Knee swelling without injury: common causes and when to seek help

Swelling of the knee usually means extra fluid in or around the joint, or thickening of the tissues around it. Joint fluid increases when the joint is irritated or inflamed, which is why swelling is a feature of many different knee problems. Swelling may appear gradually over days or weeks, which is common in wear-related arthritis, or very quickly over hours, which is more typical of gout, bleeding or infection.

The absence of injury does not make a swollen knee less important. Many of the causes listed below are conditions that develop slowly or flare without a trigger. The way the swelling behaves, together with associated features such as heat, redness, fever and pain, is what points a doctor towards the cause.

The NHS lists causes of knee pain with no obvious injury, which is the same group that causes swelling without injury. The conditions it names include pain and stiffness in both knees with mild swelling, more common in older people, which suggests osteoarthritis; a warm and red knee where kneeling or bending makes pain and swelling worse, suggesting bursitis; swelling, warmth and bruising, more likely when taking anticoagulants, suggesting bleeding in the joint; a hot and red knee with sudden attacks of very bad pain, suggesting gout or septic arthritis; and, in teenagers and young adults, pain and swelling below the kneecap, suggesting Osgood Schlatter's disease.

Two further conditions are worth knowing about. A Baker's cyst is a fluid-filled lump at the back of the knee that the NHS says usually happens in adults when another health condition causes damage to the knee joint, such as a knee injury, osteoarthritis, rheumatoid arthritis or gout. Rheumatoid arthritis itself is a long-term condition that causes pain, swelling and stiffness in the joints, according to the NHS.

The NHS says to get advice from NHS 111 now if your knee is very painful, you cannot move your knee or put any weight on it, your knee is badly swollen or has changed shape, your knee locks, gives way or painfully clicks, or you have a very high temperature, or feel hot, cold or shivery, and have redness or heat around your knee, which can be a sign of infection.

For septic arthritis in particular, the NHS describes sudden severe pain in a joint, not being able to move or put weight on it, hot, swollen or red skin over the joint and a high temperature, though not everyone has a fever, and says symptoms usually develop quickly over a few days and need to be checked straight away. It says to ask for an urgent GP appointment or get help from NHS 111, and that treatment is usually in hospital with antibiotics given into a vein and a procedure to drain the fluid. The NHS lists people at higher risk, including those with rheumatoid arthritis, a weakened immune system, a recent joint operation or an artificial joint, and diabetes.

If the lump behind the knee is accompanied by sudden worsening leg pain or swelling, a warm leg or red or darkened skin, the NHS says to ask for an urgent GP appointment or get help from NHS 111 because it could be a burst cyst or a blood clot (DVT). Call 999 or go to A&E if there is also breathlessness or chest pain.

A GP or physiotherapist will ask how quickly the swelling developed, whether it is hot or red, which activities make it worse, what other joints are affected, and about fevers, previous gout, medicines such as blood thinners, and recent illness or surgery. They will examine the knee, compare it with the other side and check movement and stability. The NHS says that in many areas you may be able to get help such as physiotherapy from NHS community musculoskeletal services without a GP referral.

Tests depend on the suspicion. For gout, the NHS says a GP may measure uric acid in the blood, and if the result is unclear may refer you to a rheumatologist, who may take a sample of fluid from inside the joint using a thin needle or arrange a scan. For a Baker's cyst, the GP may arrange an ultrasound or MRI scan. X-rays, blood tests for inflammation and joint aspiration, in which fluid is drawn off and examined, are used when the diagnosis is uncertain or infection is possible.

The NHS says that for osteoarthritis X-rays or blood tests are not usually necessary but may be used to rule out other causes such as rheumatoid arthritis or a fractured bone.

Treatment depends on the cause. For general knee pain and swelling the NHS suggests putting as little weight as possible on the knee, for example avoiding standing for a long time, using an ice pack or a bag of frozen peas wrapped in a tea towel for up to 20 minutes every 2 to 3 hours, and painkillers such as paracetamol or ibuprofen gel or tablets. The NHS says a doctor can suggest treatment based on the cause, which might include medicine, physiotherapy or referral to hospital for a scan or specialist treatment.

For gout, attacks are usually treated with an anti-inflammatory such as ibuprofen, with steroids if that does not help, and uric acid-lowering medicine to prevent attacks in people who have frequent attacks or high uric acid, according to the NHS. For a Baker's cyst, the NHS says it does not always need treatment, and options include painkillers, a steroid injection into the knee, physiotherapy, draining fluid from the cyst, or surgery to repair damage to the knee joint. For osteoarthritis, regular exercise, losing weight if overweight, suitable footwear and devices to reduce strain are recommended, with painkillers and physiotherapy for more severe symptoms.

If anticoagulant medicine is thought to be a factor, do not stop it yourself; seek advice promptly.

See a GP if knee pain or swelling does not improve within a few weeks, as the NHS advises, or if it keeps coming back. Seek earlier review if both knees or several joints are swollen, if there is morning stiffness lasting a long time, which the NHS says may suggest rheumatoid arthritis rather than osteoarthritis, or if you have other symptoms such as rash, eye redness, weight loss or fatigue.

Seeing a physiotherapist early can help with strength and movement while the cause is being established.