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Many people’s knees squeak or crack or even crackle, a phenomenon known as crepitus. In fact, most of the time it’s no big deal at all.

But a loud popping sound might give you pause — and rightfully so. This kind of pop may be accompanied by pain and swelling, signs that an injury has occurred. This noise, which is sometimes called pathological noise, may mean that something is wrong.

Sometimes, knee injuries can be mild, but they can also be severe enough to require surgery. You won’t know for sure until a doctor gets a look at your knee.

If you suddenly hear a noticeable popping sound coming from your knee, it’s pretty safe to assume that something is wrong, although the degree of injury can vary. Here are some of the possible causes.

ACL tear

One of the hallmark signs of a tear to the anterior cruciate ligament (ACL) in the knee is a loud popping sound, which is usually followed by intense pain. You might not be able to stand or put weight on the affected leg.

PCL injury

Your ACL isn’t the only knee ligament that can get injured. However, an injury to your posterior cruciate ligament (PCL) at the back of your knee joint is less common than an ACL injury. It’s also less likely than an ACL tear to make the same kind of loud popping noise at the time of injury. You’ll probably experience some pain and swelling.

MCL injury

A tear to the medial collateral ligament (MCL) in your knee can also cause some pretty severe pain and swelling, and an audible popping noise can occur when the ligament tears. The pain can vary, depending on whether you stretch it or you actually tear it. Later, you’ll likely feel some swelling and stiffness, perhaps an inability to bend and straighten your knee.

LCL injury

The last of the four big ligaments in the knee, the lateral collateral ligament (LCL) is a pretty commonly injured ligament. But it may be injured along with another ligament in the knee, so while you may hear a pop or a ripping sound if it’s torn, it could also be the sound of another ligament. You’ll feel pain, weakness, and perhaps numbness or stiffness.

Meniscus tear

A sudden pivot or rotation, or direct pressure, can tear one of the two pieces of cartilage sandwiched between your thighbone and shinbone. Known as a meniscus tear, this kind of injury can generate a popping noise.

Pain, swelling, difficulty moving, and a sensation of instability may follow. You may also feel an odd slipping sensation if the cartilage becomes loose and blocks your knee joint.

Cartilage injury

If a piece of cartilage becomes damaged, it can make a popping noise as your knee moves back and forth.

Patellar tendon tear

Your patellar tendon connects the top of your shinbone to the top of your kneecap. It can become inflamed, which is known as patellar tendonitis, or it can tear.

You may hear a tearing or popping sound if it tears. You might not be able to straighten your leg, and your kneecap may feel like it’s moving up into your thigh. You might also develop some pain, cramping, bruising, and an indentation at the bottom of your kneecap.

Knee arthritis

Knee arthritis, which is also known as osteoarthritis, develops when the cartilage that separates the bones in your knee joint breaks down. When you move your knee, you can feel the bones grating or grinding against each other, which can cause popping sounds.

None of the injuries above should be shrugged off. See a doctor right away if you experience one. In the meantime, stay off your knee and apply ice to it to keep the swelling down. If you don’t think you can put any weight on it, don’t try.

The popping noise might occur after you collide with another person, or it might happen after you rotate, pivot, or jump and land hard. Regardless of what you were doing or your level of fitness, if you hear the “pop,” get it checked out. You may need some tests to determine the cause and the extent of the damage.

Additionally, getting treatment is important, as it may help you avoid other injuries down the road. A 2016 study of nearly 1,900 adults with knee arthritis found that unstable knees made people more vulnerable to falling and fall-related injuries. But appropriate interventions could reduce that risk.

A doctor will perform a physical examination of your knee, taking note of any bruising, swelling, or other abnormalities that have developed. This will also likely include some range of motion tests to see what happens if your knee is bent or rotated.

For example, if the doctor suspects an ACL tear, they might perform certain physical diagnostic tests like the Lachman test and the anterior drawer test to test your ACL stability. If it looks like it might be a meniscus tear, they might perform a McMurray test, which puts some stress on the meniscus to see how it reacts.

Next, you’ll likely have imaging tests. An X-ray is often the next step, followed by an MRI scan, to get a better look at the ligaments and structures in your knee.

The specifics of your treatment will vary based on your diagnosis.

RICE

The RICE approach, which stands for rest, ice, compress, and elevate, is helpful for treating injuries at home, including knee injuries. Sports medicine experts typically recommend using this approach for 2 or 3 days after sustaining the injury.

Pain-relieving medications

Yep, a knee injury can hurt. Your doctor may recommend nonsteroidal anti-inflammatory drugs (NSAIDs) to help reduce the pain, or you may even be a candidate for corticosteroid injections.

Exercise

Your doctor might suggest incorporating some knee exercises into your routine to improve your flexibility and your knee’s range of motion.

Research from 2014 suggests that land-based exercise can help improve function and pain and that water-based exercise may also help with knee function.

Knee braces and other supports

If your noisy knee is the result of osteoarthritis, your doctor might suggest using a knee brace or knee sleeve to support your knee joint, as research from 2014 suggests that it might help.

Orthotic inserts in your shoes might help too. It might not eliminate the noise, but a cane might help you get around a little easier, too.

Physical therapy

In some cases, physical therapy alone after some recovery time may be enough to help you on the road to recovery from a knee injury. But it’s also often recommended after surgery to help people regain strength and mobility.

Surgery

Surgery may be necessary to repair some partially or completely torn ligaments.

For example, your doctor may recommend ACL reconstruction after you tear your ACL so you can regain strength and stability in your knee. A ligament from another part of your body or a donor will replace the torn ligament.

A torn meniscus may require arthroscopic surgery to trim away damaged meniscus tissue or sew the edges of the torn meniscus together.

Don’t grin and bear it. If your knee makes a loud popping noise (or even a series of popping sounds), get it checked out right away.