An Anterior cruciate ligament (ACL) injury is a tear or sprain of the anterior cruciate ligament (ACL). Most commons causes of ACL are sports and car accidents. Hearing or feeling a “pop” in the knee is a common initial symptom. Other symptoms include swelling, pain, inability to extend knee, pain and inability to walk or bear weight. Diagnosis is made via MRI. Treatment include compression, elevation, physical therapy and ACL reconstruction.