• An anterior cruciate ligament (ACL) injury is usually caused by a sudden twisting motion in the knee when an athlete lands or steps.
• Young women are two to eight times more likely than young men to injure the ACL.
• Landing on the balls of your feet, rather than flat-footed, may help prevent ACL injuries.

Cruciate ligaments

The articular capsule of the posterior knee is thickened by intrinsic ligaments that help to resist knee hyperextension. Inside the knee are two intracapsular ligaments, the anterior cruciate ligament and posterior cruciate ligament. These ligaments are anchored inferiorly to the tibia at the intercondylar eminence, the roughened area between the tibial condyles. The cruciate ligaments are named for whether they are attached anteriorly or posteriorly to this tibial region. Each ligament runs diagonally upward to attach to the inner aspect of a femoral condyle. The cruciate ligaments are named for the X-shape formed as they pass each other (cruciate means "cross"). The posterior cruciate ligament is the stronger ligament. It serves to support the knee when it is flexed and weight bearing, as when walking downhill. In this position, the posterior cruciate ligament prevents the femur from sliding anteriorly off the top of the tibia. The anterior cruciate ligament becomes tight when the knee is extended, and thus resists hyperextension.

Knee Joint

(a) The knee joint is the largest joint of the body. (b)–(c) It is supported by the tibial and fibular collateral ligaments located on the sides of the knee outside of the articular capsule, and the anterior and posterior cruciate ligaments found inside the capsule. The medial and lateral menisci provide padding and support between the femoral condyles and tibial condyles.

Disorders of the Knee Joints

Injuries to the knee are common. Since this joint is primarily supported by muscles and ligaments, injuries to any of these structures will result in pain or knee instability. Injury to the posterior cruciate ligament occurs when the knee is flexed and the tibia is driven posteriorly, such as falling and landing on the tibial tuberosity or hitting the tibia on the dashboard when not wearing a seatbelt during an automobile accident. More commonly, injuries occur when forces are applied to the extended knee, particularly when the foot is planted and unable to move. Anterior cruciate ligament injuries can result with a forceful blow to the anterior knee, producing hyperextension, or when a runner makes a quick change of direction that produces both twisting and hyperextension of the knee.

A worse combination of injuries can occur with a hit to the lateral side of the extended knee (Figure below). A moderate blow to the lateral knee will cause the medial side of the joint to open, resulting in stretching or damage to the tibial collateral ligament. Because the medial meniscus is attached to the tibial collateral ligament, a stronger blow can tear the ligament and also damage the medial meniscus. This is one reason that the medial meniscus is 20 times more likely to be injured than the lateral meniscus. A powerful blow to the lateral knee produces a “terrible triad” injury, in which there is a sequential injury to the tibial collateral ligament, medial meniscus, and anterior cruciate ligament.

Arthroscopic surgery has greatly improved the surgical treatment of knee injuries and reduced subsequent recovery times. This procedure involves a small incision and the insertion into the joint of an arthroscope, a pencil-thin instrument that allows for visualization of the joint interior. Small surgical instruments are also inserted via additional incisions. These tools allow a surgeon to remove or repair a torn meniscus or to reconstruct a ruptured cruciate ligament. The current method for anterior cruciate ligament replacement involves using a portion of the patellar ligament. Holes are drilled into the cruciate ligament attachment points on the tibia and femur, and the patellar ligament graft, with small areas of attached bone still intact at each end, is inserted into these holes. The bone-to-bone sites at each end of the graft heal rapidly and strongly, thus enabling a rapid recovery.

Knee Injury - A strong blow to the lateral side of the extended knee will cause three injuries, in sequence: tearing of the tibial collateral ligament, damage to the medial meniscus, and rupture of the anterior cruciate ligament.

Symptoms

You may hear a popping sound, and the leg may buckle when you try to stand on it. After the initial painful rupture, the knee develops swelling, which typically lasts three to four weeks. Once the swelling subsides, the athletes usually don't experience discomfort or giving-way with activities of daily living. However, with return to sports, the knee often buckles, causing more damage to the knee.

The doctor may perform several tests to see whether the parts of the knee stay in proper position when pressure is applied in different directions. A thorough examination is essential. An MRI is accurate in detecting a complete tear, but the only reliable means of detecting a partial one is arthroscopy (a small surgical incision through which a surgeon can use a tiny lens to see the tear).

Surgery to repair the ACL tear is usually followed by physical therapy over a period of from six to 12 months, depending on the tear.

Prevention

The anterior cruciate ligament is most often stretched or torn (or both) by a sudden twisting motion (for example, when the feet are planted one way and the knees are turned another). Although ACL tears are usually difficult to predict, their occurrence may be made less likely by thoroughly warming up before sports, learning to land on the balls of the feet (not flat-footed), and using strength training to make the muscles firmer.

Fast Facts:

• An anterior cruciate ligament (ACL) injury is usually caused by a sudden twisting motion in the knee when an athlete lands or steps.

• Young women are two to eight times more likely than young men to injure the ACL .

• Landing on the balls of your feet, rather than flat-footed, may help prevent ACL injuries.

Glossary:

Ligament

• A band of fibrous tissue that connects two or more bones at a joint.

Tendon

• Fibrous cords of tissue that connect muscle to bone.

Meniscus

• A C-shaped piece of cartilage that acts like a pad between the femur (thigh bone) and tibia (shin bone).

Sprain

• A stretch and/or tear of a ligament.

Strain

• An injury to either a muscle or a tendon