Freiberg's disease: quantitative assessment of osteonecrosis on three-dimensional CT.

BACKGROUND: The extent of necrosis is the main determining factor in the outcome of osteonecrosis. There is no method for measuring the extent of osteonecrosis of the metatarsal head in Freiberg's disease. The purpose of this study was to determine the reliability and prognostic ability of a new method for measurement of the extent of osteonecrosis in Freiberg's disease on three-dimensional computed tomography.

METHODS: A retrospective review of 11 cases with symptomatic Freiberg's disease in ten patients (5 males and 5 females; mean age, 27 years) undergoing computed tomography between July 2005 and September 2007 was performed. Two investigators (K.A.C. and H.K.O.) used a new method to measure the necrotic extent of the metatarsal head: the necrotic angle ratio of necrotic arc angle to normal arc angle of the articular surface on the sagittal reconstruction image of computed tomography. The interobserver reliability was determined for computed tomography measurement. Correlation between the Smillie staging using radiographs and the necrotic angle ratio on computed tomography was also evaluated.

RESULTS: One patient presented with stage I disease (Smillie staging), one with stage II, two with stage III, five with stage IV, and two with stage V, respectively. The necrotic angle ratio of the metatarsal head was 14% in stage I, 21% in stage II, 34% in stage III, 43% in stage IV, and 53% in stage V. The interobserver reliability for computed tomography measurement was high (Cronbach α=0.96). We found the increase of the necrotic extent in proportion to the Smillie stage. The Smillie staging using radiographs and the necrotic angle ratio on computed tomography were significantly correlated. (P<0.05)

CONCLUSIONS: Three-dimensional computed tomography measurement of the necrotic extent of the metatarsal head is a reliable and useful method in evaluating the staging of Freiberg's disease and may eventually help to optimize treatment.