Evaluation of an injectable calcium phosphate cement as an autograft substitute for transpedicular lumbar interbody fusion: a controlled, prospective study in the sheep model.

Anteroposterior procedures for lumbar interbody fusion usually combine posterior instrumentation with anterior techniques that achieve primary stability for compressive loading: tricortical strut-graft, anterior plating systems, or cages. In comparison to transpedicular lumbar interbody fusion (TLIF), these methods bear the burden of the additional anterior approach. TLIF with autograft, in contrast, does not prove to be clinically sufficient because of its lack of primary compressive stability. In a sheep model, we therefore developed a TLIF method providing primary stability for axial loading. In 24 sheep, L4-L6 were instrumented posteriorly. An endoscopically assisted L4/L5 TLIF procedure was performed via a bilateral approach. In 12 sheep, the defect was filled with an injectable calcium phosphate cement. After setting, this cement gains a stability against axial loading comparable to healthy vertebrae. Another 12 sheep were treated with autograft. The animals were killed at 8 weeks and evaluated by radiologic (plain X-ray, computed tomography), histologic and histomorphometric analysis, and fluorochrome labeling. Only ten autograft sheep were available for evaluation. Radiologically and histologically, TLIF with calcium phosphate led to a 2/12 fusion rate compared to autograft (1/10 fused) (P=0.70). Semiquantitative radiologic and histologic scoring did not reveal significant differences (P=0.88). In 4/12 calcium phosphate sheep, excessive resorption was responsible for local aseptic inflammation. The findings of this study show that calcium phosphate cement is not superior to autograft, despite enabling primary stability against compressive loading. Biointegration of the osteoconductive cement does not occur fast enough, and shear forces cause early cement fracture, subsequent fragmentation, and gross resorption with the possibility of severe inflammation.