What is Septic Arthritis?
Septic arthritis is an acute bacterial infection of the joint space that represents an orthopedic emergency, as bacterial proliferation can cause irreversible cartilage damage within hours to days if not treated immediately with joint drainage and antibiotics. 1
Definition and Pathophysiology
Septic arthritis is an acute monoarticular inflammation caused by direct invasion of the joint space by microorganisms, most commonly bacteria, though viruses and fungi can also be causative agents. 2, 3 The infection typically occurs through three main routes:
- Hematogenous seeding (most common): Bacteria reach the joint space during bacteremia and deposit in the highly vascular synovial membrane 4
- Contiguous spread: Direct extension from adjacent osteomyelitis, particularly common in neonates and infants, with concomitant joint and bone infections occurring in more than 50% of pediatric cases 4
- Direct inoculation: Through trauma, surgery, or intra-articular injections 5
The rapid destructive nature stems from bacterial virulence mechanisms, including production of toxins and proteolytic enzymes that directly degrade cartilage, plus formation of biofilms that protect bacteria from antibiotics and immune cells. 4
Clinical Presentation
The classic presentation includes acute onset of monoarticular joint pain, swelling, and fever, though this complete triad occurs in only approximately 50% of cases. 1, 4 Key clinical features include:
- Joint findings: Warm, tender, swollen joint with effusion and painful range of motion 6
- Systemic signs: Fever (>101.3°F), though may be absent in immunocompromised patients 7
- Functional impairment: Inability to bear weight on the affected limb 7
Large joints are most commonly affected, particularly the knee (most frequent), followed by hip and shoulder, though small axial joints are increasingly recognized. 6, 8
Causative Organisms
Staphylococcus aureus is the most common causative organism across all age groups, accounting for approximately 36% of cases (MSSA) plus an additional 17.6% (MRSA). 4, 8 The microbiology varies by patient demographics:
- Neonates: Group B streptococcus predominates 1, 4
- Children <4 years: Kingella kingae is common alongside S. aureus 1, 4
- Adults and older children: S. aureus predominates, with streptococcal species second most common 4
- Special populations: Salmonella species in sickle cell disease patients; polymicrobial infections and atypical organisms (fungi, mycobacteria) in immunocompromised hosts 4
MRSA is increasingly common and must be covered empirically, particularly in shoulder septic arthritis where it is the leading causative organism. 4, 8
Diagnostic Criteria
The American College of Radiology diagnostic criteria include: 1
- Fever >101.3°F
- Erythrocyte sedimentation rate (ESR) ≥40 mm/hour
- White blood cell count ≥12,000 cells/mm³
- Inability to bear weight
- C-reactive protein >2.0 mg/dL
Meeting all five criteria approaches 100% likelihood of septic arthritis. 1 However, elevated inflammatory markers (ESR and CRP) are present in most but not all cases. 6
Definitive Diagnosis
Joint aspiration is the definitive diagnostic procedure and should be performed promptly without waiting for advanced imaging. 1 Synovial fluid analysis reveals:
- White blood cell count ≥50,000 cells/mm³ is highly suggestive of septic arthritis 1
- >90% polymorphonuclear leukocytes on cytologic analysis 2
- Positive synovial fluid culture in approximately 80% of non-gonococcal cases 1
- Gram stain and culture are mandatory even if crystals are present, as crystal arthropathies can coexist with infection 1
A critical pitfall: negative synovial fluid culture does not exclude infection, occurring in approximately 20% of cases despite true infection. 4 If clinical suspicion remains high with negative cultures, consider percutaneous bone biopsy to evaluate for concurrent osteomyelitis. 1
Prognosis and Mortality
Septic arthritis carries significant morbidity and mortality:
- Mortality rate: 2-15% in most series, with 90-day mortality of 7% in patients ≤79 years and 22-69% in those >79 years 3, 5
- Poor functional outcomes (amputation, arthrodesis, prosthetic surgery, severe functional deterioration) occur in 24-33% of patients 5
- Good to excellent joint function is achieved in only 27-90% of cases with rapid and correct treatment 6
The key to preventing irreversible joint destruction is immediate recognition and treatment, as bacterial enzymes cause permanent cartilage damage within hours to days. 1, 4