Causes / typical injury mechanism: Septic arthritis of the knee is typically caused by a bacterial infection of the joint space, usually through hematogenous spread but also by direct inoculation or contiguous spread from a nearby infection. Once inside the joint space, the pathogen can trigger an immune response with subsequent joint destruction.

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.

Risk factors: Preexisting joint disease (eg, rheumatoid arthritis, osteoarthritis, gout / pseudogout, systemic lupus erythematosus [SLE], trauma, recent surgery), diabetes mellitus, cirrhosis, end-stage renal disease, intravenous (IV) drug use, immunosuppression, chemotherapy, skin diseases (eg, psoriasis, eczema, ulceration), recent bacteremia, smoking, and age older than 60 years.

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

Specific pathogens:

• 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.