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 f usion was studied in an experimental model in rabbits. Objectives. To qualitatively and quantitatively analyze the sequential histology of s pinal fusion using a previously validated animal model. Summary of Bac kground Data. Few previous studies have described the sequential histo logy during the posterolateral spinal fusion healing process using aut ogenous 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 a rthrodesis with autogenous iliac bone graft. Animals were killed 1-10 weeks after surgery, and the fusion masses were analyzed histologicall y and quantitated using a semiautomated image analysis system. Results . Three distinct phases of healing were identified (inflammatory, repa rative, and remodeling) and occurred in sequence but in a delayed fusi on in the central zone of the fusion mass compared with the outer tran sverse process zones. Membranous bone formation, evident first at the ends of the fusion eminating from the decorticated transverse processe s, was the predominant mechanism of healing. The central zone was some what different in that there was a period of endochondral bone formati on during weeks 3 and 4 in this zone where cartilage formed and was co nverted to bone. Remodeling in the central zone had equilibrated with the transverse process zones by 10 weeks. Conclusions. Lumbar intertra nsverse process spinal fusion is a complex process from a spatial and temporal standpoint. When autogenous bone is used as the graft materia l, this process critically depends on a variety of factors from the de corticated 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 sp inal fusion with autogenous bone graft will provide a foundation for o ptimizing the use of osteoinductive bone growth factors in this healin g process.