Medically reviewed on Mar 13, 2018
An ACL injury is the tearing of the anterior cruciate (KROO-she-ate) ligament (ACL) — one of the major ligaments in your knee. ACL injuries most commonly occur during sports that involve sudden stops, jumping or changes in direction — such as basketball, soccer, football, tennis, downhill skiing, volleyball and gymnastics.
Many people hear or feel a "pop" in the knee when an ACL injury occurs. Your knee may swell, feel unstable and become too painful to bear weight.
Depending on the severity of your ACL injury, treatment may include rest and rehabilitation exercises to help you regain strength and stability or surgery to replace the torn ligament followed by rehabilitation. A proper training program may help reduce the risk of an ACL injury.
The anterior cruciate ligament (ACL) is one of the key ligaments that help stabilize your knee joint. The ACL connects your thighbone (femur) to your shinbone (tibia). It's most commonly torn during sports that involve sudden stops and changes in direction — such as basketball, soccer, tennis and volleyball.
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.
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.
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.
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.
Proper training and exercise can help reduce the risk of ACL injury. A physical therapist, athletic trainer or other specialist in sports medicine can provide assessment, instruction and feedback that can help you reduce risks. Programs to reduce ACL injury include:
- Exercises that strengthen leg muscles, particularly hamstring exercises, to ensure an overall balance in leg muscle strength
- Exercises to strengthen the core: hips, pelvis and lower abdomen
- Training and exercise for proper techniques and knee position in jumping and landing
- Training to improve techniques for pivoting and cutting
Wear footwear and padding that is appropriate for your sport to help prevent injury. If you downhill ski, make sure your ski bindings are adjusted correctly by a trained professional so that your skis will release appropriately when you fall.
Wearing a knee brace does not appear to prevent ACL injury or reduce the risk of recurring injury after surgery.
During the physical exam, your doctor will check your knee for swelling and tenderness — comparing your injured knee to your uninjured knee. He or she also may move your knee into a variety of positions to assess range of motion and overall function of the joint.
Often the diagnosis can be made on the basis of the physical exam alone, but you may need tests to rule out other causes and to determine the severity of the injury. These tests may include:
- X-rays. X-rays may be needed to rule out a bone fracture. However, X-rays can't visualize soft tissues, such as ligaments and tendons.
- Magnetic resonance imaging (MRI). An MRI uses radio waves and a strong magnetic field to create images of both hard and soft tissues in your body. An MRI can show the extent of an ACL injury and signs of damage to other tissues in the knee.
- Ultrasound. Using sound waves to visualize internal structures, ultrasound may be used to check for injuries in the ligaments, tendons and muscles of the knee.
Prompt first-aid care can reduce pain and swelling immediately after an injury to your knee. Follow the R.I.C.E. model of self-care at home:
- Rest. General rest is necessary for healing and limits weight bearing on your knee.
- Ice. When you're awake, try to ice your knee at least every two hours for 20 minutes at a time.
- Compression. Wrap an elastic bandage or compression wrap around your knee.
- Elevation. Lie down with your knee propped up on pillows.
Medical treatment for an ACL injury begins with several weeks of rehabilitative therapy. A physical therapist will teach you how to do exercises that you will perform either with continued supervision or at home. You may also wear a brace to stabilize your knee and use crutches for a while to avoid putting weight on your knee.
The goal of rehabilitation is to reduce pain and swelling, restore your knee's full range of motion, and strengthen muscles. This course of physical therapy may successfully treat an ACL injury for individuals who are relatively inactive, engage in moderate exercise and recreational activities, or play sports that put less stress on the knees.
Your doctor may recommend surgery if:
- You're an athlete and want to continue in your sport, especially if the sport involves jumping, cutting or pivoting
- More than one ligament or the cartilage in your knee is injured
- You're young and active
- The injury is causing your knee to buckle during everyday activities
During ACL reconstruction, the surgeon removes the damaged ligament and replaces it with a segment of tendon — tissue similar to a ligament that connects muscle to bone. This replacement tissue is called a graft. Your surgeon will use a piece of tendon from another part of your knee or a tendon from a deceased donor. The graft will serve as scaffolding on which new ligament tissue can grow.
After surgery you'll resume another course of rehabilitative therapy. Successful ACL reconstruction paired with rigorous rehabilitation can usually restore stability and function to your knee. Athletes often can return to their sports after eight to 12 months.
Preparing for an appointment
The pain and disability associated with an ACL injury prompt many people to seek immediate medical attention. Others may make an appointment with their family doctors. Depending upon the severity of your injury, you may be referred to a doctor specializing in sports medicine or a specialist in bone and joint surgery (orthopedic surgeon).
What you can do
Before an appointment be prepared to answer the following questions?
- When did the injury occur?
- What were you doing at the time?
- Did you hear a loud "pop" or feel a "popping" sensation?
- Was there much swelling afterward?
- Have you injured your knee before?
- Have your symptoms been continuous or occasional?
- Do any specific movements seem to improve or worsen your symptoms?
- Does your knee ever "lock" or feel blocked when you're trying to move it?
- Do you ever feel that your knee is unstable or unable to support your weight?