Assay of Bone Morphogenetic Protein-2, -4, and -7 in Human Demineralized Bone Matrix

The Future of Bone Repair: Emerging Technologies and Biomaterials in Bone Regeneration

Bone defects and fractures present significant clinical challenges, particularly in orthopedic and maxillofacial applications. While minor bone defects may be capable of healing naturally, those of a critical size necessitate intervention through the use of implants or grafts. The utilization of traditional methodologies, encompassing autografts and allografts, is constrained by several factors. These include the potential for donor site morbidity, the restricted availability of suitable donors, and the possibility of immune rejection. This has prompted extensive research in the field of bone tissue engineering to develop advanced synthetic and bio-derived materials that can support bone regeneration. The optimal bone substitute must achieve a balance between biocompatibility, bioresorbability, osteoconductivity, and osteoinductivity while simultaneously providing mechanical support during the healing process. Recent innovations include the utilization of three-dimensional printing, nanotechnology, and bioactive coatings to create scaffolds that mimic the structure of natural bone and enhance cell proliferation and differentiation. Notwithstanding the advancements above, challenges remain in optimizing the controlled release of growth factors and adapting materials to various clinical contexts. This review provides a comprehensive overview of the current advancements in bone substitute materials, focusing on their biological mechanisms, design considerations, and clinical applications. It explores the role of emerging technologies, such as additive manufacturing and stem cell-based therapies, in advancing the field. Future research highlights the need for multidisciplinary collaboration and rigorous testing to develop advanced bone graft substitutes, improving outcomes and quality of life for patients with complex defects.

Effect of gamma and Ultraviolet-C sterilization on BMP-7 level of indigenously prepared demineralized freeze-dried bone allograft

The presence of bone morphogenetic proteins in demineralized freeze-dried bone allograft (DFDBA) are responsible for developing hard tissues in intraosseous defects. The most common mode of sterilization of bone allografts, i.e., Gamma rays, have dramatic effects on the structural and biological properties of DFDBA, leading to loss of BMPs. Ultraviolet-C radiation is a newer approach to sterilize biodegradable scaffolds, which is simple to use and ensures efficient sterilization. However, UV-C radiation has not yet been effectively studied to sterilize bone allografts. This study aimed to compare and evaluate the effectiveness of Gamma and Ultraviolet-C rays in sterilizing indigenously prepared DFDBA and assess their effect on the quantity of BMP-7 present in the allograft. DFDBA samples from non-irradiated, gamma irradiated, and UV-C irradiated groups were tested for BMP-7 level and samples sterilized with gamma and UV-C rays were analysed for sterility testing. The estimated mean BMP-7 level was highest in non-irradiated DFDBA samples, followed by UV-C irradiated, and the lowest in gamma irradiated samples. Our study concluded that UV-C rays effectively sterilized DFDBA as indicated by negative sterility test and comprised lesser degradation of BMP-7 than gamma irradiation.

Variability of BMP-2 content in DBM products derived from different long bone

Demineralized bone matrix (DBM) has been regarded as an ideal bone substitute as a native carrier of bone morphogenetic proteins (BMPs) and other growth factors. However, the osteoinductive properties diverse in different DBM products. We speculate that the harvest origin further contributing to variability of BMPs contents in DBM products besides the process technology. In the study, the cortical bone of femur, tibia, humerus, and ulna from a signal donor were prepared and followed demineralizd into DBM products. Proteins in bone martix were extracted using guanidine-HCl and collagenase, respectively, and BMP-2 content was detected by sandwich enzyme-linked immunosorbent assay (ELISA). Variability of BMP-2 content was found in 4 different DBM products. By guanidine-HCl extraction, the average concentration in DBMs harvested from ulna, humerus, tibia, and femur were 0.613 ± 0.053, 0.848 ± 0.051, 3.293 ± 0.268, and 21.763 ± 0.344, respectively (p < 0.05), while using collagenase, the levels were 0.089 ± 0.004, 0.097 ± 0.004, 0.330 ± 0.012, and 1.562 ± 0.008, respectively (p < 0.05). In general, the content of BMP-2 in long bones of Lower limb was higher than that in long bones of upper limb, and GuHCl had remarkably superior extracted efficiency for BMP-2 compared to collagenase. The results suggest that the origin of cortical bones harvested to fabricate DBM products contribute to the variability of native BMP-2 content, while the protein extracted method only changes the measured values of BMP-2.

Comparative Studies of Bone Graft and Orthobiologics for Foot Ankle Arthrodesis: A Critical Review

Graft materials available to supplement hindfoot and ankle arthrodesis procedures include autologous (autograft) or allogeneic bone graft (allograft) but also bone graft substitutes such as demineralized bone matrix, calcium sulfate, calcium phosphate, and tricalcium phosphate/hydroxyapatite. In addition, biologic agents, such as recombinant human bone morphogenetic protein-2 or recombinant human platelet derived growth factor-BB (rhPDGF-BB), and preparations, including platelet-rich plasma or concentrated bone marrow aspirate, have been used to facilitate bone healing in ankle or hindfoot arthrodesis. The purpose of this review was to summarize the available clinical evidence surrounding the utilization and efficacy of the above materials and biological agents in ankle or hindfoot arthrodesis procedures, with emphasis on the quality of the existing evidence to facilitate clinical decision making.

Demineralized bone matrix for repair and regeneration of maxillofacial defects: A narrative review

OBJECTIVES

Demineralized bone matrix (DBM) is a well-established bone graft material widely accepted by dentists and the public for its favorable osteoconductivity and osteoinductive potential. This article aimed to provide a narrative review of the current therapeutic applications and limitations of DBM in maxillofacial bone defects.

STUDY SELECTION, DATA, AND SOURCES

Randomized controlled trials, prospective or retrospective clinical studies, case series and reports, and systematic reviews. MEDLINE, PubMed, and Google Scholar were searched using keywords.

CONCLUSIONS

Some evidence supported the therapeutic application of DBM in periodontal intrabony defects, maxillary sinus lifts, ridge preservation, ridge augmentation, alveolar cleft repair, orthognathic surgery, and other regional maxillofacial bone defects. However, the limitations of DBM should be considered when using it, including potential low immunogenicity, instability of osteoinductive potential, handling of the graft material, and patient acceptance.

CLINICAL SIGNIFICANCE

With the increasing demand for the treatment of maxillofacial bone defects, DBM is likely to play a greater role as a promising bone graft material. Safe and effective combination treatment strategies and how to maintain a stable osteoinductive potential will be the future challenges of DBM research.

Management of non-unions of the malleolar fractures- Current Evidence

Although malleolar non-union is uncommon, it is associated with significant morbidity. Managing this problem requires understanding ankle fracture biomechanics and bone healing. We present in this article the pertinent points to be considered in evaluating and managing patients with malleolar non-union. Our discussion will focus on the important risk factors contributing to this problem, and the need to carefully consider the biomechanical stability and the biological environment to ensure successful bony unions.

Bone Graft Substitutes and Enhancement in Craniomaxillofacial Surgery

Critical-sized bone defects are a reconstructive challenge, particularly in the craniomaxillofacial (CMF) skeleton. The "gold standard" of autologous bone grafting has been the work horse of reconstruction in both congenital and acquired defects of CMF skeleton. Autologous bone has the proper balance of the protein (or organic) matrix and mineral components with no immune response. Organic and mineral adjuncts exist that offer varying degrees of osteogenic, osteoconductive, osteoinductive, and osteostimulative properties needed for treatment of critical-sized defects. In this review, we discuss the various mostly organic and mostly mineral bone graft substitutes available for autologous bone grafting. Primarily organic bone graft substitutes/enhancers, including bone morphogenic protein, platelet-rich plasma, and other growth factors, have been utilized to support de novo bone growth in setting of critical-sized bone defects. Primarily mineral options, including various calcium salt formulation (calcium sulfate/phosphate/apatite) and bioactive glasses have been long utilized for their similar composition to bone. Yet, a bone graft substitute that can supplant autologous bone grafting is still elusive. However, case-specific utilization of bone graft substitutes offers a wider array of reconstructive options.

Efficient autocrine and paracrine signaling explain the osteogenic superiority of transgenic BMP-2 over rhBMP-2

Bone morphogenetic protein-2 (BMP-2) is an osteogenic protein used clinically to enhance bone healing. However, it must be applied in very high doses, causing adverse side effects and increasing costs while providing only incremental benefit. Preclinical models of bone healing using gene transfer to deliver BMP-2 suggest that transgenic BMP-2 is much more osteogenic than rhBMP-2. Using a reporter mesenchymal cell line, we found transgenic human BMP-2 cDNA to be at least 100-fold more effective than rhBMP-2 in signaling. Moreover, a substantial portion of the BMP-2 produced by the transduced cells remained cell associated. Signaling by transgenic BMP-2 occurred via binding to the type I receptor, activating the associated kinase and generating phospho-smads. Signaling was partially resistant to noggin, an important extracellular inhibitor of BMP-2, possibly because nascent BMP-2 binds to its cell surface receptor during secretion and thus signals in a protected peri-cellular environment. Although the amounts of BMP-2 secreted by the transduced cells were too low to affect distant cells, transduced cells were able to induce signaling in a paracrine fashion that required close proximity of the cells, possibly cell-to-cell contact. The greater osteogenic potency of transgenic BMP-2 was confirmed with human bone marrow stromal cells.

Effects of demineralization mode and particle size of allogeneic bone powder on its physical and chemical properties

At present, the commonly used allogeneic bone powder in the clinic can be divided into nondemineralized bone matrix and demineralized bone matrix (DBM). Commonly used demineralizers include acids and ethylene diamine tetraacetic acid (EDTA). There may be some diversities between them. Also, the size of the bone particle can affects its cell compatibility and osteogenic ability. We produced different particle sizes i.e., < 75, 75-100, 100-315, 315-450, 450-650, and 650-1000 μm, and treated in three ways (nondemineralized, demineralized by EDTA, and demineralized by HCl). Scanning electron microscopy showed that the surface of the samples in each group was relatively smooth without obvious differences. The results of specific surface area and porosity analysis showed that they were significantly higher in demineralized bone powder than in nondemineralized bone powder, however, there was no significant difference between the two decalcification methods. The content of hydroxyproline in nondemineralized bone powder and EDTA-demineralized bone powder had no statistical difference, while HCl-demineralization had statistical significance compared with the former two, and the content increased with the decrease of particle size. The protein and BMP-2 extracted from HCl demineralized bone powder were significantly higher than that from nondemineralized bone powder and EDTA demineralized bone powder, and there were differences among different particle sizes. These results suggested the importance of demineralization mode and particle size of the allogenic bone powder and provided guidance for the choice of the most appropriate particle size and demineralization mode to be used in tissue bioengineering.

Spatial rhBMP2 delivery from hydroxyapatite scaffolds sustains bone regeneration in rabbit radius

Regenerating large bone defects requires a multi-faceted approach combining optimal scaffold designs with appropriate growth factor delivery. Supraphysiological doses of recombinant human bone morphogenetic protein 2(rhBMP2); typically used for the regeneration of large bone defects clinically in conjunction with an acellular collagen sponge (ACS), have resulted in many complications. In the current study, we develop a hydroxyapatite/collagen I (HA/Col) scaffold to improve the mechanical properties of the HA scaffolds while maintaining open connected porosity. Varying rhBMP2 dosages were then delivered from a collagenous periosteal membrane and paired with HA or HA/Col scaffolds to treat critical sized (15mm) diaphyseal radial defect in New Zealand white rabbits. The groups examined were ACS+76µg rhBMP2 (clinically used INFUSE dosage), HA+76µg rhBMP2, HA+15µg rhBMP2, HA/Col+15µg rhBMP2 and HA/Col+15µg rhBMP2+bone marrow derived stromal cells (bMSCs). After 8 weeks of implantation, all regenerated bones were evaluated using micro computed tomography, histology, histomorphometry and torsional testing. It was observed that the bone volume regenerated in the HA/Col + 15 µg rhBMP2 group was significantly higher than that in the groups with 76µg rhBMP2. The same scaffold and growth factor combination resulted in the highest bone mineral density of the regenerated bone, and the most bone apposition on the scaffold surface. Both the HA and HA/Col scaffolds paired with 15 µg rhBMP2 had sustained ingrowth of the mineralization front after 2 weeks compared to the groups with 76µg rhBMP2 which had far greater mineralization in the first 2 weeks after implantation. Complete bridging of the defect site and no significant differences in torsional strength, stiffness or angle at failure was observed across all groups. No benefit of additional bMSC seeding was observed on any of the quantified metrics, while bone-implant apposition was reduced in the cell seeded group. This study demonstrated that the controlled spatial delivery of rhBMP2 at the periosteum at significantly lower doses can be used as a strategy to improve bone regeneration around space maintaining scaffolds.

Endogenous repair theory enriches construction strategies for orthopaedic biomaterials: a narrative review

The development of tissue engineering has led to new strategies for mitigating clinical problems; however, the design of the tissue engineering materials remains a challenge. The limited sources and inadequate function, potential risk of microbial or pathogen contamination, and high cost of cell expansion impair the efficacy and limit the application of exogenous cells in tissue engineering. However, endogenous cells in native tissues have been reported to be capable of spontaneous repair of the damaged tissue. These cells exhibit remarkable plasticity, and thus can differentiate or be reprogrammed to alter their phenotype and function after stimulation. After a comprehensive review, we found that the plasticity of these cells plays a major role in establishing the cell source in the mechanism involved in tissue regeneration. Tissue engineering materials that focus on assisting and promoting the natural self-repair function of endogenous cells may break through the limitations of exogenous seed cells and further expand the applications of tissue engineering materials in tissue repair. This review discusses the effects of endogenous cells, especially stem cells, on injured tissue repairing, and highlights the potential utilisation of endogenous repair in orthopaedic biomaterial constructions for bone, cartilage, and intervertebral disc regeneration.

Preparation and Characterization of Moldable Demineralized Bone Matrix/Calcium Sulfate Composite Bone Graft Materials

Demineralized bone matrix (DBM) is a decalcified allo/xenograft retaining collagen and noncollagenous proteins, which has been extensively used because of its osteoconductive and osteoinductive properties. Calcium sulfate (CaSO₄, CS) is a synthetic bone substitute used in bone healing with biocompatible, nontoxic, bioabsorbable, osteoconductive, and good mechanical characteristics. This study aims to prepare a DBM/CS composite bone graft material in a moldable putty form without compromising the peculiar properties of DBM and CS. For this purpose, firstly, porcine femur was defatted using chloroform/methanol and extracted by acid for demineralization, then freeze-dried and milled/sieved to obtain DBM powder. Secondly, the α-form and β-form of calcium sulfate hemihydrate (CaSO₄·0.5H₂O, CSH) were produced by heating gypsum (CaSO₄·2H₂O). The morphology and particle sizes of α- and β-CSH were obtained by SEM, and their chemical properties were confirmed by EDS, FTIR and XRD. Furthermore, the DBM-based graft was mixed with α- or β-CSH at a ratio of 9:1, and glycerol/4% HPMC was added as a carrier to produce a putty. DBM/CSH putty possesses a low washout rate, good mechanical strength and biocompatibility. In conclusion, we believe that the moldable DBM/CSH composite putty developed in this study could be a promising substitute for the currently available bone grafts, and might have practical application in the orthopedics field as a potential bone void filler.

Investigating the efficacy of allograft cellular bone matrix for spinal fusion: a systematic review of the literature

OBJECTIVE

The use of allograft cellular bone matrices (ACBMs) in spinal fusion has expanded rapidly over the last decade. Despite little objective data on its effectiveness, ACBM use has replaced the use of traditional autograft techniques, namely iliac crest bone graft (ICBG), in many centers.

METHODS

In accordance with PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, a systematic review was conducted of the PubMed, Cochrane Library, Scopus, and Web of Science databases of English-language articles over the time period from January 2001 to December 2020 to objectively assess the effectiveness of ACBMs, with an emphasis on the level of industry involvement in the current body of literature.

RESULTS

Limited animal studies (n = 5) demonstrate the efficacy of ACBMs in spinal fusion, with either equivalent or increased rates of fusion compared to autograft. Clinical human studies utilizing ACBMs as bone graft expanders or bone graft substitutes (n = 5 for the cervical spine and n = 8 for the lumbar spine) demonstrate the safety of ACBMs in spinal fusion, but fail to provide conclusive level I, II, or III evidence for its efficacy. Additionally, human studies are plagued with several limiting factors, such as small sample size, lack of prospective design, lack of randomization, absence of standardized assessment of fusion, and presence of industry support/relevant conflict of interest.

CONCLUSIONS

There exist very few objective, unbiased human clinical studies demonstrating ACBM effectiveness or superiority in spinal fusion. Impartial, well-designed prospective studies are needed to offer evidence-based best practices to patients in this domain.

A multifaceted biomimetic interface to improve the longevity of orthopedic implants

The rise of additive manufacturing has provided a paradigm shift in the fabrication of precise, patient-specific implants that replicate the physical properties of native bone. However, eliciting an optimal biological response from such materials for rapid bone integration remains a challenge. Here we propose for the first time a one-step ion-assisted plasma polymerization process to create bio-functional 3D printed titanium (Ti) implants that offer rapid bone integration. Using selective laser melting, porous Ti implants with enhanced bone-mimicking mechanical properties were fabricated. The implants were functionalized uniformly with a highly reactive, radical-rich polymeric coating generated using a unique combination of plasma polymerization and plasma immersion ion implantation. We demonstrated the performance of such activated Ti implants with a focus on the coating's homogeneity, stability, and biological functionality. It was shown that the optimized coating was highly robust and possessed superb physico-chemical stability in a corrosive physiological solution. The plasma activated coating was cytocompatible and non-immunogenic; and through its high reactivity, it allowed for easy, one-step covalent immobilization of functional biomolecules in the absence of solvents or chemicals. The activated Ti implants bio-functionalized with bone morphogenetic protein 2 (BMP-2) showed a reduced protein desorption and a more sustained osteoblast response both in vitro and in vivo compared to implants modified through conventional physisorption of BMP-2. The versatile new approach presented here will enable the development of bio-functionalized additively manufactured implants that are patient-specific and offer improved integration with host tissue. STATEMENT OF SIGNIFICANCE: Additive manufacturing has revolutionized the fabrication of patient-specific orthopedic implants. Although such 3D printed implants can show desirable mechanical and mass transport properties, they often require surface bio-functionalities to enable control over the biological response. Surface covalent immobilization of bioactive molecules is a viable approach to achieve this. Here we report the development of additively manufactured titanium implants that precisely replicate the physical properties of native bone and are bio-functionalized in a simple, reagent-free step. Our results show that covalent attachment of bone-related growth factors through ion-assisted plasma polymerized interlayers circumvents their desorption in physiological solution and significantly improves the bone induction by the implants both in vitro and in vivo.

Comparing cellular bone matrices for posterolateral spinal fusion in a rat model

INTRODUCTION

Cellular bone matrices (CBM) are allograft products that provide three components essential to new bone formation: an osteoconductive scaffold, extracellular growth factors for cell proliferation and differentiation, and viable cells with osteogenic potential. This is an emerging technology being applied to augment spinal fusion procedures as an alternative to autografts.

METHODS

We aim to compare the ability of six commercially-available human CBMs (Trinity ELITE®, ViviGen®, Cellentra®, Osteocel® Pro, Bio4® and Map3®) to form a stable spinal fusion using an athymic rat model of posterolateral fusion. Iliac crest bone from syngeneic rats was used as a control to approximate the human gold standard. The allografts were implanted at L4-5 according to vendor specifications in male athymic rats, with 15 rats in each group. MicroCT scans were performed at 48 hours and 6 weeks post-implantation. The rats were euthanized 6 weeks after surgery and the lumbar spines were harvested for X-ray, manual palpation and histology analysis by blinded reviewers.

RESULTS

By manual palpation, five of 15 rats of the syngeneic bone group were fused at 6 weeks. While Trinity ELITE had eight of 15 and Cellentra 11 of 15 rats with stable fusion, only 2 of 15 of ViviGen-implanted spines were fused and zero of 15 of the Osteocel Pro, Bio4 and Map3 produced stable fusion. MicroCT analysis indicated that total bone volume increased from day 0 to week 6 for all groups except syngeneic bone group. Trinity ELITE (65%) and Cellentra (73%) had significantly greater bone volume increases over all other implants, which was consistent with the histological analysis.

CONCLUSION

Trinity ELITE and Cellentra were significantly better than other implants at forming new bone and achieving spinal fusion in this rat model at week 6. These results suggest that there may be large differences in the ability of different CBMs to elicit a successful fusion in the posterolateral spine.

The Use of Bone Grafts, Bone Graft Substitutes, and Orthobiologics for Osseous Healing in Foot and Ankle Surgery

Achieving fusion in osseous procedures about the foot and ankle presents unique challenges to the surgeon. Many patients have comorbidities that reduce osseous healing rates, and the limited space and high weightbearing demand placed on fusion sites makes the choice of bone graft, bone graft substitute, or orthobiologic agent of utmost importance. In this review, we discuss the essential characteristics of grafts, including their osteoconductive, osteoinductive, osteogenic, and angiogenic properties. Autologous bone graft remains the gold standard and contains all these properties. However, the convenience and lack of donor site morbidity of synthetic bone grafts, allografts, and orthobiologics, including growth factors and allogenic stem cells, has led to these being used commonly as augments.

A New Method for Partial Breast Reconstruction: 3-Year New Zealand Experience

In New Zealand, oncoplastic surgery is common, but partial breast reconstruction presents challenges for radiation therapy targeting. Tissue rearrangement creates ambiguity when targeting the tumor bed, with resultant overestimation of treatment volumes. Thus, adoption of advanced methods of radiation therapy have been hindered. This pilot study describes use of a novel three-dimensional implant that provides a scaffolding for tissue ingrowth during partial breast reconstruction and delineates the tumor bed more precisely to assist radiation planning and mammographic surveillance. After informed consent, 15 women were implanted with the three-dimensional bioabsorbable implant. The device was sutured to the tumor bed during lumpectomy, and tissue flaps were mobilized and attached to the implant. Visualization of the marker and radiation treatment volumes were recorded and compared. The implant provided volume replacement and helped to maintain breast contour. Cosmetic outcomes were excellent; no device- or radiation-related complications occurred. One patient had a postoperative hematoma that resolved after percutaneous drainage; there were no postoperative infections. Three-year follow-up shows no tumor recurrences and no untoward effects. When compared to conventional radiation targeting, use of the implant showed that a greater than 50 percent reduction in treatment volume was possible in some cases. Three-year mammograms show no significant artifact, normal tissue ingrowth, and minimal fibrosis. This study describes a method of oncoplastic breast reconstruction using an implantable device that marks the site of tumor excision and provides for volume replacement with tissue ingrowth. Patients tolerated it well, and radiation therapy planning, positioning, and treatment were facilitated.

Nasal Dorsal Augmentation with Freeze-Dried Allograft Bone: 10-Year Comprehensive Review

Supplemental Digital Content is available in the text.

Background:

The aim of this study was to evaluate freeze-dried cortical allograft bone for nasal dorsal augmentation. The 42-month report on 18 patients was published in 2009 in Plastic and Reconstructive Surgery with 89 percent success at level II evidence, and this article is the 10-year comprehensive review of 62 patients.

Methods:

All grafts met standards recommended by the American Association of Tissue Banks, the U.S. Food and Drug Administration, and the Centers for Disease Control and Prevention. Objective evaluation of the persistence of graft volume was obtained by cephalometric radiography, cone beam volumetric computed tomography, and computed tomography at up to 10 years. Vascularization and incorporation of new bone elements within the grafts were demonstrated by fluorine-18 sodium fluoride positron emission tomography at up to 10 years. Subjective estimation of graft volume persisting up to 10 years was obtained by patient response to a query conducted by an independent surveyor.

Results:

The authors report objective proof of persistence of volume alone or combined with proof of neovascularization in 16 of 19 allografts. The authors report the patient’s subjective opinion of volume persistence in 37 of 43 grafts. The dorsal augmentation was assessed overall to be successful in 85 percent of 62 patients evaluated between 1 and 10 years, with a mean of 4.7 years.

Conclusions:

Freeze-dried allograft bone is a safe and equal alternative for dorsal augmentation without donor-site morbidity. Further studies are needed to (1) confirm these findings for young patients needing long-term reconstruction, and (2) partially demineralize allograft bone to allow carving with a scalpel.

CLINICAL QUESTION/LEVEL OF EVIDENCE:

Therapeutic, IV.

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.

The effect of embryonic origin on the osteoinductive potential of bone allografts

STATEMENT OF PROBLEM

Allografts with osteoinduction potential are widely used to augment bone in surgical and prosthetic rehabilitations. However, osteoinduction potential varies among commercially available allografts. Donor bones are derived from different embryonic origins, either the neural crest or mesoderm. Whether the origin of the bones affects the osteoinductivity of allograftsis is unclear.

PURPOSE

The purpose of this ex vivo study was to investigate the osteoinduction potential of allografts derived from bones with distinct embryonic origins.

MATERIAL AND METHODS

Allografts were obtained from human frontal and parietal bones at 2 different ages (fetal and adult). The specimens were divided into 4 groups: adult frontal (n=5), adult parietal (n=5), fetal frontal (n=10), and fetal parietal (n=10). Two investigations were conducted to assess the osteoinductive potential of these allografts. First, the osteogenesis of human osteoblasts exposed to these allografts were evaluated by analyzing the expression of runt-related transcription factor 2 (RUNX2), collagen type 1 alpha 2 chain (COL1A2), and bone gamma-carboxyglutamate protein (BGLAP) genes using quantitative real-time polymerase chain reaction (qRT-PCR). Second, the protein content of the adult frontal and parietal bone matrices was analyzed using liquid chromatography tandem mass spectrometry (LC-MS/MS). One-way ANOVA and the t test were used for statistical analyses of the gene and protein expression of the groups (α=.05).

RESULTS

No difference was found in the gene expression of the cells exposed to frontal or parietal bones. However, all 3 genes were significantly overexpressed in cells treated with fetal bones compared with adult bones. No difference was found in protein expression between adult frontal and adult parietal bones.

CONCLUSIONS

No difference was found in the osteoinductive capacity of frontal and parietal bones used as allografts. However, the osteoinductivity of fetal bones can be higher than that of adult bones. Further microanalyses are needed to determine the protein content of fetal bones.

Interleukin 17 enhances bone morphogenetic protein-2-induced ectopic bone formation

Interleukin 17 (IL-17) stimulates the osteogenic differentiation of progenitor cells in vitro through a synergy with bone morphogenetic protein (BMP)-2. This study investigates whether the diverse responses mediated by IL-17 in vivo also lead to enhanced BMP-2-induced bone formation. Since IL-17 is known to induce osteoclastogenesis, we studied the interactions between IL-17 and BMP-2 in ceramic scaffolds either or not carrying a coating with the bisphosphonate zoledronic acid (ZOL). Histological evaluation revealed that IL-17 alone did not induce any osteoclasts at day 10. On the other hand, BMP-2 clearly stimulated early tissue ingrowth and osteoclastogenesis. Both of these processes were blocked in presence of ZOL. IL-17 signaling restored early vascularized connective tissue formation and osteoclastogenesis induced by BMP-2 in ZOL-coated scaffolds. After 12 weeks, the bone volume induced by co-delivery of BMP-2 and IL-17 was doubled as compared to that induced by BMP-2 alone. We conclude that IL-17 has osteo-stimulatory effects through a synergy with bone-inductive BMP-2. Although local and single application of IL-17 does not mediate osteoclast formation, it could promote other processes involved in bone formation such as connective tissue ingrowth. The use of IL-17 may contribute to the development of improved bone graft substitutes.

Clinical assessment of 115 cases of hindfoot fusion with two different types of graft: Allograft+DBM+bone marrow aspirate versus autograft+DBM

INTRODUCTION

Nonunion is a common complication (15%) of hindfoot and ankle arthrodesis. Autograft can improve the fusion rate because of its osteoconductive, osteoinductive and osteogenic properties. However, autograft harvesting is a source of morbidity. One alternative is to combine allograft with demineralized bone matrix (DBM) and iliac bone marrow aspirate (BMA). This combination graft has similar biological properties to healthy bone. When used alone, allograft has osteoconductive and sometimes structural properties. DBM provides osteoinduction and improves the osteconductivity. BMA adds cells and thereby osteogenic potential.

HYPOTHESIS

Given its intrinsic properties, allograft-DBM-BMA is as effective as autograft-DBM treatment while simplifying the clinical practice.

MATERIAL AND METHODS

One hundred and fifteen cases of ankle and hindfoot arthrodesis were studied in 82 patients divided in two groups: autograft-DBM vs allograft-DBM-BMA. Treatment effectiveness was assessed using clinical (time to fusion, fusion rate) and radiological (trabecular bone bridge, disappearance of joint space) criteria. A CT scan was done in 60% of cases when fusion could not be confirmed using the clinical and radiological criteria.

RESULTS

There was no significant difference between the two groups in terms of fusion rate, time to fusion, number of heterotopic ossifications, revision rate and quantity of DBM used. The nonunion rate was 18% in the autograft group and 13% in the allograft group. The infection rate was 11% in the autograft and 4% in the allograft group.

DISCUSSION

Allograft-DBM-BMA is an alternative to autograft-DBM that provides similar effectiveness without increasing the number of nonunion or complications. Osteonecrosis and surgical revision are risk factors.

LEVEL OF EVIDENCE

III retrospective study.

Human decellularized bone scaffolds from aged donors show improved osteoinductive capacity compared to young donor bone

To improve the safe use of allograft bone, decellularization techniques may be utilized to produce acellular scaffolds. Such scaffolds should retain their innate biological and biomechanical capacity and support mesenchymal stem cell (MSC) osteogenic differentiation. However, as allograft bone is derived from a wide age-range, this study aimed to determine whether donor age impacts on the ability an osteoinductive, acellular scaffold produced from human bone to promote the osteogenic differentiation of bone marrow MSCs (BM-MSC). BM-MSCs from young and old donors were seeded on acellular bone cubes from young and old donors undergoing osteoarthritis related hip surgery. All combinations resulted in increased osteogenic gene expression, and alkaline phosphatase (ALP) enzyme activity, however BM-MSCs cultured on old donor bone displayed the largest increases. BM-MSCs cultured in old donor bone conditioned media also displayed higher osteogenic gene expression and ALP activity than those exposed to young donor bone conditioned media. ELISA and Luminex analysis of conditioned media demonstrated similar levels of bioactive factors between age groups; however, IGF binding protein 1 (IGFBP1) concentration was significantly higher in young donor samples. Additionally, structural analysis of old donor bone indicated an increased porosity compared to young donor bone. These results demonstrate the ability of a decellularized scaffold produced from young and old donors to support osteogenic differentiation of cells from young and old donors. Significantly, the older donor bone produced greater osteogenic differentiation which may be related to reduced IGFBP1 bioavailability and increased porosity, potentially explaining the excellent clinical results seen with the use of allograft from aged donors.

Structural Biology and Evolution of the TGF-β Family

We review the evolution and structure of members of the transforming growth factor β (TGF-β) family, antagonistic or agonistic modulators, and receptors that regulate TGF-β signaling in extracellular environments. The growth factor (GF) domain common to all family members and many of their antagonists evolved from a common cystine knot growth factor (CKGF) domain. The CKGF superfamily comprises six distinct families in primitive metazoans, including the TGF-β and Dan families. Compared with Wnt/Frizzled and Notch/Delta families that also specify body axes, cell fate, tissues, and other families that contain CKGF domains that evolved in parallel, the TGF-β family was the most fruitful in evolution. Complexes between the prodomains and GFs of the TGF-β family suggest a new paradigm for regulating GF release by conversion from closed- to open-arm procomplex conformations. Ternary complexes of the final step in extracellular signaling show how TGF-β GF dimers bind type I and type II receptors on the cell surface, and enable understanding of much of the specificity and promiscuity in extracellular signaling. However, structures suggest that when GFs bind repulsive guidance molecule (RGM) family coreceptors, type I receptors do not bind until reaching an intracellular, membrane-enveloped compartment, blurring the line between extra- and intracellular signaling. Modulator protein structures show how structurally diverse antagonists including follistatins, noggin, and members of the chordin family bind GFs to regulate signaling; complexes with the Dan family remain elusive. Much work is needed to understand how these molecular components assemble to form signaling hubs in extracellular environments in vivo.

Dental pulp stem cells. Biology and use for periodontal tissue engineering

Inflammatory periodontal disease is a major cause of loss of tooth-supporting structures. Novel approaches for regeneration of periodontal apparatus is an area of intensive research. Periodontal tissue engineering implies the use of appropriate regenerative cells, delivered through a suitable scaffold, and guided through signaling molecules. Dental pulp stem cells have been used in an increasing number of studies in dental tissue engineering. Those cells show mesenchymal (stromal) stem cell-like properties including self-renewal and multilineage differentiation potentials, aside from their relative accessibility and pleasant handling properties. The purpose of this article is to review the biological principles of periodontal tissue engineering, along with the challenges facing the development of a consistent and clinically relevant tissue regeneration platform. This article includes an updated review on dental pulp stem cells and their applications in periodontal regeneration, in combination with different scaffolds and growth factors.

Biomimetically enhanced demineralized bone matrix for bone regenerative applications

Demineralized bone matrix (DBM) is one of the most widely used bone graft materials in dentistry. However, the ability of DBM to reliably and predictably induce bone regeneration has always been a cause for concern. The quality of DBM varies greatly depending on several donor dependent factors and also manufacturing techniques. In order to standardize the quality and to enable reliable and predictable bone regeneration, we have generated a biomimetically-enhanced version of DBM (BE-DBM) using clinical grade commercial DBM as a control. We have generated the BE-DBM by incorporating a cell-derived pro-osteogenic extracellular matrix (ECM) within clinical grade DBM. In the present study, we have characterized the BE-DBM and evaluated its ability to induce osteogenic differentiation of human marrow derived stromal cells (HMSCs) with respect to clinical grade commercial DBM. Our results indicate that the BE-DBM contains significantly more pro-osteogenic factors than DBM and enhances HMSC differentiation and mineralized matrix formation in vitro and in vivo. Based on our results, we envision that the BE-DBM has the potential to replace DBM as the bone graft material of choice.

Comparison of the osteogenic potential of OsteoSelect demineralized bone matrix putty to NovaBone calcium-phosphosilicate synthetic putty in a cranial defect model

The purpose of this study was to compare the osteogenic potential of a synthetic and a demineralized bone matrix (DBM) putty using a cranial defect model in New Zealand white rabbits. Paired, bilateral critical-size defects (10 mm) were prepared in the frontal bones of 12 rabbits and filled with either OsteoSelect DBM Putty or NovaBone calcium-phosphosilicate putty. At days 43 and 91, 6 rabbits were killed and examined via semiquantitative histology and quantitative histomorphometry. Defects filled with the DBM putty were histologically associated with less inflammation and fibrous tissue in the defect and more new bone than the synthetic counterpart at both time points. Histomorphometric analysis revealed that the defects filled with DBM putty were associated with significantly more bone formation at day 43 (70.7% vs 40.7%, P = 0.043) and at day 91 (70.4% vs 39.9%, P = 0.0044). The amount of residual implant was similar for both test groups at each time point.

Structure of bone morphogenetic protein 9 procomplex

Bone morphogenetic proteins (BMPs) belong to the TGF-β family, whose 33 members regulate multiple aspects of morphogenesis. TGF-β family members are secreted as procomplexes containing a small growth factor dimer associated with two larger prodomains. As isolated procomplexes, some members are latent, whereas most are active; what determines these differences is unknown. Here, studies on pro-BMP structures and binding to receptors lead to insights into mechanisms that regulate latency in the TGF-β family and into the functions of their highly divergent prodomains. The observed open-armed, nonlatent conformation of pro-BMP9 and pro-BMP7 contrasts with the cross-armed, latent conformation of pro-TGF-β1. Despite markedly different arm orientations in pro-BMP and pro-TGF-β, the arm domain of the prodomain can similarly associate with the growth factor, whereas prodomain elements N- and C-terminal to the arm associate differently with the growth factor and may compete with one another to regulate latency and stepwise displacement by type I and II receptors. Sequence conservation suggests that pro-BMP9 can adopt both cross-armed and open-armed conformations. We propose that interactors in the matrix stabilize a cross-armed pro-BMP conformation and regulate transition between cross-armed, latent and open-armed, nonlatent pro-BMP conformations.

Characterization of bone marrow mononuclear cells on biomaterials for bone tissue engineering in vitro

Bone marrow mononuclear cells (BMCs) are suitable for bone tissue engineering. Comparative data regarding the needs of BMC for the adhesion on biomaterials and biocompatibility to various biomaterials are lacking to a large extent. Therefore, we evaluated whether a surface coating would enhance BMC adhesion and analyze the biocompatibility of three different kinds of biomaterials. BMCs were purified from human bone marrow aspirate samples. Beta tricalcium phosphate (β-TCP, without coating or coated with fibronectin or human plasma), demineralized bone matrix (DBM), and bovine cancellous bone (BS) were assessed. Seeding efficacy on β-TCP was 95% regardless of the surface coating. BMC demonstrated a significantly increased initial adhesion on DBM and β-TCP compared to BS. On day 14, metabolic activity was significantly increased in BMC seeded on DBM in comparison to BMC seeded on BS. Likewise increased VEGF-synthesis was observed on day 2 in BMC seeded on DBM when compared to BMC seeded on BS. The seeding efficacy of BMC on uncoated biomaterials is generally high although there are differences between these biomaterials. Beta-TCP and DBM were similar and both superior to BS, suggesting either as suitable materials for spatial restriction of BMC used for regenerative medicine purposes in vivo.

Functional augmentation of naturally-derived materials for tissue regeneration

Tissue engineering strategies have utilized a wide spectrum of synthetic and naturally-derived scaffold materials. Synthetic scaffolds are better defined and offer the ability to precisely and reproducibly control their properties, while naturally-derived scaffolds typically have inherent biological and structural properties that may facilitate tissue growth and remodeling. More recently, efforts to design optimized biomaterial scaffolds have blurred the line between these two approaches. Naturally-derived scaffolds can be engineered through the manipulation of intrinsic properties of the pre-existing backbone (e.g., structural properties), as well as the addition of controllable functional components (e.g., biological properties). Chemical and physical processing techniques used to modify structural properties of synthetic scaffolds have been tailored and applied to naturally-derived materials. Such strategies include manipulation of mechanical properties, degradation, and porosity. Furthermore, biofunctional augmentation of natural scaffolds via incorporation of exogenous cells, proteins, peptides, or genes has been shown to enhance functional regeneration over endogenous response to the material itself. Moving forward, the regenerative mode of action of naturally-derived materials requires additional investigation. Elucidating such mechanisms will allow for the determination of critical design parameters to further enhance efficacy and capitalize on the full potential of naturally-derived scaffolds.

Growth factors in orthopaedic surgery: demineralized bone matrix versus recombinant bone morphogenetic proteins

During recent decades the utilisation of growth factors, especially BMPs, has received an increasing interest in orthopaedic surgery. For clinical implantation the two main options are demineralised bone matrix (DBM) and recombinant bone morphogenetic proteins (rhBMP). Many clinical studies agree on an equivalent osteoinductive effect between DBM, BMPs and autologous bone graft; however, the different origins and processing of DBM and rhBMP may introduce some fluctuations. Their respective characteristics are reviewed and possible interactions with their effectiveness are analysed. The main difference concerns the concentration of BMPs, which varies to an order of magnitude of 10(6) between DBM and rhBMPs. This may explain the variability in efficiency of some products and the adverse effects. Currently, considering osteoinductive properties, safety and availability, the DBM seems to offer several advantages. However, if DBM and rhBMPs are useful in some indications, their effectiveness and safety can be improved and more evidence-based studies are needed to better define the indications.

Evaluation of a new formulation of demineralized bone matrix putty in a rabbit posterolateral spinal fusion model

BACKGROUND CONTEXT

Alternatives to autologous bone graft (ABG) with osteoconductive, osteoinductive, and osteogenic potential continue to prove elusive. Demineralized bone matrix (DBM) however, with its osteoconductive and osteoinductive potential remains a viable option to ABG in posterolateral spine fusion.

PURPOSE

To compare the efficacy of a new formulation of DBM putty with that of ABG in a rabbit posterolateral spinal fusion model.

STUDY DESIGN

Efficacy of a new formulation of DBM was studied in an experimental animal posterolateral spinal fusion model.

METHODS

Twenty-four male New Zealand White rabbits underwent bilateral posterolateral spine arthrodesis of the L5-L6 intertransverse processes, using either ABG (control group, n=12) or DBM (DBM made from rabbit bone) putty (test group, n=12). The animals were killed 12 weeks after surgery and the lumbar spines were excised. Fusion success was evaluated by manual palpation, high resolution X-rays, microcomputed tomography imaging, biomechanical four-point bending tests, and histology.

RESULTS

Two animals were lost because of anesthetic related issues. Manual palpation to assess fusion success in the explanted lumbar spines showed no statistical significant difference in successful fusion in 81.8% (9/11) of DBM group and 72.7% (8/11) of ABG group (p=.99). Reliability of these assessments was measured between three independent observers and found near perfect agreement (intraclass correlation cofficient: 0.92 and 0.94, respectively). Fusion using high resolution X-rays was solid in 10 of the DBM group and 9 of the ABG group (p=.59). Biomechanical testing showed no significant difference in stiffness between the control and test groups on flexion, extension, and left lateral and right lateral bends, with p values accounting for .79, .42, .75, and .52, respectively. The bone volume/total volume was greater than 85% in the DBM treated fusion masses. Histologic evaluation revealed endochondral ossification in both groups, but the fusion masses were more mature in the DBM group.

CONCLUSIONS

The DBM putty achieved comparable fusion rates to ABG in the rabbit posterolateral spinal fusion model.

Demineralized bone matrix and platelet-rich plasma do not improve healing of osteochondral defects of the talus: an experimental goat study

OBJECTIVE

The purpose of this study was to evaluate the effectiveness of demineralized bone matrix (DBM) with and without platelet-rich plasma (PRP) in the treatment of osteochondral defects (OCDs) of the talus. We hypothesized that treatment with DBM would result in more bone formation than no treatment in control OCDs, and that PRP would further enhance the regenerative capacity of DBM.

METHOD

A standardized 6-mm OCD was created in each talus of 16 adult goats. According to a randomization scheme, one OCD of each goat was treated with allogeneic DBM hydrated with normal saline (n = 8) or hydrated with autologous PRP (n = 8). The contralateral OCD (n = 16) served as control. After 24 weeks, the animals were euthanized and the tali excised. Various outcome parameters were analyzed with use of macroscopic evaluation, micro-computed tomography (μCT), histology, histomorphometry, and fluorescence microscopy.

RESULTS

None of the analyses revealed statistically significant differences between the groups for any of the parameters analyzed in any volume of interest. For example, the mean bone volume fraction (BV/TV) of the defect, as measured by μCT, was 0.56 (95% confidence interval [CI], 0.44-0.68) for DBM hydrated with normal saline and 0.52 (95% CI, 0.40-0.65) for DBM hydrated with PRP, compared to 0.53 (95% CI, 0.45-0.61) and 0.54 (95% CI, 0.44-0.64) for the internal controls, respectively (P > 0.05).

CONCLUSION

In contrast to our hypotheses, no beneficial treatment effect of DBM with or without PRP was found for OCDs of the caprine talus.

Demineralized bone matrix in bone repair: history and use

Demineralized bone matrix (DBM) is an osteoconductive and osteoinductive commercial biomaterial and approved medical device used in bone defects with a long track record of clinical use in diverse forms. True to its name and as an acid-extracted organic matrix from human bone sources, DBM retains much of the proteinaceous components native to bone, with small amounts of calcium-based solids, inorganic phosphates and some trace cell debris. Many of DBM's proteinaceous components (e.g., growth factors) are known to be potent osteogenic agents. Commercially sourced as putty, paste, sheets and flexible pieces, DBM provides a degradable matrix facilitating endogenous release of these compounds to the bone wound sites where it is surgically placed to fill bone defects, inducing new bone formation and accelerating healing. Given DBM's long clinical track record and commercial accessibility in standard forms and sources, opportunities to further develop and validate DBM as a versatile bone biomaterial in orthopedic repair and regenerative medicine contexts are attractive.

Demineralized bone matrix as a vehicle for delivering endogenous and exogenous therapeutics in bone repair

As a unique human bone extract approved for implant use, demineralized bone matrix (DBM) retains substantial amounts of endogenous osteoconductive and osteoinductive proteins. Commercial preparations of DBM represent a clinically accessible, familiar, widely used and degradable bone-filling device, available in composite solid, strip/piece, and semi-solid paste forms. Surgically placed and/or injected, DBM releases its constituent compounds to bone sites with some evidence for inducing new bone formation and accelerating healing. Significantly, DBM also has preclinical history as a drug carrier by direct loading and delivery of several important classes of therapeutics. Exogenous bioactive agents, including small molecule drugs, protein and peptide drugs, nucleic acid drugs and transgenes and therapeutic cells have been formulated within DBM and released to bone sites to enhance DBM's intrinsic biological activity. Local release of these agents from DBM directly to surgical sites in bone provides improved control of dosing and targeting of both endogenous and exogenous bioactivity in the context of bone healing using a clinically familiar product. Given DBM's long clinical track record and commercial accessibility in standard forms and sources, opportunities to formulate DBM as a versatile matrix to deliver therapeutic agents locally to bone sites in orthopedic repair and regenerative medicine contexts are attractive.

Viscoelastic properties of human cortical bone tissue depend on gender and elastic modulus

Bone exhibits rate-dependent failure behavior, suggesting that viscoelasticity is a factor in the damage and fracture of bone. Microdamage initiates at scales below the macroscopic porosity in bone, and, as such, is affected by the intrinsic viscoelasticity of the bone tissue. The viscoelasticity of the bone tissue can be measured by nanoindentation and recording the creep behavior at constant load. The viscoelastic properties have been used to assess differences in tissue behavior with respect to fracture healing, aging, and mouse strains. In this study, we compared the viscoelastic behavior of human cortical bone between genders by using nanoindentation at a fixed load of 10 mN to measure the creep time constant. Bones from females had a significantly greater time constant, indicating slower creep and relaxation, than bones from males. The creep time constants decreased with increasing tissue modulus. The mineralization, collagen content, and collagen cross-link density, which were bulk measurements, were analyzed to determine if the differences in viscoelastic behavior were explained by compositional differences in the bone. However, none of the parameters differed between genders, nor were they correlated to the viscoelastic time constant. As such, the difference must depend on other matrix proteins that we did not assess or differences in the microstructural organization. This is one of the only intrinsic bone material properties that has been found to differ between males and females, and it may be important for assessing differences in fracture risk, since crack propagation is generally sensitive to viscoelastic properties.

The in vitro elution of BMP-7 from demineralized bone matrix

Demineralized bone matrix (DBM) grafts induce new bone formation by locally releasing matrix-associated growth factors, such as bone morphogenetic proteins (BMPs), to the surrounding tissue after implantation. However, the release kinetics of BMPs from DBM lack characterization. Such information can potentially help to improve processing techniques to maximize graft osteoinductive potential, as well as increase understanding of the osteoinductive process itself. We produced DBM with three particle size ranges from bovine cortical bone, i.e., <106, 106-300, and 300-710 μm and extracted 1.5 g of each size range in 40 ml of Sorensen's buffer at room temperature for up to 168 h. The BMP-7 concentration of the DBM and the buffer were measured at each time point using enzyme-linked immunosorbant assay. Based on measurement of the concentration of BMP-7 in the buffer, the 0-8 h elution rate was high, i.e., 3.3, 2.9, and 2.2 ng BMP-7/g DBM h, and for the 8-168 h interval was much lower, at 0.039, 0.15, and 0.11 ng BMP-7/g DBM h for the three size ranges, respectively. By 168 h, there was no indication that elution was nearing completion. Measurement of the residual BMP-7 remaining in the DBM as a function of time yielded unexpected results, i.e., after the BMP-7 content of the DBM declined for the first 4-6 h, it paradoxically increased for the remaining interval. We propose a two-compartment model to help explain these results in terms of the possible distribution of BMP-7 in bone matrix.

Nell-1 protein promotes bone formation in a sheep spinal fusion model

Bone morphogenetic proteins (BMPs) are widely used as bone graft substitutes in spinal fusion, but are associated with numerous adverse effects. The growth factor Nel-like molecule-1 (Nell-1) is mechanistically distinct from BMPs and can minimize complications associated with BMP therapies. This study evaluates the efficacy of Nell-1 combined with demineralized bone matrix (DBM) as a novel bone graft material for interbody spine fusion using sheep, a phylogenetically advanced animal with biomechanical similarities to human spine. Nell-1+sheep DBM or Nell-1+heat-inactivated DBM (inDBM) (to determine the osteogenic effect of residual growth factors in DBM) were implanted in surgical sites as follows: (1) DBM only (control) (n=8); (2) DBM+0.3 mg/mL Nell-1 (n=8); (3) DBM+0.6 mg/mL Nell-1 (n=8); (4) inDBM only (control) (n=4); (5) inDBM+0.3 mg/mL Nell-1 (n=4); (6) inDBM+0.6 mg/mL Nell-1 (n=4). Fusion was assessed by computed tomography, microcomputed tomography, and histology. One hundred percent fusion was achieved by 3 months in the DBM+0.6 mg/mL Nell-1 group and by 4 months in the inDBM+0.6 mg/mL Nell-1 group; bone volume and mineral density were increased by 58% and 47%, respectively. These fusion rates are comparable to published reports on BMP-2 or autograft bone efficacy in sheep. Nell-1 is an independently potent osteogenic molecule that is efficacious and easily applied when combined with DBM.

Challenges to bone formation in spinal fusion

Spinal arthrodesis continues to expand in clinical indications and surgical practice. Despite a century of study, failure of bone formation or pseudarthrosis can occur in individual patients with debilitating clinical symptoms. Here we review biological and technical aspects of spinal fusion under active investigation, describe relevant biomechanics in health and disease, and identify the possibilities and limitations of translational animal models. The purpose of this article is to foster collaborative efforts with researchers who model bone hierarchy. The induction of heterotopic osteosynthesis requires a complex balance of biologic factors and operative technique to achieve successful fusion. Anatomical considerations of each spinal region including blood supply, osteology, and biomechanics predispose a fusion site to robust or insufficient bone formation. Careful preparation of the fusion site and appropriate selection of graft materials remains critical but is sometimes guided by conflicting evidence from the long-bone literature. Modern techniques of graft site preparation and instrumentation have evolved for every segment of the vertebral column. Despite validated biomechanical studies of modern instrumentation, a correlation with superior clinical outcomes is difficult to demonstrate. In many cases, adjuvant biologic therapies with allograft and synthetic cages have been used successfully to reproduce the enhancement of fusion rates observed with cancellous and tricortical autograft. Current areas of investigation comprise materials science, stem cell therapies, recombinant growth factors, scaffolds and biologic delivery systems, and minimally invasive surgical techniques to optimize the biologic response to intervention. Diverse animal models are required to approach the breadth of spinal pathology and novel therapeutics.