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Background: Prosthetic reconstruction with extracortical bone-bridging is an effective method of limb salvage after resection of a malignant or locally invasive benign bone tumor. Use of cancellous bone graft alone is less effective in achieving extracortical bone-bridging. The present study was performed to investigate the effects of a corticocancellous onlay graft on bone and soft-tissue formation over a porous-coated replacement prosthesis in the mid-diaphyseal region of canine femora.Methods: Bilateral resection of a six-centimeter segment of the femoral diaphysis and reconstruction with a porous-coated segmental prosthesis was performed in six mongrel dogs. In one limb (the experimental side), eight strips of corticocancellous bone were evenly placed around the junctions between the femur and the prosthetic surface. Cancellous bone was placed under and between the strips of cortical bone. No graft was used in the other limb (the control side). The animals were followed for twelve weeks, with sequential assessments of load-bearing and radiographic evaluation. Biomechanical, histological, and microradiographic analyses of the specimens were performed after death.Results: On the control side, load-bearing at four weeks postoperatively was significantly decreased compared with the preoperative value (p < 0.05); no difference in these values could be detected on the experimental side. Both the area of the callus and the contact area between the bone and the prosthetic shoulder were greater on the experimental side (p < 0.05). The mechanical stiffness and the maximum torque at failure of the extracortical bridging tissue across the junction between the bone and the prosthetic shoulder were eighteen (p < 0.007) and five times greater (p < 0.05), respectively, on the experimental side.Conclusions: Extracortical bone-bridging was accomplished with corticocancellous onlay bone-grafting. Without bone-grafting, bone formed only occasionally. Bone-grafting also enhanced the formation of a soft-tissue capsule around the prosthesis.