An anterior cruciate ligament, or ACL, injury is a tear in one of the knee ligaments that joins the upper leg bone with the lower leg bone. The ACL keeps the knee stable. See a picture of the knee and the ACL.
Injuries range from mild, such as a small tear, to severe, such as when the ligament tears completely or when the ligament and part of the bone separate from the rest of the bone.
Without treatment, the injured ACL is less able to control knee movement, and the bones are more likely to rub against each other. This is called chronic ACL deficiency. The abnormal bone movement can also damage the tissue (cartilage) that covers the ends of the bones and can trap and tear the pads (menisci) that cushion the knee joints. This damage can lead to osteoarthritis.
Sometimes other knee ligaments or parts of the knee are also injured. This includes cartilage such as the menisci, or bones in the knee joint, which can be broken.
Your ACL can be injured if your knee joint is bent backward, twisted, or bent side to side. The chance of injury is higher if more than one of these movements occurs at the same time. Contact (being hit by another person or object) also can cause an ACL injury.
An ACL injury often occurs during sports. The injury can happen when your foot is firmly planted on the ground and a sudden force hits your knee while your leg is straight or slightly bent. This can happen when you are changing direction rapidly, slowing down when running, or landing from a jump. This type of injury is common in soccer, skiing, hockey, football, and other sports with lots of stop-and-go movements, jumping, or weaving. Falling off a ladder or missing a step on a staircase are other likely causes. Like any other body part, the ACL becomes weaker with age. So a tear happens more easily in people older than age 40.
Symptoms of an acute ACL injury include:
After an acute injury, you will probably have to stop whatever you are doing because of the pain, but you may be able to walk.
The main symptom of chronic ACL deficiency is the knee buckling or giving out, sometimes with pain and swelling. This can happen when an ACL injury is not treated.
Your doctor can tell whether you have an ACL injury by asking questions about your past health and examining your knee. The doctor may ask: How did you injure your knee? Have you had any other knee injuries? Your doctor will check for stability, movement, and tenderness in both the injured and uninjured knee.
You may need X-rays, which can show damage to the knee bones. Or you may need other imaging tests, such as an MRI. An MRI can show damage to ligaments, tendons, muscles, or knee cartilage. Arthroscopy may also be done. During arthroscopy, your doctor inserts surgical tools through one or more small cuts (incisions) in the knee to look at the inside of the knee.
Start first aid right away. These first-aid tips will reduce swelling and pain. Use the RICE method. The letters stand for Rest the knee, put Ice on it, use an elastic bandage to give gentle Compression to the knee, and Elevate the leg by propping it up above the level of your heart. And at first it’s also important to move your leg as little as possible. Take over-the-counter pain medicine.
You may need to walk with crutches and use a knee immobilizer to keep your knee still for the first few days after the injury.
Your knee will need to be checked by your doctor. It's important to get treatment. If you don't, the injury may become a long-lasting problem. There are two ways to treat the injury:
If you have surgery, you will also have several months of rehab afterward.
Your treatment will depend on how much of the ACL is torn, whether other parts of the knee are injured, how active you are, your age, your overall health, and how long ago the injury occurred.
There are three main treatment goals:
The best way to prevent ACL injuries is to stretch and strengthen the leg muscles, especially the front and back muscles of the thigh (quadriceps and hamstrings).
Other things you can do that may help prevent ACL injuries include:
Frequently Asked Questions
Learning about anterior cruciate ligament (ACL) injuries:
Living with an ACL injury:
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Anterior cruciate ligament (ACL) injuries are caused when the knee is straightened beyond its normal limits (hyperextended), twisted, or bent side to side.
Typical situations that can lead to ACL injuries include:
Inactive people and some older adults who have weak leg muscles may injure their knees during normal daily activities. But they usually injure bones, not ligaments.
When contact causes an ACL injury, it can be from playing a sport, from a sudden and severe accident, or from less obvious contact injuries. In football, receiving a clipping contact injury—in which the bent knee is struck from the outside—can cause an ACL injury. Clipping often damages several knee structures at the same time, including the ACL, the medial collateral ligament (MCL), and the pads in the knee (menisci) that protect and cushion the joint surface and bone ends.
An ACL injury may develop into long-lasting and recurrent (chronic) ACL deficiency that leads to an unstable knee—the knee buckles or gives out, sometimes with pain and swelling. This can occur if you had an ACL injury in the past and did not know it or if your ACL has not been treated or has been treated unsuccessfully.
Symptoms of a severe and sudden (acute) anterior cruciate ligament (ACL) injury include:
After an acute injury, you will almost always have to stop the activity you are engaged in but may be able to walk.
The main symptom of chronic (long-lasting and recurrent) ACL deficiency is an unstable knee joint. The knee buckles or gives out, sometimes with pain and swelling. This happens more often over time. But not everyone with an ACL injury develops a chronic ACL deficiency.
Other conditions with symptoms similar to ACL knee pain include injuries to other knee structures, such as:
If you have a sudden (acute) anterior cruciate ligament (ACL) injury, you typically know when it happens. You may feel or hear a pop and the knee may give out, causing you to fall. The knee swells and often is too painful or unstable to continue any activity.
An ACL injury can cause small or medium tears of the ligament, a complete tear of the ligament (rupture), a separation of the ligament from the upper or lower leg bone (avulsion), or a separation of the ligament and part of the bone from the rest of the bone (avulsion fracture). When any of these occur, the lower leg bone moves abnormally forward on the upper bone, with a sense of the knee giving out or buckling.
When the ACL ligament tears, the blood vessels around the ligament tear and blood fills the knee joint, causing swelling. When you see a doctor, he or she may not be able to examine the knee thoroughly because of the swelling.
Other parts of the knee can be injured at the same time. These may include one of the pads that act as cushions between the two leg bones (menisci), another knee ligament (medial collateral ligament or lateral collateral ligament), or the dense tissue that covers the ends of bones (cartilage). The bones of the knee joint may also be broken (fractured).
Diagnosis may not be done at the time of injury. Sometimes people think the injury is not serious, especially if the knee gets better in a few days. In this case, or if the diagnosis is missed during the initial examination, the injury may develop into a long-lasting and recurrent (chronic) ACL deficiency in which the knee moves abnormally and gives way occasionally. This can potentially cause progressive damage to the joint, including osteoarthritis. But not everyone with an ACL injury develops a chronic ACL deficiency.
The course of an ACL injury depends on:
People with minor ACL injuries usually begin treatment with a physical rehabilitation (rehab) program. Rehab exercises build strength and flexibility in the muscles on the front of the thigh (quadriceps) and strengthen and tighten the muscles in the back of the thigh (hamstrings). You may use crutches for a short time. Although knee braces may be used to stabilize the knee immediately after injury, they are not usually used long-term. Most people return to their normal activities after a few weeks of rehab.
More serious ACL injuries may require several months of rehab or surgery followed by several months of rehab to regain your knee strength, knee stability, and range of motion. You may use crutches or special knee braces, and it may take several months to a year until you can return to your previous level of activity. The rehab program is intensive—many people think of it as having a second job.
Not all ACL injuries require surgery. But whether you have surgery or not, you need to start strengthening your knee and regaining motion soon after you injure it. This prepares you for your rehab program if you choose not to have surgery and also helps prepare the knee for surgery if you choose to have it.
Things that increase your risk of anterior cruciate ligament (ACL) injuries include:
Women have more ACL injuries than men. In sports, women injure their ACL up to 8 times as often as men. There are several theories for the increased incidence in women athletes, including differences in men's and women's bodies and how they use muscles, and training that does not help make up for these differences.3, 1
Call your doctor immediately if you have an injury to your knee and:
Call your doctor today if:
Before your appointment, do not put weight on the injured knee. Use crutches if you need to. Apply ice and wrap your knee in an elastic bandage or neoprene (synthetic rubber) sleeve. Rest and elevate the knee. Take a non-prescription anti-inflammatory drug to reduce swelling. For more information on first aid steps, see the Home Treatment section of this topic.
Watchful waiting is a period of time during which you and your doctor observe your symptoms or condition without using medical treatment. Watchful waiting is not appropriate if knee pain is severe; your knee is deformed, swells, or has limited movement immediately after an injury; or you are unable to bear any weight either because of pain or instability.
Serious knee injuries need to be checked for possible broken bones as well as ligament or cartilage damage. Whenever immediate swelling follows an injury, there also may be torn blood vessels or damaged nerves in the knee. Your doctor will check your knee to make sure the blood supply to your leg is normal and the nerves are intact.
If you have occasional pain in your knee or your knee sometimes gives way or buckles, have your doctor check it. If you have damaged your ACL, it is important to get treatment so that your knee is appropriately managed, which may reduce the chance that you will get osteoarthritis in your knee.
Your family doctor, general practitioner, or an emergency medicine specialist (for acute knee injury) can diagnose most knee injuries. For treatment, you may be referred to a specialist, such as an orthopedic surgeon or a sports medicine specialist who is experienced with treating knee problems and injuries.
If surgery is considered, you may be referred to an orthopedic surgeon (possibly a sports medicine specialist) who is experienced in knee surgery.
To prepare for your appointment, see the topic Making the Most of Your Appointment.
An anterior cruciate ligament (ACL) injury is diagnosed through a medical history and a physical examination. A doctor who specializes in knee injuries (for example, an orthopedic surgeon or sports medicine specialist) will usually be able to accurately diagnose an ACL injury after:
If you see your doctor soon after your injury, the pain and the degree of swelling and muscle tenseness may make it difficult for your doctor to accurately diagnose the condition.
Other tests that may help your doctor see how badly the knee is injured include:
If your knee is red, hot, or very swollen, a knee joint aspiration (arthrocentesis) may be done, which involves removing fluid from the knee joint with a syringe (needle). This is done to:
Local anesthetic may be injected to reduce pain and make the knee easier to examine.
Arthrometric testing of the knee may also be done. In this test, your doctor uses an instrument to measure the looseness of your knee. This test is especially useful in people whose pain or physical size makes a physical examination difficult. An arthrometer has two sensor pads and a pressure handle that allows your doctor to put force on the knee. The instrument is strapped to your lower leg so that the sensor pads are placed on the knee cap and the small bump just below it (tibial tubercle). Your doctor then measures pressure by pulling or pushing on the pressure handle.
Arthroscopy is another procedure used in the diagnosis of an ACL injury and is also usually used as a method of surgery. Arthroscopy involves inserting instruments through one or more small incisions in the knee, allowing your doctor to examine the structures inside the knee joint, including the ACL.
Before arthroscopy, you and your doctor will decide what will be done if certain conditions are found. For example, you may decide in advance that if a complete tear of the ACL is found, it will be reconstructed during the arthroscopy. Or if a more severe condition is found, you and your doctor may agree to discuss the condition rather than proceeding with surgery at that time.
There are three main treatment goals. The first goal is to stabilize the knee if it is unstable—or at least stabilize it enough to suit your lifestyle. The second goal is to return your knee to normal or almost normal functioning. The third goal is to reduce the likelihood of further damage to the knee. Treating anterior cruciate ligament (ACL) injuries may also help to reduce pain, prevent osteoarthritis, and prevent loss of strength and decreased movement in the knee.
Initial treatment of an acute ACL injury consists of using first aid steps to stabilize your knee and reduce swelling and pain.
Later treatment may include several months of rehabilitation (rehab) or surgery with rehab. Not all ACL tears require surgery. Further treatment is nearly always a decision you and your doctor make between rehab only and surgery plus rehab.
If you know you have injured your ACL, initial treatment consists of:
For information on specific first aid steps to take immediately after the injury, see the Home Treatment section of this topic.
After initial treatment for an anterior cruciate ligament (ACL) injury, further treatment of the injury depends on:
Treatment options include:
For more information, see the topic:
Recovery from an ACL injury varies with each individual. Your treatment should continue until your knee is stable and strong rather than for a certain length of time.
Anterior cruciate ligament (ACL) injuries in children and teens are less common than in adults, but they do occur, especially in teens. An untreated or unsuccessfully treated ACL injury in children or teens may result in future knee problems. The knee may become more and more unstable and, over time, osteoarthritis may develop.
A child with an ACL injury can sometimes be treated without surgery in order to avoid damage to the child's still-developing bones. Non-surgical treatment includes rehabilitation (rehab) exercises, wearing a brace, and avoiding activities that require jumping or twisting. Non-surgical treatment is not always successful. A child's level of activity is a strong factor in how successful treatment is. Studies suggest that the more active a child is, the less likely non-surgical treatment will be successful and the more likely surgery will be needed in the future.2
An avulsion fracture (a separation of the ligament and a piece of the bone from the rest of the bone) is more common in young children. It can often be treated with a cast. But it sometimes needs surgery.
Surgery in a child might be necessary to prevent injury to other structures within the knee, such as the menisci. You may consider surgery if the child's knee is very unstable doing simple daily activities, if the knee's instability cannot be controlled with non-surgical methods, if the child has both an ACL injury and a meniscus tear, or if the child is a serious athlete in sports that require running, jumping, and decelerating. Post-surgery rest and a sustained rehab program are extremely important.
The main risks of surgery in a child whose bones are still growing is slowed growth (physeal arrest), which may result in one leg being longer than another. Other risks include deformity of the thigh bone (distal femoral valgus or angular limb deformity). The closer a child or teen is to skeletal maturity, the lower the risk of these conditions.
Things that you should consider about treatment options include:
Depending on how severe your injury is, surgery with rehab may offer the best chance of making your knee stable again and of continuing an active lifestyle without further pain, injury, or loss of strength and movement in your knee. Age is not a factor, although your overall health may be. Surgery may be done for adults at any age who want to continue activities that require a strong, stable knee.
If your initial injury resulted in an unstable knee that occasionally gives out (chronic ACL deficiency) and you continue to participate in activities that require a stable knee and don't have surgery, you may injure your knee again.
You will need to follow a rehab program whether or not you have surgery. If you do not complete a rehab program, even with surgery you may not regain full stability and function in your knee.
You may choose to start a rehab program to avoid or delay knee surgery by strengthening and developing flexibility in the muscles that support the knee (hamstrings and quadriceps). If you eventually need surgery, you will be much better conditioned for it and for the rehab that follows.
An avulsion fracture (a separation of the ligament and a piece of the bone from the rest of the bone) is rare in adults. But when this fracture occurs in adults, surgery may be needed to reattach the bones.
The best way to prevent anterior cruciate ligament (ACL) injuries is to stretch and strengthen the leg muscles, especially the front and back muscles of the thigh (quadriceps and hamstrings).
Other precautions that may help prevent ACL injuries include:
If you have already had an ACL injury, you can avoid another ACL injury by:
Programs to prevent ACL injuries are available. These programs generally emphasize injury awareness, avoidance techniques, and stretching, strengthening, and jumping exercises to help reduce ACL injuries.
Tips to prevent ACL injuries include practicing landing with the knees bent after jumps and crouching when pivoting and turning.
If you have an acute (sudden) anterior cruciate ligament (ACL) injury, use the following first aid steps to reduce pain and swelling:
After diagnosis of an ACL injury, your doctor may suggest exercises that help strengthen your leg and increase your range of motion. They may be the start of your non-surgical treatment program or be used to help prepare your knee for surgery. For more information, see the topic:
Medicine is used for a new anterior cruciate ligament (ACL) injury to relieve or reduce pain. Medicine also may be used for long-term (chronic) ACL deficiency and during the rehabilitation period. Non-prescription pain medicines such as acetaminophen (Tylenol, for example) or non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and naproxen, are commonly used.
NSAIDs may cause stomach upset and should be taken with food and a glass of water. They can have side effects and should not be taken with certain other medicines. Let your doctor know what other medicines you are taking. Do not give ASA to anyone younger than 20 because of the risk of Reye syndrome.
Most surgery for anterior cruciate ligament (ACL) injuries involves replacing the ACL with tissue called a graft. Usually an autograft (tendon tissue taken from another part of the body) is used. Repair is also done when the ACL has been torn from the upper or lower leg bone (avulsion). This type of injury is uncommon. In the case of an avulsion fracture, the bone fragment connected to the ACL is reattached to the bone.
Most ACL surgery is done by making small incisions in the knee and inserting instruments for surgery through these incisions (arthroscopic surgery). Open surgery (cutting a large incision in the knee) is sometimes required.
The goals of surgical treatment for anterior cruciate ligament (ACL) injuries are to:
In general, surgical techniques and rehabilitation (rehab) programs used today are successful. Most people who have ACL surgery have favourable results, with reduced pain, good knee function and stability, and a return to normal levels of activity. But some still have knee pain and instability. Athletes and those who participate in sports typically can return to their sports within months, depending on how intense and sports-focused the rehab was.
Not all ACL tears require surgery. You and your doctor will decide whether rehab only or surgery plus rehab is right for you. For more information, see the topic:
Before ACL surgery, strength and motion exercises are often done to help condition the knee for surgery and the subsequent rehabilitation program. Surgery is followed by a short period of doing home exercises, increased activity, and the use of crutches for walking. An intensive rehab program to strengthen the knee then begins. The rehab program often lasts up to a year. For more information, see the topic:
Surgery in a child might be needed to prevent injury to other structures within the knee, such as the menisci. You may consider surgery if the child's knee is very unstable doing simple daily activities, if the knee's instability cannot be controlled with non-surgical methods, if the child has both an ACL injury and a meniscus tear, or if the child is a serious athlete in sports that require running, jumping, and decelerating. Post-surgery rest and a sustained rehab program are extremely important.
The main risks of surgery in a child whose bones are still growing is slowed growth (physeal arrest), which may result in one leg being longer than another. Other risks include deformity of the thigh bone (distal femoral valgus or angular limb deformity). The risks of these conditions is lower the closer a child or teen is to skeletal maturity.
You may choose to have surgery if you:
You may choose not to have surgery if you:
Other treatment for anterior cruciate ligament (ACL) injuries includes physical rehabilitation (rehab) to:
You may choose to treat an ACL injury with rehab alone. If you have surgery, rehab will also be part of your treatment.
Physical rehabilitation (rehab) to treat ACL injuries, both with and without surgery, can be a long and rigorous program.
Physical rehab done before surgery helps your recovery after surgery.
Some people who initially choose not to have surgery eventually need to have it.
People who choose not to have ACL surgery may be less likely to return to competitive sports than those who choose to have it.
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- Honkamp NJ, et al. (2010). Anterior cruciate ligament injuries in adults. In JC DeLee et al., eds., Delee and Drez's Orthopaedic Sports Medicine: Principles and Practice, 3rd ed., vol. 2, pp. 1644–1676. Philadelphia: Saunders Elsevier.
- Shea KG, et al. (2003). Anterior cruciate ligament injury in paediatric and adolescent patients. A review of basic science and clinical research. Sports Medicine, 33(6): 455–471.
- Seroyer S, West R (2007). Anterior cruciate ligament section of Injuries specific to the female athlete. In PJ McMahon, ed., Current Diagnosis and Treatment in Sports Medicine, pp. 259–260. New York: McGraw-Hill.
Other Works Consulted
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- Biau DJ, et al. (2007). ACL reconstruction: A meta-analysis of functional scores. Clinical Orthopaedics and Related Research, 458: 180–187.
- Gilchrist J, et al. (2008). A randomized controlled trial to prevent noncontact anterior cruciate ligament injury in female collegiate soccer players. American Journal of Sports Medicine, 36(8): 1476–1483.
- Grant JA, et al. (2003). ACL reconstruction with autografts. Physician and Sportsmedicine, 31(4): 27–32, 40.
- Grudziak JS, Musahl V (2007). Anterior cruciate ligament tear section of The youth athlete. In PJ McMahon, ed., Current Diagnosis and Treatment in Sports Medicine, pp. 213–220. New York: McGraw-Hill.
- Miller SL, et al. (2002). Graft selection in anterior ligament reconstruction. Orthopedic Clinics of North America, 33(4): 675–683.
- Trees AH, et al. (2005). Exercise for treating isolated anterior cruciate ligament injuries in adults. Cochrane Database of Systematic Reviews (4).
- Trees AH, et al. (2007). Exercise for treating anterior cruciate ligament injuries in combination with collateral ligament and meniscal damage of the knee in adults. Cochrane Database of Systematic Reviews (3).
|Primary Medical Reviewer||William H. Blahd, Jr., MD, FACEP - Emergency Medicine|
|Primary Medical Reviewer||Brian D. O'Brien, MD - Internal Medicine|
|Specialist Medical Reviewer||Patrick J. McMahon, MD - Orthopedic Surgery|
|Last Revised||June 29, 2010|
Last Revised: April 29, 2012
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