Baker’s Cyst (Popliteal Cyst): Symptoms, Causes and Treatment in the UK

A Baker’s cyst, medically known as a popliteal cyst, is a fluid-filled sac that forms behind the knee. It often results in a feeling of tightness or a visible bulge, especially when the leg is fully extended or flexed. While the name might suggest a serious growth, a Baker’s cyst is non-cancerous and is typically a symptom of an underlying problem within the knee joint rather than a primary condition itself.

The cyst occurs when the synovial fluid—the lubricating fluid that helps your knee joint move smoothly—is produced in excess. This extra fluid is pushed into the popliteal bursa (a small sac of fluid behind the knee), causing it to expand and protrude. In the UK, this condition is frequently seen in adults over the age of 40, though it can affect individuals of any age who have suffered a knee injury.

In many cases, a Baker’s cyst causes no pain and may go unnoticed. However, as the swelling increases, patients often report several distinct symptoms:

• A visible lump: A soft, fluid-filled bulge behind the knee, which is often most obvious when standing.
• Knee stiffness: Difficulty bending the knee or fully straightening the leg.
• Pain and pressure: A dull ache in the knee and upper calf, which may worsen during physical activity.
• Clicking or locking: Some patients experience a sensation of the knee catching, often related to the underlying joint issue.

According to NICE (National Institute for Health and Care Excellence) guidelines, the presence of these symptoms usually warrants an assessment to rule out more serious causes of leg swelling, such as deep vein thrombosis (DVT).

A Baker’s cyst is almost always a secondary reaction to inflammation within the knee. When the joint is damaged, it reacts by producing too much synovial fluid. Common underlying causes include:

1. Osteoarthritis

This is the most frequent cause in older adults in the UK. The wear and tear of the knee cartilage leads to chronic inflammation and fluid overproduction.

2. Meniscus Tears

Tears in the crescent-shaped cartilage (meniscus) that cushions the knee can cause fluid to leak into the back of the joint.

3. Inflammatory Arthritis

Conditions such as rheumatoid arthritis or gout cause systemic inflammation that can manifest as a popliteal cyst.

4. Sports Injuries

Acute trauma to the knee ligaments (such as an ACL tear) can trigger the formation of a cyst as the body attempts to protect the injured joint.

Most Baker’s cysts will resolve on their own once the underlying cause is managed. NHS guidance suggests several self-care steps to reduce swelling and manage discomfort at home:

• RICE Protocol: Rest your knee, use ice packs for 15-20 minutes several times a day, use a compression bandage, and keep the leg elevated when sitting.
• Over-the-counter medication: Non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can help reduce both pain and inflammation. Always consult a pharmacist or doctor before starting new medication.
• Activity Modification: Avoid high-impact activities like running or jumping until the swelling subsides, but maintain gentle movement to prevent stiffness.
• Weight Management: Reducing pressure on the knee joint through weight loss can significantly decrease fluid production in patients with osteoarthritis.

If you have a persistent lump behind your knee, it is important to obtain a professional diagnosis. You can speak to a GP online in the UK to discuss your symptoms and receive guidance on the next steps. A doctor can often provide a preliminary assessment via video consultation by observing the swelling and discussing your medical history.

An online GP can provide the following services:

• Management Advice: Personalised guidance on exercises and self-care tailored to your specific knee health.
• Prescription Services: If over-the-counter options are insufficient, a doctor may prescribe stronger anti-inflammatories.
• Sick Notes: If your knee stiffness prevents you from performing your job, an online doctor can issue a private sick note for your employer.
• Referral Recommendations: If the cyst is large, painful, or persistent, a GP can advise on whether you need an ultrasound scan or a referral to a physiotherapist or orthopaedic specialist for fluid aspiration.

Occasionally, a Baker’s cyst can burst (rupture). When this happens, the synovial fluid leaks into the calf muscle, causing sudden and intense symptoms. A rupture can cause severe pain, redness, and swelling in the calf. Because these symptoms are almost identical to a Deep Vein Thrombosis (DVT)—a blood clot in the leg—it is vital to seek medical attention immediately to confirm the diagnosis through a scan. While a ruptured cyst is painful, the body eventually reabsorbs the fluid over several weeks with rest.

If conservative measures fail, UK specialists may suggest further interventions:

• Aspiration: A needle is used to drain the excess fluid from the cyst, often under ultrasound guidance.
• Corticosteroid Injections: Steroids can be injected into the knee joint to reduce inflammation and prevent the cyst from recurring.
• Physiotherapy: A tailored exercise programme can strengthen the muscles around the knee, providing better support and reducing joint strain.
• Surgery: In rare cases, if the underlying joint damage is severe (such as a large meniscus tear), surgery may be required to repair the joint, which subsequently allows the cyst to disappear.