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Water On the Knee

Last Updated: Mar 1, 2019


“Water on the knee” is common term for knee effusion, a condition in which excess fluid accumulates around the knee joint. Such effusions happen for many reasons. The most common ones are injury, overuse, and chronic health conditions like osteoarthritis.

joint capsule of the knee

Your knee joint connects your femur (upper leg bone) to the tibia and fibula (lower leg bones) with three large ligaments. On each side of the joint is a meniscus, a crescent-shaped wedge of cartilage (soft bony tissue). The meniscus is slippery and provides cushioning and lubrication to help the ends of the bones move smoothly over one another.

Like most joints, the knee is contained within a “capsule” - a membrane that normally contains less than an ounce of lubricating joint fluid. However, an injury or illness can cause extra fluid to build up around the joint. The results are swelling, pain and restricted range of motion. In severe cases, it may be impossible to walk or even put weight on the affected knee.

Water on the knee usually isn’t dangerous in itself, but it can be a sign of a more serious medical condition. In addition, prolonged swelling can cause the leg muscles to weaken. This is especially dangerous in older people, because it increases the risk of falls. Another potential complication is Baker’s cyst, a fluid filled sac that forms at the back of the knee. Occasionally these cysts get so large or bothersome that a physician must drain them.

Water on the knee is more common in people who have significant “wear and tear” on their joints. Age, previous injury, and obesity are all risk factors. People who play sports or practice very physical occupations are also more likely to experience a knee effusion.


Osteoarthritis is the most common cause of water on the knee. This condition develops as the meniscus wears away due to repetitive stress and injury over time. Without the lubricating effect of cartilage, friction increases, and the knee becomes swollen and painful to use.

Trauma is another common cause of effusion. Twisting, hyperextension, a sudden stop or a direct blow to the knee can damage the bones, ligaments and meniscus, resulting in pain and swelling.

Interestingly, many diseases can cause knee problems, including some that may have origins elsewhere in the body. Some examples:

  • Rheumatoid arthritis - An autoimmune condition in which the immune system attacks the body’s own cells.
  • Infections - Lyme disease, brucellosis, tuberculosis, and gonorrhea can all affect the knee.
  • Gout - When levels of uric acid in the blood get too high, the acid can crystallize in the joints.
  • Bursitis - Swelling of the small, fluid-filled sacs that separate your knee bones from nearby muscles and tendons.
  • Tumors - Both cancerous and benign masses sometimes infiltrate the knee.


You might have water on the knee if you notice:

  • Swelling. The skin around the kneecap looks and feels puffy compared to the other knee.
  • Stiffness. Range of motion decreases. The leg is difficult to completely bend or straighten. Depending on the cause, you might also notice instability, clicking, or locking of the knee.
  • Pain. This can range from mild to severe, depending on the cause.

Swelling that appears in the absence of an injury may be caused by an infection. Additional symptoms include fever, chills, redness, or hotness.

Recent research suggests that a swollen knee can sometimes be the first sign of Lyme disease, a potentially serious bacterial infection spread by tick bites. Knee swelling due to Lyme disease is usually pronounced and develops quickly, but is not typically painful. It can occur in the absence of other common Lyme disease symptoms like rash or fever. Left untreated, Lyme disease can lead to permanent joint damage.

Tumor-related swelling may be accompanied by fever, night pain and sweating, and unexplained weight loss.


Home Care

Mild knee swelling due to an injury or previously diagnosed condition may not require medical attention. Instead:

  • Rest. Try not to walk or put weight on the leg.
  • Ice. Place ice in a plastic bag, wrap it in a towel, and apply if to the knee for 15 to 20 minutes every two to four hours.
  • Elevate. When sitting, elevate your knee above your waist. Place pillows underneath your knee when sleeping to raise it above your heart.
  • Use pain relievers. Over-the-counter medications like acetaminophen (Tylenol®) or ibuprofen (Advil®, Motrin®) can help make you more comfortable.

When to See a Doctor

See a doctor if your knee is very swollen or doesn’t improve with home care. Also see a doctor if you notice infection symptoms like fever, redness or hotness. It’s a good idea to seek help if you don’t know what caused the swelling (in other words, it’s not related to an injury or diagnosed medical condition).

Primary care providers can diagnose and treat many straightforward knee problems. Depending on the findings, your provider may refer you to an orthopedist (skeletal system specialist) for further treatment.

A workup for water on the knee may include:

  • History and physical - Your provider will want to hear about your symptoms, the date and mechanism of injury, and how soon swelling appeared. You may also be asked about your recent activities, past injuries, and surgeries.
  • X-rays - This imaging test is good at revealing dislocated and broken bones as well as some cases of arthritis.
  • Ultrasound - Another imaging test used to detect abnormalities in the bones, ligaments and tendons.
  • Magnetic resonance imaging (MRI) - The most sensitive imaging test for soft-tissue abnormalities, including those of the tendons, ligaments, and cartilage.
  • Joint aspiration - With the knee numbed by anesthetic, the doctor uses a needle to draw out some fluid from the joint capsule. This can be analyzed for the presence of blood, bacteria, or uric acid crystals.


Next steps will vary depending on the underlying issue causing your knee effusion. Your provider’s goals include reducing swelling, relieving pain, improving function, and treating any underlying illnesses.

Common Treatments:

  • Medications - Corticosteroids are powerful drugs that fight inflammation. They can be taken orally or injected into the affected joint.
  • Physical therapy - This involves a course of exercises to improve strength and range of motion. Your therapist might also provide treatments like splinting, icing, or ultrasound to promote healing.
  • Arthrocentesis - With the knee numbed, the doctor uses a needle to draw fluid out of the joint. This helps relieve pain and pressure.
  • Arthroscopy - With the patient under sedation, the doctor threads an arthroscope (flexible tube with a camera in the end) into the knee. Tools can be passed through the tube to repair damage to the cartilage and remove loose tissue.
  • Joint replacement - In cases of severe osteoarthritis, the knee joint can be replaced with a prosthetic to relieve pain and improve function.


The course of water on the knee depends on its cause.

Osteoarthritis is incurable once it develops, but can often be managed with activity, physical therapy, and medication. In severe cases, surgery can greatly improve function.

People who have had a severe knee sprain or fracture are at increased risk for chronic (long-term) pain, stiffness, and joint instability. They may also be more likely to develop osteoarthritis as they get older.


To reduce your risk of knee pain and swelling:

  • Keep your leg muscles strong. This helps to keep your knee stable and relieve pressure on the joint. Weight-bearing exercises like walking, jogging, weight training, and even gardening are good for building strength.
  • Prevent overuse injuries. Give your knee a rest by working low-impact activities like swimming or cycling into your routine. If you’re just getting back into training, start slow and build up gradually.
  • Watch your weight. Maintaining a healthy size reduces the stress on your joints and minimizes wear and tear.


  • Diseases and Conditions: Swollen Knee (2015). Mayo Foundation for Medical Education and Research. Retrieved Dec. 4, 2015, at
  • Johnson, M. “Acute Knee Effusions: A Systemic Approach to Diagnosis.” American Family Physician (2000). Retrieved Dec. 7, 2015, at
  • Knee Effusion. University of Minnesota Medical Center. Retrieved Dec. 7, 2015, at
  • “Water on the Knee” Could Be Early Sign of Lyme Disease (2015). American Academy of Orthopedic Surgeons. Retrieved Dec. 4, 2015, at

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Tina Shahian, PhD

Tina is a writer for Innerbody Research, where she has written a large body of informative guides about health conditions.


A communication specialist in life science and biotech subjects, Tina’s successful career is rooted in her ability to convey complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. Tina Shahian’s Linkedin profile.


In her spare time, Tina enjoys drawing science-related cartoons.