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The anterior cruciate ligament (ACL) is a band of fibrous tissue that provides support to the knee joint by connecting bones or cartilages. It is one of the four important ligaments of the knee. The anterior cruciate ligament can be injured by tearing. Its fibres can be torn, or a tear can develop where the ligament attaches to the femur, which is the thighbone, or to the tibia, which is the lower leg bone.
The other three ligaments are the medial collateral ligament, the lateral collateral ligament, and the posterior cruciate ligament.
The anterior cruciate ligament keeps the tibia from slipping forward on the femur. A combined injury involving the ACL, medial collateral ligament and cartilage tears is called the "unhappy triad." This is often the result of a clipping injury in football. Usually, the anterior cruciate ligament is torn completely, not partially.
What are the signs and symptoms of the injury?
At the time of injury, the person may feel or hear a pop in the knee, and have a sense that the knee has slipped out of joint. The knee is painful, and swells with blood or joint fluid quite rapidly. The injured athlete cannot trust the stability of the knee, because it repeatedly gives way. This happens most often while pivoting, twisting or jumping. Any sudden stopping or changing of direction when running or even walking can cause the knee to give way.
What are the causes and risks of the injury?
In anterior cruciate ligament injuries, the knee may be twisted or extended beyond its normal range of motion. Lack of muscle conditioning and poor co-ordination are risk factors. Wearing the wrong shoes for the type of athletic field, such as cleats on artificial turf, can increase the risk. Skiers whose bindings, which hold the boot to the ski, are too tight are also at risk.
What can be done to prevent the injury?
Muscular conditioning and agility training before the athletic season can help prevent the injury. Proper footwear and properly adjusted ski bindings are very important.
How is the injury recognised?
Anterior cruciate ligament injury is suspected when immediate swelling and stiffness of the knee follow a painful, twisting injury with a popping sensation. The person is reluctant to bear weight on a tender, swollen knee that feels unstable. There are several tests to see whether the knee is unstable and moving abnormally. One is called the "drawer sign" because the normal knee will not budge when an examiner attempts to push and pull the tibia when the patient is seated. With severe tears there is considerable motion. Sometimes fluid is taken from the joint with a needle to see if it is bloody. X-rays are usually normal in these cases, except for signs of knee swelling. Rarely, a bone chip will show up on an x-ray if the anterior cruciate ligament is torn from its femoral or tibial attachment. A tear in the middle of its fibres is more common. Magnetic resonance imaging can show the torn ligament, joint swelling and possibly other injuries.
A knee that is unstable due to an anterior cruciate ligament injury is prone to giving way again and again. When this happens, other structures of the knee may be damaged. Other ligaments may be stretched or torn, cartilage can tear, or arthritis can develop in the joint.
What are the treatments for the injury?
The initial treatment is rest, ice, compression, and elevation, or the acronym RICE. A knee immobiliser may be used for a short time. Non-steroidal anti-inflammatory drugs (NSAIDs) can be taken. If the knee is very swollen, fluid can be drawn from the joint to help relieve the pain and prevent further damage.
Physiotherapy is very helpful in restoring the range of motion of the knee joint. It can also help the person regain strength in the thigh muscles. Initially, the patient is advised to avoid weight-bearing, by using crutches and eventually a cane.
Final treatment depends on how unstable the knee is, how active the person is, age, and other knee injuries. After physiotherapy, some people decide to stop activities that might cause re-injury. Others continue modified activities with the support of an anterior cruciate ligament knee brace. Still others undergo surgery for reconstruction of the torn ligament.
What are the side effects of the treatments?
NSAIDS may cause indigestion, ulcers or bleeding in the stomach. The drugs may also affect the liver, blood or kidneys. Surgery is usually successful, but sometimes the knee continues to be unstable, with pain, swelling or stiffness.
What happens after treatment for the injury?
After treatment, exercises for strengthening the thigh muscles are continued.
Author: John A.K. Davies, MD
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request