Why does knee bursitis cause knee swelling?

Bursae are small sacs that reduce friction between skin, tendons, muscles, and bone. When one near the knee is repeatedly pressed, rubbed, or struck, its lining can become inflamed. Extra fluid collects inside the sac, creating a soft or firm-looking lump over the kneecap, below it, or along the inner knee, depending on the affected bursa. The area may feel tender and warm, and kneeling or bending can increase discomfort.

Bursal swelling is different from fluid inside the knee joint, and several knee conditions can look similar from the outside. A clinician may check the exact location, skin changes, movement, and recent activities before deciding what evaluation is needed.

Avoid kneeling, leaning on the knee, and activities that repeatedly rub the swollen area. Use a cold pack wrapped in cloth for short periods and keep gentle movement within a comfortable range. A cushion or knee pad can reduce pressure when you must kneel. Do not massage a red, hot swelling or attempt to drain it. If symptoms continue, a clinician can discuss treatment and a physiotherapist can help adjust movement and loading.

Arrange medical review if swelling persists, returns often, limits bending, or follows an injury. Seek prompt care if the knee becomes increasingly red or hot, the swelling grows quickly, fluid drains from the skin, or you have fever or feel unwell. These signs can indicate infection or another problem requiring assessment. Get urgent help after major trauma if you cannot stand or bear weight.

Is the fluid in a bursa or inside the knee joint? Which activities are most likely to maintain the swelling? How should I protect the area at work? Which changes mean I should return sooner?