Knee Infection (Septic Arthritis): An Emergency

What to do right now

• If your knee is hot, swollen and painful and you have a fever, chills or feel unwell, call 999 or go to the nearest emergency department
• If it is a child who will not walk, or a baby who will not move the leg, go now
• If you have had knee surgery, an injection or a knee replacement and the knee becomes red, hot or starts to leak, contact your surgeon's team the same day or attend an emergency department
• Do not start leftover antibiotics at home before being seen, as they can hide the infection and make it harder to identify
• Do not wait to see whether it settles overnight

Not every hot knee is infected. Gout and other inflammatory conditions can look very similar. But only tests on the joint fluid can tell them apart reliably, so the safe approach is urgent assessment.

What septic arthritis is

Septic arthritis means bacteria have entered the joint and are multiplying in the joint fluid. The body's response, together with toxins from the bacteria, can break down the joint cartilage quickly. Bacteria usually reach the knee through the bloodstream from an infection elsewhere, such as the skin, urine or chest. Less often they enter directly through a wound, a puncture, an injection or surgery.

In children, infection can start in the bone near the growth plate and spread into the joint. In people with a knee replacement, an infection around the implant is a specific problem that needs a specialist plan.

Symptoms, in patients' own words

• The knee became swollen and hot within a day or two, often without any injury
• It hurts to move it even slightly, and you hold it bent because straightening it is painful
• You cannot put weight on it or walk normally
• The skin over the knee looks red or shiny
• Fever, shivering, sweats or simply feeling very unwell

Older adults, people with diabetes and people on medicines that lower immunity may have less fever and milder symptoms, which can delay diagnosis. Babies and young children may simply refuse to move the leg or be irritable.

Who is at higher risk

• Diabetes, especially if blood sugar is poorly controlled, which is common in Qatar and the wider Gulf
• A knee replacement or recent knee surgery or injection
• Existing joint disease such as rheumatoid arthritis, gout or advanced osteoarthritis
• Medicines or conditions that weaken the immune system, including steroids and some arthritis treatments
• Skin wounds, ulcers or infections, including cuts from manual and site work
• Older age, kidney disease and injecting drug use

Infection after an arthroscopy or injection is uncommon, but its risk is one reason careful sterile technique is used for every knee procedure.

How it is diagnosed and treated

The key test is joint aspiration: a needle is used to draw fluid from the knee, which is examined for white cells, crystals and bacteria and sent for culture. Blood tests for infection markers and blood cultures are taken, and an X-ray is usually done. Ideally, the fluid is taken before antibiotics are started, so the bacteria can be identified.

Treatment has two parts. Antibiotics are given through a vein, then usually by mouth, for several weeks depending on the bacteria and the patient. The infected fluid is also removed, most often by arthroscopic washout, in which the knee is flushed through small incisions. Some knees need more than one washout. Infected knee replacements need a tailored plan, which may range from washout with exchange of the plastic insert to staged revision surgery.

Recovery and what happens next

Once the infection is controlled, the focus moves to getting the knee moving again. Early, gentle physiotherapy helps prevent stiffness and muscle loss. Most patients need some weeks to recover strength, depending on how severe the infection was and how quickly it was treated.

Dr. Gelber and the team at Aman Hospital in Doha follow blood markers and the knee's appearance closely, and look for the original source of the infection. Control of diabetes, skin care and treatment of any other infection reduce the chance of it coming back. Some knees develop arthritis later, which is one more reason early treatment matters.