Anterior Cruciate Ligament (ACL) Injury

Summary

An acute anterior cruciate ligament (ACL) injury happens when this important ligament in your knee gets torn, often during sports like soccer or basketball when you twist your knee or land awkwardly. It can cause a loud “pop,” pain, swelling, and make the knee feel wobbly. Doctors figure out if it’s torn by asking how the injury happened, checking how the knee moves, and sometimes using an MRI to see inside the knee. Treatment often starts with rest, ice, and crutches, and many people need surgery to reconstruct the ligament, followed by months of physical therapy to help the knee get strong again. If a torn ACL is not treated some patients may develop a loose knee over time, a condition called ACL insufficiency.

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Anatomy of the Anterior Cruciate Ligament

The anterior cruciate ligament (ACL) is a strong, stretchy band of tissue located inside the knee joint (Figure 1). It connects the thigh bone (femur) to the shin bone (tibia) and crosses diagonally through the middle of the knee. The ACL helps keep the knee stable by preventing the shin bone from sliding too far forward and stops the knee from twisting too much during movements like running, jumping, or changing direction quickly. It’s an important part of the knee that helps us stay balanced and move smoothly.

ACL Injuries: How Do They Occur?

An anterior cruciate ligament (ACL) injury usually happens when the knee twists or bends in an unnatural way, often during quick stops, sudden changes in direction, or awkward landings from a jump (Figure 2). ACL tears often happen when someone’s foot is planted, and they suddenly twist their body or when they land on a bent knee and it gives out. These injuries are common in sports like soccer, basketball, football, and skiing. Teenagers and young adults who play these sports are at higher risk, especially girls, because of differences in knee anatomy and muscle strength.

Additional Potential Injuries (in Addition to an ACL Tear)

When someone tears their anterior cruciate ligament (ACL), other injuries can happen at the same time. Common ones include damage to the cartilage in the knee, like the meniscus, which acts as a cushion between the bones. The knee’s other ligaments, like the medial collateral ligament (MCL), can also be injured. Sometimes, the bones in the knee can bruise or get small fractures where they hit each other during the injury. Together, these injuries can make the knee more unstable and harder to heal.

How an Anterior Cruciate Ligament Injury is Diagnosed

When someone injures their anterior cruciate ligament (ACL), they usually feel or hear a “pop” in their knee, and it often starts to hurt and swell quickly. The knee might feel unstable or like it could give out if they try to walk. A doctor diagnoses an ACL tear by asking how the injury happened, checking for swelling of the knee, and seeing how the knee moves. The knee often swells after an ACL injury as the torn ligament bleeds which can cause the knee joint to fill up with blood. During an exam, the doctor might do special tests, like pulling on the shin bone to see if it moves more than it should, which can show if the ACL is torn. These tests include the anterior draw test and the Lachman test. X-rays of the knee joint following an ACL injury are often normal. However, an MRI scan allows the ligaments and other tissues to be reviewed and might be used to confirm the injury and check for damage to other parts of the knee.

Initial Treatment of an ACL Injury

When someone first injures their anterior cruciate ligament (ACL), the main goal is to reduce pain and swelling and protect the knee. The injured person should rest, ice the knee to reduce swelling, and keep it elevated. Wrapping the knee with a bandage can also help with swelling. Using crutches might be needed to avoid putting weight on the leg. Doctors may suggest wearing a knee brace for support and starting gentle exercises to improve movement. Pain medicine, like ibuprofen, can help with discomfort. These steps are the first part of treatment before deciding if more care, like surgery or physical therapy, is needed.

Surgical Treatment of an Anterior Cruciate Ligament Injury

To treat a torn anterior cruciate ligament (ACL), doctors often recommend surgery, especially for active people who want to return to sports. During surgery, the torn ACL is replaced with a new ligament made from tissue taken from the patient’s own body or a donor. After surgery, recovery includes months of physical therapy to strengthen the knee and regain movement. This therapy to regain knee range of motion and muscle strength is essential and is part of the reason why recovery from ACL surgery takes so long. For some people who don’t play sports, non-surgical treatments like physical therapy and wearing a knee brace may be enough. Recovery can take 6 to 12 months or longer, depending on how severe the injury is and how well the person follows their rehab plan.

Possible Long-Term Effects of an ACL Injury

An anterior cruciate ligament (ACL) injury can cause long-term problems. If the ACL injury is not treated, individuals may develop ACL insufficiency in the long run leading to looseness in their knee and possibly osteoarthritis. Even after surgical treatment people who have suffered an ACL injury are more likely to develop arthritis in the knee later in life, which can cause pain and stiffness. If the knee doesn’t heal properly or remains weak, it might feel unstable or give out during activities -a chronic condition known as ACL insufficiency. This can make it harder to play sports or do other physical activities. Even with surgery, some people may need more treatments in the future if their knee doesn’t work as well as it used to. Taking care of the knee with exercises and staying healthy can help lower these risks.

Edited by Amari Eubanks and Stephen Pinney MD, November 6th, 2025