The effect of osteogenic protein-1 in instrumented and noninstrumented posterolateral fusion in rabbits

Abstract

Background context: The use of rigid instrumentation combined with bone graft makes intuitive sense given the requirements for vascular ingrowth, bone formation and a stable environment for the cellular events of healing to develop. However, with the advances of potent osteoinductive growth factors, the role of internal fixation may come into question. Whether bone morphogenic proteins (BMPs) would benefit from a more “stable” spinal segment for bone production and modeling remains unknown. In addition, it is unknown whether BMP and rigid fixation may have an additive effect on fusion healing. Purpose: This study is proposed to test the hypothesis that rigid fixation in the lumbar spine would be advantageous to achieve fusion for autogenous bone grafting, but fusion would occur regardless of fixation with the use of osteogenic protein (OP)-1. Study design/setting: A histologic and radiographic analysis of BMP in a rabbit lumbar fusion model. Methods: Thirty-two rabbits were randomized into four groups: 1) control animals: in situ posterolateral L5–L6 arthrodesis using autogenous iliac crest bone graft; 2) fixation group: posterolateral arthrodesis L5–L6 with autogenous bone graft and interspinous fixation; 3) OP-1 group: in situ posterolateral L5–L6 arthrodesis using OP-1 and 4) combined OP-1 and fixation group. Radiographic fusion analysis was performed with computed tomography scans at 3 and 12 weeks after surgery. Decalcified histology was performed to assess tissue morphology and cellularity. Results: Minimal evidence of fusion was noted at 3 weeks with autograft or OP-1. By 12 weeks, all OP-1–treated animals had solid fusion, whereas no fusion was noted in autograft animals. The addition of fixation slightly increased radiographic fusion at 3 weeks in autograft and OP-1 groups but did not affect OP-1 animals at 12 weeks where all were fused. Decalcified histologic results confirmed the proliferative bone formation noted with OP-1 and the variable cellular response with autograft. Conclusions: The results of the present study suggest that the osteoinductive effect of OP-1 may be only minimally enhanced early in the bone healing process but does not appear to be affected in the long term by spinal fixation in the rabbit intertransverse fusion model. Fixation appeared to enhance early fusion in the autograft group.