Evaluation of OP-l as a graft substitute for intertransverse process lumbar fusion

Study Design. An established rabbit intertransverse process lumbar fusion m odel was used to evaluate osteogenic protein (OP)-1 as a potential graft su bstitute. Objectives. To determine whether OP-l is effective in producing intertransv erse process lumbar fusion in a rabbit model. Summary of Background Date, Autogenous iliac crest bone is the gold standar d in grafting material for inducing intertransverse process fusion. However , bone graft substitutes are being considered as supplementary or alternati ve means to achieve such fusion with less morbidity. Relatively little rese arch 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 st udy groups: autograft, carrier alone, and carrier with OP-1. The animals we re killed 5 weeks after surgery. Resultant fusion masses were evaluated by manual palpation, radiography, biomechanical multidirectional flexibility t esting, 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 d etected by manual palpation, none (0%) of the 8 in the carrier-alone group had fusion, and all 8 (100%) in the OP-l group had fusion, Radiographs were 55% sensitive and 92% specific for determining fusion. Biomechanical resti ng results correlated well with those of manual palpation. Histologically, autograft specimens were predominantly fibrocartilage, OP-l specimens were predominantly maturing bone, and carrier-alone specimens did not show signi ficant bone formation. Conclusions. OP-l was found to reliably induce solid intertransverse proces s fusion in a rabbit model at 5 weeks.