Symptoms and causes

Symptoms

Signs and symptoms of an ACL injury usually include:

  • A loud "pop" or a "popping" sensation in the knee
  • Severe pain and inability to continue activity
  • Swelling that begins within a few hours
  • Loss of range of motion
  • A feeling of instability or "giving way" with weight bearing

When to see a doctor

Seek immediate care if any injury to your knee causes signs or symptoms of an ACL injury. The knee joint is a complex structure of bones, ligaments, tendons and other tissues that work together. It's important to get a prompt and accurate diagnosis to determine the severity of the injury and get proper treatment.

Causes

Ligaments are strong bands of tissue that connect one bone to another. The ACL, one of two ligaments that cross in the middle of the knee, connects your thighbone (femur) to your shinbone (tibia) and helps stabilize your knee joint.

Most ACL injuries happen during sports and fitness activities that can put stress on the knee:

  • Suddenly slowing down and changing direction (cutting)
  • Pivoting with your foot firmly planted
  • Landing from a jump incorrectly
  • Stopping suddenly
  • Receiving a direct blow to the knee or collision, such as a football tackle

When the ligament is damaged, there is usually a partial or complete tear across the tissue. A mild injury may overextend the ligament but leave it intact.

Risk factors

Women are more likely to have an ACL injury than are men who participate in the same sports. Studies have suggested some reasons for these differences in risk.

In general, women athletes exhibit a strength imbalance in their thighs with the muscles at the front of the thigh (quadriceps) being stronger than the muscles at the back (hamstrings). The hamstrings help prevent the shinbone from moving too far forward — movement that can overextend the ACL.

Studies comparing jumping and landing techniques among men and women athletes have shown that women athletes are more likely to land from a jump in a way that increases stress on their knees.

Research suggests that training to strengthen muscles of the legs, hips and lower torso — as well as training to improve jumping and landing techniques — may reduce the higher ACL injury risk associated with women athletes.

Complications

People who experience an ACL injury are at higher risk of developing knee osteoarthritis, in which joint cartilage deteriorates and its smooth surface roughens. Arthritis may occur even if you have surgery to reconstruct the ligament.

Multiple factors likely influence the risk of arthritis, such as the severity of the original injury, the presence of related injuries in the knee joint or the level of activity after treatment.