Septic Arthritis

Septic arthritis is most commonly caused by haematogenous spread of bacteria to the synovium, though direct inoculation (trauma/surgery) can occur. The most common organism in the UK is Staphylococcus aureus. Other organisms include Streptococcus and, in sexually active young adults, Neisseria gonorrhoeae. It typically affects a single large joint, with the knee involved in 50% of cases. Risk factors include pre-existing joint disease (especially RA), prosthetic joints, diabetes, intravenous drug use, and immunosuppression. The mortality rate remains high (up to 10%), and survivors often suffer significant permanent joint damage. Management involves 'Sepsis Six' if systemic, urgent arthrocentesis before starting antibiotics, and surgical washout if the patient does not improve. Antibiotics are initially empirical (broad-spectrum) and then tailored to sensitivities for a total of 4-6 weeks.

The synovial membrane is highly vascular and lacks a limiting basement membrane, making it susceptible to bacterial seeding during episodes of bacteraemia. Once bacteria enter the joint space, they adhere to articular cartilage and trigger an intense acute inflammatory response. Macrophages and neutrophils release proteolytic enzymes and cytokines (e.g., TNF-α, IL-1). This, combined with increased intra-articular pressure from the effusion, leads to chondrocyte death and rapid destruction of the cartilage matrix (collagen and proteoglycans). Permanent damage to the joint can occur within 24-48 hours if the infection is not treated.

The classic presentation is the 'acute hot joint'. The patient is typically febrile and looks unwell. The affected joint is held in a position of maximum comfort (often slight flexion). Any attempt at passive movement is extremely painful. In patients with pre-existing RA, an out-of-proportion flare in a single joint should be treated as septic arthritis until proven otherwise. Complications include osteomyelitis, joint destruction leading to secondary OA, and systemic sepsis. Differential diagnoses include gout, pseudogout, and reactive arthritis.

- Bedside: Vital signs (NEWS2); Urgent joint aspiration for Gram stain, culture, and crystal microscopy.
- Bloods: FBC (leucocytosis), CRP/ESR (usually significantly elevated), Blood cultures (positive in 50%).
- Imaging: X-rays are often normal early on but may show joint space widening; later showing destruction. Ultrasound can guide aspiration of deep joints like the hip.
- Special: PCR for N. gonorrhoeae if suspected.

Emergency: Start Sepsis Six if indicated. Empirical Antibiotics: Follow local guidelines (often IV Flucloxacillin 2g QDS; Clindamycin if penicillin-allergic; Vancomycin if MRSA risk). Antibiotics should be continued IV for 2 weeks, then oral for another 2-4 weeks. Surgery: If the joint remains septic after aspiration, or for deep joints (hip), urgent surgical irrigation and debridement ('washout') via arthroscopy or arthrotomy is required. Joint splinting initially followed by early physiotherapy to regain function.