Lumbar Interbody Fusion With A Novel Injectable Bone Subsitute In Sheep: An Experimental Study

[Objective] To establish an anterior interbody fusion model on sheep lumbar spine; to evaluate the osteogenic capacity of different material fillings and disscuss the feasibility of injectable calcium phosphate cement/silk fibroin/recombinant human bone morphogenetic protein-2 (CPC/SF/rhBMP-2) composite inplated as a novel bone subsitute on spinal fusion.[Methods] Sixteen sheep were randomly divided into two groups. Eight sheep in each group were operated on left retroperitoneum approach and intervertebral discs of L1/2, L3/4 and L5/6 were exposed. Cavities were made on the midpoint of disc space perpendicular to sagittal plane, with 12 mm in depth and 7 mm in diameter. The opposite side of annular fibrosus was kept intact, nucleus pulposus was cleared completely, and endplate was processed to capillary hemorrhage. Three fusion sites in each animal were randomly assigned three of four treatments: calcium phosphate/silk fibroin composite (CPC/SF), calcium phosphate/recombinant human bone morphogenetic protein-2 composite (CPC/rhBMP-2), CPC/SF/rhBMP-2, or autogeous iliac crest cancellous bone. Sheep of the two groups were killed at 3 and 6 months postoperation respectively. The fusion sites were observed and analyzed by manual palpation, CT scan, undestructive biomechanic test, undecalcified histology, and histomorphology.[Results] All the bone subsitutes were injected succesfully while the iliac cancellous bone couldn't be injected by syringe. The structures of all the lumbar spine specimens were presvered integrity with favourable stability and without appendix destroyed. The opposite side of annular fibrosus was kept intact and no leakage of bone subsitutes was observed in the drilling side. The fusion rates of CPC/SF, CPC/rhBMP-2, CPC/SF/rhBMP-2 and autogenous iliac cancellous bone assessed by manual palpation were 0 (0/6), 16.67% (1/6), 33.33% (2/6) and 50% (3/6) at 3 months postoperation, respectively. The fusion rates were 0 (0/6), 33.33% (2/6), 50% (3/6) and 50% (3/6) at 6 months, respectively. The fusion stiffness of all directions among the four materials had significant differences (P<0.05) at 3 months postoperation, with iliac bone the greatest, CPC/SF/rhBMP-2 next, then CPC/rhBMP-2, and CPC/SF the smallest. Statistical differences were not detected between CPC/SF and intact specimen (P>0.05). It revealed similar stiffness between iliac bone and CPC/SF/rhBMP-2 at 6 months after surgery (P>0.05). The similar stiffness of the two materials were greater than CPC/rhBMP-2 and CPC/SF (P<0.05). The new bone formation among the four materials had significant differences (P<0.05) at 3 months postoperation, with iliac bone at most, CPC/SF/rhBMP-2 next, then CPC/rhBMP-2, and CPC/SF the least. The residual volume of calcium phosphate has significant differences (P<0.05) among the three bone subsitutes, with CPC/SF at most, CPC/rhBMP-2 next, and CPC/SF/rhBMP-2 the least. It revealed almost new bone formation between iliac bone and CPC/SF/rhBMP-2 at 6 months after surgery (P>0.05). The similar bone formation volume of the two materials were more than CPC/rhBMP-2 and CPC/SF (P<0.05). It showed the same trendency about residual volume of calcium phosphate with at 3 months postoperation.[Conclusion] The sheep anterior lumbar interbody fusion model, of which the cavities are made on sheep intervertebral space with 12 mm in depth and 7 mm in diameter, is an ideal fusion model to evaluate the capability of bone formation for injectable bone subsitues. The CPC/SF/rhBMP-2 composite has suitable injectable nature, cogulation time, stable supporting mechanics after cogulation, excellent osteoconduction and osteoinduction, and matching degradation and osteogenesis ability. It is a kind of ideal bone subsitute in spinal fusion.