An in-vitro study of joint geometry and loading effects on anterior cruciate ligament strain and knee kinematics

A frequent injury in both sport and recreational activities is a rupture of the anterior cruciate ligament or ACL. The ACL is a ligament in the knee that connects the femur to the tibia. It prevents excessive anterior tibial translation and contributes to overall knee joint stability. However, when excessively strained the ligament may rupture. This occurs over 250,000 times each year in the United States alone. Beyond the initial trauma of the injury, and costly reconstruction and rehabilitation, ACL injured individuals are at risk of early onset osteoarthritis and may require total knee replacement later in life. Approximately 70% of all ACL injuries are non-contact in origin, i.e. no direct contact to the knee led to the injury. Even more troubling is that non-contact anterior cruciate ligament injury is not especially well understood. This lack of understanding is likely the result of the short time period over which the injury occurs. In order to avoid risk to live subjects, and because of a lack of sufficient animal models, the most appropriate way of studying anterior cruciate ligament injury is through in-vitro simulation. In order to better understand the etiology of ACL injury and potential injury risk factors, a purpose built knee loading simulator was designed and built with the ability to simulate various athletic activities. This simulator is capable of controlling and monitoring muscle force levels, measuring strain in various knee ligaments, and measuring joint contact forces/pressure distributions in the knee during these activities. To better understand the influence of tibial geometry on ACL strain and injury, several studies of various knee-loading conditions were conducted on cadaver knees. The knees were first imaged using MRI, and measurements of their respective tibial geometries were taken. Subsequently, the knees were installed in the simulator and muscle forces were applied. In one of these studies, hip extensor-generated joint compressive forces were also applied, followed by an impulsive ground reaction force. An existing probabilistic model of injury risk, based on tibial plateau geometry was evaluated using these data. Additionally, another series of tests was conducted comparing strain in the ACL and MCL resulting from static valgus torques. The strain generated in the ACL and MCL were measured at various flexion angles under 200N of quadriceps activation and 80N of hamstrings force. The particular objective of these tests was to determine whether isolated ACL injury is possible under purely valgus loading. Another study examining the moderating effect of tibial geometry on muscle activity induced ACL strain was also conducted. The study looked at ACL strain generated from the application of quadriceps forces and hamstrings forces at different flexion angles and related these strains to the values of medial and/or lateral tibial slope in the tested knees. The results of these studies indicate that tibial slope and medial tibial depth are significant predictors of ACL strain and that pre-landing joint compression is protective of the ACL under dynamic loading. Additionally, it was shown that MCL strain increases more appreciably as a result of valgus loading as compared to the ACL. This information, coupled with the material properties of the two ligaments suggest that isolated ACL injury cannot result from purely valgus loadings. Additionally, tibial slope and medial tibial depth were shown to significantly affect ACL and MCL strain. Lastly, it was shown that medial and lateral tibial slopes moderate ACL strain due to muscle activity. The findings of the above studies are all novel. At the time of this writing, no one has related both medial tibial depth and lateral tibial slope to ACL strain under impulsive loading. Additionally, while joint compression as a means of protection for the ACL has previously been proposed, the method in which it was applied here eliminates potentially confounding posterior drawer effects from the hamstrings. The simultaneous measurement of ACL and MCL strains under valgus loading is also a significant contribution to the literature. The finding that tibial slope moderates muscle activity-induced ACL loading is also novel and serves not only as a further verification of tibial geometry as an ACL risk factor, but also shows that adequate muscle forces and joint flexion may be able to compensate for disadvantageous tibial geometries. It is the author’s hope that the information yielded by these studies can be incorporated into more effective prevention training programs so that the occurrence, severity, and overall costs of ACL injury can be reduced.