Bone healing in an aged murine fracture model is characterized by sustained callus inflammation and decreased cell proliferation

Effects of aging on the immune and periosteal response to fracture injury

Aging predisposes individuals to reduced bone mass and fragility fractures, which are costly and linked to high mortality. Understanding how aging affects fracture healing is essential for developing therapies to enhance bone regeneration in older adults. During the inflammatory phase of fracture healing, immune cells are recruited to the injury site as periosteal skeletal stem/progenitor cells (pSSPCs) rapidly proliferate and differentiate into osteochondral lineages, allowing for fibrocartilaginous callus formation and, subsequently, complete bone healing. Irrespective of age, how periosteal mesenchymal and immune cells interact during early fracture healing is incompletely understood, limiting our ability to modulate this process. To address this, we directly analyzed, in parallel, at a single-cell level, isolated murine CD45(+) and CD45(-) periosteal cells dissected from intact and fractured bones, collected three days after injury. Comprehensive analysis, corroborated by bulk RNA-sequencing, flow cytometry, and histology, demonstrated that aging decreased pSSPC proliferation, markedly reduced expression of genes required for callus formation, and increased senescence signature. During the regeneration phase, at 14 days post injury, aged mice demonstrated reduced mineralization of the callus, accompanied by elevated Sox9 expression and increased cartilage content, suggesting delayed repair. We also found that the chemokine Cxcl9 was highly upregulated in aged intact Prrx1+ pSSPCs, which has the potential to directly regulate other pSSPCs, and was associated with increased recruitment of CD8+ T cells at the fracture site. Cell-to-cell communication analysis provided further appreciation of the complex interactions among the many mesenchymal and hematopoietic cell types regulating fracture healing and highlighted the impact of aging on these interactions. Together, these results provide insight into age-induced alterations in early fracture healing, which could facilitate the development of improved therapeutic approaches for fracture repair in the elderly.

Decoding fracture healing: A scoping review of mechanistic pathways derived from transcriptional analysis in murine studies

Fracture healing is a complex biological process involving orchestrated interactions among cells, growth factors, and transcriptional pathways. Despite significant advancements in understanding bone repair, non-union and delayed healing remain prevalent, especially in patients with comorbidities such as aging, diabetes, or substance use. Murine models serve as a cornerstone in fracture healing research, offering genetic manipulability, cost-effectiveness, and biological relevance to humans. This scoping review consolidates findings from studies conducted between 2010 and 2024, focusing on mechanistic pathways derived from transcriptional analysis in secondary bone healing as identified through bulk RNA sequencing of murine models. Key mechanistic pathways were categorized and analyzed across the distinct phases of fracture healing-reactive, reparative, and remodeling-highlighting their unique roles in inflammation, ECM remodeling, cell proliferation, and tissue mineralization. The most recurrent mechanistic pathways included ECM-receptor interaction, focal adhesion, and signaling mechanisms such as MAPK and TGF-beta. Variability in methodologies and limited overlap among studies underscore the need for standardized protocols in RNA sequencing analysis. Additionally, comparisons across long bone fractures, hole defects, and craniofacial bone healing revealed shared molecular mechanisms alongside unique challenges, particularly in craniofacial models. This scoping review underscores the promise of integrating systems biology approaches with transcriptomic data to elucidate the intricate regulatory networks governing fracture repair. Addressing the identified gaps in early-phase healing and harmonizing research methodologies will advance therapeutic strategies to reduce non-union rates and improve clinical outcomes.

Osteoporosis May Not Be an Absolute Contraindication for Cementless Total Knee Arthroplasty

BACKGROUND

Cementless total knee arthroplasty (TKA) has received growing interest, particularly in younger populations, due to potential long-term survivability and improved bone preservation. Poor bone stock, as seen in osteoporosis, is considered a contraindication for this technique. This study evaluated whether osteoporotic patients < 75 years undergoing cementless TKA demonstrate similar: 1) implant-related complications, 2) medical complications, 3) readmission rates, and 4) 3-year implant survivability.

METHODS

A retrospective query of a national administrative claims database was performed between 2010 and 2022 for patients less than or equal to 75 years old who have osteoporosis and underwent primary TKA. Osteoporotic patients were divided into cementless and cemented cohorts, and propensity scores were matched based on age, sex, obesity, and the Charlson Comorbidity Index. Matching produced 7,923 patients (1,321 uncemented, 6,602 cemented). Multivariate logistic regressions evaluated the following outcomes: 90-day and 2-year implant-related complications, 90-day postoperative medical complications, and 90-day readmissions. Kaplan-Meier survival analysis was conducted to assess 3-year all-cause revision implant survivability. The significance threshold was set to P < 0.01 to minimize type 1 bias.

RESULTS

There were no statistically significant differences in implant-related complications, medical complications, readmissions, and lengths of stay between cementless and cemented TKA groups. Kaplan-Meier analysis demonstrated statistically similar 3-year survivability between cohorts (cemented: 97.6%, confidence interval 96.6 to 98.5; cementless: 97.2%, confidence interval 96.7 to 97.7; P = 0.472).

CONCLUSIONS

Patients who have osteoporosis have equivalent medical and implant-related complications as well as 3-year implant survival following cementless TKA compared with a cemented technique. Our results support cementless TKA as a viable option for patients < 75 years, regardless of prior diagnosis of osteoporosis. Intraoperative decisions regarding bone quality are still necessary to discriminate between those who are candidates for cementless TKA with those who are not.

LEVEL OF EVIDENCE

Level III, Retrospective cohort study.

Effects of Aging on the Immune and Periosteal Response to Fracture in Mice

Aging predisposes individuals to reduced bone mass and fragility fractures, which are costly and linked to high mortality. Understanding how aging affects fracture healing is essential for developing therapies to enhance bone regeneration in older adults. During the inflammatory phase of fracture healing, immune cells are recruited to the injury site as periosteal skeletal stem/progenitor cells (pSSPCs) rapidly proliferate and differentiate into osteochondral lineages, allowing for fibrocartilaginous callus formation and complete bone healing. Irrespective of age, how periosteal mesenchymal and immune cells interact during early fracture healing is incompletely understood, limiting our ability to potentially modulate these processes. To address this, we directly analyzed, in parallel, at a single-cell level, isolated murine CD45(+) and CD45(-) periosteal cells dissected from intact and fractured bones, collected three days after injury. Through comprehensive analysis, corroborated by bulk RNA-sequencing, flow cytometry, and histology, we found aging decreases pSSPCs proliferative, marked by a reduced expression of genes required for callus formation and an increased senescence signature. We found that the chemokine Cxcl9 was highly upregulated in aged intact Prrx1+ pSSPCs, predicted to interact with other pSSPCs directly, and associated with increased recruitment of CD8+ T cells at the fracture site three days after injury. Cell-to-cell communication analysis provided insight into the complexity of interactions among the many cell types regulating fracture healing and the impact of aging on these processes. Together, these results provide insight into age-induced alterations in fracture healing, informing the development of improved therapeutic approaches for fragility fractures.

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.

Systemic inflammatory and gut microbiota responses to fracture in young and middle-aged mice

Age is a patient-specific factor that can significantly delay fracture healing and exacerbate systemic sequelae during convalescence. The basis for this difference in healing rates is not well-understood, but heightened inflammation has been suggested to be a significant contributor. In this study, we investigated the systemic cytokine and intestinal microbiome response to closed femur fracture in 3-month-old (young adult) and 15-month-old (middle-aged) female wild-type mice. Middle-aged mice had a serum cytokine profile that was distinct from young mice at days 10, 14, and 18 post-fracture. This was characterized by increased concentrations of IL-17a, IL-10, IL-6, MCP-1, EPO, and TNFα. We also observed changes in the community structure of the gut microbiota in both young and middle-aged mice that was evident as early as day 3 post-fracture. This included an Enterobacteriaceae bloom at day 3 post-fracture in middle-aged mice and an increase in the relative abundance of the Muribaculum genus. Moreover, we observed an increase in the relative abundance of the health-promoting Bifidobacterium genus in young mice after fracture that did not occur in middle-aged mice. There were significant correlations between serum cytokines and specific genera, including a negative correlation between Bifidobacterium and the highly induced cytokine IL-17a. Our study demonstrates that aging exacerbates the inflammatory response to fracture leading to high levels of pro-inflammatory cytokines and disruption of the intestinal microbiota.

A Murine Delayed-Healing Model Associates Immune Response with Functional Bone Regeneration after Trauma

Nonunion and delayed-union fractures pose a significant clinical challenge, often leading to prolonged morbidity and impaired quality of life. Fracture-induced hematoma and acute inflammation are crucial for establishing the healing cascade. However, aberrant inflammatory phenotypes can suppress healing and cause bone resorption. Elucidating these mechanisms is necessary to develop potent immunomodulatory therapies and prevent nonunion. Here, we report a delayed fracture healing model enabling the modulation of interfragmentary strain that mimics the etiology of hypertrophic nonunions to elucidate the role of dysregulated immune response in poor healing outcomes. High interfragmentary strain (>15%) was associated with larger callus volumes but delayed bone healing, increased inflammation, and inferior healing outcomes, while lower strain levels (<5%) corresponded to normal bone healing. In addition, we found distinct differences in the ossification, chondrification, and fibrosis patterns between high and low-strain groups, underscoring the significant impact of strain on the healing process. A comprehensive analysis of the systemic immune response revealed dynamic alterations in immune cell populations and factors, particularly within the early hours and days post-fracture. Several immune factors exhibited significant correlations with various functional healing outcomes, indicating their potential as predictive markers for assessing fracture healing progression. Our results also highlighted the significance of timely resolution of proinflammatory signals and the elevation of pro-regenerative immune cell phenotypes in promoting bone regeneration. Multivariate analysis revealed that CD25+ T-regulatory cells were influential in predicting proper bone healing, followed by CD206+ macrophages, underscoring the pivotal role of immune cell populations in the bone healing process. In conclusion, our study provides valuable insights into the intricate interplay between interfragmentary strain, immune response, and the ultimate outcomes of fracture healing. By shedding light on the underlying mechanisms that drive hypertrophic nonunion pathogenesis, our research lays the foundation for enhanced surgical management of nonunions and offers a promising avenue for developing targeted therapeutic interventions and personalized treatment strategies for individuals suffering from fracture nonunion.

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.

Aged Callus Skeletal Stem/Progenitor Cells Contain an Inflammatory Osteogenic Population With Increased IRF and NF-κB Pathways and Reduced Osteogenic Potential

Skeletal stem/progenitor cells (SSPCs) are critical for fracture repair by providing osteo-chondro precursors in the callus, which is impaired in aging. However, the molecular signatures of callus SSPCs during aging are not known. Herein, we performed single-cell RNA sequencing on 11,957 CD45-CD31-Ter119- SSPCs isolated from young and aged mouse calluses. Combining unsupervised clustering, putative makers, and DEGs/pathway analyses, major SSPC clusters were annotated as osteogenic, proliferating, and adipogenic populations. The proliferating cluster had a differentiating potential into osteogenic and adipogenic lineages by trajectory analysis. The osteoblastic/adipogenic/proliferating potential of individual clusters was further evidenced by elevated expression of genes related to osteoblasts, adipocytes, or proliferation. The osteogenic cluster was sub-clustered into house-keeping and inflammatory osteogenic populations that were decreased and increased in aged callus, respectively. The majority of master regulators for the inflammatory osteogenic population belong to IRF and NF-κB families, which was confirmed by immunostaining, RT-qPCR, and Western blot analysis. Furthermore, cells in the inflammatory osteogenic sub-cluster had reduced osteoblast differentiation capacity. In conclusion, we identified 3 major clusters in callus SSPCs, confirming their heterogeneity and, importantly, increased IRF/NF-κB-mediated inflammatory osteogenic population with decreased osteogenic potential in aged cells.

Toll-like receptor 9 (TLR9) gene deletion-mediated fracture healing in type II diabetic osteoporosis associates with inhibition of the nuclear factor-kappa B (NF-κB) signaling pathway

Diabetes is characterized by increased fracture risk. Evidence from in vivo studies is lacking for anti-fracture strategies in diabetes. Our microarray analyses predicted association of Toll-like receptor 9 (TLR9) with both diabetes and osteoporosis, which was the focus of this work in a murine model of type II diabetic osteoporosis (T2DOP). A T2DOP model with fracture was established in TLR9 knockout (TLR9^−/−) mice, which were then treated with the NF-κB signaling pathway inhibitor (PDTC) and activator (TNF-α). The obtained data suggested that TLR9 knockout augmented regeneration of bone tissues and cartilage area in the callus, and diminished fibrous tissues in T2DOP mice. Moreover, TLR9 depletion significantly affected bone mineral density (BMD), bone volume/tissue volume (BV/TV), connectivity density, trabecular number, trabecular separation and trabecular thickness, thus promoting fracture recovery. Bone morphology and structure were also improved in response to TLR9 depletion in T2DOP mice. TLR9 depletion inactivated NF-κB signaling in T2DOP mice. PDTC was found to enhance fracture healing in T2DOP mice, while TNF-α negated this effect. Collectively, these data indicate that TLR9 depletion may hold anti-fracture properties, making it a potential therapeutic target for T2DOP.

Abbreviations: Diabetic osteoporosis (DOP); bone mineral density (BMD); Toll-like receptors (TLRs); type 2 diabetes (T2D); Toll-like receptor 9 (TLR9); nuclear factor-kappaB (NF-κB); streptozotocin (STZ); type 2 diabetic osteoporosis (T2DOP); Gene Expression Omnibus (GEO); Kyoto encyclopedia of genes and genomes (KEGG); pyrrolidine dithiocarbamate (PDTC); computed tomography (CT); Hematoxylin–eosin (HE); bone morphogenetic protein 7 (BMP7); analysis of variance (ANOVA);

RNA-seq Analysis of Peri-Implant Tissue Shows Differences in Immune, Notch, Wnt, and Angiogenesis Pathways in Aged Versus Young Mice

The number of total joint replacements (TJRs) in the United States is increasing annually. Cementless implants are intended to improve upon traditional cemented implants by allowing bone growth directly on the surface to improve implant longevity. One major complication of TJR is implant loosening, which is related to deficient osseointegration in cementless TJRs. Although poor osseointegration in aged patients is typically attributed to decreased basal bone mass, little is known about the molecular pathways that compromise the growth of bone onto porous titanium implants. To identify the pathways important for osseointegration that are compromised by aging, we developed an approach for transcriptomic profiling of peri-implant tissue in young and aged mice using our murine model of osseointegration. Based on previous findings of changes of bone quality associated with aging, we hypothesized that aged mice have impaired activation of bone anabolic pathways at the bone-implant interface. We found that pathways most significantly downregulated in aged mice relative to young mice are related to angiogenic, Notch, and Wnt signaling. Downregulation of these pathways is associated with markedly increased expression of inflammatory and immune genes at the bone-implant interface in aged mice. These results identify osseointegration pathways affected by aging and suggest that an increased inflammatory response in aged mice may compromise peri-implant bone healing. Targeting the Notch and Wnt pathways, promoting angiogenesis, or modulating the immune response at the peri-implant site may enhance osseointegration and improve the outcome of joint replacement in older patients. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The effects of bone morphogenetic protein 2 and thrombopoietin treatment on angiogenic properties of endothelial cells derived from the lung and bone marrow of young and aged, male and female mice

With an aging world population, there is an increased risk of fracture and impaired healing. One contributing factor may be aging-associated decreases in vascular function; thus, enhancing angiogenesis could improve fracture healing. Both bone morphogenetic protein 2 (BMP-2) and thrombopoietin (TPO) have pro-angiogenic effects. The aim of this study was to investigate the effects of treatment with BMP-2 or TPO on the in vitro angiogenic and proliferative potential of endothelial cells (ECs) isolated from lungs (LECs) or bone marrow (BMECs) of young (3-4 months) and old (22-24 months), male and female, C57BL/6J mice. Cell proliferation, vessel-like structure formation, migration, and gene expression were used to evaluate angiogenic properties. In vitro characterization of ECs generally showed impaired vessel-like structure formation and proliferation in old ECs compared to young ECs, but improved migration characteristics in old BMECs. Differential sex-based angiogenic responses were observed, especially with respect to drug treatments and gene expression. Importantly, these studies suggest that NTN1, ROBO2, and SLIT3, along with angiogenic markers (CD31, FLT-1, ANGPT1, and ANGP2) differentially regulate EC proliferation and functional outcomes based on treatment, sex, and age. Furthermore, treatment of old ECs with TPO typically improved vessel-like structure parameters, but impaired migration. Thus, TPO may serve as an alternative treatment to BMP-2 for fracture healing in aging owing to improved angiogenesis and fracture healing, and the lack of side effects associated with BMP-2.

Resolution of inflammation in bone regeneration: From understandings to therapeutic applications

Impaired bone healing occurs in 5-10% of cases following injury, leading to a significant economic and clinical impact. While an inflammatory response upon injury is necessary to facilitate healing, its resolution is critical for bone tissue repair as elevated acute or chronic inflammation is associated with impaired healing in patients and animal models. This process is governed by important crosstalk between immune cells through mediators that contribute to resolution of inflammation in the local healing environment. Approaches modulating the initial inflammatory phase followed by its resolution leads to a pro-regenerative environment for bone regeneration. In this review, we discuss the role of inflammation in bone repair, the negative impact of dysregulated inflammation on bone tissue regeneration, and how timely resolution of inflammation is necessary to achieve normal healing. We will discuss applications of biomaterials to treat large bone defects with a specific focus on resolution of inflammation to modulate the immune environment following bone injury, and their observed functional benefits. We conclude the review by discussing future strategies that could lead to the realization of anti-inflammatory therapeutics for bone tissue repair.

Systemically impaired fracture healing in small animal research: A review of fracture repair models

Fracture healing is a complex process requiring mechanical stability, an osteoconductive matrix, and osteoinductive and osteogenic biology. This intricate process is easily disrupted by various patient factors such as chronic disease and lifestyle. As the medical complexity and age of patients with fractures continue to increase, the importance of developing relevant experimental models is becoming paramount in preclinical research. The objective of this review is to describe the most common small animal models of systemically impaired fracture healing used in the orthopedic literature including osteoporosis, diabetes mellitus, smoking, alcohol use, obesity, and ageing. This review will provide orthopedic researchers with a summary of current models of systemically impaired fracture healing used in small animals and present an overview of the methods of induction for each condition.

A novel mouse model to study fracture healing of the proximal femur

The majority of fractures, especially in elderly and osteoporotic patients, occurs in metaphyseal bone. However, only a few experimental models exist to study metaphyseal bone healing in mice. Currently used mouse models of metaphyseal fracture healing are either based on drill hole defects, lacking adequate biomechanical stimulation at the site of fracture and therefore endochondral ossification in the fracture callus, or are introduced into the distal part of the mouse femur stabilized by a locking plate, which is challenging due to the small specimen size. Therefore, the aim of the current study was to develop a new mouse model to study metaphyseal fracture healing of the proximal femur. We chose a combination between an open osteotomy and a closed intramedullary stabilization. A 24 G needle was inserted into the femur in a closed manner, then an osteotomy was made with a 0.4-mm Gigli wire saw between the third and the lesser trochanter of the femur using an open approach. Fractured femurs were analyzed using microcomputed tomography and histology at days 14 and 21 after surgery. No animals were lost due to surgery or anesthesia. All animals displayed normal limb loading and a physiological gait pattern within the first three days after fracture. We found robust endochondral ossification during the fracture healing process with high expression of late chondrocyte and early osteogenic markers at day 14 (d14). By day 21 (d21), all fractures had a bony bridging score of 3 or more, indicating successful healing. Callus volume significantly decreased from d14 to d21, whereas high numbers of osteoclasts appeared at the fracture callus until d21, indicating that callus remodeling had already started at d21. In conclusion, we successfully developed a novel mouse model to study endochondral fracture healing of the proximal femur. This model might be useful for future studies using transgenic animals to unravel molecular mechanisms of osteoporotic metaphyseal fracture healing.

Evaluation of global gene expression in regenerate tissues during Masquelet treatment

The Masquelet induced-membrane (IM) technique is indicated for large segmental bone defects. Attributes of the IM and local milieu that contribute to graft-to-bone union are unknown. Using a rat model, we compared global gene expression profiles in critically sized femoral osteotomies managed using a cement spacer as per Masquelet to those left empty. At the end of the experiment, IM and bone adjacent to the spacer were collected from the Masquelet side. Nonunion tissue in the defect and bone next to the empty defect were collected from the contralateral side. Tissues were subjected to RNA isolation, sequencing, and differential expression analysis. Cell type enrichment analysis suggested the IM and the bone next to the polymethylmethacrylate (PMMA) spacer were comparatively enriched for osteoblastic genes. The nonunion environment was comparatively enriched for innate and adaptive immune cell markers, but only macrophages were evident in the Masquelet context. iPathwayGuide was utilized to identify cell signaling pathways and protein interaction networks enriched in the Masquelet environment. For IM vs nonunion false-discovery rate correction of P values rendered overall pathway differences nonsignificant, and so only protein interaction networks are presented. For the bone comparison, substantial enrichment of pathways and networks known to contribute to osteogenic mechanisms was revealed. Our results suggest that the PMMA spacer affects the cut bone ends that are in contact with it and at the same time induces the foreign body reaction and formation of the IM. B cells in the empty defect suggest a chronic inflammatory response to a large segmental osteotomy.

Periosteum progenitors could stimulate bone regeneration in aged murine bone defect model

Periosteal stem cells are critical for bone regeneration, while the numbers will decrease with age. This study focused on whether Prx1⁺ cell, a kind of periosteal stem cell, could stimulate bone regeneration in aged mice. Four weeks and 12 months old Prx1CreER‐GFP; Rosa26^tdTomato mice were used to reveal the degree of Prx1⁺ cells participating in the femoral fracture healing procedure. One week, 8 weeks, 12 and 24 months old Prx1CreER‐GFP mice were used to analyse the real‐time distribution of Prx1⁺ cells. Twelve months old C57BL/6 male mice (n = 96) were used to create the bone defect model and, respectively, received hydrogel, hydrogel with Prx1⁻ mesenchymal stem cells and hydrogel with Prx1⁺ cells. H&E staining, Synchrotron radiation‐microcomputed tomography and mechanical test were used to analyse the healing results. The results showed that tdTomato⁺ cells were involved in bone regeneration, especially in young mice. At the same time, GFP⁺ cells decreased significantly with age. The Prx1⁺ cells group could significantly improve bone regeneration in the murine bone defect model via directly differentiating into osteoblasts and had better osteogenic differentiation ability than Prx1⁻ mesenchymal stem cells. Our finding revealed that the quantity of Prx1⁺ cells might account for decreased bone regeneration ability in aged mice, and transplantation of Prx1⁺ cells could improve bone regeneration at the bone defect site.

The Effect of Age on Fracture Healing Time in Metacarpal Fractures

Background: Older patients are treated for fracture with increasing frequency. Although studies on animals suggest that older mice and rats heal fractures more slowly, the clinical implications remain unclear. A better understanding of differences in healing with age can help customize fracture treatment. Our purpose was to retrospectively evaluate metacarpal fractures for healing time looking specifically at age-related differences. Methods: A retrospective review of patients treated for metacarpal fractures was conducted. Patients with incomplete charts or inadequate follow-up were excluded. One hundred ninety-eight charts were analyzed. Demographic and other patient factors were documented. Fracture characteristics and treatment type were documented. Fracture healing was determined clinically. Plain radiographs and examination were used in decision making. Results: Age was not associated with fracture healing time as a continuous variable (P = .09). Patients above 75 years were not associated with increased healing time (P = .58). Fracture characteristics were related to healing time: minimally displaced and comminuted fractures healed faster than oblique fractures, spiral fractures, or transverse fractures (P = .048). Patients undergoing surgery healed faster than those without surgery (P = .046). Renal failure negatively affected fracture healing time (P = .03). Diabetes, hypothyroidism, and gender were not associated with healing time. Complications were not associated with age or other patient or fracture-related factors. Conclusions: Age does not affect clinical fracture healing time in adult. Therefore, older patients do not require disparate treatment. Other fracture-related factors and considerations such as functional demand and support systems might influence treatment decisions in fracture care.

Gender differences in tibial fractures healing in normal and muscular dystrophic mice

Duchenne muscular dystrophy (DMD) patients have a high fracture risk and poor fracture healing. The dystrophin^-/- (mdx) mouse is a murine model of DMD and exhibits delayed bone fracture healing. Since our research team has shown that adult stem cells, such as muscle-derived stem cells, display a gender difference in their osteogenic potential with the male cells being more osteogenic, we hypothesize that a potential gender differences may exist during bone healing in normal and mdx mice. To test this hypothesis, wild-type (WT) and mdx mice underwent tibial fracture surgery and microCT live scanning biweekly. The mice were sacrificed at 6 weeks post-surgery and the calluses were collected for histological analysis. To further investigate the mechanism, another two sets of mice were sacrificed at 10 days after fracture for RNA extraction and gene expression analysis and histology. MicroCT results showed, at 6 weeks post- surgery, the calluses were larger but showed less remodeling in both normal and mdx male mice when compared to females, at the same time point. However, females had higher callus bone volume density and an increase in osteoclast (OCs) number. At 10 days after fracture surgery, male mice had formed larger calluses, whereas females formed well-remodeled calluses with more osteoblasts and a greater bone area for both WT and mdx mice. Higher IGF-1 expression was observed in male mdx mice when compared to their female counterparts, whereas female WT mice had higher BMP-9 expression when compared to WT males. In conclusion, male mice formed larger bone calluses than females during tibial fracture healing for both WT and mdx mice. This may be attributed to higher IGF-1 expression, activation of Wnt/β-catennin signaling pathway and greater OB numbers during callus formation. Female mice achieved better bone remodeling in the regenerated bone with higher bone quality due to increased OC numbers that promote faster remodeling of the fracture calluses, and higher BMP-9 expression levels. Therefore, gender is one of many factors that need to be considered for both animal and human bone research.

CTRP3 Regulates Endochondral Ossification and Bone Remodeling During Fracture Healing

C1q/TNF-related protein 3 (CTRP3) is a cytokine known to regulate a variety of metabolic processes. Though previously undescribed in the context of bone regeneration, high throughput gene expression experiments in mice identified CTRP3 as one of the most highly upregulated genes in fracture callus tissue. Hypothesizing a positive regulatory role for CTRP3 in bone regeneration, we phenotyped skeletal development and fracture healing in CTRP3 knockout (KO) and CTRP3 overexpressing transgenic (TG) mice relative to wild-type (WT) control animals. CTRP3 KO mice experienced delayed endochondral fracture healing, resulting in abnormal mineral distribution, the presence of periosteal marrow compartments, and a nonunion-like state. Decreased osteoclast number was also observed in CTRP3 KO mice, whereas CTRP3 TG mice underwent accelerated callus remodeling. Gene expression profiling revealed a broad impact on osteoblast/osteoclast lineage commitment and metabolism, including arrested progression toward mature skeletal lineages in the KO group. A single systemic injection of CTRP3 protein at the time of fracture was insufficient to phenocopy the chronic TG healing response in WT mice. By associating CTRP3 levels with fracture healing progression, these data identify a novel protein family with potential therapeutic and diagnostic value. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:00-19966, 2020.

Age-related changes to macrophages are detrimental to fracture healing in mice

The elderly population suffers from higher rates of complications during fracture healing that result in increased morbidity and mortality. Inflammatory dysregulation is associated with increased age and is a contributing factor to the myriad of age-related diseases. Therefore, we investigated age-related changes to an important cellular regulator of inflammation, the macrophage, and the impact on fracture healing outcomes. We demonstrated that old mice (24 months) have delayed fracture healing with significantly less bone and more cartilage compared to young mice (3 months). The quantity of infiltrating macrophages into the fracture callus was similar in old and young mice. However, RNA-seq analysis demonstrated distinct differences in the transcriptomes of macrophages derived from the fracture callus of old and young mice, with an up-regulation of M1/pro-inflammatory genes in macrophages from old mice as well as dysregulation of other immune-related genes. Preventing infiltration of the fracture site by macrophages in old mice improved healing outcomes, with significantly more bone in the calluses of treated mice compared to age-matched controls. After preventing infiltration by macrophages, the macrophages remaining within the fracture callus were collected and examined via RNA-seq analysis, and their transcriptome resembled macrophages from young calluses. Taken together, infiltrating macrophages from old mice demonstrate detrimental age-related changes, and depleting infiltrating macrophages can improve fracture healing in old mice.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Transcriptional profiling of intramembranous and endochondral ossification after fracture in mice

Bone fracture repair represents an important clinical challenge with nearly 1 million non-union fractures occurring annually in the U.S. Gene expression differs between non-union and healthy repair, suggesting there is a pattern of gene expression that is indicative of optimal repair. Despite this, the gene expression profile of fracture repair remains incompletely understood. In this work, we used RNA-seq of two well-established murine fracture models to describe gene expression of intramembranous and endochondral bone formation. We used top differentially expressed genes, enriched gene ontology terms and pathways, callus cellular phenotyping, and histology to describe and contrast these bone formation processes across time. Intramembranous repair, as modeled by ulnar stress fracture, and endochondral repair, as modeled by femur full fracture, exhibited vastly different transcriptional profiles throughout repair. Stress fracture healing had enriched differentially expressed genes associated with bone repair and osteoblasts, highlighting the strong osteogenic repair process of this model. Interestingly, the PI3K-Akt signaling pathway was one of only a few pathways uniquely enriched in stress fracture repair. Full fracture repair involved a higher level of inflammatory and immune cell related genes than did stress fracture repair. Full fracture repair also differed from stress fracture in a robust downregulation of ion channel genes following injury, the role of which in fracture repair is unclear. This study offers a broad description of gene expression in intramembranous and endochondral ossification across several time points throughout repair and suggests several potentially intriguing genes, pathways, and cells whose role in fracture repair requires further study.

Contextual Regulation of Skeletal Physiology by Notch Signaling

PURPOSE OF REVIEW

This article reviews the past 2 years of research on Notch signaling as it relates to bone physiology, with the goal of reconciling seemingly discrepant findings and identifying fruitful areas of potential future research.

RECENT FINDINGS

Conditional animal models and high-throughput omics have contributed to a greater understanding of the context-dependent role of Notch signaling in bone. However, significant gaps remain in our understanding of how spatiotemporal context and epigenetic state dictate downstream Notch phenotypes. Biphasic activation of Notch signaling orchestrates progression of mesenchymal progenitor cells through the osteoblast lineage, but there is a limited understanding of ligand- and receptor-specific functions. Paracrine Notch signaling through non-osteoblastic cell types contributes additional layers of complexity, and we anticipate impactful future work related to the integration of these cell types and signaling mechanisms.

Mechanical Loading Promotes the Expansion of Primitive Osteoprogenitors and Organizes Matrix and Vascular Morphology in Long Bone Defects

Elucidating the effects of mechanical stimulation on bone repair is crucial for optimization of the healing process. Specifically, the regulatory role that mechanical loading exerts on the osteogenic stem cell pool and vascular morphology during healing is incompletely understood. Because dynamic loading has been shown to enhance osteogenesis and repair, we hypothesized that loading induces the expansion of the osteoprogenitor cell population within a healing bone defect, leading to an increased presence of osteogenic cells. We further hypothesized that loading during the repair process regulates vascular and collagen matrix morphology and spatial interactions between vessels and osteogenic cells. To address these hypotheses, we used a mechanobiological bone repair model, which produces a consistent and reproducible intramembranous repair response confined in time and space. Bilateral tibial defects were created in adult C57BL/6 mice, which were subjected to axial compressive dynamic loading either during the early cellular invasion phase on postsurgical days (PSDs) 2 to 5 or during the matrix deposition phase on PSD 5 to 8. Confocal and two-photon microscopy was used to generate high-resolution three-dimensional (3D) renderings of longitudinal thick sections of the defect on PSD 10. Endomucin (EMCN)-positive vessels, Paired related homeobox 1 (Prrx1+) stem cell antigen-1 positive (Sca-1+) primitive osteoprogenitors (OPCs), and osterix positive (Osx+) preosteoblasts were visualized and quantified using deep tissue immunohistochemistry. New bone matrix was visualized with second harmonic generation autofluorescence of collagen fibers. We found that mechanical loading during the matrix deposition phase (PSD 5 to 8) increased vessel volume and number, and aligned vessels and collagen fibers to the load-bearing direction of bone. Furthermore, loading led to a significant increase in the proliferation and number of Prrx1+ Sca-1+ primitive OPCs, but not Osx+ preosteoblasts within the defect. Together, these data illustrate the adaptation of both collagen matrix and vascular morphology to better withstand mechanical load during bone repair, and that the mechanoresponsive cell population consists of the primitive osteogenic progenitors. © 2019 American Society for Bone and Mineral Research.

Relative effects of age on implant integration in a rat model: A longitudinal in vivo microct study

The effect of age on implant fixation in bone is not always considered during the design of preclinical models. The decision on animal's age is often related to practical or historical reasons, which ultimately may affect the reproducibility of results. This study aimed to quantify the effect of age by monitoring the fixation of contrast-enhanced PEEK screws in rats, hypothesizing that the kinetics of fixation is impaired in older animals but that age effects are less severe than osteoporotic effects. The time course of implant fixation was investigated in healthy rats at 24, 40, and 60 weeks of age; and in ovariectomized rats. Implant fixation was monitored using in-vivo microCT and dynamic histomorphometry during 1 month. The rats were euthanized 28 days post screw insertion. The data was analyzed both in absolute value and after normalization to baseline bone mass. In absolute terms, greater age had a detrimental effect on bone implant contact, bone fraction, implant stiffness, and bone remodeling but less than ovariectomy. Interestingly, once data was normalized to baseline bone mass this effect disappeared, suggesting that the physiologic response to implant placement was not affected by age. In conclusion, implant fixation kinetics is less affected by age than by baseline bone mass in this rat model. Animals of different ages can therefore be compared but data must be construed relatively to baseline bone mass and not in absolute terms. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 9999:1-12, 2018.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.