The anterior cruciate is the main factor causing resistance to the anterior displacement of the tibia on the femur. This is demonstrated when the orthopedic surgeon pulls the tibia forward on the femur performing a test of the anterior cruciate ligament. The tibia will displace much further forward than it should when the ACL is torn. The ligament is tight when the knee is in full extension and has the least amount of tension at approximately 45’ of flexion. Because there are different bands to the anterior cruciate ligament different areas of the anterior cruciate tighten at different angles of the knee.
Examination immediately at the time of injury will reveal usually at least mild swelling of the knee, but not necessarily. The best test is called a Lachman Test where each of the examiner’s hands are placed just above and just below the knee joint. The lower bone is brought forward with the knee angled at approximately 15’ and the examiner assess the end point. Usually, there is a firm endpoint with an intact ACL when the tibia is pulled forward. When the ligament is torn that endpoint is no longer present. The examiner will also look for increased excursion of the tibia forward on the femur. A Drawer Test is when the knee is flexed to 90’. Essentially, the same test is performed. It is more difficult in an acute situation to perform this test because usually the athlete’s knee is too sore to allow the knee to bend to 90’. A Pivot Shift is a test where the knee is brought from an extended position into flexion. Usually the knee will show a slight and subtle shift as the tibia rotates on the femur and shifts back into proper position. It is actually subflexed in the full extended knee position and returns to its natural position as the knee is flexed. As it returns to its natural position there is a "pivot shift" which takes experience to detect.
Associated injuries are always assessed for at the same time. Joint line tenderness representing torn cartilage and tenderness over the lateral knee which may reflect tearing of the collateral ligaments. O’Donohue’s "terrible triad" injury involves not only the ACL, but also the medial meniscus and the medial collateral ligament. It is unfortunately fairly common.