I. Understanding the Anterior Cruciate Ligament (ACL)
The anterior cruciate ligament (ACL) is one of the four main ligaments in the knee joint, located in the center of the knee. It originates from the posterior aspect of the lateral intercondylar fossa of the femur and attaches anteriorly, distally, and medially to the anterior part of the intercondylar spine of the tibial plateau. Together with the posterior cruciate ligament (PCL), it forms the cruciate ligaments of the knee, crossing in an “X” shape at the front of the knee joint.
The core function of the ACL is to prevent excessive anterior displacement of the tibia relative to the femur, while also providing rotational stability to the knee joint. In short, the ACL acts as a “safety lock” for maintaining the dynamic stability of the knee joint—it constantly restricts the range of motion of the knee joint during running, jumping, sudden stops, and changes of direction, preventing abnormal joint displacement.
II. Epidemiology: A Number Not to Be Ignored
ACL injuries are among the most common orthopedic sports injuries. According to a long-term population-based study in the United States, the annual incidence of ACL injuries is approximately 68.6 per 100,000 person-years.
In the United States, approximately 250,000 ACL injuries occur annually, with over 175,000 requiring ACL reconstruction surgery. Other literature estimates between 100,000 and 200,000 ACL tears annually in the US. The incidence of ACL injury in the general population is approximately 1 in 3,500, equivalent to about 95,000 new ACL ruptures each year.
In terms of age distribution, athletes aged 15 to 25 account for over 50% of ACL injuries. A survey of active athletes in my country found an overall incidence of ACL rupture of 0.43%; among professional female athletes, the incidence was 0.71%, and for males, it was 0.29%, with women approximately 2.37 times more likely to be affected than men. The Institute of Sports Medicine at Peking University Third Hospital currently performs over 1,400 ACL reconstruction surgeries annually.
III. Why do ACL injuries occur?
Over 70% of ACL injuries are caused by non-contact forces. Typical scenarios include: sudden deceleration and change of direction (crossover), turning while the foot is fixed, improper landing posture, and sudden stops. Domestic data shows that 78% of ACL injuries are non-contact injuries.
Specifically, common mechanisms of ACL injury include:
Sudden stop and change of direction: Sudden deceleration and change of direction during movement.
Single-leg landing: Landing with the entire foot to stabilize the tibia, resulting in valgus displacement when the knee is in a small flexion position.
Knee hyperextension: Excessive extension of the knee joint, causing excessive stress on the ACL.
Direct impact: Direct force to the knee joint from the side (such as a tackle in soccer).
ACL injuries are rare. Approximately 50% of ACL injury patients also have meniscus tears; more than two-thirds of complete ACL ruptures are accompanied by meniscus and/or articular cartilage damage. In acute injuries, the lateral meniscus is more commonly involved; in chronic ACL injuries, medial meniscus injuries are more common.
IV. Symptoms and Diagnosis When an ACL injury occurs, patients often hear or feel a “pop” sound within the knee joint. Subsequent symptoms include:
Severe pain, inability to continue activity
Rapid swelling of the knee joint (usually occurring within minutes to 3 hours after injury, primarily due to effusion)
Limited range of motion
A feeling of “giving way” or instability in the knee when bearing weight
It is noteworthy that 70% of acute knee injuries with effusion also involve ACL injury.
For diagnosis, the Lachman test is the most effective method for examining ACL injuries. MRI has an accuracy rate of up to 95%, clearly showing ligament damage and simultaneously assessing accompanying meniscus and cartilage damage.
V. Treatment: Conservative or Surgical?
Treatment plans for ACL injuries need to be individualized based on the patient’s age, activity level, and injury severity.
Conservative treatment is suitable for patients with milder injuries (Grade I) or lower daily activity requirements (e.g., International Documentation Committee on Knee Classification III-IV). Treatment includes protection, rest, ice application, compression bandages, elevation of the affected limb (RICE principle), and systemic physical therapy.
Surgical treatment—ACL reconstruction—is the preferred option for patients with complete ACL rupture or those wishing to return to high-intensity sports. The surgery typically employs arthroscopic techniques, using tendons from other parts of the patient’s body (autologous hamstring tendons or bone-patellar tendon-bone) or allogeneic tendons to replace the damaged ligament. The timing of surgery is generally recommended within 3 months of injury to reduce the risk of secondary cartilage and meniscus damage.
In terms of long-term outcomes, the success rate of ACL reconstruction is between 75% and 95%. However, less than 50% of patients can return to normal athletic performance within one year post-surgery. For professional soccer players, over 92% can return to the field after surgery, and approximately 80% can recover to their pre-injury level.
VI. Long-term effects: More than just a ligament problem
The consequences of ACL injury extend far beyond the ligament itself. Regardless of whether surgery is performed, patients with ACL injuries have a significantly increased risk of developing knee osteoarthritis. Approximately 50% of patients with ACL injuries will develop varying degrees of osteoarthritis within 10 to 20 years of the initial injury. In the long term, the odds ratio for total knee replacement due to osteoarthritis in patients with ACL injuries has increased nearly sevenfold.
VII. Prevention: The Best Treatment
Neuromuscular training has been proven to effectively reduce the risk of ACL injuries, especially for female athletes. Training includes strengthening core muscles (hips, pelvis, lower abdomen), improving biomechanical patterns during lower limb landing, and strengthening hamstrings. Studies have shown that neuromuscular training several times a week for more than six weeks can significantly reduce non-contact ACL injuries in female athletes.
It is important to note that preventative knee braces do not prevent ACL injuries—true protection comes from scientific training and correct movement patterns.
Anterior cruciate ligament (ACL) injury is one of the most concerning injuries in sports medicine. It commonly affects young, active individuals, has far-reaching effects, and requires a long treatment period. Understanding the structure, injury mechanism, and prevention of the ACL is of significant practical importance for everyone who loves sports.