Longitudinal Quantitative Evaluation of Lesion Size Change in Osteonecrosis of the Femoral Head Using 3D-MR Volume Registration
M Takao, N Sugano, T Nishii, J Masumoto, H Miki, Y Sato, S Tamura, H Yoshikawa
Department of Orthopaedic Surgery, Osaka University Graduate School of Medicine, Osaka, Japan

There is a controversy whether lesion size of osteonecrosis of the femoral head (ONHF) decrease or not during natural course on magnetic resonance imaging (MRI). This study reports a longitudinal study of lesion volume using high-resolution serial MRI and recently developed techniques for image registration to realign the serial images. Baseline and follow-up (on minimum one year later) MRI scans were carried out on 15 patients (18 hips). Accurate subvoxel registration was performed and subtraction images were produced to reveal areas of regional necrotic lesion change. Volume-to-femoral head ratio (VFR) was calculated to normalize the measured volume to the total femoral head volume. Three of 18 hips showed spontaneous reduction in the size of the lesions. The mean decreasing volume of these 3 hips were 3.4 ± 2.0 cm3 (SD) and its VFR was 6.8 ± 3.1 % (SD). Mean necrotic lesion at baseline of decreasing lesions and unchanged lesions was 4.6 ± 2.5 cm3 (VFR, 9.1 ± 3.9 %) and 7.5 ± 5.5 cm3 (VFR, 16.7 ± 12.4 %), respectively. There was a trend for mean necrotic lesion volume to be smaller in patients who showed decreases in lesion size. Necrotic lesions within 6 months after diagnosis tended to show a reduction in size (p<0.01). In conclusion, there were some lesions which decreased in size on MRI scan in early stages and smaller lesions had a tendency to decrease in lesion size over time.