Classic history and presentation: Patients will typically present with a low-grade fever and a warm, swollen, and erythematous knee that is tender to palpation and painful to both active and passive motion. These symptoms may have progressively worsened following local trauma or a recent infection.
Incidence: Approximately 2 per 100 000 people-years for septic arthritis, and almost half of these cases will involve the knee.
- Age – On average, patients are 51 years old at presentation.
- Sex / gender – Males are most commonly affected.
Pathophysiology: Hematogenous inoculation occurs when bacteria or bacteria-laden phagocytes infiltrate the joint capsule, made possible by the high vascularity and lack of basement membrane of the synovium. The infection causes hyperemia with immune cell recruitment, releasing pro-inflammatory cytokines, which leads to joint erythema and pain. With the release of proteolytic enzymes, permanent cartilage damage can be seen as early as 8 hours after infection, which is worsened by the progressive rise of intraarticular pressure.
Grade / classification system: The Gächter classification of septic knee arthritis is used for surgical decision-making and determining prognosis. This is staged I-IV, primarily based on arthroscopic and radiographic findings.
- Stage I – purulent synovitis
- Stage II – joint empyema
- Stage III – panarthritis
- Stage IV – chronic arthritis
- Staphylococcus aureus – Most prevalent, seen in over 50% of septic arthritis cases. In the United States, methicillin-resistant S aureus (MRSA) is increasingly common.
- Neisseria gonorrhea – Associated with high-risk sexual behavior; usually polyarticular.
- Enteric gram-negative rods – Seen in elderly patients and IV drug users.
- Streptococcus agalactiae – Increasingly common in elderly populations; also seen in neonates and patients with diabetes mellitus.