The direct comparison of different commercially available demineralized bone matrix substances in an athymic rat model

Abstract

Yu-Po Lee, MD, Jeffrey C. Wang, MD, Los Angeles, CA, USA; Linda E.A. Kanim, MA, Santa Monica, CA, USA; Mark J. Jo, MD, Mark Davies, MD, Jay R. Lieberman, MD, Los Angeles, CA, USAIntroduction: Autogenous bone is used in many spinal fusion surgeries and is currently the “gold standard” against which all other graft materials are compared. However, the morbidity associated with graft harvest can include increased blood loss, chronic donor site pain and increased operative time, to name a few. In addition, the quantity of bone available for harvesting may not be sufficient for long, multilevel fusions or for use in patients who have had previous graft harvests. Consequently, these limitations have led to the development of many commercially available demineralized bone matrix substances. Currently, there are many different commercially available demineralized bone matrix substances available for use in spine surgery, each containing different amounts of osteoinductive proteins. Although each substance is reported to have osteoinductive potential, previous studies that we have done showed that there are differences in the osteoinductive capabilities of these substances. The athymic rat spine fusion model is an ideal system in which to test these substances. This rat cannot reject the human tissues and allows for each substance to be used in its direct, unaltered form. In this study, we will compare the efficacy of three different demineralized bone matrix substances, as well as rattail autograft in fusing a rat spine to our existing body of data. Previous studies done in our lab have shown that Osteofil ICM induces spinal fusion at a rate of 80%, whereas Grafton Crunch is able to induce spinal fusion at a 40% rate.Methods: Twenty mature athymic nude female rats were used (175 to 240 g, Harlan Sprague Dawley, IN). Five rats each were implanted with 1) Allomatrix (0.3 cc per side), 2) Osteofil ICM (0.3 cc per side), 3) rat tail autograft (0.3 cc per side) and 4) Grafton Crunch (0.3 cc per side). 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 0.3 cc was measured in a 1.0-cc syringe and placed into each posterolateral gutter spanning the transverse processes for a total of 0.6 cc of graft per animal. The wound was irrigated and closed. High-resolution radiographs were obtained at 1 month. The presence or absence of remodeling was evaluated on the radiographs. The lumbar spines were explanted at 1 month and 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.Results: None of the spines in the Osteofil ICM and Grafton Crunch groups showed complete fusion at 4 weeks. These spines did exhibit increased stiffness, however, and a fusion mass was evident on visual inspection. None of the lumbar spines in the rat tail autograft group exhibited spinal fusion and there was no evidence of a fusion mass on visual inspection. The Allomatrix group did not exhibit fusion in any of the explanted spines, and the matrix was actually resorbed in 80% (four of five) of the specimens. Nondecalcified histology confirmed the presence of a pseudoarthrosis or the presence of a solid fusion, and the results were highly correlated with manual testing.Conclusion: This is a controlled, prospective comparison of three commercially available demineralized bone matrix substances in a spinal fusion model. Although all of these products claim to have significant osteoinductive capabilities, this study demonstrates that there are subtle differences between them. Our previous studies have shown that Osteofil ICM induces spinal fusion 80% of the time, whereas Grafton Crunch induces fusion 40% of the time at 4 weeks in this model. The Osteofil ICM and the Grafton Crunch may eventually have gone to fusion because they did exhibit increased stiffness on manual palpation. However, none of the spines were fused at the 4-week time point. Lastly, allomatrix did not fuse any of the spines and were actually broken down and resorbed in 80% of the spines. This study demonstrates that further testing is necessary on all of the commercially available demineralized bone matrix substances, because there may actually be significant differences in their osteoinductive capabilities.