Anterior Cruciate Ligament (ACL)

The anterior cruciate ligament (ACL) is one of four ligaments critical to stabilizing the knee joint. A ligament is made of tough fibrous material and functions to control excessive motion by limiting joint mobility. Of the four major ligaments of the knee, the ACL is the most frequently injured. When you have an injury to your ACL it often feels like the knee is “giving way.”

Bandage on knee from athletic invasive knee surgery, repairing ligaments

Yenwen Lu / Getty Images

ACL Function

The anterior cruciate ligament provides the primary restraint to forward motion of the shin bone (tibia). The anatomy of the knee joint is critical to understanding this relationship. The femur (thigh bone) sits on top of the tibia (shin bone), and the knee joint allows movement at the junction of these bones. Without ligaments to stabilize the knee, the joint would be unstable and prone to dislocation. The ACL prevents the tibia from sliding too far forward.

The ACL also contributes stability to other movements at the joint including the angulation and rotation at the knee joint. The ACL performs these functions by attaching to the femur on one end, and to the tibia on the other. The other major ligaments of the knee are the posterior cruciate ligament (PCL), and the medial and lateral collateral ligaments (MCL and LCL, respectively).

Cruciate Ligament

Cruciate means cross-shaped. The anterior cruciate ligament crosses the posterior cruciate ligament (PCL) to form an X, or cross. The ACL is in front of the PCL, which is why it is named anterior while the PCL is posterior, or behind it.

When looking inside your knee arthroscopically, the ACL is typically easily visualized. The PCL is often covered by soft tissue called synovium and can be more difficult to visualize without clearing away some of this normal tissue in the knee joint. ACL tears can be readily visualized although there are times when the ACL can be torn, and the remnant ligament can be scarred down in a way that makes it looked like normal ACL tissue. Usually, a careful examination of the knee and probing of this ligament can distinguish between a normally functioning ACL and a damaged ACL.

Grades of ACL Sprains

When a ligament is injured, it is called a sprain. For the ACL, it is graded from 1 to 3:

  • Grade 1: The ligament has mild damage and the knee joint is still stable.
  • Grade 2: A partial tear with the ligament stretched and damaged.
  • Grade 3: A complete tear of the ligament—the most common type of ACL injury. Typically people with complete tears of the ACL consider surgical intervention for their injury.

ACL Tears: How They Are Treated

Tears of the ACL can happen when you land a jump or make a sudden pivot, as is typical in sports such as basketball, soccer, football, and skiing. But you can also have a tear in a fall or work-related injury. Learn about causes, symptoms, treatment, and prevention for ACL tears.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American Academy of Orthopaedic Surgeons. Management of anterior cruciate ligament injuries:

    Evidence-based clinical practice guideline.

  2. American Academy of Orthopaedic Surgeons. Anterior cruciate ligament (ACL) injuries.

  3. Johns Hopkins Medicine. Anterior cruciate ligament (ACL) injury or tear.

By Jonathan Cluett, MD
Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.