Stabilizing a Femur Osteotomy with a Plate Fixation in Ambystoma mexicanum.

The axolotl (Ambystoma mexicanum) is a promising model organism for regenerative medicine due to its remarkable ability to regenerate lost or damaged organs, including limbs, brain, heart, tail, and others. Studies on axolotl shed light on cellular and molecular pathways ruling progenitor activation and tissue restoration after injury. This knowledge can be applied to facilitate the healing of regeneration-incompetent injuries, such as bone non-union. In the current protocol, the femur osteotomy stabilization using an internal plate fixation system is described. The procedure was adapted for use in aquatic animals (axolotl, Ambystoma mexicanum). ≥20 cm snout-to-tail tip axolotls with fully ossified, mouse-size comparable femurs were used, and special attention was paid to the plate positioning and fixation, as well as to the postoperative care. This surgical technique allows for standardized and stabilized bone fixation and could be useful for direct comparison to axolotl limb regeneration and analogous studies of bone healing across amphibians and mammals.

External Mechanical Stability Regulates Hematoma Vascularization in Bone Healing Rather than Endothelial YAP/TAZ Mechanotransduction.

Bone fracture healing is regulated by mechanobiological cues. Both, extracellular matrix (ECM) deposition and microvascular assembly determine the dynamics of the regenerative processes. Mechanical instability as by inter-fragmentary shear or compression is known to influence early ECM formation and wound healing. However, it remains unclear how these external cues shape subsequent ECM and microvascular network assembly. As transcriptional coactivators, the mechanotransducers yes-associated protein 1 (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) translate physical cues into downstream signaling events, yet their role in sprouting angiogenesis into the hematoma after injury is unknown. Using bone healing as model system for scar-free regeneration, the role of endothelial YAP/TAZ in combination with tuning the extrinsic mechanical stability via fracture fixation is investigated. Extrinsically imposed shear across the gap delayed hematoma remodeling and shaped the morphology of early collagen fiber orientations and microvascular networks, suggesting that enhanced shear increased the nutrient exchange in the hematoma. In contrast, endothelial YAP/TAZ deletion has little impact on the overall vascularization of the fracture gap, yet slightly increases the collagen fiber deposition under semi-rigid fixation. Together, these data provide novel insights into the respective roles of endothelial YAP/TAZ and extrinsic mechanical cues in orchestrating the process of bone regeneration.

COMMBINI: an experimentally-informed COmputational Model of Macrophage dynamics in the Bone INjury Immunoresponse.

Bone fracture healing is a well-orchestrated but complex process that involves numerous regulations at different scales. This complexity becomes particularly evident during the inflammatory stage, as immune cells invade the healing region and trigger a cascade of signals to promote a favorable regenerative environment. Thus, the emergence of criticalities during this stage might hinder the rest of the process. Therefore, the investigation of the many interactions that regulate the inflammation has a primary importance on the exploration of the overall healing progression. In this context, an in silico model named COMMBINI (COmputational Model of Macrophage dynamics in the Bone INjury Immunoresponse) has been developed to investigate the mechano-biological interactions during the early inflammatory stage at the tissue, cellular and molecular levels. An agent-based model is employed to simulate the behavior of immune cells, inflammatory cytokines and fracture debris as well as their reciprocal multiscale biological interactions during the development of the early inflammation (up to 5 days post-injury). The strength of the computational approach is the capacity of the in silico model to simulate the overall healing process by taking into account the numerous hidden events that contribute to its success. To calibrate the model, we present an in silico immunofluorescence method that enables a direct comparison at the cellular level between the model output and experimental immunofluorescent images. The combination of sensitivity analysis and a Genetic Algorithm allows dynamic cooperation between these techniques, enabling faster identification of the most accurate parameter values, reducing the disparity between computer simulation and histological data. The sensitivity analysis showed a higher sensibility of the computer model to the macrophage recruitment ratio during the early inflammation and to proliferation in the late stage. Furthermore, the Genetic Algorithm highlighted an underestimation of macrophage proliferation by in vitro experiments. Further experiments were conducted using another externally fixated murine model, providing an independent validation dataset. The validated COMMBINI platform serves as a novel tool to deepen the understanding of the intricacies of the early bone regeneration phases. COMMBINI aims to contribute to designing novel treatment strategies in both the biological and mechanical domains.

Patients ' perspectives on bone replacement materials in a German university hospital setting.

BACKGROUND: The challenges in developing new bone replacement materials and procedures reside not solely in technological innovation and advancement, but also in a broader patient therapy acceptance. Therefore, there is a need to assess patients' perspectives on the materials and approaches in use as well as the ones being developed to better steer future progress in the field. METHODS: A self-initiating cross-sectional questionnaire aimed at people seeking treatment at the university hospital environment of Charité Berlin was formulated. The survey contained 15 close-ended questions directed toward the participant's epidemiological profile, willingness, acceptance, and agreement to receive different bone replacement materials, as well as, worries about the post-surgical consequences that can arise post bone replacement surgery. Descriptive and categorical analysis was performed to compare the observed number of subjects, their profile and each related response (Pearson's chi-square test or Fischer's test, p < 0.05). RESULTS: A total of 198 people engaged with the questionnaire, most of them Millennials. Overall patients trusted scientifically developed biomaterials designed for bone replacement, as demonstrated by their willingness to participate in a clinical trial, their acceptance of alloplastic materials, and the none/few worries about the presence of permanent implants. The data revealed the preferences of patients towards autologous sources of cells and blood to be used with a biomaterial. The data have also shown that both generation and education influenced willingness to participate in a clinical trial and acceptance of alloplastic materials, as well as, worries about the presence of permanent implants and agreement to receive a material with pooled blood and cells. CONCLUSION: Patients were open to the implantation of biomaterials for bone replacement, with a preference toward autologous sources of blood and/or tissue. Moreover, patients are concerned about strategies based on permanent implants, which indicates a need for resorbable materials. The knowledge gained in this study supports the development of new bone biomaterials.

Hydrogels with stiffness-degradation spatial patterns control anisotropic 3D cell response.

In nature, tissues are patterned, but most biomaterials used in human applications are not. Patterned biomaterials offer the opportunity to mimic spatially segregating biophysical and biochemical properties found in nature. Engineering such properties allows to study cell-matrix interactions in anisotropic matrices in great detail. Here, we developed alginate-based hydrogels with patterns in stiffness and degradation, composed of distinct areas of soft non-degradable (Soft-NoDeg) and stiff degradable (Stiff-Deg) material properties. The hydrogels exhibit emerging patterns in stiffness and degradability over time, taking advantage of dual crosslinking: Diels-Alder covalent crosslinking (norbornene-tetrazine, non degradable) and UV-mediated peptide crosslinking (matrix metalloprotease sensitive peptide, enzymatically degradable). The materials were mechanically characterized using rheology for single-phase and surface micro-indentation for patterned materials. 3D encapsulated mouse embryonic fibroblasts (MEFs) allowed to characterize the anisotropic cell-matrix interaction in terms of cell morphology by employing a novel image-based quantification tool. Live/dead staining showed no differences in cell viability but distinct patterns in proliferation, with higher cell number in Stiff-Deg materials at day 14. Patterns of projected cell area became visible already at day 1, with larger values in Soft-NoDeg materials. This was inverted at day 14, when larger projected cell areas were identified in Stiff-Deg. This shift was accompanied by a significant decrease in cell circularity in Stiff-Deg. The control of anisotropic cell morphology by the material patterns was also confirmed by a significant increase in filopodia number and length in Stiff-Deg materials. The novel image-based quantification tool was useful to spatially visualize and quantify the anisotropic cell response in 3D hydrogels with stiffness-degradation spatial patterns. Our results show that patterning of stiffness and degradability allows to control cell anisotropic response in 3D and can be quantified by image-based strategies. This allows a deeper understanding of cell-matrix interactions in a multicomponent material.

Uncovering the unique characteristics of the mandible to improve clinical approaches to mandibular regeneration.

The mandible (lower jaw) bone is aesthetically responsible for shaping the lower face, physiologically in charge of the masticatory movements, and phonetically accountable for the articulation of different phonemes. Thus, pathologies that result in great damage to the mandible severely impact the lives of patients. Mandibular reconstruction techniques are mainly based on the use of flaps, most notably free vascularized fibula flaps. However, the mandible is a craniofacial bone with unique characteristics. Its morphogenesis, morphology, physiology, biomechanics, genetic profile, and osteoimmune environment are different from any other non-craniofacial bone. This fact is especially important to consider during mandibular reconstruction, as all these differences result in unique clinical traits of the mandible that can impact the results of jaw reconstructions. Furthermore, overall changes in the mandible and the flap post-reconstruction may be dissimilar, and the replacement process of the bone graft tissue during healing can take years, which in some cases can result in postsurgical complications. Therefore, the present review highlights the uniqueness of the jaw and how this factor can influence the outcome of its reconstruction while using an exemplary clinical case of pseudoarthrosis in a free vascularized fibula flap.

The benefits of adipocyte metabolism in bone health and regeneration.

Patients suffering from musculoskeletal diseases must cope with a diminished quality of life and an increased burden on medical expenses. The interaction of immune cells and mesenchymal stromal cells during bone regeneration is one of the key requirements for the restoration of skeletal integrity. While stromal cells of the osteo-chondral lineage support bone regeneration, an excessive accumulation of cells of the adipogenic lineage is thought to promote low-grade inflammation and impair bone regeneration. Increasing evidence indicates that pro-inflammatory signaling from adipocytes is responsible for various chronic musculoskeletal diseases. This review aims to summarize the features of bone marrow adipocytes by phenotype, function, secretory features, metabolic properties and their impact on bone formation. In detail, the master regulator of adipogenesis and prominent diabetes drug target, peroxisome proliferator-activated receptor γ (PPARG), will be debated as a potential therapeutic approach to enhance bone regeneration. We will explore the possibilities of using clinically established PPARG agonists, the thiazolidinediones (TZDs), as a treatment strategy to guide the induction of a pro-regenerative, metabolically active bone marrow adipose tissue. The impact of this PPARG induced bone marrow adipose tissue type on providing the necessary metabolites to sustain osteogenic-as well as beneficial immune cells during bone fracture healing will be highlighted.

The enhancer landscape predetermines the skeletal regeneration capacity of stromal cells.

Multipotent stromal cells are considered attractive sources for cell therapy and tissue engineering. Despite numerous experimental and clinical studies, broad application of stromal cell therapeutics is not yet emerging. A major challenge is the functional diversity of available cell sources. Here, we investigated the regenerative potential of clinically relevant human stromal cells from bone marrow (BMSCs), white adipose tissue, and umbilical cord compared with mature chondrocytes and skin fibroblasts in vitro and in vivo. Although all stromal cell types could express transcription factors related to endochondral ossification, only BMSCs formed cartilage discs in vitro that fully regenerated critical-size femoral defects after transplantation into mice. We identified cell type-specific epigenetic landscapes as the underlying molecular mechanism controlling transcriptional stromal differentiation networks. Binding sites of commonly expressed transcription factors in the enhancer and promoter regions of ossification-related genes, including Runt and bZIP families, were accessible only in BMSCs but not in extraskeletal stromal cells. This suggests an epigenetically predetermined differentiation potential depending on cell origin that allows common transcription factors to trigger distinct organ-specific transcriptional programs, facilitating forward selection of regeneration-competent cell sources. Last, we demonstrate that viable human BMSCs initiated defect healing through the secretion of osteopontin and contributed to transient mineralized bone hard callus formation after transplantation into immunodeficient mice, which was eventually replaced by murine recipient bone during final tissue remodeling.

Landscape of Well-Coordinated Fracture Healing in a Mouse Model Using Molecular and Cellular Analysis.

The success of fracture healing relies on overlapping but coordinated cellular and molecular events. Characterizing an outline of differential gene regulation throughout successful healing is essential for identifying crucial phase-specific markers and may serve as the basis for engineering these in challenging healing situations. This study analyzed the healing progression of a standard closed femoral fracture model in C57BL/6N (age = 8 weeks) wild-type male mice. The fracture callus was assessed across various days post fracture (D = days 0, 3, 7, 10, 14, 21, and 28) by microarray, with D0 serving as a control. Histological analyses were carried out on samples from D7 until D28 to support the molecular findings. Microarray analysis revealed a differential regulation of immune response, angiogenesis, ossification, extracellular matrix regulation, mitochondrial and ribosomal genes during healing. In-depth analysis showed differential regulation of mitochondrial and ribosomal genes during the initial phase of healing. Furthermore, the differential gene expression showed an essential role of Serpin Family F Member 1 over the well-known Vascular Endothelial Growth Factor in angiogenesis, especially during the inflammatory phase. The significant upregulation of matrix metalloproteinase 13 and bone sialoprotein from D3 until D21 asserts their importance in bone mineralization. The study also shows type I collagen around osteocytes located in the ossified region at the periosteal surface during the first week of healing. Histological analysis of matrix extracellular phosphoglycoprotein and extracellular signal-regulated kinase stressed their roles in bone homeostasis and the physiological bone-healing process. This study reveals previously unknown and novel candidates, that could serve as a target for specific time points in healing and to remedy cases of impaired healing.

The decisive early phase of bone regeneration.

Bone has a remarkable endogenous regenerative capacity that enables scarless healing and restoration of its prior mechanical function, even under challenging conditions such as advanced age and metabolic or immunological degenerative diseases. However - despite much progress - a high number of bone injuries still heal with unsatisfactory outcomes. The mechanisms leading to impaired healing are heterogeneous, and involve exuberant and non-resolving immune reactions or overstrained mechanical conditions that affect the delicate regulation of the early initiation of scar-free healing. Every healing process begins phylogenetically with an inflammatory reaction, but its spatial and temporal intensity must be tightly controlled. Dysregulation of this inflammatory cascade directly affects the subsequent healing phases and hinders the healing progression. This Review discusses the complex processes underlying bone regeneration, focusing on the early healing phase and its highly dynamic environment, where vibrant changes in cellular and tissue composition alter the mechanical environment and thus affect the signalling pathways that orchestrate the healing process. Essential to scar-free healing is the interplay of various dynamic cascades that control timely resolution of local inflammation and tissue self-organization, while also providing sufficient local stability to initiate endogenous restoration. Various immunotherapy and mechanobiology-based therapy options are under investigation for promoting bone regeneration.

Long-term in vivo observations show biocompatibility and performance of ZX00 magnesium screws surface-modified by plasma-electrolytic oxidation in Göttingen miniature pigs.

Bioabsorbable magnesium implants for orthopedic fixation of bone have recently become available for different fields of indication. While general questions of biocompatibility have been answered, tailoring suitable degradation kinetics for specific applications as well as long-term tissue integration remain the focus of current research. The aim of this study was the evaluation of the long-term degradation behavior and osseointegration of Mg-Ca-Zn (ZX00MEO) based magnesium implants with plasma-electrolytic oxidation (PEO) surface modification (ZX00MEO-PEO) in comparison to non-surface modified implants in vivo and in vitro. Besides a general evaluation of the biological performance of the alloy over a prolonged period, the main hypothesis was that PEO surface modification significantly reduces implant degradation rate and improves tissue interaction. In vitro, the microstructure and surface of the bioabsorbable screws were characterized by SEM/EDS, cytocompatibility and degradation testing facilitating hydrogen gas evolution, carried out following ISO 10993-5/-12 and ASTM F3268-18a/ASTM G1-03 (E1:2017). In vivo, screws were implanted in the frontal bone of Minipigs for 6, 12, and 18 months, following radiological and histomorphometric analysis. A slower and more uniform degradation and improved cytocompatibility could be shown for the ZX00MEO-PEO group in vitro. A significant reduction of degradation rate and enhanced bone formation around the ZX00MEO-PEO screws in vivo was confirmed. Proficient biocompatibility and tissue integration could generally be shown in vivo regardless of surface state. The tested magnesium alloy shows generally beneficial properties as an implant material, while PEO-surface modification further improves the bioabsorption behavior both in vitro and in vivo. STATEMENT OF SIGNIFICANCE: Devices from bioabsorbable Magnesium have recently been introduced to orthopedic applications. However, the vast degradation of Magnesium within the human body still gives limitations. While reliable in-vivo data on most promising surface treatments such as Plasma-electrolytic-Oxidation is generally scarce, long-time results in large animals are to this date completely missing. To overcome this lack of evidence, we studied a Magnesium-Calzium-Zinc-alloy with surface enhancement by PEO for the first time ever over a period of 18 months in a large animal model. In-vitro, surface-modified screws showed significantly improved cytocompatibility and reduction of degradation confirmed by hydrogen gas evolution testing, while in-vivo radiological and histological evaluation generally showed good biocompatibility and bioabsorption as well as significantly enhanced reduction of degradation and faster bone regeneration in the PEO-surface-modified group.

In vivo immunomodulation of IL6 signaling in a murine multiple trauma model.

A significant number of trauma patients die during the ICU phase of care because of a severe immune response. Interleukin-6 (IL6) plays a central role within that immune response, signaling through a membrane-bound (IL6-R) and a soluble IL6 receptor (sIL6-R). IL6 and the sIL6-R can form an agonistic IL6/sIL6-R-complex, activating numerous cells that are usually not IL6 responsive, a process called trans-signaling. We attempted to demonstrate that modulation of the IL6 signaling (classic signaling and trans-signaling) can attenuate the devastating immune response after trauma in a murine multiple trauma model. Mice were allocated to three study arms: sham, fracture or polytrauma. Half of the animals had the application of an IL6-R antibody following an intervention. After a pre-set time, blood samples were analysed for IL6 and sIL6-R serum levels, organs were analysed for neutrophil infiltration and end organ damage was evaluated. IL6 and sIL6-R showed a rapid peak after fracture, and much more markedly after polytrauma. These parameters were reduced significantly by globally blocking IL6 signaling via IL6-R antibody (Mab) application. Shock organ analysis also illustrated significant neutrophil infiltration following polytrauma, which was also abated via IL6-R Mab application. Furthermore, end organ damage was reduced by IL6-R Mab application. The study results prove the regulatory role of IL6 signaling pathways in polytrauma, with haemorrhagic shock being a major trigger of inflammatory response. Modulation of IL6 signaling shows promise in the prevention of adverse events like organ failure following major trauma and might be a target for in vivo immunomodulation to reduce mortality in severely injured patients, but further evaluation regarding classic IL6 signaling and IL6 trans-signaling is needed.

Complex Spatio-Temporal Interplay of Distinct Immune and Bone Cell Subsets during Bone Fracture Healing.

BACKGROUND: The healing of a bone injury is a highly complex process involving a multitude of different tissue and cell types, including immune cells, which play a major role in the initiation and progression of bone regeneration. METHODS: We histologically analyzed the spatio-temporal occurrence of cells of the innate immune system (macrophages), the adaptive immune system (B and T lymphocytes), and bone cells (osteoblasts and osteoclasts) in the fracture area of a femoral osteotomy over the healing time. This study was performed in a bone osteotomy gap mouse model. We also investigated two key challenges of successful bone regeneration: hypoxia and revascularization. RESULTS: Macrophages were present in and around the fracture gap throughout the entire healing period. The switch from initially pro-inflammatory M1 macrophages to the anti-inflammatory M2 phenotype coincided with the revascularization as well as the appearance of osteoblasts in the fracture area. This indicates that M2 macrophages are necessary for the restoration of vessels and that they also play an orchestrating role in osteoblastogenesis during bone healing. The presence of adaptive immune cells throughout the healing process emphasizes their essential role for regenerative processes that exceeds a mere pathogen defense. B and T cells co-localize consistently with bone cells throughout the healing process, consolidating their crucial role in guiding bone formation. These histological data provide, for the first time, comprehensive information about the complex interrelationships of the cellular network during the entire bone healing process in one standardized set up. With this, an overall picture of the spatio-temporal interplay of cellular key players in a bone healing scenario has been created. CONCLUSIONS: A spatio-temporal distribution of immune cells, bone cells, and factors driving bone healing at time points that are decisive for this process-especially during the initial steps of inflammation and revascularization, as well as the soft and hard callus phases-has been visualized. The results show that the bone healing cascade does not consist of five distinct, consecutive phases but is a rather complex interrelated and continuous process of events, especially at the onset of healing.

Treatment options for critical size defects - Comparison of different materials in a calvaria split model in sheep.

Bone defects of the craniofacial skeleton are often associated with aesthetic and functional impairment as well as loss of protection to intra- and extracranial structures. Solid titanium plates and individually adapted bone cements have been the materials of choice, but may lead to foreign-body reactions and insufficient osseointegration. In contrast, porous scaffolds are thought to exhibit osteoconductive properties to support bone ingrowth. Here, we analyse in critical size defects of the calvaria in sheep whether different bone replacement materials may overcome those remaining challenges. In a critical size defect model, bilateral 20 × 20 × 5-mm craniectomies were performed on either side of the sagittal sinus in 24 adult female blackheaded sheep. Bony defects were randomised to one of five different bone replacement materials (BRMs): titanium scaffold, biodegradable poly(d,l-lactic acid) calcium carbonate scaffold (PDLLA/CC), polyethylene 1 (0.71 mm mean pore size) or 2 (0.515 mm mean pore size) scaffolds and polymethyl methacrylate (PMMA)-based bone cement block. Empty controls (n = 3) served as references. To evaluate bone growth over time, three different fluorochromes were administered at different time points. At 3, 6 and 12 months after surgery, animals were sacrificed and the BRMs and surrounding bone analysed by micro-CT and histomorphometry. The empty control group verified that the calvaria defect in this study was a reliable critical size defect model. Bone formation in vivo was detectable in all BRMs after 12 months by micro-CT and histomorphometric analysis, except for the non-porous PMMA group. A maximum of bone formation was detected in the 12-months group for titanium and PDLLA/CC. Bone formation in PDLLA/CC starts to increase rapidly between 6 and 12 months, as the BRM resorbs over time. Contact between bone and BRM influenced bone formation inside the BRM. Empty controls exhibited bone formation solely at the periphery. Overall, porous BRMs offered bone integration to different extent over 12 months in the tested calvaria defect model. Titanium and PDLLA/CC scaffolds showed remarkable osseointegration properties by micro-CT and histomorphometric analysis. PDLLA/CC scaffolds degraded over time without major residues. Pore size influenced bone ingrowth in polyethylene, emphasising the importance of porous scaffold structure.

Local immune cell contributions to fracture healing in aged individuals - A novel role for interleukin 22.

With increasing age, the risk of bone fractures increases while regenerative capacity decreases. This variation in healing potential appears to be linked to adaptive immunity, but the underlying mechanism is still unknown. This study sheds light on immunoaging/inflammaging, which impacts regenerative processes in aging individuals. In an aged preclinical model system, different levels of immunoaging were analyzed to identify key factors that connect immunoaged/inflammaged conditions with bone formation after long bone fracture. Immunological facets, progenitor cells, the microbiome, and confounders were monitored locally at the injury site and systemically in relation to healing outcomes in 12-month-old mice with distinct individual levels of immunoaging. Bone tissue formation during healing was delayed in the immunoaged group and could be associated with significant changes in cytokine levels. A prolonged and amplified pro-inflammatory reaction was caused by upregulated immune cell activation markers, increased chemokine receptor availability and a lack of inhibitory signaling. In immunoaged mice, interleukin-22 was identified as a core cell signaling protein that played a central role in delayed healing. Therapeutic neutralization of IL-22 reversed this specific immunoaging-related disturbed healing. Immunoaging was found to be an influencing factor of decreased regenerative capacity in aged individuals. Furthermore, a novel therapeutic strategy of neutralizing IL-22 may successfully rejuvenate healing in individuals with advanced immune experiences.

Histological Processing of CAD/CAM Titanium Scaffold after Long-Term Failure in Cranioplasty.

Cranioplasty is a frequently performed procedure after craniectomy and includes several techniques with different materials. Due to high overall complication rates, alloplastic implants are removed in many cases. Lack of implant material osseointegration is often assumed as a reason for failure, but no study has proven this in cranioplasty. This study histologically evaluates the osteointegration of a computer-aided design and computer-aided manufacturing (CAD/CAM) titanium scaffold with an open mesh structure used for cranioplasty. A CAD/CAM titanium scaffold was removed due to late soft tissue complications 7.6 years after cranioplasty. The histological analyses involved the preparation of non-decalcified slices from the scaffold's inner and outer sides as well as a light-microscopic evaluation, including the quantification of the bone that had formed over the years. Within the scaffold pores, vital connective tissue with both blood vessels and nerves was found. Exclusive bone formation only occurred at the edges of the implant, covering 0.21% of the skin-facing outer surface area. The inner scaffold surface, facing towards the brain, did not show any mineralization at all. Although conventional alloplastic materials for cranioplasty reduce surgery time and provide good esthetic results while mechanically protecting the underlying structures, a lack of adequate stimuli could explain the limited bone formation found. CAD/CAM porous titanium scaffolds alone insufficiently osseointegrate in such large bone defects of the skull. Future research should investigate alternative routes that enable long-term osteointegration in order to reduce complication rates after cranioplasty. Opportunities could be found in mechano-biologically optimized scaffolds, material modifications, surface coatings, or other routes to sustain bone formation.

MIF does only marginally enhance the pro-regenerative capacities of DFO in a mouse-osteotomy-model of compromised bone healing conditions.

The initial phase of fracture healing is crucial for the success of bone regeneration and is characterized by an inflammatory milieu and low oxygen tension (hypoxia). Negative interference with or prolongation of this fine-tuned initiation phase will ultimately lead to a delayed or incomplete healing such as non-unions which then requires an effective and gentle therapeutic intervention. Common reasons include a dysregulated immune response, immunosuppression or a failure in cellular adaptation to the inflammatory hypoxic milieu of the fracture gap and a reduction in vascularizing capacity by environmental noxious agents (e.g. rheumatoid arthritis or smoking). The hypoxia-inducible factor (HIF)-1α is responsible for the cellular adaptation to hypoxia, activating angiogenesis and supporting cell attraction and migration to the fracture gap. Here, we hypothesized that stabilizing HIF-1α could be a cost-effective and low-risk prevention strategy for fracture healing disorders. Therefore, we combined a well-known HIF-stabilizer - deferoxamine (DFO) - and a less known HIF-enhancer - macrophage migration inhibitory factor (MIF) - to synergistically induce improved fracture healing. Stabilization of HIF-1α enhanced calcification and osteogenic differentiation of MSCs in vitro. In vivo, only the application of DFO without MIF during the initial healing phase increased callus mineralization and vessel formation in a preclinical mouse-osteotomy-model modified to display a compromised healing. Although we did not find a synergistically effect of MIF when added to DFO, our findings provide additional support for a preventive strategy towards bone healing disorders in patients with a higher risk by accelerating fracture healing using DFO to stabilize HIF-1α.

Biomechanical Comparison of WE43-Based Magnesium vs. Titanium Miniplates in a Mandible Fracture Model in Sheep.

In fractures of the mandible, osteosynthesis with titanium plates is considered the gold standard. Titanium is an established and reliable material, its main disadvantages being metal artefacts and the need for removal in case of osteosynthesis complications. Magnesium, as a resorbable material with an elastic modulus close to cortical bone, offers a resorbable alternative osteosynthesis material, yet mechanical studies in mandible fracture fixation are still missing. The hypothesis of this study was that magnesium miniplates show no significant difference in the mechanical integrity provided for fracture fixation in mandible fractures under load-sharing indications. In a non-inferiority test, a continuous load was applied to a sheep mandible fracture model with osteosynthesis using either titanium miniplates of 1.0 mm thickness (Ti1.0), magnesium plates of 1.75 mm (Mg1.75), or magnesium plates of 1.5 mm thickness (Mg1.5). No significant difference (p > 0.05) was found in the peak force at failure, stiffness, or force at vertical displacement of 1.0 mm between Mg1.75, Mg1.5, and Ti1.0. This study shows the non-inferiority of WE43 magnesium miniplates compared to the clinical gold standard titanium miniplates.

Effects of immune cells on mesenchymal stem cells during fracture healing.

In vertebrates, bone is considered an osteoimmune system which encompasses functions of a locomotive organ, a mineral reservoir, a hormonal organ, a stem cell pool and a cradle for immune cells. This osteoimmune system is based on cooperatively acting bone and immune cells, cohabitating within the bone marrow. They are highly interdependent, a fact that is confounded by shared progenitors, mediators, and signaling pathways. Successful fracture healing requires the participation of all the precursors, immune and bone cells found in the osteoimmune system. Recent evidence demonstrated that changes of the immune cell composition and function may negatively influence bone healing. In this review, first the interplay between different immune cell types and osteoprogenitor cells will be elaborated more closely. The separate paragraphs focus on the specific cell types, starting with the cells of the innate immune response followed by cells of the adaptive immune response, and the complement system as mediator between them. Finally, a brief overview on the challenges of preclinical testing of immune-based therapeutic strategies to support fracture healing will be given.