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| Year : 2009 | Volume
: 20
| Issue : 1 | Page : 31-36 |
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| A comparison of stress distribution and flexion among various designs of bar attachments for implant overdentures: A three dimensional finite element analysis |
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Vijay Prakash1, Mariette D'Souza1, Raviraj Adhikari2
1 Department of Prosthodontics, Manipal College of Dental Sciences, Manipal, Karnataka - 576 119, India
2 Department of Mechanical Engineering, Manipal Institute of Technology, MAHE University, Manipal, Karnataka - 576 119, India
Click here for correspondence address and email
| Date of Submission | 04-Mar-2008 |
| Date of Decision | 17-Jun-2008 |
| Date of Acceptance | 14-Aug-2008 |
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 Abstract | | |
Context: Bar overdentures are popular choices among clinicians worldwide but configurations that provide an optimal biomechanical distribution of stress are still debatable.
Aims: To compare the stresses and elastic flexion between implant supported bar overdentures in various configurations using finite element analysis.
Settings and Design: A CAT scan of a human mandible was used to generate an anatomically accurate mechanical model.
Materials and Methods: Three models with bars and clips in three different configurations were constructed. Model 1 had a single bar connecting two implants, Model 2 had three bars connecting all the four implants, and Model 3 had two bars connecting the medial and distal implants on the sides only. The models were loaded under static conditions with 100N load distributed at the approximate position of the clip. The mandibular boundary conditions were modeled considering the real geometry of its muscle supporting system. Maximum von Mises stress at the level of the bar and at the bone implant interface were compared in all three models. The flexion of mandible and the bar was also compared qualitatively.
Statistical Analysis Used: The analyses were accomplished using the ANSYS software program and were processed by a personal computer. Stress on these models was analyzed after loading conditions.
Results: Qualitative comparisons showed that stress at the level of the bar and at the bone implant interface were in the following order: Model 1> Model 3> Model 2. The flexion of the mandible and the bar were in the following order: Model 2 > Model 1 > Model 3.
Conclusions: Four implant bar systems connected by bars on the sides only is a better choice than two implant bar systems and four implant bar systems with bars connecting all four implants. Keywords: Bar attachment, finite element analysis, implant overdenture, stress distribution
How to cite this article:
Prakash V, D'Souza M, Adhikari R. A comparison of stress distribution and flexion among various designs of bar attachments for implant overdentures: A three dimensional finite element analysis. Indian J Dent Res 2009;20:31-6 |
How to cite this URL:
Prakash V, D'Souza M, Adhikari R. A comparison of stress distribution and flexion among various designs of bar attachments for implant overdentures: A three dimensional finite element analysis. Indian J Dent Res [serial online] 2009 [cited 2023 Mar 27];20:31-6. Available from: https://www.ijdr.in/text.asp?2009/20/1/31/49058 |
Currently, implant overdentures have become one of the most preferred options for treating completely edentulous patients because of minimal invasiveness, a lower number of implants used, and its relative simplicity. [1],[2] Implant overdentures have various attachment systems that include bar clips, balls, O-rings, and magnets. Among them, bar overdentures is a popular choice because they contribute to load sharing. The biomechanical factors related to bar clip attachment systems include the number of implants, bar length, and material properties. [3] Mostly, implant-supported bar overdentures comprise of multiple implants splinted by the prosthesis framework where the stress distribution is much more complex than with a single tooth implant. Adding to the complexity is the flexion of the jaw bones, particularly in the mandible, which under functional loading can cause stress in the bone around the implants and ultimately can lead to bone resorption. [4],[5],[6],[7],[8],[9]
Over a period of three decades, finite element analysis (FEA) has been increasingly used [10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23] to investigate the stress distribution obtained when implants are left solitary, used with the ball attachment, or connected by a bar for clip retention in various configurations and designs. [17],[21],[23] This study was undertaken to analyze stress distribution around the implants supporting overdentures using finite element analysis. The aim of this study is to compare stresses and elastic flexion between bar-supported overdentures with two and four implants having attachments in different configurations.
| Materials and Methods | |  |
A 60-year-old female patient with four implants (3.8 mm diameter, 13 mm length) (Frialit 2, Friadent GmbH; Dentsply, Mannheim, Germany) placed anterior to the mental foramen in the anterior mandible, was selected to provide the primary data. A computerized tomography (CT) scan [Tomograph Prospeed, GE Medical system, Fairfield] of the lower jaw was taken. The mandibular section profiles were captured at 2.5 mm increments, which resulted in 34 images in the axial section. These images were then transferred to a CAD program (AutoCAD 2002; Autodesk, Mc Innis, Parkway, San Rafael, CA 94903, USA). Each section was carefully traced using the polyline feature of the software to obtain the relevant details of the cortical and cancellous bones from these images [Figure 1]. The traced sections created in AutoCAD were exported to the ANSYS software one by one and stacked in the same order as they were captured earlier [Figure 2]. Each section had several key points that were connected by line segments. The areas were created by joining these lines. These areas were joined to form partial volumes that together defined the final geometry. This procedure was repeated on the other side to complete the solid model of the bone structure. The implants (two or four depending on the relevant cases) were modeled as cylindrical volumes. Each implant was 4 mm in diameter and 13 mm in length. The implants were in the location of the canines and the second premolars. The apical end of the implant was in cancellous bone as traced from the CT scan data. The length of abutment was 2.5 mm with 5 mm of modification height and gingival height. The diameter of the bar was 2 mm. According to the requirement of the study, three 3-D solid models were made: Model 1, which had a single bar connecting two implants [Figure 3]; Model 2, which had three bars connected all four implants [Figure 4]; and Model 3, which had two bars connecting only the medial and distal implants on either side [Figure 5].
The geometric model created in ANSYS was discretized to get a finite element model (FEM) of the mandible with implants and bars. It was done with tetrahedric, isoparametric, and quadratic elements, utilizing 4 triangular faces and 10 nodes. Material properties such as the modulus of elasticity and Poisson's ratio were assigned to the model database. Mechanical properties of bone and prosthetic material were used as given in [Table 1]. Perfect Osseo-integration and linearly elastic homogenous material properties were assumed. Meshing was accomplished using both finer and coarser mesh. The total number of elements consumed in each model with degrees of freedom is given in [Table 2].
A supporting system was set up to simulate the boundary conditions. The model was supported by the muscles of mastication and temporomandibular joints. The value of forces generated by the muscles of mastication (temporalis, masseter, medial pterygoid, and lateral pterygoid) was taken from the study of Inou and co-workers. [13] The muscle positioning and the mandibular body was approximated based on descriptions found in literature. [10],[13],[20] The resultant values of the muscular forces were considered that defined the muscular action areas. The resultant values of muscular forces considered are given in [Table 3].
The forces were applied in the vertical direction along the +z axis for the masseter, temporalis, and medial pterygoid. The direction of application of force for the lateral pterygoid muscles was in the ± x axis (horizontal direction). In this study, the negative direction of the z axis of the global coordinate system was along the sagittal plane towards gravity.
To simulate the clinical situation, points at the region of the attachment of the relevant jaw muscles were loaded as if contraction of these muscles were taking place. The actual vertical bite force of edentulous patients with an implant supported overdenture was taken from the literature as 100N. [10],[11],[24],[25] The models were loaded under the static conditions with a 100N load distributed at the approximate position of the clip. The analyses were accomplished with using the ANSYS® software program (Version 7.1) and was processed by a personal computer [Wipro Infotech, Bangalore, India]. Stress on these models was analyzed after loading conditions.
| Results | | |
The stress analysis executed by ANSYS® (Version 7.1) provided results that enabled the tracing of the global and detailed graphics of the maximum (σ1) and minimum (σ3 ) principle stress and the von Mises stress field. The von Mises stress (stress equivalent) magnitude values were only considered as they summarize the effect of all the six stress components with a unique value. The maximum von Mises stress values were noted on the bar in different configurations and at the interface of the implant and the bone.
The stress analysis revealed that maximum stress, indicated by red (color-coded graph), occurred at the center of the bar in Model 1 with two implants [Figure 6]. Model 2 reportedly experienced the least stress [Figure 7] and the stress value for Model 3 [Figure 8] was slightly higher than that for Model 2. In Model 2 and Model 3, the stress was mainly concentrated at the junction of the bar and abutment. The comparative study of the maximum von Mises stress of three models at the level of the bar is shown in [Figure 9]. The magnitude of the stress in the three models is given in [Table 2] up to two decimal places.
Stress was also evaluated at the interface of the implant and the bone. The comparative evaluation of the von Mises stress field revealed that maximum stress concentration was noted in Model 1 and there was marginal variation between the values of Model 2 and Model 3 [Figure 10]. In this study, flexion of the mandible and the bar was noted respectively on static loading conditions. Model 3 exhibited the least flexion of the mandible and the bar. It was found that in Model 2, there was maximum flexion of the mandible and the bar in comparison with Model 1 and Model 3. This is shown in [Figure 11],[Figure 12],[Figure 13],[Figure 14],[Figure 15],[Figure 16]. [Table 2] represents the magnitude of flexion of the mandible and the bar(s), respectively.
| Discussion | | |
Animal experiments [26] and various clinical studies [2],[9] have shown that inappropriate loading can cause implant failure. Therefore, it is valuable to investigate the stress or strain in the bone and their relation to different parameters of implant and bone. In this study, the entire mandible was modelled three dimensionally by taking a CAT scan of the human mandible to develop an anatomically accurate model in comparison with previous attempts [11],[14],[18] that modelled tissue implant interactions using a two-dimensional analysis.
The muscular forces affect the stress on the implant and elastic flexion of the mandible suggesting that modelling of musculature is important to the accuracy of results obtained. In this study, the muscular forces with the resultant direction of action were considered as important boundary conditions in confirmation with past studies. [10],[11],[15],[20],[22],[23] The resultant force value for each muscle was taken from the study by Inou, et al.[13]
In this study, the comparative evaluation of von Mises stress at the level of the bar and bone-implant interface was done. It was observed that in both the situations maximum stress concentration occurred in Model 1 in comparison with Model 2 and Model 3. There was only a slight difference of stress between Model 2 and Model 3 in both the cases. Since Model 3 has bars connecting the medial and distal implants on the sides only, the tongue can freely move during functional movements. This could be well accepted by patients. Although, further research supported by clinical studies is desired before this could be applied clinically.
The human mandible presents with complex biomechanical behavior under functional loading. Many researchers [4],[7],[8] are of the opinion that stress around the implants was not only caused by the flexure of the bone due to movement of the implant interface relative to the surrounding bone but also by the bending of the mandible due to muscular forces. In this study, the entire mandible was modelled and the muscular forces in the respective resultant directions were applied. A comparative evaluation of the von Mises stress revealed that Model 2 experienced a higher flexion of the mandible and the bar than Model 1 and Model 3. Model 3 showed the least flexion of the mandible and the bar in comparison with the other two models. This only reconfirms that Model 3 could have clinical implications. A possible reason for the difference could be the variable elastic modulus of the bone, implant, and the bar. Also, the length of the bar in the three cases can make a difference. Further investigations on cyclic loading of the dynamic model to simulate the continuous biting functions are desired to give a clear picture.
| Conclusions | | |
Despite the limitations of the methodology, the results of static loading and linear analysis reveal that comparative stress at the bar and bone implant interface is lower in the four implant bar system than in the two implant system. The flexion of the mandible and the bar is least in the four implant system, which is connected by bars only on the sides. Considering less stress and flexion, the four implant system connected by bars only on the sides could have clinical implications.[27]
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Correspondence Address:
Vijay Prakash
Department of Prosthodontics, Manipal College of Dental Sciences, Manipal, Karnataka - 576 119
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.49058
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16]
[Table 1], [Table 2], [Table 3] |
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