What are the causes of septic arthritis?

Causes of Septic Arthritis

Septic arthritis most commonly arises from hematogenous spread of bacteria, with Staphylococcus aureus being the predominant pathogen across all age groups. 1, 2

Common Bacterial Pathogens

• Staphylococcus aureus is the most common causative organism, contributing to over two-thirds of identified cases 2, 3
• Streptococci species are the second most common bacterial cause 2
• Gram-negative bacilli, particularly in specific patient populations 2
• Age-specific pathogens include:
  • Group B streptococcus in neonates 1
  • Kingella kingae in children under 4 years of age 1, 4
  • Salmonella species in patients with sickle cell disease 1, 4
• Neisseria gonorrhoeae in sexually active young adults 2

Routes of Infection

• Hematogenous seeding (bloodstream spread) is the most common route 1, 2
• Direct introduction through trauma, injections, or surgical procedures 2
• Extension from a contiguous focus of infection, such as osteomyelitis 1, 2
• In neonates and infants, septic arthritis may occur secondary to spread of osteomyelitis into the adjacent joint 1

Risk Factors

• Advanced age (particularly >80 years) 5
• Diabetes mellitus 5, 6
• Pre-existing joint disease, especially rheumatoid arthritis 5, 6
• Recent joint surgery or presence of prosthetic joints 5
• Skin infections that can lead to bacteremia 5
• Immunosuppressive medication use 5
• Intravenous drug use 1

Pathophysiology

• Bacterial proliferation within the joint space leads to rapid cartilage damage through direct toxicity and inflammatory response 1
• The innate immune system is crucial for defense against staphylococcal infection 3
• Components of the specific immune system (T and B lymphocytes) can actually contribute to joint destruction and increased mortality rates 3
• Bacterial virulence factors that contribute to septic arthritis include:
  • Capsule polysaccharides 3
  • Toxins 3
  • Cell wall-attached adhesins 3
  • Chromosomal DNA 3

Joint Distribution

• The knee is the most commonly affected joint, followed by the hip and shoulder 6
• Over 10% of patients have polyarticular involvement, reflecting bacteremia and diminished resistance to infection 6
• More than 50% of polyarticular forms occur in patients with rheumatoid arthritis 6
• In children, the distribution varies by age:
  • Children under 2 years are more likely to have septic arthritis than osteomyelitis 1
  • Children between 2-10 years have slightly more osteomyelitis than septic arthritis 1
  • Children from 10-18 years have slightly more septic arthritis than osteomyelitis 1

Clinical Implications

• Septic arthritis is considered an orthopedic emergency due to the rapid cartilage damage that can occur 1, 4
• Mortality rates range from 2-15%, highlighting the severity of this condition 7
• Delayed diagnosis and treatment can result in permanent joint damage and functional impairment 5, 6
• Prompt recognition, aggressive antimicrobial therapy, and surgical treatment are critical for ensuring a good prognosis 2

Special Considerations

• Prosthetic joint infections are increasingly common and require different management approaches 6
• Polymicrobial infections may occur, especially in specific clinical scenarios, requiring broader antimicrobial coverage 1, 4
• Concomitant joint and bone infections are common in children and may occur in >50% of cases 1

Understanding the various causes and risk factors for septic arthritis is essential for early diagnosis and appropriate management to prevent the significant morbidity and mortality associated with this condition.