knee pain with fever

Overview

Knee pain with a fever can be a sign of an infection inside the knee joint. The medical name for this is septic arthritis. It happens when bacteria get into the joint and cause swelling, pain, and a high temperature. This is serious and needs urgent medical care, because early treatment can protect the joint and prevent long-term damage.

It is not common, but it is serious. Most people with knee pain do not have a fever. When a fever is present, doctors take it very seriously because it raises the chance of an infection.

It can affect people of all ages. Babies, young children, older adults, and people with weakened immune systems are at higher risk. People with existing joint problems, such as rheumatoid arthritis, or those who have recently had joint surgery are also more likely to get a joint infection.

Symptoms

Common symptoms

• Pain in the knee, often severe and getting worse when you move it
• Swelling and stiffness in the knee
• Redness or warmth around the knee
• A fever, which may come on suddenly
• Difficulty putting weight on the leg
• Feeling generally unwell, tired, or achy

Symptoms in children

• Limping or refusing to use the leg
• Crying when the knee is moved
• Being unusually irritable or fussy
• Running a fever
• Swelling or redness around the knee

Symptoms in older adults

• Knee pain that may be less localised
• A low-grade fever, or sometimes no fever at all
• Confusion or becoming less mobile than usual
• Swelling and warmth in the knee
• A sudden increase in joint pain or difficulty walking

Causes

Main causes

• Bacterial infection inside the knee joint (septic arthritis)
• Bacteria spreading from another infection in the body, like a skin or urinary infection
• A recent knee surgery, injection, or injury that lets germs in
• Certain types of inflammation, such as a flare of rheumatoid arthritis, which can also cause fever
• In some cases, viruses can cause joint pain and fever, but this is usually not as severe

Risk factors

• Age under 2 or over 60
• Weakened immune system from illness or certain medicines
• Diabetes or chronic kidney disease
• A recent knee operation, joint injection, or skin wound
• Rheumatoid arthritis or previous joint damage
• Use of intravenous drugs

When to see a doctor

See a doctor urgently if:

• Any knee pain with fever, even if the fever is mild
• Swelling, redness, or warmth around the knee
• Unable to straighten the knee or put weight on it
• Knee pain following recent surgery or an injection

Book a routine appointment if:

• Knee pain that lasts more than a few days with no fever
• Stiffness or swelling that is getting worse
• You have a chronic condition like arthritis and notice a change in your usual symptoms

Diagnosis

A doctor will ask about your symptoms and recent health events, then examine your knee. They will look for swelling, warmth, redness, and pain with movement. If they suspect a joint infection, they will arrange urgent tests.

Tests that may be done

• Joint aspiration — a needle is used to take a small sample of fluid from your knee; this is the most important test for infection
• Blood tests — to check for signs of infection and inflammation
• Blood cultures — to see if bacteria are in your bloodstream
• X-rays — to look at the bone and joint space
• MRI or ultrasound — to get a clearer picture of the joint and surrounding tissue

What to expect at your appointment

You may be sent to a hospital for these tests, especially if the doctor suspects an infection. The joint aspiration can be uncomfortable but is usually quick. If infection is found, treatment will start right away — sometimes before all test results come back.

Treatment

Treatment depends on the cause. If your doctor finds an infection in the knee, the main treatment is antibiotics plus removing the infected fluid. If no infection is present, treatment focuses on reducing pain and inflammation. The sooner treatment starts, the better the outcome for your joint.

Self-care at home

• Rest the knee and avoid putting weight on it until you've been checked.
• Apply a cold pack wrapped in a cloth for 15–20 minutes at a time.
• Keep your leg raised when resting to reduce swelling.
• Take paracetamol for pain only if your doctor or pharmacist says it is safe for you.
• Do not apply heat to a knee that is warm, red, or swollen.

Medical treatments

If you have a bacterial joint infection, you will be given antibiotics, usually through a vein (intravenously) in the hospital. The doctor may also drain the infected fluid from the knee with a needle. In some cases, anti-inflammatory medicines are used to reduce swelling and pain. Your healthcare team will decide the best approach based on the cause and your overall health.

When is surgery considered?

Sometimes a procedure called 'washout' is needed. This involves a surgeon flushing out the knee joint in an operating room to remove infected fluid and damaged tissue. This is done when the infection is severe, when needle drainage is not enough, or when the joint has already been damaged.

Living with this condition

After treatment, you may need to rest and gradually get back to normal activities. Follow your healthcare provider's advice on moving the knee — complete rest for too long can cause stiffness. If you are taking antibiotics, finish the whole course even if you feel better.

Lifestyle tips

• Keep your knee moving gently as advised by your care team.
• Avoid high-impact activities like running or jumping until your doctor says it is safe.
• Shower or wash your knee regularly and keep skin clean to prevent further infections.
• Manage other health conditions like diabetes carefully to support your recovery.

Diet and exercise

Eating a balanced diet helps your body fight infection and repair tissue. Gentle exercises, such as straight leg raises or ankle pumps, can keep blood flowing and maintain strength — but only once your doctor allows it. A physiotherapist can give you safe exercises for your knee.

Mental health and emotional wellbeing

Pain and illness can be exhausting and worrying. It is normal to feel anxious or low. Give yourself permission to rest, and talk about how you feel. If sadness lasts more than a couple of weeks, tell your healthcare provider.

Prevention

You cannot always prevent a joint infection, but you can reduce your risk. Clean and treat any cuts or wounds around the knee properly. If you have a skin infection, get it treated early. Manage long-term conditions like diabetes. If you have had joint surgery, follow your surgeon's care instructions closely.

Vaccines

Staying up to date with recommended vaccines, such as flu and pneumococcal vaccines, can help prevent some infections that could spread to joints. Ask your local health service what vaccines are recommended for you.

Screening programmes

There is no routine screening test for knee infections. The best way to catch a problem early is to see a doctor promptly when you have knee pain with fever.

Complications

If left untreated

• Permanent damage to the cartilage in the knee joint
• Spread of infection to the bone (osteomyelitis) or whole body (sepsis)
• Chronic pain and stiffness
• Inability to use the knee fully

Long-term outlook

With prompt treatment, most people recover well from a knee infection. The key is speed — the earlier you get medical care, the more likely your joint will heal with little or no lasting damage. Even if you need surgery or a hospital stay, the vast majority of people return to their normal lives.

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