Evaluation of OP-1 as a Graft Substitute for Intertransverse Process Lumbar Fusion

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Abstract
Study Design. An established rabbit intertransverse process lumbar fusion model was used to evaluate osteogenic protein (OP)-1 as a potential graft substitute. Objectives. To determine whether OP-1 is effective in producing intertransverse process lumbar fusion in a rabbit model. Summary of Background Data. Autogenous iliac crest bone is the gold standard in grafting material for inducing intertransverse process fusion. However, bone graft substitutes are being considered as supplementary or alternative means to achieve such fusion with less morbidity. Relatively little research has been undertaken to investigate the efficacy of OP-1 in this role. Methods. Single-level intertransverse process lumbar fusions were performed at L5–L6 of 31 New Zealand White rabbits. These were divided into three study groups: autograft, carrier alone, and carrier with OP-1. The animals were killed 5 weeks after surgery. Resultant fusion masses were evaluated by manual palpation, radiography, biomechanical multidirectional flexibility testing, and histology. Results. Seven rabbits (23%) were excluded because of complications. Of the remaining 24 rabbits, 5 (63%) of the 8 in the autograft group had fusion detected by manual palpation, none (0%) of the 8 in the carrier-alone group had fusion, and all 8 (100%) in the OP-1 group had fusion. Radiographs were 55% sensitive and 92% specific for determining fusion. Biomechanical testing results correlated well with those of manual palpation. Histologically, autograft specimens were predominantly fibrocartilage, OP-1 specimens were predominantly maturing bone, and carrier-alone specimens did not show significant bone formation. Conclusions. OP-1 was found to reliably induce solid intertransverse process fusion in a rabbit model at 5 weeks.