Orthopedics Hip & Knee ACL Injury Anterior Cruciate Ligament (ACL) By Jonathan Cluett, MD twitter linkedin Jonathan Cluett, MD, is a board-certified orthopedic surgeon with subspecialty training in sports medicine and arthroscopic surgery. Learn about our editorial process Jonathan Cluett, MD Medically reviewed by Medically reviewed by Stuart Hershman, MD on October 23, 2020 linkedin Stuart Hershman, MD, is a board-certified spine surgeon. He specializes in spinal deformity and complex spinal reconstruction. Learn about our Medical Review Board Stuart Hershman, MD on October 23, 2020 Print 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 out." 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. 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 ACL injury: A grade 1 injury is a sprain. The ligament has mild damage and the knee joint is still stable. Grade 2 ACL injury: A grade 2 ACL sprain is a partial tear with the ligament stretched and damaged. Grade 3 ACL injury: A grade 3 sprain is a complete tear of the ligament and it is the most common type of ACL injury. Typically people with complete tears of the ACL consider surgical intervention for their injury. ACL Tears - How to Treat a Torn Anterior Cruciate Ligament 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. Causes and symptoms of ACL tears: While ACL tears are common in sports, they happen most often without contact. How it is decided to have ACL surgery: Pros and cons of having ACL surgery. ACL Surgery: Learn about the timing of surgery, the different types of grafts used in ACL reconstruction, recovery from surgery and rehabilitation, and return to sports. Post-surgical rehab after ACL surgery: See the timeline from the first days after surgery through physical therapy and return to sports. Physical therapy after an ACL tear: Learn about the physical therapy goals and what ACL therapy protocols will be used after ACL surgery. ACL tears in children: The ACL attaches at the growth plate for the leg bones, making it a riskier choice to do ACL reconstruction if the child is still growing. Learn more about this dilemma. ACL tears in women: Women athletes are at a higher risk for ACL tears. Learn about why this risk is increased and what can be done to prevent them. Preventing ACL tears with neuromuscular training programs: Both children and adult athletes can reduce their risks of ACL tears with specific training programs. Was this page helpful? Thanks for your feedback! Dealing with joint pain can cause major disruptions to your day. Sign up and learn how to better take care of your body. Click below and just hit send! Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article 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. Anterior Cruciate Ligament (ACL) Injuries, American Orthopaedic Society for Sports Medicine, March, 2014.