Septic arthritis
- Emergency: proteolytic destruction of cartilage within days; hip in infants risks AVN and growth arrest.
- Ultrasound: effusion (hip: > 2 mm difference from the other side or capsule–bone distance > 5 mm), guides immediate aspiration: Gram stain, culture, cell count is the diagnostic test.
- Radiograph: effusion (widened joint space, displaced fat pads, hip subluxation in infants), then rapid, uniform joint space loss, subchondral erosion of both surfaces, osteopenia, eventually ankylosis.
- MRI: effusion, synovial thickening/enhancement, marrow oedema of both sides (osteomyelitis vs reactive), abscess; best overall assessment.
- Tom Smith arthritis: infantile septic hip with destruction of the femoral head and neck epiphysis.
- Kocher criteria (child hip): fever > 38.5 °C, non‑weight‑bearing, ESR > 40, WBC > 12,000 (CRP > 20 mg/L added). Transient synovitis is the main differential.