Baker's Cyst: Lump and Swelling Behind the Knee

What is a Baker's cyst?

A Baker's cyst, also called a popliteal cyst, is a pocket of joint fluid that bulges out at the back of the knee, usually on the inner side. The knee normally makes a small amount of lubricating fluid. When the joint is irritated, it makes more, and some of that fluid can be pushed through a natural one-way connection into a sac between the calf and hamstring tendons, where it collects.

The cyst itself is not dangerous and is not a tumour. It is better thought of as a sign that something inside the knee needs attention.

Symptoms in patients' own words

• "There's a soft lump behind my knee that gets bigger when I stand"
• "It feels tight at the back when I bend or squat"
• "I can't bend my knee all the way"
• "It aches after walking and goes down overnight"
• "My calf suddenly swelled and hurt", which can happen if the cyst leaks or bursts

Some cysts cause no symptoms and are found on a scan done for another reason.

Causes and who gets it

In adults, a Baker's cyst almost always sits alongside another knee problem. Osteoarthritis and degenerative meniscus tears are the most common, particularly from middle age. Inflammatory arthritis, gout and cartilage injuries can also cause it.

In children, Baker's cysts are less common, usually not linked to joint damage and often disappear on their own.

In Qatar we often see it in people who are on their feet for long hours at work, in runners and gym-goers with an existing meniscus problem, and in older adults with arthritis who find that kneeling for prayer or sitting on the floor makes the back of the knee feel tight.

Diagnosis

Examination often shows a soft swelling behind the knee that is more obvious with the knee straight. Dr. Gelber also examines the rest of the knee to find the cause.

An ultrasound confirms that the swelling is a fluid cyst. An MRI shows the cyst and the problem inside the knee, such as a meniscus tear or cartilage wear, and X-rays show arthritis.

It is important to rule out a blood clot in the calf (deep vein thrombosis), which can look similar to a burst cyst, so a sudden painful swollen calf should be scanned urgently. Rarely, a lump behind the knee is something other than a cyst, which is another reason to have it checked properly.

Non-surgical treatment

Most Baker's cysts are treated without surgery by calming the knee. That usually means activity modification, ice, and physiotherapy to restore movement and strengthen the thigh muscles. Anti-inflammatory medicines can help when suitable for you.

For a large, painful cyst, fluid can be drawn off with a needle under ultrasound guidance, sometimes combined with a steroid injection into the knee joint to reduce the fluid the knee produces. Cysts can come back if the underlying cause remains.

Surgical treatment

Surgery is considered when symptoms persist and the cause inside the knee can be treated. Knee arthroscopy can address a meniscus tear or loose body that is driving the fluid, and in selected cases the connection to the cyst can be treated at the same time. Removing the cyst alone without dealing with the cause often leads to recurrence. In advanced arthritis, the cyst usually settles once the arthritis itself is treated, which may include knee replacement in some patients.

Recovery and when to see a specialist in Doha

With non-surgical care, many cysts become less troublesome over weeks to a few months. After arthroscopy most patients walk the same day, return to desk work within days to a couple of weeks, and build up activity with physiotherapy.

See a knee specialist if the swelling is growing, painful or limiting bending, or if you have knee pain, catching or swelling as well. Seek urgent care for a sudden painful, swollen calf, especially after a long flight. Dr. Gelber sees patients at Aman Hospital in Doha.