Extracortical bone bridging in tumor endoprostheses. Radiographic and histologic analysis

Aseptic loosening remains a major problem following prosthetic replacement after resection of periarticular tumors. Attempts to decrease the rate of loosening led to the introduction of a composite segmental prosthesis in which the shoulder of the intramedullary stem is porous-coated to allow extracortical bone bridging and bone ingrowth. The purposes of this study were to determine the extent of extracortical bone bridging around, and the amount of bone growth into, the porous-coated shoulder of endoprostheses implanted following the resection of periarticular bone tumors and to correlate the radiographic and histologic findings.

Twenty tumor endoprostheses implanted with use of the extracortical bone-bridging technique were evaluated radiographically to determine the extent of extracortical bone and the amount of bone ingrowth. Five of these endoprostheses were retrieved and subjected to histologic analysis with backscattered electron microscopy and transmitted light microscopy to determine the extent of bone ingrowth.

At a mean of twenty-eight months postoperatively, varying amounts of extracortical bone formation were seen radiographically in all patients. Radiographs also appeared to show bone growth into the porous-coated segment of all implants. However, histologic analysis of the five retrieved prostheses revealed that none of the extracortical bone had actually grown into the porous-coated segment of the implant.

This study confirmed that autogenous bone-grafting of the bone-implant junction of a tumor endoprosthesis consistently results in the formation of extracortical bone. Although radiographs seemed to indicate that this bone grows into the porous coating, this was not confirmed histologically. Growth of extracortical bone into the extramedullary, porous-coated portion of tumor endoprostheses in humans may not be attainable with the current prosthetic design and surgical technique.