Burn and thoracic trauma alters fracture healing, systemic inflammation, and leukocyte kinetics in a rat model of polytrauma

Navigating Post-Traumatic Osteoporosis: A Comprehensive Review of Epidemiology, Pathophysiology, Diagnosis, Treatment, and Future Directions

Post-traumatic osteoporosis (PTO) presents a significant challenge in clinical practice, characterized by demineralization and decreased skeletal integrity following severe traumatic injuries. This literature review manuscript addresses the knowledge gaps surrounding PTO, encompassing its epidemiology, pathophysiology, risk factors, diagnosis, treatment, prognosis, and future directions. This review emphasizes the complexity of the etiology of PTO, highlighting the dysregulation of biomineralization processes, inflammatory cytokine involvement, hormonal imbalances, glucocorticoid effects, vitamin D deficiency, and disuse osteoporosis. Moreover, it underscores the importance of multidisciplinary approaches for risk mitigation and advocates for improved diagnostic strategies to differentiate PTO from other musculoskeletal pathologies. This manuscript discusses various treatment modalities, including pharmacotherapy, dietary management, and physical rehabilitation, while also acknowledging the limited evidence on their long-term effectiveness and outcomes in PTO patients. Future directions in research are outlined, emphasizing the need for a deeper understanding of the molecular mechanisms underlying PTO and the evaluation of treatment strategies' efficacy. Overall, this review provides a comprehensive overview of PTO and highlights avenues for future investigation to enhance clinical management and patient outcomes.

The local and systemic effects of immune function on fracture healing

The immune system plays an integral role in the regulation of cellular processes responsible for fracture healing. Local and systemic influences on fracture healing correlate in many ways with fracture-related outcomes, including soft tissue healing quality and fracture union rates. Impaired soft tissue healing, restricted perfusion of a fracture site, and infection also in turn affect the immune response to fracture injury. Modern techniques used to investigate the relationship between immune system function and fracture healing include precision medicine, using vast quantities of data to interpret broad patterns of inflammatory response. Early data from the PRECISE trial have demonstrated distinct patterns of inflammatory response in polytrauma patients, which thereby directly and indirectly regulate the fracture healing response. The clearly demonstrated linkage between immune function and fracture healing suggests that modulation of immune function has significant potential as a therapeutic target that can be used to enhance fracture healing.

Preclinical models of orthopaedic trauma: Orthopaedic Research Society (ORS) and Orthopaedic Trauma Association (OTA) symposium 2022

Orthopaedic trauma remains a leading cause of patient morbidity, mortality, and global health care burden. Although significant advances have been made in the diagnosis, treatment, and rehabilitation of these injuries, complications such as malunion, nonunion, infection, disuse muscle atrophy and osteopenia, and incomplete return to baseline function still occur. The significant inherent clinical variability in fracture care such as differing patient demographics, injury patterns, and treatment protocols make standardized and replicable study, especially of cellular and molecular based mechanisms, nearly impossible. Hence, the scientists dedicated to improving therapy and treatments for patients with orthopaedic trauma rely on preclinical models. Preclinical models have proven to be invaluable in understanding the timing between implant insertion and bacterial inoculation on the bioburden of infection. Posttraumatic arthritis (PTOA) can take years to develop clinically, but with a porcine pilon fracture model, posttraumatic arthritis can be reliably induced, so different surgical and therapeutic strategies can be tested in prevention. Conversely, the racehorse presents a well-accepted model of naturally occurring PTOA. With preclinical polytrauma models focusing on chest injury, abdominal injury, multiple fractures, and/or head injury, one can study how various injury patterns affect fracture healing can be systemically studied. Finally, these preclinical models serve as a translational bridge to for clinical application in human patients. With selection of the right preclinical model, studies can build a platform to decrease the risk of emerging technologies and provide foundational support for therapeutic clinical trials. In summary, orthopaedic trauma preclinical models allow scientists to simplify a complex clinical challenge, to understand the basic pathways starting with lower vertebrate models. Then, R&D efforts progress to higher vertebrate models to build in more complexity for translation of findings to the clinical practice.

Modulation of fracture healing by senescence-associated secretory phenotype (SASP): a narrative review of the current literature

The senescence-associated secretory phenotype (SASP) is a generic term for the secretion of cytokines, such as pro-inflammatory factors and proteases. It is a crucial feature of senescent cells. SASP factors induce tissue remodeling and immune cell recruitment. Previous studies have focused on the beneficial role of SASP during embryonic development, wound healing, tissue healing in general, immunoregulation properties, and cancer. However, some recent studies have identified several negative effects of SASP on fracture healing. Senolytics is a drug that selectively eliminates senescent cells. Senolytics can inhibit the function of senescent cells and SASP, which has been found to have positive effects on a variety of aging-related diseases. At the same time, recent data suggest that removing senescent cells may promote fracture healing. Here, we reviewed the latest research progress about SASP and illustrated the inflammatory response and the influence of SASP on fracture healing. This review aims to understand the role of SASP in fracture healing, aiming to provide an important clinical prevention and treatment strategy for fracture. Clinical trials of some senolytics agents are underway and are expected to clarify the effectiveness of their targeted therapy in the clinic in the future. Meanwhile, the adverse effects of this treatment method still need further study.

Fracture healing in a polytrauma rat model is influenced by mtDNA:cGAS complex mediated pro-inflammation

PURPOSE

The mitochondrial DNA (mtDNA) activated cyclic guanosine monophosphate-adenosine monophosphate synthase-stimulator of interferon genes (cGAS-STING) signaling pathway is a key player in mediating immune responses in autoimmune disorders and cancer. However, its role in severe trauma associated fracture healing is unknown. This study investigated if the cGAS-STING signaling pathway contributes to delayed bone healing in polytrauma (PT) fractures.

METHODS

For preliminary analyses, therapeutic dosage of RU.521 (cGAS inhibitor) (n = 2) was determined in C57BL/6 J mice by mass spectrometry, and IFNβ expression levels in serum and bronchioalveolar fluid (BALF) at 6 and 24 h (h) in RU.521/vehicle + mtDNA injected mice (n = 3/treatment and time point) was measured by ELISA. In the main study, plasma mtDNA was quantified by qPCR in a clinically relevant delayed fracture healing PT rat model with burn injury, blunt trauma, and a femoral fracture at 3 h post-trauma (hpt). Next, PT rats received either RU.521 (12 mg/kg in povidone; n = 8) or vehicle (povidone only; n = 5) immediately after injury and were followed up for 5 weeks post-trauma to assess bone regeneration by radiography and histology.

RESULTS

IFNβ levels were significantly decreased only at 24 h in BALF of RU.521 treated mice. At 3hpt mtDNA was significantly elevated in PT rats compared to rats without injury. When treated with RU.521, PT rats showed improvement in bone healing compared to vehicle control PT rats.

CONCLUSIONS

These data reveal that the cGAS-STING signaling pathway influences trauma-induced delayed bone healing. However, further evaluation of this pathway at the cellular and molecular levels to augment PT associated detrimental effects is needed.

Importance of Housekeeping Gene Optimization for the Analysis of mRNA Expression During Wound Healing in a Third-Degree Burn Injury Model

Wound healing evaluation methods in a third-degree burn injury model are categorized as histological (re-epithelialization and granulation tissue formation) and molecular (quantitative polymerase chain reaction). In general, mRNA expression is normalized to those of the housekeeping gene. Although the housekeeping gene expression is generally stable, it has been reported that the stability of these genes depends on the wound healing process and treatment method. In this study, we identified the most stable housekeeping gene (TATA-binding protein) for studying gene expression in a third-degree burn injury model, in which wound healing was promoted by grafting human amnion-derived mesenchymal cells. We investigated the wound healing effect of human amnion-derived mesenchymal cells in the injury model. The formation of granulation tissue, the differentiation from fibroblasts to myofibroblasts, and functional vascular structure were promoted in the full-thickness skin excision site by treatment with these cells. The expression of angiogenic, pro-inflammatory and anti-inflammatory related mRNA was measured and normalized to that of the housekeeping gene, showing that treatment with the cells promoted the infiltration of endothelial cells and differentiation of M1 and M2 macrophages. In conclusion, wound healing in a third-degree burn injury model can be accurately analyzed using the optimized housekeeping gene.

Neutralization of HMGB1 improves fracture healing and γδ T lymphocyte counts at the fracture site in a polytrauma rat model

Purpose

Delayed fracture healing is a common consequence of polytrauma (PT) occurring in patients with multiple injuries. We believe that when early release of high mobility group box 1 (HMGB1) molecules from necrotic tissues exceed their normal levels in blood, they dysregulate immune responses associated with normal healing. This study investigates the detrimental effect of such dysregulate immune responses by targeting HMGB1 in a PT rat model with debilitating injuries. We hypothesized that neutralization of extracellular HMGB1 immediately post-trauma would ameliorate local immune dysregulation and improve fracture healing in a PT rat model.

Methods

PT rats received a single dose of either anti-rat HMGB1 polyclonal antibody (PT-Ab HMGB1) or IgY isotype (PT-IgY), were left untreated (PT-C), or had a single injury/osteotomy only (OST). Fracture healing was evaluated by micro-computed tomography (µCT) and histology at 5 weeks; and macrophages and T cell counts within the fracture site were determined with flow cytometry  at 1 week.

Results

Notably, bone regeneration within the fracture site in PT-Ab HMGB1 rats was improved with comparable connective tissue organization than PT-C rats. Further, only γδTCR⁺ T cells, but not macrophages and CD4⁺ and CD8⁺ T cells, were diminished at the fracture site in PT-C and PT-IgY rats. Interestingly, the PT-Ab HMGB1 rats had increased γδTCR⁺ T cells compared to PT-C and PT-IgY, suggesting their potential role in regulating fracture healing.

Conclusions

Therefore, the initial burst of systemic HMGB1 following trauma may have a role in regulating bone regeneration via the modulation of a subclass of T cells within the fracture site, suggesting it’s importance as a therapeutic target in PT to combat immune dysregulation and delayed fracture healing.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40634-022-00453-3.

Influence of Thoracic Trauma on Fracture Healing in Long Bones-A Retrospective Analysis

Purpose: Pre-clinical studies indicate that concomitant thoracic trauma impairs fracture healing of long bones and reduces callus formation. The aim of this study was to investigate whether patients with accompanying chest trauma suffer from delayed fracture healing of long bones in comparison with patients with fractures of two long bones or isolated fractures. Patients and Methods: This is a clinical retrospective study from a level I trauma center. The patients were divided into three groups: (1) thoracic trauma and fracture of a long bone, (2) fractures of two long bones, (3) isolated fracture of a long bone. The fracture consolidation was defined using the radiographic union scale in tibial fractures (RUST). A RUST value of ≥10 six-to-eight months after definitive operative intervention represented complete fracture healing. Results: In the first group 19 (43.2%) fractures did not show full consolidation, in the second group 14 (45.2%) and 13 (41.9%) and in the third group 14 (36.8%). The analysis revealed no statistically significant differences between the groups regarding consolidation of the fractures six-to-eight months after definitive operative intervention (p = 0.84). Conclusions: Unlike previously reported pre-clinical data, this study did not demonstrate a negative effect on fracture consolidation in long bones when accompanied by thoracic trauma. Furthermore, the results demonstrated that concomitant fractures of two long bones does not have a negative effect on fracture consolidation.

Systemic T Cell Exhaustion Dynamics Is Linked to Early High Mobility Group Box Protein 1 (HMGB1) Driven Hyper-Inflammation in a Polytrauma Rat Model

We previously reported an early surge in high mobility group box protein 1 (HMGB1) levels in a polytrauma (PT) rat model. This study investigates the association of HMGB1 levels in mediating PT associated dysregulated immune responses and its influence on the cellular levels of receptor for advanced glycation end products (RAGE) and toll-like receptor 4 (TLR4). Using the same PT rat model treated with anti-HMGB1 polyclonal antibody, we evaluated changes in circulating inflammatory cytokines, monocytes/macrophages and T cells dynamics and cell surface expression of RAGE and TLR4 at 1, 3, and 7 days post-trauma (dpt) in blood and spleen. Notably, PT rats demonstrating T helper (Th)1 and Th2 cells type early hyper-inflammatory responses also exhibited increased monocyte/macrophage counts and diminished T cell counts in blood and spleen. In blood, expression of RAGE and TLR4 receptors was elevated on CD68+ monocyte/macrophages and severely diminished on CD4+ and CD8+ T cells. Neutralization of HMGB1 significantly decreased CD68+ monocyte/macrophage counts and increased CD4+ and CD8+ T cells, but not γδ+TCR T cells in circulation. Most importantly, RAGE and TLR4 expressions were restored on CD4+ and CD8+ T cells in treated PT rats. Overall, findings suggest that in PT, the HMGB1 surge is responsible for the onset of T cell exhaustion and dysfunction, leading to diminished RAGE and TLR4 surface expression, thereby possibly hindering the proper functioning of T cells.

What Is a Cytokine Storm and Should It Matter to Me?

The recent COVID-19 pandemic has brought attention to cytokines and the phenomenon of cytokine storm into mainstream discussions. In this disease specifically, a cytokine storm overwhelming immune response contributes to the pathophysiology and mortality of the COVID-19 infection. Analogous perturbed immune reactions are experienced in polytrauma patients, compromising local tissue healing while threatening multiple organ systems. The expanding field of osteoimmunology should contribute to the orthopaedic community's understanding of how the immune system response, whether normal or pathologic, affects the whole body outcome of our patients.

Intact Glucocorticoid Receptor Dimerization Is Deleterious in Trauma-Induced Impaired Fracture Healing

Following severe trauma, fracture healing is impaired because of overwhelming systemic and local inflammation. Glucocorticoids (GCs), acting via the glucocorticoid receptor (GR), influence fracture healing by modulating the trauma-induced immune response. GR dimerization-dependent gene regulation is essential for the anti-inflammatory effects of GCs. Therefore, we investigated in a murine trauma model of combined femur fracture and thoracic trauma, whether effective GR dimerization influences the pathomechanisms of trauma-induced compromised fracture healing. To this end, we used mice with decreased GR dimerization ability (GR^dim). The healing process was analyzed by cytokine/chemokine multiplex analysis, flow cytometry, gene-expression analysis, histomorphometry, micro-computed tomography, and biomechanical testing. GR^dim mice did not display a systemic or local hyper-inflammation upon combined fracture and thorax trauma. Strikingly, we discovered that GR^dim mice were protected from fracture healing impairment induced by the additional thorax trauma. Collectively and in contrast to previous studies describing the beneficial effects of intact GR dimerization in inflammatory models, we report here an adverse role of intact GR dimerization in trauma-induced compromised fracture healing.

Evaluation of Gas Explosion Injury Based on Analysis of Rat Serum Profile by Ultra-Performance Liquid Chromatography/Mass Spectrometry-Based Metabonomics Techniques

Gas explosion can lead to serious global public health issues. Early period gas explosion injury (GEI) can induce a series of histopathologic and specific metabolic changes. Unfortunately, it is difficult to treat GEI thoroughly. To date, the specific molecular mechanism of GEI is still unclear. To accurately diagnose and provide comprehensive clinical intervention, we performed a global analysis of metabolic alterations involved in GEI. The physiological and behavioral indicators' changes of rats after gas explosion were observed. These metabolic alterations were first investigated in a rat model using serum metabonomics techniques and multivariate statistical analysis. Significant heart rate (HR), mean blood pressure (mBP), and neurobehavioral index changes were observed in the GEI group after gas explosion. UPLC-MS revealed evident separated clustering between the control and GEI groups using supervised partial least squares discriminant analysis (PLS-DA). We designed an integrated metabonomics study for identifying reliable biomarkers of GEI using a time-course analysis of discriminating metabolites in this experiment. The metabonomics analysis showed alterations in a number of biomarkers (21 from serum). The meaningful biomarkers of GEI provide new insights into the pathophysiological changes and molecular mechanisms of GEI, including the disturbances in oxidative stress and neuroinflammatory reaction, as well as in metabolism of lipids, glucose, and amino acids in rats, suggesting that the process of GEI in humans is likely to be comprehensive and dynamic. Correlations between the GEI group and the biomarkers identified from the rat model will be further explored to elucidate the metabolic pathways responsible for GEI in the human body.

The Life of a Fracture: Biologic Progression, Healing Gone Awry, and Evaluation of Union

New knowledge about the molecular biology of fracture-healing provides opportunities for intervention and reduction of risk for specific phases that are affected by disease and medications. Modifiable and nonmodifiable risk factors can prolong healing, and the informed clinician should optimize each patient to provide the best chance for union. Techniques to monitor progression of fracture-healing have not changed substantially over time; new objective modalities are needed.

Time Course of Immune Response and Immunomodulation During Normal and Delayed Healing of Musculoskeletal Wounds

Single trauma injuries or isolated fractures are often manageable and generally heal without complications. In contrast, high-energy trauma results in multi/poly-trauma injury patterns presenting imbalanced pro- and anti- inflammatory responses often leading to immune dysfunction. These injuries often exhibit delayed healing, leading to fibrosis of injury sites and delayed healing of fractures depending on the intensity of the compounding traumas. Immune dysfunction is accompanied by a temporal shift in the innate and adaptive immune cells distribution, triggered by the overwhelming release of an arsenal of inflammatory mediators such as complements, cytokines and damage associated molecular patterns (DAMPs) from necrotic cells. Recent studies have implicated this dysregulated inflammation in the poor prognosis of polytraumatic injuries, however, interventions focusing on immunomodulating inflammatory cellular composition and activation, if administered incorrectly, can result in immune suppression and unintended outcomes. Immunomodulation therapy is promising but should be conducted with consideration for the spatial and temporal distribution of the immune cells during impaired healing. This review describes the current state of knowledge in the spatiotemporal distribution patterns of immune cells at various stages during musculoskeletal wound healing, with a focus on recent advances in the field of Osteoimmunology, a study of the interface between the immune and skeletal systems, in long bone fractures. The goals of this review are to (1) discuss wound and fracture healing processes of normal and delayed healing in skeletal muscles and long bones; (2) provide a balanced perspective on temporal distributions of immune cells and skeletal cells during healing; and (3) highlight recent therapeutic interventions used to improve fracture healing. This review is intended to promote an understanding of the importance of inflammation during normal and delayed wound and fracture healing. Knowledge gained will be instrumental in developing novel immunomodulatory approaches for impaired healing.