ACL Tear

What Is an ACL Tear? Symptoms, Causes, Diagnosis, Treatment, and Prevention

Barbara Kean
By Medically Reviewed by 
Reviewed:
Medically Reviewed

An ACL tear, or torn anterior cruciate ligament, is one of the most common knee injuries.

ACL tears are common among athletes and occur more frequently among female athletes.

Ligaments connect bones to bones. As one of the four primary ligaments of the knee, the ACL connects the front top part of the shinbone to the back bottom part of the thigh bone. The ACL also keeps the shin from sliding forward, and provides rotational stability to the knee.

This ligament provides roughly 90 percent of the stability of the knee joint.

Injured ligaments are considered “sprains.” There are three grades of sprains, and in a grade 3 sprain, the ligament is torn and the joint is very loose or unstable. In most ACL injuries, the ligament is torn completely in two, and most ACL tears occur in the middle of the ligament or when the ligament is pulled off the thigh bone. These tears can’t heal without surgery.


Signs and Symptoms of ACL Tears

Signs and symptoms of a torn ACL may include the following:

Knee Popping Many people feel or hear a “popping” in their knee when they injure their ACL.

Knee Swelling A large amount of swelling usually occurs within six hours of an ACL injury, and the knee may stay swollen for several weeks after, though the swelling tends to go down dramatically after that.

Knee Pain and Discomfort After the injury, most people have to stop whatever activity they were doing. They feel pain or tenderness in the joint area, especially when they put weight on the injured leg.

If you tear an ACL, you may also feel a loss of full range of motion and an uncomfortable feeling when walking, as well as a sensation that your knee is unstable or giving in.

You may be able to continue activity just after the injury. However, more often you’re unable to continue regular activity.

Causes and Risk Factors of ACL Tears

ACL injuries tend to happen more often in people who play high-impact sports where rapid pivoting and turning are common, such as basketball, football, skiing, and soccer.

Roughly 70 percent of ACL tears happen without direct contact to the knee.

In these instances, the tears tend to happen when a person is running and quickly changes direction, suddenly twists or cuts, or if they hyperextend their knee coming down from a jump.

The rest of ACL tears result from direct contact with an object or perhaps with another player while engaged in a sport. An example might be taking a direct hit to the knee during a football tackle.

There are a number of factors that increase your risk of an ACL injury, including:

  • Being female (it may be that hormonal differences and differences in muscle strength play a role)
  • Playing soccer, football, or basketball
  • Participating in gymnastics or downhill skiing
  • Insufficient conditioning
  • Playing with worn and poorly adjusted equipment
  • Wearing improperly fitted footwear
  • Playing on artificial turf

How Is an ACL Tear Diagnosed?

To diagnose an ACL injury, a physician will examine your knee by comparing the structures of your injured knee with your non-injured knee.

In most cases, this will allow your doctor or orthopedist to determine that the ligament is injured.

When swelling makes it difficult for a healthcare provider to diagnose a tear, further tests may be recommended, including:

  • X-Rays X-rays can show if the injury is associated with a broken bone but won’t show an injury to your ACL.
  • Magnetic Resonance Imaging (MRI) An MRI scan creates a clear image of the ACL, as well as other ligaments and cartilage, and can accurately detect a complete tear.
  • Ultrasound An ultrasound may also be used to check for injuries inside the knee.

Prognosis of ACL Tears

The prognosis of an ACL tear depends on many variables, including the kind and extent of your injury and how it is treated.

In most cases, you will need reconstruction surgery to repair the injury.

For some individuals, particularly those who are not very active, the injury is not likely to interfere with their daily activities. They may choose a strengthening program over surgery.
The prognosis for partially torn ACL without surgical intervention is often very good, with the recovery and rehabilitation period usually lasting at least three months.

But some individuals with partial ACL tears may still experience instability issues even after this recovery period.
Complete ACL tears don’t have as good a prognosis without surgery. Some who’ve completely torn an ACL are unable to participate in sports that call for pivoting. Others continue to experience instability even during normal activities such as walking.

What if you do have surgery? Studies have shown that more than 90 percent of people are able to return to their normal work and fitness activities without symptoms of knee instability.

Recovery from ACL reconstruction surgery paired with rehabilitation can usually restore stability and function to your knee. Following surgery and physical therapy, you should be able to play sports again after your leg strength, balance, and coordination have returned to near normal.

Duration of ACL Tears

In general, recovery from an ACL tear generally takes about nine months, but this varies from person to person.

It may take 8 to 12 months or more before athletes can play again.

Treatment and Medication Options for ACL Tears

Treatment for an ACL tear can include rehabilitation, followed by surgery.

Whether surgery is required to repair a tear in the anterior cruciate ligament depends on a variety of factors including:

  • Your activity level and expectations of how active you want to be
  • The type of work you do
  • Whether or not cartilage, ligaments, and other parts of the knee are also injured

Some people who are elderly or inactive may choose not to have surgery, if they can return to limited activity after rehabilitation, or by wearing a brace.

Younger people who want to maintain an active lifestyle or return to competitive sports are more likely to opt for surgery.

People who decide not to have reconstructive surgery may experience further injury to the unstable knee joint.

Reconstruction surgery is the most common technique used to treat a torn ACL. During this surgery, the torn ACL is replaced with a new ligament taken from a donor or from one of your other tendons around the knee. (Donor ligaments come from those who are no longer alive but chose, when living, to give their body to help others.)

Once a graft is chosen, the surgeon will perform arthroscopic surgery, making small incisions in the leg and inserting a pencil-shaped instrument that holds a camera called an arthroscope, as well as other tools. As time goes on, the new graft becomes a living ligament in your knee.

Physical therapy is an important part of recovery that includes stretching, balancing, and strength-training exercises.

After surgery, you’ll most likely participate in a physical therapy program for 6 to 12 months, depending on your tear and the level of activity you want to reach.

If you’re an athlete, rehabilitation may take longer to get you in physical condition to participate in your sport or activity.

The first part of physical therapy will focus on returning proper range of motion to the ACL joint and the muscles that surround it, including the quadriceps and hamstrings.

Once this is achieved, you’ll be given a strengthening program that focuses on increasing stress across the ligament.

In the last phase of rehabilitation, your physical therapist will assess which activities, if any, still create pain, discomfort, or instability in your knee joint, and will establish ways to work around such activities. It’s important to know that early return to play after ACL repair has been associated with an increased risk of re-tears and repeat injuries.

Related: Telemedicine for Physical Therapy: It Works! 

Prevention of ACL Tears

While some ACL injuries are not preventable, the following may help minimize your risk for future tears:

  • Strength training to develop strong thigh and hamstring muscles
  • Maintaining a stretching routine with your legs
  • Thoroughly warming up before playing sports, including jogging backward as a part of a warm-up routine
  • Learning to land on the balls of your feet rather than flat-footed
ACL injury prevention programs that often focus on strength and plyometric (jump training) exercises have proven to help protect against injury. A review and meta-analysis published in 2020 found that ACL injury prevention programs reduced the rates of ACL injury by 53 percent.

Complications of ACL Tears

People who experience an ACL injury are at higher risk of developing osteoarthritis, more specifically referred to as post-traumatic osteoarthritis (PTOA) in the knee.

The incidence of PTOA following ACL injury may be as high as 87 percent.

The way to prevent PTOA, which accounts for 12 percent of all cases of symptomatic osteoarthritis, remains unclear.
Multiple factors likely influence the risk of arthritis, such as age, the time between injury and surgery, and physical activity level.

Arthritis may occur even if you have surgery to reconstruct the ligament.
In addition, about half of ACL injuries occur in combination with damage to the meniscus, articular cartilage or other ligaments.

And about 65 percent of all patients with a torn ACL will eventually develop a torn meniscus (the protective cartilage that reduces friction between the bones of the knee).

Researchers believe this may also play a part in early onset arthritis.
ACL reconstruction surgery can also have potential complications. Most rarely occur. They include:

  • Infection
  • Bleeding
  • Numbness
  • Viral transmission
  • Blood clot
  • Recurrent instability
  • Knee stiffness
  • Bone growth problems in young children and adolescents (ACL surgery is sometimes delayed until a child’s bones have fully matured)
  • Kneecap pain

Research and Statistics: How Common Are ACL Tears?

It’s estimated that around 250,000 ACL injuries occur per year in the United States.

Females are 2 to 8 times more likely to suffer an ACL injury than males.

Approximately 400,000 anterior cruciate ligament reconstructions are performed each year in the United States.

Female athletes participating in contact sports like soccer, basketball, and field hockey sustained ACL injuries at 3 times the rate of male athletes in these same sports.

In addition, the highest rates of ACL injury in females resulted from gymnastics, followed by soccer and basketball.

Some of the highest rates of ACL injury in male athletes occur as the result of participating in collision sports like football or wrestling.

The higher rates among females could be due to differences in physical conditioning and muscular strength. Other possible factors include differences in pelvis and leg alignment, increased looseness in ligaments, and the effects of estrogen on ligaments.

Related Conditions and Causes of ACL Tears

As mentioned above, about half of ACL injuries occur in combination with damage to the meniscus, articular cartilage, or other ligaments.

And about 65 percent of all patients with a torn ACL will eventually develop a torn meniscus.

Resources We Love

American Orthopaedic Society for Sports Medicine (AOSSM)

This sports medicine organization’s members include orthopedic surgeons and other professionals who work to promote advances in the science and practice of sports medicine. On the organization’s Stop Sports Injuries site, you can find valuable information about ways to prevent sports injuries as well as links to tools to help you find sports medicine specialists, orthopedic surgeons, pediatric orthopedic specialists, and physical therapists.

Arthritis Foundation

A great resource for finding out more about osteoarthritis, a complication of ACL tear, including the latest treatments and therapies.

The Arthroscopy Association of North America (AANA)

AANA is an organization of more than 5,000 orthopedic surgeons and medical professionals who seek to advance the field of minimally-invasive orthopedic surgery to improve patient outcomes. If you are contemplating surgery for your ACL tear, the site can provide you with valuable information about the surgery potential risks and recovery. Their Find A Doctor tool allows you to search for board certified surgeons in your area.

OrthoInfo

This site from the American Academy of Orthopaedic Surgeons (AAOS) is a great resource for articles, videos, and resources related to bone and joint health.

Editorial Sources and Fact-Checking

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