Establishment And Analysis On Digital Three-Dimensional Finite Element Model Of Human Foot And Ankle

Background:With the modernization process, there having an unprecedented development in transportation industry and construction business, accompany with the improvement of people's health concept, foot and ankle diseases have been increasing and becoming more complex each year, which makes the orthopedics face new challenges clinically and urgently needs to be explored in depth. Foot is the base of human body's weight-bearing, and direct contact with the ground. In addition to having walking function, it's also able to absorb the movement oscillation, cushion ground reaction force to protect vital organs and organizations of the body from being harmed. Meanwhile, it can make the body's center of gravity stable. The ankle joint is the junction of human body in contact with the ground. Walking, jumping, running, and climbing all need the involvement of the ankle, even riding a bicycle or driving a car it also being indispensable to coordinate the motion. It can be said that there is no movement in daily life whithout the involvement of the ankle, so it is one of the the most susceptible joints to be hurt. The ankle joint has complex mechanical mechanism. A variety of injuries are likely to break the balance of its surrounding structures and cause mechanical instability, which can finally induce traumatic arthritis. Furthermore, in adition to leading to pain, deformity and activity disorders, structural abnormalities of foot and ankle can further affect mechanical functions of lower extremities, pelvis, spine and other body part. Therfore, it has important clinical significance to study on them.Foot and ankle biomechanics research is an important section of human biomechanics study. Many researchers pointed out that the biomechanical factors play an important role in exploring the etiology, pathogenesis, treatment and prevention of the disease of foot and ankle. In the past, studies on foot and ankle mainly used traditional experimental means of biomechanical tests. But the disadvantage of this method is the complexity of experimental means, and when carried out a variety of operating conditions, the experiment is often costly, time-consuming and low efficiency. Moreover, because it can not directly test in humans, it is difficult to accurately reflect the stress distribution after application different load. In order to solve this problem, in recent years, with the improvement of digital technology, more and more researchers use numerical analysis methods, namely, based on the theory of traditional mechanical analysis, numerical analysis methods such as finite element method (finite element method, FEM) and so on, for linear and nonlinear stress and deformation analysis. It can be simulated using a variety of foot and ankle disorders by FEM, making biomechanical studies on bones' complex geometric structures, boundary conditions and material nonuniformity of the foot and ankle issues have possible solutions.FEM has powerful capabilities of modeling and can simulate complex geometric structures, material parameters and different force in the dynamic and static state, which has increasingly been applied to the human body biomechanics. Because of the complexity of bones, joints, ligaments, tendons and other structures of foot and ankle, too much work in finite element modeling as well as the lack of detailed parameters of the various organizations, it is more difficult to simulate than other large joints by FEM. Existing finite element models of foot and ankle, most of them have not constructed anatomic structure of the ankle joint perfectly, mainly for the research related to plantar pressure. However, there is clinically growing recognition to the effects of ankle injuries on human body functions, urgent needs for more in-depth study on them.Based on previous studies, we have taken a long time to explore the effective methods of modeling, then established finite element models of nomal human foot and ankle with fine anatomical structures, through simulating many kinds of ankle joint injuries and internal fixation pattern in the models, to explore their biomechanics effect on foot and ankle.Obiective1. According to spiral CT scan images of a normal male volunteers' right foot and ankle from the plane 20 cm above the ankle down to the planta with ankle joint neutral position, to explore how to establish finite element model of nomal human foot and ankle with fine anatomical structures with Mimics, SolidWorks and ANSYS softwares. Moreover, its validity should be verified, so that it can reflect the mechanical characteristics of the normal foot and ankle.2. According to the three-dimensional finite element model of normal foot and ankle established, apply different loads to simulate the static weight-bearing state of human body in neutral position with one foot standing, and states of internal rotation of ankle, external rotation of ankle, ankle inversion, and ankle eversion, so as to explore the stress, displacement distributions of surrounding tissue, and changes of joint range of motion in main joints during various forms of normal ankle motion. At the same time, the validity of their results to be verified.3. According to the three-dimensional finite element model of normal foot and ankle established, to simulate the establishment of distal tibiofibular syndesmosis injury model, and based on distal tibiofibular joint injury models, to establish two kind of screw fixation models, in which the screw inserted 2.5 cm,5 cm above of the ankle respectively, then explore the stress, displacement distributions of surrounding tissue, and changes of joint range of motion in main joints after application of neutral position with single foot standing, internal rotation and external rotation loads. At the same time, the validity of their results to be verified. Moreover, by comparison with the results of normal model, to explore the effect of distal tibiofibular syndesmosis injury and screw fixation inserted 2.5 cm,5 cm above of the ankle respectively on stress distribution and stability of ankle.4. According to the three-dimensional finite element model of normal foot and ankle established, to simulate the establishment of posterior malleolus fracture involving 1/5,1/4,1/3 and 1/2 of the ankle joint surface injury respectively, then explore the stress, displacement distributions of surrounding tissue, and changes of joint range of motion in main joints after application of neutral position with single foot standing, internal rotation and external rotation loads. At the same time, the validity of their results to be verified. Moreover, by comparison with the results of normal model, to explore the effect of posterior malleolus fracture involving 1/5,1/4, 1/3 and 1/2 of the ankle joint surface injury respectively on stress distribution and stability of ankle.Methods1. Construction and verification of digital three-dimensional finite element model of foot and ankle:Adopting LightSpeed 16-slice spiral CT of the Imaging Center, Nanfang Hospital affiliated to Southern Medical University (scanning parameters:120～140 KV,240～300 mA, pitch of 1.375～1.75, the layer thickness 7.5 mm, matrix 512×512, reconstruction slice thickness 0.625 mm), scaned the volunteer's right foot from distal tibia and fibula 20 cm above the ankle down to the planta, with the right foot remaining neutral position. Then imported the scanned data of DICOM format into Mimics 10.01 software, by threshold segmentation automatically or manually, to reconstruct the three-dimensional structure of a complete foot and ankle composed of 28 bones and surrounding soft tissue. Finally, exported the data with point cloud format and reimported into SolidWorks 2009, using the guide of grid processing and surface generation to form geometric models and reconfigure them, then import the data into two kinds of finite element analysis softwares:(1) Method 1:Imported into Workbench module of finite element analysis software ANSYS 12.0 to establish a complete finite element model of foot and ankle. To construct 0.5 mm of articular cartilage on both sides of contact surface according to the joint space, while to use three-dimensional rod elements which were compressed only to simulate other joints cartilage, and to establish 125 springs to simulate ligaments and crural interosseous membrane, five beam elements to simulate plantar fascia.The material properties were determined with reference to documents. Then refered to Anderson's method, took the tibia and talus only as a simple model of the ankle joint for test. The normal standing status of ankle joint was simulated by application a vertical load of 600 N on the upper section of the lower tibia while the talus constrained. To measure contact pressure and contact area of inferior articular surface of the tibia, and to compare the results with the former. (2) Method 2: Imported the data into Simulation module of Solidworks to establish a simplified finite element model of the ankle. According to the research needs, to establish a 5-bones assembly finite element model containing the tibia, fibula, talus, calcaneus, and navicular, and a 9-bones assembly finite element model also including the cuboid bone and the three cuneiform bones in addition to the 5 bones mentioned above. In the models, to use tension-only springs to simulate ligaments connection. The 5-bones assembly contained 31 springs, while the 9-bones assembly was established 42 springs to simulate connected structures such as ligaments around the ankle and crural interosseous membrane. To Define the material properties of each tissue, to generate contact pair between each joints automatically or manually, and to set the corresponding boundary conditions. In the 5-bone assembly finite element model of ankle, to simulate the state of human body with one foot standing and the states of internal and external rotation of ankle, while in the 9-bones finite element model of ankle, to simulate the states of ankle inversion and ankle eversion. Then, regulated the mesh density to generate mesh, and set the simulation examples attribute for solution.2. Finite element analysis of distal tibiofibular syndesmosis injury and fixation pattern:based on the 5-bone assembly model constructed with aforementioned method 2, to establish distal tibiofibular syndesmosis injury model by suppressing the springs represented the anterior and posterior tibiofibular ligaments(ATIFL, PTIFL), the interosseous ligament(IL), along with 8 cm of the distal interosseous membrane closest to the tibiotalar joint. Then based on the distal tibiofibular syndesmosis injury model, at the plane 2.5 cm,5 cm above the ankle joint generated two￠3.5mm holes respectively, which parallel to ankle joint surface and through the center of fibula and tibia diaphysis. The cylindrical surfaces of the holes were connected with 316 stainless steel pin to simulate the establishment of two kind of screw fixation models. After that, to regulate the mesh density to generate mesh, three loads such as neutral position with single foot standing, internal rotation and external rotation of ankle were simulated for solution. Finally, to compare the results with the normal model.3. Finite element analysis of posterior malleolus fracture:based on the 5-bone assembly model constructed with aforementioned method two, to establish posterior malleolus fracture model involving 1/5,1/4,1/3 and 1/2 of the ankle joint surface injury by resection the corresponding size of fragment in accordance with the standards reported in the literature, respectively. At the same time, involving ligaments were also deleted. After that, to regulate the mesh density to generate mesh, three loads such as neutral position with single foot standing, internal rotation and external rotation of ankle were simulated for solution. Finally, to compare the results with the normal model.Results1. In the complete finite element model of foot and ankle constructed by method 1, the test results of the ankle composed of tibia and talus were taken to contrast with Anderson's study. It showed that the stress on ankle joint articular surface of the tibia mainly distributed at the central and anterolateral aspects in both of them when ankle joint was in neutral position under the vertical loading, with the maximum stress between 2.7 MPa and 4.0 MPa, substantial agreement on distribution area, distribution trend, and numerical value. It was preliminarily verified this model valid. The test results of two simplified three-dimensional finite element models of the ankle established by method 2 showed that they could simulate various load situations of ankle for solution, with moderate number of element, node and suitable amount of computing time. The stress and displacement results in each loading were reasonable and close to the actual situation. Compared with Liacouras and Wayne's model under the same boundary conditions, it showed that the rotation angle of tibia was 3.85°in our model whereas their result was 4.28°after application of ankle external rotation load, comparatively close between the two values. In adition, the contact forces of major joints were also consistent with theirs. All confirmed the validity of our models.2. The studies of distal tibiofibular syndesmosis injury model demonstrated that distal tibiofibular syndesmosis injury would reduce contact forces between the talus and fibula whereas raise other joints' contact forces, make some ligaments of ankle withstand more load resisting movement, increase the magnitude of displacement at the lower extreme of tibia and fibula, augment the rotation angle of tibia(represented the rotation of ankle in the model) from normal intactness 7.11°to injuried 12.43°after application of internal rotation load and from 3.85°to 5.46°after application of external rotation load respectively, among which, the rotation angle of ankle external rotation loading was very close to Liacouras and Wayne's result from 4.28°to 5.60°. It was further confirmed this injury model valid. While the succeeding two kinds of modes fixed with screw showed distal tibiofibular syndesmosis injury fixed with screw would reduce contact forces more or less in all joints, decrease the magnitude of displacement at the lower extreme of tibia and fibula, distinctly diminish the rotation angle of tibia, make the rotation angle of fibular close to tibia. However, there were less obvious differences between the two kinds of ways of screw fixation.3. The studies of posterior malleolar fracture models involving different size of ankle joint surface injuries demonstrated that the maximum stress of ankle joint surface increased obviously from posterior malleolar fracture involving 1/4 of ankle joint surface during application of neutral position with single foot standing load, whereas the maximum stress increased from posterior malleolar fracture involving 1/3 of ankle joint surface during application of ankle internal rotation load; each model of posterior malleolar fracture all demonstrated contact forces between the talus and fibula reduced whereas contact forces between the tibia and talus(ankle jiont) raised; In addition, the rotation angle of tibia was decreased with posterior malleolar fracture models involving ankle joint surface increased during application of ankle internal rotation load, whereas the rotation angle was increased with posterior malleolar fracture models involving ankle joint surface increased during application of ankle external rotation load.Conclusions1. This study has successfully constructed three-dimensional finite element models of normal human foot and ankle with fine anatomical structures by means of Mimics, SolidWorks, ANSYS softwares in accordance with two-dimensional images scaned by spiral CT, and reflected the basic anatomic structures and mechanical properties of the human foot and ankle.The models were proved to be valid and could simulate various load situations of ankle for mechanical analysis.2. The analytic results of distal tibiofibular syndesmosis injury model and two kinds of fixation modes with screw showed that distal tibiofibular syndesmosis injury would increase the magnitude of displacement at the lower extreme of tibia and fibula, make joint ROM increase during internal rotation and external rotation of ankle, which led to joint instability, whereas, after using the distal tibiofibular screw fixed in two different positions, the results of them both represented the magnitude of bones displacement decreased, the motion of joints limited, and contact forces of joints reduced. However, the difference between the two kinds of ways of screw fixation inserted was not significant. Therefore, the fixation with screw didn't belong to physiological, and its present role was restricted to short term applications.3. The analytic results of posterior malleolar fracture models involving different size of ankle joint surface showed that dispite the posterior malleolar fracture with small fragment (injury less than 1/4 of the joint surface) seemed not to increase the maximum stress of ankle joint, conservative treatment could be considered clinically. However, all kinds of fractures could cause contact forces and ROM of joint changed significantly.