AN EXPERIMENTAL LUMBAR INTERTRANSVERSE PROCESS SPINAL-FUSION MODEL - RADIOGRAPHIC, HISTOLOGIC, AND BIOMECHANICAL HEALING CHARACTERISTICS

Objective, The purpose of this investigation was to develop, character ize, and validate an animal model for lumbar intertransverse process f usion. Study Design, This study used a rabbit model to characterize th e radiographic, histologic, and biomechanical properties of the intert ransverse process spinal fusion healing process. Methods, Sixty adult New Zealand white rabbits underwent bilateral posterolateral spinal fu sion at L5-L6 using autogenous iliac bone graft. Four of the rabbits w ere used as negative controls: two received bone graft without decorti cation of the transverse process, and two underwent decortication with out bone grafting. Rabbits were killed at 2, 3, 4, 5, 6, or 10 weeks a nd the spinal fusions were analyzed by radiography, manual palpation, and uniaxial tensile mechanical testing or light microscopy. Results. Overall the nonunion rate was 33% in animals 4 or more weeks from surg ery. Biomechanical strength and stiffness of the fusions became statis tically different from the adjacent unfused control levels after the t hird week (P < 0.05). Tensile strength of the nonunions (1.4 times unf used control levels) was statistically less (P < 0.05) than that of th e solidly fused levels (1.8 times unfused controls) in weeks 4, 5, 6, and 10. Fusion was not achieved in any of the control animals with omi ssion of decortication or bone grafting. Light microscopic analysis sh owed three distinct and reproducible phases of spinal fusion healing. Conclusions. This animal model overcomes the limitations of previous m odels by more closely replicating the human procedure in surgical tech nique, graft healing environment, and outcome (i.e., a nonunion rate s imilar to that seen in humans). This model provides an opportunity to explore questions relevant to the biology of intertransverse process f usion and to investigate the coupling of the membranous and endochondr al mechanisms of bone formation during spinal fusion.