ACL Injury: Complete Guide to Symptoms, Causes, Diagnosis, and Treatment
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
Quick Answer: What Is an ACL Injury?
An ACL injury is a tear or sprain of the anterior cruciate ligament, one of the main ligaments that stabilizes the knee by connecting the thigh bone to the shinbone. It most often happens during sports that involve sudden stops, jumps, pivots, or direction changes — many people hear or feel a pop in the knee. Treatment ranges from rest, R.I.C.E. care, and rehab for partial tears, to ACL reconstruction surgery with a tendon graft for complete tears in active athletes, followed by up to a year or more of rehabilitation.
TL;DR
What it is: A tear or sprain of the ACL, a key ligament that stabilizes the knee joint.
The classic sign: A loud pop or popping feeling, serious pain, rapid swelling, and a knee that feels unstable or "gives out."
How it happens: Sudden stops, pivoting with a planted foot, awkward jump landings, stopping suddenly, or a direct blow.
Who's at risk: Women and girls face higher risk, along with soccer, basketball, football, volleyball, gymnastics, and downhill skiing participants; poor conditioning, wrong footwear, artificial turf, and prior ACL injuries raise risk too.
Treatment paths: Nonsurgical (R.I.C.E., bracing, crutches, physical therapy) for partial tears or less active people; surgery to reconstruct the ligament for complete tears or very active people.
Return to play: Often takes a year or more — up to one-third of athletes suffer another knee injury within two years.
ACL Injury at a Glance
Topic | Quick Summary |
What it is | A tear or sprain of the anterior cruciate ligament, one of the main ligaments in the knee |
What the ACL does | Connects the thigh bone (femur) to the shinbone (tibia) and helps keep the knee joint stable |
Classic symptom | A loud pop or popping feeling in the knee |
Other symptoms | Serious pain, rapid swelling, loss of range of motion, knee instability ("gives out") |
Common causes | Sudden stops, pivoting with foot planted, awkward jump landings, direct blows |
Higher risk | Women; soccer, basketball, football, volleyball, gymnastics, downhill skiing; poor conditioning; improper footwear or equipment; prior ACL injury |
Diagnosis | Physical exam is often enough; X-rays, MRI, and ultrasound used when needed |
Nonsurgical care | R.I.C.E. method, bracing, crutches, physical therapy, activity modification |
Surgical option | ACL reconstruction: damaged ligament removed and replaced with a tendon graft |
Recovery | Athletes can take a year or more to safely return to play; up to one-third may have another knee injury within two years |
Long-term complication | Increased risk of knee osteoarthritis, even after surgery |
What Is an ACL Injury?
The anterior cruciate ligament (ACL) is one of the key ligaments that help stabilize the knee joint. It connects the thigh bone, called a femur, to the shinbone, called a tibia.
An ACL injury is a tear or sprain of one of the main ligaments in your knee. Ligaments are strong bands of tissue that connect one bone to another. The ACL is one of two ligaments that cross in the middle of the knee, and it helps keep the knee joint stable.
ACL injuries often happen during sports or activities that involve sudden stops, jumps, pivots, or changes in direction. Examples include soccer, basketball, football, and downhill skiing. Many people hear or feel a pop in the knee when an ACL injury happens. The knee may swell, feel unstable, and become too painful to put weight on.
Symptoms: The Classic Pop
An ACL injury can happen suddenly. Common symptoms may include:
A loud pop or a popping feeling in the knee
Serious pain
Rapid swelling
Loss of range of motion
Knee instability or the feeling that the knee "gives out"
When to See a Doctor
Seek care right away if any injury to your knee causes symptoms of an ACL injury.
The knee joint is a complex structure of bones, ligaments, tendons, and other tissues working together. Often, other parts of the knee — such as the cartilage, meniscus, or other ligaments — are injured along with the ACL. A prompt and accurate diagnosis matters to determine how serious the injury is and get proper treatment.
Causes and Risk Factors
ACL injuries can happen in both contact and noncontact sports or activities. They often happen during movements that put stress on the knee, including:
Suddenly slowing down and changing direction
Pivoting with your foot firmly planted
Landing awkwardly from a jump
Stopping suddenly
Receiving a direct blow to the knee or having a collision, such as a football tackle
When the ligament is damaged, there is usually a partial or complete tear of the tissue. A mild injury may stretch the ligament but leave it intact.
Certain factors increase your risk:
Being female — women and girls are at higher risk than males, possibly due to differences in body structure, muscle strength, and hormones
Playing certain sports — soccer, basketball, football, volleyball, gymnastics, and downhill skiing
Poor conditioning or fatigue
Incorrect technique or muscle imbalances when jumping or landing
Playing on artificial turf
Footwear that does not fit properly
Poorly maintained sports equipment, such as ski bindings that aren't adjusted properly
Previous ACL injury — if you have hurt your ACL before, you are more likely to injure it again
Complications: The Arthritis Link
People who have had an ACL injury are more likely to develop osteoarthritis in the knee. Arthritis may happen even if you have surgery to reconstruct the ligament.
The risk depends on many factors, including how serious the original injury was, other related injuries in the knee, and how active you are after treatment.
How an ACL Injury Is Diagnosed
Diagnosis begins with a physical exam and detailed history. During the exam, your healthcare professional checks your knee for swelling and tenderness, compares it with your other knee, and moves your knee into a variety of positions to check range of motion and how well the knee works.
Often the physical exam alone is enough to diagnose an ACL injury. But you may need more tests to rule out other issues and see how serious the injury is:
Test | What It Does |
X-rays | Rule out a bone fracture; they do not show soft tissues such as ligaments and tendons |
MRI | Uses radio waves and a strong magnetic field to image hard and soft tissues; shows the extent of the ACL injury and any damage to other ligaments, cartilage, and the meniscus |
Ultrasound | Uses sound waves to visualize internal structures; checks for injuries in the ligaments, tendons, and muscles of the knee |
Treatment Options
Treatment depends on how serious the injury is, your activity level, and the stability of your knee. Some injuries can be treated without surgery; others need surgery to reconstruct the ligament.
Nonsurgical Care
For partial tears or for people who are less active, treatment may include the R.I.C.E. method to lessen pain and swelling:
Rest. Take it easy and avoid putting weight on your knee as you heal.
Ice. Put ice on your knee for 20 minutes every two hours while you are awake.
Compression. Wrap your knee with an elastic bandage or compression wrap.
Elevation. Lie down and prop your knee up on pillows.
Treatment without surgery also may include bracing to keep your knee stable, crutches to avoid putting weight on the knee while it heals, physical therapy to restore motion and strengthen supporting muscles, and activity modification — avoiding movements that involve cutting or pivoting.
The goal of rehabilitation is to lessen pain and swelling, restore full range of motion, and strengthen your muscles. This approach works well for people who do not have instability in the knee, are not very active, do moderate exercise, or play sports that put less stress on the knees.
Surgical Reconstruction
If you have a complete ACL tear or are very active, you may need surgery. Surgery may be recommended if:
You are an athlete who wants to keep playing, especially sports involving jumping, cutting, or pivoting
More than one ligament or the meniscus or cartilage is also injured
The injury causes your knee to give out during everyday activities
Before surgery, you will likely do some physical therapy to strengthen your knee and improve its range of motion.
During ACL reconstruction, the surgeon removes the damaged ligament and replaces it with a piece of tendon — tissue similar to a ligament that connects muscle to bone. This replacement tissue is called a graft. Your surgeon uses a piece of tendon from another part of your knee or a tendon from a deceased donor.
Physical Therapy After Surgery
Physical therapy after surgery is important. Successful ACL reconstruction combined with rigorous rehabilitation can usually restore stability and function to your knee. A physical therapist teaches you exercises you can do with the therapist's help or at home. You may also wear a brace to stabilize your knee and use crutches for a while.
Rehabilitation focuses on:
Early range of motion and pain control
Gradual strengthening of leg and core muscles
Balance and neuromuscular training to restore knee control
There is no set time frame for athletes to return to play. In general, it can take a year or more before athletes can safely return. Research shows that up to one-third of athletes may have another knee injury in the same or opposite knee within two years, especially with high-risk sports such as soccer. A longer recovery period may reduce the risk of reinjury.
Your healthcare team and physical therapists test your knee's stability, strength, function, and readiness to return at various points during rehabilitation.
Prevention: Training Programs That Work
You can lower your risk with proper training and exercise. A sports medicine physician, physical therapist, athletic trainer, or other specialist can assess how you move and offer instruction and feedback.
Programs to reduce ACL injury include:
Core exercises — strengthening the hips, pelvis, and lower abdomen to avoid moving the knee inward during a squat
Leg exercises — strengthening leg muscles, especially the hamstrings, to keep muscle strength balanced
Jumping and landing training — learning the right way to jump and land to protect the knees
Pivoting and cutting training — improving technique when making pivoting and cutting movements
Training to strengthen muscles and improve jumping and landing techniques may help reduce the higher ACL injury risk in female athletes.
For gear, wear the right shoes and padding for your sport. If you downhill ski, make sure a trained professional adjusts your ski bindings so the skis release properly if you fall.
One important note: wearing a knee brace may not prevent an ACL injury or reduce the risk of getting hurt again after surgery.
Preparing for Your Appointment
Because an ACL injury causes a lot of pain and makes the knee hard to move, many people seek medical attention right away. Others make an appointment with their family healthcare professional. You may be referred to a sports medicine specialist or an orthopedic surgeon — a specialist in bone and joint surgery.
Be prepared to answer questions such as:
When did the injury happen, and what were you doing at the time?
Did you hear a loud pop or feel a popping sensation?
Was there much swelling afterward?
Have you injured your knee before?
Do any movements improve or worsen your symptoms?
Does your knee ever lock or feel blocked when moving?
Does your knee ever feel unstable or unable to support your weight?
Frequently Asked Questions
What does an ACL injury feel like?
An ACL injury often starts with a loud pop or popping feeling in the knee, followed by serious pain, rapid swelling, loss of range of motion, and a feeling that the knee is unstable or "gives out." Many people find it too painful to put weight on the knee.
How do I know if I tore my ACL or just sprained my knee?
A torn ACL typically involves a pop at the time of injury, rapid swelling, and knee instability. However, the knee is a complex structure, and cartilage, meniscus, or other ligaments are often injured along with the ACL — so a prompt, accurate diagnosis from a healthcare professional is essential.
Do I always need surgery for an ACL injury?
No. Partial tears and less active people are often treated without surgery using the R.I.C.E. method (rest, ice, compression, elevation), bracing, crutches, and physical therapy. Surgery to reconstruct the ligament is usually recommended for complete tears, very active people, athletes who want to keep playing cutting and pivoting sports, or when other knee structures are also injured.
What is ACL reconstruction?
ACL reconstruction is surgery in which the damaged ligament is removed and replaced with a piece of tendon, called a graft. The graft comes from another part of your own knee or from a deceased donor. It is followed by structured physical therapy to restore stability and function.
How long does ACL injury recovery take?
In general, it can take a year or more for athletes to safely return to play. Research shows that up to one-third of athletes may have another knee injury in the same or opposite knee within two years, so a longer recovery period may reduce reinjury risk.
Can I prevent an ACL injury?
Proper training lowers risk: core exercises, leg strengthening (especially hamstrings), jumping and landing technique training, and pivoting and cutting technique training. Wear the right shoes and padding for your sport, and have ski bindings professionally adjusted. Note that a knee brace may not prevent an ACL injury or reinjury after surgery.
Conclusion: What to Remember About ACL Injuries
An ACL injury can sideline an athlete in a single movement — but the path back is clear. The three lessons to carry from this guide are:
The pop is the signal. A loud pop, rapid swelling, and an unstable knee mean seek care right away — other parts of the knee are often injured too, and a prompt diagnosis protects your treatment options.
Not every ACL injury needs surgery. Partial tears and less active people often heal well with R.I.C.E. care, bracing, and physical therapy; reconstruction is reserved for complete tears and those who want to return to cutting, pivoting, and jumping sports.
Rehab is the real treatment. Whether or not you have surgery, rigorous physical therapy restores stability and function — and rushing back too soon is the biggest reinjury risk, so plan on up to a year or more.
Dealing with a knee injury is stressful, but with the right diagnosis, treatment path, and rehabilitation, most people return to the activities they love. If your knee hurts or feels unstable after an injury, see a healthcare professional promptly.
Recovering from a lower-leg injury? Our guides on Achilles tendinitis and Achilles tendon rupture cover two more common sports injuries.
References
Mayo Clinic. ACL injury — Symptoms & causes. mayoclinic.org
Mayo Clinic. ACL injury — Diagnosis & treatment. mayoclinic.org
This article is for informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider for diagnosis and treatment.
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