| BackgroundThe distal tibiofibular syndesmosis plays an important role in maintaining ankle joint stability. Injury to the syndesmosis occurs through rupture or bony avulsion of the syndesmotic ligament complex. In1%to11%of the soft tissue injuries of the ankle, the syndesmosis is reported to be affected. Failure to diagnose and management syndesmotic disruption adversely affects outcome. However, evaluating syndesmotic stability in ankle is still a subject of debate as it is unclear when to stabilize an injured syndesmosis. The optimal method of syndesmosis fixation remains to be determined. Controversies surround implant selection (ie, size of screws, number of cortices engaged, composition of implants), position of the ankle joint during syndesmosis fixation, postoperative weight-bearing status, and the need and timing for implant removal. Due to the complexity of the anatomical structure of the ligament, leading to the biomechanical study of the ligaments in the human body becomes very difficult. Although many researchers have done so many cadavers mechanics research and according to the results put some guiding point of view, however, due to the limitations of methods and results in these research, it can not be completely reflect the real situation of the human body, it is necessary for the further study.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 reliable, 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 cadaver model cannot reuse; it decreased the comparability of the control study. The cost of an experiment was high. Meanwhile it was difficult to gain the cadaver now.In order to solve this problem, in recent years, with the development of the formation of imaging, improvement of the three dimension reconstruction software, and application of finite element in medical biomechanics. more and more researchers use digital technology in biomechanics research of the ankle. It is a common used method to establish a finite element model using CT images. The models are more precise, and the structure of the tissues is well represented. Meanwhile, the geometrical and biomechanical similarity are well also. Compared with the tradition biomechanical experiment, the finite element method was a more powerful mathematical tool which allows internal stress and strain analyses of complex structures with geometrical and material nonlinearities.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 attain the information and get the result which the experiment model cannot get. The finite element analysis changed the load, material parameter for individual analysis, to fully reflect the structure of the internal parts of the stress, strain and displacement and therefore to evaluated a variety of internal fixation equipment and fixed way.ObjectivesTo construct three-dimensional finite element models of the different ways of single screw internal fixation for diastasis of tibiofibular syndesmosis using CT scan data of normal adult human ankles, as well as improve the accuracy of the model and discuss the method and significance with using the digit virtual model. The static weight-bearing state of human body in neutral position with one foot standing were simulated to compare the differences between different ways of internal fixation in screw through finite element analysis and therefore to provide scientific foundation for clinic application.Methods1. Subjects and collection of imagesOne normal volunteer was choose and ankle disorders were excluded by X-ray examination, then was scanned by multi-slices computerized tomography in the neutral unloaded position. Scan plane was situated between the bottom of food and the plane of10cm upper ankle. CT images were taken with intervals of0.45millimeters. All the slices were saved in the format of the DICOM (Digital Imaging and Communications in Medicine). Therefore, the2D image data in the format of DICOM was obtained, and there were6632D-CT slices.2. Establish three dimensional finite element modelAll the CT images were imported into the three-dimensional reconstruction software of Mimics10.01(Materialise’s Interactive Medical Image Control System). According to the data, a three dimensional model of ankle and other structure was established by setting thresholding and modifying the morphology. Outcome was saved in the format of STL. Then the STL data was imported to Geomagic studio10.0software to smooth and fix surface. Finally, the surface was imported to UG software to form the solid model of ankle and other structure. The diameter was4.0mm or3.5mm screw models were constructed in UG software. The size of the screw was supplyed by AO/ASIF. The length was changed according to the actually situation. Given the complex geometry of the screws, they were modeled as simple cylinders and the thread ignored. The solid models were then imported into Hypermesh10.0software. The ligaments were constructed according to the anatomy position. Corresponding to the three different ways of single screw internal fixation for diastasis of tibiofibular syndesmosis. twelve different finite element models of surgical fixation methods were developed:screw from the ankle articular surface vertical distance2cm or3cm or4cm. screw diameter of3.5mm or4.5mm. screw through3or4layers of cortical bone. The unit of bone structure model was solid185and ligament model was link10. The diastasis of tibiofibular syndesmosis model was simulated by cutting between the tibiofibular syndesmosis in the intermediate section. Then the models were assembled, meshed and given material parameter. Finally, the finite element models were imported into the FE package ANSYS (ANSYS Inc. USA) for post-processing.3. Boundary of the finite element modelAll models were taken into finite element analysis for static module. To simulate the static weight-bearing state of human body in neutral position with one foot standing,600N were applied on the upper section of the lower tibia and fibula with the direction of gravity. The calcaneus and navicular was entirely constrained. The model was assumed that the bone was anatomical reduction, the position between the internal fixators was moveless. The plot contour of stress was attained. The Von Mises Stress of screw and the displacement of the tibia and fibula were extracted from all models.4. Evaluation indicator The von Mises stress of the screw, the displacement of the tibia and fibula were compared to estimate the biomechanical stability.Results1. Based on CT scan data, the ankle and screw three-dimensional digital simulation models were established using Mimics, Geomagic Studio, and UG software. The model has the precise structure.2. The model was meshed and given the material properties in Hypermesh10.0software. Finally, the model was imported to ANSYS11.0. A total of twelve types of diastasis of tibiofibular syndesmosis finite element models including bones, ligaments and screw were established. The number of nodes of the models was between22461and23182, and the number of elements was between75253and78108. The testing results of the models were taken to contrast with other study, for proving the validity of these models.3. The studies showed that significant differences between the different ways of fixation with regard to the biomechanical characteristics in the model. In the same bolt diameter and the position, through four layers showed minimum von Mises stress and displacement of the tibia and fibula which was lower than through three layers of cortical bone. In the same position and through the same cortical bone layers using4.5mm screw than3.5mm screw showed minimum von Mises stress and displacement of the tibia and fibula. A4.5mm screw to be fixed through four layers of cortical bone at2cm away from the horizontal plane of the ankle showed minimum von Mises stress and displacement of the tibia and fibula.Conclusions1. In this research, we established three-dimensional models of ankle using CT images. The models had good contour and the geometrical similarity was well. The physical characterize was as good as real samples. What’s more, the models were effective and accuracy. In addition, the model had the more refined and uniform grid, the greater the cell density and more accurate results. 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 ankle and its mechanical characteristics. Therefore, the three-dimensional finite element method can reflect the biomechanical properties of the overall trend, which can be used as a very good supplement for experimental specimen biomechanical study.2. The conclusion of this study was that better fixation stability may be achieved by using4.5mm screw through four layers of cortical bone at2cm away from the horizontal plane of the ankle for abruption of lower tibiofibular ligament union.3. There were some limitations in our study. We applied vertical force on the tibia and fibula only, but the traction of muscles around ankle was ignored. Secondly, we supposed that the mechanical characteristics of biological materials involved in this study were a material of homogeny, continuous and isotropy, which was different from the fact. |
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