Find Information About:

Drugs & Supplements

Get information and reviews on prescription drugs, over-the-counter medications, vitamins, and supplements. Search by name or medical condition.

Pill Identifier

Pill Identifier

Having trouble identifying your pills?

Enter the shape, color, or imprint of your prescription or OTC drug. Our pill identification tool will display pictures that you can compare to your pill.

Get Started

My Medicine

Save your medicine, check interactions, sign up for FDA alerts, create family profiles and more.

Get Started

WebMD Health Experts and Community

Talk to health experts and other people like you in WebMD's Communities. It's a safe forum where you can create or participate in support groups and discussions about health topics that interest you.

  • Second Opinion

    Second Opinion

    Read expert perspectives on popular health topics.

  • Community


    Connect with people like you, and get expert guidance on living a healthy life.

Got a health question? Get answers provided by leading organizations, doctors, and experts.

Get Answers

Sign up to receive WebMD's award-winning content delivered to your inbox.

Sign Up

Anterior Cruciate Ligament (ACL) Injuries

Font Size


Symptoms of a severe and sudden (acute) anterior cruciate ligament (ACL) injury include:

  • Feeling or hearing a "pop" in the knee at the time of injury.
  • Sudden instability in the knee. (The knee feels wobbly, buckles, or gives out.) This may happen after a jump or change in direction or after a direct blow to the side of the knee.
  • Pain on the outside and back of the knee.
  • Knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
  • Limited knee movement because of swelling and/or pain.

After an acute injury, you will almost always have to stop the activity you are doing, but you may be able to walk.

Other health problems can cause symptoms like those of an ACL injury. They include a bone break or injuries to the knee cushions (menisci) or to other ligaments in the knee.

For more information on knee injuries, see:

Chronic ACL deficiency

The main symptom of chronic (long-lasting and recurrent) ACL deficiency is an unstable knee joint. The knee buckles or gives out, sometimes with pain and swelling. This happens more often over time. But not everyone with an ACL injury develops a chronic ACL deficiency.


WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
Next Article:

Hot Topics

WebMD Video: Now Playing

Click here to wach video: Dirty Truth About Hand Washing

Which sex is the worst about washing up? Why is it so important? We’ve got the dirty truth on how and when to wash your hands.

Click here to watch video: Dirty Truth About Hand Washing

Popular Slideshows & Tools on WebMD

disciplining a boy
Types, symptoms, causes.
Remember your finger
Are You Getting More Forgetful?
fruit drinks
Eat these to think better.
No gym workout
Moves to help control blood sugar.
acupuncture needle on shoulder
10 tips to look and feel good.
Close up of eye
12 reasons you're distracted.
birth control pills
Which kind is right for you?
embarrassed woman
Do you feel guilty after eating?
Epinephrine Injection using Auto-Injector Syringe
Life-threatening triggers.
woman biting a big ice cube
Habits that wreck your teeth.
pacemaker next to xray
Treatment options.
caregiver with parent
10 tips for daily life.

Women's Health Newsletter

Find out what women really need.