ACL Tear: Signs, Diagnosis and Treatment Choices

What the ACL does

The anterior cruciate ligament runs diagonally through the middle of the knee from the thighbone (femur) to the shinbone (tibia). It prevents the tibia from sliding forward and limits rotation. Without it, the knee can "shift" or buckle when you twist, cut or land, even if walking in a straight line feels normal.

ACL injuries are common in soccer, basketball, football, skiing, netball and volleyball. Women athletes have a 2 to 8 times higher risk than men in the same sports, linked to landing mechanics, muscle activation and anatomy.

Signs of a torn ACL

• A pop felt or heard at the moment of injury
• Immediate pain and being unable to continue playing
• Swelling within a few hours, caused by bleeding into the joint
• Loss of full range of motion, especially straightening
• Tenderness along the joint line
• Instability: the knee shifts or gives way when turning, once swelling settles

Some people are surprised that they can walk soon after the injury. Walking ability does not rule out a complete tear.

Partial vs complete ACL tear

Ligament injuries are also graded 1 (stretched), 2 (partially torn) and 3 (complete). Most ACL injuries are grade 3.

Associated injuries

About half of ACL tears happen with other damage:

• Meniscus tears, which may need repair
• MCL injury on the inner side
• Bone bruises seen on MRI
• Cartilage damage
• Less often, other ligament injuries or a small fracture where the ligament attaches

How an ACL tear is diagnosed

1. History: a non-contact twisting injury, a pop and rapid swelling strongly suggest an ACL tear.
2. Examination: the Lachman test (the clinician pulls the shin forward with the knee slightly bent) is the most accurate bedside test. The pivot shift test also detects instability but is hard to do in a painful knee.
3. X-ray: to rule out fractures.
4. MRI: confirms the tear and shows meniscus, cartilage and other ligament injuries.

Examination is often easier and more accurate once the initial swelling settles, a few days to 2 weeks later.

Treatment options

First steps for everyone

• Ice, compression and elevation, and crutches if walking is painful.
• Early physical therapy to reduce swelling, regain full straightening and activate the quadriceps.
• Avoid pivoting and sports until assessed.

Rehabilitation without surgery

Structured physical therapy over several months strengthens the quadriceps, hamstrings and hips and improves balance and control. This is reasonable for:

• people who do not play pivoting or cutting sports
• those willing to change activities
• many partial tears
• some older or less active adults

Studies in young active adults have found that a substantial share managed well with rehabilitation alone, with surgery kept for those who still had instability. Ongoing giving way, however, risks damaging the meniscus and cartilage.

ACL reconstruction surgery

Surgery replaces the torn ligament with a graft, usually through keyhole (arthroscopic) surgery. A torn ACL is rarely stitched back together.

Reconstruction is usually recommended for people who:

• want to return to pivoting, cutting or jumping sports
• have repeated giving way despite rehabilitation
• have a repairable meniscus tear
• have physically demanding jobs
• are children or teenagers who are active (special techniques protect growth plates)

Surgery is often done once swelling has settled and full knee straightening has returned, a few weeks after injury. "Prehab" before surgery improves recovery.

Recovery after ACL reconstruction

• First 2 weeks: control swelling, regain full straightening, walk with crutches.
• 6 weeks: walking normally; cycling.
• 3 months: jogging in a straight line often begins.
• 6 months: agility and sport-specific drills.
• 9 to 12 months: return to pivoting sports after passing strength and functional tests. Returning before 9 months is linked with higher re-injury rates.

Complications and long-term outlook

• Re-tear of the graft or an ACL tear in the other knee, highest in young athletes returning to sport.
• Stiffness and kneecap pain.
• Osteoarthritis: people with ACL injuries have a higher risk of knee osteoarthritis 10 to 20 years later, with or without surgery, especially if the meniscus was damaged.

Most people return to everyday activities and many to their previous sport.

Preventing ACL injuries

Neuromuscular training programs, such as FIFA 11+ and similar warm-ups, teach safe landing, cutting and balance, and reduce ACL injuries substantially in young athletes, especially girls and women. They take about 15 to 20 minutes and work best when done 2 to 3 times a week.

For broader information, see ACL injury and knee injury.

Frequently asked questions

How do I know if I tore my ACL?

Typical signs are a pop at the moment of a twisting or landing injury, immediate pain, being unable to continue, swelling within hours and later a feeling that the knee gives way. A clinician can test the ligament with the Lachman test, and an MRI confirms the tear.

Can you walk with a torn ACL?

Often, yes. Many people can walk in a straight line within days of an ACL tear once swelling settles. Problems usually appear when pivoting, turning quickly or going down stairs, when the knee may give way. Being able to walk does not rule out a tear.

Can an ACL tear heal without surgery?

A complete ACL tear does not heal back together in adults, but many people regain good function with rehabilitation alone, especially if they avoid pivoting sports. Partial tears often do well without surgery. Surgery is considered when the knee keeps giving way or for return to high-demand sports.

How long is recovery from ACL surgery?

Most people walk normally by about 6 weeks and begin jogging around 3 months. Return to pivoting sports usually takes 9 to 12 months, after passing strength and functional tests. Rushing back increases the risk of a re-tear.

What is the difference between a partial and complete ACL tear?

In a partial tear, some ligament fibers remain intact, and the knee may still be stable. In a complete tear, the ligament is fully ruptured and the knee is more likely to be unstable during twisting. MRI and examination together determine which you have.