What Is ACL Injury?

Posterior Cruciate Ligament

One of the most common knee injuries, especially for athletes, is a sprain or tear of the anterior cruciate ligament (ACL). Injuries can range from mild, small tears, to severe, when the ligament tears completely or when the ligament and part of the bone separates from the rest of the bone.

The thighbone (femur), shinbone (tibia) and the kneecap (patella) meet to form your knee joint. These three bones are connected by ligaments (collateral and cruciate) that act like strong ropes to hold the bones together and keep your knee stable.

An ACL injury often occurs during sports when your foot is firmly planted and a sudden force hits your knee while your leg is straight or slightly bent. This can happen when you change direction rapidly or land from a jump. Injuries can also occur during every day activities, such as missing a step on a ladder or stepping over an item awkwardly.


An injury to your anterior cruciate ligament is usually accompanied by a "popping" noise and a feeling that your knee gives out from under you. Other signs and symptoms of an ACL injury include:
  • Pain on the outside and back of the knee
  • Knee swelling within the first few hours of the injury
  • Limited knee movement because of swelling and/or pain
  • Loss of full range of motion
  • Discomfort while walking

Treatment at Penn

Treatment for ACL injuries can vary depending on a patient's needs. Athletes, younger individuals or those who have jobs that keep them active, will most likely require surgery to safely return to their normal lifestyle. Those who are less active may be able to return to a quieter lifestyle with nonsurgical treatment, such as physical therapy or wearing a brace to protect the knee from instability.

Penn Programs & Services for ACL Injury

Man running on track
Sports Medicine Program

We know you want to get back to your favorite activities. Learn about our unique approach to the prevention, diagnosis, treatment, and rehabilitation of sports-related injuries.

Sports Rehabilitation

Penn Medicine’s Physical Medicine and Rehabilitation department diagnoses and treats a variety of sports-related injuries.

Share This Page: