Pain behind the knee: common causes and the signs that need urgent care

The back of the knee is a crowded space. Tendons, nerves and blood vessels run through it on their way to the lower leg, and a small fluid-filled pouch can develop there when the joint is irritated. Pain at the back of the knee can therefore reflect a problem inside the knee joint, a problem in the structures that pass behind it, or a circulation problem in the leg.

This page follows the NHS descriptions of the conditions most often responsible and does not replace an examination. A lump you do not recognise anywhere on the body should be checked, and certain combinations of symptoms are urgent.

The NHS describes the main symptom as a soft lump at the back of the knee that may be more visible when standing up. The lump can be the only symptom, but you may also have knee or leg pain, tightness, stiffness or aching in the knee, difficulty bending the knee, and clicking. The NHS says symptoms may be worse after walking.

A Baker's cyst does not always need treatment. Sometimes, particularly in children, it gets better on its own, and when an underlying condition is responsible, symptoms usually improve once that condition is treated. Other options listed include painkillers, a hydrocortisone injection into the knee, physiotherapy, draining fluid from the cyst and surgery to repair damage to the joint, with surgery to remove the cyst itself described as rare. The NHS says a cyst can take a few months to a few years to get better, and that a burst cyst makes symptoms worse than normal until swelling goes down, which can take a few weeks.

A deep vein thrombosis is a blood clot in a vein, usually in the leg. The NHS lists throbbing pain in one leg, usually in the calf or thigh, swelling in one leg, red, blue or darkened skin around the painful area, which may be harder to see on brown or black skin, and swollen veins. It says to ask for an urgent GP appointment or NHS 111 help if you think you have a DVT, and that a doctor who suspects it should refer you to hospital within 24 hours for an ultrasound scan.

The Baker's cyst page makes the same point from the other direction. Pain or swelling in the leg that suddenly gets worse, swelling, bruising and warm skin, red or darkened skin, or swelling that gets harder or softer could mean a burst cyst or a DVT, and both warrant an urgent call. If any of these appear together with breathlessness or chest pain, the NHS says to call 999 or go to A&E, because a clot may have reached the lungs.

Pain felt behind the knee does not always come from a cyst. The NHS knee pain page lists pain and stiffness in both knees with mild swelling in older people as typical of osteoarthritis, warmth and redness that worsen with kneeling or bending as typical of bursitis, and hot, red knees with sudden attacks of very bad pain as gout or septic arthritis. Injury-related patterns include sprains and strains, tendonitis and torn ligament, tendon or meniscus.

Because arthritis and injury are the usual reasons a Baker's cyst forms, the two often travel together: the cyst is the visible sign and the joint is the underlying problem. That is why the NHS says a GP will check for other health conditions that could be causing the cyst and may arrange an ultrasound or MRI scan, or refer you to a specialist for further tests.

The NHS says to see a GP if you have a lump anywhere on your body and do not know what it is. For knee pain without warning features it advises seeing a GP if pain does not improve within a few weeks. To ease symptoms in the meantime it suggests resting the knee gently, applying an ice pack wrapped in a tea towel for around 10 minutes at a time, raising the leg on a pillow, and taking ibuprofen or paracetamol.

Anything that looks like the clot or emergency patterns above should not wait. If the leg is hot, swollen, discoloured and painful, the NHS advice is to ask for an urgent GP appointment or contact 111 the same day.

• Soft lump at the back of the knee with aching or tightness: arrange a GP appointment.

• Sudden worsening pain or swelling in the calf or leg, warmth, colour change: urgent GP or NHS 111.

• Leg symptoms with breathlessness or chest pain: 999 or A&E.

• Pain behind the knee that persists for a few weeks without improving: see a GP or use a community MSK service.