The short answer
A Baker's cyst is a fluid-filled swelling behind the knee, often associated with a problem inside the joint. New calf pain or swelling needs urgent assessment because a ruptured cyst can resemble a blood clot.
Understanding the problem
A cyst may cause tightness or discomfort, particularly with movement. The underlying knee condition matters when planning care. A swelling behind the knee should be assessed rather than diagnosed solely from its position or appearance.
Care and next steps
Treatment can focus on symptoms and the joint problem causing excess fluid. Some cysts improve without a procedure. Do not attempt to drain the swelling yourself. Sudden worsening calf symptoms should not be assumed to be an expected part of a known cyst.
Prepare for your consultation
Note when the swelling appears and whether the knee also locks, gives way or becomes painful. Bring any imaging and previous arthritis or injury records. Ask what is driving the fluid and whether the proposed treatment addresses that cause. If you develop sudden leg symptoms, seek assessment promptly instead of waiting for an already scheduled routine consultation.
Common questions
Can I distinguish a ruptured cyst from a clot at home?
Not reliably. New calf swelling or pain needs medical assessment.
Medical sources & further reading
This patient education guide was approved for publication by Dr Shahrukh Khan on 11 September 2026. It cannot diagnose your symptoms or replace a personalised clinical assessment. Editorial policy · About the doctor.