Septic arthritis is a painful infection in a joint. It can come from germs that travel through the bloodstream from another part of the body. Septic arthritis also can happen when an injury that goes through the skin, such as an animal bite or trauma, sends germs into the joint.

There are many risk factors for septic arthritis. These include having other joint conditions and having artificial joints, called prosthetic joints.

Knees are most often affected. But septic arthritis also can affect hips, shoulders and other joints. The infection can quickly damage the bone and the tissue that connects the bone to the joint, called cartilage. So the infection needs fast treatment.

Treatment involves draining the joint with a needle or during surgery. Septic arthritis most often also needs treatment with antibiotics.

Septic arthritis often causes bad pain that comes on fast. The pain makes it hard to use the affected joint. The joint may be swollen and warm, and the skin over the joint area may have a change of color. And you might have a fever.

If septic arthritis happens in an artificial joint, it is called prosthetic joint infection. Pain and swelling may start months or years after surgery to replace a joint. Also, the joint may loosen. That causes pain when you move the joint or put weight on it.

See your healthcare professional if bad pain in a joint comes on quickly. Fast treatment can help lessen joint damage.

If you have had a joint replaced, see your healthcare professional if you have pain while using the joint.

Bacterial, viral or fungal infections can cause septic arthritis. Bacterial infection with Staphylococcus aureus (staph) is the most common cause. Staph lives on the skin and in the noses of many people.

You also can get septic arthritis when an infection, such as a skin infection or urinary tract infection, spreads through your bloodstream to a joint. Less often, a puncture wound, a shot of medicine or drugs, or surgery in or near a joint — including joint replacement surgery — can let the germs into the joint space.

Risk factors for septic arthritis include older age and:

• Having joint issues. Long-term conditions, called chronic conditions, that affect your joints can raise your risk of septic arthritis. These include osteoarthritis, gout, rheumatoid arthritis and lupus. Having had joint surgery or a joint injury also may raise the risk.
• Having an artificial joint, called a prosthetic joint. Germs can enter the joint area during joint replacement surgery. Also, germs can infect the joint by traveling through the bloodstream to the joint from another area of the body.
• Taking medicines for rheumatoid arthritis. Some medicines for rheumatoid arthritis lower the body's immune system response, making infections more likely. Diagnosing septic arthritis in people with rheumatoid arthritis may be hard. Many of the symptoms of septic arthritis are like those of rheumatoid arthritis.
• Having fragile skin. Skin that breaks easily and heals poorly can let germs into the body. Skin conditions such as psoriasis and eczema raise the risk of septic arthritis. So do skin sores that become infected.

  People who take drugs with a needle also have a higher risk of infection where the needle goes in.

• Having a weak immune system. People with weak immune systems are at higher risk of septic arthritis. This includes people with diabetes, kidney and liver issues, and those taking medicines that lower their immune system response.
• Having a joint injury. Animal bites, puncture wounds or cuts over a joint can raise the risk of septic arthritis.

Having a mix of risk factors puts you at greater risk than having just one risk factor does.

Delayed treatment for septic arthritis can lead to joint breakdown and long-term damage. In an artificial joint, septic arthritis may cause the joint to loosen or move.

To diagnose septic arthritis, your healthcare professional takes a medical history and does a physical exam. The exam may involve looking at all your joints for swelling, warmth, pain and a change of color.

Tests

The following tests can help diagnose septic arthritis:

• Study of joint fluid. Infections can change the color and other factors of the fluid within the joints. A healthcare professional can use a needle to take a sample of this fluid. Lab tests can show what germ is causing the infection. This helps your healthcare professional know which medicines to prescribe.
• Blood tests. These can show if there are signs of infection in your blood. The test uses a sample of your blood taken from a vein with a needle.
• Imaging tests. X-rays and other imaging tests of the affected joint can show damage to the joint or loosening of an artificial joint.

Joint drainage and antibiotic medicines treat septic arthritis.

Joint drainage

A healthcare professional may need to remove fluid from the infected joint. Ways to drain the fluid include the following:

• Needle. Sometimes, a healthcare professional can remove fluid from the infected joint with a needle put into the joint space.
• Scope procedure. In arthroscopy (ahr-THROS-kuh-pee), a tube with a video camera at its tip goes into the joint through a small cut called an incision. A healthcare professional puts tubes to drain the fluid through small cuts around the joint.
• Open surgery. Some joints, such as the hip, are harder to drain with a needle or arthroscopy. So you might need an open surgical procedure.

Antibiotics

You most often get an antibiotic through a vein in your arm at first. Later, you may be able to switch to an antibiotic in pill form. Treatment may last from 2 to 6 weeks or longer.

Side effects of antibiotics may include upset stomach, vomiting and loose stool. Allergic reactions also can happen. Ask your healthcare professional what side effects to expect from your medicine.

Removal of replacement joint

If a replacement joint is infected, treatment often involves removing the joint and replacing it for a time with a joint spacer. A joint spacer is made of bone cement and full of antibiotics. The spacer helps you use the joint during treatment. After a few months, you get a new replacement joint.

Sometimes a surgeon removes the implant, washes out the joint and places a new implant in a single surgery.

It's not always possible to remove the replacement joint. Then your healthcare professional may clean the joint and remove damaged tissue but keep the joint in place. You get antibiotics through a vein, then in a pill for several months to keep the infection from coming back.

You'll likely first see your main healthcare professional. Your healthcare professional may send you to a doctor who specializes in joint and bone surgery, called an orthopedist, an infectious disease specialist or a joint specialist, called a rheumatologist.

Here's some information to help you get ready for your appointment.

What you can do

When you call to make the appointment, ask if you need to do anything before you go, such as fasting before certain tests. Make a list of:

• Your symptoms, including any that may not seem linked to the reason for your appointment, and when they began.
• Key personal information, including other medical conditions you have and recent infections.
• All medicines, vitamins and supplements you take, including doses.
• Questions to ask your healthcare professional.

Take a family member or friend along, if possible, to help you remember information you get.

For septic arthritis, questions to ask include:

• What might be causing my symptoms?
• What are other possible causes?
• What tests do I need?
• Is my condition likely to go away or to last?
• What treatment do you suggest?
• How soon can I expect my symptoms to get better with treatment?
• What can I do in the meantime for my joint pain?
• Am I at risk of long-term complications from this condition?
• How can I best manage this condition with my other health issues?
• Are there brochures or other printed material that I can have? What websites do you suggest?

Be sure to ask all the questions you have.

What to expect from your doctor

Be ready to answer questions such as:

• How bad are your symptoms?
• Do they come and go or stay with you?
• What, if anything, seems to make your symptoms better?
• What, if anything, seems to make your symptoms worse?
• Have you ever had joint surgery or joint replacement?
• Do you use needles to take drugs?