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Anterior Cruciate Ligament (ACL) Injuries

What Are They, Female Predisposition, Signs, Symptoms, and More

Author:Kelsey LaFayette, DNP, ARNP, FNP-C

Editors:Ian Mannarino, MD, MBA,Alyssa Haag

Illustrator:Samantha McBundy, MFA, CMI


What is the anterior cruciate ligament (ACL?)

The anterior cruciate ligament (ACL) is a fibrous piece of connective tissue found within the knee joint. It attaches at the anteromedial intercondylar aspect of the tibial plateau and the posteromedial side of the lateral femoral condyle. The ACL is a stabilizing ligament that prevents the tibia from excessive forward movement (i.e., anterior tibial translation) and the knee joint from excessive rotational movement.  

ACL attaching at anterolateral tibial plateau and posteromedial lateral femoral condyle.

What are ACL injuries?

ACL injuries involve any type of injury that affects the integrity of the ACL and include sprains and tears. ACL sprains occur when the tissue is stretched excessively but doesn’t tear, whereas tears involve the ligament tissue rupturing and include partial and full tears. 

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What causes ACL injuries?

Both ACL sprains and tears can occur due to direct or indirect trauma involving the knee joint. Direct trauma involves the knee joint coming into contact with a physical force, like when being tackled during football. Indirect trauma involves non-contact force that can occur during a sudden change in direction, like when playing soccer; during a sudden deceleration, like when sprinting; and landing incorrectly after jumping, like when rebounding a basketball. 

ACL Injuries in Those Assigned Female at Birth

In those assigned female at birth, ACL injuries are more common and tend to occur following indirect trauma, whereas in those assigned male at birth, injuries are more commonly associated with direct trauma. The higher rate of ACL injuries seen in those assigned female at birth is thought to be multifactorial and includes anatomical and neuromuscular factors, however, no consensus has been made on the exact causes of their predisposition to ACL injuries. 

Anatomically, those assigned female at birth tend to have increased knee valgus, known as the Q angle or quadriceps angle, and a slightly smaller femoral, or intercondylar, notch that can predispose them to ACL injuries. The Q angle is the angle of two imaginary lines: one from the patellar tendon to the anterior superior iliac spine (ASIS) and the second from the tibial tubercle to the midpoint of the patella. The femur naturally slants medially from the hip joint to the knee joint, producing this angle.  In those assigned female at birth, the angle is typically between 11 and 20 degrees and in those assigned male at birth the angle is typically between 8 and 14 degrees. The larger Q angle in those assigned female at birth is thought to contribute to ACL injuries as it can increase stress on the knee joint and lead to increased foot pronation

Next, the femoral notch is a space between the lateral and medial condyles of the distal femur that houses the ACL and posterior cruciate ligament (PCL) and provides insertion points for both. Some researchers believe a narrow notch width (NWI) can predispose to ACL injuries, but the reason why this occurs is not fully understood. Researchers believe it could be in relation to the NWI encroaching on the ACL or housing a smaller ACL, which may be weaker. 

Neuromuscular factors that may contribute to ACL injuries in those assigned female at birth include altered patterns of muscle activation and decreased muscle strength in the lower extremities. Some research has shown those assigned female at birth rely more heavily on quadriceps activation during certain activities like jumping and deceleration, compared to those assigned males at birth. Normally, when the quadriceps (i.e., four muscles in the anterior thigh: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) are activated, they cause an anterior pull on the tibia that is limited by the ACL. Knee flexion, on the other hand, is able to offset some of the tibial pull and strain on the ACL that occurs during quadriceps activation. However, in those assigned female at birth with increased quadriceps activation and decreased knee flexion, the ACL can become overstretched, increasing the risk of ACL injury.

Other risk factors for ACL injuries include pelvic size, shape, and position; participation in certain activities like football, soccer, or basketball; playing on artificial turf; and using poor form during certain movements, like squatting. 

What are the signs and symptoms of an ACL injury?

Clinical manifestations of ACL injuries include:

  • Hearing a “pop” or feeling a “popping” sensation in the knee joint at the time of injury

  • Swelling of the knee joint

  • Decreased range of motion

  • Pain

  • Inability to bear weight or the feeling of knee instability

How are ACL injuries diagnosed?

Diagnosis of ACL injuries begins with a thorough history and physical examination. On exam, various musculoskeletal tests may be performed on the knee joint, like the anterior and posterior drawer tests, pivot shift test, and Lachman test. The anterior and posterior drawer tests involve pulling the tibia anteriorly or pushing the tibia posteriorly, respectively, and assessing for excessive movement. A positive anterior drawer test indicates possible ACL injury, whereas a positive posterior drawer test indicates possible PCL injury. The pivot shift test involves internally rotating the tibia, applying valgus pressure to the knee, then flexing the knee. A positive pivot shift test occurs when a “clunk” or “thunk” is heard or felt over the knee joint. Lastly, the Lachman test involves stabilizing the femur and pulling the tibia anteriorly. A positive result is concluded if there is an increase in anterior translation

Diagnosis can be confirmed with diagnostic imaging using an MRI or arthroscopy, which is surgical visualization of the joint. 

How are ACL injuries treated?

Treatment for ACL injuries involve supportive care, physical therapy, and in some cases, surgery. Supportive care can be remembered using the mnemonic RICE, which stands for rest, ice, compress, and elevate. Additional care includes avoidance of weight bearing on the affected limb and using analgesics like ibuprofen or acetaminophen for pain relief. In severe cases, surgery to repair the ligament may be necessary. With surgical intervention, it takes approximately 6 to 12 months to return to normal activity.  Physical therapy can be used and is recommended for regaining strength and mobility in all individuals who have sustained an ACL injury, regardless of type of injury or whether surgery was performed.

What are the most important facts to know about ACL injuries?

The anterior cruciate ligament (ACL) is a fibrous piece of connective tissue found within the knee joint. It attaches to the anteromedial intercondylar aspect of the tibial plateau and the posteromedial side of the lateral femoral condyle. The ACL is a stabilizing ligament meant to prevent the tibia from excessive forward movement (i.e., anterior tibial translation) and the knee joint from excessive rotational movement.  ACL injuries involve any type of injury that affects the integrity of the ACL and include sprains and tears. Risk factors for ACL injuries include being assigned female at birth; pelvic shape, size, and position; participation in certain activities; playing on artificial turf; and using improper form during certain activities. Diagnosis of ACL injuries includes history and physical examination with confirmation done via MRI or arthroscopy. Treatment includes supportive care, physical therapy, and in some cases, surgery. 

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Related links

Anatomy clinical correlates: Knee
Anatomy of the knee joint
Anatomy of the popliteal fossa

Resources for research and reference

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Cleland J, Koppenhaver S, Su J, Netter FH. Netter’s Orthopaedic Clinical Examination: An Evidence-Based Approach. Elsevier; 2022.

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