Biomechanical, Radiographic, and Histological Healing Characteristics of Anterior Spinal Fusion in a Sheep Model

Introduction: Although anterior spinal fusion has become standard spinal reconstruction technique in traumatic and degenerative spinal disorders, little is known about the details of its healing process. The objectives of this study were to investigate the time-related changes in biomechanical, radiographic, and histological properties of the segment treated with anterior spinal fusion and to elucidate the temporal relationship among them.

Methods: Twenty-four sheep underwent anterior lumbar interbody fusion at L2-3 and L4-5 with use of bioactive ceramic spacer and spinal instrumentation. Four animals each were euthonized at two, four, eight, 16, 24, and 52 weeks postoperatively. Plane radiographs and micro-computerized tomography were made to evaluate the status of fusion before mechanical testing. The surface strain of the spinal instrumentation was measured using strain-gaged rod during axial compression, flexion-extension and lateral bending. After removal of the instrumentation, mechanical tests were repeated to measure the stiffness of the segment. Histological analysis was performed to assess the status of fusion. The facet joint was also observed histologically to investigate the effect on posterior elements.

Results: The biomechanical stiffness of the operative segments increased and reached a plateau at 16 weeks postoperatively, whereas the strain on the instrumentation decreased and reached a plateau at 16 weeks postoperatively. Radiographically, bridging bone formation was confirmed after 16 weeks postoperatively. Histologically, direct bone bonding to the ceramic spacer was seen after postoperative eight weeks. Bridging bone formation was observed after postoperative 16 weeks and the volume of the bridging trabeculated bone increase continuously untill 52 weeks postoperatively. Facet joint gradually became osteoporotic and ankylosed, and fused naturally at 52 weeks postoperatively.

Discussion: This study showed that the biomechanical stabilization of the operative segment preceded the radiographic and histological appearance of a solid fusion, and that the lord-sharing of the spinal instrumentation decreased concurrently with the development of the spinal fusion. The interesting result was that maturation of fusion mass continued for at least a year without the occurrence of device-related osteoporosis in anterior aspects. These facts suggested that the fusion mass gradually became to play a major role in lord-transmission resulting in the decrease of lord-sharing of both the spinal instrumentation and the posterior spinal elements. This study leads to a better understanding of anterior spinal fusion in the clinical situation.