ACL Injury - Anterior Cruciate Ligament Injury

The anterior cruciate ligament (ACL) is a band of fibrous tissue that provides support to the knee joint. It keeps the tibia from slipping forward on the femur. The tibia is one of the bones in the lower leg. The femur is the thighbone. An ACL injury occurs when the ligament tears.

What are the causes and risks of the injury?

In ACL injuries, the knee may be twisted or extended beyond its normal range of motion. Lack of muscle conditioning and poor coordination 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 are too tight are also at risk.

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. He or she may 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.

How is the injury recognized?

Diagnosis of an ACL injury begins with a history and physical exam. There are several tests to see whether the knee is unstable and moving abnormally. Sometimes fluid is taken from the joint with a needle to see if it is bloody. Joint X-rays usually show nothing except swelling. An MRI can show the torn ligament, joint swelling, and possibly other injuries.

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.

What are the treatments for the injury?

The initial treatment is rest, ice, compression, and elevation. A knee immobilizer may be used for a short time. Medications used to treat the knee pain and swelling include:

If the knee is very swollen, fluid can be drawn from the joint to help relieve the pain and prevent further damage.

Physical therapy 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 person is advised to avoid weight-bearing activity. As the knee heals, the healthcare provider may recommend crutches and eventually a cane.

After physical therapy, some people decide to stop activities that might cause reinjury. Others continue modified activities with the support of an ACL knee brace. Still others undergo surgery for reconstruction of the torn ligament.

What are the side effects of the treatments?

Medications used to treat inflammation may cause stomach upset or allergic reactions. Surgery may cause bleeding, infection, or allergic reaction to anesthesia.

What happens after treatment for the injury?

Surgery is usually successful, but sometimes the knee continues to be unstable, with pain, swelling, or stiffness. Any new or worsening symptoms should be reported to the healthcare provider.

Author:John A.K. Davies, MD
Date Written:
Editor:Crist, Gayle P., MS, BA
Edit Date:06/01/01
Reviewer:Warren Katz, MD
Date Reviewed:06/01/01