What is septic arthritis?

Septic arthritis is an infection of the fluid and tissues inside a joint. The infection usually spreads through the blood from another part of the body, but it can also happen after an injury, surgery or injection into the joint.

It usually affects one joint. Sometimes, it can involve 2 or 3 joints at the same time.

The most common joints to get septic arthritis are the knee or hip. But any joint can be affected. People who inject drugs may get the infection in less common places such as their:

• shoulder
• chest joints
• spine
• pelvis

Septic arthritis is usually an acute condition — this means that it starts suddenly and needs urgent medical treatment. If it's not treated, septic arthritis can cause lasting joint problems or spread infection to other parts of the body. Early diagnosis and treatment are very important.

What are the symptoms of septic arthritis?

Symptoms of septic arthritis usually appear suddenly, and may include:

• joint pain and swelling, usually in one joint
• warmth or redness around the joint
• difficulty moving the joint
• fever, chills or feeling generally unwell
• delirium (sudden confusion)
• coma (loss of consciousness)
• convulsions (seizure, or a 'fit')

In young children, symptoms may be harder to notice. They may refuse to move the affected limb or joint or seem irritable. They may not have fever, especially infants.

People with weakened immune systems may have milder symptoms, such as only slight pain or swelling, even though they have a serious infection.

What causes septic arthritis?

Septic arthritis is usually caused by bacteria entering a joint. This may be caused by any bacteria, but common ones are Staphylococcus aureus (staph infection) and the Streptococcus species, including Group A streptococci and Group B streptococci. Neisseria gonorrhoeae is a common cause if you are sexually active.

Bacteria can enter the joint:

• through the bloodstream from another infection (for example, a skin or throat infection)
• after an injury or animal bite
• after an injection or medical procedure involving the joint

Who is at risk of septic arthritis?

You are more likely to get septic arthritis if you:

• have a weakened immune system (for example, if you are living with diabetes, cancer or if you take certain medicines)
• have rheumatoid arthritis, gout or other joint problems
• have had a knee or hip replacement (prosthesis)
• recently had joint surgery or an injection into your joint
• have a skin infection or open wound
• inject drugs
• are a baby or child, or are over 80 years old

People who smoke or consume large amounts of alcohol also have an increased risk of septic arthritis.

When should I see my doctor?

You should see your doctor straight away if you have:

• sudden joint pain, swelling or redness
• a fever with joint symptoms
• a child who refuses to move a limb or is irritable or upset when the limb is touched

Septic arthritis can cause permanent joint damage. It is important to get medical help immediately, so treatment can be started as early as possible.

How is septic arthritis diagnosed?

To diagnose septic arthritis, your doctor will ask about your symptoms and examine the affected joint.

Tests may include one or more of the following:

• Joint fluid test (aspiration): a small sample of fluid is taken from the joint using a needle to look for bacteria.
• Blood tests: to check for signs of infection such as full blood count (FBC), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) tests.
• Imaging tests: such as an x-ray, ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI) to look for joint damage or fluid buildup or rule out other diagnoses.

Early diagnosis and intervention are important to help prevent complications and joint damage.

How is septic arthritis treated?

Septic arthritis needs urgent hospital treatment. The main goals are to remove infected fluid, stop the infection and protect the joint.

Treatment usually involves these steps:

• Antibiotics: Your healthcare team will give you antibiotics through a vein (IV) at first. After this, you can continue taking antibiotics orally (in tablet, capsule or liquid form) at home for 2 to 4 weeks, depending on your situation.
• Drainage and surgery: the infected joint fluid is removed, and the joint is cleaned (irrigated) and any infected tissue is taken out (debrided). This can be done using either open surgery (arthrotomy) or a less invasive technique called arthroscopy. Your healthcare team may need to repeat the process of irrigation and debridement many times, to fully remove the infection.
• Follow-up: Ongoing follow-up with an orthopaedic surgeon is recommended, especially for children.

What are the complications of septic arthritis?

Without early treatment, septic arthritis can lead to serious problems such as:

• permanent joint damage
• cartilage loss
• reduced movement
• bone infection (osteomyelitis)
• sepsis

Can septic arthritis be prevented?

Septic arthritis can't always be prevented but you can lower your risk of getting it.

Tips to reduce your risk of septic arthritis include:

• keep cuts, wounds and animal or insect bites clean and covered
• see a doctor if you have any signs of infection
• manage conditions that weaken your immune system
• avoid injecting drugs, or if you need to inject for a medical condition, follow instructions from your healthcare team
• get medical treatment for bacterial infections such as staph or strep throat
• quit smoking
• drink responsibly

Resources and support

For more information:

• The Royal Children's Hospital Melbourne has a fact sheet on septic arthritis.
• The Australian Commission on Safety and Quality in Health Care has an information brochure for Aboriginal and/or Torres Strait Islander peoples: 'Could it be Sepsis?'
• You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available to speak with you 24 hours a day, 7 days a week.
• For advice during pregnancy for babies and young children call Pregnancy, Birth and Baby on 1800 882 436 or video call. Available from 7 am to midnight (AET), 7 days a week (including public holidays).