The use of an osteoinductive growth factor for lumbar spinal fusion .1. Biology of spinal fusion

Study Design. The histology of lumbar intertransverse process spinal fusion was studied in an experimental model in rabbits. Objectives. To qualitatively and quantitatively analyze the sequential histology of spinal fusion using a previously validated animal model. Summary of Background Data. Few previous studies have described the sequential histology during the posterolateral spinal fusion healing process using autogenous bone, and a basic understanding of the biology of this repair process is lacking. Methods. Fourteen adult New Zealand white rabbits underwent single-level posterolateral lumbar intertransverse process arthrodesis with autogenous iliac bone graft. Animals were killed 1-10 weeks after surgery, and the fusion masses were analyzed histologically and quantitated using a semiautomated image analysis system. Results. Three distinct phases of healing were identified (inflammatory, reparative, and remodeling) and occurred in sequence but in a delayed fusion in the central zone of the fusion mass compared with the outer transverse process zones. Membranous bone formation, evident first at the ends of the fusion eminating from the decorticated transverse processes, was the predominant mechanism of healing. The central zone was somewhat different in that there was a period of endochondral bone formation during weeks 3 and 4 in this zone where cartilage formed and was converted to bone. Remodeling in the central zone had equilibrated with the transverse process zones by 10 weeks. Conclusions. Lumbar intertransverse process spinal fusion is a complex process from a spatial and temporal standpoint. When autogenous bone is used as the graft material, this process critically depends on a variety of factors from the decorticated host bone and exposed marrow. The persistence of a central cartilage zone may be related to some types of nonunions and deserves future investigation. This enhanced understanding of the biology of spinal fusion with autogenous bone graft will provide a foundation for optimizing the use of osteoinductive bone growth factors in this healing process.