What Causes Swelling Behind the Knee? Could It Be a Baker's Cyst?

What Causes Swelling Behind the Knee? Could It Be a Baker's Cyst?

A Baker's Cyst is a fluid-filled swelling behind the knee that usually signals an underlying issue, such as arthritis, a meniscal tear, or a previous knee injury, rather than being a standalone problem. Treatment focuses on addressing that root cause through physiotherapy, alongside simple measures like rest, ice, and compression for symptom relief. Surgery is rarely needed, but persistent, worsening, or unclear symptoms are worth getting properly assessed rather than self-diagnosed.

If you've ever noticed a soft swelling at the back of your knee, especially one that feels tight or uncomfortable when you bend or straighten your leg, you might be dealing with a Baker's Cyst (also known as a popliteal cyst).

For some people it's just a mild annoyance, but for others it can interfere with walking, exercise, and everyday movement.

What Is a Baker's Cyst?

A Baker's Cyst is a fluid-filled swelling that forms behind the knee. It develops when excess synovial fluid (the lubricating fluid inside your knee joint) builds up and pushes into a small sac at the back of the knee.

This usually doesn't happen on its own. It's often a sign that something else inside the knee is irritated or inflamed.

What Causes a Baker's Cyst?

In most cases, a Baker's Cyst is linked to an underlying knee issue rather than being a standalone problem. Common causes include:

• Arthritis – particularly osteoarthritis or rheumatoid arthritis, which increase fluid production
• Meniscal tears – damage to the cartilage in the knee
• Knee injuries – trauma can trigger inflammation and fluid build-up
• Other inflammatory conditions affecting the joint

What Are the Symptoms to Look Out For?

Not everyone will have symptoms, but when they do appear, they're usually quite noticeable.

You might experience:

• A visible or palpable swelling behind the knee
• Tightness or pressure, especially when bending the knee
• Pain that worsens with activity or prolonged standing
• Stiffness or reduced range of motion
• A "full" or balloon-like sensation at the back of the knee

In some cases, the swelling can extend slightly into the calf, which can feel concerning if you're not sure what's causing it.

How Is a Baker's Cyst Diagnosed?

A physiotherapist can often identify a Baker's Cyst through a simple clinical assessment.

What to Expect

• Physical examination – checking swelling, movement, and discomfort
• Movement testing – to assess how the knee is functioning

If needed, you may be referred for ultrasound or MRI scans to confirm the diagnosis or rule out other issues.

That said, imaging isn't always necessary. Many cases can be identified and managed clinically.

What Treatment Options Actually Help?

The key to treating a Baker's Cyst is addressing the underlying cause, not just the swelling itself.

Physiotherapy

Physiotherapy plays a central role in recovery. Treatment typically includes:

• Manual therapy to reduce stiffness and improve joint movement
• Targeted exercises to strengthen the muscles around the knee
• Mobility work to restore full, pain-free movement

As symptoms settle, rehab progresses toward improving strength, control, and overall knee function, helping prevent recurrence.

Managing Pain and Swelling

In the early stages, simple strategies can make a big difference:

• Rest and activity modification – avoid aggravating movements
• Ice therapy – 15–20 minutes to reduce swelling
• Compression and elevation – to manage fluid build-up

Medical Options (If Needed)

If symptoms persist or become more severe:

• Anti-inflammatory medication may help reduce pain
• Aspiration (drainage) can relieve pressure in larger cysts
• Surgery is rarely needed but may be considered if there's significant underlying damage

When Should You Get It Checked?

It's worth getting assessed if:

• The swelling is increasing in size
• Pain is limiting your movement or activity
• You're unsure whether it's a cyst or something else
• Symptoms aren't improving with rest

Many knee conditions can feel similar, so getting the right diagnosis early can save you a lot of time and frustration.

Getting Back to Comfortable Movement

A Baker's Cyst can be uncomfortable, but the good news is that it's usually very manageable with the right approach. By:

• Treating the underlying knee issue
• Improving strength and movement
• Managing load and activity levels

…you can get back to moving comfortably and confidently again.

Remember, there are many conditions that can have similar symptoms, so searching online may not always give you a clear answer. Consulting with a trained professional is the best way to get a firm diagnosis.

If in doubt, getting it properly assessed is always the smartest first step. Your knees will thank you for it.

Frequently Asked Questions

Is a Baker's Cyst serious?

Usually not, but it's often a sign of an underlying knee issue, such as arthritis or a meniscal tear, rather than a standalone problem. Treating the root cause, not just the swelling, is key to resolving it.

Does a Baker's Cyst need to be drained or operated on?

Not usually. Most cases are managed successfully with physiotherapy and simple measures like rest, ice, and compression. Aspiration or surgery is only considered for larger cysts or significant underlying damage.

How do I know if it's a Baker's Cyst and not something else?

Many knee conditions can feel similar, so it's best not to self-diagnose. A physiotherapist can usually identify a Baker's Cyst through a clinical assessment, with imaging like ultrasound or MRI used if needed.

What are the signs a Baker's Cyst needs medical attention?

It's worth getting checked if the swelling is increasing in size, pain is limiting your movement, or symptoms aren't improving with rest.

Can physiotherapy help with a Baker's Cyst?

Yes. Physiotherapy focuses on addressing the underlying knee issue causing the cyst, through manual therapy, targeted strengthening, and mobility work, which also helps prevent recurrence.

About the Author:

Raj Gandhi

Lead Physiotherapist & Independent Non-Medical Prescriber

Raj Gandhi is a Chartered Physiotherapist and co-founder of Relive Physiotherapy, with over 20 years' specialist experience in Orthopaedics, Sports Injuries, and Musculoskeletal Medicine.

He holds a Master's degree in Sports Physiotherapy, a Postgraduate Diploma in Orthopaedic Medicine and Injection Therapy, and a Postgraduate Certificate in Independent Prescribing. Raj has a particular interest in treating back and neck pain, along with a wide range of sports injuries, and has worked as a Physiotherapist for the Commonwealth Games team. He spent over 10 years in the NHS before leaving to establish Relive Physiotherapy and is widely respected among colleagues for his expertise in Injection Therapy and his hands-on, manual approach to treatment.

Raj is a member of the Association of Chartered Physiotherapists in Orthopaedic Medicine and Injection Therapy (ACPOMIT), the Acupuncture Association of Chartered Physiotherapists (AACP), and the Association of Chartered Physiotherapists in Sports Medicine (ACPSEM). He also mentors the physiotherapy team at Relive, regularly leading training and supporting their ongoing development.

Outside the clinic, Raj is a father to two girls and enjoys playing tennis, listening to audiobooks, and photography.