| BackgroundThe talus structure, a vital connection between the human body and the foot, played a highly important role in human locomotion. Up to 60% of the talus was covered by cartilage. Knowledge about the biomechanics of the talus, will contribute to further understanding of the normal and abnormal situation talus physiological pathology practices. Abnormal distribution of load on talus articular surface was an important factor of ankle joint disease, such as osteoarthritis. The injury of ankle ligaments will disrupt the physiology structure of the talus and change the distribution of stress. The ligaments of ankle were important to the stability of ankle. The injury of ligaments will lead the changes of dislocation of the talus in the tibial mortise, which will affect the contact area and stress distribution. Abnormal distribution of stress can cause the degeneration of articular cartilage, which finally developed osteoarthritis. So it must reconstruct the ligament when the ligament injured. The anatomy of the talus was complex, so it was difficult to research the change of biomechanical characteristics.The experiment of traditional orthopedics biomechanics was based on the animal and cadaver models. There were deficiency and limit. The structure and function of animal were different from human, so the result of animal biomechanics cannot resolve the problem of human. The result in vivo was most reliability, but because of limited management, it was difficult to attain the data under the physiological state. The cadaver model gained an advantage over the geometry similarity, but it changed the characteristics of living tissue, and was difficult to attain the mechanic's characteristics. The same cadaver model cannot use again; it decreased the comparability of compared research. The cost of an experiment was high. Meanwhile it was difficult to gain the cadaver now.Following the development of digital simulation model and finite element analysis, digit virtual model applied to clinical diagnose, treatment and experiment research. Compared with tradition biomechanical experiment, the finite element method (FEM) was a powerful mathematical tool which allows internal stress and strain analyses of complex structures with geometrical and material nonlinearities. The finite element model can build the experiment model that had similar geometry and physics. The finite element method can build the load and constrain condition that other methods cannot attain, and can simulate the pathology state. The finite element model can afford the information and get the result which the experiment model cannot get. The finite element analysis changed the load, material parameter for individual analysis.Because of the complex structure, the kinetic variety and individual difference, so far, there was not a finite element model which can simulate the physiology status of the talus. Under the physiology status, the function and structure of the talus depended on the mechanic's environment. In this study, we established the mathematical model of the talus using the CT date. We researched the biomechanics characteristics and analysis the stress/strain during the stance phase of gait, the biomechanical stability of talus fracture fixed with different implants, and the changes of mechanic status after the ligaments' injury, in order to supply a new method to study the biomechanics in an ankle-foot field.Objective1. To construct the digit virtual model of the talus utilizing the mathematical technology, and discuss the method and significance with using the digit virtual model.2. To develop a detailed, three-dimensional, finite element model of a human ankle and analysis the stress distribution of the talus during different gait phases for the biomechanics studies.3. To construct a three-dimensional finite element model (FEM) of normal adult human ankle in order to supply a digital platform for biomechanical research of talar cartilage stress during gait, and understand the stress distribution of cartilage biomechanical characteristics.4. To explore the biomechanical properties of different internal fixations for talar neck fracture through finite element analysis and therefore to provide a scientific foundation for clinic application.5. To construct a three-dimensional finite element model of ankle ligament injury, analysis the changes of talus biomechanics in inversion and eversion load.Methods1. Constructing the talus digit virtual model:One common volunteer was chosen who was scanned by multi-slices computerized tomography in the neutral unloaded position. Scan plane was situated between the bottom of food and the plane of 10cm upper ankle. CT images were taken with intervals of 0.45 millimeters. All the slices were saved in the format of the DICOM (Digital Imaging and Communications in Medicine). Therefore, the 2D image data in the format of DICOM was obtained, and there were 663 2D-CT slices. Then these 2D-CT slices were imported into the three-dimensional reconstruction software of Mimics10.01. (Materialise's Interactive Medical Image Control System). From this data, a three-dimensional image about ankle was reconstructed with the software. Outcome was saved in the format of STL. Then the STL data was imported to Geomagic studio 10.0 software to fix surface. Finally, the surface was screwed in UG software to form the solid model of talus and other structure.2. Constructing a finite element model of talus and around structure on normal gait: The solid model formed in UG software was imported into Hypermesh10.0 software. The solid model of articular cartilage was constructed according the articular cartilage boundary. Then the model was meshed. The ligaments were constructed according the anatomy position. Finally a three-dimensional (3D) finite element model of talus and around structure was developed. The model was imported into ABAQUS6.9 software for post-processing. The results were compared with those from previous experimental research to validate the finite element model. The stress and strain nephograms during gait were obtained and analyzed to explore the biomechanical characteristics.3. A finite element analysis of contact characteristics of the talus articular surface on gait: according the outcome of the finite element model, the contact pressure, contact area, and the von Mises stress of articular cartilage were researched to study the contact characteristics of articular surface of the talus.4. A finite element analysis of biomechanical stability of talar neck fracture fixed with screws. One normal ankle of an adult volunteer was scanned (multi-slices computerized tomography, intervals 0.45 millimeters) in the neutral unloaded position. All the slices were saved in the format of the DICOM (Digital Imaging and Communications in Medicine). Then these 2D-CT slices were imported into the three-dimensional reconstruction software of Mimics10.01 (Materialise's Interactive Medical Image Control System). From this data, a three-dimensional image about ankle was reconstructed with the software. Outcome was saved in the format of STL. Then the STL data was imported to Geomagic studio 10.0 software to smooth and build talar neck fracture model. Simulation of talar neck fracture was achieved by an idealized planar cut between the bone segments in the neck. Then the model was fixed surface. Finally, the surface was imported and screwed in UG software to form the solid model. The size of the screw was consulted from AO/ASIF. The outside diameter was 4.0mm, the inside was 2.9mm. The length was changed according to the actually situation. The screw model was constructed in UG software. Given the complex geometry of the screws, they were modeled as simple cylinders and the thread ignored. The solid models were then imported into Hypermesh10.0 software. Then the models were assembled, meshed and given material parameter. Corresponding to the quantity and location of screws, four different finite element models of surgical fixation methods were developed: anterior-to-posterior fixation using one 4.0 mm screw, posterior-to-anterior fixation using one 4.0 mm screw, anterior-to-posterior fixation using two 4.0 mm screws, posterior-to-anterior fixation using two 4.0 mm screws. Finally, the finite element models were imported into the FE package ANSYS (ANSYS Inc., USA) for post-processing. The von Mises stress, contact area and pressure, contact gap were compared to estimate the biomechanical stability.5. The changes of biomechanics of the talus after ankle ligament injury:a finite element model of ankle ligament injury was constructed, that was simulated the injury of anterior talofibular ligament, calcaneofibular ligament, posterior talofibular ligament, anterior tibiotalar ligament, tibiocalcaneal ligament, posterior tibiotalar ligament, tibionavicular ligament. The injury models were loaded and post-processing. The stress and strain of the talus were researched to analysis the changes of talus biomechanics under inversion and eversion load. The result provides theory instruction for ligament repair in clinics.Results1. A serial precise of data of DICOM was obtained from CT scan for ankle. Using Mimics10.0, Geomagic studio10.0 and UG software, a geometric reconstruction of the ankle was developed using the data. The model has the precise structure.2. The model was meshed and given the material properties in Hypermesh10.0 software. Finally, the model was imported to ABAQUS6.9. A three-dimensional finite element model of ankle was established, which composed of 21865 nodes,73440 elements. And the stress distribution within the bone was obtained at three phases. The stress distributions of three phases were significantly different. The peak von Mises stress from the heel-strike to push-off on the talar dome was 3.0 MPa,4.3 MPa and 4.8 MPa respectively. It was 1.3 MPa,1.9 MPa and 2.8 MPa on talar neck, and 2.8 MPa,3.0 MPa,3.4 MPa on the talonavicular joint surface. It was 2.2 MPa,1.8 MPa and 1.5 MPa on subtalar joint. The von Mises stress and distribution scope were difference in different phase. The von Mises stress in the talar dome, talar neck, talonavicular joint surface gradually increased, and decreased in the subtalar joint.3. The contact pressure gradually increased in talar dome cartilage from the heel-strike to push-off. The contact pressure was 5.6,8.8 and 11.8MPa. The contact pressure distributed on the posterior medial side in the heel-strike, anterior and lateral in mid-stance, lateral and anterior-medial side. The contact pressure was distributed on medial metatarsus in talonavicular joint surface. The contact pressure was 7.8,8.6 and 8.1 MPa respectively. It was higher in mid-stance. The contact pressure was 4.5, 4.7 and 4.0 MPa in the posterior subtalar joint. The distribution of von Mises stress on talus articular cartilage was similar to contact pressure. It increased in talar dome and talonavicular joint articular cartilage and decreased in the posterior subtalar joint.4. Analysis the biomechanical behaviors of the talar neck fracture with four different fixations using finite element analysis. The distribution of von Mises stress was uneven on fixation. The values of maximum von Mises stress on screws were observed near the fracture location at each fixation. It was observed from these figures that the 2-AP experience has a low von Mises stress in mid-stance loading conditions. A similar behavior was observed in active dorsiflexion. It was 14 062MPa on neutral position, and 32.012 MPa on ankle active dorsiflexion. It was noticed that the fracture surface pressure mainly in the lateral talar neck at mid-stance,2-AP had the greatest pressure for 7.041 MPa. At the ankle dorsiflexion, the pressure concentrated in the medial,2-AP was higher than the others, as 9.165 MPa. The contact areas were of equal approximately in four fixations. The largest fracture gap in every fixation was in the plantar medial of fracture surface.1-AP had the maximum fracture gap under two loading conditions.2-AP was lowest.5. Under the internal rotation load, there was no strain/stress in posterior tibiofibular ligament, anterior talofibular ligament and posterior tibiotalar ligament. The stress in calcaneofibular ligament was higher, as 18.91 MPa. Under the external rotation load, there was no strain/stress in posterior talofibular ligament, calcaneofibular ligament, anterior tibiotalar ligament and tibionavicular ligament. The stress in anterior talofibular ligament was biggest, as 13.75 MPa. Under internal rotation, after anterior talofibular ligament injury, the von Mises peak stress was 8.56 MPa on talar dome,6.43 MPa on posterior subtalar joint,2.87 MPa on anterior subtalar joint, and 7.31 MPa on talonavicular joint. The maximum displacement was located in the posterior talus, as 1.21mm. With calcaneofibular ligament injury, the von Mises, peak stress was 9.29 MPa on talar dome,7.19 MPa on posterior subtalar joint,3.49 MPa on anterior subtalar joint, and 6.69 MPa on talonavicular joint. The maximum displacement was located in the posterior talus, as 1.72mm. With posterior talofibular injury, the von Mises peak stress was 8.86 MPa on talar dome,6.88 MPa on posterior subtalar joint,1.85 MPa on anterior subtalar joint, and 7.39 MPa on talonavicular joint. The maximum displacement was located in the posterior talus, as 1.09mm. Under the load of external rotation, the changes of von Mises and displacement was similar in anterior tibiotalar ligament, tibionavicular ligament and tibiocalcaneal ligament injury. The von Mises peak stress was 5.79 MPa on talar dome,4.91 MPa on posterior subtalar joint,1.71 MPa on anterior subtalar joint, and 6.88 MPa on talonavicular joint. The maximum displacement was located in lateral of talus body and head, as 0.63mm. With the injury of posterior tibiotalar ligament, the von Mises peak stress was 6.24 MPa on talar dome,5.23 MPa on posterior subtalar joint,1.41 MPa on anterior subtalar joint, and 6.67 MPa on talonavicular joint. The maximum displacement was located in lateral of talus body and head, as 0.72mm.Conclusions1. Based on CT scan data, the talus three-dimensional digital simulation models were established using Mimics, Geomagic Studio, and UG software. This approach was feasible, effective, faster and harmless to the human body. The model contained a large amount of information and entities with a similar geometry to the more realistic simulation of the original model.2. The three-dimensional finite element method was a biomechanical study of theories and methods to simulate the geometric model of the structure to give organizations the biological material properties. It can reflect the biomechanical properties of the overall trend, which can be used as a very good supplement for experimental specimen biomechanical study. In this study, according to the actual geometry of the skeleton, which was obtained from 3D reconstruction of computed tomography, a three-dimensional (3D) finite element model was developed using Mimics, Geomagic Studio, Hypermesh, ABAQUS software. The finite element model of had a good geometric similarity. Compared with similar studies reported in the literature, the model had the more refined and uniform grid, the greater the cell density and more accurate results. Furthermore, this model can be disassembled, with great flexibility in the choice of subjects; it can be built on the foot bones of various independent study to further expand the scope of application of the model. In addition, as a whole, compared with the anatomical structure, pathophysiology, clinical research literature, and many other biomechanical researches, it indicated that this model had good physical similarity, more accurate and complete to simulate the anatomy of the talus and its mechanical characteristics. It was beneficial for biomechanical analysis of the talus.3. The articular surface contact stress and area distribution on talus were important to the clinical research. The abnormal mechanical mechanism on cartilage was a major cause of osteoarthritis. Knowledge for the characteristics of mechanic distribution on cartilage will help to understand the mechanism of the normal talus articular cartilage and behavior of articular cartilage pathology under abnormal load.4. Finite element analysis is a useful tool for the analysis mechanical behavior of implants in fractures of bone, which can provide an experimental basis for effective fixation. Stress distribution within the implant itself and the displacement of fracture section were a standard measure of internal fixation. Internal fixation should be the ideal uniform distribution of stress as possible on the fixture, but not overly concentrated in certain parts. High stress will inevitably lead to high fracture strain, from the point of fracture healing view, this high strain was not conducive to the growth in fracture callus. One of the measures to verify the stability was to check the bone contact status at the fracture interface surface; the two factors were contact pressure and fracture gap at the fracture site. Larger contact area and the right pressure can increase the static friction between the fracture sites, reducing the fracture site clearance; increase the stiffness of fracture fixation. Talar neck fractures by double screws from front to back fixed and reliable access to the biomechanical stability.5. The injury of ankle ligament had a major impact on the stability of the talus, which will inevitably change the location of the talus in the ankle mortise, thus affecting the contact area and stress. It will lead to articular cartilage degeneration, eventually lead to osteoarthritis. In external rotation force, the injury of the posterior tibiotalar ligament, the von Mises stress, displacement and contact pressure was high. It was suggested that posterior tibiotalar ligament played an important role for ankle stability under external rotation force. Including the effect of rotation, with the ligament injury, the talus of the equivalent stress and the displacement of large, suggesting that spin with the ligament under the circumstances, including the stability of the ankle joint plays an important role. In internal rotation force, the injury of calcaneofibular ligament, the von Mises stress, displacement and contact pressure was high. It was suggested that calcaneofibular ligament played an important role for ankle stability under internal rotation force.6. The limitation of this study was that mechanical characteristics of biological materials involved in this study were assumed to be homogeneous, continuous and isotropic. Actually, bone, cartilage and ligaments material itself were not homogeneous, continuous, nor was isotropic, but anisotropy. The characteristic of soft tissue around the ankle has not considered the impact of the model. |
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