Stress analysis during post loading on implant system using finite element analysis method.
In last decade, the dental rehabilitation using dental implant has become popular treatment for aesthetic or functionality purposes. By increasing the technology, rate of success in dental implant was increased significantly. By not only influenced by improvement on method of implant placement, the success of implant which is represented by long implant stability in future also affected by internal process that is integration of bone-implant.
After implant inserted, bone will undergo remodeling and osseointegration process. This process is a complex process that involves chemical and mechanical aspect. One of the mechanical aspects that affect the remodelling/osseointegration process is the existence of stress in the area surrounding implant system. However, the distribution of stress in implant system is difficult to be measured clinically in any stage of dental implant treatment. Estimation of stress distribution through numerical modelling such as using Finite Element Analysis (FEA) method would be a good alternative to understand the process. Better understanding on the characteristic of stress distribution in the jawbone implants can help us to give a better early treatment that can support the best osseointegration process.
In the implant system, there is some significant different with natural teeth, there is no periodontal ligament support between body implant and bone as well as in the natural teeth. Hence, if body implant undergoes the stress, this stress will be transferred directly into bone. Consequently, the various loading on body implant will have direct impact on stress distribution especially at adjacent contact area of bone and nearest teeth.
Besides impacts on osseointegration or bone remodelling, the stress distribution also affect the nerves causing paraesthesia and disaesthesia  and implant stability and failure. An optimum stress distribution is required in order to maintain a strong and healthy jawbone. Furthermore, the post loading is critical stages of dental implant treatment because in this stage the bone is still in a state of remodeling and osseointegration. Hence, the stress distribution post loading is important to be known; therefore the extreme loading on implant body can be avoided to prevent unwanted risk.
Finite element analysis (FEA) method is a numerical method that commonly used to solve the mechanical problem by discretize geometry or structure of object into the so called 'finite elements' connected through nodes. This method also has been applied on implant dentistry field and reported by some authors. Chang et al.  used the finite element analysis to evaluate the stress distribution on commercial implant dental system in three dimensional. Meanwhile, Guan et al  used FEA to investigate the influence of bone and dental implant parameters on stress distribution in the mandible.
The objective of this research is to investigate the stress distribution on implant system during post loading process by using FEA
Materials and Methods
A three-dimensional(3-D) finite element model of jaw bone has been created from Cone Beam Computerized Tomography (CBCT) data of the dental implant patient. For this case, the implant treatment of the molar on the mandible was selected because of its simple shape and its clearance on the CBCT data. The model consists of some elements that are: one body implant, two neighbors of tooth and mandible bone. All of those elements are segmented from whole jaw bone from CBCT data using MIMICS software. The illustration of segmentation process is shown in the Figure 1.
In the first stage of segmentation, the jaw bone was separated from soft tissue based on threshold of HU on CBCT data. The Region of Interest (ROI) is cropped to get only interest elements. The bulk of that ROI cropped are segmented into each part (tooth, implant and bone) using Boolean operation. In the final stage, wrapping and soothing are performed to get the smooth surface and closed volume of the object.
After all components were segmented completely, all of those components are transferred into 3-Matic software to build the meshing for FEA preparation. Number of mesh was controlled by the quality of element of the mesh including the angle and size of the mesh element. Meshing is refined in the ANSYS software to get more detail and compatible with the software. The meshing of the model which is resulted from this stage is shown in the Figure 2.
The mechanical properties values which are required for material assignment on FEA study including density, n, Poisson's ratio, i and Young's modulus, E of the tooth, implant and bone are adopted from published data. Those of that value are shown in the Table 1.
In the FEA study, there are assumptions were taken such as the contact area between body of tooth and implant with bone are assumed bounded, whole area in the bottom of mandible were fixed to stabilize the model during stress analysis. Every component are assigned as homogenous material with contact inter body are assigned as rigid body. Hence, there is no movement in the interfacing of body contact during simulation of loading.
There are three different computer simulations (push out, pull in and removal torque) were performed. The stress distribution due to each different loading simulation on implant has been analyzed using ANSYS software.
Results and Discussion
The primary difficulty in simulating the mechanical behavior of dental implants is how to get the correct model for human bone tissue and its realistic elastic properties distribution in the model to response the applied mechanical forces. The complexity in the jaw bone system, either the shape or the properties distribution makes some assumption or simplification should be considered during simulation. In our study, the CBCT data of implant patient was used to get the real jaw bone model.
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The geometry of model that was generated from CBCT has some limitation on the shape precision. CBCT has high resolution on determining the size of object, unfortunately the CBCT data are based on absorption of x-ray energy where the intensity in some particular places or contrast between different part/element is not clear enough. Hence, the boundary is quite difficult to be determined. Another disadvantage on CBCT data is that there are a lot of artifacts around crown, implant, and enamel . The artifact itself was produced from scattering of x-ray energy due to hard object. This artifacts make the image more difficult to be segmented; some reduction technique should be applied manually and carefully. For material assignment, it is difficult to get the consistent relations between HU from CBCT data with elastic properties such as density. Hence, the simplification of the material properties distribution as homogenous material was taken.
Loading on implant system represented the normal process during mastication. Loading acting on body dental implants can be produced in undesirable stress within the surrounding jawbone. In this case, implant will be rejected by bone which is leading to failure of the implant [5, 6]. In order to understand the mechanism of stress distribution due to masticatory process, three different loading simulations were investigated. A significant amount of investigations have assumed the direction of the load applied to the implant is in three directions, horizontal, vertical and removal torque.
Vertical loading (Pull out) simulation
In this simulation, the normal force is stand-in vertically on dental implant and centered at the top of the dental implant. The force was used is 200 N which is a realistic value for molar during gentle biting masticatory. The simulation setup is shown in the Figure 3.
The result shown that at this loading situation, the highest stress was concentrated on bottom of body implant and bone surrounding the contact. Stress distribution also propagates strongly into cortical bone of lingua in the bone, as a whole, was concentrated in the cortical bone, around the implant. Because of a great difference between the stress values in the cortical and cancellous bone, the stress distributions in these bone regions are shown separately for better visualization. For comparison, the same scale was used in all models.
The pull-out test is performed to know the healing capability within the bone-implant interface. The maximum load capability (or failure load) is defined as the maximum force on the force-displacement plot, and the interfacial stiffness is visualized as the slope of a tangent approximately at the linear region of the force-displacement curve prior to breakpoint .
Horizontal Loading (Push out/Periotest) simulation
Horizontal loading are performed to understand the distal-mesial orthodontic movement. The significant deformation of the bone-implant unit is not measurable for most clinical situations. To overcome this limitation, the simulation study is an alternative method to understand this mechanism. Actually in clinical work, the damping characteristics during loading can be measured using Periotest measurement. The Periotest value can be correlated with the periodontal ligament characteristic. In this simulation, we performed the horizontal loading with 200 N of force that was directed into implant body horizontally. The setup and result for this simulation is shown in the Figure 4.
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The result of this simulation showed that impact of horizontal loading is more concentrated in the body of implant and some part of this stress was propagated to the lingual side. The result also showed that the propagation of stress is parallel with the direction of the loading force. There is no strong effect on both side of neighbor tooth.
Removal torque simulation
The removal torque simulation is performed to know the effect of torsional force necessary for unscrewing the fixture on bone surrounding implant. In practical, the removal torque unit is Newton-centimeters(N-cm).The value of torque will depend on the geometry and a topography of the object. In our simulation, we used 200 N-cm torque. The torque was placed at the top of the implant. The result of this response is shown in Figure 5. From this figure we can see that the removal torque give a strong effect on neighbor tooth. The maximum effect is located in area which is parallel with implant length in surrounding implant. In the horizontal slicing, it is clear that the stress was distributed into buccal and lingual side.
From all loading process simulation, the Von Mises stress was digitized in the vertical section of body implant, the results are shown in the figure 6. This figure showed that the stress distribution was strongly accumulated in the body implant below 6 mm from Cemento Enamel Junction (CEJ) level. Removal torque simulation gave very strong effect on stress distribution, in area lower than 6 mm from CEJ level, the removal torque produce hundreds MPa of Von Mises Stress. Meanwhile the pull out simulation gives the Von Mises stress is about 4-10 MPa, and push out simulation give the lowest effect on Von Mises stress, it was below 1 MPa.
The stress in the body implant will be transferred directly into bone because there is no ligament in between implant-bone as well as in natural teeth. Even though the number of stress transferred from body implant into jawbone is still affected by the quality and quantity of bone at this site and the characteristic of interfacing between implant-bone. Stress shielding around the implant may occur as mechanism of bone that protects the jawbone from stresses that it might encounter by increasing the bone density as stated as Wolff's law . Monitoring on density after implant placement to detect the stress shielding around implant might be possible to be conducted through better imaging technique.
Another possibility of high compressive stress which is exceeding 50 MPa is the resorption of cortical bone around the implant body . If the resorption is occurring for long time it will effect on the stability of implant and possibility of failure will increase. The critical stress can be estimated from critical stress curve for overload or underload, in this case the information about Von Mises stress is needed to predict that condition together with the information about density of the implant site . Stress may increase in the actual oral environment because of different bite force and skeletal pattern of the site implant. The strain and stress in the more posterior mandibular areas may be higher than anterior because of different cortical thickness and density of bone.
Higher stress distribution may occur in the area where the cortical thickness is around 2 mm compare with the area with cortical thickness around 1 mm. The thicker cancellous bone will absorb a higher proportion of the load and reducing the load to the bone . Different stress distribution will effect on osseointegration process as reported by Yun et. al. . In their study on immediately loaded implant on animal (rabbit), a stress of about 2.0 MPa had a positive effect on new bone formation, osseointegration and bone-implant interface strength. Bone loss was observed in some specimens with stress exceeding 4.0 MPa. However the limit of the stress those give a positive effect on osseointegration in human is still needed to be investigated.
The limitation of the stress analysis study based on finite element method for simulation of loading is only investigate the stress distribution in the short time period, hence the effect of continuous or repetition of the loading as actual masticatory process for long term effect is not associated. Further studies to incorporate the effects of long term loading for various possibly scenarios of loading process is still need to be conducted.
As a conclusion, the stress distribution as effect of loading process on implant system can be analyzed using 3-D Finite Element Analysis successfully. Based on the results, the following conclusions are obtained to describe the effects of different loading simulation on stress distributions in jaw bone.
(1) During vertical loading (pull out) simulation, the stress distribution was concentrated in the bottom of the implant body and it was propagated into left and right side area near neighbor tooth.
(2) During horizontal loading, the stress distribution was concentrated in the whole body of implant and propagated in small part into surrounding implant. The direction of propagation is parallel with loading force direction.
(3) Neighbor tooth was strongly inuenced by removal torque. The stress distribution due to this loading was concentrated highly in neighbor tooth. Not only propagated along the bone of jaw but also the stress was propagated into tooth body.
(4) During the simulation, the highest Von Mises stress in the body implant was produced by removal torque simulation follows by pull out simulation and the lowest Von Mises stress is occur during push out simulation.
In general, the strongest impacts of the loading process into surrounding implant area and neighbor tooth occurred during removal torque simulation.
This work was supported in part by Universiti Sains Malaysia short term Grant (grant number: 304/PPSG/61313004) and PRGS grant (grant number: 1001/PPSG/8146004).
[1.] H.-S. Chang, Y.-C. Chen, Y.-D. Hsieh, and M.-L. Hsu, Stress Distribution of Two Commercial Dental Implant Systems: A Three-Dimensional Finite Element Analysis, J. Dent. Sci., vol. 8, no. 3, pp. 261-271, Sep. 2013.
[2.] D. S. Levitt, Apicoectomy of an Endosseous Implant to Relieve Paresthesia: A Case Report, Implant Dent., vol. 12, no. 3, pp. 202-205, Sep. 2003.
[3.] H. Guan, R. Van Staden, and Y. Loo, Influence of Bone and Dental Implant Parameters on Stress Distribution in the Mandible, Int. J. Oral Maxillofac. Implants, vol. 24, no. 5, pp. 866-876, 2009.
[4.] F.S Yuan, Y.C. Sun, X.Y. Xie, Y. Wang, Quantitative assessment on artifacts of dental restorative materials in cone beam computed tomography, PubMed Beijing Da Xue Xue Bao. 2013 Dec 18;45(6):989-92.
[5.] K.S. Preeti, K.S. Kumar, J. Jins, P. Geetha, and P. Ruchi, Force Transfer and Stress Distribution in an Implant-Supported Overdenture Retained with a Hader Bar Attachment: A Finite Element Analysis, ISRN Dentistry, Vol. 2013 (2013).
[6.] R. C. Van Staden, H. Guan, and Y. C. Loo, Application of the Finite Element Method in Dental Implant Research, Comput. Methods Biomech. Biomed. Engin., vol. 9, no. 4, pp. 25770, Aug. 2006.
[7.] L. Kong, B. Liu, D. Li, Y. Song, A. Zhang, F. Dang, X. Qin, and J. Yang, Comparative Study of 12 Thread Shapes of Dental Implant Designs/: a Three-Dimensional Finite Element Analysis," world Journal Model. Simul., vol. 2, no. 2, pp. 134-140, 2006.
[8.] S. Seth, P. Kalra, Effect of Dental Implant Parameters on Stress Distribution at Bone-Implant Interface, International Journal of Science and Research (IJSR), Volume 2 Issue 6, June 2013.
[9.] H.J. Lee, K.S. Lee, M. Kim, and Y. Chun, Effect of bite force on orthodontic mini-implants in the molar region: Finite element analysis, Korean J Orthod. 2013 Oct; 43(5):218-224.
[10.] Li J, Li H, Shi L, Fok AS, Ucer C and Devlin H, A mathematical model for simulating the bone remodeling process under mechanical stimulus, Dent Mater 2007;23:1073-1078.
[11.] M. Motoyoshi, S. Ueno, K. Okazaki, N. Shimizu, Bone stress for a mini-implant close to the roots of adjacent teeth- 3D finite element analysis. Int J Oral Maxillofac Surg 2009; 38:363-368.
[12.] H.J. Yun, H. J. Xia, Z. Gang, W. Chao and F. Y. Bo, A histological and biomechanical study of bone stress and bone remodeling around immediately loaded implants, Science China, June 2014, Vol.57 No.6: 618-626.
Maya Genisa (1), ZainulAhmad Rajion (1), Solehuddin Shuib (2), Dasmawati Mohamad (1), Abdullah Pohchi (1)
(1) School of Dental Sciences, Universiti Sains Malaysia, Kubang Kerian 16150, Kelantan, Malaysia
(2) Faculty of Mechanical Engineering, Universiti Teknologi MARA 40450 Shah Alam, Selangor, Malaysia
Received 8 September 2014; Accepted 8 December 2014; Available online 2 February 2015
# Coresponding authors E- mail: email@example.com
Tabel 1: Material properties for material assignment during FEA study Material Density Poisson Young's (g/cc) Ratio Modulus (MPa) (GPa) Bone 2.17 0.3 1370 103 x 10 Implant 4.51 0.37 105000 Teeth 2.9 0.33 50000
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|Title Annotation:||Original Article|
|Author:||Genisa, Maya; Rajion, Zainul Ahmad; Shuib, Solehuddin; Mohamad, Dasmawati; Pohchi, Abdullah|
|Publication:||Trends in Biomaterials and Artificial Organs|
|Date:||Jan 1, 2015|
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