The short answer
A Baker's cyst is a fluid-filled pocket behind the knee that causes tightness, swelling, and discomfort when bending. It is almost always a sign of another problem inside the knee — usually arthritis or a meniscus tear producing extra fluid — so treatment targets that underlying cause, along with ultrasound-guided aspiration and injection when the cyst itself is bothersome.
Medically reviewed by Marc Gruner, DO, MBA, RMSK
Sports medicine physician, Mayo Clinic fellowship. Last reviewed June 2026
Overview
A Baker's (popliteal) cyst forms when the knee produces excess fluid that collects in a pouch behind the joint, creating a bulge, tightness, and pain when bending the knee fully. On its own it is rarely dangerous, but it signals that something inside the knee — commonly arthritis or a meniscus tear — is irritating the joint.
Because the cyst is a symptom rather than the root problem, Dr. Gruner evaluates the whole knee with ultrasound and treats the underlying cause. When the cyst is large or painful, it can be aspirated and injected under ultrasound guidance, which both relieves pressure and confirms accurate placement.
Common symptoms
- Swelling or a bulge behind the knee
- Tightness when bending the knee
- Aching that worsens with activity
- Occasional calf fullness
What to expect at your visit
- 1.
A focused conversation
Dr. Gruner reviews your history, prior imaging, and goals — what you want to get back to doing.
- 2.
Point-of-care ultrasound
When useful, a diagnostic musculoskeletal ultrasound evaluates the injury in real time, with no radiation.
- 3.
A clear, individualized plan
You leave understanding the diagnosis, the non-surgical options, and the expected timeline — in plain language.
Frequently asked questions
- How do you get rid of a Baker's cyst?
- The most reliable approach is to treat the underlying knee problem driving the fluid, such as arthritis or a meniscus tear. When the cyst is large or symptomatic, ultrasound-guided aspiration and injection can relieve it, and controlling the joint inflammation reduces the chance it returns.