The effect of aging on posterior intertransverse lumbar fusion: A New Zealand white rabbit model

Study Design: In vivo assessment of lumbar spinal fusion between a younger and older cohort of New Zealand white rabbits. Objective: Directly compare fusion within young and aged New Zealand white rabbits to establish an aged spinal fusion model translational research. Summary of Background Data: Prior studies have utilized skeletally mature young rabbits (6-12mo old) that may not be appropriate as an analog for studying the aging human spine. Methods: Ten aged (>36mo old) and 10 young (12mo old) New Zealand white rabbits underwent a single-level, bilateral, L5-6 posterolateral intertransverse fusion using autogenous iliac crest bone graft. The animals were killed at 6 weeks postoperatively, and the specimens were then evaluated with quantitative microcomputerized tomography and manual palpation by 6 orthopedic surgeons. The fusions were graded as either fused or not fused by each examiner. The spines were then embedded in poly(methyl methacrylate) and cut into 2-mm-thick sections for histologic analysis. Results: A higher percentage of young rabbits were determined to be successfully fused through manual palpation testing compared with the aged rabbits. Micro-computed tomography (CT) analysis revealed a significantly greater fusion mass volume in the younger rabbits than in the older cohort. In addition, the fusion density of the younger rabbits was found to be significantly lower than that of the older rabbits when normalized to the bone density in the nonfused portion of the spine. Histologic analysis showed that the quality of the bone within the fusion mass was consistent between the young and old rabbits. A greater number of young animals had bilateral continuous bone graft compared with the aged animals. Conclusions: The aged (>36 mo) New Zealand white rabbit model appears to be a valid model to evaluate the effect of aging on lumbar fusion and has the potential to more accurately model conditions that are present in the older human spine.