Water on the knee: causes and which doctor to see

For water on the knee you can first see: Orthopedics & Traumatology or Physical Medicine & Rehabilitation or Rheumatology. In large hospitals, the Rheumatology unit also deals with this. If you are not sure, a family doctor (general practitioner) can assess you first and refer you to the right specialist.

Common symptoms: marked swelling of the whole knee or around the kneecap compared with the other knee; a feeling of tightness and fullness in the knee; being unable to bend or straighten the knee fully; pain going up and down stairs and when squatting; warmth and redness of the knee (with inflammation); rapid swelling in the first hours after an injury (there may be bleeding into the joint). Symptoms vary from person to person; a doctor’s examination is needed for a diagnosis.

The most common causes: sports injuries such as a meniscus or anterior cruciate ligament tear; knee osteoarthritis; joint inflammation from crystal deposits such as gout and pseudogout; rheumatoid arthritis and other rheumatic diseases; inflammation of the sac in front of the kneecap (bursitis): a round swelling at the front of the knee; bacterial infection inside the joint (septic arthritis) – an emergency. Only a doctor’s examination can determine the exact cause.

Fluid from a mild strain goes down within a few days to a few weeks with rest, cold packs and raising the leg. The lasting solution is treating the underlying cause: physiotherapy or surgery for meniscus and ligament injuries, muscle strengthening and weight control for osteoarthritis, and medicines for gout and rheumatic diseases. When needed, the doctor can drain the fluid with a needle both to relieve it and to make a diagnosis.

It is more common in: people who play sports with sudden twisting such as football, basketball and skiing; people over 50 and overweight people (osteoarthritis); people with gout or rheumatic disease; people who work kneeling (bursitis).

Fluid in the knee is assessed by hand on examination; ultrasound shows whether fluid is present and how much. Examining fluid drawn from the knee with a needle distinguishes infection, gout crystals or bleeding into the joint. If an injury is suspected, MRI is requested for the meniscus and ligaments, and X-ray for osteoarthritis.

Exercises that strengthen the thigh muscles, an ideal weight, warming up before sport and using knee pads when working kneeling reduce the risk.

Not always. The doctor drains the fluid with a needle if the knee is very tense and painful, or when the cause needs to be found. If the cause is not treated, the fluid can build up again.

If it is due to a mild strain, it can go away within days to weeks. Swelling that recurs or lasts for weeks can be a sign of a tear, osteoarthritis or rheumatic disease, so an examination is needed.

With water on the knee, the fluid is inside the joint and the swelling is at the front and around the knee. A Baker’s cyst is a swelling behind the knee formed when joint fluid pushes out into a sac; the two can occur together.

If the answer to any of these questions is yes, call 112 without delay: “Did the knee swell suddenly, red and hot, with fever or chills? (joint infection is an emergency)” “After a blow or fall, are you unable to put any weight on the knee, or is the leg deformed?” “Is there also swelling, pain and redness in the calf, or has shortness of breath started? (possible blood clot)” “Is there confusion, fainting or difficulty waking up?” “Is there facial drooping, slurred speech or sudden weakness in an arm or leg?” “Are you struggling to breathe even at rest?”

See a doctor: the swelling has not gone down in 3–4 days or keeps coming back; after an injury, the knee gives way, locks or catches; both knees or other joints are also swollen.