Anterior Cruciate Ligament (ACL) Injury

First aid after a knee injury

1. Stop playing. Continuing on an unstable knee can damage the meniscus and cartilage.
2. Protect and rest: use crutches if walking is painful.
3. Ice: 15 to 20 minutes every 2 to 3 hours, with a cloth between ice and skin.
4. Compress: an elastic bandage helps limit swelling.
5. Elevate the leg above heart level.
6. Pain relief: acetaminophen (paracetamol) or an NSAID, following the label.
7. Get assessed by a doctor, sports medicine clinic or urgent care within a few days, sooner if the warning signs above are present.

What the ACL does

The ACL runs diagonally through the middle of the knee, connecting the thigh bone (femur) to the shinbone (tibia). It prevents the tibia from sliding forward and controls rotation. Without it, the knee may shift and "give way" during cutting, pivoting and landing, even though straight-line walking and running may feel normal.

Causes and risk factors

About 70% of ACL injuries are non-contact: decelerating suddenly, changing direction with the foot planted, landing from a jump with the knee straight or collapsing inward. Direct blows to the side of the knee, as in football tackles, cause the rest.

Risk factors:

• sports involving cutting and jumping: soccer, basketball, football, volleyball, skiing, netball
• female athletes, who have 2 to 8 times the risk of male athletes in the same sports, due to differences in landing mechanics, muscle activation and anatomy
• poor neuromuscular control, such as knees collapsing inward on landing
• fatigue, artificial turf and poorly fitting footwear
• previous ACL injury on either side

Symptoms

• a loud pop or popping feeling at the time of injury
• severe pain and inability to continue the activity
• rapid swelling, usually within 2 to 12 hours, from bleeding into the joint
• loss of full range of motion
• tenderness along the joint line
• a feeling of instability or the knee shifting, especially when turning

After the initial pain and swelling settle over 2 to 4 weeks, many people can walk normally, which leads some to underestimate the injury.

Grades of ACL injury

Diagnosis

A doctor examines the knee and compares it with the other side:

• Lachman test: the most sensitive test. With the knee slightly bent, the doctor pulls the shin forward. Excess movement and a soft end-point suggest a tear.
• Anterior drawer test and pivot shift test.
• Checks for other injuries: meniscal tears, MCL injury, PCL injury and kneecap dislocation.

Imaging:

• X-ray to rule out fractures, including a small avulsion fracture (Segond fracture) that strongly suggests an ACL tear.
• MRI confirms the tear and shows associated meniscus, cartilage and bone bruise injuries.

Treatment

Rehabilitation without surgery

May be suitable for people who:

• do not play cutting or pivoting sports, or are willing to give them up
• have a stable knee in daily activities after rehabilitation
• have partial tears, or are older and less active

Physical therapy focuses on restoring motion, then strengthening the quadriceps, hamstrings and hips, and neuromuscular and balance training. Some people do well long term. Others continue to have giving-way episodes and later choose surgery.

ACL reconstruction surgery

The torn ACL usually cannot be stitched back together. Instead, the surgeon replaces it with a graft, typically from:

• the patellar tendon (bone-tendon-bone)
• the hamstring tendons
• the quadriceps tendon
• a donor (allograft), mainly for older or less active people

The operation is done arthroscopically (keyhole surgery), usually as an outpatient. It is generally recommended for:

• athletes returning to cutting and pivoting sports
• people with repeated instability despite rehab
• combined injuries, such as a repairable meniscus tear
• children and teenagers who are active, to protect the meniscus; special techniques avoid growth plates

Recovery timeline after surgery

Returning to pivoting sports before 9 months significantly increases the risk of re-tearing the graft.

Complications

• Re-injury of the graft or tearing the ACL in the other knee; risk is highest in young athletes
• Meniscus and cartilage damage, especially with an unstable knee
• Knee stiffness, kneecap pain and loss of strength
• Knee osteoarthritis later in life, more likely with associated meniscus injuries
• Blood clots and infection after surgery (uncommon)

Prevention

Neuromuscular training programs that teach good landing and cutting technique and strengthen the hips and hamstrings reduce ACL injury rates by about half in young athletes. Examples include FIFA 11+ and PEP. They take 15 to 20 minutes as a warm-up 2 to 3 times a week.

Frequently asked questions

What does an ACL tear feel like?

Most people feel or hear a pop, followed by sharp pain and inability to continue playing. The knee swells within hours. Later, the knee may feel loose or give way during turning or pivoting.

Can you walk with a torn ACL?

Often, yes. Once early pain and swelling settle, many people can walk in a straight line. Problems tend to appear when turning, pivoting or going down stairs, when the knee may give way. Walking ability does not mean the ligament is intact.

How long does it take to recover from an ACL tear?

After reconstruction, daily activities return within 6 to 12 weeks, running at around 3 to 4 months, and competitive cutting sports usually after 9 to 12 months. Without surgery, rehab takes 3 to 6 months, but pivoting sports may remain limited.

Does a torn ACL always need surgery?

No. People who do not play pivoting sports and have a stable knee after rehabilitation may do well without surgery. Surgery is usually advised for athletes returning to cutting sports, those with repeated giving way, and young people with combined meniscus injuries.

Can a torn ACL heal on its own?

A complete ACL tear rarely heals on its own because of the joint environment and its limited blood supply. Partial tears and stretched ligaments can heal with rehabilitation. Strong surrounding muscles can compensate for a torn ACL in some people.