Baker's cyst

A Baker's cyst, also called a popliteal cyst, is a soft bulge in the hollow behind the knee. Inside it is ordinary joint fluid: too much of it has been made, and it has been squeezed out of the joint into a neighbouring sac. That is why in an adult the cyst is almost never a disease in its own right; it is a sign that something is wrong inside the knee. In children it appears on its own more often and disappears with time. Any new lump behind the knee is worth showing to a doctor: a large artery and a nerve run through there, and not everything that bulges in that hollow is a cyst.

How it forms

The knee is wrapped in a membrane holding a small amount of fluid, which lubricates the cartilage. When the joint is irritated by worn cartilage, a torn meniscus or inflammation, more fluid is produced. At the back, between the tendons, sits a sac connected to the joint cavity by a narrow passage. That passage works almost like a valve: fluid leaves easily, but comes back badly. The sac stretches, and a springy bulge appears behind the knee.

From this follows the main point about treatment: as long as the joint keeps producing surplus fluid, the cyst will fill again, however many times it is drained.

What the person feels

• a swelling in the hollow behind the knee, usually nearer the inner edge; it stands out with the leg straight and softens when the knee bends;
• a feeling of tightness and fullness at the back, stiffness after sitting;
• an aching pain in the knee or the calf, worse after a long walk and towards evening;
• the knee will not bend fully and sometimes clicks;
• with a large cyst, heaviness, tingling or numbness in the calf.

Quite often the bulge is the only complaint at all, and the person finds it by chance or hears about it from somebody else.

What lies behind a cyst in an adult

• wear of the joint, the commonest cause after forty;
• a meniscus injury, an old or recent knee trauma;
• rheumatoid arthritis and other inflammatory joint diseases;
• gout and crystal deposits inside the joint;
• a pus-forming joint infection: rare, but it is precisely the one that gives a hot, red, sharply painful knee with a high fever.

In children the cyst usually appears with a healthy joint, for no reason at all, and often goes away by itself over months or years. That is exactly why children are almost never offered an operation straight away.

When it is not a cyst

This is the most important part. The hollow behind the knee can hold swellings that look similar and mean something entirely different.

• A clot in the deep veins. Swelling and pain in the calf, skin warmer and tighter, walking hurts. By feel alone it cannot be told apart from a burst cyst.
• A widening of the popliteal artery. The swelling pulses distinctly under the fingers. It can throw off a clot and suddenly cut off the blood supply to the foot, and in that situation the count is in hours.
• A soft tissue tumour, including a malignant one. Firm, it does not soften when the knee bends, it grows gradually, sits immovably and sometimes hurts at night.
• A ganglion, a lipoma, an enlarged lymph node, inflammation of a bursa with no connection to the joint.

Hence the rule: a pulsating, hard, fast-growing or immovable lump behind the knee must be seen by a doctor without delay, while an ordinary cyst should be confirmed by ultrasound rather than by eye.

If the cyst bursts

The wall of the stretched sac can give way, and the fluid runs down between the calf muscles. The pain then rises sharply, the calf swells, the skin reddens and turns hot, and a few days later a bruise sometimes shows up near the ankle. The picture resembles a vein clot so closely that they cannot be separated without ultrasound, so this is not a situation for guessing but for being scanned. The escape of fluid is not itself dangerous: the swelling settles over one to three weeks.

Signs that call for urgent help

Call the emergency service or go to an emergency department at once if:

• breathlessness, chest pain, coughing up blood or a faint are added to the pain and swelling in the leg: that is how a clot that has reached the lungs shows itself;
• the foot has turned pale or blue, has gone cold and has lost sensation;
• the lump behind the knee pulses unmistakably;
• the knee is hot, red and sharply painful, with a high fever and chills;
• the leg has gone numb and weak to the point where you cannot put weight on it.

If a clot is suspected, do not drive yourself: ask to be taken or call for help. Take with you the list of medicines you are on.

How it is sorted out

The doctor looks at and feels the hollow behind the knee with the leg straight and bent, checks the range of movement and searches for the cause inside the joint itself. The main method is ultrasound: it shows whether there is fluid or solid tissue inside, and lets the veins be examined at the same time. Magnetic resonance imaging is needed when a meniscus or cartilage injury is suspected, when the swelling looks unusual, and before surgery. An X-ray does not show fluid, but it does show wear of the joint. Blood tests make sense if inflammatory arthritis or infection is suspected.

Putting a needle into the swelling without having looked at it on ultrasound is not acceptable: the artery and the nerve run alongside, and what passes for a cyst may turn out to be a widened vessel.

What helps

What is treated is not the bulge but the knee: when inflammation in the joint settles, the cyst shrinks by itself.

• a few days of rest during a flare, and then movement without fail, because complete stillness weakens the muscles and drags recovery out;
• cold through a cloth for ten to fifteen minutes several times a day, with the leg raised;
• pain relief: paracetamol or non-steroidal anti-inflammatory drugs in a short course, with an eye on stomach, kidney and heart conditions; the same group also comes in forms applied to the skin;
• exercises for thigh strength and knee mobility under the guidance of a physiotherapist, the most dependable investment of all where the joint is worn;
• drawing the fluid off with a needle under ultrasound guidance, sometimes with a glucocorticoid injected into the joint: it relieves the pressure, but without treating the cause the cyst fills again;
• treating the underlying condition: surgery for a torn meniscus, medicines for rheumatoid arthritis, lowering uric acid in gout;
• surgical removal of the cyst, a rare option for when everything else has failed.

It is fairer to know the timescale in advance: a cyst can persist for months and years and shrink gradually rather than within a week.

What takes the load off the knee

• losing weight: while walking the knee carries several times the weight of the body, so even a couple of kilograms less makes a difference;
• swimming, cycling and walking on the flat instead of running on tarmac and jumping;
• fewer deep squats, less time kneeling, fewer descents of steep stairs carrying a load;
• shoes with a cushioning sole and no worn-down heel;
• building the load up gradually rather than making up for everything over a weekend;
• if the knee swells after every training session, what needs changing is not the painkiller but the training.

Online consultation

At an online appointment the doctor will ask how long the bulge has been there and how it has changed, will separate the signs that call for urgent investigation from a calm situation, will explain where to start, with an ultrasound or with an orthopaedic examination, and will advise which exercises are safe for the particular state of your knee.

This material is for information only and does not replace medical advice.