Evaluation of carriers of bone morphogenetic protein for spinal fusion

Application of Cytokines of the Bone Morphogenetic Protein (BMP) Family in Spinal Fusion - Effects on the Bone, Intervertebral Disc and Mesenchymal Stromal Cells

Low back pain is a prevalent socio-economic burden and is often associated with damaged or degenerated intervertebral discs (IVDs). When conservative therapy fails, removal of the IVD (discectomy), followed by intersomatic spinal fusion, is currently the standard practice in clinics. The remaining space is filled with an intersomatic device (cage) and with bone substitutes to achieve disc height compensation and bone fusion. As a complication, in up to 30% of cases, spinal non-fusions result in a painful pseudoarthrosis. Bone morphogenetic proteins (BMPs) have been clinically applied with varied outcomes. Several members of the BMP family, such as BMP2, BMP4, BMP6, BMP7, and BMP9, are known to induce osteogenesis. Questions remain on why hyper-physiological doses of BMPs do not show beneficial effects in certain patients. In this respect, BMP antagonists secreted by mesenchymal cells, which might interfere with or block the action of BMPs, have drawn research attention as possible targets for the enhancement of spinal fusion or the prevention of non-unions. Examples of these antagonists are noggin, gremlin1 and 2, chordin, follistatin, BMP3, and twisted gastrulation. In this review, we discuss current evidence of the osteogenic effects of several members of the BMP family on osteoblasts, IVD cells, and mesenchymal stromal cells. We consider in vitro and in vivo studies performed in human, mouse, rat, and rabbit related to BMP and BMP antagonists in the last two decades. We give insights into the effects that BMP have on the ossification of the spine. Furthermore, the benefits, pitfalls, and possible safety concerns using these cytokines for the improvement of spinal fusion are discussed.

Comparative Efficacy of Commonly Available Human Bone Graft Substitutes as Tested for Posterolateral Fusion in an Athymic Rat Model

Background

Insufficient data exist on bone graft substitute materials efficacy; two thirds lack any clinical data.1,2 This prospective animal study identified efficacy differences among commercially available materials of several classes.

Methods

Historically validated muscle pouch osteoinduction study (OIS) and posterolateral fusion (PLF) were performed in an athymic rat model. Grafting material products implanted were demineralized bone matrix (DBM)-based allografts (Accell EVO3, DBX Mix, DBX Strip, Grafton Crunch, Grafton Flex, Grafton Matrix, Grafton Putty, Magnifuse, and Progenix Plus), allografts (OsteoSponge, MinerOss), cellular allograft (Osteocel Plus), ceramics (Mozaik Strip), or activated ceramics (Actifuse ABX Putty, Vitoss BA). After 4 weeks, OIS specimens were evaluated ex vivo by histologic osteoinductivity. After 8 weeks, PLF ex vivo specimens were evaluated for fusion by manual palpation (F_MP), radiography (F_XR), and histology (F_HISTO).

Results

OIS: No materials exhibited a rejection reaction on histology. All DBM-based materials exhibited osteoinductive potential as new bone formation at > 88% of implanted sites. One plain allograft (OsteoSponge) formed bone at 25% of sites. No bone formed for one ceramic (Mozaik Strip), three activated ceramics (Actifuse ABX Putty), or one cellular allograft, regardless of human bone marrow aspirate (hBMA) when added. PLF: Among the 10 DBMs, 6 had F_MP of 100% (Accell EVO3, DBX Mix, DBX Strip, Grafton Flex, Grafton Putty, Magnifuse), 2 had F_MP of 94% (Grafton Crunch, Grafton Matrix), and 2 conditions had F_MP of 0% (Progenix Plus, Progenix Plus + athymic rat iliac crest bone graft [arICBG]). Ceramics (Mozaik Strip), activated ceramics (Actifuse ABX Putty, Vitoss BA), plain allograft (OsteoSponge, MinerOss (PLF study), and cellular allograft (Osteocel Plus) demonstrated 0% F_MP. ArICBG demonstrated 13% F_MP.

Conclusions

Eight DBM-based materials (Accell EVO3, DBX Mix, DBX Strip, Grafton Crunch, Grafton Flex, Grafton Matrix, Grafton Putty, Magnifuse) demonstrated excellent (> 90% F_MP) efficacy in promoting fusion via bone healing. Two DBM conditions (Progenix Plus, Progenix Plus + arICBG) showed no manual palpation fusion (F_MP). Systematically, over the 2 studies (OIS and PLF), cellular (Osteocel Plus), plain allografts (OsteoSponge, MinerOss; PLF study), ceramic (Mozaik Strip), and activated ceramics (Actifuse ABX Putty, Vitoss BA) demonstrated poor F_MP efficacy (< 10%).

Clinical Relevance

When selecting DBMs, clinicians must be cognizant of variability in DBM efficacy by product and lot. While theoretically osteoinductive, cellular allograft and activated ceramics yielded poor in vivo efficacy. Whole allograft and ceramics may provide osteoconductive scaffolding for mixed-material grafting; however, surgeons should be cautious in using them alone. Direct clinical data are needed to establish efficacy for any bone graft substitute.

The effect of ketorolac on posterior thoracolumbar spinal fusions: a prospective double-blinded randomised placebo-controlled trial protocol

INTRODUCTION

Ketorolac has been shown to provide quality postoperative pain control and decrease opioid requirement with minimal side effects following spinal surgery. However, the literature addressing its use in spinal fusions is highly variable in both its effectiveness and complications, such as pseudarthrosis. Recent literature postulates that ketorolac may not affect fusion rates and large randomised controlled trials are needed to demonstrate ketorolac as a safe and effective adjuvant treatment to opioids for postoperative pain control.

METHODS AND ANALYSIS

This is a multihospital, prospective, double-blinded, randomised placebo-controlled trial. Data concerning fusion rates, postoperative opioid use, pain scores, length of stay will be recorded with the aim of demonstrating that the use of ketorolac does not decrease thoracolumbar spinal fusion rates while identifying possible adverse events related to short-term minimal effective dose compared with placebo. Additionally, this investigation aims to demonstrate a decrease in postoperative opioid use demonstrated by a decrease in morphine equivalence while showing equivalent postoperative pain control and decrease the average length of stay.

ETHICS AND DISSEMINATION

Ethical approval was obtained at all participating hospitals by the institutional review board. The results of this study will be submitted for publication in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

NCT03278691.

Biomimetic delivery of signals for bone tissue engineering

Bone tissue engineering is an exciting approach to directly repair bone defects or engineer bone tissue for transplantation. Biomaterials play a pivotal role in providing a template and extracellular environment to support regenerative cells and promote tissue regeneration. A variety of signaling cues have been identified to regulate cellular activity, tissue development, and the healing process. Numerous studies and trials have shown the promise of tissue engineering, but successful translations of bone tissue engineering research into clinical applications have been limited, due in part to a lack of optimal delivery systems for these signals. Biomedical engineers are therefore highly motivated to develop biomimetic drug delivery systems, which benefit from mimicking signaling molecule release or presentation by the native extracellular matrix during development or the natural healing process. Engineered biomimetic drug delivery systems aim to provide control over the location, timing, and release kinetics of the signal molecules according to the drug's physiochemical properties and specific biological mechanisms. This article reviews biomimetic strategies in signaling delivery for bone tissue engineering, with a focus on delivery systems rather than specific molecules. Both fundamental considerations and specific design strategies are discussed with examples of recent research progress, demonstrating the significance and potential of biomimetic delivery systems for bone tissue engineering.

The accelerated effect of recombinant human bone morphogenetic protein 2 delivered by β-tricalcium phosphate on tendon-to-bone repair process in rabbit models

BACKGROUND

Bone morphogenetic protein 2 (BMP-2) plays an important role in the tendon-to-bone repair process. However, there is no previous literature on acceleration of the tendon-to-bone repair process by BMP-2 delivered by β-tricalcium phosphate (β-TCP). The aim of this study was to investigate the accelerated effect of recombinant human BMP-2 (rhBMP-2) delivered by β-TCP on the tendon-to-bone repair process.

METHODS

The infraspinatus tendon of elderly female Japanese white rabbits was detached from its insertion site on the humerus. A bone tunnel (4.2 mm) was created at the original insertion site of the tendon, which was repaired using the McLaughlin procedure after filling in β-TCP (porosity 75%) without BMP-2 (control group) or with 10 µg rhBMP-2 (BMP group). The rabbits were sacrificed at the second, fourth, and eighth weeks after surgery for histologic analysis and biomechanical testing. We also evaluated the maturity of the tendon-to-bone junction using the tendon-to-bone maturity score.

RESULTS

Histologic analysis revealed no significant difference between the groups at 2 and 8 weeks but a more abundant organized fibrocartilage at the tendon-to-bone junction in the BMP group at 4 weeks. The tendon-to-bone maturity score improved sequentially. The interface of the BMP group at 4 weeks had significantly improved biomechanical properties than that of the control group.

CONCLUSION

The tendon-to-bone repair process was facilitated by the use of rhBMP-2 delivered by β-TCP at 4 weeks.

Enhancement of posterolateral lumbar spine fusion using recombinant human bone morphogenetic protein-2 and mesenchymal stem cells delivered in fibrin glue

Mesenchymal stem cells have shown great potential for accelerating bone healing. In the present study, we evaluate the efficacy of fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 composite for posterolateral spinal fusion in a rabbit model. Forty adult rabbits underwent posterolateral intertransverse fusion at the L5–L6 level. The animals were randomly divided into four groups based on the implant material: fibrin glue, fibrin glue/mesenchymal stem cells composite, fibrin glue-recombinant human bone morphogenetic protein-2 (fibrin glue/recombinant human bone morphogenetic protein-2) composite, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 composite. After six weeks, the rabbits were euthanized for manual palpation, radiographic examination, biomechanical testing, and histology. Manual palpation results showed that the fusion rate for fibrin glue, fibrin glue/mesenchymal stem cells, fibrin glue/recombinant human bone morphogenetic protein-2, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 was 0, 0, 40%, and 70%, respectively. Moreover, fusion rate determined by radiographic examination for fibrin glue, fibrin glue/mesenchymal stem cells, fibrin glue/recombinant human bone morphogenetic protein-2, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 was 0, 0, 40%, and 80%, respectively. Gray analysis showed that fibrin glue/recombinant human bone morphogenetic protein-2 group had higher ossification area and density than fibrin glue group; and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 group had higher ossification area and density than fibrin glue/recombinant human bone morphogenetic protein-2 group. Formation of continuous bone masses between L5 and L6 level in mesenchymal stem cells/recombinant human bone morphogenetic protein-2/fibrin glue group was further confirmed by computed tomography scanning and three-dimensional reconstruction. Biomechanical testing demonstrated that the fusion strength (flexion, extension, lateral bending, and axial rotation) in fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 group is significantly higher than that in fibrin glue/recombinant human bone morphogenetic protein-2 group. The formation of mature bone tissues between transverse processes of the fused specimens from both fibrin glue/recombinant human bone morphogenetic protein-2, and fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 groups was confirmed by HE staining, and quantitative real-time polymerase chain reaction results showed the upregulation of CD31, type I collagen, osteocalcin, and osteonectin in the fibrin glue/mesenchymal stem cells/recombinant human bone morphogenetic protein-2 group. In conclusion, our findings show that mesenchymal stem cells delivered with recombinant human bone morphogenetic protein-2 using fibrin glue as carrier are more effective in enhancing spine fusion than recombinant human bone morphogenetic protein-2 without mesenchymal stem cells in the rabbit model.

A Biphasic Calcium Sulphate/Hydroxyapatite Carrier Containing Bone Morphogenic Protein-2 and Zoledronic Acid Generates Bone

In orthopedic surgery, large amount of diseased or injured bone routinely needs to be replaced. Autografts are mainly used but their availability is limited. Commercially available bone substitutes allow bone ingrowth but lack the capacity to induce bone formation. Thus, off-the-shelf osteoinductive bone substitutes that can replace bone grafts are required. We tested the carrier properties of a biphasic, calcium sulphate and hydroxyapatite ceramic material, containing a combination of recombinant human bone morphogenic protein-2 (rhBMP-2) to induce bone, and zoledronic acid (ZA) to delay early resorption. In-vitro, the biphasic material released 90% of rhBMP-2 and 10% of ZA in the first week. No major changes were found in the surface structure using scanning electron microscopy (SEM) or in the mechanical properties after adding rhBMP-2 or ZA. In-vivo bone formation was studied in an abdominal muscle pouch model in rats (n = 6/group). The mineralized volume was significantly higher when the biphasic material was combined with both rhBMP-2 and ZA (21.4 ± 5.5 mm³) as compared to rhBMP-2 alone (10.9 ± 2.1 mm³) when analyzed using micro computed tomography (μ-CT) (p < 0.01). In the clinical setting, the biphasic material combined with both rhBMP-2 and ZA can potentially regenerate large volumes of bone.

Bone Regeneration in the Rat Mandible with Bone Morphogenetic Protein-2: A Comparison of Two Carriers

OBJECTIVE: To compare mandibular bone regeneration with bone morphogenetic protein-2 (BMP-2) delivered with two carriers: a hyaluronic acid polymer (HY), and a collagen carrier complexed with calcium hydroxyapatite and tricalcium phosphate (collagen/HA/TCP).

STUDY DESIGN: Defects were created in the bilateral mandibular bodies of 16 Sprague-Dawley rats. The defects were filled with the HY carrier, the HY carrier loaded with BMP-2, the collagen/HA/TCP carrier, or the collagen/HA/TCP carrier loaded with BMP-2. Animals were euthanatized after 6 weeks, and the hemi-mandibles were analyzed histomorphologically.

RESULTS: Specimens containing BMP-2 had significantly larger new bone and marrow volumes than control specimens. Specimens in the hyaluronan/BMP-2 group tended to have larger volumes of new bone and osteoid than collagen/HA/TCP/BMP-2 specimens, though these differences were not statistically significant.

CONCLUSION: The HY and collagen/HA/TCP carriers had comparable efficacy for bone healing with BMP-2.

SIGNIFICANCE: Bone morphogenetic proteins can be delivered with commercially available alloplasts as osteogenic bone substitutes for the repair of craniofacial bone defects. EBM rating: B-2.

Mesenchymal stem cells expressing baculovirus-engineered BMP-2 and VEGF enhance posterolateral spine fusion in a rabbit model

BACKGROUND CONTEXT

Mesenchymal stem cell (MSC)-based cell therapy and gene transfer have converged and show great potential for accelerating bone healing. Gene therapy can provide more sustained expression of osteogenic factors such as bone morphogenetic protein-2 (BMP-2). We previously demonstrated that low-dose BMP-2 enhanced spinal posterolateral fusion by MSCs in a rabbit model. Herein, we genetically modified rabbit MSCs with a recombinant baculovirus encoding BMP-2 (Bac-CB) and vascular endothelial growth factor (Bac-VEGF) seeded into porous scaffolds to enhance spinal fusion.

PURPOSE

This study evaluates the success rate of the MSC-based cell therapy and gene transfer approach for single-level posterolateral spine fusion. We hypothesize that combining three-dimensional tricalcium phosphate (TCP) scaffolds and genetically modified allogeneic MSCs with baculovirus-mediated growth factor expression would increase the success rate of spinal fusion.

STUDY DESIGN

The study design was based on an animal model (approved by the Institutional Animal Care and Use Committee) using 18 adult male New Zealand rabbits.

METHODS

This study included 18 male New Zealand rabbits, weighing 3.5 to 4 kg. Allogeneic bone marrow-derived MSCs were isolated and genetically modified with Bac-CB and Bac-CV seeded onto TCP scaffolds (MSC/Bac/TCP). The animals were divided into three groups according to the material implanted into the bilateral L4-L5 intertransverse space: TCP scaffold (n=6), MSC/TCP (n=6), and MSC/Bac/TCP (n=6). After 12 weeks, the rabbits were euthanized for radiographic examination, manual palpation, and histologic study.

RESULTS

Bilateral fusion areas in each animal were evaluated independently. The radiographic fusion rates at 12 sites were 0 of 12 in the TCP scaffold group, 4 of 12 in the MSC/TCP group, and 10 of 12 in the MSC/Bac/TCP group. By manual palpation, there were zero solid fusions in the TCP scaffold group, two solid fusions in the MSC/TCP group, and five solid fusions in the MSC/Bac/TCP group. Fusion rates were significantly greater in the MSC/Bac/TCP group.

CONCLUSIONS

The results indicate the potential of using baculovirus as a vector for gene/cell therapy approaches to improve bone healing and support the feasibility of using allogeneic MSCs for inducing bone formation and intertransverse process fusion.

Comparison of a novel oxysterol molecule and rhBMP2 fusion rates in a rabbit posterolateral lumbar spine model

BACKGROUND CONTEXT

The nonunion rate after lumbar spinal fusion is as high as 25%. Recombinant human bone morphogenetic protein 2 (rhBMP2) has been used as a biological adjunct to promote bony fusion. However, recently there have been concerns about BMP2. Oxysterol 133 (Oxy133) has been shown to promote excellent fusion rates in rodent lumbar spine models and offers a potential alternative to rhBMP2.

PURPOSE

The purpose of this study was to compare the fusion rate of rhBMP2 and Oxy133 in a randomized controlled trial using a posterolateral lumbar rabbit spinal fusion model.

STUDY DESIGN

This was a randomized control animal study.

METHODS

Twenty-four male adult white New Zealand rabbits (3-3.5 kg) underwent bilateral posterolateral lumbar spinal fusion at L4-L5. Rabbits were divided into four groups: control (A), 30-μg rhBMP2 (B), 20-mg Oxy133 (C), and 60-mg Oxy133 (D). At 4 weeks, fusion was evaluated by fluoroscopy, and at 8 weeks, the rabbits were sacrificed and fusion was evaluated radiographically, by manual palpation, and with microcomputed tomography.

RESULTS

Fusion rates by radiographic analysis at 8 weeks were Group A, 40.0%; Group B, 91.7%; Group C, 91.7%; and Group D, 100%. Evaluation of fusion masses by manual palpation of excised spines after sacrifice showed the following fusion rates: Group A, 0%; Group B, 83.3%; Group C, 83.3%; and Group D, 90%. Microcomputed tomography scanning confirmed these findings.

CONCLUSIONS

These findings in a rabbit model demonstrate that both 20- and 60-mg Oxy133 doses promote fusion that is equivalent to fusion induced by 30-μg rhBMP2 and significantly greater than the control group. The present findings confirm that Oxy133 is a promising candidate for therapeutic development as an alternative to rhBMP2 to promote spinal fusion.

Complications with the use of bone morphogenetic protein 2 (BMP-2) in spine surgery

BACKGROUND CONTEXT

Recombinant human bone morphogenetic protein 2 (rhBMP-2) is a very potent osteogenic growth factor that has been used successfully in various spine fusions, obviating the need for autologous iliac crest bone graft harvest and therefore avoiding the associated morbidities.

PURPOSE

In the past few years, a tremendous increase in rhBMP-2 usage was noted, and concerns regarding costs, benefits, and safety issues were raised by many. The goal of this work was to provide a comprehensive review of the adverse events and complications associated with use of rhBMP-2.

STUDY DESIGN

Literature review.

METHODS

This is a review of the current literature on the reported adverse events, complications, and concerns associated with rhBMP-2 use.

RESULTS

This article discusses the wide spectrum of adverse outcomes related to rhBMP-2 use in the lumbar and the cervical spine; retrograde ejaculation, antibodies formation, postoperative radiculitis, postoperative nerve root injury, ectopic bone formation, vertebral osteolysis/edema, dysphagia and neck swelling, hematoma formation, interbody graft lucency, and wound healing complications are reviewed. Cost-related concerns, dosage considerations, carrier types, and theoretical carcinogenesis concerns were also presented.

CONCLUSIONS

Despite the excellent spinal fusion rates promoted by this powerful molecule, the increasingly reported adverse outcomes associated with bone morphogenetic protein usage have created real concerns. This article will provide the reader with a good understanding of the reported complications associated with rhBMP-2 use and ultimately help recognize its safety spectrum and limits for better clinical application.

Spinal fusion in the next generation: gene and cell therapy approaches

Bone fusion represents a challenge in the orthopedics practice, being especially indicated for spine disorders. Spinal fusion can be defined as the bony union between two vertebral bodies obtained through the surgical introduction of an osteoconductive, osteoinductive, and osteogenic compound. Autogenous bone graft provides all these three qualities and is considered the gold standard. However, a high morbidity is associated with the harvest procedure. Intensive research efforts have been spent during the last decades to develop new approaches and technologies for successful spine fusion. In recent years, cell and gene therapies have attracted great interest from the scientific community. The improved knowledge of both mesenchymal stem cell biology and osteogenic molecules allowed their use in regenerative medicine, representing attractive approaches to achieve bone regeneration also in spinal surgery applications. In this review we aim to describe the developing gene- and cell-based bone regenerative approaches as promising future trends in spine fusion.

Factors influencing arthrodesis rates in a rabbit posterolateral spine model with iliac crest autograft

PURPOSE

The rabbit posterolateral intertransverse spine arthrodesis model has been widely used to evaluate spinal biologics. However, to date, the validity and reproducibility of performance of iliac crest bone graft, the most common and critical control group, has not been firmly established. We evaluated original research publications that utilized this model, identified which experimental conditions affected fusion rates, and developed an algorithm to predict fusion rates for future study designs.

METHODS

A MEDLINE search was performed for publications through December, 2011 that utilized this model to evaluate fusion rates elicited by iliac crest autograft. All study parameters were recorded, and logistic regression analyses were performed to determine the effects of these variables on fusion rates as determined by either manual palpation or radiographs.

RESULTS

Seventy studies with 959 rabbits in 102 groups met the inclusion criteria. Excluding studies that measured fusion at 4 or fewer weeks or intentionally tried to decrease the fusion rate, the overall fusion rate for autograft was 58.3 ± 16.3 % (mean ± SD) as determined by manual palpation and 66.4 ± 17.8 % by plain radiographs. Regression analysis demonstrated a difference between these outcome measures with a trend towards significance (p = 0.09). Longer time points and larger volumes of autograft resulted in significantly greater reported fusion rates (p < 0.0001 and p < 0.05, respectively). Neither strain, age, weight, nor vertebral level significantly affected fusion rates.

CONCLUSIONS

Although experimental conditions varied across studies, time point evaluation and autograft volume significantly affected fusion rates. Despite some variability demonstrated across certain studies, we demonstrated that when the time point and volume of autograft were controlled for, the iliac crest control group of the rabbit posterolateral spinal arthrodesis model is both reliable and predictably affected by different experimental conditions.

Characterization of scaffold carriers for BMP9-transduced osteoblastic progenitor cells in bone regeneration

Successful bone tissue engineering at least requires sufficient osteoblast progenitors, efficient osteoinductive factors, and biocompatible scaffolding materials. We have demonstrated that BMP9 is one of the most potent factors in inducing osteogenic differentiation of mesenchymal progenitors. To facilitate the potential use of cell-based BMP9 gene therapy for bone regeneration, we characterize the in vivo osteoconductive activities and bone regeneration potential of three clinically used scaffold materials, type I collagen sponge, hydroxyapatite-tricalcium phosphate (HA-TCP), and demineralized bone matrix (DBM), using BMP9-expressing C2C12 osteoblastic progenitor cells. We find that recombinant adenovirus-mediated BMP9 expression effectively induces osteogenic differentiation in C2C12 cells. Although direct subcutaneous injection of BMP9-transduced C2C12 cells forms ectopic bony masses, subcutaneous implantation of BMP9-expressing C2C12 cells with collagen sponge or HA-TCP scaffold yields the most robust and mature cancellous bone formation, whereas the DBM carrier group forms no or minimal bone masses. Our results suggest that collagen sponge and HA-TCP scaffold carriers may provide more cell-friendly environment to support the survival, propagation, and ultimately differentiation of BMP9-expressing progenitor cells. This line of investigation should provide important experimental evidence for further preclinical studies in BMP9-mediated cell-based approach to bone tissue engineering.

Recombinant human bone morphogenetic protein-2 stimulates the osteogenic potential of the Schneiderian membrane: a histometric analysis in rabbits

This study evaluated the osteoinductive effect of the recombinant human bone morphogenetic protein-2 (rhBMP-2)-coated biphasic calcium phosphate (BCP) carrier system on the grafted sinus area, including surrounding tissues and the Schneiderian membrane. A total of 18 male rabbits were used in this study; two for in vitro and 16 for in vivo experiments. Schneiderian membranes taken from two animals were cultured with or without rhBMP-2, and quantitative reverse transcriptase-polymerase chain reaction analysis was performed. Both maxillary sinuses in each of the 16 animals were used to compare the in vivo effects of rhBMP-2-coated BCP (experimental group) and BCP alone (control group). In each animal, rhBMP-2-coated BCP was grafted into one of the maxillary sinuses, and the same amount of BCP alone was grafted into the contralateral site in random order. Radiologic and histometric analyses were performed at 2 and 8 weeks after surgery. After 2 days of culturing with or without rhBMP-2, a significant increase in the expression of early osteoblasts (RUNX2, type I collagen, alkaline phosphatase, and osteopontin) could be observed. Different histologic healing patterns were observed in experimental and control sites: newly formed bone lining the reflected sinus membrane without bone formation was observed at the central areas of experimental sites (window=0.06%; center=0%; membrane=20.86% of new bone), whereas evenly distributed new bone formation was observed at the control sites (window=7.27%; center=7.41%; membrane=15.58% of new bone).The augmented volume was well maintained at both the experimental and control sites during the experimental period, but at 2 weeks, the augmented volume was greater at the experimental sites than at the control sites (232.62 and 195.29 mm(3), respectively; p<0.001). These results suggest that good space maintenance in sinus augmentation is achieved with BCP, while the osteoinductive potential of the sinus membrane is activated at the early stage of healing with rhBMP-2.

Posterolateral inter-transverse lumbar fusion in a mouse model

BACKGROUND

Spinal fusion is a common orthopaedic procedure that has been previously modeled using canine, lapine, and rodent subjects. Despite the increasing availability of genetically modified mouse strains, murine models have only been infrequently described.

PURPOSE

To present an efficient and minimally traumatic procedure for achieving spinal fusion in a mouse model and determine the optimal rhBMP-2 dose to achieve sufficient fusion mass.

METHOD

MicroCT reconstructions of the unfused mouse spine and human spine were compared to design a surgical approach. In phase 1, posterolateral lumbar spine fusion in the mouse was evaluated using 18 animals allocated to three experimental groups. Group 1 received decortication only (n=3), Group 2 received 10 μg rhBMP-2 in a collagen sponge bilaterally (n=6), and Group 3 received 10 μg rhBMP-2 + decortication (n=9). The surgical technique was assessed for intra-operative safety, efficacy, access and reproducibility. Spines were harvested for analysis at 3 weeks (Groups 1, 2) and 1, 2, and 3 weeks (Group 3). In phase 2, a dose response study was carried out in an additional 18 animals with C57BL6 mice receiving sponges containing 0, 0.5, 1, 2.5, 5 μg of rhBMP-2 per sponge bilaterally.

RESULTS

The operative procedure via midline access was rapid and reproducible, and fusion of the murine articular processes was found to be analogous to the human procedure. Unlike reports from other species, decortication alone (Group 1) yielded no new bone formation. Addition of rhBMP-2 (Groups 2 and 3) yielded a significant bone mass that bridged the L4-L6 vertebrae. The subsequent dose response experiment revealed that 0.5 μg rhBMP-2 per sponge was sufficient to create a fusion mass.

CONCLUSION

We describe a new approach for mouse lumbar spine fusion that is safe, efficient, and highly reproducible. The technique we employed is analogous to the human midline procedure and may be highly suitable for genetically modified mouse models.

Critical size defect regeneration using PEG-mediated BMP-2 gene delivery and the use of cell occlusive barrier membranes - the osteopromotive principle revisited

OBJECTIVE

The objective of this study was to investigate if osseous regeneration can be accelerated by involvement of periosteal tissue. Bone defect regeneration could be accelerated by the involvement of periosteal tissue if osteogenic cell signalling is maintained within the defect. It was questioned if local cell-mediated BMP-2 gene delivery makes a cell occlusive membrane dispensable during bone critical size defect regeneration.

METHODS

PEG matrix (degradation time 10 days) and PEG membrane (degradation time 120 days) were used in the pig calvarial model. Cylindrical (1 × 1 cm) critical size defects (CSD) (9 per animal; 20 animals) were filled with: (i) particulated autologous bone, covered with PEG membrane (group 1); (ii) HA/TCP, covered with PEG membrane (group 2); (iii) HA/TCP, mixed with PEG matrix (group 3); and (iv) HA/TCP mixed with BMP-2-transfected osteoblasts and PEG matrix (group 4). BMP-2/4 gene transfer: liposomal in vitro transfection of BMP-2/V5-tag fusion-protein. Quantitative histomorphometry (toluidine blue staining) after 2, 4 and 12 weeks assessed bone formation. Semiquantitative immunohistochemistry estimated the expression of BMP-2, V5-tag, Runx-2 and Sox9.

RESULTS

PEG matrix embedded BMP-2 expressing cells presented higher bone formation (P < 0.05) than HA/TCP + PEG matrix defect filling or PEG membrane covering (HA/TCP filling) after 12 weeks. Highest expression of BMP-2, Runx-2 and lowest expression of fibrous tissue marker Sox9 was seen in the BMP-2 group.

CONCLUSION

PEG matrix embedded BMP-2 expressing cells are capable to maintain osteogenic signalling and to accelerate osseous defect regeneration in absence of a cell occlusive membrane.

Human mesenchymal stem cells and biomaterials interaction: a promising synergy to improve spine fusion

PURPOSE

Spine fusion is the gold standard treatment in degenerative and traumatic spine diseases. The bone regenerative medicine needs (i) in vitro functionally active osteoblasts, and/or (ii) the in vivo induction of the tissue. The bone tissue engineering seems to be a very promising approach for the effectiveness of orthopedic surgical procedures, clinical applications are often hampered by the limited availability of bone allograft or substitutes. New biomaterials have been recently developed for the orthopedic applications. The main characteristics of these scaffolds are the ability to induce the bone tissue formation by generating an appropriate environment for (i) the cell growth and (ii) recruiting precursor bone cells for the proliferation and differentiation. A new prototype of biomaterials known as "bioceramics" may own these features. Bioceramics are bone substitutes mainly composed of calcium and phosphate complex salt derivatives.

METHODS

In this study, the characteristics bioceramics bone substitutes have been tested with human mesenchymal stem cells obtained from the bone marrow of adult orthopedic patients.

RESULTS

These cellular models can be employed to characterize in vitro the behavior of different biomaterials, which are used as bone void fillers or three-dimensional scaffolds.

CONCLUSIONS

Human mesenchymal stem cells in combination with biomaterials seem to be good alternative to the autologous or allogenic bone fusion in spine surgery. The cellular model used in our study is a useful tool for investigating cytocompatibility and biological features of HA-derived scaffolds.

Efficacy of interspinous process lumbar fusion with recombinant human bone morphogenetic protein-2 delivered with a synthetic polymer and β-tricalcium phosphate in a rabbit model

INTRODUCTION

As a powerful bone-inducing cytokine, rhBMP-2 has been used as a bone graft substitute in combination with animal-derived collagen to achieve interbody or posterolateral spinal fusion. Successful interspinous process fusion using rhBMP-2 in combination with synthetic carrier materials would offer a safe, minimally invasive spinal fusion option for the treatment of spinal disorders. The aims of the present study were to achieve interspinous process fusion by implanting rhBMP-2-retaining degradable material instead of bone grafting and to evaluate efficacy for vertebral stabilization.

MATERIALS AND METHODS

A polymer gel (200 mg), β-tricalcium phosphate powder (400 mg), and rhBMP-2 (0, 30, 60 or 120 μg) were mixed to generate a plastic implant, which was then placed during surgery to bridge the L5-6 interspinous processes of 58 rabbits. Control animals received implants either without rhBMP-2 or with autogenous bone chips from the iliac crest. L5-6 vertebrae were recovered 8 weeks postoperatively. Interspinous process fusion was evaluated by radiography, biomechanical bending test, intradiscal pressure (IDP) measurement, and histology.

RESULTS

In bending tests, strength of fusion was significantly greater in BMP60 and BMP120 groups than in sham, BMP0, BMP30 or autogenous bone groups. IDP at L5-6 was significantly reduced in BMP60 and BMP120 groups compared to sham, BMP0, BMP30, and autograft groups. Histologically, coronal sections of the fusion mass showed a bone mass bridging both spinous processes.

CONCLUSION

Solid interspinous process fusion was achieved in rabbit models by 8 weeks after implanting the biodegradable bone-inducing material. These results suggest a potential new less-invasive option without bone grafting for the treatment of lumbar disorders.

Improved spinal fusion efficacy by long-term delivery of bone morphogenetic protein-2 in a rabbit model

BACKGROUND AND PURPOSE

Various new delivery systems for recombinant human bone morphogenetic protein-2 (rhBMP-2) have been introduced to improve its efficacy in osteogenesis. Of these, we have previously developed heparin-conjugated PLGA nanospheres (HCPN) as a long-term delivery system for BMP-2. In vitro studies have shown that the BMP-2 long-term delivery system enhances the level of bone formation. However, the long-term effects of BMP-2 on spinal fusion have not been assessed. Therefore, we now tested the hypothesis that the long-term delivery of BMP-2 using HCPN improves spinal fusion compared to short-term delivery in a rabbit fusion model.

METHODS

24 adult New Zealand White rabbits underwent posterolateral fusion (6 animals in 4 groups). The autograft group received an autologous iliac chip bone graft as a positive control. The BMP-2-PN group received rhBMP-2 (20 μg per implant) and PLGA nanospheres (PN) suspended in fibrin gel, and served as a short-term release group. The HCPN group received HCPN suspended in fibrin gel without BMP-2 as a negative control. The BMP-2-HCPN group received rhBMP-2 (20 μg per implant)-bound HCPN suspended in fibrin gel and served as a long-term release group. All animals were killed 12 weeks after surgery. Manual palpation, axial tensile tests, radiography, and histological evaluations were then performed.

RESULTS

The spinal fusion rate and Young's modulus of the fusion mass were better in the BMP-2 long-term delivery group than in the short-term delivery group at an equivalent dose. However, the outcome of the long-term delivery was inferior to that of the autograft group.

INTERPRETATION

The HCPN system showed potential as an effective carrier that might improve the osteogenic efficacy of BMP-2 for spinal fusion.

Graft options in posterolateral and posterior interbody lumbar fusion

STUDY DESIGN

Review article, review of literature.

OBJECTIVE

To review the bone graft options that exist for posterolateral and posterior interbody lumbar fusion.

SUMMARY OF BACKGROUND DATA

As the number of lumbar fusion surgeries has increased over the last decade, alternative methods of grafting have been developed. Iliac crest autograft bone has traditionally been used for lumbar fusion. The downside to this graft option, however, is donor site morbidity.

METHODS

The current literature on alternatives to iliac crest autograft bone for obtaining lumbar fusion was reviewed.

RESULTS

Platelet gels, demineralized bone matrix, synthetic bone graft, and bone morphogenetic protein are potential options for bone graft supplementation or substitution. In preclinical studies, platelet gels have been beneficial to bone growth when combined with autograft, but clinical studies do not support the use of platelet gel in posterolateral lumbar fusion. Preclinical studies of demineralized bone matrix have shown significant variability in the osteoinductive properties of the available products, and clinical data showing efficacy is limited. The use of synthetic bone graft material (ceramics) in lumbar fusion surgery is increasing. Calcium phosphate compounds (i.e., beta-tricalcium phosphate and hydroxyapetite) are most commonly used and are often combined with type I collagen to form a matrix. These materials provide an osteoconductive scaffold for bony ingrowth and can be combined with bone marrow aspirate or used as a carrier for osteogenic factors. Bone morphogenetic protein (rhBMP-2) has been shown to provide similar or even increased fusion rates over autograft iliac crest bone. There are, however, potential safety concerns associated with the use of bone morphogenetic protein that are not fully understood.

CONCLUSION

Several alternatives to iliac crest autograft bone provide promising early clinical results in achieving posterolateral and posterior interbody lumbar fusion.

Ceramic/polymer nanocomposites with tunable drug delivery capability at specific disease sites

Pharmaceutical agents are often used to stimulate new bone formation for the treatment of bone injuries or diseases (such as osteoporosis). However, there are several problems associated with current orthopedic drug delivery methods. First, conventional systemic administration of pharmaceutical agents may not effectively reach targeted sites and, thus, they can cause nonspecific bone formation in areas not affected by injury or disease. Second, even if intentionally delivered or implanted locally to the damaged bone tissue, these agents tend to rapidly diffuse into adjacent tissues due to weak physical bonding to their drug carriers, which limits their potential to promote prolonged bone formation in targeted areas of bone disease. Therefore, in this study, biodegradable ceramic/polymer nanocomposites were explored as novel drug carriers for orthopedic applications to prolong local drug release and, thus, improve drug effectiveness at bone disease sites. Specifically, a bone morphogenetic protein (BMP-7) derived peptide (DIF-7c) was used as a model drug in this study and was first loaded onto nanocrystalline hydroxyapatite (nano-HA) by either covalent chemical attachment or physical adsorption. These drug-carrying nano-HA particles were then dispersed into a degradable polymer (poly-lactide-co-glycolide or PLGA) matrix to create an implantable system capable of long-term drug release. The aminophase silane covalent chemical immobilization process was utilized in this study. These nanocomposite-based drug delivery systems were then characterized for drug loading efficiency and in vitro drug release. Results demonstrated that DIF-7c was successfully immobilized onto nano-HA placed in PLGA. Moreover, a greater prolonged two-phase release profile (of more than 3 months) was achieved when using aminophase silane chemical immobilization to nano-HA particles. Since previous studies have demonstrated greater in vivo bone growth on nano- compared with micron-HA particles placed in rat calvaria, this study continued to demonstrate that ceramic/polymer nanocomposites are promising candidates as novel orthopedic materials to promote bone regeneration.

The design and use of animal models for translational research in bone tissue engineering and regenerative medicine

This review provides an overview of animal models for the evaluation, comparison, and systematic optimization of tissue engineering and regenerative medicine strategies related to bone tissue. This review includes an overview of major factors that influence the rational design and selection of an animal model. A comparison is provided of the 10 mammalian species that are most commonly used in bone research, and existing guidelines and standards are discussed. This review also identifies gaps in the availability of animal models: (1) the need for assessment of the predictive value of preclinical models for relative clinical efficacy, (2) the need for models that more effectively mimic the wound healing environment and mass transport conditions in the most challenging clinical settings (e.g., bone repair involving large bone and soft tissue defects and sites of prior surgery), and (3) the need for models that allow more effective measurement and detection of cell trafficking events and ultimate cell fate during the processes of bone modeling, remodeling, and regeneration. The ongoing need for both continued innovation and refinement in animal model systems, and the need and value of more effective standardization are reinforced.

Delivery systems for bone growth factors — the new players in skeletal regeneration

Given the challenge of an increasing elderly population, the ability to repair and regenerate traumatised or lost tissue is a major clinical and socio-economic need. Pivotal in this process will be the ability to deliver appropriate growth factors in the repair cascade in a temporal and tightly regulated sequence using appropriately designed matrices and release technologies within a tissue engineering strategy. This review outlines the current concepts and challenges in growth factor delivery for skeletal regeneration and the potential of novel delivery matrices and biotechnologies to influence the healthcare of an increasing ageing population.

Successful spinal fusion by E. coli-derived BMP-2-adsorbed porous beta-TCP granules: a pilot study

Bone morphogenetic proteins (BMPs) were originally identified as osteoinductive proteins. With cloning of BMP genes, studies of BMPs and their clinical application have advanced. However, with increasing clinical applications, drug delivery systems and production costs have become more important issues. To address these issues, we asked whether E. coli-derived rhBMP-2 (E-BMP-2)-adsorbed porous beta-TCP granules could achieve posterolateral lumbar fusion in a rabbit model similar to autogenous bone grafts. Lumbar spinal fusion masses were evaluated by 3-D computed tomography, mechanical testing, and histological analyses 8 weeks after surgery. By these measures E-BMP-2-adsorbed beta-TCP granules achieved lumbar spinal fusion in dose-dependent fashion in a rabbit model as well as autogenous bone graft. Our preliminary findings suggest E-BMP-2-adsorbed porous beta-TCP could be a novel, effective alternative to autogenous bone grafting for generating new bone and promoting regenerative repair of bone, and potentially utilizable in the clinical setting for treating spinal disorders.

Biological basis of bone formation, remodeling, and repair-part I: biochemical signaling molecules

The bony biochemical environment is an active and dynamic system that permits and promotes cellular functions that lead to matrix production and ossification. Each component is capable of conveying important regulatory cues to nearby cells, thus effecting gene expression and changes at the cytostructural level. Here, we review the various signaling molecules that contribute to the active and dynamic nature of the biochemical system. These components include hormones, cytokines, and growth factors. We describe their role in regulating bone metabolism. Certain growth factors (i.e., TGF-beta, IGF-1, and VEGF) are described in greater detail because of their potential importance in developing successful tissue-engineering strategies.

Posterolateral spinal fusion in a rabbit model using a collagen-mineral composite bone graft substitute

Choosing the appropriate graft material to participate in the healing process in posterolateral spinal fusion continues to be a challenge. Combining synthetic graft materials with bone marrow aspirate (BMA) and autograft is a reasonable treatment option for surgeons to potentially reduce or replace the need for autograft. FormaGraft, a bone graft material comprising 12% bovine-derived collagen and 88% ceramic in the form of hydroxyapatite (HAp) and beta tricalcium phosphate (beta-TCP) was evaluated in three possible treatment modalities for posterior spinal fusion in a standard rabbit model. These three treatment groups were FormaGraft alone, FormaGraft soaked in autogenous BMA, and FormaGraft with BMA and iliac crest autograft. No statistically demonstrable benefits or adverse effects of the addition of BMA were found in the current study based on macroscopic, radiology or mechanical data. This may reflect, in part, the good to excellent results of the collagen HA/TCP composite material alone in a well healing bony bed. Histology did, however, reveal a benefit with the use of BMA. Combining FormaGraft with autograft and BMA achieved results equivalent to autograft alone. The mineral and organic nature of the material provided a material that facilitated fusion between the transverse processes in a standard preclinical posterolateral fusion model.

Enhancement of posterolateral lumbar spine fusion using low-dose rhBMP-2 and cultured marrow stromal cells

We tested the hypothesis that the dose of recombinant human bone morphogenetic protein-2 (rhBMP-2) required to induce spine fusion can be reduced by combination with mesenchymal stem cells (MSCs). Twenty-four adult rabbits underwent posterolateral intertransverse fusion at the L4-L5 level. The animals were divided into four groups based on the implant material: autologous iliac graft, Alginate-MSCs composite, Alginate-BMP-2-MSCs composite, and Alginate-BMP-2 composite. After 16 weeks, the rabbits were euthanized for radiographic examination, manual palpation, biomechanical testing, and histology. Radiographic union of 12 intertransverse fusion areas for the autogenous iliac graft, Alginate-MSCs, Alginate-BMP-2-MSCs, and Alginate-BMP-2 groups was 11, 8, 11, and 0, respectively. Moreover, manual palpation of six fusion segments in each subgroup found solid union to be 6, 1, 5, and 0, respectively. The average torques at failure of the first three groups were 2278 +/- 135, 1943 +/- 140, and 2334 +/- 187 N-mm, respectively. The failure torque did not differ significantly between the autograft and Alginate-BMP-2-MSCs groups; both groups were significantly higher than the Alginate-MSCs group. The results indicate that MSCs delivered with in vitro cellular doses of rhBMP-2 are more osteoinductive than MSCs without rhBMP-2. In combination with MSCs, a low dose (2.5 microg) of rh-BMP-2 could enhance bone formation and posterolateral spine fusion success in the rabbit model.

Acceleration of spinal fusion using syngeneic and allogeneic adult adipose derived stem cells in a rat model

Posterolateral spinal fusion is the standard treatment for lumbar compression fractures. Adult adipose tissue-derived stem cells (ASCs) promote osteogenesis in vivo and in vitro. The hypothesis tested in this study was that syngeneic and allogeneic ASCs on a biomaterial scaffold composed of tricalcium phosphate and collagen I will accelerate spinal fusion in a rat model. ASCs from male Fischer or ACI rats were loaded onto scaffolds (53,571 cells/mm(3)) and cultured in stromal media for 48 h. Male Fisher rats were assigned to 4 cohorts (n = 14/cohort) after bilateral decortication of the L4 and L5 transverse processes: (1) No treatment; (2) scaffold only; (3) scaffold + syngeneic ASCs; or (4) scaffold + allogeneic ASCs. Half of each cohort was harvested 4 or 8 weeks after surgery. Spinal fusion was evaluated with radiographs, microcomputed tomography, and light microscopy. Callus did not form in spines without scaffolds. There were no significant differences in callus formation among scaffold cohorts 4 weeks after surgery. Callus formation was more mature in both ASC cohorts versus scaffold alone 8 weeks after surgery based on microstructure as well as radiographic and microcomputed tomographic evidence of active bone formation. Inflammatory cell infiltrate was significantly lower in both ASC cohorts (syngeneic = 18.3 +/- 0.85%; allogeneic = 23.5 +/- 2.33%) versus scaffold alone (46.8 +/- 11.8%) 4 weeks after surgery. Results of this study support syngeneic and allogeneic ASC acceleration of posterior lumbar spinal fusion in a rat model.

Bone regeneration by using scaffold based on mineralized recombinant collagen

Bone regeneration was achieved in the 15-mm segmental defect model in the radius of rabbit by using the scaffold based on mineralized recombinant collagen for the first time. The recombinant collagen was recombinant human-like type I collagen, which was produced by cloning a partial cDNA that was reversed by mRNA from human collagen alpha1(I) and transferred to E. coli. The scaffold material nano-hydroxyapatite/recombinant human-like collagen/poly(lactic acid) (nHA/RHLC/PLA) was developed by biomimetic synthesis. Thermo gravimetric analysis, X-ray diffraction and scanning electron microscopy were applied to exhibit that the scaffold showed some features of natural bone both in main component and hierarchical microstructure. The percentages of organic phase and inorganic phase of nHA/RHLC were similar to that of natural bone. The three-dimensional porous scaffold materials mimic the microstructure of cancellous bone. In the implantation experiment, the segmental defect was healed 24 weeks after surgery, and the implanted composite was completely substituted by new bone tissue. The results of the implantation experiment were very comparable with that of the scaffold based on mineralized animal-sourced collagen. It is concluded that the scaffold based on mineralized recombinant collagen maintains the advantages of mineralized animal-sourced collagen, while avoids potential virus-dangers. The scaffold is a promising material for bone tissue engineering.

Ketorolac and spinal fusion: does the perioperative use of ketorolac really inhibit spinal fusion?

STUDY DESIGN

Retrospective review.

OBJECTIVE

To evaluate the effect of postoperative use of ketorolac (Toradol) on spinal fusion in humans.

SUMMARY OF BACKGROUND DATA

The value of parenteral ketorolac in postoperative analgesia has been well documented across surgical specialties. However, some studies have shown that ketorolac may adversely affect osteogenic activity and fracture healing.

METHODS

A total of 405 consecutive patients who underwent primary lumbar posterolateral intertransverse process fusion with pedicle screw instrumentation were included in this retrospective study. A subtotal of 228 patients received Toradol after surgery for adjunctive analgesia. Each patient received a mandatory dose of 30 mg intravenously every 6 hours for 48 hours. The same surgeon performed the fusion procedure on all of these patients. Historical controls included 177 patients who did not receive Toradol after surgery. The minimum follow-up period was 24 months. Nonunions were diagnosed by analyzing sequential radiographs, flexion-extension radiographs, and computed tomography with multiplanar reconstructions. The gold standard of surgical exploration was performed in symptomatic patients with diagnostic ambiguity or nonunions diagnosed by imaging.

RESULTS

There were no smokers in the study population. Pseudarthrosis was identified in 12 of 228 patients (5.3%) who received Toradol after surgery, and in 11 of 177 patients (6.2%) who did not. There was no significant difference detected in the nonunion rates between the two groups (P > 0.05, chi2 method).

CONCLUSION

Use of ketorolac after spinal fusion surgery in humans, limited to 48 hours after surgery for adjunctive analgesia, has no significant effect on ultimate fusion rates.

Evaluation of insoluble bone gelatin as a carrier for enhancement of osteogenic protein-1-induced intertransverse process lumbar fusion in a rabbit model

STUDY DESIGN

Postero-lateral lumbar fusion in a rabbit model was performed to compare the bone induction potential of autograft, insoluble bone gelatin (ISBG), osteogenic protein-1 (OP-1), and the combination of ISBG and OP-1.

OBJECTIVE

To evaluate the efficiency of ISBG as a carrier/enhancer for OP-1 in a rabbit spinal fusion model.

SUMMARY OF BACKGROUND DATA

OP-1 or recombinant human BMP-7 has been shown to be effective in inducing new bone formation in surgical applications such as spinal arthrodesis. However, the lack of an ideal carrier contributes to its associated comorbidities (e.g., uncontrolled bone growth, local inflammatory over-response, nonfusion) and limits its use clinically.

METHODS

Adult New Zealand white rabbits (n = 32) underwent bilateral lumbar intertransverse process fusion procedures at L5 to L6 and were randomized to receive: (1) autograft; (2) ISBG; (3) OP-1; or (4) ISBG in combination with OP-1 (ISBG + OP-1). Spinal fusion masses were evaluated by manual palpation, biomechanical testing, radiographic assessment, microcomputer tomography scanning and histologic examination at 6 weeks after surgery.

RESULTS

Treatment of ISBG + OP-1 resulted in higher spinal fusion rates (7 of 7, 100%) than that of autograft (3 of 7, 43%), ISBG (2 of 8, 25%), and OP-1 (2 of 7, 29%) based on manual palpation (P < 0.01). Greater fusion rates in the ISBG + OP-1 group were also evidenced by radiographic examination (P < 0.01), microcomputer tomography bone volume analysis (P < 0.01), and biomechanical testing (P < 0.05). Histologic assessment demonstrated that treatment of ISBG + OP-1 induces new contiguous bone formation in the interval between the transverse processes which was absent in the other groups.

CONCLUSION

In this study, ISBG + OP-1 resulted in more effective lumbar intertransverse process fusion than autograft, OP-1 putty or ISBG alone. ISBG is capable of enhancing OP-1-induced bone formation.

Modification of bone graft by blending with lecithin to improve hydrophilicity and biocompatibility

Lecithin was blended to improve the hydrophilicity and biocompatibility of bone graft containing poly(l-lactic acid) (PLLA). Solution blending and freeze drying were used to fabricate symmetrical scaffolds containing different percentages of lecithin (lecithin: PLLA = 0, 5, 10 wt%). Scanning electron microscopy showed that the scaffolds maintained the three-dimensional porous structure. A water uptake experiment proved the significant improvement of hydrophilicity of the blend scaffold. With the addition of lecithin, the compressive strength and compressive modulus decreased. When the weight ratio of lecithin to PLLA was up to 10%, the compressive strength was still more than the lower limit of natural cancellous bone. To test the biocompatibility of the scaffolds, cell culture in vitro and subcutaneous implantation in vivo were performed. MC3T3-E1 preosteoblastic cells were cultured on the scaffolds for 7 days. Methylthiazol tetrazolium assay and laser scanning confocal microscopy were used to exhibit proliferation and morphology of the cells. The subcutaneous implantation in rats tested inflammatory response to the scaffolds. The results proved the better biocompatibility and milder inflammatory reactions of the blend scaffold (lecithin: PLLA = 5%) compared with the scaffold without lecithin. The modified scaffold containing lecithin is promising for bone tissue engineering.

Enhancing effects of a prostaglandin EP4 receptor agonist on recombinant human bone morphogenetic protein-2 mediated spine fusion in a rabbit model

STUDY DESIGN

An experimental animal study aimed at achieving posterolateral intertransverse process fusion with rhBMP-2 in combination with the local delivery of an EP4 receptor agonist.

OBJECTIVE

To determine whether an EP4 receptor agonist (EP4A) can reduce the amount of BMP required to achieve posterolateral spinal fusion in rabbits.

SUMMARY OF BACKGROUND DATA

In the clinic, BMP retaining implants are used for spinal fusion and the treatment of pseudarthrosis after long bone fracture. However, the requirement of high doses of BMP-2 for bone formation in humans makes the implants expensive and limits their use in the clinic. Previous studies in our laboratory using a new delivery system involving a synthetic polymer/beta-TCP powder composite had shown it was possible to reduce the total BMP-2 amount to 30 microg per fusion in a rabbit model. To further reduce the dose of BMP required for a successful fusion, we explored the use of a chemical compound to enhance the bone-inducing action of BMP-2.

METHODS

In order to prepare 1 implant to bridge the unilateral L5 and L6 transverse processes, 300 mg of polymer gel (PLA-DX-PEG block copolymer), 300 mg of beta-TCP powder, rhBMP-2 (7.5, 3.75, or 0 microg), with or without EP4A (45 microg) were mixed and manually shaped to resemble a rod. Through a posterolateral approach, 2 implants were placed on both sides (1 per side) by surgery in order to bridge the transverse processes of adult New Zealand white rabbits (n = 48). The lumbar vertebrae were recovered 6 weeks after surgery. The posterolateral fusion was examined by manual palpation, radiography, biomechanical testing, and histology.

RESULTS

All of 8 rabbits that received 7.5 microg of BMP-2 and EP4A consistently showed fusion by significant amount of new bone formation. However, solid fusion was seen in only 3 of 8 rabbits that received 7.5 microg of BMP-2 without the EP4 receptor agonist.

CONCLUSION

Local administration of an EP4 receptor agonist enhanced the bone-inducing activity of BMP-2 in a rabbit posterolateral lumbar spinal fusion model and as a result, the dose of BMP-2 required for this outcome was reduced by 50% compared with our previous report. The coadministration of this compound via a local delivery system may help to reduce the costs of spine fusion with use of BMP-2 in the clinic.

In vitro degradation characteristics of photocrosslinked anhydride systems for bone augmentation applications

In the past decade, injectable biomaterials that are capable of in situ formation have garnered increased interest for use in restorative orthopedic procedures. In this study, the in vitro degradation of photocrosslinked polyanhydride matrices, derived from methacrylic anhydrides of 1,6-bis(p-carboxyphenoxy)hexane (MCPH) and sebacic acid (MSA) were evaluated over a 6-week period under physiological conditions. These matrices were augmented with two additives--the reactive diluent poly(ethylene glycol) diacrylate (PEGDA) and the buffering agent calcium carbonate (CaCO3). Disk shaped specimens were produced by crosslinking the components using both chemical and photoinitiators and exposure to visible light. The experimental variables studied included: MCPH:MSA ratio, PEGDA molecular weight and weight fraction, and incorporation of CaCO3. The effects of these variables on local pH, water uptake, mass loss, and mechanical properties were explored. Increasing the MCPH:MSA ratio decreased the mass loss and water uptake at predetermined endpoints, and decreased buffer acidity during degradation. Both PEGDA and CaCO3 were found to decrease acidity and to reduce water uptake during degradation. Incorporation of CaCO3 enabled maintenance of compressive modulus during degradation. These results demonstrate that incorporation of reactive diluents and nonreactive additives into networks of photocrosslinked anhydrides can improve system properties as a material for bone replacement.

The effects of bone morphogenetic protein and basic fibroblast growth factor on cultured mesenchymal stem cells for spine fusion

STUDY DESIGN

Posterolateral lumbar transverse process fusion was carried out using cultured mesenchymal stem cells with or without bone morphogenetic protein (BMP) and basic fibroblast growth factor (FGF).

OBJECTIVES

To determine the ability of BMP and basic FGF to enhance the efficacy of bone marrow-derived mesenchymal stem cells in lumbar arthrodesis.

SUMMARY OF BACKGROUND DATA

Our previous study hypothesized that it would be important to differentiate into osteogenic cells and to implant a large number of cells for achieving solid spinal fusions.

METHODS

Thirty-six adult rabbits underwent single-level bilateral posterolateral intertransverse process fusions at L4-L5. Animals were divided into 5 groups, each according to the material implanted: (1) autologous bone (autograft; n = 8); (2) mesenchymal stem cells (n = 7); (3) mesenchymal stem cells with recombinant human bone morphogenetic protein (rhBMP)-2 (mesenchymal stem cell-BMP; n = 7); (4) mesenchymal stem cells with basic FGF (mesenchymal stem cell-FGF; n = 7); and (5) mesenchymal stem cells with rhBMP-2 and basic FGF (mesenchymal stem cell-BMP-FGF; n = 7). Fresh bone marrow cells from the iliac crest of each animal were cultured in a standard medium for 2 weeks. For an additional week, the mesenchymal stem cells (1 x 10(6) cells/mL) were cultured in 10(-8) M dexamethasone, type I collagen gel and porous hydroxyapatite particles with or without rhBMP-2 (2 microg/mL) and basic FGF (5 microg/mL). Animals were killed 6 weeks after surgery. Radiograph, manual palpation, and histology were used to evaluate spinal fusions.

RESULTS

Fusion rates were 5/7 in the autograft group, 0/7 in the mesenchymal stem cell group, 2/7 in the mesenchymal stem cell-BMP group, 3/7 in the mesenchymal stem cell-FGF group, and 6/7 in the mesenchymal stem cell-BMP-FGF group. The histology in some of both mesenchymal stem cell-BMP and mesenchymal stem cell-FGF groups demonstrated that fibrous tissues and cartilages remained in grafted areas. In the mesenchymal stem cell-BMP-FGF group, each grafted fragment was connected with new bone ingrowths.

CONCLUSIONS

This study showed that bone marrow-derived mesenchymal stem cells cultured with rhBMP-2 and basic FGF act as a substitute for autograft in lumbar arthrodesis. This technique may yield a more consistent quality of fusion bone as compared to that with autograft.

Bone repair in rat mandible by rhBMP-2 associated with two carriers

This study evaluated the quantity and quality of newly formed bone, stimulated by rhBMP-2 in combination with monoolein or chitosan gel as carriers, in critical bone defects created in 36 Wistar rat mandibles. Two weeks after surgery, the animals were anesthetized with 37.5% urethane submitted to perfusion and the hemi-mandibles removed for histological and histomorphometrical analysis. The results showed that there was a statistical difference between groups of animals receiving or not rhBMP-2 (p<0.05). Newly formed bone was more intense in the occlusal region, followed by the basal and middle regions, respectively. Both carriers, monoolein and chitosan gels were adequate for defect filling and control of protein release.

Bone morphogenetic protein gene therapy using a fibrin scaffold for a rabbit spinal-fusion experiment

OBJECTIVE

To observe the feasibility of using fibrin gel as a scaffold during rabbit spinal fusion with rat kidney cells carrying bone morphogenetic protein-2 (BMP-2) expression vector.

METHODS

Normal Rattusnorvegicus (rat) kidney (NRK) cells were transfected with pCMV-BMP2 vectors by using a nonviral reagent (FuGENE6), following which the transfected cells (1.5 x 10(7)) were encapsulated and evenly suspended in a fibrin scaffold, and implanted to either side of the L5-L6 intertransverse space for six test rabbits. For the control group (six rabbits), the transfected cells (1.5 x 10(7)) were added on and absorbed in a surgical gelatin sponge, and implanted in an analogous location. Radiographs of the spine of all animals were taken at six, ten, and 12 weeks subsequent to fusion surgery. Gross and histological examination of the fusion masses for all the animals were performed subsequent to animals having been sacrificed at 12 weeks.

RESULTS

Expression of BMP-2 was verified in the pCMV-BMP2 transfected NRK cells which were used for the subsequent rabbit experiment. For all 12 rabbits, no evidence of implanted-tissue rejection was seen during the postoperative course. Neither residual scaffold nor inflammatory granulation tissue was seen in the harvested spinal segments. For the six study-group rabbits, radiographic examinations revealed that four individuals (67%) featured prominent new-bone formation (good fusion), and two (33%) revealed moderate new-bone formation (fair fusion) at the implanted sites, whereas all six control-group rabbits revealed no evidence of new-bone formation (nonfusion) at the implanted sites. For the histological examinations, all animals in the study group revealed the presence of osseous tissue at the sites corresponding to the sites of radiographically demonstrated new-bone formation, whereas for the control group, no osseous tissue was seen at the implanted sites for any animal

CONCLUSION

Fibrin gel, being a biocompatible and biodegradable matrix, can encapsulate pCMV-BMP2 transfected NRK cells and adhere to the intertransverse lumbar-spine spaces of test rabbits. With the above characteristics, it plays an important role in the successful delivery of pCMV-BMP2-transfected cells for this rabbit spinal-fusion experiment. Being a natural matrix and able to be obtained readily from patients' own blood, fibrin gel seems to be a promising scaffold for future gene therapy in human trials.

Assessing the stiffness of spinal fusion in animal models

The clinical goal of spinal fusion is to reduce motion and the associated pain. Therefore, measuring motion under loading is critical. The purpose of this study was to validate four-point bending as a means to mechanically evaluate simulated fusions in dog and rabbit spines. We hypothesized that this method would be more sensitive than manual palpation and would be able to distinguish unilateral vs bilateral fusion. Spines from four mixed breed dogs and four New Zealand white rabbits were used to simulate posterolateral fusion with polymethyl methacrylate as the fusion mass. We performed manual palpation and nondestructive mechanical testing in four-point bending in four planes of motion: flexion, extension, and right and left bending. This testing protocol was used for each specimen in three fusion modes: intact, unilateral, and bilateral fusion. Under manual palpation, all intact spines were rated as not fused, and all unilateral and bilateral simulated fusions were rated as fused. In four-point bending, dog spines were significantly stiffer after unilateral fusion compared with intact in all directions. Additionally, rabbit spines were stiffer in flexion and left bending after unilateral fusion. All specimens exhibited significant differences between intact and bilateral fusion except the rabbit in extension. For unilateral vs bilateral fusion, significant differences were present for right bending in the dog model and for flexion in the rabbit. Unilateral fusion can provide enough stability to constitute a fused grade by manual palpation but may not provide structural stiffness comparable to bilateral fusion.

The effect of acute smoking on spinal fusion: an experimental study among rabbits

BACKGROUND

The establishment of an environment to force animals to inspire cigarette smoke is mandatory to study the true effects of smoking. This model has been used to study long-bone healing but has not yet been used to study spinal fusion.

METHODS

Forty male rabbits were divided into four equal groups. All the animals underwent bilateral intertransverse-process fusion at L5-L6 using the 1995 fusion model of Boden et al. The first (C8-week) group did not undergo cigarette smoke inhalation and individual rabbits were killed at 8 weeks; the second (S8-week) group underwent intermittent cigarette smoke inhalation and individual rabbits were killed at 8 weeks; the third (C6-week) group did not undergo cigarette smoke inhalation, and animals were killed at 6 weeks; and the fourth (S6-week) group underwent intermittent smoke inhalation and group-included rabbits were killed at 6 weeks. Subsequent to the animals having been killed, the fusion masses were harvested for a series of studies including manual palpation, biomechanical testing, radiographic examination, and histologic analysis.

RESULTS

Six rabbits died shortly after the operation. Of the remaining 34 rabbits, the lumbar spine specimen was harvested for study. Manual palpation, radiographic examination, and histologic analysis of the fusion masses revealed no statistically significant difference in fusion results between the control and the corresponding smoking group killed at either 8 weeks or 6 weeks. Biomechanical testing of the fusion masses also revealed no statistically significant difference in fusion results between the control and the smoking group killed at 8 weeks, although it did indicate that smoking resulted in a 44% decrease in mean flexion stiffness and a 32% decrease in mean extension stiffness among the animals killed at 6 weeks. The former (decrease in flexion stiffness) was statistically significant (p < 0.05).

CONCLUSION

The results of the biomechanical testing conducted as a part of the current study demonstrate that acute cigarette smoke inhalation delays but does not prevent the spinal fusion process for rabbits.

Effect of recombinant human bone morphogenetic protein-2 in an experimental model of spinal fusion in a radiated area

STUDY DESIGN

An animal model of posterolateral intertransverse process spine fusion was used.

OBJECTIVES

To investigate whether recombinant human bone morphogenetic protein-2 (rhBMP-2) can overcome the adverse effects of radiation treatment (RT) on spine fusion.

SUMMARY OF BACKGROUND DATA

Spinal metastases are common. Some of these patients are candidates for spinal cord decompression and vertebral reconstruction; however, radiation has significant adverse effects on bone healing.

METHODS

A posterolateral fusion model was used with rhBMP-2 or iliac crest bone graft (ICBG). Eighty one-year-old rabbits were divided into eight groups: 1) RT 14 days before surgery, rhBMP-2; 2) RT 14 days before surgery, ICBG; 3) RT 2 days after surgery, rhBMP-2; 4) RT 2 days after surgery, ICBG; 5) RT 14 days after surgery, rhBMP-2; 6) RT 14 days after surgery, ICBG; 7) no RT, rhBMP-2; 8) no RT, ICBG. Animals were killed approximately 35 days after surgery. Manual palpation was the definitive test of fusion. Biomechanical and histologic assessments were also performed.

RESULTS

All rhBMP-2 groups had significantly greater fusion rates versus respective ICBG control groups: 1 (86%) versus 2 (0%) (P = 0.005), 3 (100%) versus 4 (0%) (P < 0.0001), 5 (100%) versus 6 (0%) (P < 0.0001), and 7 (100%) versus 8 (60%) (P = 0.003). Stiffness and ultimate strength did not differ significantly between the experimental and control groups. Histologic assessment confirmed new bone formation in the fusion masses from rhBMP-2 groups.

CONCLUSIONS

Use of rhBMP-2 produced a significantly greater rate of fusion compared with ICBG in a previously radiated area in an animal model, without the morbidity of ICBG harvesting and without the risk of inadvertently using autograft contaminated by micrometastases.

Animal models for spinal fusion

Animal models for spinal fusion are essential for preclinical testing of new fusion methods and adjuncts. They allow for control of individual variables and quantification of outcome measures. Model characteristics are considered. Preclinical experiments to evaluate proof of concept, feasibility, and efficacy are generally studied in an orderly progression from smaller to larger animal models with an evolving cascade of evidence which has become known as the "burden of proof". Methods of fusion analysis include manual palpation, radiographs, computed tomography, histology, biomechanical testing, and molecular analysis. Models which have been developed in specific species are reviewed. This sets the stage for the interpretation of studies evaluating bone graft materials such as allograft, demineralized bone matrices, bone morphogenetic proteins, ceramics, and others with consideration of the variables affecting their success. As evidence accumulates, clinical trials and applications are defined.

Tricalcium phosphate and glutaraldehyde crosslinked gelatin incorporating bone morphogenetic protein--a viable scaffold for bone tissue engineering

Bone defects caused by various etiologies must be filled with suitable substances to promote bone repair. Autogenous iliac crest graft is most frequently used, but is often associated with morbidities. Several bone graft substitutes have been developed to provide osteoconductive matrices as well as to enhance osteoinductivity. A tricalcium phosphate and glutaraldehyde crosslinked gelatin (GTG) scaffold, incorporated with bone morphogenetic proteins (BMPs), was developed to provide an alternative mean of bone tissue engineering. This study investigated differences between GTG and BMP-4 immobilized GTG (GTG-BMP) scaffolds on neonatal rat calvaria osteoblast activities. The GTG scaffold possessed an average pore size of 200 microm and a porosity of 75%. HE staining revealed uniform cell distribution throughout the scaffold 24 h post cell seeding. Alkaline phosphatase (ALP) activity of the GTG samples increased initially and then stabilized at 3 weeks postseeding. ALP activity of the GTG-BMP samples was similar to that of the GTG samples in the second and third weeks, but it continued increasing and became significantly greater than that of the GTG samples by the fourth week. Gla-type osteocalcin (Gla-OC) activity of the GTG-BMP samples was initially lower, but also became significantly greater than that of the GTG samples by the fourth week. An HE stain revealed greater numbers of attached cells and a richer matrix deposits in the GTG-BMP samples. A von Kossa stain showed larger mineralizing nodules, in greater numbers, after 4 weeks of in vitro cultivation. These findings suggest that the GTG scaffold provides an excellent porous structure, conductive to greater cell attachment and osteoblast differentiation, and that utility can be significantly enhanced by the inclusion of BMPs. A GTG-BMP scaffold holds promise as a superior bioactive material for bone tissue engineering.

Experimental spinal fusion with recombinant human bone morphogenetic protein-2 delivered by a synthetic polymer and beta-tricalcium phosphate in a rabbit model

STUDY DESIGN

An experimental animal study to achieve posterolateral intertransverse process spine fusion with recombinant bone morphogenetic protein in combination with a new delivery system.

OBJECTIVE

To evaluate the efficacy of a new synthetic biodegradable bone-inducing material containing recombinant human bone morphogenetic protein-2 (rhBMP-2) as a bone-graft substitute for posterolateral intertransverse process fusion in a rabbit model.

SUMMARY OF BACKGROUND DATA

rhBMP-2, a powerful bone-inducing cytokine, has been used as a bone graft substitute in combination with animal-derived collagen to achieve spinal fusion in animal models. However, the minimum dose of rhBMP-2 required to obtain solid posterolateral intertransverse process fusion was high on the basis of previous reports (>100 microg in rabbit models). To improve the efficacy, performance of rhBMP-2, and the safety of the delivery system for this protein, a more sophisticated system is required.

METHODS

To fabricate one implant for one-side L4-L5 intertransverse process fusion, beta-tricalcium phosphate (beta-TCP) powder (300 microg), a polymer gel (PLA-DX-PEG block copolymer; 300 microg) and rhBMP-2 (7.5, 15, or 30 microg) were mixed and manually shaped to resemble a rod. Through a posterolateral approach, two implants were placed on both sides (1 per side) by surgery so as to bridge the transverse processes of adult New Zealand white rabbits (n = 27). In control animals, implants without rhBMP or autogenous cortico-cancellous bone chips from the iliaccrest were placed in a similar location. The lumbar vertebrae were recovered 6 weeks after surgery. The posterolateral fusion was examined by manual palpation, radiography, biomechanical testing, and histology.

RESULTS

Rabbits that received 15 or 30 microg of rhBMP-2 showed consistent fusion. However, solid fusion was seen in 2 of 5 rabbits with autografting and rabbits that received 7.5 microg of rhBMP-2. Fusion was not observed in the rabbits that did not receive rhBMP-2.

CONCLUSIONS

Consistent spinal fusion was obtained by implanting a biodegradable bone-inducing implant composed of beta-TCP, PLA-DX-PEG, and rhBMP-2 within a period of 6 weeks. The rhBMP-2 doses required for the spinal fusion were significantly lower than those reported previously.

The use of cultured bone marrow cells in type I collagen gel and porous hydroxyapatite for posterolateral lumbar spine fusion

STUDY DESIGN

Posterolateral lumbar transverse process fusion was completed using the cultured bone marrow cells in type I collagen gel and porous hydroxyapatite.

OBJECTIVE

To compare the efficacy of cultured bone marrow cells with that of bone morphogenetic protein (BMP) as a graft alternative to autologous bone for posterolateral spine fusion.

SUMMARY OF BACKGROUND DATA

The clinical application of BMP for spinal fusion may be limited by high dose and cost. Recently, mesenchymal stem cells have been studied in various fields because of their capability to differentiate into various cells, including those in the osteogenic lineage.

METHODS

Thirty adult rabbits were used. Each underwent single-level, bilateral, posterolateral intertransverse process fusions at L4-L5. The animals were divided into 4 groups, each according to the material implanted: (1) autologous bone (autograft, n = 9); (2) porous hydroxyapatite (HA) particles and type I collagen sheet with 100 microg rhBMP-2 (BMP-HA, n = 7); (3) bone marrow cells (1 x 10(6) cells/mL, low-marrow-HA, n = 7); and (4) bone marrow cells (1 x 10(8) cells/mL, high-marrow-HA, n = 7). Before implantation for groups 3 and 4, fresh bone marrow cells from the iliac crest of each animal were cultured in a standard medium for 2 weeks. For one additional week, the marrow cells were cultured in 10(-8) M dexamethasone, type I collagen gel, and HA. Animals were euthanized 6 weeks after surgery. Spinal fusions were evaluated by radiograph, manual palpation, and histology.

RESULTS

The fusion rates were 4 of 7 in the autograft group, 7 of 7 in the BMP-HA group, 0 of 7 in the low-marrow-HA group, and 5 of 7 in the high-marrow-HA group. The histology in the BMP-HA and high-marrow-HA groups showed that grafted HA fragments were connected with mature new bone. The pores of HA fragments were filled up with bone matrix. In the low-marrow-HA group, fibrous tissue was predominant in the grafted fragments.

CONCLUSIONS

This study shows that the cultured bone marrow cells can act as a substitute for autograft or BMP in spine fusion. The current formulation may yield improved fusion success and better quality of fusion bone as compared to autograft.