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Malignant bone-forming tumors that arise from the surfaces of long bones are far less common than those that arise from within bone. These surface osteosarcomas are clinically and radiographically similar, yet histologically they are quite distinct. In reviewing the literature, we classified the tumors according to three subgroups: parosteal (juxtacortical) osteosarcoma, periosteal osteosarcoma, and high-grade surface osteosarcoma. We also studied the clinical, radiographic, and histological findings in eighty patients who had been treated for an osteosarcoma over a forty-four-year period. Adequate follow-up data existed for forty-eight of the eighty patients. The duration of follow-up ranged from two to fifteen years after the initial operation (amputation or resection). Patients who had a parosteal osteosarcoma had the best prognosis; those who had a periosteal osteosarcoma, the next best; and those who had a high-grade surface osteosarcoma, the poorest. Because of these widely varying prognoses, the lesions require different treatment.