|Year : 2020 | Volume
| Issue : 2 | Page : 203-208
|Biomechanical finite element analysis of a single implant threaded in anterior and posterior regions of maxilla bone
Laith A Sabri1, Falah A Hussein2, Abdulsalam R AL-Zahawi3, Besaran Y Abdulrahman4, Kareem N Salloomi5
1 Department of Mechatronics, Al-Khwarizmi College of Engineering, University of Baghdad, Iraq
2 Department of Maxillofacial Surgery, School of Dentistry, University of Sulaimani, Iraq
3 Department of Restorative Dentistry, School of Dentistry, University of Sulaimani, Iraq
4 Darashishi Dental Clinical Center, School of Dentistry, University of Sulaimani, Iraq
5 Automated Manufacturing Engineering Department, Al-Khwarizmi College of Engineering, University of Baghdad, Iraq
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|Date of Submission||19-Jun-2018|
|Date of Acceptance||22-Jun-2019|
|Date of Web Publication||19-May-2020|
| Abstract|| |
Context: The ability of implant dentistry to be a successful alternative for edentulous patients has increased in the last decade. Clinical features such as osseointegration and stability, in addition to the endurance of the integration urged the researchers towards a better understanding of the design parameters that control long term success of the implants. It is therefore necessary to quantify the effect of changing implant design parameters on interface stress distribution within the maxilla bone. Methods and Materials: A 3D-finite element study was conducted to investigate the effect of changing implant shape parameters (implant body design and implant thread depth) on stress distribution while insertion of the implant in two different regions of maxilla bone (anterior (type III bone) and posterior (type IV bone)). A 3D-CAD geometry of implant-maxilla bone was created through importing digitally visualized CT skull images of a human adult, and then converted into a workable solid body through using a collection of engineering software. Tapered and cylindrical implant models with three different implant V-shaped thread depths (0.25 mm, 0.35 mm, 0.45 mm) were threaded into maxilla bone to investigate the design parameters effect on the final stress status. The proposed implant was of commercial dimensions of 10 mm length and 4 mm in diameter. A vertical static load of 250N was directly applied to the center of the suprastructure of the implant for each model. Results: Evaluations were performed for stress distribution patterns and maximum equivalent Von Mises (EQV) stresses for implants in two regions of maxilla bone under 250N vertical static loading. The obtained results throughout this work showed that, for all models, the highest stresses were located at the crestal cortical bone around the implant neck. The von-Mises stress distribution patterns at different models were similar and higher peak von-Mises stresses of cortical bone were seen in tapered implant body compared to cylinder body in all models. Conclusions: Within the restrictions of the current model, the results obtained can be applied clinically to select properly both implant thread depth and body shape design for a foreseeable success of implant therapy.
Keywords: Finite element analysis, implant design, maxilla bone, stresses
|How to cite this article:|
Sabri LA, Hussein FA, AL-Zahawi AR, Abdulrahman BY, Salloomi KN. Biomechanical finite element analysis of a single implant threaded in anterior and posterior regions of maxilla bone. Indian J Dent Res 2020;31:203-8
|How to cite this URL:|
Sabri LA, Hussein FA, AL-Zahawi AR, Abdulrahman BY, Salloomi KN. Biomechanical finite element analysis of a single implant threaded in anterior and posterior regions of maxilla bone. Indian J Dent Res [serial online] 2020 [cited 2020 Aug 6];31:203-8. Available from: http://www.ijdr.in/text.asp?2020/31/2/203/284576
| Introduction|| |
Recent developments in implant design and surgical technique qualified implant dentistry to be more effective and reliable scientific method for the rehabilitation of totally or partially edentulous patients. The developed dental implant represents an artificial tooth root made of biocompatible material mostly titanium which is surgically set into a jawbone to provide a full support to prosthetic tooth crown during replacing of a missing tooth or teeth. In regard to long-term success, some research studies, showed that a high success rate that exceeds 95% retention over a 5-year period might be achieved if the implants are well designed, manufactured and installed. The implemented researches within this field revealed that design parameters such as thread geometry, implant shape, diameter, length, angulation,, surface treatment, bone quality, and surgical technique, have a great influence on the quality of the osseointegration between implant and bone. To investigate the effect of the process design parameters, multiple implant designs were examined in dental practice, and many important features were considered like thread or hollow cylindrical shape and stepped implant type.,, Moreover, the total contact area between the implant and bone was noticed as one of the design parameters that played a major role in the osseointegration strength of implant-bone interface and this area is affected by the implant surface treatment and implant thread pitch, depth, and width.,, In addition to the aforementioned factors, it was found that the quality of the bone around implants also has a great influence on the outcome of the implant treatment, in which the increase in bone density enhances the mechanical properties of the interface region.
To assess the effect of design parameters on the quality of osseointegration, different experimental and theoretical analysis methods were used to achieve this goal. Among these methods is the Finite Element Analysis method (FEA), which offers a suitable degree of reliability and accuracy without the risk and expense of implantation. The method had been used as a tool to predict for example stresses in the peri-implant region and in the components of implant-supported restorations.,,, Mathematically, the method depends on using numerical techniques in solving partial differential equations that govern the simulation problem. With FEM, the structures whether it is simple or complicated are to be converted into meshes using a computer software. The resulting models consist of elements, nodes, and pre-defined boundary conditions. During the simulation, the loads are to be applied on specific nodes or elements specified by the user and then the displacement and stresses will be evaluated as a result for running the simulation analysis. The FEA was rapidly applied in many aspects of implant dentistry such as the shape and design of restorations, crowns, dental implants, dowels, retention pins, removable dentures, fixed restoration and many other applications.
Clinically, both maxilla and mandible show different physical characteristics in terms of structure and density during insertion of the implant. The maxilla is spongier compared to the mandible or the mandible is more compact than the maxilla. In literature, researchers investigated the influence of different shape implant parameters on stress distribution in mandible bone much more than maxilla bone and showed that the parameters such as implant diameter, insertion depth, and loading angle were a significant factor influencing the stress created in the bone.,,,, Lately, and due to bone quality, maxilla received more attention from investigators to evaluate stress distribution due to the insertion of the implant by using FEM as an analysis method.,,, The researches showed that if an implant is placed in poor-quality bone with the thin cortex and low density trabeculae (Type D4 bone) has a greater chance of failure compared with the other types of bones. This low-density bone is often found in the posterior maxilla and several studies report higher implant failure rates in this region.
There is a shortage in the literature that deals with the effect of design parameters on stress prediction around implant-maxilla bone using real bone geometry. Therefore, the objective of the current work is to evaluate the influence of implant thread depth on stress distribution in alveolar bone types (D3 (type III) and D4 type (IV)) by FEA and compare the influence of implant shaft designs (taper and cylinder) on stress distribution in both types of alveolar bone.
| Materials and Methods|| |
To achieve the goal of this study, a computerized tomography Scanner, Autodesk Meshmixer, SolidWorks, and ANSYS Workbench software were used as tools to create the CAD model and run the FEA analysis for the implant-Maxilla bone. The key behind achieving a successful modulization throughout this work was the use of computed tomography (CT), which offers an advantage for realistic modeling in not only the development of anatomic structures but also in the inclusion of material properties according to different bone density values. In this study, a real jaw bone picture for a 65-year-old completely edentulous patient was first created by using computed tomography skull images. A total of 209 CT skull slices with a pixel size of 0.488 mm and slice increment of 1.0 mm were generated and used in a form of (DICOM) file data. Features like segmentation and threshold were used to separate maxilla bone from the other parts of the skull then the cancellous bone from the cortical bone of the separated maxillae. Different hard and soft tissues of the skull had been identified by using the feature of image density thresholding. In this work we tried to model maxilla bone in a very realistic manner, however this increased the level of geometric detail which resulted in increased working and computing time and therefore only half of the maxilla bone was considered in the modulization process.
The process of separation of cortical bone from the spongy bone in upper jaw bones was little onerous, besides the process needs more attention and prolongs working time because the bones in the area are finer and there are bony projections that were not easy to separate the spongy bone from cortical. The completed 3D model file of the maxilla was exported as series of stereolithography (STL) file for use by other software and/or rapid prototype manufacturing technologies.
In order to convert the STL 3D models of the cortical and spongy bone of maxillae to solid 3D model (SAT file) and to export the 3D model in a format acceptable by ANSYS Workbench software, the STL files of the 3D maxillary model were imported into a designing software (SolidWorks) – which is a 3D mechanical CAD (computer-aided design) program that runs on Microsoft Windows, and exported as a solid 3D model in (SAT) file format.
Insertion of non-living structures (Implants)
The non-living structures (implants) can substantially influence the calculated stress and strain values, similar to the living structures. These materials can be digitally modeled in FEA studies using isotropic property material. In an isotropic material, the relevant material properties are the same in all directions, resulting in only two independent material constants – such as (Young's modulus) and (Poisson's ratio). In this simulation study, six dental implants were designed by SolidWorks and consequently inserted into the maxilla. Implants were of two design groups (cylinder and taper), with a standard length of 10 mm, diameter 4 mm, pitch 0.8 mm, and standard V-shape threads with different thread depth (0.25 mm, 0.35 mm, 0.45 mm), as shown in [Figure 1].
|Figure 1: The illustration shows group A (cylinder shaft) implants; group B (taper shaft) implants|
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For each implant, a conical shape supra structure of (5 mm) was connected. It is also well to mention that during geometry creation a state of optimal osseointegration was achieved (i.e 100% of the interface was satisfied) – which means cortical and trabecular bone was assumed to be perfectly bonded to the implant.
Application of finite element method
Ansys workbench was used to compute the stress analysis by creating an assembly of all model parts (master model), defining the material properties, meshing, applying boundary conditions and loading. Before starting the FEA analysis, the master model parts: maxillary bone models (cortical and spongy bone), and implants were first assembled. The SAT files of each 3D model that have been prepared in advance were imported into ANSYS workbench. In the “Assembly” model, Boolean operations were performed to merge different model parts together with the retaining boundaries and subtracting option was used to eliminate the corresponding volumes and produce a perfect contact area between all the assembly parts (bone and implants).
The properties of all materials used were assumed isotropic homogeneous and linear elastic behavior was considered. Values of Young's modulus and the Poisson's ratio for each material cortical bone, cancellous bone, titanium are summarized in [Table 1].
The problem of creating a mesh for such complex model as one piece was solved by sectioning of the model into a series of parallel slices. These slices were meshed separately and then combined automatically.
Now, because the maxilla is fixed to the base of the skull, the model boundary conditions were applied as follows: the nodes along the external surface areas of the cortical bone of the oral and nasopharyngeal cavities from the section plane were fixed in all directions [Figure 2].
Regarding load application, the amounts of vertical forces applied were different in previous studies, ranged from (100N-300N).,, In this study, an axial load of 250N was directly applied to the center of the supra structure of the implant for each model. After finishing all the steps of the pre-processing stage, the models were submitted to the ANSYS workbench solver to obtain a solution. The program calculates the displacement and then the stresses at each node present in the model. Frequently von Mises stress (equivalent tensile stress), minimum principal, and maximum principal is used to evaluate the effect of loading forces on the peri-implant region or prosthesis structure.
| Results|| |
The transmitted forces from the supra structure to the surrounding bone via implants were studied. The values of Von Mises stress that resulted from occlusal loading conditions in vertical directions of force applied to all FEA models were considered to determine the stresses transmitted to the underlying supporting bone. Generated equivalent Von Mises stresses were calculated numerically and plotted graphically. Results were displayed as colored stress contour plots to identify regions of different stress concentrations.
As we outlined earlier, three different thread depths (0.25, 0.35, and 0.45) with two different shaft designs (Tapered and Cylindrical) were considered as parameters that might affect the status of interface stresses between implant and bone. The implant was of commercial dimensions of 10 mm length and 4 mm diameter. A thread pitch of 0.8 mm was used as the optimal thread pitch for achieving primary stability and optimum stress production on implants with V-shape threads. In fact, the proposed dimensions of the threaded implant used in this study were chosen in such a way to maximize initial contact, increase the surface area, and to satisfy smooth dissipation of loads at the bone-implant interface. [Figure 3] shows the values of maximum equivalent stresses values of supporting bone in Type III models in the maxillary anterior area (D3 Bone) resulting from the application of (250 N) vertical load on the implant. The highest EQV stress was seen in the T3 implant (tapered implant with thread depth 0.45 mm), followed by the T1 implant. The pattern of stress distribution is similar for all models and maximum equivalent Mises stress areas were located in the distal aspect of the implant at the cortical bone in all Type III models [Figure 4] a-f. As shown the highest stresses were located at the crestal cortical bone around the implant neck. The reason behind this fact is the irregular geometry we are dealing with which indicate the necessity for doing an optimization for the structural shape of both neck area of the implant and the crestal cortical zone of the bone to increase the area of osseointegration and consequently decrease stress in the surrounding bone tissues.
|Figure 3: The chart shows Maximum EQV stresses in Type III models (anterior maxilla bone) with different thread depth implants|
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|Figure 4: Distribution of EQV stress pattern under 250N vertical load in Type III model for a-(C1) implant, b- (C2) implant, c- (C3) implant, d- (T1) implant, e- (T2) implant, and f- (T3) implant|
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The values of maximum EQV stresses on supporting bone in Type IV models (posterior maxilla D4 bone) are shown in [Figure 5]. The highest value of maximum EQV stress was found in the T3 implant (tapered shaft with 0.45 mm thread depth) followed by (T1) implant. The maximum stress bearing areas were located in the distal side of implants at the cortical bone in all Type II models as shown in [Figure 6]a-f.
|Figure 5: The chart shows Maximum EQV stresses in Type IV models (posterior maxilla bone) with different thread depth implants|
Click here to view
|Figure 6: Distribution of EQV stress pattern under 250N vertical load in Type IV model for a-(C1) implant, b- (C2) implant, c- (C3) implant, d- (T1) implant, e- (T2) implant, and f- (T3) implant|
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The success rate of cylindrical implants in both D3 and D4 bones was higher than that of the tapered implants. This was assigned to the fact that cylindrical implants generate less lateral force in spongy D3 and D4 bones than the tapered implants.
| Discussion|| |
This study aimed to evaluate the influence of implant design parameters such as thread depth, implant body shapes (cylindrical and taper), and standard V-shape threads on stress distribution in two different regions (anterior and posterior) of maxilla bone. The FEM is used as a numerical method to accomplish the structural analysis to predict stresses resulting due to applying a vertical load. The structure is discretized into the so called “finite elements” connected through nodes. The accomplishment of simulation process with a high percentage of success depends mainly on accuracy in modeling the geometry and surface structure of the implant, the material characteristics of the implant and jawbone, the loading and boundary conditions in addition to the biomechanical implant jawbone interface. The computed tomography and magnetic resonance imaging were used to construct models for FEA. The model construction method used in the present study was non-destructive and allowed to construct finite element models with accurate size and structure using actual patient CT data along with the combination of different CAD software.
In this study, we used finite element modeling of dental implants via spirally threaded implants. The models and results were closer to the real conditions and much more accurate in predicting the stress patterns. The current FEA results show that all models had a peak stress localized in the crestal region of the cortical bone, which is co-approved by previous studies., The peak stress on the cortical bone was highest in the threaded implants. However, it should not be concluded that threads lead in a greater amount of implant failure. In reality, threads have three main functions. These are to maximize initial contact, enhance the functional surface area and facilitate dissipations of stress at the interfacial area. The thread design used in this work shows a wavy interfacial stress pattern along the surface of the implant in the trabecular bone while the cylindrical non-thread model showed one large high stress area. So, although the threaded forms had a higher peak of stress, the benefits of threads cannot be neglected. In evaluating the best form of thread for dental implants, three factors should be taken into account – thread shape, thread depth, and thread pitch.
Considering the taper body of the implant, it was shown that the tapered implant body increased peak tensile stress of cortical bone compared to the straight (cylinder) body with standard thread depth and bone quality and this agrees with, who reported that the tapered thread design of brand mark implant exhibited higher stress levels in bone than the cylinder implants which seemed to distribute stress more evenly. In fact, the tapered body form has been a place for the challenge in the study.
| Conclusions|| |
Based on the FEA results obtained within this work, the following conclusions can be drawn regarding the effect of both thread depth and implant shaft design on implant-maxilla bone interface stress distribution:
- Taper body form had a higher peak of Von Mises stress than that cylinder body implants in all types of bones. It is also worthy to mention that the Stress distribution pattern at supporting bone structure did not change with changing implant designs and thread depth in all models. The maximum Von Mises stresses of cortical bone were located at the crestal cortical bone around the implants. In all models, the maximum Von Mises stresses were located in the distal side of implants.
- Cortical bone and bone structure adjacent to first thread bear more von Mises stresses than spongious bone.
- In relation to implant thread depth, 0.35 mm thread had minimum Von Mises stress as compared with 0.25 mm and 0.45 mm thread depth almost in all types of bones, while 0.45 mm thread depth showed the highest peak of maximum EQV stress for Type III and Type IV models.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Dr. Laith A Sabri
Department of Mechatronics, Al-Khwarizmi College of Engineering, University of Baghdad
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
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