Anterior cruciate ligament tears occur when the knee is forcefully twisted or stressed during impact. the anterior cruciate ligament prevents anterior translation of the tibia on the femur. This means it prevents the lower leg moving forward relative to the upper thigh. When torn sometimes a pop is heard and there is significant swelling. ACL injuries often require surgical intevention and repair particularly in the sporting population. The most common graft used to replace the ACL is the hamstring tendon graft but patella tendons and synthetic versions are also used.