Septic Arthritis

Overview – Septic Arthritis

Septic arthritis is a medical emergency involving acute infection of a joint space, most commonly caused by Staphylococcus aureus or Neisseria gonorrhoeae. Rapid recognition and intervention are vital, as delayed treatment can lead to irreversible joint damage, avascular necrosis, and even systemic sepsis. This page provides a concise yet detailed overview of the causes, clinical signs, diagnosis, and management of septic arthritis — a critical topic for any final-year medical student.

Definition

Septic arthritis refers to infection and inflammation of a joint, caused by the direct invasion of pathogens into the synovial space.

Aetiology

• Causative Organisms:
  • Staphylococcus aureus → most common overall
  • Neisseria gonorrhoeae → common in young, sexually active adults
  • Others: Streptococci, Gram-negative bacilli, Mycobacteria, Fungi (rare)

Pathogenesis

• Routes of Infection:
  • Haematogenous spread (most common)
  • Direct extension from nearby infection (e.g. osteomyelitis, cellulitis)
  • Iatrogenic (e.g. post-arthroscopy, joint injection)

Clinical Features

Septic arthritis is a true emergency — early treatment is essential.

• Typical presentation:
  • Acute monoarthritis (commonly the knee)
  • Joint held in slight flexion to reduce pain
  • Swelling, redness, warmth, and marked ↓ROM
  • Severe joint pain
• Systemic features:
  • Fever, malaise, lethargy
  • May show signs of sepsis: fever, chills, dehydration
• If Gonococcal origin:
  • Often presents with migratory polyarthritis
  • Accompanied by maculopapulovesicular skin lesions
  • Eventually localises to monoarthritis (typically knee)

Investigations

Management

Urgent joint drainage + appropriate antibiotics = cornerstone of care

• Antibiotics (IV)
  • If gonococcal:
    • Azithromycin, Ceftriaxone, or Doxycycline
  • If staphylococcal:
    • Ampicillin, Erythromycin, or Vancomycin
• Procedures
  • Aspiration or arthroscopic washout of joint
  • Surgical debridement (if necessary)
  • Do NOT use intra-articular steroids
• Supportive
  • Analgesia
  • Joint immobilisation (initially)

Complications

• Avascular necrosis (esp. femoral head due to raised intra-articular pressure)
• Destruction of cartilage and epiphyseal plates
• Osteomyelitis (spread to adjacent bone)
• Joint deformity and chronic disability (if untreated)

Differential Diagnosis

• Crystal arthropathies (e.g. gout, pseudogout)
• Reactive arthritis
• Autoimmune arthropathies (e.g. rheumatoid arthritis, psoriatic arthritis)
• Trauma-related haemarthrosis
• Viral arthritis (e.g. parvovirus)

Summary – Septic Arthritis

Septic arthritis is an acutely painful and potentially destructive joint infection that requires prompt recognition and treatment. Diagnosis relies on joint aspiration, inflammatory markers, and microbiological studies. Management involves empirical IV antibiotics and joint washout. If untreated, complications such as osteomyelitis and joint destruction may occur. For more on musculoskeletal infections and acute joint conditions, visit our Musculoskeletal Overview page.