An ACL tear is a common but severe knee injury that occurs when you abruptly stop or change direction while running. In some cases, jumping and landing may also tear the ACL ( anterior cruciate ligament) — the ligament that stabilizes the knee. Whatever the cause may be, the injury is most likely to put an end to your sports life, or daily living activities for that matter, until you heal from it. An MRI scan is used to confirm its diagnosis, and the next step is the treatment, which is often the surgery.