Baker's Cyst
A fluid-filled swelling behind the knee
Quick Facts
- Type: Joint (knee) condition
- Location: Back of the knee (popliteal area)
- Common causes: Arthritis, knee injury, cartilage tears
- Treatment: Treating the underlying knee problem
Overview
A Baker's cyst, also called a popliteal cyst, is a fluid-filled sac that forms behind the knee. The knee joint normally contains a small amount of lubricating fluid. When a problem in the knee causes the joint to make too much fluid, the excess can push into a sac at the back of the knee, creating a noticeable bulge.
A Baker's cyst is usually a sign of another knee problem, such as arthritis or a cartilage tear, rather than a disease on its own. Many cysts cause little trouble and improve when the underlying issue is treated. Occasionally a cyst becomes large or bursts, which can cause pain and swelling in the calf.
Symptoms
Some Baker's cysts cause no symptoms and are found by chance. When symptoms occur, they may include:
- A bulge or feeling of fullness behind the knee
- Swelling behind the knee that may extend into the calf
- Tightness or stiffness, especially when fully bending or straightening the knee
- Mild aching or discomfort, often worse with activity
- A lump that feels softer when the knee is bent
If a cyst bursts, fluid can leak into the calf, causing sudden pain, swelling, redness, and a sensation of water running down the leg. Because these signs can resemble a blood clot, they should be checked promptly.
Causes
A Baker's cyst forms when the knee produces too much joint fluid, which collects in a sac behind the knee. Common underlying causes include:
- Osteoarthritis: Wear-and-tear arthritis is a frequent cause in adults.
- Inflammatory arthritis: Such as rheumatoid arthritis.
- Cartilage tears: A torn meniscus can irritate the knee and increase fluid.
- Knee injury: Trauma that inflames the joint.
- Other knee conditions: That cause swelling and excess fluid.
In children, Baker's cysts can occur without an obvious underlying joint problem and often resolve on their own.
Risk Factors
- Osteoarthritis or rheumatoid arthritis of the knee
- A previous knee injury or cartilage tear
- Older age
- Activities or sports that stress the knees
Diagnosis
A clinician can often suspect a Baker's cyst from a physical examination of the knee.
- Physical examination: Feeling the swelling behind the knee and checking how it changes with bending and straightening.
- Ultrasound: Confirms that the lump is fluid-filled and helps rule out a solid mass.
- MRI: Provides detailed images and can reveal underlying problems such as a cartilage tear.
If a blood clot in the leg is a concern, imaging may be used to exclude that, since the symptoms can overlap.
Treatment
Treatment focuses on the underlying knee problem, which usually reduces the cyst. Many cysts need no specific treatment.
- Rest and activity changes: Reducing activities that aggravate the knee.
- Ice and elevation: To ease swelling and discomfort.
- Pain relief: Over-the-counter anti-inflammatory medications when appropriate.
- Treating the cause: Managing arthritis or repairing a cartilage tear often shrinks the cyst.
- Fluid drainage or injection: A clinician may drain the cyst or inject a corticosteroid to reduce inflammation.
- Physical therapy: Strengthening and stretching to support the knee.
Surgery to remove a cyst is uncommon and is usually considered only if it keeps coming back and causes significant problems.
Prevention
- Manage underlying knee arthritis with appropriate treatment
- Strengthen the muscles around the knee to support the joint
- Maintain a healthy weight to reduce stress on the knees
- Warm up and use good technique during sports to avoid knee injuries
- Address knee injuries early before they cause ongoing swelling
When to See a Doctor
See a clinician if you notice persistent swelling or a lump behind the knee, or if knee pain and stiffness are interfering with daily activities. Seek prompt care if you have sudden calf pain, swelling, redness, or warmth, since a burst Baker's cyst can mimic a blood clot, and a clot needs urgent evaluation. Also see a doctor for a rapidly growing lump, fever, or a hard, fixed mass.
Frequently Asked Questions
What causes a Baker's cyst?
It forms when the knee makes too much joint fluid, which collects in a sac behind the knee. This usually happens because of an underlying problem such as osteoarthritis, rheumatoid arthritis, or a cartilage tear that irritates the joint.
Is a Baker's cyst dangerous?
Most are harmless and cause only mild swelling or discomfort. The main concern is if a cyst bursts, causing calf pain and swelling that can resemble a blood clot. Because a clot is serious, sudden calf pain and swelling should be checked promptly.
Will a Baker's cyst go away on its own?
Many cysts shrink or disappear when the underlying knee problem is treated, and in children they often resolve without treatment. Some persist, and a clinician may drain the cyst or treat the cause if it is bothersome.
How is a Baker's cyst different from a blood clot?
A Baker's cyst is a fluid-filled sac behind the knee, while a blood clot (deep vein thrombosis) is a clot in a leg vein. A burst cyst can cause similar calf swelling and pain, so imaging such as an ultrasound may be used to tell them apart.
Do I need surgery for a Baker's cyst?
Usually not. Most are managed by treating the underlying knee condition, with rest, ice, and sometimes drainage or a steroid injection. Surgery is reserved for cysts that keep returning and cause significant problems.
References
- Mayo Clinic. Baker's cyst — Symptoms and causes.
- MedlinePlus, U.S. National Library of Medicine. Popliteal cyst.
- American Academy of Orthopaedic Surgeons (OrthoInfo). Popliteal (Baker's) Cyst.
- Arthritis Foundation. Baker's Cyst.