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ACL Injury

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Last updated May 4, 2022

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An ACL injury is a partial or full tear of the anterior cruciate ligament in the knee. It can cause pain and swelling and make the knee unstable. Depending on how severe the injury is, you may need surgery.

What is an ACL injury?

The ACL (anterior cruciate ligament) is a strong band of tissue in the center of the knee that helps connect the thigh bone to the shin bone and keeps your knee stable. ACL tears often happen when you step awkwardly or try to change direction quickly when running. Skiing injuries can also damage the ACL. You may hear a loud popping sound when the injury occurs.

The ACL can tear if your knee bends backward (hyperextends), making it feel unstable. Because the ACL is so important to the function of your knee joint, injuries to this ligament often need surgery. When the ACL is torn, it’s common to also have other injuries around the knee that need to be treated.

Dr. Rx

How a patient might describe it: “I heard a pop in the knee.” “My knee swelled and I had a hard time bending it.” “I feel OK walking straight ahead, but my knee feels unstable if I turn or pivot.” —Dr. Ben Schwartz


ACL injuries range from sprains (partial tearing) to complete tears. An ACL sprain causes swelling and pain but usually doesn't affect the stability of your knee.

ACL tears cause pain and swelling, but they can also make it feel like your knee bones are shifting. You’re more likely to feel this sensation if you try to turn quickly or pivot on the leg rather than when you’re walking straight ahead. You may have pain when you put weight on the leg or feel like the leg is going to buckle or give out on you.

Main symptoms

  • Swelling, which usually comes on within a few hours of injury
  • Buildup of fluid (blood) in the knee due to tearing of a small blood vessel that runs through the ACL
  • Stiffness
  • Pain
  • Difficulty putting full weight on the leg
  • A sensation of looseness or bones shifting in the knee
  • Buckling or giving way of the leg


ACL injuries usually happen from an awkward movement of the knee. Sudden pivoting or stopping can cause ACL injuries. Many ACL injuries occur if the knee bends backwards forcefully. This can happen if you step or land awkwardly on the leg or you have a direct blow to the knee.

Athletes are more likely to have ACL tears, especially female athletes that play basketball or volleyball, and skiers. Football players may tear their ACL with a direct blow to the knee.

Can an ACL tear heal by itself?

Most ACL tears don’t heal on their own and almost always require surgery. Sprains (partial tears may heal on their own).

In rare cases, ACL tears may be treated with physical therapy to strengthen the muscles around the knee. This treatment is recommended only if you’re older and don’t plan on returning to physical activity or exercise.

Pro Tip

ACL repair surgery is recommended for patients who wish to return to playing sports or moderate- to high-impact exercise. Patients who are less active or who only participate in low-impact exercise (walking, biking) may consider non-surgical treatment. —Dr. Schwartz


Many ACL sprains can be treated without surgery. Treatment includes a period of rest until symptoms improve (6–8 weeks), followed by physical therapy and sometimes wearing a brace.

Some partial ACL tears may require surgery if they cause the knee to feel unstable.

Most complete ACL tears require surgery to make the knee stable again. You may need to have physical therapy before the surgery to help increase the motion of your knee. This will help reduce stiffness that can occur after surgery.

Your doctor may also recommend removing fluid from the knee before surgery to help you regain motion, and it can also relieve some of your pain. Wearing a brace on the knee can provide support while you heal.


Non-steroidal anti-inflammatories (NSAIDs)

NSAIDs can be used before or after surgery to reduce swelling and inflammation:

  • OTC NSAIDs like Ibuprofen (Motrin, Advil) and naproxen (Aleve)
  • Prescription NSAIDs such as meloxicam (Mobic), nabumetone (Relafen), naproxen (Naprosyn), and celecoxib (Celebrex)

Opioid pain medications

Stronger types of pain pills may be prescribed after surgery to help with severe pain. These are usually taken for a short period of time to reduce the risk of addiction.

  • Hydrocodone (Vicodin, Norco)
  • Oxycodone (Percodan, Roxicet, Percocet)
  • Hydromorphone (Dilaudid)

ACL surgery

For an ACL surgery, your ligament is replaced with a new ligament that’s created from one of your own hamstring tendons or a donor (cadaver) tendon. The surgeon makes tunnels in your knee bones and secures the new ligament within them.

You will likely go home the same day you have ACL surgery. Your doctor will give you a knee brace to wear for 6–12 weeks after surgery to protect the healing ligament.

Your doctor will likely recommend that you start physical therapy right away to build up the strength in your knee muscles. It may be a few weeks before you can begin to bend the knee. Full healing can take a year or even longer in some cases.

ACL surgery is usually very effective at making your knee stable again. In rare cases, you might develop an infection after surgery, which requires a second surgery to clean out the knee and several weeks of antibiotics. It’s also possible to tear the new ligament if you return to high-risk sports. In this case, it will need to be replaced.

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Pro Tip

A new ligament created from one of your own tendons is called an “autograft.”  In other cases, the surgeon may use a cadaver tendon to reconstruct the ACL.  This is called a tendon “allograft.”  There is some debate as to which approach is better.  —Dr. Schwartz

Preventive tips

Though ACL injuries usually happen because of an awkward movement or accident, there are a few things you can do to help prevent them.

First, if you participate in high impact activities, it’s important to make sure that the muscles around your knees are strong, especially the hamstring muscles that are located on the back of your thighs. This helps keep stress off of the knee.

If you play sports, make sure your equipment is in good condition and that you’re wearing it correctly. Using proper form and technique when exercising or playing sports can also help reduce the risk of an ACL tear.

Wearing a knee brace has not been definitely proven to prevent ACL injuries.

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The stories shared below are not written by Buoy employees. Buoy does not endorse any of the information in these stories. Whenever you have questions or concerns about a medical condition, you should always contact your doctor or a healthcare provider.
Dr. Schwartz is a board-certified Orthopedic Surgeon and Member of the Buoy Medical Advisory Board. He graduated Magna Cum Laude from the College of William and Mary (1998) with a B.S. in Biology, then obtained his medical degree from the Medical College of Virginia (2002) where he was elected to the Alpha Omega Alpha Medical Honor Society. After completing his Orthopedic Surgery Residency at Bost...
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