Successful transpedicular lumbar interbody fusion by means of a composite of osteogenic protein-1 (rhBMP-7) and hydroxyapatite carrier - A comparison with autograft and hydroxyapatite in the sheep spine

Study Design. Transpedicular lumbar interbody fusion (TLIF) was performed in a sheep model comparing three treatment groups: a composite of osteogenic protein (OP)-1 and hydroxyapatite carrier (HA), HA without OP-1, and autograft. Objective. To evaluate the efficacy of the composite of OP-1 and HA (HA-OP-1) in achieving reliable TLIF. Summary of Background Data. Anterior fusion techniques directly address disc-related problems and achieve primary axial stability. However, they are characterized by high morbidity. Alternatively, the theoretically advantageous posterior TLIF technique using autograft fails clinically because it lacks compressive stability. Methods. In 36 sheep, lumbar vertebrae L4 to L6 were instrumented posteriorly. Endoscopically assisted TLIF of L4 to L5 was performed. In 12 sheep, the defect was filled with injectable HA-OP-1. Another 12 sheep were treated with HA and another 12 with autograft. Animals were killed at 8 weeks and evaluated by radiologic, histologic, and histomorphometric analysis and by fluorochrome labeling. Results. Only 10 autograft sheep were available for evaluation. Radiologically and histologically, TLIF with HA-OP-1 led to a fusion rate of 10 in 12 compared with autograft (one in 10 fused) and HA (two in 12 fused) (P = 0.0016). Semiquantitative radiologic and histologic scoring also revealed significant differences with superiority of HA-OP-1 (P = 0.0011). Compared with HA, HA-OP-1 presented significantly more ossification at the bone-cement interface (P = 0.0003) and less cement resorption was responsible for local aseptic inflammation. Conclusions. Biointegration of the osteoconductive HA does not occur, because shear forces cause early HA fracture, subsequent fragmentation, and gross resorption (initiating severe inflammation in four of 12 sheep). In contrast, osteoinductive effects of HA-OP-1 enable bio-integration, resulting in full osseous composite sheathing and solid fusion. By use of this composite, TLIF is successfully applied in sheep. Harvesting autograft and the anterior approach are avoided.