Comparing human “local” bone versus human cancellous bone for spinal fusion in an athymic rat model

Abstract

Yu-Po Lee, MD, Los Angeles, CA, USA; Linda E.A. Kanim, MA, Santa Monica, CA, USA; Jeffrey C. Wang, MD, Los Angeles, CA, USAIntroduction: “Local” bone graft harvested from spinous processes and lamina is sometimes used as a source of graft for accompanying spinal fusion procedures. Local bone graft is periodically used as a substitute for iliac crest bone in order to avoid the morbidity associated with harvesting iliac crest bone graft. Local bone has also been used to augment the available iliac crest bone or mixed with other grafting materials. Local bone is characteristically more cortical compared with iliac crest bone, which is more cancellous. The ability of this cortical bone to induce de novo bone growth and eventual fusion has not been thoroughly tested in spite of its increasing use. Further, the relative effect between local versus iliac crest on the rapidity and/or the rate of fusion remains unknown. The athymic rat is a useful animal model to study the relative differences of these bone grafts, because it provides a nonimmunogenic environment for in vivo evaluation of human tissues for bone induction and spinal fusion. This is a preliminary study to compare both the rapidity and the rate of fusion between local bone and iliac crest bone.Methods: Mature athymic nude female rats were used in this study (175 to 240 g, Harlan Sprague Dawley, IN). The rats each were implanted with either human bone (0.6 cc per side in one group) obtained from the spinous processes of patients undergoing laminectomy (cortical bone) or human bone (0.6 cc per side in one group) obtained from the iliac crests of patients (cancellous bone). All procedures were performed through a midline incision, exposing the transverse processes. Decortication was performed on the L4 and L5 transverse processes (lamina and facet joints were left intact without decortication). An aliquot equal to 0.6 cc was measured in a 1.0 cc syringe and placed into each posterolateral gutter spanning the transverse processes for a total of 1.2 cc of graft per animal. The wound was irrigated and closed. High-resolution radiographs were obtained monthly. The presence or absence of remodeling was evaluated on the radiographs. Explanted lumbar spines were manually tested for intersegmental motion after sacrifice. Any motion detected by manual testing was considered a failure of fusion. Absence of any motion was considered a successful fusion. Histological analysis consisted of nondecalcified sections.Results: All of the animals implanted with human iliac crest bone showed radiographic evidence of new osseous bone formation and incorporation up to 8 weeks time. Conversely, none of the animals implanted with human local bone showed significant new osseous bone formation or incorporation up to the 8-week time point. Comparing the radiograph, the animals implanted with human iliac crest bone appeared more mature than the other group. Manual palpation and visual inspection of the explanted spines confirmed these results. At each time point, the human iliac crest bone demonstrated greater stiffness with more restriction of motion. However, none of the animals showed complete fusion by manual palpation at 8 weeks.Conclusion: This is the first study to directly test the ability of cortical versus cancellous human bone to induce spinal fusion in an athymic rat model. The athymic nude rat model allows for human tissues to be tested, because the rat does not reject the human tissue. This study demonstrated differences in the osteoinductive ability of human local bone versus human cancellous bone. These data suggest that human local bone is inferior to human iliac crest bone. Hence, more research is needed before clinical substitution of human iliac crest bone with human local bone.