Biomechanical Analysis of Anterior Cruciate Ligament Injury Mechanisms

摘要

The anterior cruciate ligament is the most frequently injured knee ligament. The ligament can be injured due to a sudden external impact like a traffic accident, but in most cases the injury is a result of people participating in athletic activities. This ligament has a significant impact on people’s quality of life, because it is a basis for a normal knee function. The patients are mostly young people and the consequences often affect them for the rest of their lives. Regardless of the great number of injuries, the trauma mechanisms are still unclear. A better understanding of the aetiology might increase the possibilities to prevent the injuries and improve the rehabilitation strategies. The objective of this project was to determine which trauma mechanisms have the potential to rupture the anterior cruciate ligament by quantifying the strain in the ligament during both voluntary and forced movements. Although a ligament injury may appear to have been caused by a single inciting event, it may be a complex interaction between internal and external risk factors. The mere presence of these risk factors is not sufficient to produce injury, but they predispose the athlete for the injury to occur in a given situation. The inciting event is the final link in the chain that causes an injury. The project did not attempt to determine the factors that increase the risk of sustaining an injury, but focuses on the inciting event - the injury mechanism. Anterior cruciate ligament injury mechanisms were studied with four musculoskeletal models made with The AnyBody Modelling SystemTM. AnyBody is a general musculoskeletal modelling and optimisation software system based on inverse dynamics. The inverse dynamic analysis determines the unknown forces from the equations of the known motion. Due to the redundancy of the muscle actuator configuration, the muscle recruitment problem is formulated as an optimisation problem. The musculoskeletal models made it possible to determine the knee shear force during various sports movements and explore the elongation of the anterior cruciate ligament during both natural and forced movements. In order to investigate which movements that have the potential to rupture the anterior cruciate ligament, it was chosen to quantify the strain in the ligaments and muscles around the knee joint during a forward lunge. The dynamic analysis was applied to a model with the anterior cruciate ligament intact, and to a model without the ligament. It had been expected that there would be a significant difference between the two models, because studies have shown that anterior cruciate ligament deficient subjects perform a forward lunge differently from healthy subjects. However, the dynamic analysis showed that there was no difference in muscle activity or joint reactions between the two models. The analysis revealed that the knee joint reaction produced an anterior pull in the proximal tibia. In other words, the anterior cruciate ligament was unstrained. An analysis of a male runner showed that sprint strains the anterior cruciate ligament. However, the knee shear force, which was used to evaluate the ligament strain, was well below the ultimate tensile strength of the ligament. Considering that sprint probably is one of the most intense sagittal plane sports movements, it appears that voluntary contraction is insufficient to injure a healthy cruciate ligament. Even though intense voluntary contraction might be insufficient to injure the anterior cruciate ligament during sprint, the analysis does not rule out the possibility that other sagittal plane movements may put more strain on the ligament. Therefore, a representative selection of various feasible sagittal plane movements were analysed with the sagittal model. The analysis of the sagittal model demonstrated that it is unlikely that sagittal plane mechanisms will rupture the anterior cruciate ligament. In the lunge model, the runner model and the sagittal model the knee joint was approximated as an ideal hinge. But the relative movements between femur and tibia are far more complex and are related to a complicated interaction between muscles, ligaments and bones. In addition to the knee joint’s natural movement, flexion/extension, it can also be forced into hyperextension, valgus or varus positions, increased internal/external rotation and anterior/posterior translation of the tibia. The advanced knee model made it possible to investigate the elongation of the anterior cruciate ligament for various knee positions and thereby evaluate which movements are most likely to tear the ligament. The analysis of the model showed that: The ligament is strained the most when the knee joint is flexed 5 – 25deg. Anterior translation of the tibia increases the strain significantly. Valgus and especially varus positions can increase the strain in the ligament significantly and the ligament is therefore likely to tear if the knee joint is forced into either varus or valgus. Rotation of the tibia about its longitudinal axis only produces minor strain and it seems implausible that this mechanism will injure the ligament. It was found that visual analysis of injury situations does not produce the information necessary to evaluate the strain level in the anterior cruciate ligament at failure.