What is septic arthritis?

Septic arthritis is an infection in the joint fluid (also known as synovial fluid) and joint tissues. The infection usually reaches the joints though the bloodstream, although some joints may become infected after surgery or injury of an involved joint, which can expose the joint space to germs or infection.

In rare instances, an infection of the bone can spread to an adjacent joint causing septic arthritis. Bacteria, fungus, or viruses are capable of infecting the joint, although “septic arthritis” is typically used to describe non-viral causes of joint infections, which are usually caused by bacteria.

In most children, septic arthritis only affects one joint in the body. The most common sites for septic arthritis include the:

• Knee
• Hip
• Ankle
• Elbow
• Wrist
• Shoulder
• Pelvis

Causes

Many different types of bacteria can cause septic arthritis. The type of bacteria depends on the site of infection, age of the patient, and any underlying medical conditions that may make a child more likely to acquire a joint infection.

Bacteria most often associated with septic arthritis include, but are not limited to:

• Staphylococci
• Haemophilus influenzae
• Gram-negative bacilli
• Streptococci

These bacteria can enter a child’s body in a variety of ways, including:

• Infection that spreads from another source inside the body, such as a skin or bone infection adjacent to a joint
• Infected wounds
• Open fractures, or bones that penetrate through the skin
• Foreign object penetrating the skin
• Trauma

Signs and symptoms

Symptoms of septic arthritis vary from child to child and may differ depending on which joint is affected, the age of the child, and the type of organism causing the infection.

Septic arthritis typically has a fast onset with symptoms progressing to the point of severe pain and immobility of the affected joint within hours.

Common symptoms include:

• Joint pain, usually severe
• Joint swelling
• Redness in the area of the affected joint
• Warmth around the affected joint
• Limping
• Refusal to walk or bear weight on the affected joint
• Limited use of the affected joint
• Guarding or protecting the affected joint to prevent it from being touched or moved
• Other symptoms of illness, such as fever, vomiting, sore throat or headache

Symptoms of septic arthritis may resemble other medical conditions or problems. Always consult your child's doctor for a diagnosis.

Testing and diagnosis

Prompt diagnosis of septic arthritis is necessary to prevent permanent damage to the joint.

In addition to a complete medical history and physical examination, clinical experts at The Children’s Hospital of Philadelphia use a variety of methods to diagnose septic arthritis:

• Removing and culturing joint fluid to determine the type of bacteria causing the infection, how well your child’s body is fighting the infection, and the appropriate antibiotic to treat the infection.
• Bacterial cultures can be obtained if there is concern an infection in another part of the body has spread to the joint.
• Blood tests can determine if there is evidence of systemic inflammation.
• X-rays visualize bones and surrounding tissues of an affected joint.
• Ultrasounds can visualize extra fluid in a joint and can help guide a needle into a joint to obtain joint fluid.
• Bone scans may be used to identify other sites of infection if the joint is not suspected to be the primary site of infection
• Magnetic resonance imaging (MRI) produces detailed images of organs and structures within the body.

Treatment

Specific treatment for septic arthritis will be determined by your child's doctor based on:

• Your child's overall health and medical history
• The type of bacteria causing the infection, location of infection, and predisposing medical conditions
• Your child's tolerance for specific medications, procedures, and therapies
• Expectation for the course of the disease
• Your opinion or preference

Septic arthritis usually requires treatment with antibiotics, which are often given intravenously to ensure prompt response to therapy. Antibiotics are only effective if your child’s condition is caused by a bacteria.

If your child’s infection is caused by fungi, he will need antifungal medications.

If a viral infection caused your child’s septic arthritis, the virus will usually need to run its course without treatment, although medications can be used to help alleviate pain.

In some cases, the infected joint must be drained and cleaned because antibiotics cannot penetrate deep enough into a severely infected joint. This may require drainage by a needle, tube or surgery.

Other symptom-based treatments may include:

• Medications for pain and fever
• Physical therapy to maintain muscle strength and joint range of motion
• Splinting the joint to relieve pain

Follow-up care

After beginning treatment, patients with septic arthritis will be monitored closely for clinical improvement. Further evaluation or a change in therapy may be required if a child does not improve.

If your child is able to take oral antibiotics and they are effective, he may be discharged from the hospital after a good response to therapy is observed.

After discharge from the hospital, your child will be followed by his regular doctor or an infectious disease specialist. He will be monitored for continued symptom improvement. Further treatment or evaluation of complications from septic arthritis may be warranted after the infection is cleared.

Possible complications may include:

• Increased or decreased mobility of affected joint
• Poor growth of the affected limb if the growth plate was involved
• Long-term damage to the bone surrounding the affected joint
• Increased risk of fracture to bones surrounding the affected joint

Outlook

If septic arthritis is treated fast enough, long-term damage to the joint is usually minimal. However, the risk of permanent damage to the joint or surrounding tissues is increased if: 

• There is knee, hip or shoulder involvement
• Your child is younger than one year old when diagnosed
• Your child’s symptoms were left untreated for too long

Permanent damage could lead to decreased mobility, growth problems, chronic pain, or increased risk of fracture.

With prompt recognition and treatment of septic arthritis, however, the risk of these negative long-term effects is minimal.