Pathogenesis of Metaphyseal Radiolucent Changes Following Ischemic Necrosis of the Capital Femoral Epiphysis in Immature Pigs: A Preliminary Report

Background: Although metaphyseal radiolucent changes are often seen in patients with Legg-Calvé-Perthes disease, the pathogenesis of these changes remains controversial. The purpose of the present study was to determine the prevalence and histopathological characteristics of these metaphyseal radiolucent changes in a piglet model of ischemic necrosis of the capital femoral epiphysis. Methods: Ischemic necrosis of the right femoral head was produced in fifty piglets by surgically placing a ligature tightly around the femoral neck. The contralateral, left hip of each animal was used as a control. Radiographs and histological sections of the femoral heads were examined at two, four, and eight weeks. The radiographs were used to measure the femoral neck length in order to assess growth disturbance. Results: Thirteen of the fifty animals were found to have radiolucent changes in the proximal femoral metaphysis on the side of the infarcted femoral head. These changes were observed in none of the twelve animals that were evaluated at two weeks, in one of the fourteen animals that were evaluated at four weeks, and in twelve of the twenty-four animals that were evaluated at eight weeks. The radiolucent changes ranged from a focal cystic lesion to a diffuse area of radiolucency around the proximal femoral physis. Three distinct types of histological changes were observed in the metaphysis. Type-I changes were characterized by focal thickening of the physeal cartilage extending down into the metaphysis. Some of these lesions demonstrated cystic degeneration of the thickened cartilage. Type-II changes were characterized by central disruption of the physis and resorption and replacement of the metaphyseal bone in the region by fibrovascular tissue. Type-III changes were characterized by diffuse resorption of the physeal cartilage and resorption of the adjacent metaphyseal and epiphyseal bone. The mean femoral neck length on the infarcted side in animals with metaphyseal radiolucent changes was significantly shorter than that in animals without metaphyseal radiolucent changes (p = 0.02). Conclusions: Metaphyseal radiolucent changes frequently were observed in this piglet model at eight weeks after the induction of ischemia. The metaphyseal radiolucent changes were associated with histopathological lesions of the physis. The present study suggests that the presence of metaphyseal lesions can result in a greater growth disturbance of the proximal femoral physis than is seen in the absence of metaphyseal lesions. Clinical Relevance: The present study supports the clinical observation that the presence of diffuse metaphyseal radiolucent changes may be associated with substantial growth disturbance of the proximal part of the femur in patients with Legg-Calvé-Perthes disease. The study provides a histopathological basis for proximal femoral physeal growth disturbance that has not been clearly demonstrated in the past. These findings also may provide a histopathological basis for the fluid-filled metaphyseal cysts that sometimes are observed on the magnetic resonance imaging scans of these patients.