Bioabsorbable Interbody Cages in a Sheep Cervical Spine Fusion Model

Study Design. An experimental study using a sheep cervical spine interbody fusion model. Objectives. To compare interbody fusion of an autologous tricortical iliac crest bone graft with two bioabsorbable cages and to determine whether there are differences between the three interbody fusion techniques in 1) the ability to preserve postoperative distraction, 2) the biomechanical stability, and 3) the histologic characteristics of intervertebral bone matrix formation. Summary and Background Data. Bioabsorbable cages would be beneficial compared with metallic cages; however, currently no suitable bioabsorbable interbody fusion cage is available. Method. Twenty-four sheep underwent C3/C4 discectomy and fusion. The following stabilization techniques were used: Group 1) autologous tricortical iliac crest bone graft (n = 8); Group 2) bioabsorbable cage made of 70/30 poly(l-lactide-co-d,l-lactide) (experimental) filled with autologous cancellous bone graft (n = 8); Group 3) bioabsorbable cage made of a polymer-calciumphosphate composite (Biomet Europe, Dordrecht, The Netherlands) filled with autologous cancellous bone graft (n = 8). Radiographic scans to determine disc space height were performed before and after surgery and after 1, 2, 4, 8, and 12 weeks, respectively. After 12 weeks, animals were killed, and fusion sites were evaluated using functional radiographic views in flexion and extension. Quantitative computed tomographic scans were used to assess bone mineral density, bone mineral content, and bony callus volume. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending to determine stiffness, ROM, and neutral and elastic zone. Histomorphological and histomorphometrical analysis were performed to evaluate fusion and foreign body reactions associated with the bioabsorbable cages. Results. Over a 12-week period, the polymer-calciumphosphate composite cage showed significantly higher values for disc space height compared with the bone graft and the poly(l-lactide-co-d,l-lactide) cage. Additionally, the polymer-calciumphosphate composite cage demonstrateda significantly higher stiffness and lower ROM, neutral zone, and elastic zone in axial rotation and lateral bending than any other group. However, quantitative computed tomographic scans demonstrated cracks in six of the eight polymer-calciumphosphate composite cages after 12 weeks. Histologically, the highest bone volume/total volume ratio and the highest fusion rate were found in the polymer-calciumphosphate composite cage group. Although the poly(l-lactide-co-d,l-lactide) cage showed grade I through III foreign body reactions in all fusion areas, only two animals developed grade I foreign body reactions with the polymer-calciumphosphate composite cage. Conclusion. After 12 weeks, there was no significant difference between the bioabsorbable poly(l-lactide-co-d,l-lactide) cage and the tricortical bone graft. In comparison to the tricortical bone graft, the bioabsorbable polymer-calciumphosphate composite cage showed significantly better distractive properties, a significantly higher biomechanical stiffness, and an advanced interbody fusion; however, six of eight polymer-calciumphosphate composite cages cracked. Although the fate of the foreign body reactions and the cracks is currently unclear for both bioabsorbable cages, the early appearance of large osteolysis associated with use of the poly(l-lactide-co-d,l-lactide) cage allows skepticism regarding the value of this bioabsorbable implant.