| 1. Background and PurposeAnterior cruciate ligament (ACL) is the important structure to maintain knee joint steadiness, and is the easily injured ligament as well. With the rapid development of Chinese economy and society, people's living standard is improved and traffic injuries and exercise injuries present more and more, causing the occurrence rate of ACL injuries rises year by year. Anterior tibial displacement is not limited by positive mechanics and dynamical reflexion after ACL injuried, which resulting in knee joint unsteadiness and change of movement trajectory, and therefore leading to joint biomechnical change and abnormity of stress distribution, followed by meniscal and cartilage injury and degeneration of joint. Function of knee joint is affected seriously and motor ability is degraded. Although many researches have been reported on ACL anatomy, function and treatment, and early reconstruction operation after ACL injure has been accepted by most of researchers, few studies are conducted on clinical natural history of ACL injure, damage location, degree and general pattern of incidence and development of secondary meniscal and cartilage injure after ACL injure. None of researchers can explicate the quantitative change of knee joint biomechanics after ACL injure. Traditional experimental biomechanical research has intrinsic defects such as high expense, dificulty in modeling the whole condition and inability to obtain mechanics distribution in whole scope. But for biological tissue with irregular boundary condition and structural shape, three-dimensional finite element simulation analysis is an effective method to analyze mechanics of the structure. Therefore, the present research intends to study the effects on meniscus and cartilage and biomechnical factors after ACL function deficiency by means of combining clinical epidemiological study with three dimensional finite simulation element analysis.2. Methods2.1 Patients of ACL-injury were selected for clinical epidemical study to analyze the location, degree, general pattern of incidence and development of meniscus of knee joint and cartilage damage when ACL was deficient.2.2 3.0T MR images of straight position of healthy adult volunteers'left knee joints were taken as source data. By means of Mimics, Geomagic and Hypermesh software, the three dimensional finite element analysis platform of whole knee joint was constructed by getting knee joint point cloud data, predisposing the data, geometry reconstructing and mesh generating.2.3 With Ansys software the models with divided mesh were recorded in material mechanical chracterisics, boundary conditions were restrained and the models were loaded to conduct finite element structure nonlinear claculation. We observed spatial stress of normal knee joint under load, and then observed that of ACL deficiency so at to analyze the stress change on meniscus and cartilage.3. Results3.1 Of 366 ACL-deficient cases, 223 patients developed meniscal damage in different degree with incidence rate of 60.9%. The most common damage was on posterior horn of medial meniscus (110, 30.1%). There were 75 patients suffering from one or more cartilage injuries, and the incidence rate was 20.5%. Medial femoral condyle was the most common damage (51, 38.9%), followed by medial tibial plateau (27, 20.6%).3.2 The incidence rate of lateral meniscus was highest and that of medial meniscus was lowest when TFI<6 weeks at early stage of damage, which suggested that the injury of lateral meniscus might be mainly caused by direct force as injured. As time went on, the incidence rate of lateral meniscus kept stable on the whole, while the rate of medial meniscus and cartilage rose significantly, which indicated that the injuries of medial meniscus and cartilage could be mainly caused by unsteadiness of knee joint because of ACL deficiency.3.3 Incidence of meniscus and cartilage were kept in classification at time points of 6 weeks, 3 months, 6 months, 12 months and 2 years after ACL was deficient. The statistical data showed that there was significant statistical difference (p<0.01) before and after each time point, which also indicated that it was better to reconstruct ACL as soon as possible in order to avoid secondary meniscal and cartilage damage in ACL deficiency. But the best time we suggested for reconstruction was 4-6 weeks after injured to decrease the possible risk of joint fibrosis in reconstructing at week 2-3 after ACL was deficient.3.4 Taking 3.0 T MR images of straight position of healthy volunteers'knee joints as source data, three dimensional finite element model of whole knee, including distal femur, proximal tibiofibula, whirbone, patellar ligament, medial and lateral collateral ligament, anterior and posterior cruciate ligament, meniscus and cartilage, was successfully set up by applying Mimics, Geomagic and Hypermesh software. There are 106293 points and 326935 units in the whole model.3.5 By means of Ansys software and introducing material attribute, boundary condition and load, we accomplished finite element analysis and calculation successfully for normal knee joint and ACL-deficiency, and obtained the greatest stress area of meniscus and cartilage. According to the finite element analysis, the stress could map of spatial stress and stress change tendency when ACL was deficient conformed to clinical research conclusion.4. Conclusion4.1 Large sample of Chinese ACL-deficiencies was collected for clinical epidemical analysis. Arthroscope was taken as the gold standard to analyze the incidence, development and general raw of meniscal and cartilage damage. It was found that the incidence rate of lateral meniscal injury was highest at early stage of ACL-deficiency and kept rather stable with TFI prolonged, while the incidence rate of medial meniscal and cartilage increased obviously with TFI prolonged. We supposed that the injury of lateral meniscus was mainly caused by direct force when injured but the medial meniscus and cartilage damage were mainly caused by unsteadiness of knee joint because of ACL-deficiency. To avoid secondary meniscal and cartilage injury, ACL reconstruction was suggested as early as possible. The optimal time we suggested was 4 to 6 weeks after injured in order to decrease the possible risk of joint fibrosis in reconstructing at 2 to 3 weeks after ACL was deficient.4.2 Taking high quality of MR image as source data, more real three dimensional finite element model of knee joint could be set up rapidly and efficiently, and the shape and relative position of ligament, cartilage and meniscus in knee joint accorded with the actual condition. The model could be used for finite element analysis of various conditions and acted as primitive model in future research.4.3 Biomechanical research on knee joint could be rather well conducted with three dimensional finite element method. By introducing mesh generating, material attribute, constrained condition and load, stress change tendency of each tissue was fairly precisely simulated in ACL-deficient knees, and the results obtained better confirmed the clinical conclusions. The simulation of experiment platform set up in the present study would be generally applied in research on bone and cartilage injury, meniscal injury, ligament injury and recovery biomechanics. The outcome of the present study would be successively applied in research of operation method for knee joint, research of prosthetic replacement and rehabilitation care with good prospect.
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