Critical biological determinants of incorporation of non-vascularized cortical bone grafts. Quantification of a complex process and structure

Our goal in this study was to evaluate the effects of and the interaction between the hypothesized principal determinants of the incorporation of grafts: antigenicity and treatment of the graft. We implanted fresh and frozen cortical bone grafts that were matched for both major and non-major histocompatibility complex antigens (syngeneic grafts), matched for major but not for non-major histocompatibility complex antigens (minor mismatch), and mismatched for both major and non-major histocompatibility complex antigens (major mismatch). We used a rat model with an eight-millimeter segmental defect in the femur. The construct was stabilized with a plastic plate, threaded Kirschner wires, and cerclage wires. We evaluated the grafts at one, two, and four months after implantation. We measured the immune response; assessed the incorporation of the graft with use of histological examination, biomechanical testing, and quantitative isotopic kinetics; and statistically analyzed the effects of and the interactions among three independent variables : time, the degree of matching for major histocompatibility complex antigens, and the treatment of the graft (whether it was fresh or frozen). These three independent variables had profound effects on the pattern, rate, and quality of the incorporation of the graft. Two-way and three-way interactions among these variables were also noted. Serial changes in every dependent variable were observed with time. Systemic antibody specific for donor antigens was measurable only in the serum of animals that had a major mismatch, but freezing markedly attenuated the systemic antibody response. Revascularization was profoundly affected by histocompatibility-antigen matching; the syngeneic grafts were revascularized more quickly and to a greater degree than the grafts with either a minor or a major mismatch. Freezing significantly (p < 0.001) reduced the revascularization of the syngeneic grafts but had no discernible effect on the grafts with a minor mismatch. CLINICAL RELEVANCE: The findings of this investigation are clinically important because they help to explain the unpredictability of incorporation of cortical bone grafts. The graft that is most commonly implanted clinically, the frozen (or processed) mismatched allograft, had the least predictable process of incorporation. However, our findings suggest that the process of incorporation may be manipulated; for example, by the addition or removal of cells and, indirectly, of cytokines.