Harvinder S. Sandhu, MD(1), Frank P. Cammisa, MD (1), Stephen E. Heim, MD (2), Edgar G. Dawson, MD(3), Rick B. Delamarter, MD(3), Eduardo F. Luque, MD(4)
1. New York, 2. Chicago, 3. Los Angeles, 4. Mexico City

Introduction
An arthrodesis (fusion) of adjacent vertebral segments of the spinal column is surgically created in nearly 200,000 patients per year in the United States. Despite recent technical advances, the incidence of failure of such intervention is still significant. Furthermore, substantial amounts of autogenous bone graft must be procured and transplanted from the ileum to the spine to enable consolidation of a fusion mass with elements of the spinal column. The additional morbidity associated with this procedure is well known. Finally, secondary attempts at spinal fusion following initial failure have even a poorer prognosis and pose greater challenges.

Advances in spinal fixation devices have addressed some mechanical considerations. However, osteogenesis and skeletal healing (fusion consolidation) are biologic processes which are facilitated but not replaced by instrumented fixation. Recently, considerable attention has been directed toward biologic considerations and, in particular, novel growth factors that regulate and promote these biologic processes. This renewed focus has been a consequence of the successful isolation, purification, and characterization of an entire family of bone morphogenetic proteins (BMPs), and the demonstration that a singular molecular species of recombinant human BMP (rhBMP) could induce the entire cascade of endochondral osteogenesis. Optimism for using such growth factors as effective bone-graft substitutes increased following a series of valuable preclinical animal experiments.

Preclinical Studies with rhBMPs
During the past four years several investigators have demonstrated that rhBMP-2 and rhBMP-7, combined with appropriate delivery vehicles, can induce successful posterolateral spinal fusion in rabbits, dogs, and rhesus monkeys without need for autogenous bone graft. The fusion rate and strength of fusion mass was equal or superior to the rate and strength of fusions produced with autogenous graft from the iliac crest. Cook and associates demonstrated that when using osteogenic protein-1 (OP-1 or rhBMP-7), posterior lumbar interlaminar fusions in dogs occurred in 12 weeks compared to 26 weeks when iliac crest bone was used. Sandhu and David, in separate studies, reported a 100% rate of lumbar intertransverse process fusion in beagles within six months of implantation of rhBMP-2 compared to 0% and 33% rate of fusion with autogenous iliac crest. Boden, using a rabbit lumbar intertransverse process fusion model with a four week postoperative endpoint, reported a 100%-fusion rate with rhBMP-2 and a 42%-fusion rate with autogenous graft.

The efficacy of rhBMP-2 for producing anterior spinal column fusions was also explored. Sandhu et al demonstrated that implantation of threaded titanium intervertebral “cages” containing rhBMP-2 without autograft in a sheep anterior lumbar interbody fusion (ALIF) model consistently resulted in rapid intervertebral osseous fusion through the fixation device. Similarly, the same investigators showed that when rhBMP-2 was added to porous tantalum ALIF cages in the same model, significantly greater bone ingrowth into the porous macrostructure occurred.

Data obtained from these lower animal models prompted further investigation in the higher nonhuman primate model. Boden et al recently demon-strated the efficacy of rhBMP-2 in a rhesus monkey ALIF model. Laparoscopic assisted L5-S1 ALIF procedures with threaded, tapered, titanium cages were performed in seven animals. A type I collagen sponge carrying a high dose of rhBMP-2, a low dose of rhBMP-2, or no growth factor was placed into the cage prior to implantation. Animals implanted with a cage containing rhBMP-2 (high or low dose) all had successful fusions. The control animals implanted with a cage containing collagen alone did not fuse. In another recently presented study, Fishgrund et al reported successful anterior cervical interbody fusions in a primate model using cadaveric allograft bone dowels filled with rhBMP-2 also carried by a type I collagen sponge. Histologic analysis of the rhBMP-2 associated cervical fusions revealed complete resorption of the allograft by six months, an observation not seen when the allograft bone dowels were filled with autogenous bone graft from the iliac crest.

Clinical Trials with rhBMP-2 - Early Observations
Based upon the promising results obtained from the preclinical studies, carefully selected and controlled clinical trials with rhBMP-2 have begun. Each of the studies is still in progress and formal interpretation and presentation of the data has not been performed. Some preliminary observations have been made and are included in the summary below.

A clinical trial examining the use of a rhBMP-2/collagen sponge composite combined with a threaded titanium interbody fusion cage for anterior lumbar interbody fusion has been completed. No autogenous bone graft was used in any patient. A total of fourteen patients were enrolled. CT images of the fusions were obtained three and six months following surgery. These images demonstrated de novo bone formation within the cages bridging adjacent vertebral segments. Bone formation anterior, posterior, and lateral to the cages was also noted (Figs. 2a,b and Figs. 3a,b). Formal evaluation of fusion status and clinical outcome has not yet been completed.

A clinical trial examining the use of rhBMP-2 in posterior lumbar fusion surgery for the treatment of degenerative disease of the lumbar spine is underway. Evidence of de novo bone formation attributable to the growth factor is present in follow-up plain radiographs of the lumbar spine (Fig. 4). Formal evaluation of these cases and their outcomes is currently in progress.

Conclusions
At this time, evaluations of the clinical applications of the growth factor rhBMP-2 for the spinal column are in the earliest stages of study. Early observations suggest that this growth factor is capable of inducing de novo bone formation in the spinal environments. Formal examination of these clinical cases, however, is necessary and pending.

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