Baker's cyst

Also called Popliteal cyst

A fluid swelling behind the knee, caused by something inside the joint. A burst one looks exactly like a blood clot.

What it is

A swelling of fluid behind the knee, formed when fluid from an irritated knee joint is pushed backwards into a pouch.

It is almost always a symptom rather than a disease. In adults the underlying cause is usually osteoarthritis, a meniscus tear, or an inflammatory arthritis such as rheumatoid arthritis. Treating the cyst without addressing the knee simply lets it refill.

In children they are common, usually unrelated to any joint problem, and almost always resolve without treatment.

The clinically important part is what happens when one bursts. Fluid tracks down into the calf, causing sudden pain, swelling, warmth and sometimes bruising around the ankle. **It is indistinguishable from a deep vein thrombosis without a scan**, and it must be treated as a possible clot until an ultrasound says otherwise. Anybody presenting with a painful swollen calf, even with a known Baker's cyst, needs that excluded.

Most cysts need no treatment. Where they are troublesome, aspiration with a steroid injection helps temporarily, and definitive treatment is treating the knee.

Signs you might notice

A soft, sometimes firm, swelling behind the knee, most obvious when standing with the leg straight.

Tightness or aching behind the knee, worse on bending fully or after activity.

Sometimes no symptoms at all.

A burst cyst: sudden pain behind the knee, then swelling, warmth and tenderness in the calf, and sometimes bruising around the ankle a day or two later.

**Same-day assessment for any swollen painful calf.** Do not assume it is a burst cyst; deep vein thrombosis must be excluded by ultrasound.

Also urgent: a hot, red, very painful knee with fever, which may be septic arthritis; or numbness, pins and needles or a cold foot, which suggests pressure on nerves or vessels.

A firm, hard or rapidly growing lump behind the knee needs imaging; not everything there is a cyst.

How it can affect day-to-day life

The name misleads people into thinking it is a lump that should be removed. Surgery to remove the cyst without treating the knee has a high recurrence rate, and the joint is the target.

Because it is caused by knee fluid, anything that settles the knee settles the cyst: reducing aggravating load, quadriceps strengthening, weight management, and treating the arthritis.

A ruptured cyst is painful for a week or two and then settles; the anxiety about whether it was a clot is often the worse part, and getting the scan resolves it.

In children, reassurance and watchful waiting is right, and imaging is only needed if the swelling is unusual, painful or growing.

For anybody with an inflammatory arthritis, a recurring cyst is a sign the disease is not controlled and is worth reporting to rheumatology rather than treating locally.

Supporting someone well

Get a swollen painful calf assessed the same day, every time. Assume a clot until proven otherwise.

Treat the knee, not the cyst: ask for physiotherapy and address the underlying arthritis or meniscal problem.

Build quadriceps strength; it protects the joint and reduces effusion.

Use ice and simple pain relief for a flare.

Ask about aspiration and steroid injection if it is genuinely troublesome, understanding it may recur.

Ask for imaging if the lump is hard, fixed or growing.

For an inflammatory arthritis, report a recurring cyst as a marker of disease activity.

In a child, expect it to resolve on its own.

Where to get help

A GP practice or self-referral to physiotherapy.

Same-day assessment or urgent care for a swollen painful calf.

A and E for a hot, red, painful knee with fever.

Arthritis UK for knee exercises and self-management.