Is my swelling bursitis or fluid inside the knee? How can I tell?

Where the fluid is makes the difference

A bursa is a sac outside the joint. When it swells, the fluid collects in one spot, most often directly over the kneecap or on the inner shin below the knee. The joint itself is not affected, so bending and straightening are usually only mildly limited.

Water on the knee, which doctors call an effusion, is extra fluid inside the joint capsule. It is a sign that something inside the knee is irritated or injured, such as a meniscus tear, ligament injury, arthritis or gout.

Telling them apart

• Bursitis: a soft, often well-defined bump in one place; the kneecap outline may be hidden under it but the rest of the knee looks normal
• Bursitis: bending is usually comfortable apart from pressure on the bump when kneeling
• Effusion: the whole knee looks swollen, especially above and beside the kneecap
• Effusion: the knee feels tight, bending is limited and it may be hard to straighten fully
• Effusion after injury: swelling within hours suggests bleeding into the joint and needs prompt assessment
• Behind the knee: a lump there is more likely a Baker's cyst, which is linked to fluid inside the joint

Why it matters

Bursitis is usually caused by pressure or friction and is treated by protecting the area. An effusion points to a problem inside the joint that needs its own diagnosis and treatment. Both can be infected, and a hot, red knee with fever is urgent in either case.

What to do and when to be seen

For a bump over the kneecap without redness, reduce kneeling, use padding and ice, and watch it for two to three weeks. For a swollen whole knee, rest it, elevate and ice, and book an assessment, sooner if it followed an injury. Seek same-day care for heat, redness, fever, or a knee you cannot bend or bear weight on. At Aman Hospital in Doha, Dr. Gelber examines the knee and uses ultrasound, X-ray or MRI depending on what he finds.