The ACL is a primary restraint

to anterior translation of the tibia relative to the femur.29In vitro studies demonstrated that an anterior shear force applied on the tibia was the primary ACL loading mechanism. 30 and 31 The magnitude of anterior shear force applied on the tibia and its effect on ACL loading are largely affected by the posterior ground reaction force and knee flexion angle during a movement. The posterior ground reaction force on the foot during a movement creates a flexion moment at the knee that needs to be balanced by an extension moment at the knee. The quadriceps SB203580 order is the primary generator of knee extension moments. While generating knee moments, the quadriceps applies an anterior shear force at the proximal end of the tibia that is a primary cause of anterior tibial translation and ACL loading mechanism. 30 and 31 A previous study demonstrated see more that the peak impact posterior ground reaction force was significantly correlated to the peak impact knee extension moment and proximal tibial anterior shear force during the landing of a stop-jump task. 32 Knee flexion angle affects ACL loading through its relationships with patella tendon-tibia shaft

angle and ACL elevation angle.33, 34 and 35 Studies showed that ACL loading decreased when knee flexion angles increased.30 and 36 Taylor et al.37 recently quantified in vivo ACL length during a landing task using a combined fluoroscopic, magnetic resonance imaging (MRI), and videographic technique. They found that knee flexion angle and ACL length were negatively correlated, and that others the peak ACL length actually occurred prior to landing when the knee flexion angle was minimal. Taylor et al. 38 also found that knee flexion angles explained 61% of the variance in ACL length, and that peak ACL length occurred in mid-stance during walking when the knee was close to full extension. Using the same technique,

Brown et al. 39 found that landing with an increased initial knee flexion angle decreased peak ACL length during both pre-landing and landing phases of a drop vertical jump task. Kim et al. 40 recently estimated knee kinematics at the time of ACL injury for eight patients following ACL injuries through reconstruction of the relative positions of the femur and tibia at the time of ACL injury by maximizing the contact of bone bruise areas between the femur and tibia in MRI. Their results showed a mean tibial anterior translation of 22 mm, a mean knee flexion angle of 12°, and a mean knee valgus angle of 5° at the time of ACL injury. These findings clearly demonstrate that anterior translation of the tibia relative to the femur is the primary mechanism of ACL injury, and that a small knee flexion angle is responsible for an increased anterior shear force at the knee and thus anterior translation of the tibia relative to the femur.