Anatomy and Pathophysiology

The bone structure of the knee joint is formed by the femur, the tibia, and the patella. The ACL is one of the four main ligaments within the knee that connect the femur to the tibia. The knee is essentially a hinged

joint that is held together by the medial collateral (MCL), lateral collateral (LCL), anterior cruciate (ACL) and posterior cruciate (PCL) ligaments. The ACL runs diagonally in the middle of the knee, preventing the tibia

from sliding out in front of the femur as well as providing rotational stability to the knee (Figure 1).
The weight-bearing surface of the knee is covered by a layer of articular cartilage. On either side of the joint, between the cartilage surfaces of the femur and tibia, are the medial meniscus and lateral meniscus. The menisci act as shock absorbers and work with the cartilage to reduce the stresses between the tibia and the femur. It is estimated that 70 percent of ACL injuries occur through non-contact mechanisms while 30 percent result from direct contact with another player or object. The mechanism of injury is often associated with deceleration coupled with cutting, pivoting or sidestepping maneuvers, awkward landings or “out of control” play. Several studies have shown that female athletes have a higher incidence of ACL injury than male athletes in certain sports. It has been proposed that this is due to differences in physical conditioning, muscular strength, and neuromuscular control. Other hypothesized causes of this gender-related difference in ACL injury rates include pelvis and lower extremity (leg) alignment, increased ligamentous laxity and the effects of estrogen on ligament properties. Approximately 50 percent of ACL injuries occur in combination with damage to the meniscus, articular cartilage, or other ligaments. Additionally, patients may have bone bruises of the bone beneath the cartilage surface. These may be seen on an MRI and may indicate injury to the overlying articular cartilage.

Immediately after the injury, patients usually experience pain and swelling and the knee feels unstable. Within a few hours after a new ACL injury, patients often have a large amount of knee swelling, a loss of full range of motion, pain or tenderness along the joint line and discomfort while walking. In addition to performing special tests for identifying meniscus tears and injury to other ligaments of the knee, the physician will often perform the Lachman’s test to see if the ACL is intact. If the ACL is torn, the examiner will feel increased forward (upward or anterior) movement of the tibia in relation to the femur (especially when compared to the normal leg) and a soft, mushy endpoint (because the ACL is torn) when this movement ends. Here is a link to other Lachman’s test movies. Another test for ACL injury is the pivot shift test. In this test, if the ACL is torn, the tibia will start forward when the knee is fully straight and then will shift back into the correct position in relation to the femur when the knee is bent past 30 degrees