The complexity of the knee joint allows it to support a wide range of movements — but it also makes the area vulnerable to a variety of injuries and chronic conditions.

Each year in the United States, about 12 million people see doctors for knee pain.

Researchers found that among all people seeking medical treatment in the country between 2007 and 2011, 1.5–7.3% sought care for pain in the front of the knee alone.

Below, we describe common causes of sudden knee pain and the symptoms that tend to accompany it. We also look into the options for diagnosis, treatment, and prevention.

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A number of conditions can cause sudden knee pain.

Some of the more common causes of sudden knee pain are:

  • Anterior cruciate ligament, or ACL, injuries. These often occur during sports that involve jumping or other sudden changes in direction. They can cause instability or prevent the knee from bearing weight.
  • Posterior cruciate ligament, or PCL, injuries. These can develop after a blow to the front of the knee.
  • Collateral ligament injuries. These contact injuries often happen during sports.
  • Meniscus tears. Tackles, twisting, aging, and arthritis can each damage the knee’s meniscus, its shock-absorbing cartilage. A person may feel the knee “locking” or experience discomfort going down the stairs.
  • Fractures. The three bones of the knee joint can fracture or break. The patella, or kneecap, does so most frequently.
  • Patellofemoral pain syndrome. Sometimes called runner’s or jumper’s knee, this mainly causes pain at the front of the knee that gets worse with climbing the stairs, squatting, or bending. It sometimes causes popping sounds or swelling.
  • Dislocation. This can occur when the thighbone, shinbone, and patella are out of alignment, due to structural problems or injuries.
  • Tendinitis. This issue, inflammation of the tendons around the joint, stems from overuse and is more common among middle-aged athletes.
  • Bursitis. Repetitive motions, like running, can cause sacs of fluid around the knee to become irritated, painful, and swollen.
  • Osgood-Schlatter disease. This overuse injury is most common in teenage boys, particularly in athletes. The symptoms include soreness around the knee and pain when extending the leg.
  • Arthritis. This develops when the cartilage protecting the joints breaks down and leaves the bones rubbing against each other. It can cause swelling, stiffness, and a reduced range of movement.

Sometimes, physicians cannot identify what is causing knee pain. For about 30% of teenagers who seek treatment for pain at the front of the knee, a doctor cannot diagnose the cause.

In the list above, we describe specific symptoms of issues that commonly cause sudden knee pain. But in general, one or more of the following tend to accompany this pain:

  • swelling
  • stiffness
  • a clicking sound with the pain
  • pain when rising after sitting
  • pain climbing the stairs
  • pain descending the stairs
  • the knee “locking” or not bending
  • the knee giving way

The exact location of the pain can help identify the cause and the best way to treat it.

For example, pain between the kneecap and the shin can be a sign of a repetitive use injury, such as from running.

Above, we list some risk factors for specific causes of sudden knee pain. Overall, the following tend to increase the chances of developing this pain:

  • increasing activity levels suddenly, which can put too much stress on the body too quickly
  • having poor alignment and a weak core, which can place stress on the knees
  • having weakness in one or both hips, which can lead to an uneven gait and eventual knee problems
  • having tight hamstrings and iliotibial bands, which can limit the body’s ability to recover from a workout
  • having an imbalance of strength and flexibility
  • wearing poorly fitting or worn out shoes
  • having overweight or obesity

Unless sudden knee pain clearly results from an injury or accident or the pain is severe, most people can safely wait for a few days before seeing a doctor.

During this time, people should avoid or limit any activities that cause discomfort.

Resting, applying ice packs, and using nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Advil), can provide relief. Symptoms may then start improving. However, this does not always happen.

A visit to a physician typically begins with a complete medical history and an overview of the person’s activity level.

The doctor then manipulates the person’s leg, checking for pain, tenderness, and swelling. They also do this to assess the range of motion.

To make a diagnosis, the doctor may need imaging, such as MRI scans or X-rays.

The treatment for sudden knee pain depends on its cause and severity. Some examples are below:

  • Arthritis. NSAIDs, physical therapy, yoga, and Pilates can help reduce pain, but if the condition is severe, a person may need knee replacement surgery.
  • Meniscus tears. When a tear is smaller, rest and over-the-counter pain relievers can help. More severe cases may require arthroscopic surgery to repair the tear or remove pieces of torn tissue.
  • Tendinitis. Thoroughly stretching the affected leg can increase blood flow, reduce inflammation, and help correct alignment shifts.

When a person is in too much pain to do physical therapy, the doctor may recommend cortisone shots. These provide a degree of relief that usually allows a person to start their rehabilitation exercises.

One of the most important treatments for knee pain — physical therapy — is not strictly a home remedy, as a trained physical therapist “prescribes” certain movements and exercises. But people typically do these at home.

In order to see benefits, a person must commit to a regular physical therapy routine, which often requires daily or twice-daily sessions. People can do these exercises on their own.

Although it takes time to see improvements, the results are often long-lasting.

Learn more about exercises for strengthening the knee here.

Braces and bandages can immobilize injured parts of the knee and provide support. The aim is to help minimize pain and prevent further damage.

Manufacturers market a number of supplements for knee pain, such as glucosamine and various herbal compounds. However, little scientific evidence supports their use.

Wear and tear on the body is inevitable, and injuries can be hard to avoid entirely.

Still, the following can help people can maintain or improve their balance, strength, and flexibility, offering some protection from sudden knee pain:

  • strengthening the core and hips
  • maintaining flexibility in the joints
  • improving alignment
  • increasing levels of activity slowly
  • wearing shoes that fit well
  • stretching regularly

Sudden knee pain is a common issue — the complex structure of the knee means that it can be injured in various ways.

Often, recovery from a knee problem just involves rest and self-care.

However, people with severe or persistent pain may require physical therapy or surgery.