My knee is hot, red and swollen and I have a fever. Is it an emergency?

Why this combination is urgent

Bacteria inside a joint multiply in the joint fluid and release substances that break down cartilage. The sooner the infection is identified and treated, the better the chance of protecting the joint. Septic arthritis can also spread into the bloodstream and make you seriously unwell.

Some people are at higher risk: anyone who has recently had a knee injection, arthroscopy or knee replacement, people with diabetes, those on steroids or other medicines that lower immunity, older adults, and anyone with a skin infection or wound near the knee.

Other causes that can look the same

• Gout or pseudogout: crystal attacks can make the knee just as hot, red and swollen, sometimes with a mild temperature
• Infected prepatellar bursitis: a hot, red swelling on the front of the kneecap rather than throughout the joint, common after kneeling or a graze
• Cellulitis: a skin infection spreading over the knee
• Inflammatory arthritis flares

It is not possible to tell these apart safely at home. The key test is drawing fluid from the knee with a needle and examining it in the laboratory, usually alongside blood tests.

What to do and what not to do

• Go to an emergency department the same day, or call your surgeon urgently if you recently had knee surgery
• Do not take antibiotics left over from a previous illness before being seen, because they can hide the infection and make the joint fluid test unreliable
• Paracetamol for fever and pain is reasonable while you arrange to be seen
• Do not massage, heat or exercise the knee
• Bring a list of your medicines and any recent procedures

What treatment usually involves

If infection is confirmed, treatment usually means washing out the joint, often by keyhole surgery (knee arthroscopy), together with antibiotics, first through a drip and then by mouth for several weeks. If crystals are found instead, gout is treated with anti-inflammatory medicine. After an infection settles, Dr. Gelber follows up with physiotherapy to recover movement and strength, and checks the knee over the following months for any lasting cartilage damage.