A swollen knee occurs when excess fluid accumulates in or around your knee joint. Your doctor might refer to this condition as an effusion (ih-FYU-zhen) in your knee joint. Some people call this condition "water on the knee."

A swollen knee may be the result of trauma, overuse injuries, or an underlying disease or condition. To determine the cause of the swelling, your doctor might need to obtain a sample of the fluid to test for infection, disease or injury.

Removing some of the fluid also helps reduce the pain and stiffness associated with the swelling. Once your doctor determines the underlying cause of your swollen knee, appropriate treatment can begin.

Signs and symptoms typically include:

• Swelling. The skin around your kneecap can puff up noticeably, especially when you compare the affected knee to the normal one.
• Stiffness. When your knee joint contains excess fluid, you might not be able to bend or straighten your leg completely.
• Pain. Depending on the cause of the fluid buildup, the knee might be very painful — to the point that it's difficult or impossible to bear weight on it.

When to see a doctor

See your doctor if:

• Self-care measures or prescribed medications don't relieve the pain and swelling
• One knee becomes red and feels warm to the touch compared to your other knee

Many types of problems, ranging from traumatic injuries to diseases and other conditions, can cause a swollen knee.

Injuries

Damage to any part of your knee can cause excess joint fluid to accumulate. Injuries that can cause fluid buildup in and around the knee joint include:

• Torn ligament, particularly the anterior cruciate ligament (ACL)
• Cartilage (meniscus) tear
• Irritation from overuse
• Broken bones

Diseases and conditions

Underlying diseases and conditions that can produce fluid buildup in and around the knee joint include:

• Osteoarthritis
• Rheumatoid arthritis
• Infection
• Gout
• Pseudogout
• Bursitis
• Cysts
• Tumors

• Age. Your likelihood of developing a swollen knee related to arthritis increases as you age.
• Sports. People who participate in sports that involve twisting the knee, such as basketball, are more likely to experience the types of knee injuries that cause swelling.
• Obesity. Excess weight puts added stress on the knee joint, contributing to the tissue and joint overload and knee degeneration that can lead to a swollen knee. Obesity increases your risk of osteoarthritis, one of the more frequent causes of knee swelling.

Complications of a swollen knee can include:

• Muscle loss. Fluid in the knee can harm the working of your muscles and cause thigh muscles to weaken and atrophy.
• Fluid-filled sac (Baker's cyst). The buildup of fluid in your knee can lead to the formation of a Baker's cyst in the back of your knee. A swollen Baker's cyst can be painful, but usually improves with icing and compression. If the swelling is severe, you might need to have fluid removed (cyst aspiration).

A swollen knee is typically the result of an injury or chronic health condition. To manage your overall health and prevent injuries:

• Strengthen the muscles around your knee. Strong muscles around a joint can help ease pressure on the joint itself.
• Choose low-impact exercise. Certain activities, such as water aerobics and swimming, don't place continuous weight-bearing stress on your knee joints.
• Maintain a healthy weight. Excess weight contributes to the wear-and-tear damage that can lead to a swollen knee.

Your doctor is likely to start with a detailed history and physical examination. After that you likely will need tests to determine the underlying problem that is causing your swollen knee.

Imaging tests

Imaging tests can help show where the problem is located. Options include:

• X-ray. An X-ray can rule out broken or dislocated bones, and determine if you have arthritis.
• Ultrasound. This test can check for arthritis or disorders affecting the tendons or ligaments.
• MRI. This test can detect tendon, ligament and soft tissue injuries that aren't visible on X-rays.

Joint aspiration (arthrocentesis)

Your doctor withdraws fluid from inside your knee to check for the presence of:

• Blood, which may stem from injuries or bleeding disorders
• Bacteria, which may be causing an infection
• Crystals common to gout or pseudogout

Treatment varies, depending on the cause of the swollen knee, its severity and your medical history. Treatment generally involves pain medication and procedures to remove fluid from the knee joint.

Medications

Your doctor might prescribe oral pain medication, if over-the-counter pain relievers aren't enough. To ease inflammation, your doctor might suggest an oral corticosteroid, such as prednisone. Other types of steroids can be injected directly into your knee joint.

Surgical and other procedures

Treating the underlying cause of a swollen knee might require:

• Arthrocentesis. Removing fluid from the knee can help relieve pressure on the joint. After aspirating joint fluid, your doctor might inject a corticosteroid into the joint to treat inflammation.
• Arthroscopy. A lighted tube (arthroscope) is inserted through a small incision into your knee joint. Tools attached to the arthroscope can remove loose tissue or repair damage in your knee.
• Joint replacement. If bearing weight on your knee joint becomes intolerable, you might need knee replacement surgery.

Your doctor might also recommend physical therapy to improve your knee's function and strength.

Taking care of yourself when you have a swollen knee includes:

• Rest. Avoid weight-bearing activities as much as possible.
• Ice and elevation. To control pain and swelling, apply ice to your knee for 15 to 20 minutes every two to four hours. When you ice your knee, raise your knee higher than the level of your heart, using pillows for comfort.
• Pain relievers. Over-the-counter medicines such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others) can help reduce your knee pain.

You are likely to be referred to a doctor specializing in musculoskeletal and joint problems.

What you can do

• Write down your symptoms, and when they began.
• Write down your key medical information, including other conditions.
• Write down key personal information, including any major changes or stressors in your life.
• Make a list of all your medications, vitamins or supplements.
• Find out if anyone in your family has had an autoimmune disease.
• Ask a relative or friend to accompany you, to help you remember what the doctor says.
• Write down questions to ask your doctor.

Questions to ask your doctor

• What's the most likely cause of my symptoms?
• What kinds of tests do I need?
• What treatments are available?
• I have other health conditions. How can I best manage them together?

In addition to the questions that you've prepared to ask your doctor, don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you a number of questions. Being ready to answer them may leave time to go over points you want to discuss in depth. You may be asked:

• Have you injured your knee recently? If so, describe the injury in detail.
• Does your knee "lock" or feel unstable?
• Has your knee felt warm or looked red? Do you have a fever?
• Do you play recreational sports? If so, what sports?
• Do you have any type of arthritis?
• Do you have a family history of autoimmune disease?