Can decreased femoral head enhancement differentiate between septic hip arthritis and transient synovitis?

Objective To determine whether decreased femoral head enhancement on MRI differentiates septic arthritis from transient synovitis. Materials and methods This retrospective study included children < 10 years old with hip effusion on post-contrast MRI for suspected musculoskeletal infection. Two pediatric radiologists independently assessed femoral head enhancement. Kocher and modified Kocher scores were calculated from clinical and lab data. Differences between septic arthritis and transient synovitis were analyzed using Student’s t -tests and Fisher’s exact tests. Sensitivity and specificity for diagnosing septic arthritis were calculated for Kocher scores, their individual components, decreased femoral head enhancement, and muscle edema. Interobserver agreement was assessed. Results Thirty-four children were included (20 transient synovitis, 14 septic arthritis). Kocher and modified Kocher scores were significantly higher in septic arthritis ( p = 0.003, 0.008). Interobserver agreement for femoral head enhancement was substantial (kappa = 0.70). On consensus read, decreased femoral head enhancement was seen in 71.4% of septic arthritis and 50.0% of transient synovitis cases ( p = 0.296). Bone marrow edema was present in two septic arthritis cases. Muscle edema had moderate to high sensitivity (71.4%, 92.9%) but moderate to low specificity (75.0%, 50.0%) for septic arthritis. Conclusion Decreased femoral head enhancement does not reliably distinguish septic arthritis from transient synovitis. Relying on this finding alone may lead to unnecessary interventions in children with transient synovitis. Muscle edema and bone marrow edema may support the diagnosis of septic arthritis. Clinical evaluation and inflammatory markers remain critical in guiding decisions for hip aspiration.