What is an ACL injury?

An ACL injury stands for an anterior cruciate ligament injury. It may also be known as a torn ACL. It is one of the most common types of knee injuries. There are about 200,000 people in the United States who have an ACL injury every year.


The anterior cruciate ligament is a ligament in the knee that sits right below the patella, which is the knee bone. An ACL injury happens when you tear or overstretch the anterior collateral ligament on the inside of your knee. The tear can either be a partial tear or a full tear. People get an ACL injury most often while they are playing sports or participating in physical activities that cause you to make sudden stops or changes in direction. If you have an ACL injury, you usually hear a loud popping sound at the time of the injury. You will also most likely experience severe pain or swelling in the knee. An ACL injury may require surgery to reconstruct the ACL.

There are three bones that meet to form your knee joint: the femur, the tibia, and the patella. The patella is the kneecap which sits in front of the joint to provide protection. The knee bones are connected to other bones by ligaments. There are four main ligaments in your knee which hold the bones together and keep the knee stable. The collateral ligaments are found on the sides of the knee. The medial collateral ligament is on the inside and the lateral collateral ligament is on the outside. The cruciate ligaments are found inside the knee joint and control the back and forth motion of the knee. The anterior cruciate ligament runs diagonally in the middle of the knee. It prevents the tibia from sliding out in front of the femur. It also provides rotational stability to the knee.

The most common signs and symptoms of an ACL injury include knee pain, swelling, tenderness along the joint line, a loud popping sound at the time of injury, loss of full range of motion, discomfort while walking, and a feeling that your knee is not stable. The risk factors that increase your risk for developing an ACL injury include being female and playing a sport that puts pressure on your knee or requires sudden changes of direction. These types of sports include basketball, soccer, football, tennis, or skiing.

The anterior cruciate ligament can be injured in a number of ways. Causes of an ACL injury may include changing direction rapidly, stopping suddenly, slowing down while running, landing from a jump incorrectly, or direct contact or collision (such as a football tackle).