Biopsy specimens from 19 patients with transient regional osteoporosis were studied. All patients presented with pain. There were nine patients with transient osteoporosis of the hip. Six of these specimens were therapeutic core biopsies, and three were femoral heads removed during total hip replacement. The other patients with osteoporosis in different locations had biopsies to rule out infection or neoplasm. Five of these patients had transient osteoporosis of the knee. Three had ankle involvement and two had involvement of the tibial shaft. Plain radiographs were available for study in all cases, and either a bone scan or an MRI was studied in each case. Except for one patient who was lost to follow-up, all had resolution of symptoms and radiographic changes. The histologic changes in the biopsies were distinctive, although they were present in varying degrees. There was edema and reactive bone formation in the marrow spaces. In addition, osteoclastic bone resorption was active in 14 of the 19 cases. Although lipid cysts were sometimes found in the marrow spaces, there was no evidence of fat necrosis or bone necrosis. The high bone turnover and absence of fat necrosis suggests that this disorder is a vasomotor response rather than an early stage of osteonecrosis. Awareness of these characteristic histologic changes should enable the pathologist to make a specific diagnosis of transient regional osteoporosis when a biopsy is required
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