Rodent animal models of delayed bone healing and non-union formation: a comprehensive review

A rhPDGF-BB/bovine type I collagen/β-TCP mixture for the treatment of critically sized non-union tibial defects: An in vivo study in rabbits

Non-union during healing of bone fractures affects up to ~5% of patients worldwide. Given the success of recombinant human platelet-derived growth factor-B chain homodimer (rhPDGF-BB) in promoting angiogenesis and bone fusion in the hindfoot and ankle, rhPDGF-BB combined with bovine type I collagen/β-TCP matrix (AIBG) could serve as a viable alternative to autografts in the treatment of non-unions. Defects (~2 mm gaps) were surgically induced in tibiae of skeletally mature New Zealand white rabbits. Animals were allocated to one of four groups-(1) negative control (empty defect, healing for 8 weeks), (2 and 3) acute treatment with AIBG (healing for 4 or 8 weeks), and (4) chronic treatment with AIBG (injection 4 weeks post defect creation and then healing for 8 weeks). Bone formation was analyzed qualitatively and semi-quantitatively through histology. Samples were imaged using dual-energy X-ray absorptiometry and computed tomography for defect visualization and volumetric reconstruction, respectively. Delayed healing or non-healing was observed in the negative control group, whereas defects treated with AIBG in an acute setting yielded bone formation as early as 4 weeks with bone growth appearing discontinuous. At 8 weeks (acute setting), substantial remodeling was observed with higher degrees of bone organization characterized by appositional bone growth. The chronic healing, experimental, group yielded bone formation and remodeling, with no indication of non-union after treatment with AIBG. Furthermore, bone growth in the chronic healing group was accompanied by an increased presence of osteons, osteonal canals, and interstitial lamellae. Qualitatively and semiquantitatively, chronic application of AI facilitated complete bridging of the induced non-union defects, while untreated defects or defects treated acutely with AIBG demonstrated a lack of complete bridging at 8 weeks.

Implant retention in a rabbit model of fracture-related infection

Aims

Fracture-related infection (FRI) is commonly classified based on the time of onset of symptoms. Early infections (< two weeks) are treated with debridement, antibiotics, and implant retention (DAIR). For late infections (> ten weeks), guidelines recommend implant removal due to tolerant biofilms. For delayed infections (two to ten weeks), recommendations are unclear. In this study we compared infection clearance and bone healing in early and delayed FRI treated with DAIR in a rabbit model.

Methods

Staphylococcus aureus was inoculated into a humeral osteotomy in 17 rabbits after plate osteosynthesis. Infection developed for one week (early group, n = 6) or four weeks (delayed group, n = 6) before DAIR (systemic antibiotics: two weeks, nafcillin + rifampin; four weeks, levofloxacin + rifampin). A control group (n = 5) received revision surgery after four weeks without antibiotics. Bacteriology of humerus, soft-tissue, and implants was performed seven weeks after revision surgery. Bone healing was assessed using a modified radiological union scale in tibial fractures (mRUST).

Results

Greater bacterial burden in the early group compared to the delayed and control groups at revision surgery indicates a retraction of the infection from one to four weeks. Infection was cleared in all animals in the early and delayed groups at euthanasia, but not in the control group. Osteotomies healed in the early group, but bone healing was significantly compromised in the delayed and control groups.

Conclusion

The duration of the infection from one to four weeks does not impact the success of infection clearance in this model. Bone healing, however, is impaired as the duration of the infection increases.

Temporal dynamics of immune-stromal cell interactions in fracture healing

Bone fracture repair is a complex, multi-step process that involves communication between immune and stromal cells to coordinate the repair and regeneration of damaged tissue. In the US, 10% of all bone fractures do not heal properly without intervention, resulting in non-union. Complications from non-union fractures are physically and financially debilitating. We now appreciate the important role that immune cells play in tissue repair, and the necessity of the inflammatory response in initiating healing after skeletal trauma. The temporal dynamics of immune and stromal cell populations have been well characterized across the stages of fracture healing. Recent studies have begun to untangle the intricate mechanisms driving the immune response during normal or atypical, delayed healing. Various in vivo models of fracture healing, including genetic knockouts, as well as in vitro models of the fracture callus, have been implemented to enable experimental manipulation of the heterogeneous cellular environment. The goals of this review are to (1): summarize our current understanding of immune cell involvement in fracture healing (2); describe state-of-the art approaches to study inflammatory cells in fracture healing, including computational and in vitro models; and (3) identify gaps in our knowledge concerning immune-stromal crosstalk during bone healing.

Computational models of bone fracture healing and applications: a review

Fracture healing is a very complex physiological process involving multiple events at different temporal and spatial scales, such as cell migration and tissue differentiation, in which mechanical stimuli and biochemical factors assume key roles. With the continuous improvement of computer technology in recent years, computer models have provided excellent solutions for studying the complex process of bone healing. These models not only provide profound insights into the mechanisms of fracture healing, but also have important implications for clinical treatment strategies. In this review, we first provide an overview of research in the field of computational models of fracture healing based on CiteSpace software, followed by a summary of recent advances, and a discussion of the limitations of these models and future directions for improvement. Finally, we provide a systematic summary of the application of computational models of fracture healing in three areas: bone tissue engineering, fixator optimization and clinical treatment strategies. The application of computational models of bone healing in clinical treatment is immature, but an inevitable trend, and as these models become more refined, their role in guiding clinical treatment will become more prominent.

Cilostazol promotes blood vessel formation and bone regeneration in a murine non-union model

Non-unions represent a major complication in trauma and orthopedic surgery. Many factors contribute to bone regeneration, out of which an adequate vascularization has been recognized as crucial. The phosphodiesterase-3 (PDE-3) inhibitor cilostazol has been shown to exert pro-angiogenic and pro-osteogenic effects in a variety of preclinical studies. Hence, we herein investigated the effects of cilostazol on bone regeneration in an atrophic non-union model in mice. For this purpose, a 1.8 mm femoral segmental defect was stabilized by pin-clip fixation and the animals were treated daily with 30 mg/kg body weight cilostazol or saline (control) per os. At 2, 5 and 10 weeks after surgery the healing of femora was analyzed by X-ray, biomechanics, photoacoustic imaging, and micro-computed tomography (µCT). To investigate the cellular composition and the growth factor expression of the callus tissue additional histological, immunohistochemical and Western blot analyses were performed. Cilostazol-treated animals showed increased bone formation within the callus, resulting in an enhanced bending stiffness when compared to controls. This was associated with a more pronounced expression of vascular endothelial growth factor (VEGF), a higher number of CD31-positive microvessels and an increased oxygen saturation within the callus tissue. Furthermore, cilostazol induced higher numbers of tartrate-resistant acidic phosphate (TRAP)-positive osteoclasts and CD68-positive macrophages. Taken together, these findings demonstrate that cilostazol is a promising drug candidate for the adjuvant treatment of atrophic non-unions in clinical practice.

Parathyroid hormone stimulates bone regeneration in an atrophic non-union model in aged mice

BACKGROUND

Non-union formation still represents a major burden in trauma and orthopedic surgery. Moreover, aged patients are at an increased risk for bone healing failure. Parathyroid hormone (PTH) has been shown to accelerate fracture healing in young adult animals. However, there is no information whether PTH also stimulates bone regeneration in atrophic non-unions in the aged. Therefore, the aim of the present study was to analyze the effect of PTH on bone regeneration in an atrophic non-union model in aged CD-1 mice.

METHODS

After creation of a 1.8 mm segmental defect, mice femora were stabilized by pin-clip fixation. The animals were treated daily with either 200 mg/kg body weight PTH 1-34 (n = 17) or saline (control; n = 17) subcutaneously. Bone regeneration was analyzed by means of X-ray, biomechanics, micro-computed tomography (µCT) imaging as well as histological, immunohistochemical and Western blot analyses.

RESULTS

In PTH-treated animals bone formation was markedly improved when compared to controls. This was associated with an increased bending stiffness as well as a higher number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts and CD31-positive microvessels within the callus tissue. Furthermore, PTH-treated aged animals showed a decreased inflammatory response, characterized by a lower number of MPO-positive granulocytes and CD68-positive macrophages within the bone defects when compared to controls. Additional Western blot analyses demonstrated a significantly higher expression of cyclooxygenase (COX)-2 and phosphoinositide 3-kinase (PI3K) in PTH-treated mice.

CONCLUSION

Taken together, these findings indicate that PTH is an effective pharmacological compound for the treatment of non-union formation in aged animals.

Sildenafil, a phosphodiesterase-5 inhibitor, stimulates angiogenesis and bone regeneration in an atrophic non-union model in mice

Non-union formation represents a major complication in trauma and orthopedic surgery. The phosphodiesterase-5 (PDE-5) inhibitor sildenafil has been shown to exert pro-angiogenic and pro-osteogenic effects in vitro and in vivo. Therefore, the aim of the present study was to analyze the impact of sildenafil in an atrophic non-union model in mice. After creation of a 1.8 mm segmental defect, mice femora were stabilized by pin-clip fixation. Bone regeneration was analyzed by means of X-ray, biomechanics, photoacoustic and micro-computed tomography (µCT) imaging as well as histological, immunohistochemical and Western blot analyses at 2, 5 and 10 weeks after surgery. The animals were treated daily with either 5 mg/kg body weight sildenafil (n = 35) or saline (control; n = 35) per os. Bone formation was markedly improved in defects of sildenafil-treated mice when compared to controls. This was associated with a higher bending stiffness as well as an increased number of CD31-positive microvessels and a higher oxygen saturation within the callus tissue. Moreover, the bone defects of sildenafil-treated animals contained more tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts and CD68-positive macrophages and exhibited a higher expression of the pro-angiogenic and pro-osteogenic markers cysteine rich protein (CYR)61 and vascular endothelial growth factor (VEGF) when compared to controls. These findings demonstrate that sildenafil acts as a potent stimulator of angiogenesis and bone regeneration in atrophic non-unions.

A new method for segmentation and analysis of bone callus in rodent fracture models using micro-CT

Fracture burden has created a need to better understand bone repair processes under different pathophysiological states. Evaluation of structural and material properties of the mineralized callus, which is integral to restoring biomechanical stability is, therefore, vital. Microcomputed tomography (micro-CT) can facilitate noninvasive imaging of fracture repair, however, current methods for callus segmentation are only semiautomated, restricted to defined regions, time/labor intensive, and prone to user variation. Herein, we share a new automatic method for segmenting callus in micro-CT tomograms that will allow for objective, quantitative analysis of the bone fracture microarchitecture. Fractured and nonfractured mouse femurs were scanned and processed by both manual and automated segmentation of fracture callus from cortical bone after which microarchitectural parameters were analyzed. All segmentation and analysis steps were performed using CTAn (Bruker) with automatic segmentation performed using the software's image-processing plugins. Results showed automatic segmentation reliably and consistently segmented callus from cortical bone, demonstrating good agreement with manual methods with low bias: tissue volume (TV): -0.320 mm3 , bone volume (BV): 0.0358 mm3 , and bone volume/tissue volume (BV/TV): -3.52%, and was faster and eliminated user-bias and variation. Method scalability and translatability across rodent models were verified in scans of fractured rat femora showing good agreement with manual methods with low bias: TV: -3.654 mm3 , BV: 0.830 mm3 , and BV/TV: 7.81%. Together, these data validate a new automated method for segmentation of callus and cortical bone in micro-CT tomograms that we share as a fast, reliable, and less user-dependent tool for application to study bone callus in fracture, and potentially elsewhere.

Sequential application of small molecule therapy enhances chondrogenesis and angiogenesis in murine segmental defect bone repair

The increasing incidence of physiologic/pathologic conditions that impair the otherwise routine healing of endochondral bone fractures and the occurrence of severe bone injuries necessitate novel approaches to enhance clinically challenging bone fracture repair. To promote the healing of nonunion fractures, we tested an approach that used two small molecules to sequentially enhance cartilage development and conversion to the bone in the callus of a murine femoral segmental defect nonunion model of bone injury. Systemic injections of smoothened agonist 21k (SAG21k) were used to stimulate chondrogenesis through the activation of the sonic hedgehog (SHH) pathway early in bone repair, while injections of the prolyl hydroxylase domain (PHD)2 inhibitor, IOX2, were used to stimulate hypoxia signaling-mediated endochondral bone formation. The expression of SHH pathway genes and Phd2 target genes was increased in chondrocyte cell lines in response to SAG21k and IOX2 treatment, respectively. The segmental defect responded to sequential systemic administration of these small molecules with increased chondrocyte expression of PTCH1, GLI1, and SOX9 in response to SAG and increased expression of hypoxia-induced factor-1α and vascular endothelial growth factor-A in the defect tissues in response to IOX2. At 6 weeks postsurgery, the combined SAG-IOX2 therapy produced increased bone formation in the defect with the bony union over the injury. Clinical significance: This therapeutic approach was successful in promoting cartilage and bone formation within a critical-size segmental defect and established the utility of a sequential small molecule therapy for the enhancement of fracture callus development in clinically challenging bone injuries.

Tobacco heating system has less impact on bone metabolism than cigarette smoke

Cigarette smoking promotes osteoclast activity, thus increasing the risk of secondary osteoporosis, leading to osteoporosis-associated fracture and impaired fracture healing. Heated tobacco products (HTP) are considered potential reduced-risk alternatives to cigarettes. However, their impact on bone metabolism remains to be elucidated. We developed an in vitro model that mimics in vivo bone cell interactions to comparatively evaluate the effects of HTPs and cigarette smoke on bone cell functionality and viability. We generated an in vitro coculture system with SCP-1 and THP-1 cells (1:8 ratio) cultured on a decellularized Saos-2 matrix with an optimized coculture medium. We found that, following acute or chronic exposure, particulate matter extract from the aerosol of an HTP, the Tobacco Heating System (THS), was less harmful to the bone coculture system than reference cigarette (1R6F) smoke extract. In the fracture healing model, cultures exposed to the THS extract maintained similar osteoclast activity and calcium deposits as control cultures. Conversely, smoke extract exposure promoted osteoclast activity, resulting in an osteoporotic environment, whose formation could be prevented by bisphosphonate coadministration. Thus, THS is potentially less harmful than cigarette smoke to bone cell differentiation and bone mineralization - both being crucial aspects during the reparative phase of fracture healing.

Photoacoustic imaging for the study of oxygen saturation and total hemoglobin in bone healing and non-union formation

Non-union formation represents a major complication in trauma surgery. Adequate vascularization has been recognized as vital for bone healing. However, the role of vascularization in the pathophysiology of non-union formation remains elusive. This is due to difficulties in studying bone microcirculation in vivo. Therefore, we herein studied in a murine osteotomy model whether photoacoustic imaging may be used to analyze vascularization in bone healing and non-union formation. We found that oxygen saturation within the callus tissue is significantly lower in non-unions compared to unions and further declines over time. Moreover, the amount of total hemoglobin (HbT) within the callus tissue was markedly reduced in non-unions. Correlation analyses showed a strong positive correlation between microvessel density and HbT, indicating that photoacoustically determined HbT is a valid parameter to assess vascularization during bone healing. In summary, photoacoustic imaging is a promising approach to study vascular function and tissue oxygenation in bone regeneration.

Fracture Healing in the Setting of Endocrine Diseases, Aging, and Cellular Senescence

More than 2.1 million age-related fractures occur in the United States annually, resulting in an immense socioeconomic burden. Importantly, the age-related deterioration of bone structure is associated with impaired bone healing. Fracture healing is a dynamic process which can be divided into four stages. While the initial hematoma generates an inflammatory environment in which mesenchymal stem cells and macrophages orchestrate the framework for repair, angiogenesis and cartilage formation mark the second healing period. In the central region, endochondral ossification favors soft callus development while next to the fractured bony ends, intramembranous ossification directly forms woven bone. The third stage is characterized by removal and calcification of the endochondral cartilage. Finally, the chronic remodeling phase concludes the healing process. Impaired fracture healing due to aging is related to detrimental changes at the cellular level. Macrophages, osteocytes, and chondrocytes express markers of senescence, leading to reduced self-renewal and proliferative capacity. A prolonged phase of "inflammaging" results in an extended remodeling phase, characterized by a senescent microenvironment and deteriorating healing capacity. Although there is evidence that in the setting of injury, at least in some tissues, senescent cells may play a beneficial role in facilitating tissue repair, recent data demonstrate that clearing senescent cells enhances fracture repair. In this review, we summarize the physiological as well as pathological processes during fracture healing in endocrine disease and aging in order to establish a broad understanding of the biomechanical as well as molecular mechanisms involved in bone repair.

Effects of Amnion Lyophilization Sterile Radiation against the Number of Osteoblasts and Osteocytes in Nonunion Fractures: An Experimental Research Study

Background: This study used an experimental posttest-only control group design that involved white rats of the Sprague Dawley strain. Methods: The samples used were 8-week-old rats, weighing 250–350 grams of the male sex. The study was conducted on five groups of rats with a total of seven rats per group. Results: The mean value of osteocytes in the control group was 00.00 ± 00.00 and in the ALSR group it was 87.14 ± 44.85. The mean value of osteoblasts in the control group was 50.06 ± 5.76 and in the ALSR group it was 283.63 ± 22.86. This study showed that there were differences in the number of osteocytes and osteoblasts in the two groups. Conclusion: The study reported that the ALSR group had significantly different numbers of osteoblasts and osteocytes than the control group.

Comparison of two non-union models with damaged periosteum in mice: Segmental defect and pin-clip fixation versus transverse fracture and K-wire stabilization

Despite growing knowledge about the mechanisms of fracture healing, non-union formation still represents a major complication in trauma and orthopedic surgery. Non-union models in mice gain increasing interest, because they allow investigating the molecular and cellular mechanisms of failed fracture healing. These models often use segmental defects to achieve non-union formation. Alternatively, failed fracture healing can be induced by transverse fractures with additional periosteal injury. The present study systematically compared the reliability of these two approaches to serve as non-union model. A 0.6 mm K-wire was inserted into the femora of CD-1 mice in a retrograde fashion and a closed transverse femoral fracture was created. Subsequently, the fracture site was exposed and the periosteum was cauterized. This approach was compared with a well-established non-union model involving the pin-clip fixation of a 1.8 mm segmental defect. The callus tissue was analyzed by means of radiography, biomechanics, histology and Western blotting. At 10 weeks after surgery 10 out of 12 femora (83.3 %) of the K-wire group showed a non-union formation. The pin-clip model resulted in 100 % non-union formation. The K-wire group showed increased bone formation, osteoclast activity and bending stiffness when compared to the group with pin-clip fixation. This was associated with a higher expression of bone formation markers. However, the number of CD31-positive microvessels was reduced in the K-wire group, indicating an impaired angiogenic capacity after periosteal cauterization. These findings suggest that the pin-clip model is more reliable for the study of non-union formation in mice. The K-wire model including periosteal injury by cauterization however, may be particularly applied in preclinical studies which explore the effects of damaged periosteum and reduced angiogenic capacity to trauma-induced fractures.

Drill-Hole Bone Defects in Animal Models of Bone Healing: Protocol for a Systematic Review

Background

Bone fractures are common conditions of the musculoskeletal system. Several animal models of bone fractures have been established to help elucidate the complex process of bone healing. In the last decades, drill-hole bone defects have emerged as a method to study bone healing. Animal models of drill-hole defects are easy to standardize and do not require external fixation of the bone. However, current studies of drill-hole bone defects lack detailed descriptions of techniques and interstudy standardization.

Objective

This systematic review aims to present a detailed description of the different methods used to induce drill-hole bone defects in long bones of laboratory animals and to provide a comprehensive overview of their methodology and potential for investigation of bone healing.

Methods

A systematic search of PubMed and Embase will be performed of abstracts containing variations of the following four keywords: “long bone,” “drill-hole,” “regeneration,” and “animal model.” Abstract screening and full-text screening will be performed independently by 2 reviewers, and data will be extracted to a predesigned extraction protocol. The primary outcome of the included studies is the technique used to create the drill-hole bone defect, and secondary outcomes are any measurements or analyses of bone defect and regeneration. A narrative synthesis will be used to present the primary outcome, while information on secondary outcomes will be displayed graphically. The study protocol follows the PRISMA-P (Preferred Reporting Items for Systematic Review and Meta-analysis Protocols) guidelines.

Results

Abstract and full-text screening is ongoing and is expected to be completed by October 2022. Data extraction will commence immediately after, and the manuscript is expected to be completed by December 2023. The systematic review will follow the PRISMA statement.

Conclusions

The strength of this systematic review is that it provides a comprehensive methodological overview of the different drill-hole methods and their advantages and disadvantages. This will assist researchers in choosing which model to use when studying different aspects of bone healing.

Trial Registration

International Prospective Register of Systematic Reviews CRD42020213076; https://tinyurl.com/bp56wdwe

International Registered Report Identifier (IRRID)

PRR1-10.2196/34887

The specialist in regeneration-the Axolotl-a suitable model to study bone healing?

While the axolotl's ability to completely regenerate amputated limbs is well known and studied, the mechanism of axolotl bone fracture healing remains poorly understood. One reason might be the lack of a standardized fracture fixation in axolotl. We present a surgical technique to stabilize the osteotomized axolotl femur with a fixator plate and compare it to a non-stabilized osteotomy and to limb amputation. The healing outcome was evaluated 3 weeks, 3, 6 and 9 months post-surgery by microcomputer tomography, histology and immunohistochemistry. Plate-fixated femurs regained bone integrity more efficiently in comparison to the non-fixated osteotomized bone, where larger callus formed, possibly to compensate for the bone fragment misalignment. The healing of a non-critical osteotomy in axolotl was incomplete after 9 months, while amputated limbs efficiently restored bone length and structure. In axolotl amputated limbs, plate-fixated and non-fixated fractures, we observed accumulation of PCNA+ proliferating cells at 3 weeks post-injury similar to mouse. Additionally, as in mouse, SOX9-expressing cells appeared in the early phase of fracture healing and amputated limb regeneration in axolotl, preceding cartilage formation. This implicates endochondral ossification to be the probable mechanism of bone healing in axolotls. Altogether, the surgery with a standardized fixation technique demonstrated here allows for controlled axolotl bone healing experiments, facilitating their comparison to mammals (mice).

Effects of Systemic or Local Administration of Mesenchymal Stem Cells from Patients with Osteoporosis or Osteoarthritis on Femoral Fracture Healing in a Mouse Model

The purpose of this study was to analyze the regenerative capacity of mesenchymal stem cells (MSCs) in the treatment of fractures. MSCs extracted from patients with osteoporotic hip fractures or hip osteoarthritis undergoing hip replacement surgeries were cultured and injected into mice with femoral fracture. Two experimental models were established, one for the systemic administration of MSCs (n = 29) and another one for local administration (n = 30). Fracture consolidation was assessed by micro-CT and histology. The degree of radiological consolidation and corticalization was better with MSCs from osteoporosis than from osteoarthritis, being significant after systemic administration (p = 0.0302 consolidation; p = 0.0243 corticalization). The histological degree of consolidation was also better with MSCs from osteoporosis than from osteoarthritis. Differences in histological scores after systemic infusion were as follows: Allen, p = 0.0278; Huo, p = 0.3471; and Bone Bridge, p = 0.0935. After local administration at the fracture site, differences in histological scores were as follows: Allen, p = 0.0764; Huo, p = 0.0256; and Bone Bridge, p = 0.0012. As osteoporosis and control groups were similar, those differences depended on an inhibitory influence by MSCs from patients with osteoarthritis. In conclusion, we found an unexpected impairment of consolidation induced by MSCs from patients with osteoarthritis. However, MSCs from patients with osteoporosis compared favorably with cells from patients with osteoarthritis. In other words, based on this study and previous studies, MSCs from patients with osteoporosis do not appear to have worse bone-regenerating capabilities than MSCs from non-osteoporotic individuals of similar age.

Effect of Angiogenesis in Bone Tissue Engineering

The reconstruction of large skeletal defects is still a tricky challenge in orthopedics. The newly formed bone tissue migrates sluggishly from the periphery to the center of the scaffold due to the restrictions of exchange of oxygen and nutrition impotent cells osteogenic differentiation. Angiogenesis plays an important role in bone reconstruction and more and more studies on angiogenesis in bone tissue engineering had been published. Promising advances of angiogenesis in bone tissue engineering by scaffold designs, angiogenic factor delivery, in vivo prevascularization and in vitro prevascularization are discussed in detail. Among all the angiogenesis mode, angiogenic factor delivery is the common methods of angiogenesis in bone tissue engineering and possible research directions in the future.

Cryogel Scaffold-Mediated Delivery of Adipose-Derived Stem Cells Promotes Healing in Murine Model of Atrophic Non-Union

Non-union is defined as the permanent failure of a bone to heal and occurs clinically in 5% of fractures. Atrophic non-unions, characterized by absent/minimal callus formation, are poorly understood and difficult to treat. We recently demonstrated a novel murine model of atrophic non-union in the 3.6Col1A1-tk (Col1-tk) mouse, wherein dosing with the nucleoside analog ganciclovir (GCV) was used to deplete proliferating osteoprogenitor cells, leading to a radiographic and biomechanical non-union after the mid-shaft femur fracture. Using this Col1-tk atrophic non-union model, we hypothesized that the scaffold-mediated lentiviral delivery of doxycycline-inducible BMP-2 transgenes would induce osteogenesis at the fracture site. Cryogel scaffolds were used as a vehicle for GFP+ and BMP-2+ cell delivery to the site of non-union. Cryogel scaffolds were biofabricated through the cross-linking of a chitosan–gelatin polymer solution at subzero temperatures, which results in a macroporous, spongy structure that may be advantageous for a bone regeneration application. Murine adipose-derived stem cells were seeded onto the cryogel scaffolds, where they underwent lentiviral transduction. Following the establishment of atrophic non-unions in the femurs of Col1-tk mice (4 weeks post-fracture), transduced, seeded scaffolds were surgically placed around the site of non-union, and the animals were given doxycycline water to induce BMP-2 production. Controls included GFP+ cells on the cryogel scaffolds, acellular scaffolds, and sham (no scaffold). Weekly radiographs were taken, and endpoint analysis included micro-CT and histological staining. After 2 weeks of implantation, the BMP-2+ scaffolds were infiltrated with cartilage and woven bone at the non-union site, while GFP+ scaffolds had woven bone formation. Later, timepoints of 8 weeks had woven bone and vessel formation within the BMP-2+ and GFP + scaffolds with cortical bridging of the original fracture site in both groups. Overall, the cell-seeded cryogels promoted osseous healing. However, while the addition of BMP-2 promoted the endochondral ossification, it may provide a slower route to healing. This proof-of-concept study demonstrates the potential for cellularized cryogel scaffolds to enhance the healing of non-unions.

Effect of Intramedullary Nailing Patterns on Interfragmentary Strain in a Mouse Femur Fracture: A Parametric Finite Element Analysis

Delayed long bone fracture healing and nonunion continue to be a significant socioeconomic burden. While mechanical stimulation is known to be an important determinant of the bone repair process, understanding how the magnitude, mode, and commencement of interfragmentary strain (IFS) affect fracture healing can guide new therapeutic strategies to prevent delayed healing or nonunion. Mouse models provide a means to investigate the molecular and cellular aspects of fracture repair, yet there is only one commercially available, clinically-relevant, locking intramedullary nail (IMN) currently available for studying long bone fractures in rodents. Having access to alternative IMNs would allow a variety of mechanical environments at the fracture site to be evaluated, and the purpose of this proof-of-concept finite element analysis study is to identify which IMN design parameters have the largest impact on IFS in a murine transverse femoral osteotomy model. Using the dimensions of the clinically relevant IMN as a guide, the nail material, distance between interlocking screws, and clearance between the nail and endosteal surface were varied between simulations. Of these parameters, changing the nail material from stainless steel (SS) to polyetheretherketone (PEEK) had the largest impact on IFS. Reducing the distance between the proximal and distal interlocking screws substantially affected IFS only when nail modulus was low. Therefore, IMNs with low modulus (e.g., PEEK) can be used alongside commercially available SS nails to investigate the effect of initial IFS or stability on fracture healing with respect to different biological conditions of repair in rodents.

Development and characterization of a predictive microCT-based non-union model in Fischer F344 rats

INTRODUCTION

Non-unions remain a clinical problem and are characterised by the failure to heal after a defined period of time. Current preclinical non-union models apply a wide variety of techniques to diminish intrinsic healing potential deviating from the clinical situation. The aim of this study was to develop and characterise a non-union model in rats using internal plate fixation without the need for additional healing insults, whereby bone healing can be longitudinally assessed using microCT. It was hypothesized that healing/non-unions can be accurately predicted at early time points by microCT.

MATERIALS AND METHODS

Female, skeletally mature Fischer F344 rats received a 2 mm or 1 mm femoral osteotomy, stabilized with either a 2 mm thick plate or a 1.25 mm thick plate. Healing was monitored by microCT over 14 weeks and histological analysis at euthanasia. The mechanical environment was characterised using finite element (FE) modelling and biomechanical testing.

RESULTS

The majority of animals receiving the 2 mm thick plate displayed poor healing responses in both the 2 mm and 1 mm defect size groups. Bone and cartilage formation were markedly improved using the 1.25 mm thick plate. MicroCT could accurately predict bone forming capacity at early time points (3-4 weeks).

CONCLUSIONS

The 2 mm thick plating system confers poor healing responses in female Fischer F344 rats, comparable to atrophic non-unions. By reducing plate thickness to increase interfragmentary strain within the defect site healing is improved, leading to borderline healing situations or increased abundance of cartilage tissue present in the defect site with ultimate failure to bridge the defect (hypertrophic non-union). Furthermore, microCT can reliably identify delayed/non-healing animals within 4 weeks, thereby allowing their selective targeting for the testing of novel, clinically relevant treatment strategies in different clinical situations aimed at restoring impaired bone healing.

Animal models of impaired long bone healing and tissue engineering- and cell-based in vivo interventions

Bone healing after injury typically follows a systematic process and occurs spontaneously under appropriate physiological conditions. However, impaired long bone healing is still quite common and may require surgical intervention. Various complications can result in different forms of impaired bone healing including nonunion, critical-size defects, or stress fractures. While a nonunion may occur due to impaired biological signaling and/or mechanical instability, a critical-size defect exhibits extensive bone loss that will not spontaneously heal. Comparatively, a stress fracture occurs from repetitive forces and results in a non-healing crack or break in the bone. Clinical standards of treatment vary between these bone defects due to their pathological differences. The use of appropriate animal models for modeling healing defects is critical to improve current treatment methods and develop novel rescue therapies. This review provides an overview of these clinical bone healing impairments and current animal models available to study the defects in vivo. The techniques used to create these models are compared, along with the outcomes, to clarify limitations and future objectives. Finally, rescue techniques focused on tissue engineering and cell-based therapies currently applied in animal models are specifically discussed to analyze their ability to initiate healing at the defect site, providing information regarding potential future therapies. In summary, this review focuses on the current animal models of nonunion, critical-size defects, and stress fractures, as well as interventions that have been tested in vivo to provide an overview of the clinical potential and future directions for improving bone healing.

Mouse Digit Tip Regeneration Is Mechanical Load Dependent

Amputation of the mouse digit tip results in blastema-mediated regeneration. In this model, new bone regenerates de novo to lengthen the amputated stump bone, resulting in a functional replacement of the terminal phalangeal element along with associated non-skeletal tissues. Physiological examples of bone repair, such as distraction osteogenesis and fracture repair, are well known to require mechanical loading. However, the role of mechanical loading during mammalian digit tip regeneration is unknown. In this study, we demonstrate that reducing mechanical loading inhibits blastema formation by attenuating bone resorption and wound closure, resulting in the complete inhibition of digit regeneration. Mechanical unloading effects on wound healing and regeneration are completely reversible when mechanical loading is restored. Mechanical unloading after blastema formation results in a reduced rate of de novo bone formation, demonstrating mechanical load dependence of the bone regenerative response. Moreover, enhancing the wound-healing response of mechanically unloaded digits with the cyanoacrylate tissue adhesive Dermabond improves wound closure and partially rescues digit tip regeneration. Taken together, these results demonstrate that mammalian digit tip regeneration is mechanical load-dependent. Given that human fingertip regeneration shares many characteristics with the mouse digit tip, these results identify mechanical load as a previously unappreciated requirement for de novo bone regeneration in humans. © 2021 American Society for Bone and Mineral Research (ASBMR).

Experimental Rat Model of Bony Defects in the Facet Joint Maintained with Bone Wax for the Study of Spinal Pain

Purpose

Studies using experimental rat models for low back pain due to facet-joint defects are scarce. This study used a novel experimental rat model to determine whether bony defects induced by facetectomy could be maintained by bone wax, thus mimicking spondylolysis, and to analyze the effect of the facetectomy on rat behavior.

Patients and Methods

Twelve 10-week-old male Wistar rats weighing 300–350 g were divided into group A (n = 6) that underwent unilateral facetectomy of the right L5-6 facet joint and group B (n = 6) that additionally applied water-soluble bone wax at the facetectomy site. The difference in the left and right stride length, detected by the footprint test, and change in the left and right facet joint area were compared before and 4 weeks after the experiment.

Results

Even though the difference between the left and right stride lengths of groups A and B was not statistically significant, in contrast to group A, group B showed a shorter stride length on the right side (p = 0.22 and 0.46, in group A and group B, respectively). The right facet joint area, where the facetectomy was performed, was significantly smaller in group B 4 weeks after surgery, but not in group A (p = 0.50 and < 0.01, in group A and group B, respectively).

Conclusion

Based on the results, we concluded that the bony defects, induced by facetectomy at the L5-6 facet joint, were maintained with bone wax. This study will provide an experimental model for bony defects in the facet joint.

What Did We Learn About Fracture Pain from Animal Models?

Progress in bone fracture repair research has been made possible due to the development of reproducible models of fracture in rodents with more clinically relevant fracture fixation, where there is considerably better assessment of the factors that affect fracture healing and/or novel therapeutics. However, chronic or persistent pain is one of the worst, longest-lasting and most difficult symptoms to manage after fracture repair, and an ongoing challenge remains for animal welfare as limited information exists regarding pain scoring and management in these rodent fracture models. This failure of adequate pre-clinical pain assessment following osteotomy in the rodent population may not only subject the animal to severe pain states but may also affect the outcome of the bone healing study. Animal models to study pain were also mainly developed in rodents, and there is increasing validation of fracture and pain models to quantitatively evaluate fracture pain and to study the factors that generate and maintain fracture pain and develop new therapies for treating fracture pain. This review aims to discuss the different animal models for fracture pain research and characterize what can be learned from using animal models of fracture regarding behavioral pain states and new molecular targets for future management of these behaviors.

Challenges in the Development of Biological Approaches for the Treatment of Degenerative Disc Disease

There are numerous innovative and promising approaches aimed at slowing, reversing, or healing degenerative disc disease. However, multiple treatment-specific impediments slow progress toward realizing the benefits of these therapies. First, the exact pathophysiology underlying degenerative disc disease remains complicated and challenging to study. In addition, the study of the spine and intervertebral disc in animal models is difficult to translate to humans, hindering the utility of preclinical research. Biological treatments are subject to the complex biomechanical environment in which native discs degenerate. The regulatory approval environment for these therapeutics will likely involve a high degree of scrutiny. Finally, patient selection and assessment of outcomes are a particular challenge in this clinical setting.

Reliability of Goldberg Scoring System in the Radiographic Evaluation of Bony Union after Bone Grafting

Background

Evaluation of bony union after bone grafting is very important in orthopedic surgery. The aim of this study was to verify inter- and intraobserver reliability of the Goldberg scoring system for radiographic evaluation of bony union after bone grafting in various situations of animal models.

Methods

Twenty-seven male C57/BL6 mice, which lack the ability to synthesize galactose-alpha-1,3-galactose (GalT KO mice), and 9 C57/BL6 mice carrying a wild-type gene were used as animal models. We divided the mice into four groups. In group 1, syngenic bone grafting and intramedullary fixation were performed (9 wild type C57BL/6 mice). In group 2, allogenic bone grafting was performed (9 GalT KO mice). In group 3, an alpha-galactosidase-treated porcine xenograft was transplanted into the femur to reduce the antigenicity (9 GalT KO mice). In group 4, a non-treated porcine xenobone grafting was performed (9 GalT KO mice). The level of radiographic bony union (Goldberg method) was assessed by three orthopedic surgeons. Intra- and interobserver reliability for radiographic evaluation was assessed.

Results

In the Goldberg scoring system, most of the radiographic measurements showed substantial to almost perfect intraobserver reliability. The total score showed substantial intraobserver reliability. The kappa coefficient (κ) of the first examiner was 0.603, the κ of the second examiner was 0.790, and the κ of the third examiner was 0.758. The scoring system showed substantial interobserver reliability. The κ of the first session was 0.641 and the κ of the second session was 0.649.

Conclusions

The Goldberg scoring system is a reliable tool for radiographic evaluation of bony union after bone grafting.

Skeletal regeneration for segmental bone loss: Vascularised grafts, analogues and surrogates

Massive segmental bone defects (SBD) are mostly treated by removing the fibula and transplanting it complete with blood supply. While revolutionary 50 years ago, this remains the standard treatment. This review considers different strategies to repair SBD and emerging potential replacements for this highly invasive procedure. Prior to the technical breakthrough of microsurgery, researchers in the 1960s and 1970s had begun to make considerable progress in developing non autologous routes to repairing SBD. While the breaktthrough of vascularised bone transplantation solved the immediate problem of a lack of reliable repair strategies, much of their prior work is still relevant today. We challenge the assumption that mimicry is necessary or likely to be successful and instead point to the utility of quite crude (from a materials technology perspective), approaches. Together there are quite compelling indications that the body can regenerate entire bone segments with few or no exogenous factors. This is important, as there is a limit to how expensive a bone repair can be and still be widely available to all patients since cost restraints within healthcare systems are not likely to diminish in the near future. STATEMENT OF SIGNIFICANCE: This review is significant because it is a multidisciplinary view of several surgeons and scientists as to what is driving improvement in segmental bone defect repair, why many approaches to date have not succeeded and why some quite basic approaches can be as effective as they are. While there are many reviews of the literature of grafting and bone repair the relative lack of substantial improvement and slow rate of progress in clinical translation is often overlooked and we seek to challenge the reader to consider the issue more broadly.

Ablation of Proliferating Osteoblast Lineage Cells After Fracture Leads to Atrophic Nonunion in a Mouse Model

Nonunion is defined as the permanent failure of a fractured bone to heal, often necessitating surgical intervention. Atrophic nonunions are a subtype that are particularly difficult to treat. Animal models of atrophic nonunion are available; however, these require surgical or radiation-induced trauma to disrupt periosteal healing. These methods are invasive and not representative of many clinical nonunions where osseous regeneration has been arrested by a "failure of biology". We hypothesized that arresting osteoblast cell proliferation after fracture would lead to atrophic nonunion in mice. Using mice that express a thymidine kinase (tk) "suicide gene" driven by the 3.6Col1a1 promoter (Col1-tk), proliferating osteoblast lineage cells can be ablated upon exposure to the nucleoside analog ganciclovir (GCV). Wild-type (WT; control) and Col1-tk littermates were subjected to a full femur fracture and intramedullary fixation at 12 weeks age. We confirmed abundant tk+ cells in fracture callus of Col-tk mice dosed with water or GCV, specifically many osteoblasts, osteocytes, and chondrocytes at the cartilage-bone interface. Histologically, we observed altered callus composition in Col1-tk mice at 2 and 3 weeks postfracture, with significantly less bone and more fibrous tissue. Col1-tk mice, monitored for 12 weeks with in vivo radiographs and micro-computed tomography (μCT) scans, had delayed bone bridging and reduced callus size. After euthanasia, ex vivo μCT and histology showed failed union with residual bone fragments and fibrous tissue in Col1-tk mice. Biomechanical testing showed a failure to recover torsional strength in Col1-tk mice, in contrast to WT. Our data indicates that suppression of proliferating osteoblast-lineage cells for at least 2 weeks after fracture blunts the formation and remodeling of a mineralized callus leading to a functional nonunion. We propose this as a new murine model of atrophic nonunion. © 2021 American Society for Bone and Mineral Research (ASBMR).

Effect of Bone-Marrow-Derived Mesenchymal Stem Cells on the Healing of Bone Fractures

This study was performed to evaluate the effectiveness of mesenchymal stem cells (MSCs) on bone healing and to assess the role of various chemical stimulants and mediators in healing. Forty female mice were randomly assigned to 4 groups (10 mice each) after the induction of fixed fractures: group I: received fixation only; group II: received phosphate-buffered saline (PBS); group III: received intralesion MSCs (IL-MSCs); and group IV: received intraperitoneal MSCs (IP-MSCs). Serum alkaline phosphatase (ALP) levels and the expression of the osteocalcin (OCN), bone morphogenetic protein-2 (BMP-2), and stromal-derived factor-1 (SDF-1) genes were measured. ALP reached baseline level only in IL-MSCs, whereas OCN reached baseline level in MSCs recipients (IL-MSCs and IP-MSCs). BMP-2 significantly increased in MSCs recipients 3 weeks postfracture and increased in all groups 8 weeks postfracture with significant increases in MSC recipients than the fixation and PBS groups. The highest BMP-2 expression was reached in IL-MSC group. MSCs either locally or systemically improves or accelerates the healing of bone fractures with better results obtained after local injection, as shown by biochemical, radiological, and histological findings. MSCs are effective candidates for bone regeneration.

Handheld bioprinting strategies for in situ wound dressing

Handheld bioprinting has recently attracted considerable attention as a technology to deliver biomaterials and/or cells to injury sites by using freeform, user-instructed deposition approaches, specifically targeted towards in situ wound dressing and healing. In this review, we present a concise introduction of handheld bioprinting, and a thorough discussion on design and manufacture of handheld bioprinters and choice over bioinks. Finally, the advantages, challenges, and prospective of the said technologies are elaborated. It is believed that handheld bioprinting will play an essential role in the field of in situ wound healing mainly due to its excellent portability, user-friendliness, cost-effectiveness, and amenability to various wound needs.

Non-union bone fractures

The human skeleton has remarkable regenerative properties, being one of the few structures in the body that can heal by recreating its normal cellular composition, orientation and mechanical strength. When the healing process of a fractured bone fails owing to inadequate immobilization, failed surgical intervention, insufficient biological response or infection, the outcome after a prolonged period of no healing is defined as non-union. Non-union represents a chronic medical condition not only affecting function but also potentially impacting the individual's psychosocial and economic well-being. This Primer provides the reader with an in-depth understanding of our contemporary knowledge regarding the important features to be considered when faced with non-union. The normal mechanisms involved in bone healing and the factors that disrupt the normal signalling mechanisms are addressed. Epidemiological considerations and advances in the diagnosis and surgical therapy of non-union are highlighted and the need for greater efforts in basic, translational and clinical research are identified.

Gene activated scaffolds incorporating star-shaped polypeptide-pDNA nanomedicines accelerate bone tissue regeneration in vivo

Increasingly, tissue engineering strategies such as the use of biomaterial scaffolds augmented with specific biological cues are being investigated to accelerate the regenerative process. For example, significant clinical challenges still exist in efficiently healing large bone defects which are above a critical size. Herein, we describe a cell-free, biocompatible and bioresorbable scaffold incorporating a novel star-polypeptide biomaterial as a gene vector. This gene-loaded scaffold can accelerate bone tissue repair in vivo in comparison to a scaffold alone at just four weeks post implantation in a critical sized bone defect. This is achieved via the in situ transfection of autologous host cells which migrate into the implanted collagen-based scaffold via gene-loaded, star-shaped poly(l-lysine) polypeptides (star-PLLs). In vitro, we demonstrate that star-PLL nanomaterials designed with 64 short poly(l-lysine) arms can be used to functionalise a range of collagen based scaffolds with a dual therapeutic cargo (pDual) of the bone-morphogenetic protein-2 plasmid (pBMP-2) and vascular endothelial growth factor plasmid (pVEGF). The versatility of this polymeric vector is highlighted in its ability to transfect Mesenchymal Stem Cells (MSCs) with both osteogenic and angiogenic transgenes in a 3D environment from a range of scaffolds with various macromolecular compositions. In vivo, we demonstrate that a bone-mimetic, collagen-hydroxyapatite scaffold functionalized with star-PLLs containing either 32- or 64- poly(l-lysine) arms can be used to successfully deliver this pDual cargo to autologous host cells. At the very early timepoint of just 4 weeks, we demonstrate the 64-star-PLL-pDual functionalised scaffold as a particularly efficient platform to accelerate bone tissue regeneration, with a 6-fold increase in new bone formation compared to a scaffold alone. Overall, this article describes for the first time the incorporation of novel star-polypeptide biomaterials carrying two therapeutic genes into a cell free scaffold which supports accelerated bone tissue formation in vivo.

Fracture Healing Research-Shift towards In Vitro Modeling?

Fractures are one of the most frequently occurring traumatic events worldwide. Approximately 10% of fractures lead to bone healing disorders, resulting in strain for affected patients and enormous costs for society. In order to shed light into underlying mechanisms of bone regeneration (habitual or disturbed), and to develop new therapeutic strategies, various in vivo, ex vivo and in vitro models can be applied. Undeniably, in vivo models include the systemic and biological situation. However, transferability towards the human patient along with ethical concerns regarding in vivo models have to be considered. Fostered by enormous technical improvements, such as bioreactors, on-a-chip-technologies and bone tissue engineering, sophisticated in vitro models are of rising interest. These models offer the possibility to use human cells from individual donors, complex cell systems and 3D models, therefore bridging the transferability gap, providing a platform for the introduction of personalized precision medicine and finally sparing animals. Facing diverse processes during fracture healing and thus various scientific opportunities, the reliability of results oftentimes depends on the choice of an appropriate model. Hence, we here focus on categorizing available models with respect to the requirements of the scientific approach.

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

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

Three dimensional printed nanostructure biomaterials for bone tissue engineering

The suffering from organ dysfunction due to damaged or diseased tissue/bone has been globally on the rise. Current treatment strategies for non-union bone defects include: the use of autografts, allografts, synthetic grafts and free vascularized fibular grafts. Bone tissue engineering has emerged as an alternative for fracture repair to satisfy the current unmet need of bone grafts and to alleviate the problems associated with autografts and allografts. The technology offers the possibility to induce new functional bone regeneration using synergistic combination of functional biomaterials (scaffolds), cells, and growth factors. Bone scaffolds are typically made of porous biodegradable materials that provide the mechanical support during repair and regeneration of damaged or diseased bone. Significant progress has been made towards scaffold materials for structural support, desired osteogenesis and angiogenesis abilities. Thanks for innovative scaffolds fabrication technologies, bioresorbable scaffolds with controlled porosity and tailored properties are possible today. Despite the presence of different bone scaffold fabrication methods, pore size, shape and interconnectivity have not yet been fully controlled in most of the methods. Moreover, scaffolds with tailored porosity for specific defects are still difficult to manufacture. Nevertheless, such scaffolds can be designed and fabricated using three dimensional (3D) printing approaches. 3D printing technology, as an advanced tissue scaffold fabrication method, offers the opportunity to produce complex geometries with distinct advantages. The technology has been used for the production of various types of bodily constructs such as blood vessels, vascular networks, bones, cartilages, exoskeletons, eyeglasses, cell cultures, tissues, organs and novel drug delivery devices. This review focuses on 3D printed scaffolds and their application in bone repair and regeneration. In addition, different classes of biomaterials commonly employed for the fabrication of 3D nano scaffolds for bone tissue engineering application so far are briefly discussed.

Glycyrrhizic Acid Promotes Osteogenic Differentiation of Human Bone Marrow Stromal Cells by Activating the Wnt/β-Catenin Signaling Pathway

Glycyrrhizic acid (GA) is a major triterpene glycoside isolated from liquorice root that has been shown to inhibit osteoclastogenesis. However, there have been no reports regarding the effect of GA on osteogenic differentiation. Therefore, this study was performed to explore the effects and mechanism of action of GA on osteogenesis. A CCK-8 array was used to assess cell viability. The osteogenic capability was investigated by real-time quantitative PCR, western blotting and immunofluorescence analyses. ALP staining and ARS were used to evaluate ALP activity and mineralization, respectively. GA-GelMA hydrogels were designed to verify the therapeutic effects of GA in vivo by radiographic analysis and histological evaluation. Our results show that GA had no significant influence on the viability or proliferation of human bone marrow stromal cells (hBMSCs). GA promoted osteogenic differentiation and enhanced calcium deposition. Furthermore, ratio of active β-catenin and total β-catenin protein increased after treatment with GA. Wnt/catenin signaling inhibitor partially attenuated the effects of GA on osteogenic differentiation. In a mouse femoral fracture model, GA-GelMA hydrogels accelerated bone healing. Our results show that GA promotes the osteogenic differentiation of hBMSCs by modulating the Wnt/β-catenin signaling pathway. GA-GelMA hydrogels promoted bone fracture healing. GA has potential as a cost-effective treatment of bone defects.

Development of a tibial experimental non-union model in rats

BACKGROUND

Many non-union animal models have been developed to explore the problems surrounding fracture healing. However, the existing models are not perfect and cannot satisfy all non-union studies. This study aimed to make a non-union model of the tibia in rats by cauterization of the posterior of 2 mm on both sides of the fracture end after open osteotomy of the tibia and fixing the fractured tibia with a Kirschner wire 0.8 mm in diameter.

METHODS

For this study, 96 female adult Sprague-Dawley (SD) rats were used. The rats underwent surgery to produce a tibial open fracture and were fixed with a 0.8-mm diameter Kirschner wire. In 48 of the rats, the periosteum proximal and distal to the fracture end was cauterized.

RESULTS

At 2, 4, 6, and 8 weeks after surgery, radiological and histological analysis showed typical physiological healing in the control group, and the healing rate was 100% at 6 weeks. But the non-union group was characterized by resorption of the fracture ends with few callus formations and no bridging callus formation, and the healing rate was 0% at 8 weeks.

CONCLUSIONS

This method represents a reproducible model to create atrophic non-unions. This model provides a new option for studying the basic healing mechanisms and evaluating new therapies for bone regeneration and treatment of non-unions.

Silicon Oxynitrophosphide Nanoscale Coating Enhances Antioxidant Marker-Induced Angiogenesis During in vivo Cranial Bone-Defect Healing

Critical-sized bone defects are challenging to heal because of the sudden and large volume of lost bone. Fixative plates are often used to stabilize defects, yet oxidative stress and delayed angiogenesis are contributing factors to poor biocompatibility and delayed bone healing. This study tests the angiogenic and antioxidant properties of amorphous silicon oxynitrophosphide (SiONPx) nanoscale-coating material on endothelial cells to regenerate vascular tissue in vitro and in bone defects. in vitro studies evaluate the effect of silicon oxynitride (SiONx) and two different SiONPx compositions on human endothelial cells exposed to ROS (eg, hydrogen peroxide) that simulates oxidative stress conditions. in vivo studies using adult male Sprague Dawley rats (approximately 450 g) were performed to compare a bare plate, a SiONPx-coated implant plate, and a sham control group using a rat standard-sized calvarial defect. Results from this study showed that plates coated with SiONPx significantly reduced cell death, and enhanced vascular tubule formation and matrix deposition by upregulating angiogenic and antioxidant expression (eg, vascular endothelial growth factor A, angiopoetin-1, superoxide dismutase 1, nuclear factor erythroid 2-related factor 2, and catalase 1). Moreover, endothelial cell markers (CD31) showed a significant tubular structure in the SiONPx coating group compared with an empty and uncoated plate group. This reveals that atomic doping of phosphate into the nanoscale coating of SiONx produced markedly elevated levels of antioxidant and angiogenic markers that enhance vascular tissue regeneration. This study found that SiONPx or SiONx nanoscale-coated materials enhance antioxidant expression, angiogenic marker expression, and reduce ROS levels needed for accelerating vascular tissue regeneration. These results further suggest that SiONPx nanoscale coating could be a promising candidate for titanium plate for rapid and enhanced cranial bone-defect healing. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

BMP-2 delivery strategy modulates local bone regeneration and systemic immune responses to complex extremity trauma

Bone nonunions arising from large bone defects and composite injuries remain compelling challenges for orthopedic surgeons. Biological changes associated with nonunions, such as systemic immune dysregulation, can contribute to an adverse healing environment. Bone morphogenetic protein 2 (BMP-2), an osteoinductive and potentially immunomodulatory growth factor, is a promising strategy; however, burst release from the clinical standard collagen sponge delivery vehicle can result in adverse side effects such as heterotopic ossification (HO) and irregular bone structure, especially when using supraphysiological BMP-2 doses for complex injuries at high risk for nonunion. To address this challenge, biomaterials that strongly bind BMP-2, such as heparin methacrylamide microparticles (HMPs), may be used to limit exposure and spatially constrain proteins within the injury site. Here, we investigate moderately high dose BMP-2 delivered in HMPs within an injectable hydrogel system in two challenging nonunion models exhibiting characteristics of systemic immune dysregulation. The HMP delivery system increased total bone volume and decreased peak HO compared to collagen sponge delivery of the same BMP-2 dose. Multivariate analyses of systemic immune markers showed the collagen sponge group correlated with markers that are hallmarks of systemic immune dysregulation, including immunosuppressive myeloid-derived suppressor cells, whereas the HMP groups were associated with immune effector cells, including T cells, and cytokines linked to robust bone regeneration. Overall, our results demonstrate that HMP delivery of moderately high doses of BMP-2 promotes repair of complex bone nonunion injuries and that local delivery strategies for potent growth factors like BMP-2 may positively affect the systemic immune response to traumatic injury.

Modeling Complex Orthopedic Trauma in Rodents: Bone, Muscle and Nerve Injury and Healing

Orthopedic injury can occur from a variety of causes including motor vehicle collision, battlefield injuries or even falls from standing. Persistent limb pain is common after orthopedic injury or surgery and presents a unique challenge, as the initiating event may result in polytrauma to bone, muscle, and peripheral nerves. It is imperative that we understand the tissue-specific and multicellular response to this unique type of injury in order to best develop targeted treatments that improve healing and regeneration. In this Mini Review we will first discuss current rodent models of orthopedic trauma/complex orthotrauma. In the second section, we will focus on bone-specific outcomes including imaging modalities, biomechanical testing and immunostaining for markers of bone healing/turnover. In the third section, we will discuss muscle-related pathology including outcome measures of fibrosis, muscle regeneration and tensile strength measurements. In the fourth section, we will discuss nervous system-related pathology including outcome measures of pain-like responses, both reflexive and non-reflexive. In all sections we will consider parallels between preclinical outcome measures and the functional and mechanistic findings of the human condition.

Effect of Implantation Ti-6Al-4V ELI in femoral bone defect regeneration of Sprague Dawley rat

Ti-6Al-4V ELI is one of the most commonly used dental implant restore function. The solution treatment temperature variation can significantly increase the strength, but it is not yet known the effect of these temperature variations on the alloy's biocompatibility properties. Twelve female Sprague Dawley rats were divided into six groups as follows: the treated group, the control group, and the defect group without implant material. In the treated group, the femur bone defect was implanted with as-cast Ti-6Al-4V ELI, 850°C, 950°C, and 1050°C heat-treated Ti-6Al-4V ELI implant material. The rats were euthanized after 30 days postimplantation and evaluated histologically. The results show that the histological scoring of the specimen for femur defect without implant material is 2 (fibrous union and fibrocartilaginous), score with implant as-cast is 2.5, the sample with 850°C heat treatment material is 2.5, 950°C is 2.5, and the temperature at 1050°C is 2.5. The score of 2.5 is between score 2 and score 3: hemorrhage, fibrous union, fibrocartilaginous microhemorrhage, and mineralized cartilage union. In conclusion, there is no effect of different heat treatment temperatures for Ti-6Al-4V ELI implant material in rat bone regeneration's maturation level.

Smoking Alters Inflammation and Skeletal Stem and Progenitor Cell Activity During Fracture Healing in Different Murine Strains

Smokers are at a higher risk of delayed union or nonunion after fracture repair. Few specific interventions are available for prevention because the molecular mechanisms that result in these negative sequelae are poorly understood. Murine models that mimic fracture healing in smokers are crucial in further understanding the local cellular and molecular alterations during fracture healing caused by smoking. We exposed three murine strains, C57BL/6J, 129X1/SvJ, and BALB/cJ, to cigarette smoke for 3 months before the induction of a midshaft transverse femoral osteotomy. We evaluated fracture healing 4 weeks after the osteotomy using radiography, micro-computed tomography (μCT), and biomechanical testing. Radiographic analysis demonstrated a significant decrease in the fracture healing capacity of smoking 129X1/SvJ mice. μCT results showed delayed remodeling of fracture calluses in all three strains after cigarette smoke exposure. Biomechanical testing indicated the most significant impairment in the functional properties of 129X1/SvJ in comparison with C57BL/6J and BALB/cJ mice after cigarette smoke exposure. Thus, the 129X1/SvJ strain is most suitable in simulating smoking-induced impaired fracture healing. Furthermore, in smoking 129X1/SvJ murine models, we investigated the molecular and cellular alterations in fracture healing caused by cigarette smoking using histology, flow cytometry, and multiplex cytokine/chemokine analysis. Histological analysis showed impaired chondrogenesis in cigarette smoking. In addition, the important reparative cell populations, including skeletal stem cells and their downstream progenitors, demonstrated decreased expansion after injury as a result of cigarette smoking. Moreover, significantly increased pro-inflammatory mediators and the recruitment of immune cells in fracture hematomas were demonstrated in smoking mice. Collectively, our findings demonstrate the significant cellular and molecular alterations during fracture healing impaired by smoking, including disrupted chondrogenesis, aberrant skeletal stem and progenitor cell activity, and a pronounced initial inflammatory response. © 2020 American Society for Bone and Mineral Research (ASBMR).

An overview of de novo bone generation in animal models

Some of the earliest success in de novo tissue generation was in bone tissue, and advances, facilitated by the use of endogenous and exogenous progenitor cells, continue unabated. The concept of one health promotes shared discoveries among medical disciplines to overcome health challenges that afflict numerous species. Carefully selected animal models are vital to development and translation of targeted therapies that improve the health and well-being of humans and animals alike. While inherent differences among species limit direct translation of scientific knowledge between them, rapid progress in ex vivo and in vivo de novo tissue generation is propelling revolutionary innovation to reality among all musculoskeletal specialties. This review contains a comparison of bone deposition among species and descriptions of animal models of bone restoration designed to replicate a multitude of bone injuries and pathology, including impaired osteogenic capacity.

Improvement of clinical fracture healing - What can be learned from mechano-biological research?

The most significant predictors of reoperation following operative management of fractures are the presence of a third degree open fracture, remaining fracture gaps and a transverse fracture. However clinical studies provide no information regarding the involvement of various soft tissues or how the mechanical environment affects revascularisation and bone healing. Here the results of experimental and numerical mechano-biological studies on fracture healing are summarized to provide guidance toward clinical treatment of fractures. In experimental studies, isolated muscle crush appeared to only temporarily impair fracture healing, with no significant effect to the final bone healing, whereas a more severe muscle trauma significantly reduced callus formation and biomechanical properties of the healed bones. An intraoperative trauma can furthermore impede vascularization. Surgical removal of the haematoma or periosteum disturbs fracture healing. While reaming for intramedullary nailing reduced blood flow in the bone during the early phase of bone healing, it did not affect the stiffness or strength of the final bone healing. The optimal conditions for rapid vascularization and bone healing result from fracture fixation that minimizes shearing movements in the healing zone while allowing moderate compressive movements. Bone healing is increasingly delayed with increasing fracture gap size and critical-size defects do not heal sufficiently independent of the mechanical environment. The stiffness of fracture fixation systems like nails and external fixators applied in clinical treatments frequently display a too low stiffness, whereas plate systems often cause a too stiff fixation that suppresses bone healing.

Ex vivo regional gene therapy with human adipose-derived stem cells for bone repair

BACKGROUND

The treatment of complex bone loss scenarios remains challenging. This study evaluates the efficacy of ex vivo regional gene therapy using transduced human adipose-derived stem cells (ASCs) overexpressing bone morphogenetic protein-2 (BMP-2) to treat critical-sized bone defects.

METHODS

Critical-sized femoral defects created surgically in immunocompromised rats were treated with ASCs transduced with a lentivirus encoding BMP-2 (Group 1, n = 14), or green fluorescent protein (Group 2, n = 5), nontransduced ASCs (Group 3, n = 5), or rhBMP-2 (Group 4, n = 14). At 12 weeks, femurs were evaluated for quantity and quality of bone formation with plain radiographs, micro-computed tomography, histology/histomorphometry, and biomechanical strength testing.

RESULTS

Thirteen of 14 samples in Group 1 and all 14 samples in Group 4 showed radiographic healing, while no samples in either Groups 2 or 3 healed. Groups 1 and 4 had significantly higher radiographic scores (p < 0.001), bone volume fraction (BV/TV) (p < 0.001), and bone area fraction (BA/TA) than Groups 2 and 3 (p < 0.001). Radiographic scores, BV/TV, and BA/TA were not significantly different between Groups 1 and 4. No difference with regards to mean torque, rotation at failure, torsional stiffness, and energy to failure was seen between Groups 1 and 4.

CONCLUSIONS

Human ASCs modified to overexpress BMP-2 resulted in abundant bone formation, with the quality of bone comparable to that of rhBMP-2. This strategy represents a promising approach in the treatment of large bone defects in the clinical setting.

CLINICAL RELEVANCE

Large bone defects may require sustained protein production to induce an appropriate osteoinductive response. Ex vivo regional gene therapy using a lentiviral vector has the potential to be part of a comprehensive tissue engineering strategy for treating osseous defects.

Individualized Techniques of Implant Coating with an Antibiotic-Loaded, Hydroxyapatite/Calcium Sulphate Bone Graft Substitute

BACKGROUND

The treatment of fracture- or non-union-related infections has persistently been a major challenge for both patients and treating surgeons. With rising aging of patients and increasing comorbidities, combined with the heterogeneity of germs and any number of multi-resistance against standard antibiotics, a successful treatment is increasingly difficult. One potential solution could be a custom-made individualized antibacterial coating of standard implants with a biphasic degradable biocarrier (Cerament G/V, supplied by Bonesupport AB, Lund, Sweden) that releases high doses of antibiotics around the bone-implant-interface. Here, we describe our technique of coating intramedullary nails, plates and press-fit shoulder endoprostheses which may prevent bacterial adhesion and biofilm formation. So far, there is very limited experience in individual coating of implants in hip or knee endoprostheses to prevent reoccurrence of surgical-site infection. Currently, no reports are available for coating of stems of shoulder prosthesis and nails or plates for fracture fixation.

METHODS

Here, we show our first experiences with a new individualized surgical technique of coating these implants with a resorbable antibiotic-loaded hydroxyapatite/calcium sulphate biocomposite to prevent biofilm formation and thereby recurrence of bone or joint infection. We describe three cases for coating of plates and nails for fracture fixation and coating of stems of a shoulder prosthesis.

RESULTS

No adverse events of the resorbable bone graft substitute were observed. In all of the cases, no recurrence of the infection was observed and osseointegration was achieved. After implant coating of the shoulder prosthesis, no radiological signs of loosening were detected.

CONCLUSION

We present a new surgical approach of a surface coating of plates, intramedullary nails or prostheses. The osteoconductive- and anti-inflammatory effect of the gentamicin- or vancomycin-loaded hydroxyapatite/calcium sulphate bone graft substitutes shows promising results.

Vascularization Strategies in the Prevention of Nonunion Formation

Delayed healing and nonunion formation are major challenges in orthopedic surgery, which require the development of novel treatment strategies. Vascularization is considered one of the major prerequisites for successful bone healing, providing an adequate nutrient supply and allowing the infiltration of progenitor cells to the fracture site. Hence, during the last decade, a considerable number of studies have focused on the evaluation of vascularization strategies to prevent or to treat nonunion formation. These involve (1) biophysical applications, (2) systemic pharmacological interventions, and (3) tissue engineering, including sophisticated scaffold materials, local growth factor delivery systems, cell-based techniques, and surgical vascularization approaches. Accumulating evidence indicates that in nonunions, these strategies are indeed capable of improving the process of bone healing. The major challenge for the future will now be the translation of these strategies into clinical practice to make them accessible for the majority of patients. If this succeeds, these vascularization strategies may markedly reduce the incidence of nonunion formation. Impact statement Delayed healing and nonunion formation are a major clinical problem in orthopedic surgery. This review provides an overview of vascularization strategies for the prevention and treatment of nonunions. The successful translation of these strategies in clinical practice is of major importance to achieve adequate bone healing.