Morphology of the lumbar intertransverse process fusion mass in the rabbit model: A comparison between two bone graft materials-rhBMP-2 and autograft

We used computed tomography and the rabbit model to seek an explanation for the biomechanical superiority of recombinant human bone morphogenetic protein-2 (rhBMP-2) as compared with autogenous iliac bone in lumbar intertransverse process fusions, i.e., the rhBMP-2 fusions demonstrated significantly higher strength and stiffness. The results show that the fusion masses using rhBMP-2 have higher volume, and higher minimum contact area of fusion mass attachment to the transverse process. For both bone graft materials there was a correlation between site of failure and the fusion mass quadrant that contained the minimum contact area of attachment to the transverse process. The ideal graft material would be one that produced strong, mature, early fusions that failed along their length at random positions, rather than failing with high frequency at the transverse process. The fusions using rhBMP-2 were nearer to the ideal than those using autograft: These were stronger, more mature, and only 6 of 9 rhBMP-2 failed in the quadrant that contained the minimum contact area of attachment to the transverse process, as opposed to 11 of 12 for autograft.