Experimental fusion of the sheep cervical spine.: Part I:: Effect of cage design on interbody fusion

Introduction. There has been a rapid increase in the use of interbody fusion cages as an adjunct to spondylodesis, although experimental data are lacking. A sheep cervical spine interbody fusion model was used to determine the effect of different cage design parameters (endplate-implant contact area, maximum contiguous pore) on interbody fusion. Material and Method. In vitro evaluation: 24 sheep cadaver specimens (C2-C5) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive flexibility method using a non-constrained testing apparatus. Four different groups were examined: (1) control group (intact) (n=24), (2) autologous tricortical iliac crest bone graft (n=8),(3) Harms cage (n=8), and (4) SynCage-C (n=8). In vivo evaluation: 24 sheep underwent C3/4 discectomy and fusion: group 1: autologous tricortical iliac crest bone graft (n=8), group 2: Harms cage filled with autologous cancellous iliac crest bone grafts (n=8), and group 3: SynCage-C filled with autologous cancellous iliac crest bone grafts (n=8). Radiographic scans were performed pre- and postoperatively and after 1, 2,4,8, and 12 weeks, respectively. At the same time points, disc space height (DSH), height index (HI), intervertebral angle (IVA), and endplate angle (EA) were measured. After 12 weeks the animals were killed and fusion sites were evaluated using biomechanical testing in flexion, extension, axial rotation, and lateral bending. Additionally, histomorphological and histomorphometrical analyses were performed. Results. Over a 12-week period the cage groups showed significantly higher values for DSH, HI, IVA, and EA compared to the bone graft. In vivo stiffness was significantly higher for the tricortical iliac crest bone graft and Harms cage than in vitro stiffness. However, there was no difference between in vitro and in vivo stiffness of the SynCage-C. Histomorphometrical evaluation showed a more progressed bone matrix formation in the Harms cage group than in both other groups. Conclusion. The parameter endplate-implant contact area was not able to determine subsidence of cages. In contrast,the maximum contiguous pore of a cage significantly correlates with interbody bone matrix formation inside the cage. Additionally, there was no correlation between in vitro and in vivo stiffness of interbody fusion cages. Therefore, biomechanical in vitro studies are not able to determine in vivo outcome of fusion cages. Animal experimental evaluations of interbody fusion cages are essential prior to clinical use.