Baker's cyst

A Baker’s cyst (also known as a popliteal cyst) is a swelling that forms behind the knee when the tissue at the back of your knee fills with synovial fluid – the lubricating fluid from the joint. This makes it swollen and inflamed.

Cysts can vary in size and can, very occasionally, develop behind both knees at the same time.

Baker’s cyst is much more common in adults over the age of 40, but they can develop at any age and even in children. It’s also more common in women because they’re more likely to develop conditions that are associated with Baker’s cyst, including osteoarthritis and rheumatoid arthritis.

Anatomy of the knee joint

The knee joint capsule is a thick structure that supports the joint. It surrounds the knee including the kneecap (patella) and the bones of the upper and lower leg (the femur, tibia and fibula). The capsule has an important membrane lining called the synovium which produces synovial fluid. This fluid performs like a lubricant, cushioning the knee joint during movement.

You also have several small pouches filled with synovial fluid next to the knee called bursae (one is called a bursa). Bursae are normally found near joints and their purpose is to reduce friction and allow maximal movement of the joint. The bursa at the back of your knee is called the popliteal bursa.

Each knee joints also contains two pads of cartilage tissue that sit on top of the tibia bone in the lower leg. These are called the medial and lateral menisci. Each meniscus is a shock absorber, they reduce the impact between your upper and lower leg, as well as help to keep the knee joint stable and improve smooth movement.

In some cases there are no symptoms, but an MRI scan for another reason finds the cyst. However, the larger the cyst, the more likely it is that you'll notice symptoms other than the swelling itself.

These symptoms can include:

• Swollen joints
• A build-up of fluid around your knee
• Crepitus (a clicking noise in your knee)
• An ache behind your knee and in your calf
• Difficulty bending your knee
• Tightness and/or swelling at your knee joint

Occasionally a Baker’s cyst ruptures which can be painful, with swelling and redness in your calf and may become infected.

Your GP will examine your knee and ask questions about the swelling and how you’re feeling.

You'll usually get a diagnosis from your GP without any further tests. However, in some cases you might have an ultrasound scan or MRI scan. This is to rule out other causes – for example, deep vein thrombosis (DVT), a tumour or an aneurysm (a bulge in a blood vessel), or to check the extent of the cyst.

Your GP can also shine a light through your swollen area to diagnose a Baker’s cyst. If the light passes through the swollen area, then it’s full of fluid and it’s a cyst.

A child may be referred to a specialist paediatric orthopaedic surgeon if there’s the possibility of an underlying joint disease.

In some cases, a cyst may develop in a healthy knee. This type is called a primary Baker’s cyst and is more common in younger people. You're more likely to develop Baker's cyst if you have an underlying problem with your knee. This is known as a secondary Baker’s cyst. The most common cause of Baker’s cyst in adults is osteoarthritis, caused by wear and tear of the joints, and rheumatoid arthritis, caused by the immune system.

It can also affect people who have gout, a type of arthritis that is caused by the build-up of uric acid.

Other possible causes of Baker’s cyst include a sports-related injury such as:

• A meniscus tear
• Anterior cruciate ligament damage
• Cartilage damage

In children, the cyst may relate to injury or irritation of the bursa. Cysts may also occur with conditions such as juvenile idiopathic arthritis.

There are things you can do yourself to treat Baker’s cyst including rest and taking non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, or painkillers like paracetamol, to reduce swelling and relieve pain if advised by your doctor.

In many cases, the swelling goes away by itself. Using compression bandages and an ice pack can also help. If you don’t have an ice pack, wrapping a bag of frozen peas in a tea towel will work just as well.

Your GP may also suggest using crutches to take the weight off your painful knee until your symptoms improve. A physiotherapist can also give you some gentle exercises to strengthen the muscles around your knee.

A cyst may sometimes be aspirated. This means a fine needle is inserted to remove the fluid. Aspiration may not be appropriate if you have a long-term (chronic) cyst, as it will be filled with jelly rather than a fluid.

In some cases, surgery may be recommended to repair damage around the joint. If this is the case, knee arthroscopy is usually performed ­­– a type of keyhole surgery.

If there’s an underlying condition, such as osteoarthritis, treating this condition will usually shrink the cyst.

Other treatments related to Baker’s cyst include:

• Knee joint repair
• Steroid joint injections

If your cyst has ruptured, your doctor will likely recommend resting your affected leg and keeping it elevated when you can. The fluid will slowly be reabsorbed into your body within a couple of weeks. Your GP may also prescribe some painkillers, such as paracetamol or codeine, to reduce the pain of a ruptured cyst.

What is the best treatment for a Baker's cyst?

You may find that you don’t need treatment for your Baker’s cyst as the swelling often goes away by itself.

Your GP may also recommend things for you to do at home to help reduce the swelling like wearing compression bandages, applying an ice pack or using crutches. A physiotherapist can also give you some gentle exercises to strengthen the muscles around your knee too.

A cyst may need to be aspirated – where your doctor drains the fluid using a needle. In some cases, surgery may be recommended to repair damage around the joint.

What causes a Baker's cyst?

Bakers cysts are caused when fluid from within the knee joint fills up tissues at the back of the knee causing them to swell. This can happen in a healthy knee but usually occurs if you have an underlying condition that affects your joints or a sports-related injury.

What happens if a Baker's cyst goes untreated?

If left untreated, the cyst may go away on its own. However, you may find that the pain gets worse or the cyst increases in size. The cyst can also burst and cause bruising in the lower leg.

Is walking good for a Baker's cyst?

Walking is generally good for overall strength and stability of the knee joint, but you may find your symptoms are aggravated by walking because it causes fluid to pass between the knee joint and the cyst.

How long does it take for a Baker's cyst to heal on its own?

While Baker’s cysts tend to disappear with time, this may take several months or even years.