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

• Feeling or hearing a pop in the knee at the time of injury.
• Pain on the outside and back of the knee.
• The knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the knee joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
• Limited knee movement because of pain or swelling or both.
• The knee feeling unstable, buckling, or giving out.

After an acute injury, you will probably have to stop whatever you are doing because of the pain. But you may be able to walk.

You can tear your ACL when you plant your foot and then push off, change direction, or pivot. This can happen during certain sports, like soccer or basketball.

You can also tear it from getting hit in your leg or knee during a contact sport like football or in high-speed sports like skiing.

Injuries like those from a car crash, stepping in a hole, or jumping or falling from a height can also cause an ACL tear.

To diagnose an ACL injury, your doctor will ask you to describe how you injured your knee and what you felt. The doctor will check your knee for swelling or tenderness. They may gently push and pull on your leg to see if the knee joint moves in an abnormal way. The exam is usually done on both legs so the doctor can compare one leg to the other to see what's normal for you.

You may have an X-ray to help make sure there isn't a different injury, like a broken bone. Ligaments can't be seen on an X-ray. An MRI is an imaging test that can help show the ACL. It can help your doctor see if you have an ACL tear. Often an injury that causes an ACL tear also injures other ligaments or the cartilage called the meniscus. An MRI can help your doctor diagnose these other injuries.

Treatment for an ACL injury includes:

• First aid right away. This is to help take care of symptoms like pain and swelling.
  • Put ice or a cold pack on your knee for 10 to 20 minutes several times a day. Put a thin cloth between the ice and your skin.
  • Elevate your injured knee when possible.
  • Use crutches and a knee immobilizer.
  • Take over-the-counter pain medicine, such as acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs). Be safe with medicines. Read and follow all instructions on the label.
• Nonsurgical treatment. This is also called rehab. It can include controlling swelling, increasing knee mobility, strengthening the muscles, and working on balance. If you're returning to a sport or intense activity, you may also work on techniques to reduce the risk of injury. It takes several months of rehab for your knee to get better.
• Surgery. You and your doctor can decide if rehab is enough or if surgery is right for you. If you have surgery, you will also have several months of rehab afterward.

You don't have to decide about surgery right away. Usually, your doctor would not do surgery for at least several weeks after your injury. This allows time for the swelling in your knee to go down. It also gives you time to strengthen the muscles around the knee and make sure you can move your knee well.

• Follow your doctor's directions for wearing a brace or an immobilizer, which limits movement of your knee. Wrapping your knee with an elastic bandage may help reduce or prevent swelling.
• Use crutches as directed in the first few days after your injury if your doctor recommends them.
• Rest your knee when possible. But try to flex your calf muscles often to help blood circulate in your legs.
• Put ice or a cold pack on your knee for 10 to 20 minutes at a time. Try to do this every 1 to 2 hours during the next 3 days (when you are awake) or until the swelling goes down. Put a thin cloth between the ice and your skin.
• Prop up your leg on a pillow when you ice it or anytime you sit or lie down during the next 3 days. Try to keep it above the level of your heart. This will help reduce swelling.
• Take pain medicines exactly as directed.
  • If the doctor gave you a prescription medicine for pain, take it as prescribed.
  • If you are not taking a prescription pain medicine, take an over-the-counter medicine to reduce pain and swelling. These include ibuprofen (Advil, Motrin) and naproxen (Aleve). Read and follow all instructions on the label.
• Follow your doctor's or physical therapist's directions for strength exercises. Exercises to make your thigh muscles stronger and increase knee motion can help you get ready for a physical rehabilitation (rehab) program or surgery with a rehab program. Here are a few exercises you can do if your doctor says it is okay.
  • Quad sets: Lie down on the floor or the bed with your injured leg straight. Fully extend your leg—there should be no or little bend in your knee. Tighten the thigh (quadriceps) of your injured leg for 10 seconds. Do not lift your heel up. Relax your quadriceps for 10 seconds. Repeat 8 to 12 times several times during the day.
  • Straight-leg raises: Lie down on the floor or the bed with your injured leg flat and your uninjured leg bent so that the bottom of your foot is on the floor or bed. Tighten the quadriceps of your injured leg. Keeping your knee as straight as possible, lift your injured leg off the bed until it is about 18 inches above the bed or floor. Lower your leg back down and relax for 5 seconds. Do 3 sets of 8 to 12 repetitions.
  • Heel slides: Lie down on the floor or the bed with your leg flat. Slowly begin to slide your heel toward your rear end (buttocks), keeping your heel on the floor. Your knee will begin to bend. Slide your heel and bend your knee until it becomes a little sore and you can feel a small amount of pressure inside your knee. Hold this position for 15 to 30 seconds. Slide your heel back down until your leg is straight on the floor. Relax for 10 seconds. Repeat 2 to 4 times several times during the day.
  • Side-lying leg lifts: Lie on your side with your legs straight and the injured leg on top. Keeping your leg straight, raise your injured leg up and backward about 6 inches. Lower the leg to the starting position. Do 3 sets of 20 repetitions, or if you tire quickly, 3 sets of 8 to 12 repetitions.