Septic arthritis is an intensely painful infection in a joint. The joint can become infected with germs that travel through your bloodstream from another part of your body. Septic arthritis can also occur when a penetrating injury brings germs directly into the joint.

Infants and older adults are most likely to develop septic arthritis. The most common joints affected are the knees and hips. Septic arthritis can quickly and severely damage the cartilage and bone within the joint, so prompt treatment is crucial.

Treatment involves draining the joint with a needle or via an operation. Intravenous antibiotics also may be necessary to stop the infection.

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Septic arthritis typically causes extreme discomfort and difficulty using the affected joint. The joint may be swollen, red and warm, and you might have a fever.

When to see a doctor

See your doctor if you have sudden onset of severe pain in a joint. Prompt treatment may help minimize joint damage.

Source: http://www.mayoclinic.com

Septic arthritis can be caused by bacterial, viral or fungal infections. Bacterial infection with Staphylococcus aureus (staph) is the most common cause. Staph commonly lives on even healthy skin.

Septic arthritis may develop when an infection elsewhere in your body, such as an upper respiratory tract infection or urinary tract infection, spreads through your bloodstream to a joint. Less commonly, a puncture wound, drug injection or surgery in or near a joint may give the germs a pathway into the joint space.

The lining of your joints (synovium) has little ability to protect itself from infection. Your body's reaction to the infection — including inflammation that can increase pressure and reduce blood flow within the joint — contributes to the damage.

Source: http://www.mayoclinic.com

The following tests typically help diagnose septic arthritis:

Joint fluid analysis. To discover exactly what bacterium is causing your infection, your doctor usually takes a sample of the fluid within your joint (synovial fluid) through a needle inserted in the space around your joint. Synovial fluid normally appears clear and thick. Bacterial infections can alter the color, consistency, volume and makeup of the synovial fluid. Lab analysis of your synovial fluid iincludes tests to determine what organism is causing your infection. Blood tests. Your doctor may order blood tests to see if bacteria are present in your bloodstream. Imaging tests. X-rays and other imaging tests of the affected joint also may be ordered to assess any damage to the joint.

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If treatment is delayed, septic arthritis can quickly lead to joint degeneration and permanent damage.

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Risk factors for septic arthritis include:

• Existing joint problems. Chronic diseases and conditions that affect your joints — such as osteoarthritis, gout, rheumatoid arthritis or lupus — may increase your risk of septic arthritis. An artificial joint, previous joint surgery and joint injury also increase your risk.
• Taking medications for rheumatoid arthritis. People with rheumatoid arthritis have a further increase in risk because of the medications they take. Rheumatoid arthritis medications may suppress the immune system, making infections more likely to occur. Also, diagnosing septic arthritis in people with rheumatoid arthritis is difficult because many of the signs and symptoms are similar.
• Skin fragility. If your skin breaks easily and heals poorly, bacteria may have constant access to your body. Skin conditions such as psoriasis and eczema increase your risk of septic arthritis, as do infected skin wounds. People who regularly inject drugs also have a higher risk of infection at the site of injection.
• Weak immune system. A weak immune system may give you a higher risk of septic arthritis because your body can't defend itself against infections. People with diabetes, kidney and liver problems, and those taking drugs that suppress their immune system have an increased risk of infections.

Having a combination of risk factors usually puts you at a greater risk than having just one risk factor.

Source: http://www.mayoclinic.com