What is an anterior cruciate ligament (ACL) injury?

An anterior cruciate ligament, or ACL, injury is a tear in one of the knee ligaments that joins the upper leg bone with the lower leg bone. The ACL keeps the knee stable. See a picture of the knee and the ACL.

Injuries range from mild, such as a small tear, to severe, such as when the ligament tears completely or when the ligament and part of the bone separate from the rest of the bone.

Without treatment, the injured ACL is less able to control knee movement, and the bones are more likely to rub against each other. This is called chronic ACL deficiency. The abnormal bone movement can also damage the tissue (cartilage) that covers the ends of the bones and can trap and tear the pads (menisci) that cushion the knee joints. This damage can lead to osteoarthritis.

Sometimes other knee ligaments or parts of the knee are also injured. This includes cartilage such as the menisci, or bones in the knee joint, which can be broken.

What causes an ACL injury?

Your ACL can be injured if your knee joint is bent backward, twisted, or bent side to side. The chance of injury is higher if more than one of these movements occurs at the same time. Contact (being hit by another person or object) also can cause an ACL injury.

An ACL injury often occurs during sports. The injury can happen when your foot is firmly planted on the ground and a sudden force hits your knee while your leg is straight or slightly bent. This can happen when you are changing direction rapidly, slowing down when running, or landing from a jump. This type of injury is common in soccer, skiing, football, and other sports with lots of stop-and-go movements, jumping, or weaving. Falling off a ladder or missing a step on a staircase are other likely causes. Like any other body part, the ACL becomes weaker with age. So a tear happens more easily in people older than age 40.

What are the symptoms?

Symptoms of an acute ACL injury include:

• Feeling or hearing a pop in the knee at the time of injury.
• Pain on the outside and back of the knee.
• The knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the knee joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
• Limited knee movement because of pain or swelling or both.
• The knee feeling unstable, buckling, or giving out.

After an acute injury, you will probably have to stop whatever you are doing because of the pain, but you may be able to walk.