What Is an ACL? Your Knee Ligament Explained

If you’re wondering, “what is an ACL?” then you’ve come to the right place. Whether you’re here with questions about knee anatomy or an ACL injury or tear, this article covers the basics of your ACL, its strengths and weaknesses, and what happens when this part of the body gets injured and needs treatment or ACL reconstruction.

What Is an ACL? What Does ACL Stand For?

ACL stands for anterior cruciate ligament, and it’s one of the four primary ligaments in your knee. Your knee joint has three bones that come together: your femur (thigh bone), tibia (shin bone), and your patella (kneecap). Your kneecap is not unlike Captain America’s shield, there to protect the joint by sitting in front of it and guarding against injury. The other two bones are connected by an intricate system of cartilage, tendons, and ligaments, which act as sturdy ropes holding your bones in alignment and keeping your knee joint stable. The ligaments are of two different types.

Collateral Ligaments

The collateral ligaments are on the sides of the knee, and control the sideways motions of the knee, bracing it against unnatural movement (the hinge of your knee goes forward and back, not side to side, and these ligaments help hold it in that alignment). They are:

• The medial collateral ligament (MCL) located on the inside of your knee.
• The lateral collateral ligament (LCL) located on the outside of your knee.

Cruciate Ligaments

The cruciate ligaments are inside of your knee joint, and cross opposite to one another to form an X shape. These control the forward and backward movements of the knee. They are:

• The anterior cruciate ligament (ACL) crosses in front (but still behind the kneecap).
• The posterior cruciate ligament (PCL) crosses on the backside of the knee joint.

So, where is your ACL? Your anterior cruciate ligament, or ACL, runs diagonally behind the kneecap, and is in place to keep your shin bone from sliding out in front of your thigh bone (thankfully). It also provides the stability to allow knee rotation.

ACL or Anterior Cruciate Ligament Injuries

It’s likely that you are here researching ACL injuries, because an ACL tear or sprain is one of the most common knee injuries, especially in sports medicine. Athletes who participate in diverse high demand sports from football, to basketball, to soccer, to skiing and ice-skating can run a higher risk of damaging their ACL. Other risk factors include the following.

• Participating in certain sports that involve sudden knee adjustments: High-risk sports include basketball, soccer, football, gymnastics, or winter sports involving icy conditions like downhill skiing or ice-skating.
• Being female: Whether it’s due to relative muscle strength, neuromuscular control, hormonal influences, or a difference in anatomy, female athletes are statistically more likely to suffer an ACL injury.
• Insufficient conditioning: Poor conditioning could lead to weaknesses and damage during activity.
• Improper footwear: Wearing footwear that doesn’t fit properly could contribute to an injury occurrence.
• Artificial turf: Playing on artificial turf surfaces could lead to unnatural motions and increase the likelihood of injury.
• Using poorly maintained equipment: Ski bindings or other protective gear that isn’t properly fitted or maintained could lead to mishap and injury.

Causes of ACL Injury

While playing sports, it may be easy to move your knee joint in a way that inadvertently injures it, but ACL tears can also occur by accident (car crashes, stepping into an unexpected hole in the ground, etc.). The ACL could be damaged by any of the following movements or circumstances.

• Sudden direction change, or “cutting”
• Sudden stopping, or slowing down when running
• Receiving a direct collision or contact blow to the knee (as one might in a football tackle)
• Incorrectly landing after a jump
• Pivoting on a firmly planted foot

ACL injuries can occur in any sport or fitness activity that puts extra stress or strain on the knee. Damage could be anywhere from a mild form of over-stretching (which nevertheless leaves the ligament intact), a partial tear, or a complete tear of the ligament tissue. Beware of the symptoms of this injury, as it could be serious.

ACL Injury Symptoms

Many people feel or hear a “pop” or “snap” sound at the moment of injury. However, there are other signs to watch for, such as swelling or knee instability while walking (not to mention pain).

Depending on how severe the injury is, you may not need surgery, merely rest and some rehabilitation exercises to get you back on your feet. Be advised that this determination should be made by a trusted medical professional, because if you underestimate this injury and don’t seek a doctor’s advice, it could worsen and require surgery. The symptoms may include the following.

• Rapid, possibly painful swelling: Rest, ice, compress, and elevate (RICE) your knee, but if your knee swelling does not resolve within 24 hours, seek medical attention.
• A loss of full range of motion: Your knee “catches” or is otherwise inhibited.
• Instability: A sensation that your knee is “giving way.”
• Tenderness or pain: A twinge of discomfort to severe pain, which causes a limp or an inability to continue your normal activities.

It’s important that you see a doctor immediately if you suspect the signs or symptoms of an ACL injury. What could be a minor injury that first day or two might not be so minor if you aren’t aware of the injury being exacerbated by a too-soon return to normal movement.

ACL Tear and Sprain Severity

ACL tears are considered “sprains” and have a severity scale on which they are graded. The following scale describes the level of damage.

• Grade 1 sprains: Mild damage indicating the ligament has been overly stretched but is still able to perform its function and stabilize the knee joint.
• Grade 2 sprains: Partial tears fall into this category, when the ligament stretches to the point that it becomes loose.
• Grade 3 sprains: Complete tears fall under grade 3, wherein the ligament has split into two pieces and the knee joint is unstable as a result.

It is rare to find a partial tear of the ACL. Most tears are near or complete tears that will need medical intervention and possibly ACL surgery.

Diagnosis

In diagnosing your knee damage, whether it’s sports-related or otherwise, a doctor will ask for your symptoms and perform a physical examination. On comparison with the undamaged knee, most doctors will be able to determine whether you have a ligament injury at this stage.

Further imaging tests may be ordered to evaluate the state of your bones and soft tissue before deciding on treatment options. These may include an X-ray to make sure there is no broken bone involved in the injury or an MRI to assess the level of injury done to the surrounding soft tissue and ligaments.

ACL Injury Treatment Options

Those who are fortunate enough to have a grade 1 sprain may be able to heal with nonsurgical treatments. Those with more extreme tears will have to make a decision with their doctor and/or physical therapist on the best course of action, which may involve ACL reconstruction surgery.

Nonsurgical Treatment

While a partial or complete ACL tear will not heal on its own, nonsurgical treatments may still be more effective if surgery is too risky or if the knee is still stable enough to function. Nonsurgical options may include the following.

• Bracing gear: A brace or crutch might be recommended to protect the knee while it heals.
• Physical therapy: A rehabilitation program can help stretch and restrengthen your leg muscles, allowing for a quicker return to functioning without causing any more undo injury.
• NSAIDs for pain: Nonsteroidal anti-inflammatory drugs may help alleviate pain and swelling in the early days of recovery.

Surgical Treatment

Surgery is not always the best option, even in cases where surgery is the only next step. A young athlete undergoing surgery will most likely have high chances of a full recovery and a return to sporting activities, but an older, less-active person might not experience many gains from surgery, and it might not be worth the cost and discomfort.

Knee surgery for an ACL tear rarely means reattaching the ligament via suturing the two ends back together. Instead a surgical repair involves reconstruction using a tissue graft made from another tendon in the body, often the patellar tendon (which goes between your shin bone and kneecap), or from a cadaver (an allograft). This graft is the scaffolding on which a new ligament will grow into place.

An ACL or anterior cruciate ligament reconstruction is a fairly significant procedure, and that decision will have to be made personally and in consultation with a doctor or orthopedic surgeon.

Recovery, Rehabilitation, and Prevention

Torn ACL recovery time can be as long as 6 months after surgery, but recovery depends on an individual’s age, general health, and fitness level. Rehabilitation will most likely involve a physical therapist to help you regain range of motion and strength as quickly as possible. The final phase of a physical therapy plan will be tailored to your specific habits and needs, which for an athlete would involve a functional return to their sport.

Future injury prevention may involve specific gear going forward, such as specialty footwear, a regular knee brace during sporting activities, or exercises you may want to make a habit of including in your routine. Those exercises may include the following focal points.

• Leg strength: Exercises designed to not only strengthen your leg muscles, but also improve overall balance in your leg muscle strength.
• Core strength: Exercises meant to increase strength in your lower abdomen, pelvis, and hips.
• Jump technique: Exercises and training tools to help your body learn proper jump and landing form to prevent future injuries.

You may also look into exercises that avoid impacting your knees as much as possible.

All-Around ACL Knowledge

The American Academy of Orthopaedic Surgeons (AAOS) encourages you to stay informed on common knee injuries like ACL or meniscus tears, and to seek medical advice to avoid ending up with a chronically unstable knee. Your knees are literally what get you up in the morning, so take the best care of them you can!