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 Table of Contents  
ORIGINAL RESEARCH
Year : 2018  |  Volume : 10  |  Issue : 2  |  Page : 88-93

Comparison between two materials for the fabrication of modified design for posterior inlay-retained fixed dental prosthesis: A finite element study


1 Restorative Dentistry Department, Al-Farabi Dental College, Jeddah, Saudi Arabia; Department of Crown and Bridge, Faculty of Oral and Dental Medicine, Al Azhar University, Cairo, Egypt
2 Fixed and Removable Prosthodontics Department, Oral and Dental Division, National Research Centre, Giza; Department of Fixed and Removable Prosthodontics, Faculty of Oral and Dental Medicine, Al Nahda University (NUB), Beni Suef, Egypt
3 Restorative Dentistry Department, Al-Farabi Dental College, Jeddah, Saudi Arabia
4 Al-Farabi Dental College, Jeddah, Saudi Arabia

Date of Web Publication23-Apr-2018

Correspondence Address:
Dr. Rami M Galal
59 4th, Touristic District, 6th of October, Giza
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jioh.jioh_246_17

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  Abstract 

Aims: This study was done to test materials to restore missing mandibular first molar with a new prosthetic design. Materials and Methods: Two three-dimensional finite element models were prepared, especially to simulate missing mandibular first molar. Models geometry was created on commercial engineering computer-aided design/computer-aided manufacturing package and then transferred to ANSYS for the stress analysis. Posterior inlay-retained fixed dental prosthesis (FDP) was modeled as one piece and as multiple pieces containing metallic substructure, coating layer, and wings resin coating. The dental prosthesis materials tested in this study are full zirconia as one piece or first case study, in addition to chromium cobalt as a substructure, porcelain coating, and adhesive resin as wings coating to be the second case study. Results: Compressive load of 400N was applied on the buccal cusp, and resultant stresses were compared between the two models. Von Misses stress distributions in the two models showed safe values. The zirconia prosthesis showed less stresses. Conclusions: The two case studies showed two equivalent alternatives. Both cases are suitable for FDP manufacturing. Other factors such as lifetime and color matching may govern the dentist selection of material to be used.

Keywords: Finite element method, inlay-retained prosthesis, zirconia


How to cite this article:
Yossef SA, Galal RM, Alqahtani WM, Alluqmani AA, Abdulsamad MA, Alsharabi OH, Smurqandi EM. Comparison between two materials for the fabrication of modified design for posterior inlay-retained fixed dental prosthesis: A finite element study. J Int Oral Health 2018;10:88-93

How to cite this URL:
Yossef SA, Galal RM, Alqahtani WM, Alluqmani AA, Abdulsamad MA, Alsharabi OH, Smurqandi EM. Comparison between two materials for the fabrication of modified design for posterior inlay-retained fixed dental prosthesis: A finite element study. J Int Oral Health [serial online] 2018 [cited 2022 Dec 4];10:88-93. Available from: https://www.jioh.org/text.asp?2018/10/2/88/230860


  Introduction Top


When a decision is made to replace a missing single posterior tooth in bounded saddle area by a fixed dental prosthesis (FDP), a number of prosthesis designs are available. These include traditional full veneer FDP, inlay-retained FDP (IRFDP), and implant-supported FDP.

One of the basic principles of tooth preparation in fixed prosthodontics is conservation of sound tooth structure to preserve tooth vitality and reduce postoperative sensitivity.

Conservative preparation designs such as IRFDPs are much less invasive than conventional full-coverage crown preparations. Several investigations introduced and examined different designs of conservative FPDs with the aim of conserving tooth structure.[1],[2]

However, a major disadvantage of full coverage FDP is the removal of significant amount of sound tooth structure of the abutment teeth estimated between 63% and 73%.[3] When treatment with dental implants is not considered or contraindicated for a patient, restoration of the existing dental space by IRFDP would be a more favorable treatment option, assuming that sufficient sound tooth structure is available in the abutments teeth. The conventional design of IRFDP includes a mesial and distal inlay wings used as retainers for the pontic. In the current study, a finite element experiment was used to analyze the stresses generated on metal and zirconia pontic areas. Zirconia inlay-retained prostheses give us good esthetic results in addition to the conservative nature of the prosthesis.[4] It is well proven that the implant supported prostheses are preferred regarding satisfaction and biological issues.[5] However, sometimes, patients preferred other treatment options other than the implant supported type.[6] This may be due to financial causes or fear from surgery. Furthermore, due to the fact that the patients now are more aware of the oral hygiene measures because of the social media and other sources of information, the inlay-retained prostheses is regaining its popularity. Being preservative type of prosthesis with less loss of natural tooth structure is the main cause of being preferred by patients.[7] These types of prostheses can bond to minimally prepared teeth well, but the documented survival rates are for short-term periods.[8],[9] Hence, when it is contraindicated in a certain patient to do implant-supported prosthesis, it would be more favorable to perform inlay-supported prosthesis to him rather than to do full coverage prosthesis. There should be enough sound tooth structure in the abutment teeth. The original design of the inlay-retained prostheses is composed of a pontic with 2 wings of inlay retainers with cavities prepared in the adjacent teeth. The inlays fill the whole volume of the cavities. This type of prostheses can be constructed from metal, resin, or all ceramic materials. A proposed modified design of the inlay-retained prostheses is studied here using the finite element analysis method. In this research, two analyses were done to evaluate the effect of the restoration materials alternatives on stresses generated under compressive loading.


  Materials and Methods Top


Two three-dimensional (3D) geometric models were prepared on commercial engineering general purpose computer-aided design/computer-aided manufacturing (CAD/CAM) software (Solid Works 2013 - Dassault Systèmes SolidWorks Corporation, Waltham, MA, USA). The geometric models were transferred as IGES file to the meshing and finite element analysis package (ANSYS Workbench version 14.0 - ANSYS Inc., Canonsburg, PA, USA).

Mesh density is a parameter that improves the results accuracy and reduces artificial peak stresses by improving the representation of the actual geometry. The mesh density effect was evaluated before extracting results that the final simulations were carried out. The number of nodes and elements in each component are listed in [Table 1]. The generated cobalt chromium pontic contains composite covering layer of 1.5 mm thickness as a separate part for simulating veneering material, while a resin cement layer of 30 μm was also created.
Table 1: Number of nodes and elements in all meshed components

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Isotropic, homogenous, and linearly elastic materials' properties were assumed and fed into the finite element software based on previous studies and manufacturer's information [Table 2]. The first model's pontic was one piece made from only one material (zirconia), while the second one consists of four pieces (cobalt chromium core, veneering porcelain, and two wings' veneering composite). Compressive load of 400N was applied on the buccal cusp, while the base of the two side teeth was set to be fixed in place as loading and boundary conditions, respectively. Linear static analysis was performed on Workstation HP ProLaint ML150, with Intel Xeon 3.2 GHz processors (with 1MB L2 cache), 10GB RAM, using ANSYS version 14.0.
Table 2: Material properties of used in the finite element models

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  Results Top


Von Misses stress distributions in the two models showed safe values within the physiological limits, while the zirconia prosthesis showed lower stress values by about 47% as presented in [Figure 1], [Figure 2], [Figure 3], [Figure 4]. The vertical deformation (negative Z axis direction) is three times higher with porcelain fused to metal prosthesis in comparison to zirconia one [Figure 5], [Figure 6], [Figure 7], [Figure 8]. This may be referred to thinner wings in the second model where the adhesive resin coating on both wings is negligibly stressed.
Figure 1: Model #1 and Pontic Von Mises stress distribution

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Figure 2: Model #1and Pontic Von Mises stress distribution (The prosthesis only is shown)

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Figure 3: Model #2 Prosthesis (coating and core) Von Mises stress distribution

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Figure 4: Model #2 Prosthesis (coating and core) Von Mises stress distribution (The prosthesis only is shown)

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Figure 5: Model #1 and Teeth vertical deformation distribution

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Figure 6: Model #1and Teeth vertical deformation distribution (without the prosthesis)

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Figure 7: Model #2 and Teeth vertical deformation distribution

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Figure 8: Model #2 and Teeth vertical deformation distribution (without the prosthesis)

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  Discussion Top


Is it a safe design regarding the stresses generated on the IRFDP to use metal alloy substructure veneered with porcelain and with extended rests or wings seated in the neighboring teeth cavities and covered with resin filling?

Clinical reports about IRFDP indicated good survival rates up to 7 years with debonding as the main problem occurring.[10],[11]

In general, all finite element models are just simplification of reality. It should be done with care and should be appropriate and related to the tested problem. The boundary conditions should be modeled in an appropriate way. However, this is very difficult in case of boundary conditions of teeth as the periodontium is a very complex structure of fibers attached to the root.[12] The root is embedded in the jaw bone and surrounded by the gingiva so its geometry cannot be captured by optical scanning. Hence, published approaches to model the entire root and the surrounding materials cannot be applied.[13] Artificially modeled roots can be used but they would lead to high computational times that are not suitable with the planned coupling with CAD/CAM system. The scanned data contained parts of the gingiva. That's why the outer contour is slightly different from the clinical crown. Teeth are considered as rigid bodies,[14] so it is assumed that the gingiva has a very little effect on the stresses falling on the prosthesis. With loading, the most subjected part to deformation is the periodontal ligament. And it has viscoelastic and shock absorbing behavior, and it was not considered in this study. To simplify the effect of periodontal ligament's resiliency on the stresses over the FDP, its shock-absorbing effect is simply neglected. Only the elastic properties of the materials were considered. In this finite element study, the results are affected by the load angle and boundary conditions. Furthermore, the point of loading has a great effect on the results as confirmed by Zhang et al.[16] The results of the finite element analysis with the load magnitude 400 N are shown in [Figure 8] and [Figure 9] for model #1 and in [Figure 10] and [Figure 11] for model #2. The calculations showed higher stresses in the IRFDP made from chrome cobalt alloy veneered with porcelain compared to that formed at the all zirconia prostheses. However, both within the safe limits. This corresponds to the findings of a case report by Samran et al. in 2015[17] that suggested this new design and found it was successful within the limitations of this report and suggested studying the stresses on this design using the finite element analysis method. Zirconia was tested by Kermanshah et al. in 2012[18] in a finite element analysis study by using it as a bar to reinforce the IPS Empress II bridge at the connector area which is the main site of fracture and the most critical area in all ceramic FDPs. It decreased the Von Mises stresses distribution. This may explain why in our study, zirconia had better results than the metal substructure model. Zirconia had better esthetics, biocompatibility, and resistance to masticatory forces. It has high fracture toughness, flexural strength, and desirable properties that resist prosthesis chipping and fracture.[19],[20] With lower bending and less stresses in the analysis. Comparisons are made in terms of Von Mises criteria and principle stresses. Von Mises is a formula for combining the 3 principle uniaxial stresses (x, y, and z planes). It provides a measure of the overall stress. It takes into consideration shear energy as a result of an applied load. When an elastic body is subjected to 3D loads, we can find that although none of the uniaxial principle stresses is more than the yield strength of the material but yielding can occur as a result of the 3 stresses together. The equivalent stress which is called the “von Mises stress” if it is more than the yield strength of the material, ductile failure will occur. The maximum and minimum stresses are determined to show areas of compression and tension. The zirconia model showed a more favorable stress distribution pattern mainly due to the fact that the material is in one bulk being able to absorb and distribute stresses; not multiple layers as the second model. Von Mises stresses were 47.3% higher for the metal whereas the stresses predict that the tensile stresses in metal connectors were 56% greater. This is explained by the fact that it has thin thickness due to the need for subsequent veneering. Finite element analysis studies were needed according to Bömicke et al.[21] to assess the performance of this type of prostheses to solve the problem of fractured abutments in the in vitro studies. In the present simulation, a load of only 400 N was chosen. If greater loads were done, the maximum value would be expected to scale directly with the increasing load as the modeling conducted here was an elastic analysis. Hence, with maximum biting force in the molar region mainly stated between 600 and 750 N,[22] we merely need to scale the stresses in this finite element model by factor of 1.75 to find the equivalent Von Mises 1.08 Mpa for the zirconia model versus 2.05 for the chrome cobalt model. Both materials alternatives can be used.
Figure 9: 4Pontic Von Misses stress distribution

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Figure 10: Model #2 Prosthesis (coating and core) Von Mises stress distribution

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Figure 11: Model #2 Prosthesis (coating and core) Von Mises stress distribution, connector area

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Clinical use of zirconia is now preferred due to its better esthetic and biocompatible results. The results of the finite element models detailed above indicated safe levels of stress for both models but more favorable for the zirconia model. This is also confirmed by the results of a systematic review by Chen et al.[23] who found that there are studies showing 21 cases of fractured glass ceramic frameworks compared to only 1 fractured case with metal ceramic framework and 0 cases with the zirconia frameworks. Raghavan et al.[24] also supported the preference of zirconia regarding fracture resistance in their finite element analysis. Zirconia is preferred for this design of IRFDP. Using veneered zirconia was not recommended for this type of prostheses according to Rathmann et al.[25] who found in a prospective clinical study that there is large incidence of complications with it. In a systematic review by Castillo-Oyagüe et al.,[26] it was found that the zirconia inlay retained prostheses in most of the studied clinical researches showed suitable results compared to the traditional metal prostheses. The studied researches by the systematic review by El-Khodary et al.[27] stated that monolithic zirconia can replace the veneered zirconia in this type of prostheses. Hence, zirconia frameworks without veneering ceramic were recommended for more studies as done here in our study. Furthermore, more future clinical research is needed to support the results.

Other factors such as cost, maintenance periods, and life time will affect the dentist decision to use the studied design or conventional bridge.


  Conclusions Top


The two models studied showed two equivalent alternatives for the prosthesis. Both materials are suitable for inlay-retained fixed partial denture manufacturing. Zirconia is preferred and can withstand more stresses than cobalt chromium for this design. Other factors such as lifetime and color matching may govern the dentist selection of material to be used.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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27.
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
 
 
    Tables

  [Table 1], [Table 2]


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[Pubmed] | [DOI]



 

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