Bone morphogenetic proteins and tissue engineering: future directions

Comparative analysis of clinical outcome and quality of life between amputations and combined bone and flap reconstructions at the lower leg

PURPOSE

At the lower leg, soft tissue defects with exposed bones, tendons, or hardware require flap coverage. In this retrospective study, we analyzed combined bone and soft tissue reconstructions compared to amputations of the lower leg in a civilian setting.

MATERIALS AND METHODS

Patients who underwent combined bone and flap reconstruction (LR) or amputation (LA) of the lower leg were eligible for the study. Bone conditions included fractures and bony defects due to posttraumatic osteomyelitis and non-union. Besides the analysis of the medical history, the study included clinical examination including extremity functional scale (LEFS) and SF-36-questionnaire.

RESULTS

LEFS score was significantly higher in the LR group compared to the LA group. Importantly, 42% in the LR group as opposed to 80% in the LA group could not return to their occupation. Mean hospitalization was 119 in the LR and 49 days in the LA group. SF-36 body item scores were significantly higher in the LR group as compared to LA.

CONCLUSIONS

Patients undergoing complex extremity reconstructions, including flap transfer to the lower leg have better functionality and higher quality of life than amputated patients. These data emphasize the advantages of these procedures and justify reconstructive efforts for limb salvage. Level of Evidence III.Implications for RehabilitationAmputation and combined bone and flap reconstruction in severe injuries of the lower leg can imply functional disabilities even after successful treatment.Albeit longer hospitalizations, patients with complex reconstructions showed better functional outcomes and had a higher quality of life.Limb salvage showed better functional outcomes and a higher rate in reintegration to work as compared to limb amputation.These data emphasize the importance of complex bone and soft tissue reconstruction in this patient cohort.

Effect of Bone Morphogenetic Protein-2 in the Treatment of Long Bone Non-Unions

Background: Delayed fracture healing continues to cause significant patient morbidity and an economic burden to society. Biological stimulation of non-unions includes application of recombinant bone morphogenetic protein-2 (rhBMP-2). However, rhBMP-2 use continues to be a matter of controversy as literature shows scarce evidence for treatment effectiveness. Questions: The objective of this study was to evaluate the effectiveness of rhBMP-2 treatment on long bone non-unions measuring union rate and time to union. Furthermore, we assess risk factors for treatment failure. Methods and patients: A total of 91 patients with non-unions of long bones were treated with rhBMP-2 (n = 72) or standard care without BMP (n = 19) at our institution. Patient characteristics, comorbidities, nicotine consumption, and complications were recorded. Bone healing was assessed by plane X-rays and clinical examination. Patients were followed up with for 24 months. Results: Overall, there was significantly faster bone healing after rhBMP-2 application compared to the no-BMP group (p < 0.001; HR = 2.78; 95% CI 1.4–5.6). Union rates differed significantly between rhBMP-2 compared to the no-BMP group (89% vs. 47%; p < 0.001). At the humerus, there was neither a significantly higher union rate in the rhBMP-2 (83%) compared to the no-BMP group (50%) (p = 0.26; n = 12) nor a faster bone healing with a median time of 9 months in both groups (HR = 2.01; 95% CI 0.49–8.61; p = 0.315). The 33 femora treated using rhBMP-2 healed significantly faster than 9 femora in the no-BMP group (HR = 2.93; 95% CI 1.00–8.4; p = 0.023) with significant differences in union rate with 85% and 44%, respectively (p = 0.022). Regarding tibia non-unions, 25 out of 27 (93%) healed with a median of 9 months after rhBMP-2 application with no significant difference in the no-BMP group (33%) in time to union (p = 0.097) but a significantly higher union rate (p = 0.039). There was no effect of comorbidities, age, sex, soft tissue damage, or nicotine use on time to union, union rate, or secondary interventions. Conclusion: Consistent with the literature, overall, significantly higher union rates with reduced time to union were achieved after rhBMP-2 application. Femoral and tibial non-unions in particular seem to profit from rhBMP-2 application.

Three-dimensional bioactive hydrogel-based scaffolds for bone regeneration in implant dentistry

Bone tissue requires a range of complex mechanisms to allow the restoration of its structure and function. Bone healing is a signaling cascade process, involving cells secreting cytokines, growth factors, and pro-inflammatory factors in the defect site that will, subsequently, recruit surrounding stem cells to migrate, proliferate, and differentiate into bone-forming cells. Bioactive functional scaffolds could be applied to improve the bone healing processes where the organism is not able to fully regenerate the lost tissue. However, to be optimal, such scaffolds should act as osteoconductors - supporting bone-forming cells, providing nutrients, and sustaining the arrival of new blood vessels, and act as osteoinducers - slowly releasing signaling molecules that stimulate mesenchymal stem cells to differentiate and deposit mineralized bone matrix. Different compositions and shapes of scaffolds, cutting-edge technologies, application of signaling molecules to promote cell differentiation, and high-quality biomaterials are reaching favorable outcomes towards osteoblastic differentiation of stem cells in in vitro and in vivo researches for bone regeneration. Hydrogel-based biomaterials are being pointed as promising for bone tissue regeneration; however, despite all the research and high-impact scientific publications, there are still several challenges that prevent the use of hydrogel-based scaffolds for bone regeneration being feasible for their clinical application. Hence, the objective of this review is to consolidate and report, based on the current scientific literature, the approaches for bone tissue regeneration using bioactive hydrogel-based scaffolds, cell-based therapies, and three-dimensional bioprinting to define the key challenges preventing their use in clinical applications.

The Effects of Stably Tethered BMP-2 on MC3T3-E1 Preosteoblasts Encapsulated in a PEG Hydrogel

Bone morphogenetic protein-2 (BMP-2) is a clinically used osteoinductive growth factor. With a short half-life and side effects, alternative delivery approaches are needed. This work examines thiolation of BMP-2 for chemical attachment to a poly(ethylene glycol) hydrogel using thiol-norbornene click chemistry. BMP-2 retained bioactivity post-thiolation and was successfully tethered into the hydrogel. To assess tethered BMP-2 on osteogenesis, MC3T3-E1 preosteoblasts were encapsulated in matrix metalloproteinase (MMP)-sensitive hydrogels containing RGD and either no BMP-2, soluble BMP-2 (5 nM), or tethered BMP-2 (40-200 nM) and cultured in a chemically defined medium containing dexamethasone for 7 days. The hydrogel culture supported MC3T3-E1 osteogenesis regardless of BMP-2 presentation, but tethered BMP-2 augmented the osteogenic response, leading to significant increases in osteomarkers, Bglap and Ibsp. The ratio, Ibsp-to-Dmp1, highlighted differences in the extent of differentiation, revealing that without BMP-2, MC3T3-E1 cells showed a higher expression of Dmp1 (low ratio), but an equivalent expression with tethered BMP-2 and more abundant bone sialoprotein. In addition, this work identified that dexamethasone contributed to Ibsp expression but not Bglap or Dmp1 and confirmed that tethered BMP-2 induced the BMP canonical signaling pathway. This work presents an effective method for the modification and incorporation of BMP-2 into hydrogels to enhance osteogenesis.

Functionalized Scaffold and Barrier Membrane with Anti-BMP-2 Monoclonal Antibodies for Alveolar Ridge Preservation in a Canine Model

Introduction

The aim of this study was to investigate the ability of anti-bone morphogenetic protein 2 monoclonal antibody (anti-BMP-2 mAb) to functionalize scaffolds to mediate bone regeneration in a canine model.

Materials and Methods

The mandibular right premolar 4 (PM4) was extracted in eight beagle dogs and grafted with anti-BMP-2 mAb+anorganic bovine bone mineral with 10% collagen (ABBM-C) and porcine bilayer native collagen membrane (CM). The ABBM-C and CM were functionalized with either anti-BMP-2 mAb (test group) or an isotype matched control mAb (control group). Animals were euthanized at 12 weeks for radiographic, histologic, and histomorphometric analyses. Outcomes were compared between groups.

Results

3D imaging using cone beam computed tomography (CBCT) revealed that sites treated with ABBM-C and CM functionalized with anti-BMP-2 mAb exhibited significantly more remaining bone width near the alveolar crest, as well as buccal bone height, compared with control groups. Histologic and histomorphometric analyses demonstrated that in anti-BMP-2 mAb-treated sites, total tissue volume was significantly higher in the coronal part of the alveolar bone crest compared with control sites. In anti-BMP-2 mAb-treated sites, bone formation was observed under the barrier membrane.

Conclusion

Functionalization of the ABBM-C scaffold and CM appeared to have led to bone formation within healing alveolar bone sockets.

Design of Bio-Conjugated Hydrogels for Regenerative Medicine Applications: From Polymer Scaffold to Biomolecule Choice

Bio-conjugated hydrogels merge the functionality of a synthetic network with the activity of a biomolecule, becoming thus an interesting class of materials for a variety of biomedical applications. This combination allows the fine tuning of their functionality and activity, whilst retaining biocompatibility, responsivity and displaying tunable chemical and mechanical properties. A complex scenario of molecular factors and conditions have to be taken into account to ensure the correct functionality of the bio-hydrogel as a scaffold or a delivery system, including the polymer backbone and biomolecule choice, polymerization conditions, architecture and biocompatibility. In this review, we present these key factors and conditions that have to match together to ensure the correct functionality of the bio-conjugated hydrogel. We then present recent examples of bio-conjugated hydrogel systems paving the way for regenerative medicine applications.

Growth Factor Engineering Strategies for Regenerative Medicine Applications

Growth factors are critical molecules for tissue repair and regeneration. Therefore, recombinant growth factors have raised a lot of hope for regenerative medicine applications. While using growth factors to promote tissue healing has widely shown promising results in pre-clinical settings, their success in the clinic is not a forgone conclusion. Indeed, translation of growth factors is often limited by their short half-life, rapid diffusion from the delivery site, and low cost-effectiveness. Trying to circumvent those limitations by the use of supraphysiological doses has led to serious side-effects in many cases and therefore innovative technologies are required to improve growth factor-based regenerative strategies. In this review, we present protein engineering approaches seeking to improve growth factor delivery and efficacy while reducing doses and side effects. We focus on engineering strategies seeking to improve affinity of growth factors for biomaterials or the endogenous extracellular matrix. Then, we discuss some examples of increasing growth factor stability and bioactivity, and propose new lines of research that the field of growth factor engineering for regenerative medicine may adopt in the future.

Tissue Engineering; Current Status & Futuristic Scope

Almost 30 years have passed since the term ‘tissue engineering’ was created to represent a new concept that focuses on the regeneration of neotissues from cells with the support of biomaterials and growth factors. This interdisciplinary engineering has attracted much attention as a new therapeutic means that may overcome the drawbacks involved in the current artificial organs and organ transplantation that have also been aiming at replacing lost or severely damaged tissues or organs. However, the tissues regenerated by tissue engineering and widely applied to patients are still minimal, including skin, bone, cartilage, capillary, and periodontal tissues. What are the reasons for such slow advances in clinical applications of tissue engineering? This article gives a brief overview of the current state of tissue engineering, covering the fundamentals and applications. The fundamentals of tissue engineering involve cell sources, scaffolds for cell expansion and differentiation, as well as carriers for growth factors. Animal and human trials are a major part of the applications. Based on these results, some critical problems to be resolved for the advances of tissue engineering are addressed from the engineering point of view, emphasizing the close collaboration between medical doctors and biomaterials scientists.

Spatiotemporal Control Strategies for Bone Formation through Tissue Engineering and Regenerative Medicine Approaches

Global increases in life expectancy drive increasing demands for bone regeneration. The gold standard for surgical bone repair is autografting, which enjoys excellent clinical outcomes; however, it possesses significant drawbacks including donor site morbidity and limited availability. Although collagen sponges delivered with bone morphogenetic protein, type 2 (BMP2) are a common alternative or supplement, they do not efficiently retain BMP2, necessitating extremely high doses to elicit bone formation. Hence, reports of BMP2 complications are rising, including cancer promotion and ectopic bone formation, the latter inducing complications such as breathing difficulties and neurologic impairments. Thus, efforts to exert spatial control over bone formation are increasing. Several tissue engineering approaches have demonstrated the potential for targeted and controlled bone formation. These approaches include biomaterial scaffolds derived from synthetic sources, e.g., calcium phosphates or polymers; natural sources, e.g., bone or seashell; and immobilized biofactors, e.g., BMP2. Although BMP2 is the only protein clinically approved for use in a surgical device, there are several proteins, small molecules, and growth factors that show promise in tissue engineering applications. This review profiles the tissue engineering advances in achieving control over the location and onset of bone formation (spatiotemporal control) toward avoiding the complications associated with BMP2.

Non-unions

Non-union of long bones is a significant consequence of fracture treatment. Bone regeneration is a complex physiological process of bone formation which can be seen during normal fracture healing. An improved understanding of the molecular and cellular events that occur during bone repair and remodelling has led to the development of biologic agents that can augment the biological microenvironment and enhance bone repair. Currently, there are different strategies to augment the impaired or "insufficient" bone-regeneration process, including the "gold standard" autologous bone graft, free fibula vascularised graft, allograft implantation, and use of growth factors, osteoconductive scaffolds, osteoprogenitor cells and distraction osteogenesis. A lack of standardized outcome measures for comparison of biologic agents in clinical fracture repair trials, frequent off-label use and a limited understanding of the biological activity of these agents at the bone repair site have limited their efficacy in clinical applications.

Stem cell-derived exosomes: A promising strategy for fracture healing

OBJECTIVES

To describe the biological characteristics of exosomes and to summarize the current status of stem cell-derived exosomes on fracture healing. Meanwhile, future challenges, limitations and perspectives are also discussed.

METHODS

Search and analyze the related articles in pubmed database through the multi-combination of keywords like "stem cells","exosomes","bone regeneration" and "fracture healing".

CONCLUSION

Stem cell-derived exosome therapy for fracture healing has been enjoying popularity and is drawing increasing attention. This strategy helps to promote proliferation and migration of cells, as well as osteogenesis and angiogenesis, in the process of bone formation. Although the exact mechanisms remain elusive, exosomal miRNAs seem to play vital roles. Future studies are required to solve multiple problems before clinical application, including comprehensive and thorough understanding of exosomes, the exact roles of exosomes in regulating bone formation, and the optimal source, dose and frequency of treatment, as well as technical and safety issues. Moreover, studies based on fracture models of large animals are could offer guidance and are in demand.

Repair of critical sized cranial defects with BMP9-transduced calvarial cells delivered in a thermoresponsive scaffold

Large skeletal defects caused by trauma, congenital malformations, and post-oncologic resections of the calvarium present major challenges to the reconstructive surgeon. We previously identified BMP-9 as the most osteogenic BMP in vitro and in vivo. Here we sought to investigate the bone regenerative capacity of murine-derived calvarial mesenchymal progenitor cells (iCALs) transduced by BMP-9 in the context of healing critical-sized calvarial defects. To accomplish this, the transduced cells were delivered to the defect site within a thermoresponsive biodegradable scaffold consisting of poly(polyethylene glycol citrate-co-N-isopropylacrylamide mixed with gelatin (PPCN-g). A total of three treatment arms were evaluated: PPCN-g alone, PPCN-g seeded with iCALs expressing GFP, and PPCN-g seeded with iCALs expressing BMP-9. Defects treated only with PPCN-g scaffold did not statistically change in size when evaluated at eight weeks postoperatively (p = 0.72). Conversely, both animal groups treated with iCALs showed significant reductions in defect size after 12 weeks of follow-up (BMP9-treated: p = 0.0025; GFP-treated: p = 0.0042). However, H&E and trichrome staining revealed more complete osseointegration and mature bone formation only in the BMP9-treated group. These results suggest that BMP9-transduced iCALs seeded in a PPCN-g thermoresponsive scaffold is capable of inducing bone formation in vivo and is an effective means of creating tissue engineered bone for critical sized defects.

Fabrication of a three-dimensional β-tricalcium-phosphate/gelatin containing chitosan-based nanoparticles for sustained release of bone morphogenetic protein-2: Implication for bone tissue engineering

Fabrication of an ideal scaffold having proper composition, physical structure and able to have sustained release of growth factors still is challenging for bone tissue engineering. Current study aimed to design an appropriate three-dimensional (3-D) scaffold with suitable physical characteristics, including proper compressive strength, degradation rate, porosity, and able to sustained release of bone morphogenetic protein-2 (BMP2), for bone tissue engineering. A highly porous 3-D β-tricalcium phosphate (β-TCP) scaffolds, inside of which two perpendicular canals were created, was fabricated using foam-casting technique. Then, scaffolds were coated with gelatin layer. Next, BMP2-loaded chitosan (CS) nanoparticles were dispersed into collagen hydrogel and filled into the scaffold canals. Physical characteristics of fabricated constructs were evaluated. Moreover, the capability of given construct for bone regeneration has been evaluated in vitro in interaction with human buccal fat pad-derived stem cells (hBFPSCs). The results showed that gelatin-coated TCP scaffold with rhBMP2 delivery system not only could act as a mechanically and biologically compatible framework, but also act as an osteoinductive graft by sustained delivering of rhBMP2 in a therapeutic window for differentiation of hBFPSCs towards the osteoblast lineage. The proposed scaffold model can be suggested for delivering of cells and other growth factors such as vascular endothelial growth factor (VEGF), alone or in combination, for future investigations.

Strategies for delivering bone morphogenetic protein for bone healing

Bone morphogenetic proteins (BMPs) are the most significant growth factors that belong to the Transforming Growth Factor Beta (TGF-β) super-family. Though more than twenty members of this family have been identified so far in humans, Food and Drug Administration (FDA) approved two growth factors: BMP-2 and BMP-7 for treatments of spinal fusion and long-bone fractures with collagen carriers. Currently BMPs are clinically used in spinal fusion, oral and maxillofacial surgery and also in the repair of long bone defects. The efficiency of BMPs depends a lot on the selection of suitable carriers. At present, different types of carrier materials are used: natural and synthetic polymers, calcium phosphate and ceramic-polymer composite materials. Number of research articles has been published on the minute intricacies of the loading process and release kinetics of BMPs. Despite the significant evidence of its potential for bone healing demonstrated in animal models, future clinical investigations are needed to define dose, scaffold and route of administration. The efficacy and application of BMPs in various levels with a proper carrier and dose is yet to be established. The present article collates various aspects of success and limitation and identifies the prospects and challenges associated with the use of BMPs in orthopaedic surgery.

Non-mulberry silk fibroin grafted poly (Є-caprolactone)/nano hydroxyapatite nanofibrous scaffold for dual growth factor delivery to promote bone regeneration

HYPOTHESIS

This study aims at developing biodegradable, mineralized, nanofibrous scaffolds for use in bone regeneration. Scaffolds are loaded with combinations of bone morphogenic protein-2 (rhBMP-2) and transforming growth factor beta (TGF-β) and evaluated in vitro for enhancement in osteoinductivity.

EXPERIMENTS

Poly(Є-caprolactone) (PCL) doped with different portions of nano-hydroxyapatite is electrospun into nanofibrous scaffolds. Non-mulberry silk fibroin (NSF) obtained from Antheraea mylitta is grafted by aminolysis onto them. Scaffolds prepared have three concentrations of nano-hydroxyapatite: 0% (NSF-PCL), 25% (NSF-PCL/n25), and 50% (NSF-PCL/n50). Growth factor loading is carried out in three different combinations, solely rhBMP-2 (BN25), solely TGF-β (TN25) and rhBMP-2+TGF-β (T/B N25) via carbodiimide coupling.

FINDINGS

NSF-PCL/n25 showed the best results in examination of mechanical properties, bioactivity, and cell viability. Hence only NSF-PCL/n25 is selected for loading growth factors and subsequent detailed in vitro experiments using MG-63 cell-line. Both growth factors show sustain release kinetics from the matrix. The T/B N25 scaffolds support cellular activity, proliferation, and triggering of bone-associated genes' expression better and promote earlier cell differentiation. Dual growth factor loaded NSF grafted electrospun PCL/nHAp scaffolds show promise for further development into a suitable scaffold for bone tissue engineering.

Adenoviral Mediated Expression of BMP2 by Bone Marrow Stromal Cells Cultured in 3D Copolymer Scaffolds Enhances Bone Formation

Selection of appropriate osteoinductive growth factors, suitable delivery method and proper supportive scaffold are critical for a successful outcome in bone tissue engineering using bone marrow stromal cells (BMSC). This study examined the molecular and functional effect of a combination of adenoviral mediated expression of bone morphogenetic protein-2 (BMP2) in BMSC and recently developed and characterized, biodegradable Poly(L-lactide-co-є-caprolactone){poly(LLA-co-CL)}scaffolds in osteogenic molecular changes and ectopic bone formation by using in vitro and in vivo approaches. Pathway-focused custom PCR array, validation using TaqMan based quantitative RT-PCR (qRT-PCR) and ALP staining showed significant up-regulation of several osteogenic and angiogenic molecules, including ALPL and RUNX2 in ad-BMP2 BMSC group grown in poly(LLA-co-CL) scaffolds both at 3 and 14 days. Micro CT and histological analyses of the subcutaneously implanted scaffolds in NOD/SCID mice revealed significantly increased radiopaque areas, percentage bone volume and formation of vital bone in ad-BMP2 scaffolds as compared to the control groups both at 2 and 8 weeks. The increased bone formation in the ad-BMP2 group in vivo was paralleled at the molecular level with concomitant over-expression of a number of osteogenic and angiogenic genes including ALPL, RUNX2, SPP1, ANGPT1. The increased bone formation in ad-BMP2 explants was not found to be associated with enhanced endochondral activity as evidenced by qRT-PCR (SOX9 and FGF2) and Safranin O staining. Taken together, combination of adenoviral mediated BMP-2 expression in BMSC grown in the newly developed poly(LLA-co-CL) scaffolds induced expression of osteogenic markers and enhanced bone formation in vivo.

Bone morphogenetic protein-2 and bone therapy: successes and pitfalls

Objectives

Bone morphogenetic proteins (BMPs), more specifically BMP-2, are being increasingly used in orthopaedic surgery due to advanced research into osteoinductive factors that may enhance and improve bone therapy. There are many areas in therapy that BMP-2 is being applied to, including dental treatment, open tibial fractures, cancer and spinal surgery. Within these areas of treatment, there are many reports of successes and pitfalls. This review explores the use of BMP-2 and its successes, pitfalls and future prospects in bone therapy.

Methods

The PubMed database was consulted to compile this review.

Key findings

With successes in therapy, there were descriptions of a more rapid healing time with no signs of rejection or infection attributed to BMP-2 treatment. Pitfalls included BMP-2 ‘off-label’ use, which lead to various adverse effects.

Conclusions

Our search highlighted that optimising treatment with BMP-2 is a direction that many researchers are exploring, with areas of current research interest including concentration and dose of BMP-2, carrier type and delivery.

3D bioprinting of BMSC-laden methacrylamide gelatin scaffolds with CBD-BMP2-collagen microfibers

Three-dimensional (3D) bioprinting combines biomaterials, cells and functional components into complex living tissues. Herein, we assembled function-control modules into cell-laden scaffolds using 3D bioprinting. A customized 3D printer was able to tune the microstructure of printed bone mesenchymal stem cell (BMSC)-laden methacrylamide gelatin scaffolds at the micrometer scale. For example, the pore size was adjusted to 282 ± 32 μm and 363 ± 60 μm. To match the requirements of the printing nozzle, collagen microfibers with a length of 22 ± 13 μm were prepared with a high-speed crusher. Collagen microfibers bound bone morphogenetic protein 2 (BMP2) with a collagen binding domain (CBD) as differentiation-control module, from which BMP2 was able to be controllably released. The differentiation behaviors of BMSCs in the printed scaffolds were compared in three microenvironments: samples without CBD-BMP2-collagen microfibers in the growth medium, samples without microfibers in the osteogenic medium and samples with microfibers in the growth medium. The results indicated that BMSCs showed high cell viability (>90%) during printing; CBD-BMP2-collagen microfibers induced BMSC differentiation into osteocytes within 14 days more efficiently than the osteogenic medium. Our studies suggest that these function-control modules are attractive biomaterials and have potential applications in 3D bioprinting.

Recent biological trends in management of fracture non-union

Bone regeneration is a complex, well-orchestrated physiological process of bone formation, which can be seen during normal fracture healing, and is involved in continuous remodelling throughout adult life. Currently, there is a plethora of different strategies to augment the impaired or “insufficient” bone-regeneration process, including the “gold standard” autologous bone graft, free fibula vascularised graft, allograft implantation, and use of growth factors, osteoconductive scaffolds, osteoprogenitor cells and distraction osteogenesis. Improved “local” strategies in terms of tissue engineering and gene therapy, or even “systemic” enhancement of bone repair, are under intense investigation, in an effort to overcome the limitations of the current methods, to produce bone-graft substitutes with biomechanical properties that are as identical to normal bone as possible, to accelerate the overall regeneration process, or even to address systemic conditions, such as skeletal disorders and osteoporosis. An improved understanding of the molecular and cellular events that occur during bone repair and remodeling has led to the development of biologic agents that can augment the biological microenvironment and enhance bone repair. Orthobiologics, including stem cells, osteoinductive growth factors, osteoconductive matrices, and anabolic agents, are available clinically for accelerating fracture repair and treatment of compromised bone repair situations like delayed unions and nonunions. A lack of standardized outcome measures for comparison of biologic agents in clinical fracture repair trials, frequent off-label use, and a limited understanding of the biological activity of these agents at the bone repair site have limited their efficacy in clinical applications.

Bone Morphogenetic Proteins: Promising Molecules for Bone Healing, Bioengineering, and Regenerative Medicine

Bone morphogenetic proteins (BMPs), glycoproteins secreted by some cells, are members of the TGF-β superfamily that have been implicated in a wide variety of roles. Currently, about 20 different BMPs have been identified and grouped into subfamilies, according to similarities with respect to their amino acid sequences. It has been shown that BMPs are secreted growth factors involved in mesenchymal stem cell differentiation, also being reported to control the differentiation of cancer stem cells. BMPs initiate signaling from the cell surface by binding to two different receptors (R: Type I and II). The heterodimeric formation of type I R and II R may occur before or after BMP binding, inducing signal transduction pathways through SMADs. BMPs may also signal through SMAD-independent pathways via mitogen-activated protein kinases (ERK, p38MAPKs, JNK). BMPs may act in an autocrine or paracrine manner, being regulated by specific antagonists, namely: noggin and chordin. Genetic engineering allows the production of large amounts of BMPs for clinical use, and clinical trials have shown the benefits of FDA-approved recombinant human BMPs 2 and 7. Several materials from synthetic to natural sources have been tested as BMP carriers, ranging from hydroxyapatite, and organic polymers to collagen. Bioactive membranes doped with BMPs are promising options, acting to accelerate and enhance osteointegration. The development of smart materials, mainly based on biopolymers and bone-like calcium phosphates, appears to provide an attractive alternative for delivering BMPs in an adequately controlled fashion. BMPs have revealed a promising future for the fields of Bioengineering and Regenerative Medicine. In this chapter, we review and discuss the data on BMP structure, mechanisms of action, and possible clinical applications.

Novel hedgehog agonists promote osteoblast differentiation in mesenchymal stem cells

Hedgehog (Hh) family members are involved in multiple cellular processes including proliferation, migration, differentiation, and cell fate determination. Recently, the novel Hh agonists Hh-Ag 1.3 and 1.7 were identified in a high-throughput screening of small molecule compounds that activate the expression of Gli1, a target of Hh signaling. This study demonstrates that Hh-Ag 1.3 and 1.7 strongly activate the expression of endogenous Gli1 and promote osteoblast differentiation in the mesenchymal stem cell line C3H10T1/2. Both compounds stimulated alkaline phosphatase activity in a dose-dependent manner, and induced osteoblast marker gene expression in C3H10T1/2 cells, which indicated that they had acquired an osteoblast identity. Of the markers, the expression of osterix/Sp7, a downstream target of runt-related transcription factor (Runx)2, was induced by Hh-Ag 1.7, which also rescued the osteoblast differentiation defect of RD-127, a mesenchymal cell line from Runx2-deficient mice. Hh-Ags also activated canonical Wnt signaling and synergized with low doses of BMP-2 to enhance osteoblastic potential. Thus, Hh-Ag 1.7 could be useful for bone healing in individuals with abnormalities in osteogenesis, such as osteoporosis patients and the elderly, and can contribute to the development of novel therapeutics for the treatment of bone fractures and defects.

Optimization of entrapping conditions to improve the release of BMP-2 from PELA carriers by response surface methodology

A microcapsule prepared from triblock copolymer poly(lactic acid)-poly(ethylene glycol)-poly(lactic acid) (PLA-PEG-PLA, PELA) was investigated as a controlled release carrier for recombinant human bone morphogenetic protein-2 (rhBMP-2). The rhBMP-2/PELA microspheres were prepared using the water-in-oil-in-water (W/O/W) solvent evaporation method. This work was conducted to optimize the entrapping conditions of the rhBMP-2 loaded PELA copolymer. The effects on encapsulation efficiency (EE) of different molecular weights (MW) of PEG in the copolymer, the amount of PELA, the amount of rhBMP-2, the span-20 concentration, the polyvinyl alcohol (PVA) concentration and stirring time were tested. On the basis of single-factor experiments, the optimum parameters were achieved using response surface methodology (RSM). The results showed that the highest EE of BMP-2 was achieved with a span-20 concentration of 0.5%, PEG MW 4000 Da, a stirring time of 30 min at 800 rpm min(-1), 282.3 mg of PELA, 1 μg of rhBMP-2 and PVA concentration 0.79%. Under these optimal conditions, it was predicted that the highest EE to be achieved would be 76.5%; the actual EE achieved was 75%.

Composite chitosan and calcium sulfate scaffold for dual delivery of vancomycin and recombinant human bone morphogenetic protein-2

A biodegradable, composite bone graft, composed of chitosan microspheres embedded in calcium sulfate, was evaluated in vitro for point-of-care loading and delivery of antibiotics and growth factors to prevent infection and stimulate healing in large bone injuries. Microspheres were loaded with rhBMP-2 or vancomycin prior to mixing into calcium sulfate loaded with vancomycin. Composites were evaluated for set time, drug release kinetics, and bacteriostatic/bactericidal activity of released vancomycin, induction of ALP expression by released rhBMP-2, and interaction of drugs on cells. Results showed the composite set in under 36 min and released vancomycin levels that were bactericidal to S. aureus (>MIC 8-16 μg/mL) for 18 days. Composites exhibited a 1 day-delayed release, followed by a continuous release of rhBMP-2 over 6 weeks; ranging from 0.06 to 1.49 ng/mL, and showed a dose dependent release based on initial loading. Released rhBMP-2 levels were, however, too low to induce detectable levels of ALP in W20-17 cells, due to the affinity of rhBMP-2 for calcium-based materials. With stimulating amounts of rhBMP-2 (>50 ng/mL), the ALP response from W-20-17 cells was inhibited when exposed to high vancomycin levels (1,800-3,600 μg/mL). This dual-delivery system is an attractive alternative to single delivery or preloaded systems for bone regeneration since it can simultaneously fight infection and deliver a potent growth factor. Additionally, this composite can accommodate a wide range of therapeutics and thus be customizable for specific patient needs, however, the potential interactive effects of multiple agents must be investigated to ensure that functional activity is not altered.

Synergistic induction of early stage of bone formation by combination of recombinant human bone morphogenetic protein-2 and epidermal growth factor

This study evaluates whether the combination of the rhBMP-2 and various types of growth factors including EGF, FGF, PDGF and VEGF increases osteoinductivity compared to the single use of rhBMP-2 through in vitro and in vivo study. Cultured human MSCs were treated with rhBMP-2 only or in combination with growth factors. For in vivo evaluation, rhBMP-2 only or with growth factors was implanted into the calvarial defect made on SD rats. Both EGF and PDGF significantly increased both ALP activity and expression level in hMSCs when treated in combination with rhBMP-2 at 3 and 7 days of differentiation and significantly raised the accumulation of the calcium at day 14. Furthermore, micro-CT scanning revealed that the EGF an FGF groups show significantly increased new bone surface ratio compared to the rhBMP-2 only group and, the EGF treatment significantly up regulated percent bone volume and trabecular number at two weeks after the surgery. VEGF treatment also significantly raised trabecular number and FGF treatment significantly increased the trabecular thickness. Histological examination revealed that the EGF combination group showed enhanced bone regeneration than the rhBMP-2 only group two weeks after the implantation. Even though the treatment of rhBMP-2 with PDGF and FGF failed to show enhanced osteogenesis in vitro and in vivo simultaneously, these results suggest that the positive effect of the combination of EGF and rhBMP-2 is expected to induce the bone formation earlier compared to the single use of rhBMP-2 in vitro and in vivo. © 2014 The Authors. Journal of Tissue Engineering and Regenerative Medicine published by John Wiley & Sons Ltd.

Effects of combinations of BMP-2 with FGF-2 and/or VEGF on HUVECs angiogenesis in vitro and CAM angiogenesis in vivo

Angiogenesis, a complex biologic process, is regulated by a large number of angiogenic factors, including vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2). Whether Bone morphogenetic proteins-2 (BMP-2), the osteoinductive factor, could significantly reinforce the effect of VEGF and FGF-2 on angiogenesis has not been studied in detail. To study the positive effects of multiple growth factors on angiogenesis, HUVECs were treated with BMP-2, VEGF, or FGF-2 singly and in binary and ternary combinations. This study further investigates the optimal timing of the ternary combination of BMP-2, VEGF and FGF-2 for angiogenesis in the chorioallantoic membrane (FGF-2 CAM). Results of single applications of BMP-2, VEGF, or FGF-2 suggested that HUVECs angiogenesis could be promoted in a dose-dependent manner and that the optimal concentration of BMP, VEGF and FGF-2 was 10, 50 and 1 ng/mL, respectively. These results indicated that the angiogenic activity of VEGF and FGF-2 was amplified by combining with BMP-2. The ternary combination of BMP-2, VEGF and FGF-2 exhibited a positive and synergistic effect on HUVECs angiogenesis, with the lower concentrations of each factor (1 ng/mL of BMP-2, 25 ng/mL of VEGF and 0.1 ng/mL of FGF-2) being sufficient to show synergistic promotion. When VEGF and FGF-2 were added in the initial activation stage and BMP-2 was added in the maturation stage, both HUVECs angiogenesis in vitro and CAM angiogenesis in vivo could be enhanced more effectively. These results could provide a basis for the controlled release systems capable of delivering multiple factors sequentially to promote angiogenesis in tissue engineering.

Reconstitution of bone-like matrix in osteogenically differentiated mesenchymal stem cell-collagen constructs: A three-dimensional in vitro model to study hematopoietic stem cell niche

Mesenchymal stem/stromal cells (MSCs) and osteoblasts are important niche cells for hematopoietic stem cells (HSCs) in bone marrow osteoblastic niche. Here, we aim to partially reconstitute the bone marrow HSC niche in vitro using collagen microencapsulation for investigation of the interactions between HSCs and MSCs. Mouse MSCs (mMSCs) microencapsulated in collagen were osteogenically differentiated to derive a bone-like matrix consisting of osteocalcin, osteopontin, and calcium deposits and secreted bone morphogenic protein 2 (BMP2). Decellularized bone-like matrix was seeded with fluorescence-labeled human MSCs and HSCs. Comparing with pure collagen scaffold, significantly more HSCs and HSC–MSC pairs per unit area were found in the decellularized bone-like matrix. Moreover, incubation with excess neutralizing antibody of BMP2 resulted in a significantly higher number of HSC per unit area than that without in the decellularized matrix. This work suggests that the osteogenic differentiated MSC–collagen microsphere is a valuable three-dimensional in vitro model to elucidate cell–cell and cell–matrix interactions in HSC niche.

Effects of bilayer gelatin/β-tricalcium phosphate sponges loaded with mesenchymal stem cells, chondrocytes, bone morphogenetic protein-2, and platelet rich plasma on osteochondral defects of the talus in horses

Osteochondrosis (OC) is a common and clinically important joint disorder in horses. However, repair of the OC region is difficult because of the avascular nature of cartilage. This study aimed to evaluate the efficacy of bilayer gelatin/β-tricalcium phosphate (GT) sponges loaded with mesenchymal stem cells (MSCs), chondrocytes, bone morphogenetic protein-2 (BMP-2), and platelet rich plasma (PRP) for the repair of osteochondral defects of the talus in horses. Full-thickness osteochondral defects were created on both the lateral trochlear ridges of the talus (n = 6). In the test group, a basic GT sponge loaded with MSCs and BMP-2 (MSC/BMP2/GT) was inserted into the lower part of the defect, and an acidic GT sponge loaded with chondrocyte, MSCs, and PRP (Ch/MSC/PRP/GT) was inserted into the upper part of the defect. In the control group, the defect was treated only with bilayer GT sponges. Repair of osteochondral defects was assessed by radiography, quantitative computed tomography (QCT), and macroscopic and histological evaluation. The test group showed significantly higher radiographic, QCT, macroscopic, and histological scores than the control group. This study demonstrated that the bilayer scaffolds consisting of Ch/MSC/PRP/GT for the chondrogenic layer and MSC/BMP2/GT for the osteogenic layer promoted osteochondral regeneration in an equine model. The bilayer scaffolds described here may be useful for treating horses with OC.

Controlled release strategies for bone, cartilage, and osteochondral engineering--Part II: challenges on the evolution from single to multiple bioactive factor delivery

The development of controlled release systems for the regeneration of bone, cartilage, and osteochondral interface is one of the hot topics in the field of tissue engineering and regenerative medicine. However, the majority of the developed systems consider only the release of a single growth factor, which is a limiting step for the success of the therapy. More recent studies have been focused on the design and tailoring of appropriate combinations of bioactive factors to match the desired goals regarding tissue regeneration. In fact, considering the complexity of extracellular matrix and the diversity of growth factors and cytokines involved in each biological response, it is expected that an appropriate combination of bioactive factors could lead to more successful outcomes in tissue regeneration. In this review, the evolution on the development of dual and multiple bioactive factor release systems for bone, cartilage, and osteochondral interface is overviewed, specifically the relevance of parameters such as dosage and spatiotemporal distribution of bioactive factors. A comprehensive collection of studies focused on the delivery of bioactive factors is also presented while highlighting the increasing impact of platelet-rich plasma as an autologous source of multiple growth factors.

Monotherapy vs. polytherapy in the treatment of forearm non-unions and bone defects

OBJECTIVE

To determinate the efficacy of "polytherapy", a surgical technique that utilize all the components of the diamond concept (mesenchymal stem cells, bone morphogenetic proteins and scaffold) versus a "monotherapy", a surgical technique that utilize only one component of the diamond conceptin the treatment of severe forearm non-unions.

METHODS

We studied a database of 52 patients with 52 forearm non-unions; we classified the patients with the NUSS SCORE and we divided the patients in two group according to the treatment received. So we distinguished a group of patients treated according to the principles of "monotherapy" (33 patients) and a group of patients treated according to the principles of "polytherapy" (19 patients). The minimum follow up was 12 months.

RESULTS

In the monotherapy group 21/33 non-unions (63.64%) went on to develop a radiographic and clinical healing within a period of 12 months, the calculated DASH SCORE showed a mean value of 55.15 points. In the polytherapy group 17/19 (89.47%) nonunions went on to develop clinical and radiographic healing within 12 months, and the average DASH score showed a mean value of 45.47 points.

CONCLUSION

The polytherapy technique with the use of recombinant morphogenetic proteins, autologous MSCs and scaffold in the same surgical time appears to be an effective treatment for patients with severe forearm non-unions.

High levels of ephrinB2 over-expression increases the osteogenic differentiation of human mesenchymal stem cells and promotes enhanced cell mediated mineralisation in a polyethyleneimine-ephrinB2 gene-activated matrix

Gene therapy can be combined with tissue engineering constructs to produce gene-activated matrices (GAMs) with enhanced capacity for repair. Polyethyleneimine (PEI), a non-viral vector, has previously been optimised for high efficiency gene transfer in rat mesenchymal stem cells (rMSCs). The use of PEI to transfect human MSCs (hMSCs) with ephrinB2 is assessed here. Recently a role for the ephrinB2 ligand and EphB4 receptor duo has been proposed in bone remodelling. Herein, over-expression of the ephrinB2 ligand resulted in increased osteogenic differentiation in hMSCs. As ephrinB2 is a cell surface anchored ligand which only interacts with cells expressing the cognate EphB4 receptor through direct contact, we have shown that direct cell-cell contact between two neighbouring cells is responsible for enhanced osteogenesis. In an effort to begin to elucidate the molecular mechanisms at play downstream of ephrinB2 over-expression, RT-PCR was performed on the GAMs which revealed no significant changes in runx2 or BMP2 expression but an upregulation of osterix (Osx) and Dlx5 expression prompting the belief that the mode of osteogenesis is independent of the BMP2 pathway. This select interaction, coupled with the transient gene expression profile of PEI, makes the PEI-ephrinB2 GAM an ideal candidate matrix for a bone targeted GAM.

Gene therapy approaches to regenerating bone

Bone formation and regeneration therapies continue to require optimization and improvement because many skeletal disorders remain undertreated. Clinical solutions to nonunion fractures and osteoporotic vertebral compression fractures, for example, remain suboptimal and better therapeutic approaches must be created. The widespread use of recombinant human bone morphogenetic proteins (rhBMPs) for spine fusion was recently questioned by a series of reports in a special issue of The Spine Journal, which elucidated the side effects and complications of direct rhBMP treatments. Gene therapy - both direct (in vivo) and cell-mediated (ex vivo) - has long been studied extensively to provide much needed improvements in bone regeneration. In this article, we review recent advances in gene therapy research whose aims are in vivo or ex vivo bone regeneration or formation. We examine appropriate vectors, safety issues, and rates of bone formation. The use of animal models and their relevance for translation of research results to the clinical setting are also discussed in order to provide the reader with a critical view. Finally, we elucidate the main challenges and hurdles faced by gene therapy aimed at bone regeneration as well as expected future trends in this field.

Perspective on the evolution of cell-based bone tissue engineering strategies

Despite the compelling clinical needs in enhancing bone regeneration and the potential offered by the field of tissue engineering, the adoption of cell-based bone graft substitutes in clinical practice is limited to date. In fact, no study has yet convincingly demonstrated reproducible clinical performance of tissue-engineered implants and at least equivalent cost-effectiveness compared to the current treatment standards. Here, we propose and discuss how tissue engineering strategies could be evolved towards more efficient solutions, depicting three different experimental paradigms: (i) bioreactor-based production; (ii) intraoperative manufacturing, and (iii) developmental engineering. The described approaches reflect the need to streamline graft manufacturing processes while maintaining the potency of osteoprogenitors and recapitulating the sequence of biological steps occurring during bone development, including vascularization. The need to combine the assessment of efficacy of the different strategies with the understanding of their mechanisms of action in the target regenerative processes is highlighted. This will be crucial to identify the necessary and sufficient set of signals that need to be delivered at the injury or defect site and should thus form the basis to define release criteria for reproducibly effective engineered bone graft substitutes.

Design, fabrication and in vitro evaluation of a novel polymer-hydrogel hybrid scaffold for bone tissue engineering

The development of a bone mechanically-compatible and osteoinductive scaffold is important for bone tissue engineering applications, particularly for the repair and regeneration of large area critically-sized bone defects. Although previous studies with weight-bearing scaffolds have shown promising results, there is a clear need to develop better osteoinductive strategies for effective scaffold-based bone regeneration. In this study, we designed and fabricated a novel polymer-hydrogel hybrid scaffold system in which a load-bearing polymer matrix and a peptide hydrogel allowed for the synergistic combination of mechanical strength and great potential for osteoinductivity in a single scaffold. The hybrid scaffold system promoted increased pre-osteoblastic cell proliferation. Further, we biotinylated human recombinant bone morphogenetic protein 2 (rhBMP2), and characterized the biotin addition and its effect on rhBMP2 biological activity. The biotinylated rhBMP2 was tethered to the hybrid scaffold using biotin-streptavidin complexation. Controlled release studies demonstrated increased rhBMP2 retention with the tethered rhBMP2 hybrid scaffold group. In vitro evaluation of the hybrid scaffold was performed with rat bone marrow stromal cells and mouse pre-osteoblast cell line MC3T3-E1 cells. Gene expression of alkaline phosphatase (ALP), collagen I (Col I), osteopontin (OPN), bone sialoprotein (BSP), Runx-2 and osteocalcin (OC) increased in MC3T3-E1 cells seeded on the rhBMP2 tethered hybrid scaffolds over the untethered counterparts, demonstrating osteoinductive potential of the hybrid graft. These findings suggest the possibility of developing a novel polymer-hydrogel hybrid system that is weight bearing and osteoinductive for effective bone tissue engineering.

Bone graft substitutes: What are the options?

Currently, a number of bone grafting materials are available in the clinical setting to enhance bone regeneration, varying from autologous bone to several bone graft substitutes. Although autologous bone remains the "gold standard" for stimulating bone repair and regeneration, the morbidity from its harvesting and its restricted availability generated the need for the development of other materials or strategies either to substitute autologous bone graft or expand its limited supply. Bone graft substitutes can possess one or more components: an osteoconductive matrix, acting as a scaffold; osteoinductive proteins and other growth factors to induce differentiation and proliferation of bone-forming cells; and osteogenic cells for bone formation. Based on their distinct properties, all these bone grafting alternatives have specific indications, and can be used either alone or in combination. In this review, we summarise the available bone grafting materials, focussing mainly on the various bone substitutes and their characteristics, in an effort to specify the indications for their use.

Enhancement of fracture healing with the diamond concept: the role of the biological chamber

Bone regeneration presents a unique challenge to both clinicians and scientists. Recently, a vast amount of knowledge has been attained with regard to the molecular mediators, cell populations and the overall cascade of events participating in the bone repair processes. For the treatment of bone non-unions or bone defects, the 'diamond concept' for biological enhancement supports the implantation of mesenchymal stem cells, a scaffold and a growth factor. Prior to the implantation of any or all of these materials however, the surgeon must develop the ideal biological environment (non-union bed) where molecular and physiological processes will evolve facilitating an early and successful osteogenesis leading to bone continuity and functional restoration of the affected limb. At the end of the surgical procedure the non-union bed should have been transformed to a 'biological chamber' active enough to support efficiently all the necessary physiological processes for a successful outcome. The notion of creating the optimum 'biological chamber' represents the centre of the highest biological activity and in a sense the heart of the diamond concept.

Human adipose derived stem cells reduce callus volume upon BMP-2 administration in bone regeneration

INTRODUCTION

The demand for new therapeutic approaches to treat bone defects and fractures is increasing in trauma surgery and orthopaedics because the number of patients with degenerative diseases is continuously growing. "Tissue Engineering" offers promising new technologies that combine the three components - cells, growth factors and matrix. Efforts are targeted at improving and accelerating recovery, especially for long bone fractures, and reducing the risk of delayed bone healing or pseudoarthrosis. Adult human adipose-derived stem cells (ASC) can differentiate into osteoblasts in an osteogenic surrounding. Bone morphogenetic protein-2 (BMP-2) accelerates and initiates this differentiation. Fibrin, a matrix that promotes wound healing, is a promising carrier for ASCs and BMP-2.

MATERIALS AND METHODS

In this study, a 2mm transcortical drill hole in the femur of male rats served as a small non-critical size defect model for fracture simulation. In vivo bone healing was investigated upon administration of the growth factor BMP-2 embedded with ASCs in a locally applied fibrin matrix. Groups with the components alone were also investigated. After 2 and 4 weeks, μCT and histology were performed to determine the bone and callus volume.

RESULTS AND DISCUSSION

After only a short period of time (2 and 4 weeks), this animal model discloses comparative information about the osteogenetic potential and bone regeneration with little effort (no osteosynthesis necessary). The most significant result found in this model is that the combination of ASCs and BMP-2 in a fibrin matrix significantly reduces callus formation after 2 weeks compared to BMP-2 alone. BMP-2 alone significantly increased callus formation. ASCs embedded alone in the fibrin matrix did not lead to increased bone regeneration.

CONCLUSION

Transplantation of ASC modulated the callus induction by BMP-2 to a normal volume.

Treatment of long bone non-unions with polytherapy: indications and clinical results

The incidence of long bone non-unions has been estimated to range between 5 and 10%. Autologous bone graft usually harvested from the iliac crest continues to be the gold standard for biological enhancement of atrophic non-unions. However, its use has been hampered by minor and major donor site complications. Moreover despite possessing the properties of osteogenecity, osteoconductivity and some osteoinductivity the overall results of treatment have not been consistent with disappointing success rates at times. The concept of polytherapy for the treatment of non-unions, namely the simultaneous application of the three fundamental elements of the diamond concept, osteoprogenitor cells, growth factor and osteoconductive scaffold, appears to be an attractive alternative but more studies are desirable to validate this strategy.

Bone regeneration: current concepts and future directions

Bone regeneration is a complex, well-orchestrated physiological process of bone formation, which can be seen during normal fracture healing, and is involved in continuous remodelling throughout adult life. However, there are complex clinical conditions in which bone regeneration is required in large quantity, such as for skeletal reconstruction of large bone defects created by trauma, infection, tumour resection and skeletal abnormalities, or cases in which the regenerative process is compromised, including avascular necrosis, atrophic non-unions and osteoporosis. Currently, there is a plethora of different strategies to augment the impaired or 'insufficient' bone-regeneration process, including the 'gold standard' autologous bone graft, free fibula vascularised graft, allograft implantation, and use of growth factors, osteoconductive scaffolds, osteoprogenitor cells and distraction osteogenesis. Improved 'local' strategies in terms of tissue engineering and gene therapy, or even 'systemic' enhancement of bone repair, are under intense investigation, in an effort to overcome the limitations of the current methods, to produce bone-graft substitutes with biomechanical properties that are as identical to normal bone as possible, to accelerate the overall regeneration process, or even to address systemic conditions, such as skeletal disorders and osteoporosis.

Evaluation of an injectable silk fibroin enhanced calcium phosphate cement loaded with human recombinant bone morphogenetic protein-2 in ovine lumbar interbody fusion

The objective of this study was to investigate the efficacy of an injectable calcium phosphate cement/silk fibroin/human recombinant bone morphogenetic protein-2 composite (CPC/SF/rhBMP-2) in an ovine interbody fusion model. Twenty-four mature sheep underwent anterior lumbar interbody fusion at the levels of L1/2, L3/4, and L5/6 with random implantation of CPC/SF, CPC/rhBMP-2, CPC/SF/rhBMP-2, or autogenous iliac bone. After the sheep were sacrificed, the fusion segments were evaluated by manual palpation, CT scan, undestructive biomechanical testing, undecalcified histology, and histomorphology. The fusion rates of CPC/SF/rhBMP-2 were 55.56% and 77.78% at 6 and 12 months, respectively. The fusion was superior to all the biomaterial grafts in stiffness, and reached the same stiffness as the autograft at 12 months. The new bone formation was less than autograft at 6 months, but similar with that at 12 months. However, the ceramic residue volume of CPC/SF/rhBMP-2 was significantly decreased compared with CPC/SF and CPC/rhBMP-2 at both times. The results indicated that CPC/SF/rhBMP-2 composite had excellent osteoconduction and osteoinduction, and balanced degradation and osteogenesis.