rhBMP-6 stimulated osteoprogenitor cells enhance posterolateral spinal fusion in the New Zealand white rabbit

Long-term posterolateral spinal fusion in rabbits induced by rhBMP6 applied in autologous blood coagulum with synthetic ceramics

Autologous bone graft substitute (ABGS) containing rhBMP6 in autologous blood coagulum (Osteogrow) is a novel therapeutic solution for bone regeneration. This study is aimed to investigate the long-term outcome of ABGS with synthetic ceramics (Osteogrow-C) in rabbit posterolateral spinal fusion (PLF) model. Osteogrow-C implants were implanted bilaterally between rabbit lumbar transverse processes. We compared the outcome following implantation of ABGS with ceramic particles of different chemical composition (TCP and biphasic ceramics containing both TCP and HA) and size (500–1700 µm and 74–420 µm). Outcome was analyzed after 14 and 27 weeks by microCT, histology, and biomechanical analyses. Successful bilateral spinal fusion was observed in all animals at the end of observation period. Chemical composition of ceramic particles has impact on the PLF outcome via resorption of TCP ceramics, while ceramics containing HA were only partially resorbed. Moreover, persistence of ceramic particles subsequently resulted with an increased bone volume in implants with small particles containing high proportion of HA. ABGS (rhBMP6/ABC) with various synthetic ceramic particles promoted spinal fusion in rabbits. This is the first presentation of BMP-mediated ectopic bone formation in rabbit PLF model with radiological, histological, and biomechanical features over a time course of up to 27 weeks.

Bone Morphogenetic Proteins, Carriers, and Animal Models in the Development of Novel Bone Regenerative Therapies

Bone morphogenetic proteins (BMPs) possess a unique ability to induce new bone formation. Numerous preclinical studies have been conducted to develop novel, BMP-based osteoinductive devices for the management of segmental bone defects and posterolateral spinal fusion (PLF). In these studies, BMPs were combined with a broad range of carriers (natural and synthetic polymers, inorganic materials, and their combinations) and tested in various models in mice, rats, rabbits, dogs, sheep, and non-human primates. In this review, we summarized bone regeneration strategies and animal models used for the initial, intermediate, and advanced evaluation of promising therapeutical solutions for new bone formation and repair. Moreover, in this review, we discuss basic aspects to be considered when planning animal experiments, including anatomical characteristics of the species used, appropriate BMP dosing, duration of the observation period, and sample size.

Stem Cells and Spinal Fusion

BACKGROUND

This manuscript is a review of the literature investigating the use of mesenchymal stem cells (MSCs) being applied in the setting of spinal fusion surgery. We mention the rates of pseudarthrosis, discuss current bone grafting options, and examine the preclinical and clinical outcomes of utilizing MSCs to assist in successfully fusing the spine.

METHODS

A thorough literature review was conducted to look at current and previous preclinical and clinical studies using stem cells for spinal fusion augmentation. Searches for PubMed/MEDLINE and ClinicalTrials.gov through January 2021 were conducted for literature mentioning stem cells and spinal fusion.

RESULTS

All preclinical and clinical studies investigating MSC use in spinal fusion were examined. We found 19 preclinical and 17 clinical studies. The majority of studies, both preclinical and clinical, were heterogeneous in design due to different osteoconductive scaffolds, cells, and techniques used. Preclinical studies showed promising outcomes in animal models when using appropriate osteoconductive scaffolds and factors for osteogenic differentiation. Similarly, clinical studies have promising outcomes but differ in their methodologies, surgical techniques, and materials used, making it difficult to adequately compare between the studies.

CONCLUSION

MSCs may be a promising option to use to augment grafting for spinal fusion surgery. MSCs must be used with appropriate osteoconductive scaffolds. Cell-based allografts and the optimization of their use have yet to be fully elucidated. Further studies are necessary to determine the efficacy of MSCs with different osteoconductive scaffolds and growth/osteogenic differentiation factors.

LEVEL OF EVIDENCE

3.

Growth Factors, Carrier Materials, and Bone Repair

This chapter provides an overview of the growth factors active in bone regeneration and healing. Both normal and impaired bone healing are discussed, with a focus on the spatiotemporal activity of the various growth factors known to be involved in the healing response. The review highlights the activities of most important growth factors impacting bone regeneration, with a particular emphasis on those being pursued for clinical translation or which have already been marketed as components of bone regenerative materials. Current approaches the use of bone grafts in clinical settings of bone repair (including bone grafts) are summarized, and carrier systems (scaffolds) for bone tissue engineering via localized growth factor delivery are reviewed. The chapter concludes with a consideration of how bone repair might be improved in the future.

Supramolecular self-assembling peptides to deliver bone morphogenetic proteins for skeletal regeneration

Recombinant human bone morphogenetic proteins (BMPs) have shown clinical success in promoting bone healing, but they are also associated with unwanted side effects. The development of improved BMP carriers that can retain BMP at the defect site and maximize its efficacy would decrease the therapeutic BMP dose and thus improve its safety profile. In this review, we discuss the advantages of using self-assembling peptides, a class of synthetic supramolecular biomaterials, to deliver recombinant BMPs. Peptide amphiphiles (PAs) are a broad class of self-assembling peptides, and the use of PAs for BMP delivery and bone regeneration has been explored extensively over the past decade. Like many self-assembling peptide systems, PAs can be designed to form nanofibrous supramolecular biomaterials in which molecules are held together by non-covalent bonds. Chemical and biological functionality can be added to PA nanofibers, through conjugation of chemical moieties or biological epitopes to PA molecules. For example, PA nanofibers have been designed to bind heparan sulfate, a natural polysaccharide that is known to bind BMPs and potentiate their signal. Alternatively, PA nanofibers have been designed to synthetically mimic the structure and function of heparan sulfate, or to directly bind BMP specifically. In small animal models, these bio-inspired PA materials have shown the capacity to promote bone regeneration using BMP at doses 10-100 times lower than established therapeutic doses. These promising results have motivated further evaluation of PAs in large animal models, where their safety and efficacy must be established before clinical translation. We conclude with a discussion on the possiblity of combining PAs with other materials used in orthopaedic surgery to maximize their utility for clinical translation.

Evaluation of synthetic ceramics as compression resistant matrix to promote osteogenesis of autologous blood coagulum containing recombinant human bone morphogenetic protein 6 in rabbit posterolateral lumbar fusion model

Posterolateral lumbar fusion (PLF) is a commonly performed surgical procedure for the treatment of pathological conditions of the lumbosacral spine. In the present study, we evaluated an autologous bone graft substitute (ABGS) containing rhBMP6 in autologous blood coagulum (ABC) and synthetic ceramics used as compression resistant matrix (CRM) in the rabbit PLF model. In the pilot PLF rabbit experiment, we tested four different CRMs (BCP 500-1700 μm, BCP 1700-2500 μm and two different TCP in the form of slabs) which were selected based on achieving uniform ABC distribution. Next, ABGS implants composed of 2.5 mL ABC with 0.5 g ceramic particles (TCP or BCP (TCP/HA 80/20) of particle size 500-1700 μm) and 125 μg rhBMP6 (added to blood or lyophilized on ceramics) were placed bilaterally between transverse processes of the lumbar vertebrae (L5-L6) following exposition and decortication in 12 New Zealand White Rabbits observed for 7 weeks following surgery. Spinal fusion outcome was analysed by μCT, palpatory segmental mobility testing and selected specimens were either tested biomechanically (three-point bending test) and/or processed histologically. The total fusion success rate was 90.9% by both μCT analyses and by palpatory segmental mobility testing. The volume of newly formed bone between experimental groups with TCP or BCP ceramics and the different method of rhBMP6 application was comparable. The newly formed bone and ceramic particles integrated with the transverse processes on histological sections resulting in superior biomechanical properties. The results were retrospectively found superior to allograft devitalized mineralized bone as a CRM as reported previously in rabbit PLF. Overall, this novel ABGS containing rhBMP6, ABC and the specific 500-1700 μm synthetic ceramic particles supported new bone formation for the first time and successfully promoted posterolateral lumbar fusion in rabbits.

Stem Cells in Spinal Fusion

STUDY DESIGN

Review of literature.

OBJECTIVES

This review of literature investigates the application of mesenchymal stem cells (MSCs) in spinal fusion, highlights potential uses in the development of bone grafts, and discusses limitations based on both preclinical and clinical models.

METHODS

A review of literature was conducted looking at current studies using stem cells for augmentation of spinal fusion in both animal and human models.

RESULTS

Eleven preclinical studies were found that used various animal models. Average fusion rates across studies were 59.8% for autograft and 73.7% for stem cell-based grafts. Outcomes included manual palpation and stressing of the fusion, radiography, micro-computed tomography (μCT), and histological analysis. Fifteen clinical studies, 7 prospective and 8 retrospective, were found. Fusion rates ranged from 60% to 100%, averaging 87.1% in experimental groups and 87.2% in autograft control groups.

CONCLUSIONS

It appears that there is minimal clinical difference between commercially available stem cells and bone marrow aspirates indicating that MSCs may be a good choice in a patient with poor marrow quality. Overcoming morbidity and limitations of autograft for spinal fusion, remains a significant problem for spinal surgeons and further studies are needed to determine the efficacy of stem cells in augmenting spinal fusion.

Stem cells in preclinical spine studies

BACKGROUND CONTEXT

The recent identification and characterization of mesenchymal stem cells have introduced a shift in the research focus for future technologies in spinal surgery to achieve spinal fusion and treat degenerative disc disease. Current and past techniques use allograft to replace diseased tissue or rely on host responses to recruit necessary cellular progenitors. Adult stem cells display long-term proliferation, efficient self-renewal, and multipotent differentiation.

PURPOSE

This review will focus on two important applications of stem cells in spinal surgery: spine fusion and the management of degenerative disc disease.

STUDY DESIGN

Review of the literature.

METHODS

Relevant preclinical literature regarding stem cell sources, growth factors, scaffolds, and animal models for both osteogenesis and chondrogenesis will be reviewed, with an emphasis on those studies that focus on spine applications of these technologies.

RESULTS

In both osteogenesis and chondrogenesis, adult stem cells derived from bone marrow or adipose show promise in preclinical studies. Various growth factors and scaffolds have also been shown to enhance the properties and eventual clinical potential of these cells. Although its utility in clinical applications has yet to be proven, gene therapy has also been shown to hold promise in preclinical studies.

CONCLUSIONS

The future of spine surgery is constantly evolving, and the recent advancements in stem cell-based technologies for both spine fusion and the treatment of degenerative disc disease is promising and indicative that stem cells will undoubtedly play a major role clinically. It is likely that these stem cells, growth factors, and scaffolds will play a critical role in the future for replacing diseased tissue in disease processes such as degenerative disc disease and in enhancing host tissue to achieve more reliable 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.

Tissue engineering strategies in spinal arthrodesis: the clinical imperative and challenges to clinical translation

Skeletal disorders requiring the regeneration or de novo production of bone present considerable reconstructive challenges and are one of the main driving forces for the development of skeletal tissue engineering strategies. The skeletal or mesenchymal stem cell is a fundamental requirement for osteogenesis and plays a pivotal role in the design and application of these strategies. Research activity has focused on incorporating the biological role of the mesenchymal stem cell with the developing fields of material science and gene therapy in order to create a construct that is not only capable of inducing host osteoblasts to produce bone, but is also osteogenic in its own right. This review explores the clinical need for reparative approaches in spinal arthrodesis, identifying recent tissue engineering strategies employed to promote spinal fusion, and considers the ongoing challenges to successful clinical translation.

BMP signaling in mesenchymal stem cell differentiation and bone formation

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and have diverse functions during development and organogenesis. BMPs play a major role in skeletal development and bone formation, and disruptions in BMP signaling cause a variety of skeletal and extraskeletal anomalies. Several knockout models have provided insight into the mechanisms responsible for these phenotypes. Proper bone formation requires the differentiation of osteoblasts from mesenchymal stem cell (MSC) precursors, a process mediated in part by BMP signaling. Multiple BMPs, including BMP2, BMP6, BMP7 and BMP9, promote osteoblastic differentiation of MSCs both in vitro and in vivo. BMP9 is one of the most osteogenic BMPs yet is a poorly characterized member of the BMP family. Several studies demonstrate that the mechanisms controlling BMP9-mediated osteogenesis differ from other osteogenic BMPs, but little is known about these specific mechanisms. Several pathways critical to BMP9-mediated osteogenesis are also important in the differentiation of other cell lineages, including adipocytes and chondrocytes. BMP9 has also demonstrated translational promise in spinal fusion and bone fracture repair. This review will summarize our current knowledge of BMP-mediated osteogenesis, with a focus on BMP9, by presenting recently completed work which may help us to further elucidate these pathways.

Application of resorbable poly(lactide-co-glycolide) with entangled hyaluronic acid as an autograft extender for posterolateral intertransverse lumbar fusion in rabbits

Facilitating fusion between bony segments in a reliable and reproducible manner using a synthetic bone graft material has a number of benefits for the surgeon as well as the patient. Although autograft remains the gold standard, associated comorbidities continue to drive the development of new biomaterials for use in spinal fusion. The ability of autograft alone and autograft combined with a radiolucent biomaterial composed of resorbable osteoconductive poly(lactide-co-glycolide) with entangled hyaluronic acid to facilitate fusion was examined in a single-level noninstrumented posterolateral intertransverse lumbar fusion model in New Zealand White rabbits. Progressive bone formation was demonstrated radiographically for the extender group (synthetic biomaterial plus autograft) between 3 and 6 months. Computed tomography revealed a new cortical shell in the fusion mass at 3 and 6 months for both study groups. Tensile testing at 6 months demonstrated that the quality of bone formed between the intertransverse space was equivalent for both study groups. Histologic evaluation of the fusion mass revealed new bone on and adjacent to the transverse processes with the synthetic biomaterial group that extended laterally, supporting the osteoconductive nature of the material. Histological evidence of endochondral bone growth in the intertransverse space was observed for the autograft plus synthetic biomaterial group. Bone remodeling, new marrow spaces, and peripheral cortices were observed for each study group at 3 months that matured by 6 months. These findings support the use of a radiolucent biosynthetic material comprising poly(lactide-co-glycolide) with integrated hyaluronic acid as an autograft extender for lumbar intertransverse fusion.

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.