Femur osteoid osteoma in children: are there location-dependent differences in MRI findings?

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Osteoid osteomas are most commonly found in the femur and preferentially affect the pediatric population. Magnetic resonance imaging (MRI) findings of femoral osteoid osteomas are not well described. To systematically characterize pretreatment MRI findings of clinically confirmed femur osteoid osteomas in children and determine location-dependent differences. The current study included children (< 18years) with clinically confirmed femoral osteoid osteomas, who underwent pretreatment MRI in the last 14years (2009-2023). Two radiologists retrospectively reviewed all examinations and recorded skeletal maturity, lesion size, epicenter, perilesional dark rim, and concomitant subjacent findings. Chi-squared, Mann-Whitney U, and Kruskal-Wallis tests were used to compare MRI findings between femoral neck and shaft lesions and for subgroup analyses among lesions along the proximal, mid, and distal femoral neck. Our study group included 35 pediatric patients (19 boys, 16 girls, mean age 10.1 ± 3.7; range 4.0-17.3years) with 23 femoral neck and 12 femoral shaft lesions. Radiologists' agreement on MRI findings ranged between 49-100%. Neck lesions were more common among older patients while shaft lesions were more common among younger patients (11.6 ± 3.2 vs. 7.0 ± 2.8years, P< 0.01). Complete perilesional dark rim sign was more common in the femoral shaft than in the femoral neck (92%, 11/12 vs. 17%, 4/23, P< 0.01). Within the femoral neck, a complete dark rim was more commonly observed with lesions in the distal thirdcompared to mid and proximal third (50% vs. 0%, 0%, P= 0.04). Concomitant findings were common, but only effusion-synovitis was significantly more common in femoral neck lesions than in shaft lesions (65% vs. 0%, P< 0.01). And within the neck, effusion-synovitis was more common with proximal and mid than distal neck lesions (100%, 85% vs. 25%, P= 0.01). Differential MRI findings were observed not only between osteoid osteomas in the femoral neck and shaft, but also between lesions located within the proximal and distal neck. Patterns of findings follow the known anatomic boundary of the femoral neck/shaft and these findings can help improve pretreatment diagnostic accuracy.