Long Bone Defect Models for Tissue Engineering Applications: Criteria for Choice

Cell sheet-basedin vitrobone defect model for long term evaluation of bone repair materials

Development of tissue-engineeredin vitrohuman bone defect models for evaluation of bone repair materials (BRMs) is a promising approach for addressing both translational and ethical concerns regarding animal models. In this study, human bone marrow mesenchymal stem cell sheets were stacked to form a periosteum like tissue. HE staining showed a cell-dense, multilayered structure. BRMs were implanted in the defect area of the three-dimensional (3D) model. The CCK-8 test demonstrated that the 3D model was stronger in resisting the cytotoxicity of three kinds of commercial BRMs than the 2D culture model, which was consistent within vivoresults. After 28 d implantation in the 3D model, western blot and RT-qPCR showed that three materials induced increased expressions of RUNX2, OSX, OCN, OPN, while Materials B and C seemed to have stronger osteoinductivity than A.In vivoexperiments also confirmed the osteoinductivity of the BRMs after 28 and 182 d implantation. Alizarin red staining proved that the mineralized nodules of Materials B and C were more than that of A. The differences of osteogenic properties among three BMRs might be attributed to calcium ion release. This cell sheet-based bone tissue model can resist cytotoxicity of BRMs, demonstrating the priority of long-term evaluation of osteoinductivity of BRMs. Further, the osteoinduction results of the 3D model corresponded to that ofin vivoexperiments, suggesting this model may have a potential to be used as a novel tool for rapid, accurate evaluation of BRMs, and thus shorten their research and development process.

Electric/Magnetic Intervention for Bone Regeneration: A Systematic Review and Network Meta-Analysis

Electric/magnetic material or field is a promising strategy for bone regeneration. The aim of this systematic review and network meta-analysis was to analyze the evidence regarding the efficacy of electric and magnetic intervention for bone regeneration and provide directions for further research. A comprehensive search was performed to identify the rats/rabbits/mice research that involved the electric/magnetic treatment with quantitative radiographic assessment of bone formation. Network meta-analyses were also conducted to assess different interventions and outcomes for osteogenesis. In total, there were 51 articles included in the systematic review and 19 articles in the network meta-analyses. The majority used microcomputerized tomography bone volume/tissue volume (BV/TV) to evaluate outcomes in rats. Results showed that placing electric/magnetic materials in situ had more prominent effects than the electric/magnetic field on bone regeneration. For all species, electrical materials with zeta potential of -53 mV proved to be the most effective in increasing BV (mean difference [MD]: 4.20 mm³, 95% confidence interval [CI]: [1.72-6.68]) and bone mineral density (MD: 312 mg/cm³, 95% CI: [172.43-451.57]). Magnetic materials with external magnetic fields topped in BV/TV (MD: 43%, 95% CI: [36.04-49.96]). It also led in trabecular number (MD: 2.00 mm^-1, 95% CI: [1.45-2.55]), trabecular thickness (MD: 61.00 μm, 95% CI: [44.31- 77.69]), and trabecular separation (MD: -0.40 mm, 95% CI: [-0.56 to -0.24]) on the condition of lacking electric materials. Biomaterials implantation is the most effective method for stimulating osteogenesis in rats, especially in electrical materials with negative charge. The combination of diverse interventions shows promising effects but needs further research, so does the underlying mechanism. Impact Statement Bone defect, especially for the large defect from aging, trauma, or pathology, which cannot be completely healed, remains a clinical challenge. Mimicking physical microenvironment has emerged as a new strategy for tissue regeneration. Electric and magnetic material and field used as the physical stimulation for bone regeneration have attracted interest due to their potential and facile application in clinic. This article reviewed related animal studies and carried out a network meta-analysis to thoroughly understand how electric and magnetic interventions impacted on tissues and created an osteogenic microenvironment.

Angle-stable interlocking nailing in a canine critical-sized femoral defect model for bone regeneration studies: In pursuit of the principle of the 3R’s

Introduction: Critical-sized long bone defects represent a major therapeutic challenge and current treatment strategies are not without complication. Tissue engineering holds much promise for these debilitating injuries; however, these strategies often fail to successfully translate from rodent studies to the clinical setting. The dog represents a strong model for translational orthopedic studies, however such studies should be optimized in pursuit of the Principle of the 3R’s of animal research (replace, reduce, refine). The objective of this study was to refine a canine critical-sized femoral defect model using an angle-stable interlocking nail (AS-ILN) and reduce total animal numbers by performing imaging, biomechanics, and histology on the same cohort of dogs.

Methods: Six skeletally mature hounds underwent a 4 cm mid-diaphyseal femoral ostectomy followed by stabilization with an AS-ILN. Dogs were assigned to autograft (n = 3) or negative control (n = 3) treatment groups. At 6, 12, and 18 weeks, healing was quantified by ordinal radiographic scoring and quantified CT. After euthanasia, femurs from the autograft group were mechanically evaluated using an established torsional loading protocol. Femurs were subsequently assessed histologically.

Results: Surgery was performed without complication and the AS-ILN provided appropriate fixation for the duration of the study. Dogs assigned to the autograft group achieved radiographic union by 12 weeks, whereas the negative control group experienced non-union. At 18 weeks, median bone and soft tissue callus volume were 9,001 mm³ (range: 4,939–10,061) for the autograft group and 3,469 mm³ (range: 3,085–3,854) for the negative control group. Median torsional stiffness for the operated, autograft treatment group was 0.19 Nm/° (range: 0.19–1.67) and torque at failure was 12.0 Nm (range: 1.7–14.0). Histologically, callus formation and associated endochondral ossification were identified in the autograft treatment group, whereas fibrovascular tissue occupied the critical-sized defect in negative controls.

Conclusion: In a canine critical-sized defect model, the AS-ILN and described outcome measures allowed refinement and reduction consistent with the Principle of the 3R’s of ethical animal research. This model is well-suited for future canine translational bone tissue engineering studies.

Preparation of gamma poly-glutamic acid/hydroxyapatite/collagen composite as the 3D-printing scaffold for bone tissue engineering

Background

All types of movements involve the role of articular cartilage and bones. The presence of cartilage enables bones to move over one another smoothly. However, repetitive microtrauma and ischemia as well as genetic effects can cause an osteochondral lesion. Numerous treatment methods such as microfracture surgergy, autograft, and allograft, have been used, however, it possesses treatment challenges including prolonged recovery time after surgery and poses a financial burden on patients. Nowadays, various tissue engineering approaches have been developed to repair bone and osteochondral defects using biomaterial implants to induce the regeneration of stem cells. 

Methods

In this study, a collagen (Col)/γ-polyglutamate acid (PGA)/hydroxyapatite (HA) composite scaffold was fabricated using a 3D printing technique. A Col/γ-PGA/HA 2D membrane was also fabricated for comparison. The scaffolds (four layers) were designed with the size of 8 mm in diameter and 1.2 mm in thickness. The first layer was HA/γ-PGA and the second to fourth layers were Col/γ-PGA. In addition, a 2D membrane was constructed from hydroxyapatite/γ-PGA and collagen/γ-PGA with a ratio of 1:3. The biocompatibility property and degradation activity were investigated for both scaffold and membrane samples. Rat bone marrow mesenchymal stem cells (rBMSCs) and human adipose-derived stem cells (hADSCs) were cultured on the samples and were tested in-vitro to evaluate cell attachment, proliferation, and differentiation. In-vivo experiments were performed in the rat and nude mice models.

Results

In-vitro and in-vivo results show that the developed scaffold is of well biodegradation and biocompatible properties, and the Col-HA scaffold enhances the mechanical properties for osteochondrogenesis in both in-vitro and animal trials.

Conclusions

The composite would be a great biomaterial application for bone and osteochondral regeneration.

Novel Techniques and Future Perspective for Investigating Critical-Size Bone Defects

A critical-size bone defect is a challenging clinical problem in which a gap between bone ends will not heal and will become a nonunion. The current treatment is to harvest and transplant an autologous bone graft to facilitate bone bridging. To develop less invasive but equally effective treatment options, one needs to first have a comprehensive understanding of the bone healing process. Therefore, it is imperative to leverage the most advanced technologies to elucidate the fundamental concepts of the bone healing process and develop innovative therapeutic strategies to bridge the nonunion gap. In this review, we first discuss the current animal models to study critical-size bone defects. Then, we focus on four novel analytic techniques and discuss their strengths and limitations. These four technologies are mass cytometry (CyTOF) for enhanced cellular analysis, imaging mass cytometry (IMC) for enhanced tissue special imaging, single-cell RNA sequencing (scRNA-seq) for detailed transcriptome analysis, and Luminex assays for comprehensive protein secretome analysis. With this new understanding of the healing of critical-size bone defects, novel methods of diagnosis and treatment will emerge.

Does zinc oxide nanoparticles potentiate the regenerative effect of platelet-rich fibrin in healing of critical bone defect in rabbits?

Background

Many encouraging studies confirmed the ability of Zinc Oxide Nanoparticles (ZnONPs) in accelerating bone growth and mineralization. The use of Platelet Rich-Fibrin (PRF) as a sole filling material for large segmental bone defects remains questionable. The objectives are to investigate the regenerative efficacy of autologous Platelet Rich-Fibrin (PRF) and Zinc Oxide Nanoparticles (ZnONPs) in repairing large segmental bone ulnar defects in a randomized controlled study in rabbits using computed tomographic interpretations. A 12 mm critical size defect was surgically induced in the ulna of 30 rabbits (n = 10/ group). In the control group, the defect was left empty. In the PRF group, the defect is filled with PRF. In the PRF/ZnONPs group, the defect is filled with PRF that was inoculated with 0.1 ml of 0.2% ZnONPs. Radiologic healing capacity was evaluated at the first, second, and third postoperative months.

Results

Statistical analysis showed significant differences in the radiologic healing scores between the groups (P = 0.000–0.0001) at all-time points (P = 0.000–0.047) during the study.

Conclusion

Rabbits in the PRF/ZnONPs group showed the highest appreciable bone quality and quantity followed by the PRF group with high quantity but low bone quality meanwhile, rabbits in the control group showed minimal quantity but medium bone quality. Interestingly, the addition of ZnONPs to PRF can accelerate the healing of ulnar critical-size defects in rabbits.

Effects of Channels and Micropores in Honeycomb Scaffolds on the Reconstruction of Segmental Bone Defects

The reconstruction of critical-sized segmental bone defects is a key challenge in orthopedics because of its intractability despite technological advancements. To overcome this challenge, scaffolds that promote rapid bone ingrowth and subsequent bone replacement are necessary. In this study, we fabricated three types of carbonate apatite honeycomb (HC) scaffolds with uniaxial channels bridging the stumps of a host bone. These HC scaffolds possessed different channel and micropore volumes. The HC scaffolds were implanted into the defects of rabbit ulnar shafts to evaluate the effects of channels and micropores on bone reconstruction. Four weeks postoperatively, the HC scaffolds with a larger channel volume promoted bone ingrowth compared to that with a larger micropore volume. In contrast, 12 weeks postoperatively, the HC scaffolds with a larger volume of the micropores rather than the channels promoted the scaffold resorption by osteoclasts and bone formation. Thus, the channels affected bone ingrowth in the early stage, and micropores affected scaffold resorption and bone formation in the middle stage. Furthermore, 12 weeks postoperatively, the HC scaffolds with large volumes of both channels and micropores formed a significantly larger amount of new bone than that attained using HC scaffolds with either large volume of channels or micropores, thereby bridging the host bone stumps. The findings of this study provide guidance for designing the pore structure of scaffolds.

Schwann Cells Accelerate Osteogenesis via the Mif/CD74/FOXO1 Signaling Pathway In Vitro

Schwann cells have been found to promote osteogenesis by an unclear molecular mechanism. To better understand how Schwann cells accelerate osteogenesis, RNA-Seq and LC-MS/MS were utilized to explore the transcriptomic and metabolic response of MC3T3-E1 to Schwann cells. Osteogenic differentiation was determined by ALP staining. Lentiviruses were constructed to alter the expression of Mif (macrophage migration inhibitory factor) in Schwann cells. Western blot (WB) analysis was employed to detect the protein expression. The results of this study show that Mif is essential for Schwann cells to promote osteogenesis, and its downstream CD74/FOXO1 is also involved in the promotion of Schwann cells on osteogenesis. Further, Schwann cells regulate amino acid metabolism and lipid metabolism in preosteoblasts. These findings unveil the mechanism for Schwann cells to promote osteogenesis where Mif is a key factor.

Conductive Scaffolds for Bone Tissue Engineering: Current State and Future Outlook

Bone tissue engineering strategies attempt to regenerate bone tissue lost due to injury or disease. Three-dimensional (3D) scaffolds maintain structural integrity and provide support, while improving tissue regeneration through amplified cellular responses between implanted materials and native tissues. Through this, scaffolds that show great osteoinductive abilities as well as desirable mechanical properties have been studied. Recently, scaffolding for engineered bone-like tissues have evolved with the use of conductive materials for increased scaffold bioactivity. These materials make use of several characteristics that have been shown to be useful in tissue engineering applications and combine them in the hope of improved cellular responses through stimulation (i.e., mechanical or electrical). With the addition of conductive materials, these bioactive synthetic bone substitutes could result in improved regeneration outcomes by reducing current factors limiting the effectiveness of existing scaffolding materials. This review seeks to overview the challenges associated with the current state of bone tissue engineering, the need to produce new grafting substitutes, and the promising future that conductive materials present towards alleviating the issues associated with bone repair and regeneration.

In Vivo Efficacy of Neutrophil-Mediated Bone Regeneration Using a Rabbit Calvarial Defect Model

Reconstruction of bone due to surgical removal or disease-related bony defects is a clinical challenge. It is known that the immune system exerts positive immunomodulatory effects on tissue repair and regeneration. In this study, we evaluated the in vivo efficacy of autologous neutrophils on bone regeneration using a rabbit calvarial defect model. Methods: Twelve rabbits, each with two surgically created calvarial bone defects (10 mm diameter), were randomly divided into two groups; (i) single application of neutrophils (SA-NP) vs. SA-NP control, and (ii) repetitive application of neutrophils (RA-NP) vs. RA-NP control. The animals were euthanized at 4 and 8 weeks post-operatively and the treatment outcomes were evaluated by micro-computed tomography, histology, and histomorphometric analyses. Results: The micro-CT analysis showed a significantly higher bone volume fraction (bone volume/total volume) in the neutrophil-treated groups, i.e., median interquartile range (IQR) SA-NP (18) and RA-NP (24), compared with the untreated controls, i.e., SA-NP (7) and RA-NP (14) at 4 weeks (p < 0.05). Similarly, new bone area fraction (bone area/total area) was significantly higher in neutrophil-treated groups at 4 weeks (p < 0.05). Both SA-NP and RA-NP had a considerably higher bone volume and bone area at 8 weeks, although the difference was not statistically significant. In addition, immunohistochemical analysis at 8 weeks revealed a higher expression of osteocalcin in both SA-NP and RA-NP groups. Conclusions: The present study provides first hand evidence that autologous neutrophils may have a positive effect on promoting new bone formation. Future studies should be performed with a larger sample size in non-human primate models. If proven feasible, this new promising strategy could bring clinical benefits for bone defects to the field of oral and maxillofacial surgery.

Biodegradable metals for bone defect repair: A systematic review and meta-analysis based on animal studies

Biodegradable metals are promising candidates for bone defect repair. With an evidence-based approach, this study investigated and analyzed the performance and degradation properties of biodegradable metals in animal models for bone defect repair to explore their potential clinical translation. Animal studies on bone defect repair with biodegradable metals in comparison with other traditional biomaterials were reviewed. Data was carefully collected after identification of population, intervention, comparison, outcome, and study design (PICOS), and following the inclusion criteria of biodegradable metals in animal studies. 30 publications on pure Mg, Mg alloys, pure Zn and Zn alloys were finally included after extraction from a collected database of 2543 publications. A qualitative systematic review and a quantitative meta-analysis were performed. Given the heterogeneity in animal model, anatomical site and critical size defect (CSD), biodegradable metals exhibited mixed effects on bone defect repair and degradation in animal studies in comparison with traditional non-degradable metals, biodegradable polymers, bioceramics, and autogenous bone grafts. The results indicated that there were limitations in the experimental design of the included studies, and quality of the evidence presented by the studies was very low. To enhance clinical translation of biodegradable metals, evidence-based research with data validity is needed. Future studies should adopt standardized experimental protocols in investigating the effects of biodegradable metals on bone defect repair with animal models.

In vivo study on the healing of bone defect treated with non-thermal atmospheric pressure gas discharge plasma

Medical treatment using non-thermal atmospheric pressure plasma (NTAPP) is rapidly gaining recognition. NTAPP is thought to be a new therapeutic method because it could generate highly reactive species in an ambient atmosphere which could be exposed to biological targets (e.g., cells and tissues). If plasma-generated reactive species could stimulate bone regeneration, NTAPP can provide a new treatment opportunity in regenerative medicine. Here, we investigated the impact of NTAPP on bone regeneration using a large bone defect in New Zealand White rabbits and a simple atmospheric pressure plasma (helium microplasma jet). We observed the recovery progress of the large bone defects by X-ray imaging over eight weeks after surgery. The X-ray results showed a clear difference in the occupancy of the new bone of the large bone defect among groups with different plasma treatment times, whereas the new bone occupancy was not substantial in the untreated control group. According to the results of micro-computed tomography analysis at eight weeks, the most successful bone regeneration was achieved using a plasma treatment time of 10 min, wherein the new bone volume was 1.51 times larger than that in the plasma untreated control group. Using H&E and Masson trichrome stains, nucleated cells were uniformly observed, and no inclusion was confirmed, respectively, in the groups of plasma treatment. We concluded the critical large bone defect were filled with new bone. Overall, these results suggest that NTAPP is promising for fracture treatment.

Honeycomb Scaffold-Guided Bone Reconstruction of Critical-Sized Defects in Rabbit Ulnar Shafts

Reconstruction of critical-sized defects (CSDs) in bone shafts remains a major challenge in orthopedics. Honeycomb (HC) scaffolds are considered promising as their uniaxial channels bridge the amputation stumps of bones and promote the ingrowth of bone and blood vessels (BV) into the scaffolds. In this study, the ability of the HC scaffolds, composed of the bone mineral or carbonate apatite (CAp), was evaluated by reconstructing 10, 15, and 20 mm segmental defects in the rabbit ulnar shaft. Radiographic and μ-computed tomography evaluations showed that bony calluses were formed around the scaffolds at 4 weeks post-surgery in all defects, whereas no callus bridged in the ulna without scaffolds. At 12 weeks post-surgery, the scaffolds were connected to the host bone in 10 and 15 mm defects, while a slight gap remained between the scaffold and host bone in the 20 mm defect. New bone formation and scaffold resorption progressed over 12 weeks. Histological evaluations showed that mature bones (MB) and BV were already formed at the edges of the scaffolds at 4 weeks post-surgery in 10, 15, and 20 mm defects. In the central region of the scaffold, in the 10 mm defect, MB and BV were formed at 4 weeks post-surgery. In the 15 mm defect, although BV were formed, a few MB were formed. It is concluded that CAp HC scaffolds have good potential value for the reconstruction of CSDs.

Musculoskeletal tissue engineering: Regional gene therapy for bone repair

Bone loss associated with fracture nonunion, revision total joint arthroplasty (TJA), and pseudoarthrosis of the spine presents a challenging clinical scenario for the orthopaedic surgeon. Current treatment options including autograft, allograft, bone graft substitutes, and bone transport techniques are associated with significant morbidity, high costs, and prolonged treatment regimens. Unfortunately, these treatment strategies have proven insufficient to safely and consistently heal bone defects in the stringent biological environments often encountered in clinical cases of bone loss. The application of tissue engineering (TE) to musculoskeletal pathology has uncovered exciting potential treatment strategies for challenging bone loss scenarios in orthopaedic surgery. Regional gene therapy involves the local implantation of nucleic acids or genetically modified cells to direct specific protein expression, and has shown promise as a potential TE technique for the regeneration of bone. Preclinical studies in animal models have demonstrated the ability of regional gene therapy to safely and effectively heal critical sized bone defects which otherwise do not heal. The purpose of the present review is to provide a comprehensive overview of the current status of gene therapy applications for TE in challenging bone loss scenarios, with an emphasis on gene delivery methods and models, scaffold biomaterials, preclinical results, and future directions.

Spatio-temporal characterization of fracture healing patterns and assessment of biomaterials by time-lapsed in vivo micro-computed tomography

Thorough preclinical evaluation of functionalized biomaterials for treatment of large bone defects is essential prior to clinical application. Using in vivo micro-computed tomography (micro-CT) and mouse femoral defect models with different defect sizes, we were able to detect spatio-temporal healing patterns indicative of physiological and impaired healing in three defect sub-volumes and the adjacent cortex. The time-lapsed in vivo micro-CT-based approach was then applied to evaluate the bone regeneration potential of functionalized biomaterials using collagen and bone morphogenetic protein (BMP-2). Both collagen and BMP-2 treatment led to distinct changes in bone turnover in the different healing phases. Despite increased periosteal bone formation, 87.5% of the defects treated with collagen scaffolds resulted in non-unions. Additional BMP-2 application significantly accelerated the healing process and increased the union rate to 100%. This study further shows potential of time-lapsed in vivo micro-CT for capturing spatio-temporal deviations preceding non-union formation and how this can be prevented by application of functionalized biomaterials. This study therefore supports the application of longitudinal in vivo micro-CT for discrimination of normal and disturbed healing patterns and for the spatio-temporal characterization of the bone regeneration capacity of functionalized biomaterials.

Resveratrol and Angiogenin-2 Combined With PEGDA/TCS Hydrogel for the Targeted Therapy of Hypoxic Bone Defects via Activation of the Autophagy Pathway

The guarantee of cell survival under hypoxic conditions and rapid vascularization is a key in tissue engineering strategies for treating bone defects. Our study aimed to establish the protective role of bone marrow mesenchymal stem cells (BMSCs) and human umbilical vein endothelial cells (HUVECs) in hypoxic conditions and realize rapid vascularization in bone defects. Resveratrol (Res), a non-flavonoid polyphenolic compound, and angiopoietin-2 (ANG2), a vascular activating factor, were applied to enhance BMSC and HUVEC survival, osteogenesis, and angiogenesis. The morphology, autophagy, viability, apoptosis, cycle, and osteogenic differentiation of BMSCs treated with Res were analyzed. The results indicated that Res could improve BMSC survival and differentiation via the autophagy pathway under hypoxic conditions. In addition, Res maintained HUVEC growth and proliferation in a hypoxic and ANG2 double-adverse environment via the autophagy pathway. To simulate a relatively hypoxic environment, small-aperture PEGDA/TCS hydrogels containing Res and ANG2 were prepared. BMSCs were cultured in the PEGDA/TCS scaffold and transplanted into a large tibial defect. CD31 immunofluorescence showed that the density and size of new blood vessels in the bone defect were significantly enhanced by ANG2 and Res at 8 weeks after surgery. H&E, Masson, and immunohistochemical staining results indicated that ANG2 combined with Res could promote new bone formation in defects. All these results suggested that Res combined with ANG2 may be a novel strategy for the targeted therapy of hypoxic bone defects with tissue engineering scaffolds.

Time-lapsed imaging of nanocomposite scaffolds reveals increased bone formation in dynamic compression bioreactors

Progress in bone scaffold development relies on cost-intensive and hardly scalable animal studies. In contrast to in vivo, in vitro studies are often conducted in the absence of dynamic compression. Here, we present an in vitro dynamic compression bioreactor approach to monitor bone formation in scaffolds under cyclic loading. A biopolymer was processed into mechanically competent bone scaffolds that incorporate a high-volume content of ultrasonically treated hydroxyapatite or a mixture with barium titanate nanoparticles. After seeding with human bone marrow stromal cells, time-lapsed imaging of scaffolds in bioreactors revealed increased bone formation in hydroxyapatite scaffolds under cyclic loading. This stimulatory effect was even more pronounced in scaffolds containing a mixture of barium titanate and hydroxyapatite and corroborated by immunohistological staining. Therefore, by combining mechanical loading and time-lapsed imaging, this in vitro bioreactor strategy may potentially accelerate development of engineered bone scaffolds and reduce the use of animals for experimentation.

Schädli et al. present a bioreactor system that combines mechanical loading with longitudinal microCT imaging to assess bone mineralization in a poly(lactic-co-glycolic acid) (PLGA) scaffold reinforced with nanoparticles. This approach allows rapid and rigorous evaluation of engineered bone scaffolds performance in vitro and might reduce the use of animals for experimentation.

A systematic review of preclinical in vivo testing of 3D printed porous Ti6Al4V for orthopedic applications, part I: Animal models and bone ingrowth outcome measures

For Ti6Al4V orthopedic and spinal implants, osseointegration is often achieved using complex porous geometries created via additive manufacturing (AM). While AM porous titanium (pTi) has shown clinical success, concerns regarding metallic implants have spurred interest in alternative AM biomaterials for osseointegration. Insights regarding the evaluation of these new materials may be supported by better understanding the role of preclinical testing for AM pTi. We therefore asked: (a) What animal models have been most commonly used to evaluate AM porous Ti6Al4V for orthopedic bone ingrowth; (b) What were the primary reported quantitative outcome measures for these models; and (c) What were the bone ingrowth outcomes associated with the most frequently used models? We performed a systematic literature search and identified 58 articles meeting our inclusion criteria. We found that AM pTi was evaluated most often using rabbit and sheep femoral condyle defect (FCD) models. Additional ingrowth models including transcortical and segmental defects, spinal fusions, and calvarial defects were also used with various animals based on the study goals. Quantitative outcome measures determined via histomorphometry including ''bone ingrowth'' (range: 3.92-53.4% for rabbit/sheep FCD) and bone-implant contact (range: 9.9-59.7% for rabbit/sheep FCD) were the most common. Studies also used 3D imaging to report outcomes such as bone volume fraction (BV/TV, range: 4.4-61.1% for rabbit/sheep FCD), and push-out testing for outcomes such as maximum removal force (range: 46.6-3092 N for rabbit/sheep FCD). Though there were many commonalities among the study methods, we also found significant heterogeneity in the outcome terms and definitions. The considerable diversity in testing and reporting may no longer be necessary considering the reported success of AM pTi across all model types and the ample literature supporting the rabbit and sheep as suitable small and large animal models, respectively. Ultimately, more standardized animal models and reporting of bone ingrowth for porous AM materials will be useful for future studies.

Preclinical studies on pleiotropic functions of erythropoietin on bone healing

Erythropoietin (ЕPО) is a glycoprotein hormone, mainly known for its haemopoietic function. For orthopaedics, its pleiotropic effects – osteogenic and angiogenic potential, are of primary interest. The exact mechanism of EPO action is still unclear. The effects of EPO on bone healing were investigated through experiments with rats, mice, rabbits and pigs. Each of used models for experimental bone defects (calvarial models, long bone segmental defects, posterolateral spinal fusion and corticosteroid-induced femoral head osteonecrosis) has specific advantages and flaws. Obtaining specific and correct results is largely dependent on the used model. The brief evaluation of models could serve for standardisation of preclinical studies on bone regeneration.

Bone regenerative medicine: metatarsus defects in sheep to evaluate new therapeutic strategies for human long bone defect. A systematic review

INTRODUCTION

Large bone defects in long bone are not able to repair themselves and require grafts. Although autograft is the gold standard, it is associated with some disadvantages. Consequently, the application of tissue engineering (TE) techniques help with the use of allogenic biological and artificial scaffolds, cells and growth factors (GFs). Following 3Rs and in vitro testing strategies, animal models are required in preclinical in vivo studies to evaluate the therapeutic effects of the most promising TE techniques.

MATERIALS AND METHODS

A systematic review was performed from 2000 to 2019 to evaluate bone regeneration sheep metatarsus defects.

RESULTS

Eleven in vivo studies on sheep metatarsus defect were retrieved. The mid-diaphysis of metatarsus was the region most employed to perform critical size defects. Natural, synthetic and hybrid scaffolds were implanted, combined with bone marrow mesenchymal stem cells (BMSCs), GFs such as osteogenic protein 1 (OP1) and platelet rich plasma (PRP). The maximum follow-up period was 4 and 6 months in which radiography, histology, histomorphometry, computed tomography (CT) and biomechanics were performed to evaluate the healing status.

CONCLUSIONS

the sheep metatarsus defect model seems to be a suitable environment with a good marriage of biological and biomechanical properties. Defects of 3 cm are treated with natural scaffolds (homologous graft or allografts), those of 2.5 cm with natural, synthetic or composite scaffolds, while little defects (0.5 × 0.5 cm) with composite scaffolds. No difference in results is found regardless of the defect size.

Quantifying Vascular Changes Surrounding Bone Regeneration in a Porcine Mandibular Defect Using Computed Tomography

Angiogenesis is a critical process essential for optimal bone healing. Several in vitro and in vivo systems have been previously used to elucidate some of the mechanisms involved in the process of angiogenesis, and at the same time, to test potential therapeutic agents and bioactive factors that play important roles in neovascularization. Computed tomography (CT) is a noninvasive imaging technique that has recently allowed investigators to obtain a diverse range of high-resolution, three-dimensional characterization of structures, such as bone formation within bony defects. Unfortunately, to date, angiogenesis evaluation relies primarily on histology, or ex vivo imaging and few studies have utilized CT to qualitatively and quantitatively study the vascular response during bone repair. In the current study a clinical CT-based technique was used to evaluate the effects of rhBMP-2 eluting graft treatment on soft tissue vascular architecture surrounding a large segmental bone defect model in the minipig mandible. The objective of this study was to demonstrate the efficacy of contrast-enhanced, clinical 64-slice CT technology in extracting quantitative metrics of vascular architecture over a 12-week period. The results of this study show that the presence of rhBMP-2 had a positive effect on vessel volume from 4 to 12 weeks, which was explained by a concurrent increase in vessel number, which was also significantly higher at 4 weeks for the rhBMP-2 treatment. More importantly, analysis of vessel architecture showed no changes throughout the duration of the study, indicating therapeutic safety. This study validates CT analysis as a relevant imaging method for quantitative and qualitative analysis of morphological characteristics of vascular tissue around a bone healing site. Also important, the study shows that CT technology can be used in large animal models and potentially be translated into clinical models for the development of improved methods to evaluate tissue healing and vascular adaptation processes over the course of therapy. This methodology has demonstrated sensitivity to tracking spatial and temporal changes in vascularization and has the potential to be applied to studying changes in other high-contrast tissues as well. Impact Statement Tissue engineering solutions depend on the surrounding tissue response to support regeneration. The inflammatory environment and surrounding vascular supply are critical to determining if therapies will survive, engraftment occurs, and native physiology is restored. This study for the first time evaluates the blood vessel network changes in surrounding soft tissue to a bone defect site in a large animal model, using clinically available computed tomography tools and model changes in vessel number, size, and architecture. While this study focuses on rhBMP2 delivery impacting surrounding vasculature, this validated method can be extended to studying the vascular network changes in other tissues as well.

Calvarial Versus Long Bone: Implications for Tailoring Skeletal Tissue Engineering

Tissue-engineered graft substitutes have shown great potential to treat large bone defects. While we usually assume that therapeutic approaches developed for appendicular bone healing could be similarly translated for application in craniofacial reconstruction and vice versa, this is not necessarily accurate. In addition to those more well-known healing-associated factors, such as age, lifestyle (e.g., nutrition and smoking), preexisting disease (e.g., diabetes), medication, and poor blood supply, the developmental origins and surrounding tissue of the wound sites can largely affect the fracture healing outcome as well as designed treatments. Therefore, the strategies developed for long bone fracture repair might not be suitable or directly applicable to skull bone repair. In this review, we discuss aspects of development, healing process, structure, and tissue engineering considerations between calvarial and long bones to assist in designing the tailored bone repair strategies. Impact Statement We summarized, in this review, the existing body of knowledge between long bone and calvarial bone with regard to their development and healing, tissue structure, and consideration of current tissue engineering strategies. By highlighting their similarities and differences, we propose that tailored tissue engineering strategies, such as scaffold features, growth factor usage, and the source of cells for tissue or region-specific bone repair, are necessary to ensure an optimized healing outcome.

Use of a three-dimensional printed polylactide-coglycolide/tricalcium phosphate composite scaffold incorporating magnesium powder to enhance bone defect repair in rabbits

Background

The repair of large bone defects remains challenging for orthopaedic surgeons. Bone grafting remains the method of choice; such grafts fill spaces and enhance bone repair. Therapeutic agents also aid bone healing. The objective of this study is to develop a composite bioactive scaffold composed of polylactide-coglycolide (PLGA) and tricalcium phosphate (TCP) (the basic carrier) incorporating osteogenic, bioactive magnesium metal powder (Mg).

Method

Porous PLGA/TCP scaffolds incorporating Mg were fabricated using a low-temperature rapid-prototyping process. We term the PLGA/TCP/Mg porous scaffold (hereafter, PPS). PLGA/TCP lacking Mg served as the control material when evaluating the efficacy of PPS. A total of 36 New Zealand white rabbits were randomly divided into blank, PLGA/TCP (P/T) and PPS group, with 12 rabbits in each group. We established bone defects 15 mm in length in rabbit radii to evaluate the in vivo osteogenic potential of the bioactive scaffold in terms of the direct controlled release of osteogenic Mg ion during in vivo scaffold degradation. Radiographs of the operated radii were taken immediately after implantation and then at 2, 4, 8 and 12 weeks. Micro-computed tomography of new bone formation and remaining scaffold and histological analysis were performed at 4, 8, 12 weeks after operation.

Results

X-ray imaging performed at weeks 4, 8 and 12 post-surgery revealed more newly formed bone within defects implanted with PPS and PLGA/TCP scaffolds than blank group (p < 0.05). And micro-computed tomography performed at weeks 4 and 8 after surgery revealed more newly formed bone within defects implanted with PPS scaffolds than PLGA/TCP scaffolds (p < 0.05). Histologically, the PPS group had more newly mineralized bone than controls (p < 0.05). The increases in new bone areas (total implant regions) in the PPS and PLGA/TCP groups were 19.42% and 5.67% at week 4 and 48.23% and 28.93% at week 8, respectively. The percentages of remaining scaffold material in total implant regions in the PPS and PLGA/TCP groups were 53.30% and 7.65% at week 8 and 20.52% and 2.70% at week 12, respectively.

Conclusion

Our new PPS composite scaffold may be an excellent orthopaedic substitute; it exhibits good biocompatibility and may potentially have clinical utility.

Translational potential of this article

Magnesium and beta-tricalcium phosphate had osteoinduction. It is significant to print a novel bone composite scaffold with osteoinduction to repair segmental bone defects. This study evaluated efficacy of PPS in the rabbit radius segmental bone defect model. The results showed that the novel scaffold with good biocompatibility may be an excellent graft and potentially have clinical utility.

Wireless Implantable Sensor for Noninvasive, Longitudinal Quantification of Axial Strain Across Rodent Long Bone Defects

Bone development, maintenance, and regeneration are remarkably sensitive to mechanical cues. Consequently, mechanical stimulation has long been sought as a putative target to promote endogenous healing after fracture. Given the transient nature of bone repair, tissue-level mechanical cues evolve rapidly over time after injury and are challenging to measure noninvasively. The objective of this work was to develop and characterize an implantable strain sensor for noninvasive monitoring of axial strain across a rodent femoral defect during functional activity. Herein, we present the design, characterization, and in vivo demonstration of the device's capabilities for quantitatively interrogating physiological dynamic strains during bone regeneration. Ex vivo experimental characterization of the device showed that it possessed promising sensitivity, signal resolution, and electromechanical stability for in vivo applications. The digital telemetry minimized power consumption, enabling extended intermittent data collection. Devices were implanted in a rat 6 mm femoral segmental defect model, and after three days, data were acquired wirelessly during ambulation and synchronized to corresponding radiographic videos, validating the ability of the sensor to noninvasively measure strain in real-time. Together, these data indicate the sensor is a promising technology to quantify tissue mechanics in a specimen specific manner, facilitating more detailed investigations into the role of the mechanical environment in dynamic bone healing and remodeling processes.

A pilot study: Alternative biomaterials in critical sized bone defect treatment

BACKGROUND

Critical-sized bone defects are a significant challenge with limited effective reconstructive options. The Masquelet Technique (MT) offers a solution to help restore form and function. Although this technique has produced promising results; a clear mechanism has not been determined. Theories include that the induced membrane has osteogenic potential or the membrane acts as a physical barrier to prevent fibrous tissue ingrowth. We hypothesize the induced membrane acts primarily as a physical barrier and that a synthetic non-biological membrane will allow a comparable amount of bone volume in the defect site.

METHODS

Ten New Zealand rabbit forelimbs (n=10) were divided into three study groups. A critical sized defect of 3.5cm in the ulna was created. In the control group, a traditional MT was performed (n=4). The experimental arm varied by replacement of the PMMA with a non-porous (n=3) or porous (150um) (n=3) polytetrafluoroethylene (PTFE) membrane filled with allograft. Micro-CT analysis was done to compare bone volume to tissue volume ratios (BV/TV). Defect sections were examined histologically with alkaline phosphatase (ALP), tartrate-resistant acid phosphatase (TRAP) and von kossa (VK) staining.

RESULTS

MicroCT analysis comparing BV/TV between the control and experimental arms showed no difference. BV/TV of the MT was 7.77%±2.34 compared to porous 9.12%±3.66 and nonporous 9.76%±1.57 PTFE membranes (p1=0.761, p2=0.572, respectively). Histological sections from both samples stained for ALP and TRAP displayed osteoblastic and osteoclastic activity. There was a higher amount of ALP and TRAP positively stained cells near the native bone ends in comparison to the center of the defect, in both sample types.

CONCLUSION AND SIGNIFICANCE

Replacing the induced membrane from the MT with a synthetic PTFE membrane illustrated that the membrane acts primarily as a functional barrier. Compared to the induced membrane, the PTFE membrane was able to display similar osteointegrative properties. These results allow for future optimization of the technique with the potential to further streamline towards a single stage procedure.

The Effects of Systemic Therapy of PEGylated NEL-Like Protein 1 (NELL-1) on Fracture Healing in Mice

Fractures are common, with an incidence of 13.7 per 1000 adults annually. Systemic agents have been widely used for enhancing bone regeneration; however, the efficacy of these therapeutics for the management and prevention of fracture remains unclear. NEL-like protein 1 (NELL-1) is a potent pro-osteogenic cytokine that has been modified with polyethylene glycol (PEG)ylation [PEGylated NELL-1 (NELL-PEG)] to enhance its pharmacokinetics for systemic therapy. Our aim was to investigate the effects of systemic administration of NELL-PEG on fracture healing in mice and on overall bone properties in uninjured bones. Ten-week-old CD-1 mice were subjected to an open osteotomy of bilateral radii and treated with weekly injections of NELL-PEG or PEG phosphate-buffered saline as control. Systemic injection of NELL-PEG resulted in improved bone mineral density of the fracture site and accelerated callus union. After 4 weeks of treatment, mice treated with NELL-PEG exhibited substantially enhanced callus volume, callus mineralization, and biomechanical properties. NELL-PEG injection significantly augmented bone regeneration, as confirmed by high expression of bone turnover rate, bone formation rate, and mineral apposition rate. Consistently, the immunohistochemistry results also confirmed a high bone remodeling activity in the NELL-PEG-treated group. Our findings suggest that weekly injection of NELL-PEG may have the clinical potential to accelerate fracture union and enhance overall bone properties, which may help prevent subsequent fractures.

Osteogenic differentiation of equine cord blood multipotent mesenchymal stromal cells within coralline hydroxyapatite scaffolds in vitro

Objective: To investigate the osteogenic differentiation potential of equine umbilical cord blood-derived multipotent mesenchymal stromal cells (CB-MSC) within coralline hydro-xyapatite scaffolds cultured in osteogenic induction culture medium.

Methods: Scaffolds seeded with equine CBMSC were cultured in cell expansion culture medium (control) or osteogenic induction medium (treatment). Cell viability and distribution were confirmed by the MTT cell viability assay and DAPI nuclear fluorescence staining, respectively. Osteogenic differentiation was evaluated after 10 days using reverse transcription polymerase chain reaction, alkaline phosphatase activity, and secreted osteocalcin concentration. Cell morphology and matrix deposition were assessed by scanning electron microscopy (SEM) after 14 days in culture.

Results: Cells showed viability and adequate distribution within the scaffold. Successful osteogenic differentiation within the scaffolds was demonstrated by the increased expression of osteogenic markers such as Runx2, osteopontin, osteonectin, collagen IA increased levels of alkaline phosphatase activity increased osteocalcin protein secretion and bone-like matrix presence in the scaffold pores upon SEM evaluation.

Clinical significance: These results demonstrate that equine CB-MSC maintain viability and exhibit osteogenic potential in coralline hydroxyapatite scaffolds when induced in vitro. Equine CB-MSC scaffold constructs deserve further investigation for their potential role as biologically active fillers to enhance bone-gap repair in the horse.

Neutrophil-mediated enhancement of angiogenesis and osteogenesis in a novel triple cell co-culture model with endothelial cells and osteoblasts

Repair and regeneration of critical-sized bone defects remain a major challenge in orthopaedic and craniomaxillofacial surgery. Until now, attempts to bioengineer bone tissue have been hindered by the inability to establish proper angiogenesis and osteogenesis in the tissue construct. In the present study, we established a novel triple cell co-culture model consisting of osteoblasts, endothelial cells, and neutrophils and conducted a systematic investigation of the effects of neutrophils on angiogenesis and osteogenesis. Neutrophils significantly increased angiogenesis in the tissue construct, evidenced by the formation of microvessel-like structures with an extensive lattice-like, stable tubular network in the co-culture model. Moreover, neutrophils significantly induced the expression of pro-angiogenic markers, such as VEGF-A, CD34, EGF, and FGF-2 in a dose- and time-dependent manner. Subsequently, PCR arrays corroborated that neutrophils upregulate the important angiogenic markers and MMPs. Moreover, neutrophils also enhanced osteogenic markers, such as ALP, OCN, OPN, and COL-1 compared with the controls. As shown by the osteogenic gene arrays, neutrophils significantly regulated major osteogenic markers such as BMP2, BMP3, BMP4, BMP5, TGF-β2, RUNX2, and ECM proteins. Significantly higher mineralization was observed in triple cell co-culture compared with controls. Foregoing data indicate that the triple cell co-culture model can be used to stimulate the growth of microvasculature within a bone bioengineering construct to improve cell viability. Neutrophil-mediated enhancement of angiogenesis and osteogenesis could be a viable, clinically relevant tissue engineering strategy to obtain optimal bone growth in defect sites, in the field of oral and maxillofacial surgery.

3D biomimetic artificial bone scaffolds with dual-cytokines spatiotemporal delivery for large weight-bearing bone defect repair

It is a great challenge to prepare “functional artificial bone” for the repair of large segmental defect, especially in weight-bearing bones. In this study, bioactive HA/PCL composite scaffolds that possess anatomical structure as autogenous bone were fabricated by CT-guided fused deposition modeling technique. The scaffolds can provide mechanical support and possess osteoconduction property. Then the VEGF-165/BMP-2 loaded hydrogel was filled into biomimetic artificial bone spatially to introduce osteoinduction and angioinduction ability via sustained release of these cytokines. It has been revealed that the cytokine-loaded hydrogel possessed good biodegradability and could release the VEGF-165/BMP-2 sustainedly and steadily. The synergistic effect of these two cytokines showed significant stimulation on the osteogenic gene expresssion of osteoblast in vitro and ectopic ossification in vivo. The scaffolds were then implanted into the rabbit tibial defect sites (1.2 cm) for bone regeneration for 12 weeks, indicating the best repair of defect in vivo, which was superior to the pure hydrogel/scaffolds or one-cytokine loaded hydrogel/scaffolds and close to autogenous bone graft. The strategy to construct an “anatomy-structure-function” trinity system as functional artificial bone shows great potential in replacing autogenous bone graft and applying in large bone defect repair clinically in future.

The Role of Three-Dimensional Scaffolds in Treating Long Bone Defects: Evidence from Preclinical and Clinical Literature-A Systematic Review

Long bone defects represent a clinical challenge. Bone tissue engineering (BTE) has been developed to overcome problems associated with conventional methods. The aim of this study was to assess the BTE strategies available in preclinical and clinical settings and the current evidence supporting this approach. A systematic literature screening was performed on PubMed database, searching for both preclinical (only on large animals) and clinical studies. The following string was used: "(Scaffold OR Implant) AND (Long bone defect OR segmental bone defect OR large bone defect OR bone loss defect)." The search retrieved a total of 1573 articles: 51 preclinical and 4 clinical studies were included. The great amount of preclinical papers published over the past few years showed promising findings in terms of radiological and histological evidence. Unfortunately, this in vivo situation is not reflected by a corresponding clinical impact, with few published papers, highly heterogeneous and with small patient populations. Several aspects should be further investigated to translate positive preclinical findings into clinical protocols: the identification of the best biomaterial, with both biological and biomechanical suitable properties, and the selection of the best choice between cells, GFs, or their combination through standardized models to be validated by randomized trials.

Minipig-BMSCs Combined with a Self-Setting Calcium Phosphate Paste for Bone Tissue Engineering

Calcium phosphate cements (CPCs) are a new generation of bone repair materials with good biocompatibility for various stem cells. The minipig is a recommended large animal model for bone engineering research. This study aimed to evaluate the feasibility of utilizing CPC scaffolds for the adhesion, proliferation, and osteogenic differentiation of minipig's bone marrow mesenchymal stem cells (pBMSCs). Passage 3 pBMSCs were seeded on the CPC scaffold and cultured with osteogenic culture medium (osteogenic group) or normal medium (control group). The density of viable cells increased in both groups, and pBMSCs firmly attached and spread well on the CPC scaffold. The alkaline phosphatase (ALP) activity in the osteogenic group had significantly increased on day 7 (D7) and peaked on D14. qRT-PCR revealed that mRNA levels of ALP and three osteogenic marker genes were significantly higher on D4, D7, and D14 in the osteogenic group. Alizarin Red S staining showed a significantly higher degree of bone mineralization from D7, D14 to D21 in the osteogenic group. These results indicated that pBMSCs can attach, proliferate well on CPC scaffold, and be successfully induced to differentiate into osteogenic cells. Our findings may be helpful for bone tissue engineering and the studies of bone regeneration.

Angiogenesis Assays for the Evaluation of Angiogenic Properties of Orthopaedic Biomaterials - A General Review

Vascularization is an essential process in bone formation, remodeling and regeneration during both bone development and fracture repair. Vascularization remains a big challenge directly leading to the final success of newly regenerated bone. In this review, the advantages and disadvantages of different angiogenesis assays and bone defect models are described in details for investigating revascularization of materials of interest. Unlike conventional angiogenesis study with growth factors or pharmaceutical molecules performed in two-dimension, special considerations are taken into account whether these assays can be translated for testing three-dimensional implantable devices. Over the years, accurate and quantifiable in vitro, ex vivo and in vivo assays have been extensively demonstrated to be useful in examining how new blood vessels grow. These methods can contribute to the fundamental understanding of angiogenic properties of the materials, but a bone defect model is still pivotal in order to understand the cascade actions of angiogenesis along with bone formation. Finally, angiogenesis and osteogenesis are both complex processes interacting with each other, the choice of which assay to be performed should adequately address the clinical relevance and reflect the sequence of responses of revascularization of the test materials.

Tissue engineered vascularized periosteal flap enriched with MSC/EPCs for the treatment of large bone defects in rats

Vascularized periosteal flaps are used for complex cases if the reconstruction of large bone defects is necessary in modern trauma and orthopedic surgery. In this study, we combined this surgical procedure with β-TCP scaffold and mesenchymal stem cells (MSCs) + endothelial progenitor cells (EPCs) as a tissue engineering approach to obtain optimum conditions for bone healing in rats. A critical size femoral defect was created in 80 rats allocated into 4 groups. Defects were treated according to the following protocol: i) vascularized periosteal flap alone; ii) vascularized periosteal flap + β-TCP scaffold; iii) vascularized periosteal flap + β-TCP scaffold + ligated vascular pedicle; and ii) vascularized periosteal flap + β-TCP scaffold + MSCs/EPCs. After 8 weeks, femur bones were extracted and analyzed for new bone formation, vascularization, proliferation and inflammatory processes and strength. Bone mineral density (BMD) and biomechanical stability at week 8 were highest in group 4 (flap + β-TCP scaffold + MSCs/EPCs) compared to all the other groups. Stability was significantly higher in group 4 (flap + β-TCP scaffold + MSCs/EPCs) in comparison to group 3 (ligated flap + β-TCP scaffold). BMD was found to be significantly lower in group 3 (ligated flap + β-TCP scaffold) compared to group 1 (flap) and group 4 (flap + β-TCP scaffold + MSCs/EPCs). The highest density of blood vessels was observed in group 4 (flap + β-TCP + MSCs/EPCs) and the values were significantly increased in comparison to group 3 (ligated flap), but not to group 1 (flap) and group 2 (flap + β-TCP). The highest amounts of proliferating cells were observed in group 4 (flap + β-TCP scaffold + MSC/EPCs). The percentage of proliferating cells was significantly higher in group 4 (flap + β-TCP scaffold + MSCs/EPCs) in comparison to all the other groups after 8 weeks. Our data thus indicate that critical size defect healing could be improved if MSCs/EPCs are added to β-TCP scaffold in combination with a periosteal flap. Even after 8 weeks, the amount of proliferating cells was increased. The flap blood supply is essential for bone healing and the reduction of inflammatory processes.

Establishment of a Segmental Femoral Critical-size Defect Model in Mice Stabilized by Plate Osteosynthesis

The use of tissue-engineered bone constructs is an appealing strategy to overcome drawbacks of autografts for the treatment of massive bone defects. As a model organism, the mouse has already been widely used in bone-related research. Large diaphyseal bone defect models in mice, however, are sparse and often use bone fixation which fills the bone marrow cavity and does not provide optimal mechanical stability. The objectives of the current study were to develop a critical-size, segmental, femoral defect in nude mice. A 3.5-mm mid-diaphyseal femoral ostectomy (approximately 25% of the femur length) was performed using a dedicated jig, and was stabilized with an anterior located locking plate and 4 locking screws. The bone defect was subsequently either left empty or filled with a bone substitute (syngenic bone graft or coralline scaffold). Bone healing was monitored noninvasively using radiography and in vivo micro-computed-tomography and was subsequently assessed by ex vivo micro-computed-tomography and undecalcified histology after animal sacrifice, 10 weeks postoperatively. The recovery of all mice was excellent, a full-weight-bearing was observed within one day following the surgical procedure. Furthermore, stable bone fixation and consistent fixation of the implanted materials were achieved in all animals tested throughout the study. When the bone defects were left empty, non-union was consistently obtained. In contrast, when the bone defects were filled with syngenic bone grafts, bone union was always observed. When the bone defects were filled with coralline scaffolds, newly-formed bone was observed in the interface between bone resection edges and the scaffold, as well as within a short distance within the scaffold.

The present model describes a reproducible critical-size femoral defect stabilized by plate osteosynthesis with low morbidity in mice. The new load-bearing segmental bone defect model could be useful for studying the underlying mechanisms in bone regeneration pertinent to orthopaedic applications.

Repair of segmental radial defects in dogs using tailor-made titanium mesh cages with plates combined with calcium phosphate granules and basic fibroblast growth factor-binding ion complex gel

Repair of large segmental defects of long bones are a tremendous challenge that calls for a novel approach to supporting immediate weight bearing and bone regeneration. This study investigated the functional and biological characteristics of a combination of a tailor-made titanium mesh cage with a plate (tTMCP) with tetrapod-shaped alpha tricalcium phosphate granules (TB) and basic fibroblast growth factor (bFGF)-binding ion complex gel (f-IC gel) to repair 20-mm segmental radial defects in dogs. The defects were created surgically in 18 adult beagle dogs and treated by implantation of tTMCPs with TB with (TB-gel group) or without (TB group) f-IC gel. Each tTMCP fitted the defect well, and all dogs could bear weight on the affected limb immediately after surgery. Dogs were euthanized 4, 8 and 24 weeks after implantation. Histomorphometry showed greater infiltration of new vessels and higher bone union rate in the TB-gel group than in the TB group. The lamellar bone volume and mineral apposition rate did not differ significantly between the groups, indicating that neovascularization may be the primary effect of f-IC gel on bone regeneration. This combination method which is tTMCP combined with TB and f-IC gel, would be useful for the treatment of segmental long bone defects.

A biodegradable antibiotic-eluting PLGA nanofiber-loaded deproteinized bone for treatment of infected rabbit bone defects

We fabricated a biodegradable antibiotic-eluting poly(d,l)-lactide-co-glycolide nanofiber-loaded deproteinized bone (ANDB) scaffold that provided sustained delivery of vancomycin to repair methicillin-resistant Staphylococcus aureus bone defects. To fabricate the biodegradable ANDB, poly(d,l)-lactide-co-glycolide and vancomycin were first dissolved in 1,1,1,3,3,3-hexafluoro-2-propano. The solution was then electrospun to produce biodegradable antibiotic-eluting membranes that were deposited on the surface of bovine deproteinized cancellous bone. We used scanning electron microscopy to determine the properties of the scaffold. Both elution and high-performance liquid chromatography assays were used to evaluate the in vitro vancomycin release rate from the ANDB scaffold. Three types of scaffolds were co-cultured with bacteria to confirm the in vitro antibacterial activity. The infected bone defect rabbit model was induced by injecting 10(7) colony forming units of a methicillin-resistant Staphylococcus aureus strain into the radial defect of rabbits. Animals were then separated into treatment groups and implanted according to the following scheme: ANDB scaffold in group A, poly(d,l)-lactide-co-glycolide nanofiber-loaded deproteinized bone (NDB) scaffold with intravenous (i.v.) vancomycin in group B, and NDB scaffold alone in group C. Treatment efficacy was evaluated after eight weeks using radiological, microbiological, and histological examinations. In vitro results revealed that biodegradable ANDB scaffolds released concentrations of vancomycin that were greater than the minimum inhibitory concentration for more than four weeks. Bacterial inhibition tests also confirmed antibacterial efficacy lasted for approximately four weeks. Radiological and histological scores obtained in vivo revealed significant differences between groups A, B and C. Importantly, group A had significantly lower bacterial load and better bone regeneration when compared to either group B or C. Collectively, these results show that our fabricated ANDB scaffolds possess: (1) effective bactericidal activity against methicillin-resistant Staphylococcus aureus, (2) the ability to promote site-specific bone regeneration, and (3) the potential for use in the treatment of infected bone defects.

Preclinical testing of drug delivery systems to bone

Bone defects do not heal in 5-10% of the fractures. In order to enhance bone regeneration, drug delivery systems are needed. They comprise a scaffold with or without inducing factors and/or cells. To test these drug delivery systems before application in patients, they finally need to be tested in animal models. The choice of animal model depends on the main research question; is a functional or mechanistic evaluation needed? Furthermore, which type of bone defects are investigated: load-bearing (i.e. orthopedic) or non-load-bearing (i.e. craniomaxillofacial)? This determines the type of model and in which type of animal. The experiments need to be set-up using the 3R principle and must be reported following the ARRIVE guidelines.

Animal Models for Evaluation of Bone Implants and Devices

Bone implants and devices are a rapidly growing field within biomedical research, and implants have the potential to significantly improve human and animal health. Animal models play a key role in initial product development and are important components of nonclinical data included in applications for regulatory approval. Pathologists are increasingly being asked to evaluate these models at the initial developmental and nonclinical biocompatibility testing stages, and it is important to understand the relative merits and deficiencies of various species when evaluating a new material or device. This article summarizes characteristics of the most commonly used species in studies of bone implant materials, including detailed information about the relevance of a particular model to human bone physiology and pathology. Species reviewed include mice, rats, rabbits, guinea pigs, dogs, sheep, goats, and nonhuman primates. Ultimately, a comprehensive understanding of the benefits and limitations of different model species will aid in rigorously evaluating a novel bone implant material or device.

Engraftment of Prevascularized, Tissue Engineered Constructs in a Novel Rabbit Segmental Bone Defect Model

The gold standard treatment of large segmental bone defects is autologous bone transfer, which suffers from low availability and additional morbidity. Tissue engineered bone able to engraft orthotopically and a suitable animal model for pre-clinical testing are direly needed. This study aimed to evaluate engraftment of tissue-engineered bone with different prevascularization strategies in a novel segmental defect model in the rabbit humerus. Decellularized bone matrix (Tutobone) seeded with bone marrow mesenchymal stromal cells was used directly orthotopically or combined with a vessel and inserted immediately (1-step) or only after six weeks of subcutaneous “incubation” (2-step). After 12 weeks, histological and radiological assessment was performed. Variable callus formation was observed. No bone formation or remodeling of the graft through TRAP positive osteoclasts could be detected. Instead, a variable amount of necrotic tissue formed. Although necrotic area correlated significantly with amount of vessels and the 2-step strategy had significantly more vessels than the 1-step strategy, no significant reduction of necrotic area was found. In conclusion, the animal model developed here represents a highly challenging situation, for which a suitable engineered bone graft with better prevascularization, better resorbability and higher osteogenicity has yet to be developed.

Bioabsorbable fish scale for the internal fixation of fracture: a preliminary study

Fish scales, which consist of type I collagen and hydroxyapatite (HA), were used to fabricate a bioabsorbable bone pin in this study. Fresh fish scales were decellularized and characterized to provide higher biocompatibility. The mechanical properties of fish scales were tested, and the microstructure of an acellular fish scale was examined. The growth curve of a myoblastic cell line (C2C12), which was cultured on the acellular fish scales, implied biocompatibility in vitro, and the morphology of the cells cultured on the scales was observed using scanning electron microscopy (SEM). A bone pin made of decellularized fish scales was used for the internal fixation of femur fractures in New Zealand rabbits. Periodic X-ray evaluations were obtained, and histologic examinations were performed postoperatively. The present results show good cell growth on decellularized fish scales, implying great biocompatibility in vitro. Using SEM, the cell morphology revealed great adhesion on a native, layered collagen structure. The Young's modulus was 332 ± 50.4 MPa and the tensile strength was 34.4 ± 6.9 MPa for the decellularized fish scales. Animal studies revealed that a fish-scale-derived bone pin improved the healing of bone fractures and degraded with time. After an 8-week implantation, the bone pin integrated with the adjacent tissue, and new extracellular matrix was synthesized around the implant. Our results proved that fish-scale-derived bone pins are a promising implant material for bone healing and clinical applications.

Templated repair of long bone defects in rats with bioactive spiral-wrapped electrospun amphiphilic polymer/hydroxyapatite scaffolds

Effective repair of critical-size long bone defects presents a significant clinical challenge. Electrospun scaffolds can be exploited to deliver protein therapeutics and progenitor cells, but their standalone application for long bone repair has not been explored. We have previously shown that electrospun composites of amphiphilic poly(d,l-lactic acid)-co-poly(ethylene glycol)-co-poly(d,l-lactic acid) (PELA) and hydroxyapatite (HA) guide the osteogenic differentiation of bone marrow stromal cells (MSCs), making these scaffolds uniquely suited for evaluating cell-based bone regeneration approaches. Here we examine whether the in vitro bioactivity of these electrospun scaffolds can be exploited for long bone defect repair, either through the participation of exogenous MSCs or through the activation of endogenous cells by a low dose of recombinant human bone morphogenetic protein-2 (rhBMP-2). In critical-size rat femoral segmental defects, spiral-wrapped electrospun HA-PELA with preseeded MSCs resulted in laminated endochondral ossification templated by the scaffold across the longitudinal span of the defect. Using GFP labeling, we confirmed that the exogenous MSCs adhered to HA-PELA survived at least 7 days postimplantation, suggesting direct participation of these exogenous cells in templated bone formation. When loaded with 500 ng of rhBMP-2, HA-PELA spirals led to more robust but less clearly templated bone formation than MSC-bearing scaffolds. Both treatment groups resulted in new bone bridging over the majority of the defect by 12 weeks. This study is the first demonstration of a standalone bioactive electrospun scaffold for templated bone formation in critical-size long bone defects.

A modified rabbit ulna defect model for evaluating periosteal substitutes in bone engineering: a pilot study

The present work defines a modified critical size rabbit ulna defect model for bone regeneration in which a non-resorbable barrier membrane was used to separate the radius from the ulna to create a valid model for evaluation of tissue-engineered periosteal substitutes. Eight rabbits divided into two groups were used. Critical defects (15 mm) were made in the ulna completely eliminating periosteum. For group I, defects were filled with a nanohydroxyapatite poly(ester urethane) scaffold soaked in PBS and left as such (group Ia) or wrapped with a tissue-engineered periosteal substitute (group Ib). For group II, an expanded-polytetrafluoroethylene (e-PTFE) (GORE-TEX^®) membrane was inserted around the radius then the defects received either scaffold alone (group IIa) or scaffold wrapped with periosteal substitute (group IIb). Animals were euthanized after 12–16 weeks, and bone regeneration was evaluated by radiography, computed microtomography (μCT), and histology. In the first group, we observed formation of radio-ulnar synostosis irrespective of the treatment. This was completely eliminated upon placement of the e-PTFE (GORE-TEX^®) membrane in the second group of animals. In conclusion, modification of the model using a non-resorbable e-PTFE membrane to isolate the ulna from the radius was a valuable addition allowing for objective evaluation of the tissue-engineered periosteal substitute.