AUGMENTATION OF AUTOGRAFT USING RHBMP-2 AND DIFFERENT CARRIER MEDIA IN THE CANINE SPINAL-FUSION MODEL

This study evaluated the use of recombinant human bone morphogenetic p rotein (rhBMP-2) with various types of carrier media, and the effect o f rhBMP-2 as an adjunct to autogenous iliac crest bone graft in the ca nine spinal fusion model. BMP induces mesenchymal cells to differentia te into cartilage and bone. The recent availability of rhBMP-2 has cre ated the opportunity to evaluate this material's properties in augment ing autogenous bone graft in spinal fusion. Currently, the most approp riate type of carrier media for rhBMP-2 is undetermined. Bilateral int ertransverse spinal fusions were performed on six canine lumbar spines at L1-L2, L3-L4, and L5-L6, using autogenous posterior iliac crest bo ne graft at each level, creating a total of is segmental fusion sites. All 18 sites were then randomly assigned to one of six fusion methods : autogenous bone graft (ABG) alone, ABG + rhBMP-2, ABG + collagen (He listat) ''sandwich'' + rhBMP-2, ABG + collagen (Helistat) morsels + rh BMP-2, ABG + polylactic/glycolic acid sponge (PLGA) sandwich + rhBMP-2 , and ABG + open-pore polylactic acid morsels + rhBMP-2. Each material was evaluated for ease of handling and application at the index surge ry. The animals underwent computed tomography (CT) scanning of the lum bar fusion sites after 8 weeks. Volumetric measurements of total fusio n mass at each level were performed using two-dimensional CT scan slic es and a volumetric program supplied by the Siemens Medical System. Th e animals were killed after imaging studies. The lumbar spine fusion s ites were evaluated for integrity of the fusion mass, both visually an d with manual mechanical stressing. Crossover of the fusion mass to ad joining levels was also evaluated. Histologic evaluation of all fusion sites was performed. The addition of rhBMP-2 significantly increased bone graft volume as noted on CT scan. Carrier that could be mixed wit h morselized bone graft offered easier handling and application and al l spine segments fused. Polylactic/glycolic acid (PLGA) sites were ass ociated with a greater incidence of voids within the fusion mass. No s ignificant difference in carrier media for rhBMP-2 could be determined . However, PLGA was associated with a higher rate of fusion mass void formation. rhBMP-2, when added to autograft, significantly increased t he volume and the maturity of the resulting fusion mass.