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

Abstract

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.

Keywords: Decreased enhancement; MRI; Septic arthritis; Transient synovitis.

Fig. 1

Flowchart of children with septic hip arthritis and transient synovitis; > 50,000 indicates white blood cells per milliliter in the aspirated synovial fluid. eMR, electronic medical records; JIA, juvenile idiopathic arthritis; GPA, granulomatosis with polyangiitis; SLE, systemic lupus erythematosus

Fig. 2

A 6-year-old boy with transient synovitis. The patient developed flu-like symptoms 10 days before admission and presented with 1 day of left hip pain, unable to bear weight on the day of admission. He was afebrile, with a white blood cell (WBC) count of 12,400 cells/mL, an erythrocyte sedimentation rate (ESR) of 43 mm/h, and a C-reactive protein (CRP) level of 3.5 mg/dL. Left hip aspiration yielded a WBC count of 1,150 cells/mL with a negative culture. He was treated with a non-steroidal anti-inflammatory drug (NSAID), resulting in symptom improvement. A coronal MRI STIR shows a large left hip effusion. B coronal MRI SET1 with fat suppression post-contrast shows decreased enhancement of the left femoral head

Fig. 3

A 3-year-old boy with septic hip arthritis and myositis. The patient presented with a 3-day history of fever (38.9 °C) and refusal to bear weight. He had a normal white blood cell (WBC) count, an erythrocyte sedimentation rate (ESR) of 67 mm/h, and a C-reactive protein (CRP) level of 15 mg/dL. Left hip aspiration revealed a WBC count of 11,539 cells/mL, with a positive culture for Staphylococcus aureus. A coronal MRI STIR shows moderate left hip effusion and marked edema in the left gluteal muscles. B coronal MRI SET1 with fat suppression post-contrast shows normal enhancement of the left femoral head and enhancement of the left gluteal muscles

Fig. 4

A 3-year-old girl with septic hip arthritis. The patient initially presented with 3 days of limping followed by a refusal to bear weight. She had a fever of 39.6 °C, a normal white blood cell (WBC) count, an erythrocyte sedimentation rate (ESR) of 80 mm/h, and a C-reactive protein (CRP) level of 4.3 mg/dL. Right hip aspiration revealed a WBC count of 175,000 cells/mL, with a positive culture for Staphylococcus aureus. A coronal MRI STIR shows moderate right hip effusion and edema in the adductor minimus muscle. B coronal MRI SET1 with fat suppression post-contrast shows decreased enhancement of the right femoral head