| Background and purpose:Anterior cruciate ligament (ACL) is an important anterior stabilizing mechanism for the knee joint. ACL injury may cause apparent anterior instability of the knee joint, seriously affecting knee joint function. Moreover, ACL injury may result in secondary lesions of main structures such as articular cartilage and menisci and early development of joint degeneration and osteoarthrosis. The anatomy and function of ACL and the treatments of ACL injury have been addressed in many researches; however, the biomechanical mechanisms by which ACL maintains joint stability and gets injured remain undefined. In addition, the effects of ACL reconstruction following injury are not ideal yet, which is fundamentally due to a poor understanding of the anatomy and function of normal ACL and biomechanical distribution under various movement states. In recent years, researchers have begun to emphasize the study on ACL biomechanics. According to various study objectives, some biomechanical data were obtained from corpse studies. Nevertheless, biomechanical experiments have some intrinsic defects such as high costs, difficulty in simulating all work conditions, and failure to obtain global mechanical distribution information of ACL. For ACL with irregular boundary conditions, structure and shape, the three-dimensional finite element method is effective in the mechanical analysis of ACL structure. Nevertheles, relative studies in this regard have not been reported yet in civile country. The finite element models of ACL established by foreign research institutes were far from flawless. This study was aimed to establish a digital model that vividly reflects the true geometric structure of ACL. Based on this, meshing was performed to establish a finite element model of ACL, so that a platform for mechanical analysis and simulation test of the ACL structure can be provided. Meanwhile, material properties were introduced, boundary conditions set and load added to carry out the finite elements analysis and reveal the spatial stress distribution of ACL, so as to provide detailed and complete biomechanical basis for the prevention, treatment and rehabilitation of ACL injury.Methods:1. Knee joints were obtained from healthy male Chinese corpses and immobilized on half-ring sulcated fixators with the joints in a straightened position. Then the joint capsule, patella, patellar tendon and retinaculum, quadriceps tendon, popliteus tendon, posterior cruciate ligament, menisci, and medial and lateral collateral ligaments were removed, and distal femur, proximal tibia, ACL and its complete attachment points were retained. The specimens were scanned by using a Cyberware 3030 3-D laser scanner to obtain point cloud data. By using reverse enginering technology and Imageware software, contour extraction, match, and parametric adjustment were carried out for distal femur, proximal tibia, ACL and its insertion points. Then three-dimensional solid modeling was carried out by using the UG software to construct the model function through Curves feature.2. The three-dimensional solid model established by the UG software was saved as an IGES file, and free meshing was carried out by using the good interface processing and meshing features of ABAQUS in a tetrahedron solid unit. Since ligaments are a nonlinear material, the selected unit type was C3D10MH. Based on fineness and economical considerations, the mesh size parameters were set so that the units were enough to guarantee a certain simulation precision and also to reduce the time of analysis and the requirements on computer hardware.3. Nonlinear calculation of finite element structure was performed on ABAQUS finite element software by entering the material's mechanical properties and constrained boundary conditions into the meshed model and adding enforced displacement load to the model to observe the force and spatial stress distribution of ACL under load.Results:1. ACL was scanned for the first time by using a laser scanner in a rotatory way, and a three-dimensional solid model that involved the soft and hard tissues including the knee joint was successfully established by using the UG software. The model can be observed at any angle, in any view, and from any cross-section; moreover, it can be entered into the finite element software for further biomechanical analysis of ACL.2. A three-dimensional finite element model including distal femur, ACL, proximal tibia was established by using ABAQUS software. In the model, there were 4989 nodes and 21890 units for the femur element, 3193 nodes and 15680 units for the tibia element, and 13943 nodes and 8841 units for the ACL element. The model had 22125 nodes and 46411 units in all; hence, the model was significantly superior to previously established finite element models in terms of mesh precision.3. Finite element analysis and calculation was successfully performed by introducing material properties, boundary conditions, and load. The relationship between overall nodal force and displacement of ACL and the stress cloud chart reflecting stress spatial distribution of ACL under anterior tibial load were determined. The results of finite element analysis were in accordance with the conclusions of previous experiments and clinical studies. The obtained data can be further used for the study of ACL injury, the development of artificial substitute, and rehabilitative treatment of ACL injury.Conclusion:By using reverse engineering technology and a laser scanner, geometric data were obtained to establish a more real three-dimensional solid model of the knee joint, with more actual ACL shape and relative position. This model can be used for finite element analysis under various work conditions and as an original model for further in-depth study.The three-dimensional finite element method can be well used for the biomechanical study of ACL. Through meshing, introduction of material properties, constrained conditions, and load, ACL stress distribution under anterior tibial load of straightened knee joint was accurately simulated. The calculation results were of practical clinical significance. Hence, this is a practical method of biomechanical study and can act as a beneficial supplement to experimental biomechanical methods. |
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