria. The models displayed stress both numerically and by color coding. The results obtained are interpreted.

The data obtained were exported to ANSYS 11 using the IGES format. Areas corresponding to each structure were plotted and meshed with isoparametric nodes, according to mechanical properties of each structure and of materials used.

The mesh process involves divisions of system to be studied into a set of small discrete elements defined by nodes. The number of elements generated varied depending on the different geometric that were meshed so that the final mesh accurately represented the original geometry. The models had 125,114 elements and 24,424 nodes. These data are summarized in [Table 2]. All the tooth structures and materials used in models were considered homogeneous, with different material properties for fiberglass in parallel and perpendicular directions.{Table 2}

 Results



Values of maximum von Mises stresses occurring in materials of models under oblique load and vertical load are presented in [Table 3].{Table 3}

von Mises stress in an intact tooth was concentrated in the crown near the cingulum under oblique load and on the incisal edge with vertical load.

With oblique load

In the crown region, maximum von Mises stress was observed in both the cast post and fiber post (29.039 MPa and 30.795 MPa, respectively) [Figure 3] and [Figure 4]Within the post region, maximum von Mises stress was observed in cast post (22.926 MPa) when compared to fiber post (5.177 MPa) [Figure 5] and [Figure 6]In the root dentin, more von Mises stress was observed in fiber post (18.357 MPa) when compared to cast post (5.952 MPa) [Figure 7] and [Figure 8].{Figure 3}{Figure 4}{Figure 5}{Figure 6}{Figure 7}{Figure 8}

With vertical load

In the crown region, stress observed in the cast post is 9.673 MPa and in fiber post is 7.986 MPaWithin the post, stress in the cast post is 9.673 MPa and in fiber post is 8.687 MPaIn the dentin, stress in the cast post is 4.700 MPa and in fiber post is 4.236 MPa.

 Discussion



An important advantage of using a finite element method in stress analysis is that all conditions can be kept identical (such as tooth morphology, mechanical properties, load, and periodontal support). Furthermore, the numeric method ensures that the root canal preparation has the same size and taper in each model, which would have been impossible to achieve in an experimental study in human teeth. However, the result of this method is an approximate solution. Thus, the results in this study are presented in a qualitative rather than a quantitative manner.

The stress values in posts depend on the elastic modulus of the post material. The present study demonstrated that equivalent von Mises stresses in the cast posts increased with the increasing elastic modulus of the post materials. With regard to the oblique load, the more rigid the post, the lower the von Mises stresses in the postdentin interface. Oblique load was given in the cingulum region and the vertical load was given in incisal region. For oblique load, the maximum von Mises stress was observed in cingulum and in the cervical region. For the vertical load, the maximum von Mises stress was observed in incisal region and in the cervical region. The stresses observed in the crown region were less than the tensile strength of feldspathic ceramics.

In this present study, fracture resistance of glass fiber post and cast metal post was compared. Results showed that fiber post exerted more stress on dentin than that of cast post and resisted forces better. Hence, the cast post has better fracture resistance when compared to the fiber post.

In the crown region, both cast post and fiber post exerted maximum stress during the oblique load. In the post region, more stress is observed by cast post when compared with the fiber post. When the dentin is considered, more stress is dissipated to dentin in the fiber post when compared to the cast post.

When cast post is considered, most of the stress is taken by the post (26N) and remaining stress is dissipated to the dentin and the crown region. On the contrary, when a fiber post is considered, comparatively very minimal stress is taken up by the post (6N), and therefore, most of the stress is dissipated to the dentin and crown.

This inference can be summarized by the fact that metal post resists more stress and takes up prime proportion of the load applied to the tooth compared to that of the fiber post. When considering the stress pattern, maximum stress is observed in the cervical region of the crown and also in the area where the stress is given, i.e., for oblique load, load is applied in the cingulum region, and for the vertical load, load is applied in the incisal region. The inference can be summarized by the fact that both cast metal post and fiber post have a rick of fracture near the cervical region of the tooth.

 Conclusion



Stress distribution is similar between cast post and fiber post during vertical loadCast metal post dissipates less stress to the dentin and gives better fracture resistance to the tooth when compared to glass fiber post during oblique loadingWhen considering the stress pattern, maximum stress was observed in the cervical region of the crown and in the area of load application during oblique and vertical loading. The inference can be summarized by the fact that both cast metal post and fiber post have a rick of fracture near the cervical region of the tooth.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

[6]

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