Joint Infection (Septic Arthritis)

How joints become infected

Joints are lined by a membrane (synovium) that produces lubricating fluid. Bacteria can reach the joint:

• through the bloodstream from an infection elsewhere, such as skin, urinary tract or teeth, or from injection drug use (most common)
• directly, from a wound, animal bite, joint injection or surgery
• from nearby bone infection (osteomyelitis), particularly in children

Once inside, bacteria multiply and trigger intense inflammation. Enzymes released by bacteria and white blood cells can destroy cartilage within days.

Causes and risk factors

Organisms

• Staphylococcus aureus, including MRSA
• streptococci
• Neisseria gonorrhoeae (gonococcal arthritis), often with skin pustules and pain moving between joints and tendons
• gram-negative bacteria in older adults, people with weakened immunity and those who inject drugs
• Kingella kingae in young children
• rarely: tuberculosis, fungi, Lyme disease (which usually causes a less acute arthritis)

Risk factors

• age over 80 and under 5
• existing joint disease: rheumatoid arthritis, osteoarthritis, gout
• a joint replacement
• recent joint surgery or injection
• diabetes, kidney failure, liver disease, cancer
• immune-suppressing medicines
• injection drug use
• skin infections and ulcers

Symptoms

• sudden onset of severe joint pain
• swelling, warmth and redness
• inability to move the joint or bear weight
• fever and chills (absent in some, especially older adults)
• in babies: irritability, not moving a limb, crying when the diaper is changed (hip infection)

Usually one joint is affected; several joints may be involved in gonococcal infection, rheumatoid arthritis or sepsis.

Prosthetic joint infection

Infection of an artificial joint may develop soon after surgery (wound redness, drainage, fever) or months to years later (gradually increasing pain, loosening, or a sudden hot swollen joint after a bloodstream infection).

Diagnosis

The key test is joint aspiration: drawing fluid from the joint with a needle before antibiotics are started if possible.

Other tests:

• blood tests: white blood cell count, CRP and ESR, blood cultures
• X-rays to look for joint damage or fractures; ultrasound to detect fluid, especially in the hip
• MRI for bone involvement or deep joints
• gonorrhea testing of urine, throat and genitals in sexually active patients

Treatment

• Drainage: removing infected fluid is essential. Options include repeated needle aspiration, arthroscopic washout, or open surgery (especially for the hip or prosthetic joints).
• Intravenous antibiotics: started immediately after samples are taken, typically covering Staphylococcus aureus and streptococci (with MRSA cover where needed), then tailored to culture results. Treatment usually lasts 2 to 6 weeks, often switching to oral antibiotics after improvement.
• Pain relief and joint rest, followed by early physical therapy to restore movement.
• Prosthetic joint infection: surgery ranging from washout with retention of the implant to removal and replacement in one or two stages, plus prolonged antibiotics.

Complications

• permanent cartilage damage and early arthritis
• joint stiffness or fusion
• osteomyelitis
• sepsis
• growth disturbance in children
• death, particularly in older adults with other illnesses

Outlook

Prompt treatment greatly improves outcomes. Even so, a significant proportion of patients have lasting joint problems, and mortality is meaningful in older and medically frail people. Delays of more than a few days increase the risk of permanent damage.

Frequently asked questions

What are the signs of a joint infection?

A joint infection usually causes sudden, severe pain in one joint, with swelling, warmth, redness and difficulty moving it or putting weight on it. Fever and chills are common. Anyone with these symptoms should go to the emergency department.

How is septic arthritis different from gout?

Both can cause a sudden hot, swollen, painful joint. Gout is caused by uric acid crystals, often in the big toe, and is not an infection. Because they look alike and can occur together, doctors test joint fluid for both crystals and bacteria.

How long does it take to recover from a joint infection?

Antibiotic treatment usually lasts 2 to 6 weeks. Pain and swelling improve within days of drainage and antibiotics, but full recovery of strength and movement can take months with physical therapy. Some people have lasting stiffness or arthritis.

Can a joint replacement get infected years later?

Yes. Bacteria from an infection elsewhere in the body can travel through the blood to an artificial joint at any time. New pain, swelling, warmth or fever in a replaced joint should be checked promptly.