ACL Injury

Overview

The anterior cruciate ligament (ACL) is one of two ligaments that cross inside the knee, connecting the thighbone to the shinbone. It is a key stabilizer that keeps the knee from sliding too far forward and helps control twisting and pivoting movements. An ACL injury is a sprain or tear of this ligament.

ACL injuries are among the most common serious knee injuries, especially in sports that involve sudden stops, jumps, and changes of direction such as soccer, basketball, and skiing. Injuries range from a mild overstretch to a complete tear. A torn ACL does not heal back together on its own, so treatment focuses on restoring knee stability and function, with or without surgery depending on the person's needs.

Symptoms

An ACL injury often happens suddenly during activity. Typical signs include:

• A loud pop or popping sensation in the knee at the moment of injury.
• Rapid swelling, usually within a few hours.
• Severe pain and difficulty continuing the activity.
• A feeling that the knee is unstable or gives way, especially with pivoting.
• Loss of full range of motion.
• Tenderness around the knee and trouble bearing weight.

Some people have a milder partial tear with less dramatic symptoms, but instability with twisting movements is a common clue that the ACL is involved.

Causes

ACL injuries usually result from forces that twist or overstretch the knee. Common mechanisms include:

• Sudden changes of direction or cutting movements while running.
• Stopping abruptly while moving quickly.
• Landing awkwardly from a jump.
• A direct blow to the knee, such as during a tackle.
• Sudden slowing combined with a pivot, as in skiing accidents.

Many ACL tears are non-contact injuries, meaning they happen without anyone touching the knee, simply from the way the body twists and lands.

Risk Factors

Factors that may raise the risk of an ACL injury include:

• Participating in pivoting or jumping sports such as soccer, basketball, football, volleyball, and skiing.
• Poor conditioning, muscle weakness, or imbalance around the hip and thigh.
• Faulty movement patterns, such as landing with the knees turned inward.
• Worn or poorly fitting footwear, or playing surfaces that increase grip.
• Previous ACL injury, which raises the risk of another.
• Female athletes have a higher rate of ACL injury in some sports.

Diagnosis

Diagnosis is based on the history of the injury, examination of the knee, and imaging:

• Physical examination, including specific maneuvers that test how much the shinbone moves forward, which indicates ACL function.
• Checking for swelling, range of motion, and tenderness, and assessing for damage to other knee structures.
• MRI, which shows the ACL clearly and reveals tears as well as injury to the meniscus, cartilage, or other ligaments.
• X-rays, which do not show the ligament but help rule out a fracture.

ACL injuries often occur with other damage, such as a meniscus tear, so a full assessment of the knee is important.

Treatment

Initial care follows the standard approach for acute injuries: rest, ice, compression, and elevation to control pain and swelling, along with pain relievers as needed. Longer-term treatment depends on the severity of the tear, the person's activity level and goals, and whether other structures are injured.

• Nonsurgical treatment with structured physical therapy may be enough for partial tears or for less active people, focusing on strengthening the muscles around the knee and restoring stability.
• Surgical reconstruction rebuilds the torn ligament using a tendon graft and is often recommended for athletes and people who want to return to pivoting sports or who have an unstable knee.
• Rehabilitation is essential either way, typically lasting several months, to rebuild strength, motion, and control.
• A brace may be used during recovery or activity.

Recovery from surgery and return to sport generally takes many months and requires committed rehabilitation.

Prevention

Targeted training programs can lower the risk of ACL injury, especially in athletes. Helpful measures include:

• Neuromuscular training that teaches safe jumping, landing, and cutting techniques.
• Strengthening the muscles of the hips, thighs, and core.
• Improving balance and body control.
• Proper warm-up before activity and gradual increases in training.
• Using appropriate footwear for the sport and surface.

When to See a Doctor

See a doctor promptly if you feel or hear a pop in your knee during activity, the knee swells quickly, or you feel the knee is unstable or cannot bear weight. Early evaluation helps confirm the diagnosis and plan treatment.

Seek urgent care if the knee is severely deformed, intensely painful, numb, cold, or pale, or if you cannot move it at all, as these may signal a fracture, dislocation, or damage to nerves or blood vessels that needs immediate attention.

Related Topics

Frequently Asked Questions

References

1. American Academy of Orthopaedic Surgeons (AAOS).
2. Mayo Clinic.
3. MedlinePlus, U.S. National Library of Medicine.