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.
What causes an ACL injury?
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, 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.
What are the symptoms?
Symptoms of an acute ACL injury include:
Feeling or hearing a pop in the knee at the time of injury.
Pain on the outside and back of the knee.
The knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the knee joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
Limited knee movement because of pain or swelling or both.
The knee feeling unstable, buckling, or giving out.
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.
How is an ACL injury diagnosed?
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, and 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.
How is it treated?
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. Be safe with medicines. Read and follow all instructions on the label.
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:
Exercises and training, also called rehab. It takes several months of rehab for your knee to get better.
Surgery. You and your doctor can decide if rehab is enough or if surgery is right for you.
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:
Make the knee stable if it is unsteady, or at least make it stable enough to do your daily activities.
Make your knee strong enough to do all the activities you used to do.
Reduce the chance that your knee will be damaged more.
How can you prevent ACL injuries?
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).
Here are other things you can do that may help prevent ACL injuries:
Avoid wearing shoes with cleats in contact sports.
Avoid wearing high-heeled shoes.
Avoid sports that involve lots of twisting and contact.
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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:
Changing direction quickly or cutting around an obstacle or another player with one foot solidly planted on the ground. (This can happen in sports that put high demand on the ACL, such as basketball, football, soccer, skiing, and gymnastics.)
Landing after a jump with a sudden slowing down, especially if the leg is straight or slightly bent (such as in basketball).
Falling off a ladder, stepping off a curb, jumping from a moderate or extreme height, stepping into a hole, or missing a step when walking down a staircase. Injuries like these tend to be caused by stopping suddenly, with the leg straight or slightly bent.
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.
Symptoms
Symptoms of a severe and sudden (acute) anterior cruciate ligament (ACL) injury include:
Feeling or hearing a "pop" in the knee at the time of injury.
Sudden instability in the knee. (The knee feels wobbly, buckles, or gives out.) This may happen after a jump or change in direction or after a direct blow to the side of the knee.
Pain on the outside and back of the knee.
Knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
Limited knee movement because of swelling and/or pain.
After an acute injury, you will almost always have to stop the activity you are doing, but you may be able to walk.
Other health problems can cause symptoms like those of an ACL injury. They include a bone break or injuries to the knee cushions (menisci) or to other ligaments in the knee.
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.
What Happens
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 for you 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.
How an anterior cruciate ligament (ACL) injury is treated and how it heals depends on:
The condition of the ACL before the injury. This includes prior injuries, partial tears, ACL deficiency, and changes due to age.
The general condition and health of the rest of your knee before this injury.
The amount of damage or injury to the ACL. Injuries are usually grouped into grade I, II, or III sprains (tears) according to the amount of damage.
Other injuries to the knee joint, such as to the cartilage or menisci, or to bones in the knee.
Your age, how active you are, and how committed you are to treatment and rehabilitation (rehab).
The time of diagnosis. If the ACL diagnosis is not made soon after the injury, the knee may be further damaged with use.
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 didn't know it or if your ACL has not been treated or has been treated unsuccessfully. ACL deficiency can cause damage to the joint, including osteoarthritis. But not everyone with an ACL injury gets ACL deficiency.
People with minor ACL injuries usually begin treatment with a physical 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). Most people return to their normal activities after a few weeks of rehab.
More serious ACL injuries may need several months of rehab or surgery followed by several months of rehab to regain your knee strength, knee stability, and range of motion.
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. It also helps prepare the knee for surgery if you choose to have it.
What Increases Your Risk
Things that increase your risk of anterior cruciate ligament (ACL) injuries include:
Playing sports that involve sudden changes in direction or cutting around other players or obstacles, such as skiing, football, soccer, basketball, baseball, and tennis.
Making accidental movements that may twist your knee. Examples include falling off a ladder, jumping from an extreme height, stepping into a hole, or missing a step on a staircase.
Losing muscle tone in legs (from aging or inactivity).
Having unbalanced leg muscle strength, such as if the muscles in the front of your thigh (quadriceps) are stronger than the muscles at the back of your thigh (hamstrings).
Previous ACL injuries, especially if your knee sometimes gives out or buckles (chronic ACL deficiency).
Call your doctor immediately if you have an injury to your knee and:
You have severe pain in your knee.
Your knee appears to be deformed.
You have signs of damage to the nerves or blood vessels. Signs include numbness, tingling, a "pins-and-needles" sensation below the injury, an inability to move your leg below the injury, pale or bluish skin, or your leg feels cold.
You have severe swelling in your knee right after the injury.
Call your doctor today if:
Your knee begins to swell within 2 hours of the injury.
You hear or feel a pop in your knee during an injury.
Your knee won't bear weight.
You are unable to straighten your leg completely.
Your knee is unstable, buckles, or gives out.
Your knee "locks" in one position.
You have had an anterior cruciate ligament (ACL) injury in the past, and you have reinjured your knee.
Before your appointment, don't 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 nonsteroidal anti-inflammatory drug, such as ibuprofen (Advil) or naproxen (Aleve), to reduce swelling. For more information on first aid steps, see Home Treatment.
Watchful waiting
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; if your knee is deformed, swells, or has limited movement immediately after an injury; or if you are unable to bear any weight because of either 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. This may reduce the chance that you will get osteoarthritis in your knee.
If surgery is considered, you may be referred to an orthopedic surgeon (possibly a sports medicine specialist) who is experienced in knee surgery.
Exams and Tests
An anterior cruciate ligament (ACL) injury is diagnosed through a medical history and a physical exam. 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:
Taking your medical history. You will be asked how you injured your knee, about your symptoms at the time of injury, whether you have had any other knee injuries, and general questions about your health.
Checking your knees for stability, strength, range of movement, swelling, and tenderness. Tests for stability include a Lachman test and a pivot shift test. The Lachman test compares the degree of looseness (laxity) in your knees.
Looking at an X-ray, which is usually done for any knee injury if there is pain, swelling, or you cannot put your weight on the leg. Although an ACL injury cannot be directly diagnosed by an X-ray, an X-ray can show whether a bone is broken, any bone fragments are in the knee, the ACL is torn from the bone along with a little piece of the bone (avulsion fracture), or blood is present in the knee (effusion).
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.
More imaging tests
Other tests that may help your doctor see how badly the knee is injured include:
An MRI. It can identify an ACL tear or other problems, such as meniscus tears or other ligament injuries.
A CT scan. It can be done to see any small breaks in the bones.
Looking at fluid in the knee
If your knee looks red, feels warm to the touch, or is very swollen, a knee joint aspiration (arthrocentesis) may be done. This involves removing fluid from the knee joint with a needle. It is done to:
Help relieve pain and pressure. This may make the physical exam easier and make you more comfortable.
Check joint fluid for possible infection or inflammation.
Look for blood, which may mean there is a tear.
Look for drops of fat, which may mean there is a broken bone.
Local anesthetic may be injected to reduce pain and make the knee easier to examine.
Other tests
Arthrometric testing: In this test, your doctor uses a tool to measure the looseness of your knee. This test is especially useful in people whose pain or size makes a physical exam difficult. An arthrometer has two sensor pads and a pressure handle that allows your doctor to put force on the knee.
Arthroscopy: This can be used to diagnose an ACL injury and as a method of surgery. It involves inserting tools through one or more small incisions in the knee, which allows 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.
Treatment Overview
The goals of treatment for an anterior cruciate ligament (ACL) injury are to:
Restore normal or almost normal stability in the knee.
Restore the level of function you had before the knee injury.
Limit loss of function in the knee.
Prevent injury or more damage to other knee structures.
Reduce pain.
You'll need to work with your doctor to decide whether you should have several months of rehabilitation (rehab) or surgery with rehab. Not all ACL tears need surgery.
Treatment right after an injury
If you know you have injured your ACL, the first treatment consists of:
Using crutches and/or splints in the first few days. If crutches or splints are used for too long, the muscles will become weaker from too little activity. Then movement of the knee will become stiff and restricted.
Strength and motion exercises to help prepare you for treatment.
When the injury occurred and how stable your knee is.
Whether other parts of the knee are injured. If they are, it will be harder for the strong parts of your knee to compensate and protect the injured parts.
Whether you had other knee problems before, such as injuries that caused long-term (chronic) ACL deficiency, or osteoarthritis.
How active you are.
Your age and overall health.
Your willingness and ability to complete a long and rigorous rehab.
Treatment options include:
Nonsurgical treatment only, such as a physical rehab program.
ACL surgery to reconstruct the ACL or to reconstruct it and repair other injuries, such as a meniscus tear.
Recovery from an ACL injury varies for each person. Your treatment should continue until your knee is stable and strong rather than for a certain length of time.
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).
You may help prevent ACL injuries if you:
Avoid wearing shoes with cleats in contact sports.
Avoid wearing high-heeled shoes.
Avoid sports that involve lots of twisting and contact.
If you have already had an ACL injury, you can avoid another one by:
Strengthening the injured knee through rehabilitation (rehab) exercises.
Changing your sports techniques to avoid motions that might stress the injured knee.
Wearing a knee brace during high-risk activities. But braces should be used only if rehab is also being done. Wearing a brace alone may be of little benefit and may give you a false sense of security.
Programs to prevent ACL injuries are available. These programs typically emphasize injury awareness, avoidance techniques, and stretching, strengthening, and jumping exercises to help reduce ACL injuries.
You can help prevent ACL injuries by practicing landing with the knees bent after jumps and crouching when pivoting and turning.
Home Treatment
If you have an acute (sudden) anterior cruciate ligament (ACL) injury, use the following first aid steps to reduce pain and swelling:
Rest and reduce your activity level. If it hurts to put weight on your knee, use crutches until you can see your doctor. Crutches can be rented from most drugstores. Crutches should not be used for long, because a lack of activity can cause muscle tissue to waste away and cause restricted movement of the knee.
Ice your knee. To avoid a freeze-burn, don't put the ice directly on your skin. Put a cloth or towel between the ice and your knee.
Elevate your knee while applying ice or anytime you are sitting or lying down.
Wrap your knee with an elastic bandage or neoprene sleeve (available at a drugstore). This may help ease pain during movement and reduce fluid inside the knee. Don't wrap your knee too tightly, as this may cause swelling below the bandage. Loosen the bandage if it is too tight. Signs of an overly tight bandage include numbness, tingling, increased pain, and coolness in the foot.
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 nonsurgical treatment program or be used to help prepare your knee for surgery.
Nonprescription pain medicines such as acetaminophen (Tylenol, for example) or nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and naproxen, are commonly used. Be safe with medicines. Read and follow all instructions on the label.
Surgery
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 surgery typically is used only in the case of an avulsion fracture (a separation of the ligament and a piece of the bone from the rest of the bone). In this case, 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 surgical tools through these incisions (arthroscopic surgery). Open surgery (cutting a large incision in the knee) is sometimes required.
Goals of surgery
The goals of surgical treatment for ACL injuries are to:
Restore normal or almost normal stability in the knee.
Restore the level of function you had before the knee injury.
Limit loss of function in the knee.
Prevent injury or degeneration to other knee structures.
Reduce pain.
Most people who have ACL surgery have favorable 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 take part in sports typically can return to their sports within months. But this may depend on how intense and sports-focused the rehab was.
Before ACL surgery, strength and motion exercises are often done to help get the knee ready for surgery and for rehab after surgery. Surgery is followed by a short period of 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.
Depending on how bad your injury is, surgery with rehab may offer the best chance of making your knee stable again. It also may help you return to an active lifestyle without further pain, injury, or loss of strength and movement in your knee.
If your injured knee gives out now and then (chronic ACL deficiency) and you continue to do activities that require a stable knee, you may injure your knee again. That may be another reason to consider surgery.
You will need to follow a rehab program whether or not you have surgery. If you don't complete a rehab program, even with surgery you may not regain full stability and function in your knee.
Protect the ACL and your knee joint from further injury.
Allow you to return to most activities that you did before the injury. If rehab is done without surgery, the knee might not be stable during some movements.
You may choose to treat an ACL injury with rehab alone. If you have surgery, rehab will also be part of your treatment.
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.
Credits
ByHealthwise Staff Primary Medical ReviewerWilliam H. Blahd, Jr., MD, FACEP - Emergency Medicine Adam Husney, MD - Family Medicine E. Gregory Thompson, MD - Internal Medicine Kathleen Romito, MD - Family Medicine Patrick J. McMahon, MD - Orthopedic Surgery, Sports Medicine Freddie H. Fu, MD - Orthopedics
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.
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