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ACL Injury (Anterior Cruciate Ligament) What to Know

ACL Injury (Anterior Cruciate Ligament) What to Know

An ACL injury is a tear or sprain of the anterior cruciate (KROO-she-ate) ligament (ACL) — one of the strong bands of tissue that help connect your thigh bone (femur) to your shinbone (tibia Injuries to the ACL are relatively common knee injuries among athletes. They occur most frequently in those who play sports involving pivoting (e.g. football, basketball, netball, soccer, European team handball, gymnastics, downhill skiing)..

Athletes who participate in high demand sports like soccer, football, and basketball are more likely to injure their anterior cruciate ligaments.

One of the most common ways people hurt their knees is by injuring their ACL (anterior cruciate ligament). This is one of the bands of tissue that holds the bones together within your knee.

What Is The ACL (Anterior Cruciate Ligament)

The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee. In general, the incidence of ACL injury is higher in people who participate in high-risk sports, such as basketball, football, skiing, and soccer.

Approximately half of ACL injuries occur in combination with damage to the meniscus, articular cartilage, or other ligaments. Additionally, patients may have bruises of the bone beneath the cartilage surface. These may be seen on a magnetic resonance imaging (MRI) scan and may indicate injury to the overlying articular cartilage.

The knee is a hinge joint held together by four ligaments. A ligament is a structure in the knee that holds the bones together and helps to control joint movement or motion. There is a ligament on each side of the knee (the collateral ligaments) and two ligaments deep inside the knee. The two ligaments inside the knee that “cross” each other are called the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). Both ligaments attach on one side to the end of the thighbone (femur) and on the other to the top of the shinbone (tibia).

During activity, the ACL controls how far forward the tibia can “slide” relative to the femur: it essentially acts to prevent too much forward movement. While some degree of motion or sliding is normal and is required for knee function, too much motion may damage other structures in the knee which can lead to long term problems in some patients.

ACL Injury How Does It Happen?

Athletes often get ACL injuries when they stop and quickly change directions while they’re running. People who play soccer, football, tennis, basketball or volleyball, or who do gymnastics are more likely to twist their knees by mistake when they compete than, say, cross-country runners, who simply move forward at a steady pace. Your speed — combined with the way that you twist or turn your knee — makes it likely that you’ll stretch or tear your ACL.

ACL injuries are more common among women than men.

Types of ACL Injuries

There are several conditions that cause varying levels of anterior cruciate ligament (ACL) instability. These include: sprains, avulsion fractures, ACL deficiencies, and complex ligament injuries. More than half of all types of ACL injuries occur with associated damage to another part of the knee – most commonly, another ligament, articular cartilage (bone lining), or meniscus (cushion pad). Types of ACL injuries can be diagnosed by a thorough examination, X-rays, and magnetic resonance imaging (MRI).

ACL Sprains

A sprain occurs when the fibers or threads of the ligament are stretched, partially torn, or in severe cases, completely ruptured. ACL sprains are classified by a grade as follows:

Grade I ACL Sprain

Grade I sprains are considered the most mild of the different types of ACL injuries. While patients will experience symptoms, they can usually be treated without surgery. This type of ACL sprain occurs when the fibers of the ligament are stretched, but a tear does not exist. For the most part, the knee will remain stable. Symptoms of a Grade I sprain include tenderness, swelling, moderate knee pain, and some limit to mobility. A period of rest, along with anti-inflammatory medications, crutches, and ice can usually remedy this type of injury.

Grade II ACL Sprain

A Grade II sprain refers to an ACL injury where the fibers of the ligament are partially torn. This particular injury occurs occasionally, but in most cases the tears will be complete. The same symptoms apply as a Grade I sprain, however they tend to be more severe. The difference is in the instability, because some fibers of the ligament are actually torn, and the joint may feel unstable or as if it will give out during activity. In other cases, the joint will feel stable, so not all Grade II sprains will require surgery.

Reconstruction of a partially torn ACL will depend on the patient’s age, activity level, and sports or fitness goals, and will primarily depend on the patient’s symptoms. In cases of instability, partial, or complete, ACL reconstruction is typically recommended.

Grade III ACL Sprain

Grade III sprains are the most common types of ACL injuries among athletes. This occurs when the fibers of the ligament are completely torn. This is often referred to as a rupture, meaning the ligament has completely torn apart into two sections. Symptoms of a Grade III sprain are frequently more severe. Swelling and tenderness may be immediate, knee pain can be severe, and stiffness may result.

Rarely, a complete ACL tear can occur without pain, swelling, or stiffness Grade III sprains almost always leave the knee unstable. For this particular grade of injury, reconstructive surgery is usually recommended, once full extension is achieved and the swelling decreases.

ACL Avulsion Fracture

ACL avulsion fracture is more rare than an ACL sprain, and occurs when the ACL tears by breaking a piece of bone off from where the ligament attaches to the thigh or leg. In most cases, it’s the tibia (leg bone) that is affected—meaning, the ACL rips by breaking (fracturing) a piece of bone from where it attaches to the shinbone. This is usually the result of excessive overuse and muscular contraction during sports. Direct trauma through a blunt force or hit can also cause this type of injury. ACL avulsion fractures are more common in children than in adults, but can occur in adults, and are quite common in skiers.

ACL Deficient Knee

Knees that do not have an anterior cruciate ligament, are in most cases unstable, and instability results in unwanted, ongoing symptoms for patients. The most prominent symptom is knee buckling, which will be felt during running and cutting activities, as well as walking down stairs, and sometimes during other everyday activities. In other cases, there is a sensation that the knee will buckle if the patient is not careful and attentive. While patients can live with ACL deficiency by decreasing activity, this can lead to meniscus tearing, cartilage damage, and eventually, the onset and progression of osteoarthritis.

These additional knee injuries are due to buckling, the sensation of buckling, or even as a result of micro-instability. Therefore, surgery is usually recommended for younger patients, active patients, or patients who live in areas or have occupations which require climbing or walking on uneven ground.

Complex and Multi-Ligament Knee Injuries

Sometimes, in conjunction with an ACL injury, other damage may exist within the knee. The ACL may become injured along with meniscus damage (which occurs in the vast majority of cases), or with damage to cartilage. Damage may also occur to the ACL plus another ligament. Multi-ligament damage usually occurs when a traumatic event is severe. If it is suspected that the ACL is injured along with another knee structure, a thorough exam, followed by an X-ray and an MRI is required.

When multiple knee ligament injuries occur, sometimes, a knee dislocation or fracture can also be present. In order to correct the overlapping injuries, your physician will need to perform a multi-ligament reconstruction surgery to repair damaged ligaments, as well as use a graft to replace ligaments and tendons that cannot be saved.

What Are the Symptoms?

Many people hear a popping noise in their knee when they get hurt. But it doesn’t happen to everyone. More common symptoms include:

Pain. If you have a minor injury, you may not feel pain. You may feel sore along your knee’s joint line. Some people have trouble standing or putting pressure on the hurt leg.

Swelling. This is most likely to happen during the first 24 hours. You can reduce swelling by putting ice on your knee and elevating (raising) your leg by propping it up on a pillow.

Trouble walking. If you’re able to put pressure on your hurt leg, you may notice that it’s harder than normal to walk. Some people find that the knee joint feels looser than it should.

Less range of motion. After you damage your ACL, it’s very likely that you won’t be able to bend and flex your knee like you normally would.


It is estimated that the majority of ACL injuries occur through non-contact mechanisms, while a smaller percent result from direct contact with another player or object.

The mechanism of injury is often associated with deceleration coupled with cutting, pivoting or sidestepping maneuvers, awkward landings or “out of control” play.

Several studies have shown that female athletes have a higher incidence of ACL injury than male athletes in certain sports. It has been proposed that this is due to differences in physical conditioning, muscular strength, and neuromuscular control. Other hypothesized causes of this gender-related difference in ACL injury rates include pelvis and lower extremity (leg) alignment, increased ligamentous laxity, and the effects of estrogen on ligament properties.

How Is An ACL Injury Diagnosed?

An ACL tear can be diagnosed by a physician through a history and physical examination. On physical examination, the physician can specifically assess the amount of motion present and determine if the ACL is torn. Additionally, evaluation of other structures within the knee is done also, as ACL tears are often found in association with injury to other structures within the knee, such as the cartilage and collateral ligaments.

X-rays are taken to evaluate for the presence of any fractures. In many patients, an MRI scan of the knee may be ordered. The scan can clarify the question of an ACL tear if the history and examination are inconclusive. The scan is also useful for evaluating the cartilage or meniscus tissue in the knee if this information is necessary to make decisions regarding the best treatment for a specific patient.

What Is The Treatment For An ACL Injury Or Tear?

Treatment options are based on the patient’s symptoms, examination, the growth remaining in his or her growth plates, type of injury to the ligament, and the type of sports and activity goals. Learn more about ACL tear treatment.


Nonsurgical treatment is most appropriate for grade 1 injuries. This would include immobilization or bracing, physical therapy, and a gradual progression back to regular activities and sports.


Surgical treatment is recommended for individuals with a grade 3 or complete ACL tear. Surgical options may vary based on the type of ACL injury, whether the patient has open or closed growth plates, and the type.


Proper training and exercise can help reduce the risk of ACL injury. A sports medicine physician, 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 to strengthen the core — including the hips, pelvis and lower abdomen — with a goal of training athletes to avoid moving the knee inward during a squat
  • Exercises that strengthen leg muscles, particularly hamstring exercises, to ensure an overall balance in leg muscle strength
  • Training and exercise emphasizing proper technique and knee position when jumping and landing from jumps
  • Training to improve technique when performing pivoting and cutting movements

Training to strengthen muscles of the legs, hips and core — as well as training to improve jumping and landing techniques and to prevent inward movement of the knee — may help to reduce the higher ACL injury risk in female athletes.

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