Comparison of Subchondral Fracture on Radiographs with MR Images in Evaluation of Osteonecrosis in LCP Patients

ABSTRACT

PURPOSE: To compare the subchondral fracture on plain radiography and MR image as a method for assessing osteonecrosis in Legg-Calve-Perthes(LCP) pateients. MATERIALS AND METHODS: We retrospetively reviewed 15 hip joint MR images and plain radiography which visualized subchondral fracture. With basis of the Salter-Thompson classification, extent of necrosis was graded group A to B, as follows; Group A= 50%. On PACS workstation, necrotic area of each MR image was measured to calculate the volume of necrotic portion volume = necrotic area x slice thickness. Necrotic areas on MR images were graded group A to B and results were compared with that measured in Salter-Thompson classification. On follow up, bone resorption was measured and the extent was compared with subchondral fracture representing necrotic area and that on MR volume method respectively. RESULTS: In 9 joints of 15 hip joints (60%), the degree of necrosis in Salter-Thompson classification on plain radiographs was different from that on MR volume method. Based on plain radiographs by Salter-Thompson classification, the degree of necrosis was overestimated in 6 (67%) joints, and underestimated in 3(33%) joints compared with MR volume method. On follow up study, bone resorption was not correlated with necrotic extent of subchondral fracture and MR volume method. CONCLUSION: The extent of femoral head necrosis measured by subchondral fracture was different from that measured by MR and was not correlated with bone resorption on follow up. Therefore, usefulness of subchondral fracture as a prognostic factor may be limited.