Technical ReportrhBMP-6 stimulated osteoprogenitor cells enhance posterolateral spinal fusion in the New Zealand white rabbit
Introduction
One of the most important factors in achieving spinal fusion is the use of an osteogenic or osteoinductive agent to stimulate the bridging of bone from one vertebra to the next. Currently, autogenous iliac crest bone graft (ICBG) is the material of choice for this application; however, because of the complications and limitations associated with its use [1], [2], several strategies have been explored to enhance its function, or replace it altogether. Early efforts included the use of autograft substitutes, such as allograft bone and calcium phosphate ceramics, while subsequent groups have experimented with cytokines, growth factors, and gene therapy [3], [4], [5], [6], [7]. More recently, several investigators have evaluated strategies involving the use of cell-based therapies [8], [9], [10], [11].
For the past several years, we have been developing a technique for stimulating marrow-derived osteoprogenitor cells by exposing them to a matrix rich in recombinant human bone morphogenetic protein-6 (rhBMP-6) secreted by a transformed murine cell line. Our work is an extension of other studies that have shown that treatment of osteoprogenitor cells with bone morphogenetic proteins (BMPs) in vitro induces differentiation towards an osteoblastic lineage [12], [13], [14]. Our preliminary data, some of which have been published in abstract form (see Babat et al. Trans. 46th Orthopaedic Research Society, 2000;25:363), suggest that this exposure induces osteoblastic differentiation and that the induced cells are capable of increasing new bone formation.
This study was performed to evaluate the osteogenic capacity of two concentrations of BMP-6 stimulated osteoprogenitor cells (rhBMP-6 stOPCs) in posterolateral intertransverse spinal fusions in the New Zealand white rabbit. In particular, we hypothesized that the use of rhBMP-6 stOPCs would improve the rate of fusion and the biomechanical properties of the arthrodesed intertransverse levels in the rabbit spinal fusion model, compared with autogenous iliac crest bone graft.
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Materials and methods
After Institutional Animal Care and Use Committee (IACUC) approval, single-level, bilateral intertransverse process spinal fusions were performed on 69, 8-month-old New Zealand white rabbits (3.8±0.5 kg) [15]. The animals were randomly assigned to one of five experimental groups: Groups I and II received osteoprogenitor cells in guanidine extracted demineralized bone matrix (gDBM), Group III received morselized autogenous iliac crest bone graft (ICBG), Group IV received gDBM carrier alone, and
Results
Of the 69 animals originally enrolled in the study, 53 were ultimately available for evaluation; 16 animals were excluded because of technical problems and perioperative morbidity, including anesthetic complications (4), wrong level fusion (3), infection (3), seromas (3), and immediate postoperative hind limb palsy (3).
Palpation. The fusion rate was highest in the two groups of animals that received grafts of rhBMP-6 stOPCs (Groups I and II). In both, the fusion rates were 77%, which was
Discussion
This study was performed to evaluate the osteogenic capacity of autologous osteoprogenitor cells exposed to an extracellular matrix containing rhBMP-6. Single-level (L5–L6) spinal fusions were performed in two groups of rabbits treated with rhBMP-6 stOPCs (30×106 cells and 60×106 cells, respectively), and the fusion rates and biomechanical properties of the fusion masses were compared with those of animals grafted with autogenous iliac crest bone graft, gDBM, and untreated controls. Our results
Acknowledgments
The authors thank Scott A. McAllister, BS, Michael J. Barnum, MD, and L. Brett Babat, MD for their technical assistance, and Jason T. Machan, PhD for consultation regarding the statistical analysis.
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FDA device/drug status: not approved for this indication (rhBMP-6-rich extracellular matrix; autologous marrow cells cultured ex vivo and reimplanted).
Funding for this work was provided by the RIH Orthopaedic Foundation, Inc. and University Orthopedics, Inc.
Portions of this work were presented at the 48th and 49th Annual Meetings of the Orthopaedic Research Society.