Anterior Cruciate Ligament Injuries

The knee joint provides mobility and stability for your legs during walking and running activities. However, these functions can be compromised if the knee is injured. With the increased popularity of and participation in sports and fitness activities, the number of knee injuries has increased. The severity of these injuries varies from mild strains (injury to a muscle or its tendon, which connects muscle to bone) or sprains (injury to a ligament, which connects two bones) to complete tears of the ligaments and other soft tissue structures of the knee.
 

Anatomy

The knee joint comprises the cartilage-covered surfaces of three bones: the femur (thigh bone), the patella (knee cap), and the tibia (shin bone). Four main ligaments help stabilize the knee; the medial (inner side) (Fig. 1A) and lateral (outer side) (Fig. 1B) collateral ligaments resist side-to-side motion, and the anterior (front) and posterior (back) cruciate ligaments resist forward and backward motion, respectively (Fig. 1C ). The ligaments work together with the medial and lateral menisci (crescent-shaped cartilage) (Fig. 1D) and the leg muscles to stabilize the joint and allow the knee to generate and deliver the large quantities of power required for activities.     The anterior cruciate ligament (ACL) lies inside the knee joint (Figs. 2A and 2B). It consists of strong fibers (or collagen) that function like the strands of a rope or cable. This ligament provides most of the support that prevents the tibia from slipping forward against the femur.

Mechanism of injury
When it functions normally, the ACL can handle large forces with little or no problem. If, however, the knee receives forces of a high magnitude and the muscles cannot help absorb the stress, the ACL may take all the load, and it may tear. High-magnitude loading can occur during a slip and fall, sudden change in direction, landing off balance while jumping, or hyperextension of the knee (Fig. 3A). When the ligament tears, it generally ruptures like a rope, and the knee momentarily slides out of place (Fig. 3B).

Signs of injury
Most people who have torn their ACL say that they heard a "pop" in their knee as the ligament tore. Usually, the knee swells within the first hour after injury and is quite painful. The injured person cannot continue his or her activity.

Treatment
Treatment for an acute (recent) ACL tear involves icing the knee and seeking prompt medical attention. Do not try to walk on the knee without assistance. You must protect the knee against further injury, which will likely occur without appropriate treatment.
A doctor who is familiar with knee injuries can confirm the diagnosis of an ACL injury through a physical examination. He or she will tailor your treatment to the severity of the instability and to the types of activities in which you plan to participate.
If your activities will place only low demands on your injured knee, you may not need surgery. You may have good results with nonoperative treatment, which may involve using crutches, wearing a knee brace, and participating in physical therapy. If you plan to have an active lifestyle, you probably will need surgery. Through surgical treatment, the doctor can rebuild or reconstruct the ligament to recreate a maximally stable joint that can meet the demands of work and play.

How can you prevent these injuries?
Unfortunately, completely protecting your knee against ACL injury is impossible. However, if you have a strenuous job or play sports hard, then strengthening and conditioning programs are your best ally. So, before heading to the mountains for a snow-skiing trip or making your debut on the basketball court, talk to a doctor, physical therapist, or athletic trainer to find out how to best prepare for the demands you will soon face.

Kurt E. Jacobson, M.D.
Columbus, Georgia