Septic Arthritis: A Joint Infection That Needs Urgent Care

What Is Septic Arthritis?

Septic arthritis is an infection of a joint that develops over a few hours or days. It starts in the lining of the joint or the tissues around it, and it is usually caused by bacteria. In young, sexually active adults, the bacteria that cause gonorrhoea are a known cause. Other bacteria, such as those that live on the skin, are more common in older people and can destroy the joint quickly.

Germs can reach a joint through the bloodstream, through a nearby wound or skin infection, or, rarely, after an injection or operation on the joint.

Septic arthritis is a medical emergency. With fast treatment most people recover well, but if it is left untreated, the infection can permanently damage the cartilage within days and can spread into the blood.

Symptoms

Over a few hours to a few days, people with septic arthritis develop moderate to severe joint pain, warmth, tenderness and swelling, and the joint becomes hard to move, both on their own and when someone else moves it. The skin over the joint is sometimes red. Fever and feeling unwell are common, but in some people, especially older adults, these general symptoms are mild or missing.

Usually only one joint is affected, most often the knee, followed by the hip, shoulder, ankle or wrist.

In Babies and Children

In infants and children, septic arthritis may show up as discomfort, refusing to use a limb (the arm or leg hangs still, as if it were paralysed), irritability and poor feeding. Fever may be high, mild or absent. A child who suddenly will not put weight on a leg should be seen the same day.

Who Is at Higher Risk

• Older adults
• People with diabetes, rheumatoid arthritis or kidney disease
• People with a joint replacement
• People who recently had an injection or operation on the joint
• People with skin infections or ulcers
• People whose immune system is weakened by illness or medicines

How It Is Diagnosed

The key test is to draw fluid from the joint with a fine needle. The fluid is examined under a microscope and grown in the laboratory (a culture) to find the germ and the right antibiotic. Blood tests look for signs of infection, and blood cultures check whether germs have entered the bloodstream. X-rays or an ultrasound scan help check the joint.

Gout and some other conditions can look very similar, which is why testing the joint fluid matters so much.

Treatment

Treatment has two parts, and both start quickly.

If a joint replacement becomes infected, the treatment is different and often involves further surgery to the implant. Read about revision joint replacement.

Recovery

Most people feel better within a few days of starting treatment, although the joint can stay stiff and sore for weeks. Regular check-ups and blood tests make sure the infection has fully cleared. If the joint was damaged before treatment started, arthritis can develop in it later, which is one more reason not to wait.