What causes swelling behind the knee?

Whether the swelling appeared suddenly, follows an injury, changes with knee bending, or extends into the calf helps determine how quickly it should be assessed.

A Baker's cyst is a fluid-filled swelling behind the knee that can form when the joint produces extra fluid. Knee osteoarthritis, a meniscus tear, gout, or other joint inflammation can contribute to that fluid. Knee bursitis causes swelling around a cushioning sac, although its exact position varies.

An ACL tear or meniscus injury may present with swelling after twisting, instability, locking, or difficulty bearing weight. Calf and knee swelling can also occur with a blood clot, while a pulsating lump can indicate an artery problem. Those vascular possibilities require prompt medical assessment rather than home treatment.

Reduce activities that increase the pressure or pain, and avoid deep squatting or forceful massage over an unexplained lump. A wrapped cool pack and leg elevation may help after a minor strain if there are no urgent warning signs. Do not try to puncture or drain swelling yourself.

A clinician may inspect both legs, feel the swelling, test knee motion and stability, and check circulation. Ultrasound or other imaging may be considered according to the examination. Joint aspiration is sometimes used to analyse or relieve fluid inside a swollen joint, but it is not suitable for every lump behind the knee.

Seek urgent assessment for sudden swelling with calf pain, warmth, redness, or one leg becoming noticeably larger. Call emergency services if leg symptoms occur with chest pain, breathlessness, coughing blood, faintness, or a racing heartbeat.

Urgent care is also appropriate for a pulsating mass, a cold or pale foot, fever with a hot red knee, major injury, or inability to bear weight. Arrange a routine appointment for a lump that persists, enlarges, restricts bending, or repeatedly returns.

Ask whether the swelling is inside the joint, a cyst, a bursa, or a vascular structure. Mention recent travel, reduced mobility, injury, gout attacks, medicines, and previous knee problems because each detail can alter the assessment.