Jeffrey H. Schimandle

Experimental spinal fusion with recombinant human bone morphogenetic protein-2.

Study Design. Lumbar intertransverse process arthrodesis using recombinant human bone morphogenetic protein‐2 was performed in a previously established rabbit model for posterolateral spinal fusion and compared with fusions achieved using autogenous bone graft. Objectives. To qualitatively compare different recombinant human bone morphogenetic protein‐2 dosages and carriers and to determine the efficacy of recombinant human bone morphogenetic protein‐2 as a bone graft substitute to produce lumbar intertransverse process fusion in a validated rabbit model for posterolateral spinal fusion. Summary of Background Data. Autogenous bone was considered the most successful bone graft material used for spinal arthrodesis. Problems with its use may occur in 25‐30% of patients and prompted the search for and investigation of bone graft substitutes and osteoinductive growth factors, such as bone morphogenetic proteins. Recombinant human bone morphogenetic protein‐2 was used successfully in orthotopic sites to generate bone in animal mandibular and long bone defect models. Methods. Posterolateral intertransverse process arthrodeses were performed at L5‐L6 in 56 rabbits using recombinant human bone morphogenetic protein‐2 or autogenous bone graft. Rabbits were killed either 5 weeks later to qualitatively compare fusions achieved using different recombinant human bone morphogenetic protein‐2 dosages and carriers or 4 weeks later to compare the efficacy of recombinant human bone morphogenetic protein‐2 in achieving spinal fusion compared with using autogenous bone graft. Inspection, manual palpation, radiography, histology, and biomechanic testing were used to assess the fusion. Results. All rabbits implanted with recombinant human bone morphogenetic protein‐2 achieved solid spinal fusion by manual palpation and were fused radiographically, whereas only 42% of the autograft control fusions were solid. More mature fusions with greater trabecular bone formation were shown radiographically and histologically in rabbits implanted with the highdose recombinant human bone morphogenetic protein‐2 than with the low‐dose recombinant human bone morphogenetic protein‐2. Fusions achieved using recombinant human bone morphogenetic protein‐2 delivered in the collagen carrier were more remodeled and homogeneous compared with using recombinant human bone morphogenetic protein‐2 delivered in autograft ± collagen carrier. Fusions achieved with recombinant human bone morphogenetic protein‐2 were biomechanically stronger and stiffer than fusions achieved using autogenous bone graft. Conclusions. Recombinant human bone morphogenetic protein‐2 successfully and reliably achieved lumbar intertransverse process fusion in a validated rabbit model for posterolateral spinal fusion. Radiographically and histologically, greater and more rapid bone formation, consolidation, and remodeling were shown with recombinant human bone morphogenetic protein‐2 compared with autogenous bone graft. Fusions achieved with recombinant human bone morphogenetic protein‐2 were biomechanically stronger and stiffer than autograft fusions.Study Design Neurophysiologic reactions of cauda equina nerve roots to intermittently appliced compression were assessed for two different modes of compression using a porcine model. Objective To assess the neurephysiologic reactions of cauda equina nerve roots to intermittently applied compression. Summary of Background Data A number of experimental studies have been presented recently regarding the reaction pattern of spinal nerve roots to compression. These studies have used a continuous pressure level. For studies of pathophysiologic mechanisms behind neurogenic claudication, however, it would be more relevant to study the effects of intermittently applied compression. Methods The cauda equina was exposed and compression was applied by two inflatable balloons. Two different modes of compression were used. Fither the two balloons were inflated and deflated simultaneously (Intermittent compression), or just the caudal balloon was inflated and deflated while the cranial balloon was kept continuously inflated (continuous/intermittent compression). The experimental series were: intermittent compression at 10 mm Hg (n = 5) and 50 mm Hg (n = 5), and continuous/intermittent compression at 10 mm Hg (n = 5) and 50 mm Hg (n = 5). For both modes of compression the pressure in the balloons with intermittent inflation was maintained for 10 minutes and deflated for 5 minutes. This procedure was repeated in 8 cycles for 2 hours. Muscle action potentials were recorded in the tail muscles. Results Compression at 10 mm Hg induced similar reductions of muscle action potentials for both compression modes. At 50 mm Hg, the effects were more pronounced at continuous/intarmittent compression than at intermittent compression. The reduction of muscle action potentials was slightly more pronounced for 50 than for 10 mm Hg at intermittent compression. However, a statistically significant difference in the results was founds only between 10 and 50 mm Hg at the continuous/intermittent compression mode. Conclusions The established model allows investigation of the effects of intermittent cauda equina compression, which might be clinically more relevant than continuous compression regarding the pathophysiologic mechanisms behind neurogenic claudication.

An experimental lumbar intertransverse process spinal fusion model. Radiographic, histologic, and biomechanical healing characteristics.

Objective. The purpose of this investigation was to develop, characterize, and validate an animal model for lumbar intertransverse process fusion. Study Design. This study used a rabbit model to characterize the radiographic, histologic, and biomechanical properties of the intertransverse process spinal fusion healing process. Methods. Sixty adult New Zealand white rabbits underwent bilateral posterolateral spinal fusion at L5-L6 using autogenous iliac bone graft. Four of the rabbits were used as negative controls: two received bone graft without decortication of the transverse process, and two underwent decortication without bone grafting. Rabbits were killed at 2, 3, 4, 5, 6, or 10 weeks and the spinal fusions were analyzed by radiography, manual palpation, and uniaxial tensile mechanical testing or light microscopy. Results. Overall the nonunion rate was 33% in animals 4 or more weeks from surgery. Biomechanical strength and stiffness of the fusions became statistically different from the adjacent unfused control levels after the third week (P < 0.05). Tensile strength of the nonunions (1.4 times unfused control levels) was statistically less (P < 0.05) than that of the solidly fused levels (1.8 times unfused controls) in weeks 4, 5, 6, and 10. Fusion was not achieved in any of the control animals with omission of decortication or bone grafting. Light microscopic analysis showed three distinct and reproducible phases of spinal fusion healing. Conclusions. This animal model overcomes the limitations of previous models by more closely replicating the human procedure in surgical technique, graft healing environment, and outcome (i.e., a nonunion rate similar to that seen in humans). This model provides an opportunity to explore questions relevant to the biology of intertransverse process fusion and to investigate the coupling of the membranous and endochondral mechanisms of bone formation during spinal fusion.

The Use of an Osteoinductive Growth Factor for Lumbar Spinal Fusion: Part II

Study Design The histology of lumbar intertransverse process spinal fusion was studied in an experimental model in rabbits. Objectives To qualitatively and quantitatively analyze the sequential histology of spinal fusion using a previously validated animal model. Summary of Background Data Few previous studies have described the sequential histology during the posterolateral spinal fusion healing process using autogenous bone, and a basic understanding of the biology of this repair process is lacking. Methods Fourteen adult New Zealand white rabbits underwent single-level posterolateral lumbar intertransverse process arthodesis with autogenous iliac bone graft. Animals were killed 1–10 weeks after surgery, and the fusion masses were analyzed histologically and quantitated using a semiautomate image analysis system. Results Three distinct phases of healing were identified (inflammatory, reparative, and remodeling) and occurred in sequence but in a delayed fashion in the central zone of the fusion mass compared with the outer transverse process zones. Membranous bone formation, evident first at the ends of the fusion eminating from the decorticated transverse processes, was the predominant mechanism of healing. The central zone was somewhat different in that there was a period of endochondral bone formation during weeks 3 and 4 in this zone where cartilage formed and was converted to bone. Remodelling in the central zone had equilibrated with the transerse process zones by 10 weeks. Conclusions Lumbar intertransverse process spinal fusion is a complex process form a spatial and temporal standpoint. When autogenous bone is used as the graft material, this process critically depends on a variety of factors from the decorticated host bone and exposed marrow. The persistence of a central cartilage zone may be related to some types of nonunions and deserves future investigation. This enhanced understanding of the biology of spinal fusion with autogenous bone graft will provide a foundation for optimizing the use of osteoinductive bone growth factors in this healing process.

Lumbar Intertransverse-process Spinal Arthrodesis with Use of a Bovine Bone-derived Osteoinductive Protein. A Preliminary Report

The use of a bovine bone-derived osteoinductive protein extract as a bone-graft substitute was evaluated in a rabbit model of intertransverse process arthrodesis of the lumbar spine. Forty-five adult New Zealand White rabbits had arthrodesis between the fifth and sixth lumbar vertebrae with use of one of three graft materials: autogenous iliac-crest bone, osteoinductive protein delivered in an allogeneic demineralized bone matrix/collagen carrier, or demineralized bone matrix/collagen carrier or demineralized bone matrix/collagen carrier without osteoinductive protein. Fusion was assessed by manual palpation, radiography, biomechanical testing, and light microscopy at two and five weeks after the operation. At two weeks, light microscopic analysis of the arthrodesis site in which osteoinductive protein had been used showed that most of the demineralized bone matrix was still present, with small amounts of membranous and endochondral bone formation at the peripheral margins of the implant. Light microscopic analysis of the five-week specimens showed increased new-bone formation and a more homogeneous and mature fusion mass with the osteoinductive bone protein than with the autogenous bone graft. At five weeks, the fusions with the osteoinductive protein extract were characterized by more secondary spongiosa, with formation of bone marrow centrally and a cortical rim peripherally. Of the thirty-five rabbits that were examined at five weeks, all ten in the group that had received osteoinductive bone protein had a solid fusion, but the rate of fusion was significantly less in the other two groups: eight of thirteen rabbits (p = 0.05) in the group that had received autogenous bone graft and two of twelve rabbits (p = 0.0001) in the group that had received demineralized bone matrix/collagen carrier without osteoinductive bone protein. The use of osteoinductive bone protein resulted in stronger (p = 0.02) and stiffer (p = 0.005) fusions compared with those obtained with the use of autogenous iliac-crest graft.

Biologic enhancement of spinal fusion

STUDY DESIGN Literature review. OBJECTIVES To review the available animal and clinical data on biologic enhancements of spinal fusion. SUMMARY OF BACKGROUND DATA Lumbar spinal arthrodesis may result in pseudarthrosis in 5% to 35% of patients. Although much research has focused on the mechanical factors affecting spinal fusion, the use of internal fixation has not eliminated the problem of spinal nonunions. Accordingly, biologic enhancement of spinal fusion has become an important focus of spinal research. METHODS Medline and hand searches. RESULTS Electric stimulation, bone graft substitutes, and bone growth factors have been researched most extensively. Electric stimulation and early attempts at bone graft substitutes (allograft, xenograft) have yielded variable results. The feasibility of biologic enhancement of spinal fusion with osteoinductive growth factors has been shown in animals. CONCLUSION The efficacy of bone growth factors for lumbar fusion remains to be definitively established in humans.

In vivo evaluation of a resorbable osteoinductive composite as a graft substitute for lumbar spinal fusion

The purpose of this prospective animal study was to evaluate the efficacy of a resorbable coral particulate to serve as a carrier with several doses of a bovine-derived osteoinductive bone protein mixture. A previously validated rabbit model for posterolateral intertransverse process lumbar spinal fusion was used. Posterolateral intertransverse process arthrodeses were performed at L5-L6 in 64 adult New Zealand white rabbits. The bone graft substitute evaluated consisted of a Biocoral/collagen composite with one of four doses (0, 100, 300, or 1,000 micrograms) of a bovine-derived osteoinductive bone protein extract (BP). Fusions were assessed at 5 weeks by manual palpation, radiography, histology, and biomechanical testing. Use of the Biocoral/collagen carrier without BP resulted in no solid fusions. Biocoral/collagen with 100 micrograms BP resulted in solid fusions in 31% (4 of 13) of the rabbits. Biocoral with 300 micrograms or 1,000 micrograms BP resulted in solid spinal fusion in all rabbits (27 of 27). There were no neurologic complications related to the growth factor or carrier. Small subcutaneous collections of serous fluids were noted in 26% of the animals in the 1st postoperative week, but resolved without problems by the 5th week. Such seromas may limit the clinical utility of this growth factor-carrier combination. Biocoral/collagen combined with the appropriate dose of bovine-derived osteoinductive bone protein was efficacious as a bone graft substitute for achieving posterolateral lumbar spinal fusion in the rabbit model. A dose of 300 micrograms BP was determined as the threshold to reliability produce solid spinal fusions.

Spine update. Animal use in spinal research.

Progress in biomedical research often has depended on the use of animals as a testing ground for both experimental and clinical hypotheses. Animal models have been widely used in all specialties of medicine and have been crucial for acquiring basic science and clinical knowledge pertaining to spinal surgery. In addition to overcoming the many ethical and societal restrictions normally encountered in human studies, the use of animal models permits certain methodologic approaches inapplicable in humans. The purpose of this article is to: 1) review the general concepts of models, 2) discuss recommendations and regulations regarding the use of animals in biomedical research, and 3) present guidelines for the selection of the most suitable model for a particular study. Animal data are only as applicable as the model from which it is derived. Thus, future animal models must be carefully chosen using rational guidelines and should overcome the deficiencies and limitations of previous models.

Bone substitutes for lumbar fusion:present and future

Abstract Recent advances in our knowledge regarding the spinal fusion process, as well as the potential for biological manipulation of bone formation, make it timely to review the use of bone grafts, graft adjuncts, and bone graft substitutes in the lumbar spinal fusion process. Despite technical advances such as rigid segmental internal fixation, the success of a spine fusion depends largely on the effective use of bone graft materials and the application of sound bone grafting techniques. In this article, the properties and use of various graft materials are reviewed. In addition, results from studies using animal models to study graft materials and data from limited clinical studies are briefly presented. Lastly, gaps in our existing knowledge are highlighted and areas of future research discussed. An understanding of the biology and properties of bone graft materials, including adjuncts and substitutes, will enable the surgeon to apply sound principles in minimizing the current problems associated with achieving a successful spinal fusion.