Explore ACL Injuries

What is the Anterior Cruciate Ligament (ACL)?

One of the four principle ligaments in the knee, the anterior cruciate ligament (ACL) is a tough band of fibrous tissue, similar to a rope, connecting bones together at a joint.

Two ligaments on the sides of the knee (collateral ligaments) provide stability during sideways motions: the medial collateral ligament (MCL) on the inner side and the lateral collateral ligament (LCL) on the outer side. Two ligaments cross in the center (cruciate) of the knee joint. The forward ligament is the anterior cruciate ligament (ACL) and the ligament behind is the posterior cruciate ligament (PCL). The ACL ensures the tibia doesn't slide forward, allowing pivoting, twisting and jumping. The PCL prevents the tibia from moving backward relative to the femur.

What is an ACL Injury?

During physical activity, the ACL helps stabilize the knee. An ACL injury can range from a minor strain to a full ACL tear of the ligament, resulting in pain, swelling and instability of the knee.

Types of ACL injuries

ACL injuries are graded at one of three levels:

  • Grade 1: This ACL sprain is a mild stretching of the ligament, but the ACL remains intact and still offers knee stability.
  • Grade 2: The stretching of the ACL ligament that results in a partial tear. This is the least common of the three grades.
  • Grade 3: A complete tear of the ACL. In this situation, the ACL is in two pieces and can no longer stabilize the knee.

Are ACL Injuries More Common in Females?

Studies indicate that females injure their ACL at a higher rate than males. A variety of factors can be the reason for this, including the unique design of a female's pelvis and leg alignment, looser ligaments in females, the effect of estrogen on ligaments, leg muscle strength and neuromuscular control.

Discover how our Women’s Sports Medicine Program can help prevent ACL injuries in female athletes.

What causes an ACL injury?

An ACL injury is often caused by an acute injury, typically seen during sports. When athletes twist, pivot and jump, they rely on the ACL to stabilize their movements. An injury can occur if a person suffers an impact to the knee during the motion or if too much stress is placed on the ACL.

Common causes of ACL injuries include:

  • Quick change of direction
  • Sudden stops
  • Landing jumps incorrectly
  • Collisions or impacts

How can the ACL tear?

The most common cause of an ACL tear is a combination of a sudden stopping motion on the leg while quickly twisting at the knee. The planted foot remains stationary while the body rotates toward the opposite side of the body. This can happen in a sport like basketball when a player lands on the leg while coming down from a rebound or runs down the court and makes an abrupt stop to pivot.

ACL Injury Symptoms

Patients typically feel the moment an ACL injury occurs. Common symptoms of an ACL injury happen both during an acute injury and following the injury.

  • Hearing or feeling a popping in the knee
  • Knee instability or the knee giving out
  • Pain in the knee
  • Swelling
  • Tenderness around the knee
  • Loss of range of motion

What does a torn ACL feel like?

When the ACL tears, the person feels the knee go out of the joint and often hears or feels a "pop." Signs of a torn ACL include instability when trying to stand on the leg, excessive swelling within two hours, growing severity of pain and difficulty walking.

Can a torn ACL be painless?

If no structure other than the ACL is injured, the knee may regain range of motion and be painless after six or eight weeks. The knee can then feel completely normal during regular daily activity. However, it can be a "trick knee." If a knee does not have an ACL, it can give way or be unstable when the person pivots or changes direction. Patients can usually run straight ahead and ride a bicycle without difficulty, but when making a quick turn away from the planted foot, the knee gives way and collapses. This abnormal motion can damage the menisci or articular cartilage and cause further knee problems.

If a person does not participate in pivoting sports and is relatively inactive, the knee can feel quite normal without an ACL. Many patients, especially those over the age of 30 not involved in sports, may elect not to have the ACL reconstructed.

In young, athletic patients, the knee will tend to give way during activities requiring quick changes of direction. Each time the knee gives way, the menisci and articular cartilage are exposed to abnormal stresses. Damage to the joint surfaces and menisci can result in early arthritis.

What other knee structures can be injured when the ACL tears?

There are two types of cartilage in the knee. One type of fibrous cartilage is called the meniscus. Each knee has two crescent-shaped menisci that act as shock absorbers between the femur and tibia. There is a medial (inner) and lateral (outer) meniscus. The menisci are attached to the tibia. The ACL prevents excessive forward tibial motion at the knee joint. When the ACL tears, the knee can partially dislocate (subluxate). Either meniscus can become compressed between the femur and tibia, resulting in meniscus tears. This abnormal motion of the joint can also bruise the bones.

The second type of cartilage in the knee joint is called articular cartilage. This joint cartilage is the hard white tissue that is seen at the end of a chicken bone. It is a smooth, white glistening surface that covers the ends of the bones. The surface is slick and much smoother than ice, and as a result, there is very little friction when the joint moves. This joint cartilage can get damaged when the ACL tears. If this articular cartilage is injured, the joint no longer moves smoothly. Stiffness, pain, swelling and grinding can occur. Eventually, arthritis can develop.

With severe contact injuries, such as trauma from motor vehicle accidents, other ligaments can also be injured. The medial collateral ligament tears when the knee is hit from the outside. The lateral collateral ligament is torn when the knee is hit from the inside. When multiple ligaments are torn, the knee may also have injuries to the nerves and blood vessels.

ACL Injury Diagnosis and Treatments

The initial treatment of an ACL injury includes ice and gentle compression to control swelling, along with rest. Patients may need to use a brace or crutches to keep weight off the knee.

Consult a healthcare provider for a proper diagnosis of a suspected ACL injury. Trained professionals will run a physical exam to test the stability of the ACL. In some cases, X-rays can rule out any bone damage, and an MRI can provide detail on the extent of a potential ACL tear.

Nonsurgical Treatment: Do all ACL tears need surgery?

Not every ACL tear requires surgery. For patients over 30 who aren't active in sports, good knee function can be achieved without surgery. Patients younger than 25 years old, regardless of activity level, tend to have problems with instability and have frequent episodes of the knee giving way, leading to the need for ACL surgery.

Usually, the best treatment following acute ACL injury is to protect the joint, apply ice and use crutches for several weeks. As the swelling and pain subside, the patient can put weight on the leg and reduce the use of a brace or crutches. Patients then start therapeutic exercises to regain motion. Resistive exercises to build up strength should not be done during this time to prevent damaging the kneecap (patella).

Surgery for ACL Tears

Surgical reconstruction of a torn ACL is usually recommended for patients younger than 25. It is best to wait for the pain and swelling to subside and allow associated injuries to heal before performing surgery for the ACL. If surgery is done too soon after injury, rehabilitation is difficult, and the knee may get stiff and have permanent loss of motion. The athlete can usually get back to sports much more quickly if the knee is allowed to recover from the initial injury. Surgery can be performed once a painless full range of motion is regained.

If the knee also has an injured medial collateral ligament (MCL), it is best to allow the MCL to heal completely (usually six to eight weeks) before reconstructing the ACL. The torn MCL usually does not need to be repaired surgically.

There may be instances when a surgeon wants to perform surgery immediately following an injury, such as with a knee dislocation or when multiple ligaments are torn.

Surgical Options: There are different surgical options to repair a torn ACL. You and your doctor can discuss which is best for your unique case. The options include one of four ACL grafts — Allograft Graft, Bone-Patellar Tendon-Bone Graft, Hamstring Graft or Quadriceps Tendon Graft — or an Anatomic Footprint ACL Reconstruction.

If you have had surgery to repair a torn ACL and think you might need revision ACL surgery, our Sports Medicine surgeons consult on and perform revision ACL surgery frequently.

Rehabilitation after an ACL injury

Rehabilitation is essential to proper recovery following an ACL injury, both for patients who underwent ACL surgery and those who did not. A full physical therapy plan can help rebuild muscle around the knee and ensure a full of range of motion of the ACL and knee.

Immediately following ACL reconstruction surgery, rehabilitation works on improving motion and knee mobility. Therapy then expands to include strengthening leg muscles to protect the repaired ligament. The ACL rehabilitation process, which can last up to one year, steadily builds upon strength exercises and agility to allow the patient to return to their chosen activities or sports. Learn more about the Mass General ACL rehabilitation protocol.

How to prevent ACL injuries

Athletes may never be able to eliminate all injuries, but the following steps can help decrease the risk of an ACL injury:

  • Strengthen muscles in the leg and core evenly, but especially focus on the hamstrings and quadriceps
  • Improve muscle flexibility
  • Warm-up and stretch properly before intense activities
  • Don't work out or engage in sports if overly fatigued
  • Use proper technique in your sports movements and work on improving your balance

ACL Injuries in Sports

Athletes experience the most ACL injuries due to the types of movements they perform frequently. A sport like basketball can put players at risk because of the pivoting, jumping and twisting required. Other sports have additional complications that can impact the ACL.

In football, soccer or lacrosse, cleats prevent the foot from slipping. As a result, all the stress is transferred to the ACL. When this stress exceeds the strength of the ACL, the ligament tears.

In skiing, the ACL is commonly injured when the skier sits back while falling. The modern ski boot is stiff, high and tilted forward. The boot holds the tibia forward, and the weight of the body can quickly shift backward. When the body rotates away from the weight-bearing ski, the knee is twisted, and the ski prevents the foot from moving, causing all the stress to be absorbed by the ACL. When force exceeds the strength of the ligament, it fails (tears).


When should I see a doctor for my ACL pain?

See your doctor if an injury caused the knee pain or you're experiencing swelling, redness or tenderness of the knee. Mass General Brigham Sports Medicine specialists diagnose and treat a full spectrum of knee injuries, including torn ACL, PCL, MCL, LCL, and multi-ligament knee injuries. Request an appointment or more information today.

How long does it take to heal after ACL surgery?

Physical therapy often begins within days following ACL reconstruction surgery. The recovery timeline can last around nine months, sometimes longer to return to specific sports activities.

Are ACL injuries serious for younger people?

The younger the patient with an ACL injury, the more likely they will develop additional knee problems if left untreated. Long-term effects can include damage to other ligaments and an early onset of arthritis.

What should I not do if I think my ACL is injured?

You should not engage in activities that require twisting, jumping, pivoting or deep knee bends if you suspect you've injured your ACL. Rest and brace the ACL and have it examined by a healthcare provider before returning to activities.

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