The anterior cruciate ligament (ACL) is most frequently injured during non-contact activities when there is a sudden change in the direction of movement (cutting or pivoting) (Fig. 6.81). Contact sports may also result in ACL injury due to sudden twisting, hyperextension, and valgus force related to direct collision. The injury usually affects the mid-portion of the ligament and manifests itself as a complete or partial discontinuity of the fibers or abnormal orientation and contour of the ligament. With an acute ACL tear, a sudden click or pop can be heard and the knee becomes rapidly swollen. Several tests are used to clinically assess the injury, and the diagnosis is usually confirmed by MRI. A full thickness ACL tear causes instability of the knee joint. The treatment depends on the desired level of activity of the patient. In those with high activity levels, surgical reconstruction of the ligament is required. Those with low activity levels may opt for knee bracing and physiotherapy; however, in the long term the internal damage to the knee leads to the development of early osteoarthritis.