REVASCULARIZED INTERCALARY BONE ALLOGRAFTS WITH SHORT-TERM IMMUNOSUPPRESSION WITH CYCLOSPORINE IN THE CANINE

To study the healing process of vascularized intercalary bone allograf t after withdrawal of immunosuppressive drugs, allotransplantation of the tibia diaphysis with a vascular pedicle was performed in eight adu lt mongrel dogs (group 2) and assessments were made both during admini stration and after discontinuation of cyclosporin A. As controls, simi lar grafts with the vascular pedicles were removed and reimplanted bac k to the same animals (five dogs, group 1). Allotransplantation of fro zen stored bone without a vascular pedicle (10 dogs, groups 3A and 3B) were also compared. No union occurred in most cases of frozen stored bone allotransplant because the transplanted bone was resorbed, leadin g to loosening and subsequent failure of osteosynthesis with the plate and screws used. Under cyclosporin A immunosuppression, bony union (i .e., when trabeculae were seen crossing the graft-recipient junction w ith obliteration of the junction line) occurred at almost similar time intervals in all dogs of group 2 (bone allotransplant with a vascular pedicle) by 3 months postoperatively, which was similar to those of g roup 1. No systemic side effects of cyclosporin A were observed. Cyclo sporin A was discontinued 3 months following graft implantation. The b one graft became avascular within a week following withdrawal of cyclo sporin A. However, bone union was maintained, and the transplanted bon e never showed bone resorption, sclerosis, or fracture on serial radio graphs up to the time the animals were sacrificed, between 5 and 14 mo nths later. Histology at sacrifice showed that the transplanted allogr afts were being replaced at both ends by fresh bone derived from the t ransplantation bed. We conclude on the basis of the results of this st udy that solid bony union can be obtained in allotransplanted bone wit h a vascular pedicle if cyclosporin A is given for a brief period. Aft er cyclosporin A is withdrawn, although the bone becomes nonviable sec ondary to rejection occurring in the blood vessels, its skeletal struc ture remains intact, enabling it to maintain its structural support wh ile awaiting replacement by bony ingrowth from both ends of the graft.