Swelling at the back of the knee: causes and next steps

Swelling behind the knee may be joint fluid or a Baker's cyst, but injury, inflammation, and circulation problems also need consideration. The fullness may feel tight when you bend or straighten the knee, and it may change with activity. Its location is useful information, but it cannot identify the cause by itself.

A Baker's cyst is a fluid-filled swelling at the back of the knee that can develop when the joint produces extra fluid. Knee osteoarthritis, a meniscus tear, gout, or bursitis may irritate the joint and contribute to fluid. An ACL tear can cause swelling after a twisting injury, usually with pain or a feeling that the knee gives way. A tender, warm, red area or sudden calf symptoms requires a different assessment.

Notice whether the lump is soft or tense, whether it changes with knee position, and whether the whole knee is swollen. Record any recent twist, fall, long journey, new exercise, or fever.

Rest from movements that increase pressure, such as deep squats, kneeling, sprinting, or repeated stairs. Keep gentle, comfortable movement rather than holding the knee rigid for long periods. Elevation and a wrapped cold pack may ease symptoms. Do not try to puncture, massage forcefully, or repeatedly squeeze the swelling.

Book an assessment when the swelling persists, returns, restricts bending, or follows an injury. A clinician may examine the knee and calf and decide whether imaging or joint aspiration is appropriate.

Get urgent help for sudden calf or leg swelling, increasing warmth or redness, severe pain, breathlessness, chest pain, fever, inability to bear weight, or a knee that is locked. A lump behind the knee can sometimes resemble other problems, so sudden changes should be assessed rather than treated with home massage.

Ask whether the swelling is inside the joint or in the tissues behind it, whether a Baker's cyst is likely, and whether a scan is needed. Mention any injury, calf symptoms, fever, locking, or instability. Ask which exercises are safe and what change should prompt an earlier review.