Patients often describe it as water in the knee. What they are describing is an effusion: extra fluid inside the joint. It is a sign, not a diagnosis, and the useful question is always what produced it.
The knee is lined by a membrane that normally makes a small amount of lubricating fluid. When the joint is irritated, that membrane produces far more. Swelling is therefore the joint reporting that something inside it is inflamed, injured or worn.
Swelling inside the joint behaves differently from swelling around it. Fluid inside makes the knee feel tight and hard to bend fully, and the kneecap can feel as though it is floating. Swelling in the soft tissues outside the joint pits when pressed and spreads beyond the knee.
Swelling that appears within an hour or two of a twisting injury usually means bleeding inside the joint. The common causes are an anterior cruciate ligament tear, a fracture involving the joint surface, or a tear in the vascular part of a meniscus.
Swelling that takes a day or two to build after an injury more often reflects a meniscal tear or a sprain. The timeline matters, so it is worth telling your doctor exactly when the swelling appeared.
In an older patient with no injury, recurrent swelling after activity is most commonly osteoarthritis. The joint surface is worn, activity irritates the lining, and fluid follows. It typically settles with rest and returns with load.
Swelling in several joints at once, or swelling with prolonged morning stiffness, raises the possibility of inflammatory arthritis and deserves blood tests rather than reassurance. Gout and pseudogout produce dramatic, acutely painful swelling that often comes on overnight.
A knee that is hot, markedly swollen, very painful to move even slightly, and accompanied by fever needs assessment the same day. Infection in a joint is uncommon but it damages cartilage quickly, and it is diagnosed by aspirating the fluid and examining it.
This also applies to a replaced knee that becomes hot and swollen after having been comfortable. That is not a flare to wait out.
A Baker's cyst is fluid from the joint tracking into a pouch behind the knee. It is a consequence of the joint producing excess fluid, not a separate disease, so treating the cyst without treating the cause tends to be futile. A cyst that bursts can cause calf pain and swelling that mimics a clot, which is why that combination is always examined properly.
Relative rest from the activity that triggered it, ice for fifteen to twenty minutes, elevation, and a short course of anti-inflammatory treatment if it suits you medically. Compression with a simple sleeve helps some patients.
Aspirating fluid gives quick relief and, more importantly, gives information when the diagnosis is uncertain. It is not a long term treatment on its own.
A detailed guide on knee swelling is available here: https://www.mayurajcc.com/knee-pain-guides/knee-swelling-water-in-knee
See a doctor if swelling recurs more than twice, if it follows an injury, if the knee locks or gives way, or if it is hot with fever. A clinical examination and a weight-bearing X-ray usually explain it, and aspiration is added when the cause is unclear.
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