Define Septic (Pyogenic) Arthritis. Describe its clinical features, investigations, management and complications.

Definition & Importance

Septic (pyogenic) arthritis is infection within a joint. It is an orthopaedic emergency because pus and bacterial enzymes destroy articular cartilage within days, leading to permanent joint damage. It is commonest in young children and in immunocompromised or prosthetic-joint adults; the usual organism is Staphylococcus aureus (in sexually active young adults consider Neisseria gonorrhoeae).

Routes of Infection & Pathology

The joint is infected haematogenously, by spread from adjacent osteomyelitis (especially where the metaphysis is intracapsular — hip, shoulder, elbow, ankle), or by direct inoculation (penetrating injury, injection, surgery). Bacterial multiplication provokes a purulent effusion; enzymes and the host inflammatory response digest the proteoglycan and collagen of cartilage, and raised intra-articular pressure can compromise the epiphyseal blood supply.

Clinical Features

Acute onset of a hot, swollen, exquisitely painful joint held still, with severe pain on the least passive movement and inability to bear weight, accompanied by fever and systemic upset. In an infant the only signs may be pseudoparalysis, irritability and refusal to feed. The hip, being deep, may show only a flexed, abducted, externally rotated posture with pain.

Investigations

Urgent joint aspiration is the key investigation: the fluid is turbid/purulent with a very high white-cell count (predominantly neutrophils); send it for Gram stain, culture and crystals (to exclude gout/pseudogout). Blood tests show raised WCC, CRP and ESR; take blood cultures. Ultrasound confirms an effusion and guides aspiration of the hip; radiographs are often normal early but exclude other pathology.

Management

This is an emergency requiring urgent joint decompression and antibiotics. The joint is drained and thoroughly washed out — by arthroscopic or open arthrotomy lavage (repeated aspiration is an option in accessible joints); the hip must be formally drained without delay. Empirical IV antibiotics against Staph. Aureus are begun after cultures and adjusted to sensitivities, then continued (IV then oral) for several weeks. The joint is rested then mobilised early to preserve movement once infection is controlled.

Complications

Cartilage destruction and secondary osteoarthritis, joint stiffness or ankylosis, avascular necrosis (notably of the femoral head), growth disturbance in children, dislocation of the septic hip, and systemic sepsis.

Differential Diagnosis & Special Situations

The main differentials are transient synovitis (in children), acute osteomyelitis, crystal arthropathy (gout/pseudogout — hence always send fluid for crystals), reactive arthritis and haemarthrosis. Special situations to recognise: gonococcal arthritis in sexually active young adults (may be migratory with tenosynovitis and a rash), and prosthetic joint infection, which needs specialist combined medical and surgical management (debridement with implant retention, or one-/two-stage revision). Immunocompromise, diabetes and intravenous drug use predispose to atypical organisms and sites (e.g. Sternoclavicular, sacroiliac).

Aspirate every acutely hot swollen joint — delay destroys cartilage.

• Orthopaedic emergency — pus destroys cartilage within days.
• Commonest organism Staph. Aureus; gonococcus in sexually active young adults.
• Hot, painful, swollen joint held still; Kocher criteria differentiate the child’s septic hip.
• Urgent aspiration (Gram stain, culture, crystals) then emergency drainage + IV antibiotics.
• Complications: cartilage loss/OA, stiffness, AVN, growth disturbance.