Programmed removal of chondrocytes during endochondral fracture healing

Additively manufactured porous scaffolds by design for treatment of bone defects

There has been increasing attention to produce porous scaffolds that mimic human bone properties for enhancement of tissue ingrowth, regeneration, and integration. Additive manufacturing (AM) technologies, i.e., three dimensional (3D) printing, have played a substantial role in engineering porous scaffolds for clinical applications owing to their high level of design and fabrication flexibility. To this end, this review article attempts to provide a detailed overview on the main design considerations of porous scaffolds such as permeability, adhesion, vascularisation, and interfacial features and their interplay to affect bone regeneration and osseointegration. Physiology of bone regeneration was initially explained that was followed by analysing the impacts of porosity, pore size, permeability and surface chemistry of porous scaffolds on bone regeneration in defects. Importantly, major 3D printing methods employed for fabrication of porous bone substitutes were also discussed. Advancements of MA technologies have allowed for the production of bone scaffolds with complex geometries in polymers, composites and metals with well-tailored architectural, mechanical, and mass transport features. In this way, a particular attention was devoted to reviewing 3D printed scaffolds with triply periodic minimal surface (TPMS) geometries that mimic the hierarchical structure of human bones. In overall, this review enlighten a design pathway to produce patient-specific 3D-printed bone substitutions with high regeneration and osseointegration capacity for repairing large bone defects.

Research advances of nanomaterials for the acceleration of fracture healing

The bone fracture cases have been increasing yearly, accompanied by the increased number of patients experiencing non-union or delayed union after their bone fracture. Although clinical materials facilitate fracture healing (e.g., metallic and composite materials), they cannot fulfill the requirements due to the slow degradation rate, limited osteogenic activity, inadequate osseointegration ability, and suboptimal mechanical properties. Since early 2000, nanomaterials successfully mimic the nanoscale features of bones and offer unique properties, receiving extensive attention. This paper reviews the achievements of nanomaterials in treating bone fracture (e.g., the intrinsic properties of nanomaterials, nanomaterials for bone defect filling, and nanoscale drug delivery systems in treating fracture delayed union). Furthermore, we discuss the perspectives on the challenges and future directions of developing nanomaterials to accelerate fracture healing.

FOXO 1 deletion in chondrocytes rescues diabetes-impaired fracture healing by restoring angiogenesis and reducing apoptosis

Introduction

Diabetes mellitus is associated with higher risks of long bone and jaw fractures. It is also associated with a higher incidence of delayed union or non-union. Our previous investigations concluded that a dominant mechanism was the premature loss of cartilage during endochondral bone formation associated with increased osteoclastic activities. We tested the hypothesis that FOXO1 plays a key role in diabetes-impaired angiogenesis and chondrocyte apoptosis.

Methods

Closed fractures of the femur were induced in mice with lineage-specific FOXO1 deletion in chondrocytes. The control group consisted of mice with the FOXO1 gene present. Mice in the diabetic group were rendered diabetic by multiple streptozotocin injections, while mice in the normoglycemic group received vehicle. Specimens were collected 16 days post fracture. The samples were fixed, decalcified, and embedded in paraffin blocks for immunostaining utilizing anti cleaved caspase-3 or CD31 specific antibodies compared with matched control IgG antibody, and apoptosis by the TUNEL assay. Additionally, ATDC5 chondrocytes were examined in vitro by RT-PCR, luciferase reporter and chromatin immunoprecipitation assays.

Results

Diabetic mice had ~ 50% fewer blood vessels compared to normoglycemic mice FOXO1 deletion in diabetic mice partially rescued the low number of blood vessels (p < 0.05). Additionally, diabetes increased caspase-3 positive and apoptotic chondrocytes by 50%. FOXO1 deletion in diabetic animals blocked the increase in both to levels comparable to normoglycemic animals (p < 0.05). High glucose (HG) and high advanced glycation end products (AGE) levels stimulated FOXO1 association with the caspase-3 promoter in vitro, and overexpression of FOXO1 increased caspase-3 promoter activity in luciferase reporter assays. Furthermore, we review previous mechanistic studies demonstrating that tumor necrosis factor (TNF) inhibition reverses impaired angiogenesis and reverses high levels of chondrocyte apoptosis that occur in fracture healing.

Discussion

New results presented here, in combination with recent studies, provide a comprehensive overview of how diabetes, through high glucose levels, AGEs, and increased inflammation, impair the healing process by interfering with angiogenesis and stimulating chondrocyte apoptosis. FOXO1 in diabetic fractures plays a negative role by reducing new blood vessel formation and increasing chondrocyte cell death which is distinct from its role in normal fracture healing.

Bioceramics/Electrospun Polymeric Nanofibrous and Carbon Nanofibrous Scaffolds for Bone Tissue Engineering Applications

Bone tissue engineering integrates biomaterials, cells, and bioactive agents to propose sophisticated treatment options over conventional choices. Scaffolds have central roles in this scenario, and precisely designed and fabricated structures with the highest similarity to bone tissue have shown promising outcomes. On the other hand, using nanotechnology and nanomaterials as the enabling options confers fascinating properties to the scaffolds, such as precisely tailoring the physicochemical features and better interactions with cells and surrounding tissues. Among different nanomaterials, polymeric nanofibers and carbon nanofibers have attracted significant attention due to their similarity to bone extracellular matrix (ECM) and high surface-to-volume ratio. Moreover, bone ECM is a biocomposite of collagen fibers and hydroxyapatite crystals; accordingly, researchers have tried to mimic this biocomposite using the mineralization of various polymeric and carbon nanofibers and have shown that the mineralized nanofibers are promising structures to augment the bone healing process in the tissue engineering scenario. In this paper, we reviewed the bone structure, bone defects/fracture healing process, and various structures/cells/growth factors applicable to bone tissue engineering applications. Then, we highlighted the mineralized polymeric and carbon nanofibers and their fabrication methods.

Molecular enhancement of fracture healing - Is there a role for Bone Morphogenetic Protein-2, parathyroid hormone, statins, or sclerostin-antibodies?

Enhancement of fracture healing has been a hot topic over the last two decades. This narrative review article is aimed to provide an update on current clinical use and evidence on four clinically available agents in the treatment of fracture healing: bone morphogenetic proteins-2 (BMP-2), parathyroid hormone, statins and sclerostin-antibodies. After first promising results from animal and clinical studies in the early 2000s, BMP-2 was studied mainly in open tibia shaft fractures treated with intramedullary nailing. There are conflicting results from different randomized clinical trials (RCTs) regarding fracture healing time and complications compared to BMP-2 free control treatment in open tibia fractures, as BMP-2 could not show significant differences in patients treated with reamed nails compared to BMP-2 free control treatment with reamed nailing only. Given that fact, its official use was limited in Europe to open tibia shaft fractures treated with unreamed tibial nailing by the European Medical Agency (EMA). Another more recent RCT failed to show equivalence of BMP- 2 together with allograft versus autograft for the treatment of tibia fractures with critical size defects. Recombinant human parathyroid hormone has proven anabolic effects on bone metabolism and is commonly used in treatment of severe osteoporosis. Different animal trials suggested an enhancement effect in fracture healing by PTH. In several clinical trials, PTH seems to have a stimulative effect for lower limb fractures. Statins, commonly used in treatment of dyslipidemia, could also enhance fracture healing in animal trials, especially when they were applied locally at the fracture site. For statins, there is only one RCT that failed to show significant effects for the oral administration of statins in undisplaced distal radius fractures. The role of sclerostin in fracture healing has more and more been understood. Application of sclerostin antibodies has been shown to be beneficial for fracture healing in animal trials. However, no RCTs on the effect of sclerostin antibodies on fracture healing have been performed yet. In conclusion, the "magic bullet" for molecular enhancement of fracture healing has not been identified yet, at least not with its optimal dosage and delivery method.

Chondrogenesis Defines Future Skeletal Patterns Via Cell Transdifferentiation from Chondrocytes to Bone Cells

PURPOSE OF REVIEW

The goal of this review is to obtain a better understanding of how chondrogenesis defines skeletal development via cell transdifferentiation from chondrocytes to bone cells.

RECENT FINDINGS

A breakthrough in cell lineage tracing allows bone biologists to trace the cell fate and demonstrate that hypertrophic chondrocytes can directly transdifferentiate into bone cells during endochondral bone formation. However, there is a knowledge gap for the biological significance of this lineage extension and the mechanisms controlling this process. This review first introduces the history of the debate on the cell fate of chondrocytes in endochondral bone formation; then summarizes key findings obtained in recent years, which strongly support a new theory: the direct cell transdifferentiation from chondrocytes to bone cells precisely connects chondrogenesis (for providing a template of the future skeleton, classified as phase I) and osteogenesis (for finishing skeletal construction, or phase II) in a continuous lineage-linked process of endochondral bone formation and limb elongation; and finally outlines nutrition factors and molecules that regulate the cell transdifferentiation process during the relay from chondrogenesis to osteogenesis.

Review of Animal Models of Comorbidities in Fracture-Healing Research

There is clinical evidence that patient-specific comorbidities like osteoporosis, concomitant tissue injury, and ischemia may strongly interfere with bone regeneration. However, underlying mechanisms are still unclear. To study these mechanisms in detail, appropriate animal models are needed. For decades, bone healing has been studied in large animals, including dogs, rabbits, pigs, or sheep. However, large animal models display a limited ability to study molecular pathways and cellular functions. Therefore in recent years, mice and rats have become increasingly popular as a model organism for fracture healing research due to the availability of molecular analysis tools and transgenic models. Both large and small animals can be used to study comorbidities and risk factors, modelling the human clinical situation. However, attention has to be paid when choosing an appropriate model due to species differences between large animals, rodents, and humans. This review focuses on large and small animal models for the common comorbidities ischemic injury/reduced vascularization, osteoporosis, and polytrauma, and critically discusses the translational and molecular aspects of these models. Here, we review material which was presented at the workshop "Animal Models of Comorbidities in Fracture Healing Research" at the 2019 ORS Annual Meeting in Austin Texas. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2491-2498, 2019.

Role of Nrf2 in Fracture Healing: Clinical Aspects of Oxidative Stress

Fracture healing is a natural process that recapitulates embryonic skeletal development. In the early phase after fracture, reactive oxygen species (ROS) are produced under inflammatory and ischemic conditions due to vessel injury and soft tissue damage, leading to cell death. Usually, such damage during the course of fracture healing can be largely prevented by protective mechanisms and functions of antioxidant enzymes. However, intrinsic oxidative stress can cause excessive toxic radicals, resulting in irreversible damage to cells associated with bone repair during the fracture healing process. Clinically, patients with type-2 diabetes mellitus, osteoporosis, habitual drinkers, or heavy smokers are at risk of impaired fracture healing due to elevated oxidative stress. Although increased levels of oxidative stress markers upon fracture and effects of antioxidants on fracture healing have been reported, a detailed understanding of what causes impaired fracture healing under intrinsic conditions of oxidative stress is lacking. Nuclear factor erythroid 2-related factor 2 (Nrf2) has been identified as a key transcriptional regulator of the expression of antioxidants and detoxifying enzymes. It further not only plays a crucial role in preventing degenerative diseases in multiple organs, but also during fracture healing. This narrative review evaluates the influence of intrinsic oxidative stress on fracture healing and sheds new light on the intriguing role of Nrf2 during bone regeneration in pathological fractures.

Trauma in the Diabetic Limb

Poorly controlled diabetes with comorbid manifestations negatively affects outcomes in lower extremity trauma, increasing the risk of short-term and long-term complications. Management strategies of patients with diabetes that experience lower extremity trauma should also include perioperative management of hyperglycemia to reduce adverse and serious adverse events.

FOXO1 Deletion Reverses the Effect of Diabetic-Induced Impaired Fracture Healing

Type 1 diabetes impairs fracture healing. We tested the hypothesis that diabetes affects chondrocytes to impair fracture healing through a mechanism that involves the transcription factor FOXO1. Type 1 diabetes was induced by streptozotocin in mice with FOXO1 deletion in chondrocytes (Col2α1Cre+FOXO1L/L) or littermate controls (Col2α1Cre-FOXO1L/L) and closed femoral fractures induced. Diabetic mice had 77% less cartilage and 30% less bone than normoglycemics evaluated histologically and by micro-computed tomography. Both were reversed with lineage-specific FOXO1 ablation. Diabetic mice had a threefold increase in osteoclasts and a two- to threefold increase in RANKL mRNA or RANKL-expressing chondrocytes compared with normoglycemics. Both parameters were rescued by FOXO1 ablation in chondrocytes. Conditions present in diabetes, high glucose (HG), and increased advanced glycation end products (AGEs) stimulated FOXO1 association with the RANKL promoter in vitro, and overexpression of FOXO1 increased RANKL promoter activity in luciferase reporter assays. HG and AGE stimulated FOXO1 nuclear localization, which was reversed by insulin and inhibitors of TLR4, histone deacetylase, nitric oxide, and reactive oxygen species. The results indicate that chondrocytes play a prominent role in diabetes-impaired fracture healing and that high levels of glucose, AGEs, and tumor necrosis factor-α, which are elevated by diabetes, alter RANKL expression in chondrocytes via FOXO1.

Hybrid fracture fixation systems developed for orthopaedic applications: A general review

Orthopaedic implants are applied daily in our orthopaedic clinics for treatment of musculoskeletal injuries, especially for bone fracture fixation. To realise the multiple functions of orthopaedic implants, hybrid system that contains several different materials or parts have also been designed for application, such as prosthesis for total hip arthroplasty. Fixation of osteoporotic fracture is challenging as the current metal implants made of stainless steel or titanium that are rather rigid and bioinert, which are not favourable for enhancing fracture healing and subsequent remodelling. Magnesium (Mg) and its alloys are reported to possess good biocompatibility, biodegradability and osteopromotive effects during its in vivo degradation and now tested as a new generation of degradable metallic biomaterials. Several recent clinical studies reported the Mg-based screws for bone fixation, although the history of testing Mg as fixation implant was documented more than 100 years ago. Truthfully, Mg has its limitations as fixation implant, especially when applied at load-bearing sites because of rather rapid degradation. Currently developed Mg-based implants have only been designed for application at less or non-loading-bearing skeletal site(s). Therefore, after years research and development, the authors propose an innovative hybrid fixation system with parts composed of Mg and titanium or stainless steel to maximise the biological benefits of Mg; titanium or stainless steel in this hybrid system can provide enough mechanical support for fractures at load-bearing site(s) while Mg promotes the fracture healing through novel mechanisms during its degradation, especially in patients with osteoporosis and other metabolic disorders that are unfavourable conditions for fracture healing. This hybrid fixation strategy is designed to effectively enhance the osteoporotic fracture healing and may potentially also reduce the refracture rate. The translational potential of this article: This article systemically reviewed the combination utility of different metallic implants in orthopaedic applications. It will do great contribution to the further development of internal orthopaedic implants for fracture fixation. Meanwhile, it also introduced a titanium-magnesium hybrid fixation system as an alternative fixation strategy, especially for osteoporotic patients.

The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

Bone grafts and biomaterials substitutes for bone defect repair: A review

Bone grafts have been predominated used to treat bone defects, delayed union or non-union, and spinal fusion in orthopaedic clinically for a period of time, despite the emergency of synthetic bone graft substitutes. Nevertheless, the integration of allogeneic grafts and synthetic substitutes with host bone was found jeopardized in long-term follow-up studies. Hence, the enhancement of osteointegration of these grafts and substitutes with host bone is considerably important. To address this problem, addition of various growth factors, such as bone morphogenetic proteins (BMPs), parathyroid hormone (PTH) and platelet rich plasma (PRP), into structural allografts and synthetic substitutes have been considered. Although clinical applications of these factors have exhibited good bone formation, their further application was limited due to high cost and potential adverse side effects. Alternatively, bioinorganic ions such as magnesium, strontium and zinc are considered as alternative of osteogenic biological factors. Hence, this paper aims to review the currently available bone grafts and bone substitutes as well as the biological and bio-inorganic factors for the treatments of bone defect.

Augmentation of Fracture Healing Using Soft Callus

OBJECTIVES

This study sought to investigate the effect of soft callus removal and reapplication in a rat closed femur fracture model. We hypothesized that removing soft callus will impair fracture healing, whereas reapplication will facilitate healing.

METHODS

A closed midshaft femur fracture was created in 78 rats and stabilized with an intramedullary wire. Seven days later, rats were equally divided and fractures surgically exposed. In the control group, no callus was removed, whereas in the callus removal group CR(-) group, the callus was removed and in the callus replaced group CR(+), callus was removed and replaced. Half of the rats were killed at 4 and 7 weeks. Fracture healing was graded with radiographs and callus volume measured with micro-CT. Mechanical torsion properties were measured, and histologic analysis was conducted.

RESULTS

At both end points, evidence of delayed healing was found on radiographs and micro-CT in CR(-) rats (P = 0.0001), whereas CR(+) rats showed normal fracture healing compared with controls. The normalized callus volume was similar in all groups at both end points. At 7 weeks, the maximum stiffness in CR(-) rats was 68% less than control (P = 0.0001). Stiffness increased 55% in CR(+) rats from CR(-) rats (P = 0.0017). Histology supported our findings with complete endochondral ossification in CR(+) rats but wide areas of hyaline cartilage in CR(-) rats at 7 weeks.

CONCLUSIONS

Removal of soft callus in a rat model delays fracture healing at early and late time points, whereas replacement mitigates these negative consequences. Replacing the soft callus should be considered in all osteosynthesis procedures.

Bone fracture in a rat femoral fracture model is associated with the activation of autophagy

Autophagy, which is a mechanism for the turnover of intracellular molecules and organelles, protects cells during stress responses; however, the role of autophagy in the stages of bone fracture remains to be elucidated. The aim of the present study was to investigate the process of autophagy in bone tissue at different time-points after fracture. A femur fracture model was established in male adult Wistar rats via surgery. The protein expression of microtubule-associated protein II light chain 3 (LC3-II) was analyzed in a femur fracture (experimental) group and a sham-surgery group using immunofluorescence. The protein expression of proliferating cell nuclear antigen (PCNA) was used to investigate the cell proliferation in bone tissue following fracture via immunohistochemical analysis. The correlation between cell proliferation and autophagy was analyzed using linear regression. LC3-II protein was constitutively expressed in the sham-surgery group; however, compared with the expression in the sham-surgery group, the LC3-II expression in the experimental group was significantly increased at each time-point (P<0.05). Similarly, immunohistochemistry revealed that the number of PCNA-positive cells in each section was significantly increased following fracture injury (P<0.01). A comparison of the LC3-II- and PCNA-positive rates in the experimental group rats at each time-point revealed a linear correlation (R²=0.43, P<0.01). In conclusion, surgically induced fracture in rats is associated with an increase in LC3-II and PCNA protein expression during the initial stages of fracture injury, and a correlation exists between the expression of the two proteins. These results suggest that potential treatment aimed at improving fracture healing should target the process of autophagy.

Tendon regeneration and scar formation: The concept of scarless healing

Tendon healing is characterized by the formation of fibrovascular scar tissue, as tendon has very little intrinsic regenerative capacity. This creates a substantial clinical challenge in the setting of large, chronic tears seen clinically. Interest in regenerative healing seen in amphibians and certain strains of mice has arisen in response to the biological behavior of tendon tissue. Bone is also a model of tissue regeneration as healing bone will achieve the mechanical and histologic characteristics of the original tissue. The ultimate goal of the study of genes and mechanisms that contribute to true tissue regeneration is to ultimately attempt to manipulate the expression of those genes and activate these mechanisms in the setting of tendon injury and repair. Clearly, further research is needed to bring this to the forefront, however, study of scarless healing has potential to have meaningful application to tendon healing.

Fracture healing: mechanisms and interventions

Fractures are the most common large-organ, traumatic injuries to humans. The repair of bone fractures is a postnatal regenerative process that recapitulates many of the ontological events of embryonic skeletal development. Although fracture repair usually restores the damaged skeletal organ to its pre-injury cellular composition, structure and biomechanical function, about 10% of fractures will not heal normally. This article reviews the developmental progression of fracture healing at the tissue, cellular and molecular levels. Innate and adaptive immune processes are discussed as a component of the injury response, as are environmental factors, such as the extent of injury to the bone and surrounding tissue, fixation and the contribution of vascular tissues. We also present strategies for fracture treatment that have been tested in animal models and in clinical trials or case series. The biophysical and biological basis of the molecular actions of various therapeutic approaches, including recombinant human bone morphogenetic proteins and parathyroid hormone therapy, are also discussed.

The temporal and spatial development of vascularity in a healing displaced fracture

Underlying vascular disease is an important pathophysiologic factor shared among many co-morbid conditions associated with poor fracture healing, such as diabetes, obesity, and age. Determining the temporal and spatial patterns of revascularization following a fracture is essential for devising therapeutic strategies to augment this critical reparative process. Seminal studies conducted in the last century have investigated the pattern of vascularity in bone following a fracture. The consensus model culminating from these classical studies depicts a combination of angiogenesis emanating from both the intact intramedullary and periosteal vasculature. Subsequent to the plethora of experimental fracture angiography in the early to mid-20th century there has been a paucity of reports describing the pattern of revascularization of a healing fracture. Consequently the classical model of revascularization of a displaced fracture has remained largely unchanged. Here, we have overcome the limitations of animal fracture models performed in the above described classical studies by combining novel techniques of bone angiography and a reproducible murine femur fracture model to demonstrate for the first time the complete temporal and spatial pattern of revascularization in a displaced/stabilized fracture. These studies were designed specifically to i) validate the classical model of fracture revascularization of a displaced/stabilized fracture, ii) assess the association between intramedullary and periosteal angiogenesis and iii) elucidate the expression of VEGF/VEGF-R in relation to the classical model. From the studies, in conjunction with classic studies of angiogenesis during fracture repair, we propose a novel model (see abstract graphic) that defines the process of bone revascularization subsequent to injury to guide future approaches to enhance fracture healing. This new model validates and advances the classical model by providing evidence that during the process of revascularization of a displaced fracture 1) periosteal angiogenesis occurs in direct communication with the remaining intact intramedullary vasculature as a result of a vascular shunt and 2) vascular union occurs through an intricate interplay between intramembranous and endochondral VEGF/VEGF-R mediated angiogenesis.

Molecular mechanisms underlying skeletal growth arrest by cutaneous scarring

In pediatric surgeries, cutaneous scarring is frequently accompanied by an arrest in skeletal growth. The molecular mechanisms responsible for this effect are not understood. Here, we investigated the relationship between scar contracture and osteogenesis. An excisional cutaneous wound was made on the tail of neonatal mice. Finite element (FE) modeling of the wound site was used to predict the distribution and magnitude of contractile forces within soft and hard tissues. Morphogenesis of the bony vertebrae was monitored by micro-CT analyses, and vertebral growth plates were interrogated throughout the healing period using assays for cell proliferation, death, differentiation, as well as matrix deposition and remodeling. Wound contracture was grossly evident on post-injury day 7 and accompanying it was a significant shortening in the tail. FE modeling indicated high compressive strains localized to the dorsal portions of the vertebral growth plates and intervertebral disks. These predicted strain distributions corresponded to sites of increased cell death, a cessation in cell proliferation, and a loss in mineralization within the growth plates and IVD. Although cutaneous contracture resolved and skeletal growth rates returned to normal, vertebrae under the cutaneous wound remained significantly shorter than controls. Thus, localized contractile forces generated by scarring led to spatial alterations in cell proliferation, death, and differentiation that inhibited bone growth in a location-dependent manner. Resolution of cutaneous scarring was not accompanied by compensatory bone growth, which left the bony elements permanently truncated. Therefore, targeting early scar reduction is critical to preserving pediatric bone growth after surgery.

Role of Fas and Treg cells in fracture healing as characterized in the fas-deficient (lpr) mouse model of lupus

Previous studies showed that loss of tumor necrosis factor α (TNFα) signaling delayed fracture healing by delaying chondrocyte apoptosis and cartilage resorption. Mechanistic studies showed that TNFα induced Fas expression within chondrocytes; however, the degree to which chondrocyte apoptosis is mediated by TNFα alone or dependent on the induction of Fas is unclear. This question was addressed by assessing fracture healing in Fas-deficient B6.MRL/Fas(lpr) /J mice. Loss of Fas delayed cartilage resorption but also lowered bone fraction in the calluses. The reduced bone fraction was related to elevated rates of coupled bone turnover in the B6.MRL/Fas(lpr) /J calluses, as evidenced by higher osteoclast numbers and increased osteogenesis. Analysis of the apoptotic marker caspase 3 showed fewer positive chondrocytes and osteoclasts in calluses of B6.MRL/Fas(lpr) /J mice. To determine if an active autoimmune state contributed to increased bone turnover, the levels of activated T cells and Treg cells were assessed. B6.MRL/Fas(lpr) /J mice had elevated Treg cells in both spleens and bones of B6.MRL/Fas(lpr) /J but decreased percentage of activated T cells in bone tissues. Fracture led to ∼30% to 60% systemic increase in Treg cells in both wild-type and B6.MRL/Fas(lpr) /J bone tissues during the period of cartilage formation and resorption but either decreased (wild type) or left unchanged (B6.MRL/Fas(lpr) /J) the numbers of activated T cells in bone. These results show that an active autoimmune state is inhibited during the period of cartilage resorption and suggest that iTreg cells play a functional role in this process. These data show that loss of Fas activity specifically in chondrocytes prolonged the life span of chondrocytes and that Fas synergized with TNFα signaling to mediate chondrocyte apoptosis. Conversely, loss of Fas systemically led to increased osteoclast numbers during later periods of fracture healing and increased osteogenesis. These findings suggest that retention of viable chondrocytes locally inhibits osteoclast activity or matrix proteolysis during cartilage resorption.

Can platelet-rich plasma (PRP) improve bone healing? A comparison between the theory and experimental outcomes

The increased concentration of platelets within platelet-rich plasma (PRP) provides a vehicle to deliver supra-physiologic concentrations of growth factors to an injury site, possibly accelerating or otherwise improving connective tissue regeneration. This potential benefit has led to the application of PRP in several applications; however, inconsistent results have limited widespread adoption in bone healing. This review provides a core understanding of the bone healing mechanisms, and corresponds this to the factors present in PRP. In addition, the current state of the art of PRP preparation, the key aspects that may influence its effectiveness, and treatment outcomes as they relate specifically to bone defect healing are presented. Although PRP does have a sound scientific basis, its use for bone healing appears only beneficial when used in combination with osteoconductive scaffolds; however, neither allograft nor autograft appear to be appropriate carriers. Aggressive processing techniques and very high concentrations of PRP may not improve healing outcomes. Moreover, many other variables exist in PRP preparation and use that influence its efficacy; the effect of these variables should be understood when considering PRP use. This review includes the essentials of what has been established, what is currently missing in the literature, and recommendations for future directions.

Long bone fracture repair in mice harboring GFP reporters for cells within the osteoblastic lineage

GFP reporter mice previously developed to assess levels of osteoblast differentiation were employed in a tibial long bone fracture model using a histological method that preserves fluorescent signals in non-decalcified sections of bone. Two reporters, based on Col1A1 (Col3.6GFPcyan) and osteocalcin (OcGFPtpz) promoter fragments, were bred into the same mice to reflect an early and late stage of osteoblast differentiation. Three observations were apparent from this examination. First, the osteoprogenitor cells that arise from the flanking periosteum proliferate and progress to fill the fracture zone. These cells differentiate to osteoblasts, chondrocytes, to from the outer cortical shell. Second, the hypertrophic chondrocytes are dispersed and the cartilage matrix mineralized by the advancing Col3.6+ osteoblasts. The endochondral matrix is removed by the following osteoclasts. Third, a new cortical shell develops over the cartilage core and undergoes a remodeling process of bone formation on the inner surface and resorption on the outer surface. The original fractured cortex undergoes resorption as the outer cortical shell remodels inward to become the new diaphyseal bone. The fluorescent microscopy and GFP reporter mice used in this study provide a powerful tool for appreciating the molecular and cellular processes that control these fundamental steps in fracture repair, and may provide a basis for understanding fracture nonunion.

TNF-alpha mediates diabetes-enhanced chondrocyte apoptosis during fracture healing and stimulates chondrocyte apoptosis through FOXO1

To gain insight into the effect of diabetes on fracture healing, experiments were carried out focusing on chondrocyte apoptosis during the transition from cartilage to bone. Type 1 diabetes was induced in mice by multiple low-dose streptozotocin injections, and simple transverse fractures of the tibia or femur was carried out. Large-scale transcriptional profiling and gene set enrichment analysis were performed to examine apoptotic pathways on total RNA isolated from fracture calluses on days 12, 16, and 22, a period of endochondral bone formation when cartilage is resorbed and chondrocyte numbers decrease. Tumor necrosis factor alpha (TNF-alpha) protein levels were assessed by ELISA and caspase-3 by bioactivity assay. The role of TNF was examined by treating mice with the TNF-specific inhibitor pegsunercept. In vitro studies investigated the proapoptotic transcription factor FOXO1 in regulating TNF-induced apoptosis of chondrogenic ATDC5 and C3H10T1/2 cells as representative of differentiated chondrocytes, which are important during endochondral ossification. mRNA profiling revealed an upregulation of gene sets related to apoptosis in the diabetic group on day 16 when cartilage resorption is active but not day 12 or day 22. This coincided with elevated TNF-alpha protein levels, chondrocyte apoptosis, enhanced caspase-3 activity, and increased FOXO1 nuclear translocation (p < .05). Inhibition of TNF significantly reduced these parameters in the diabetic mice but not in normoglycemic control mice (p < .05). Silencing FOXO1 using siRNA in vitro significantly reduced TNF-induced apoptosis and caspase activity in differentiated chondrocytes. The mRNA levels of the proapoptotic genes caspase-3, caspase-8, caspase-9, and TRAIL were significantly reduced with silencing of FOXO1 in chondrocytic cells. Inhibiting caspase-8 and caspase-9 significantly reduced TNF-induced apoptosis in chondrogenic cells. These results suggest that diabetes causes an upregulation of proapoptotic genes during the transition from cartilage to bone in fracture healing. Diabetes increased chondrocyte apoptosis through a mechanism that involved enhanced production of TNF-alpha, which stimulates chondrocyte apoptosis and upregulates mRNA levels of apoptotic genes through FOXO1 activation.

Accelerated cartilage resorption by chondroclasts during bone fracture healing in osteoprotegerin-deficient mice

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG), a decoy receptor of RANKL, maintain bone mass by regulating the differentiation of osteoclasts, which are bone-resorbing cells. Endochondral bone ossification and bone fracture healing involve cartilage resorption, a less well-understood process that is needed for replacement of cartilage by bone. Here we describe the role of OPG produced by chondrocytes in chondroclastogenesis. Fracture healing in OPG(-/-) mice showed faster union of the fractured bone, faster resorption of the cartilaginous callus, and an increased number of chondroclasts at the chondroosseous junctions compared with that in wild-type littermates. When a cultured pellet of OPG(-/-) chondrocytes was transplanted beneath the kidney capsule, the pellet recruited many chondroclasts. The pellet showed the ability to induce tartrate-resistant acid phosphatase-positive multinucleated cells from RAW 264.7 cells in vitro. Finally, OPG(-/-) chondrocytes (but not wild-type chondrocytes) cultured with spleen cells induced many tartrate-resistant acid phosphatase-positive multinucleated cells. The expression of RANKL and OPG in chondrocytes was regulated by several osteotropic factors including 1,25-dihydroxyvitamin D(3), PTHrP, IL-1alpha, and TNF-alpha. Thus, local OPG produced by chondrocytes probably controls cartilage resorption as a negative regulator for chondrocyte-dependent chondroclastogenesis.

Advances in the establishment of defined mouse models for the study of fracture healing and bone regeneration

The availability of a broad spectrum of antibodies and gene-targeted animals caused an increasing interest in mouse models for the study of molecular mechanisms of fracture healing and bone regeneration. In most murine fracture models, the tibia or the femur is fractured using a 3-point bending device (closed models) or is osteotomized using an open surgical approach (open models). For fracture studies in mice, the tibia has to be considered less appropriate compared with the femur because the stabilization of the fracture is more difficult due to its triangular, distally declining caliber and its bowed longitudinal axis. Biomechanical factors critically influence the bone healing process. Thus, the use of stable osteosynthesis techniques is also of interest in murine fracture models. To achieve stable fixation, several biomechanically standardized implants have recently been introduced, including a locking nail and an intramedullary compression screw. Other implants, such as a pin-clip, an external fixator, and a locking plate, additionally allow the stabilization of fractures with distinct gap sizes. This enables the study of healing of critical size defects and nonunions. The use of these implants further allows a rigid fixation of fractures in bridle bones, which is essential for fracture studies in animals suffering from metabolic bone diseases like osteoporosis. In general, the analysis of bone healing in these models includes different imaging techniques and histologic, immunohistochemical, biomechanical, and molecular methods. To evaluate the impact of different osteosynthesis techniques on physical activity and rehabilitation, gait analysis may additionally be performed. By this, the gait of the animals can be visualized and quantitatively analyzed using modified running wheels and dynamic high-resolution radiography systems. Taken together, a variety of different murine femur fracture models have become available, providing defined biomechanical conditions for fracture research. The use of these mouse models may now allow studying the influence of fracture stabilization techniques on molecular mechanisms of bone healing.

Stress fracture healing: fatigue loading of the rat ulna induces upregulation in expression of osteogenic and angiogenic genes that mimic the intramembranous portion of fracture repair

Woven bone is formed in response to fatigue-induced stress fractures and is associated with increased local angiogenesis. The molecular mechanisms that regulate this woven bone formation are unknown. Our objective was to measure the temporal and spatial expression of osteo- and angiogenic genes in woven bone formation in response to increasing levels of fatigue-induced damage. We used the rat forelimb compression model to produce four discrete levels of fatigue damage in the right ulna of 115 male Fischer rats. Rats were killed at 0 (1 h), 1, 3 and 7 days after loading. Using qRT-PCR, we quantified gene expression associated with osteogenesis (BMP2, Msx2, Runx2, Osx, BSP, Osc), cell proliferation (Hist4), and angiogenesis (VEGF, PECAM-1) from the central half of the ulna. The spatial distribution of BMP2, BSP and PCNA was assessed by immunohistochemistry or in situ hybridization in transverse histological sections 1, 4, and 7 mm distal to the ulnar mid-diaphysis. One hour after loading, BMP2 was significantly upregulated in neurovascular structures in the medial ulnar periosteum. Expression of angiogenic markers (VEGF, PECAM-1) increased significantly between Day 0 and 1 and, as with BMP2 expression, remained upregulated through Day 7. While Osx and BSP were upregulated on Day 1, the other osteogenic genes (Msx2, Runx2, Osx, BSP and Osc) were induced on Day 3 in association with the initiation of periosteal woven bone formation and continued through Day 7. The magnitude of osteogenic gene expression, particularly matrix genes (BSP, Osc) was significantly proportional the level of fatigue damage. The woven bone response to fatigue injury is remarkably similar to the "intramembranous" portion of fracture repair - rapid formation of periosteal woven bone characterized by early BMP2 expression, cell proliferation, and upregulation of osteogenic genes. We speculate that woven bone repair of fatigue damage may be an abbreviated fracture response without the requirement for endochondral repair. We conclude that bone fatigue repair is a process similar to intramembranous fracture repair characterized by increases in the expression of genes associated with angiogenesis, cell proliferation and osteoblastogenesis, and that the response from the local vasculature precedes the osteogenic response to fatigue loading.

Diabetes causes the accelerated loss of cartilage during fracture repair which is reversed by insulin treatment

Fracture healing in diabetic individuals and in animal models of diabetes is impaired. To investigate mechanisms by which diabetes may affect fracture healing we focused on the transition from cartilage to bone, a midpoint in the fracture healing process. Femoral fractures were induced in mice rendered diabetic by multiple low dose streptozotocin treatment and compared to matching normoglycemic mice. One group of diabetic animals was treated with slow release insulin to maintain normal serum glucose levels. The results indicate that there was relatively little difference in the initial formation of the fracture callus on day 10. However, on day 16 the diabetic group had significantly smaller callus, greater loss of cartilage and enhanced osteoclastogenesis that was normalized by treatment with insulin when assessed by histomorphometric analysis. Chondrocyte apoptosis was significantly higher in diabetic mice and this increase was blocked by insulin. These changes were accompanied by diabetes-increased mRNA levels of RANKL, TNF-alpha, and ADAMTS-4 and -5 measured by real-time PCR, which was reversed by insulin treatment. On days 16 and 22 bone formation within the callus of diabetic mice was significantly less than the normoglycemic and brought to normal levels by insulin treatment. These results suggest that a significant effect of diabetes on fracture healing is increased chondrocyte apoptosis and osteoclastogenesis that accelerates the loss of cartilage and reduces the anlage for endochondral bone formation during fracture repair. That insulin reverses these effects demonstrates that they are directly related to the diabetic condition.

Optimization of allograft implantation using scaffold-free chondrocyte plates

If a tissue-engineered cartilage transplant is to succeed, it needs to integrate with the host tissue, to endure physiological loading, and to acquire the phenotype of the articular cartilage. Although there are many reported treatments for osteochondral defects of articular cartilage, problems remain with the use of artificial matrices (scaffolds) and the stage of implantation. We constructed scaffold-free three-dimensional tissue-engineered cartilage allografts using a rotational culture system and investigated the optimal stage of implantation and repair of the remodeling site. We evaluated the amounts of extracellular matrix and gene expression levels in scaffold-free constructs and transplanted the constructs for osteochondral defects using a rabbit model. Allografted 2-week constructs expressed high levels of proteoglycan and collagen per DNA content, integrated with the host cartilage successfully, and were able to counter physiological loads, and the chondrocyte plate contributed reparative mesenchymal stem cells to the final phenotype of the articular cartilage.

Determining the most important cellular characteristics for fracture healing using design of experiments methods

Computational models are employed as tools to investigate possible mechanoregulation pathways for tissue differentiation and bone healing. However, current models do not account for the uncertainty in input parameters, and often include assumptions about parameter values that are not yet established. The objective of this study was to determine the most important cellular characteristics of a mechanoregulatory model describing both cell phenotype-specific and mechanobiological processes that are active during bone healing using a statistical approach. The computational model included an adaptive two-dimensional finite element model of a fractured long bone. Three different outcome criteria were quantified: (1) ability to predict sequential healing events, (2) amount of bone formation at early, mid and late stages of healing and (3) the total time until complete healing. For the statistical analysis, first a resolution IV fractional factorial design (L(64)) was used to identify the most significant factors. Thereafter, a three-level Taguchi orthogonal array (L(27)) was employed to study the curvature (non-linearity) of the 10 identified most important parameters. The results show that the ability of the model to predict the sequences of normal fracture healing was predominantly influenced by the rate of matrix production of bone, followed by cartilage degradation (replacement). The amount of bone formation at early stages was solely dependent on matrix production of bone and the proliferation rate of osteoblasts. However, the amount of bone formation at mid and late phases had the rate of matrix production of cartilage as the most influential parameter. The time to complete healing was primarily dependent on the rate of cartilage degradation during endochondral ossification, followed by the rate of cartilage formation. The analyses of the curvature revealed a linear response for parameters related to bone, where higher rates of formation were more beneficial to healing. In contrast, parameters related to fibrous tissue and cartilage showed optimum levels. Some fibrous connective tissue- and cartilage formation was beneficial to bone healing, but too much of either tissue delayed bone formation. The identified significant parameters and processes are further confirmed by in vivo animal experiments in the literature. This study illustrates the potential of design of experiments methods for evaluating computational mechanobiological model parameters and suggests that further experiments should preferably focus at establishing values of parameters related to cartilage formation and degradation.

Proliferative Activity, Apoptosis, and Histogenesis in the Early Stages of Rat Tooth Extraction Wound Healing

We examined the proliferative activity, apoptosis, and histogenesis in the early stages of the rat healing socket from just after extraction until new bone formation occurs. Thirty 11-week-old male Wistar rats underwent bilateral maxillary first molar tooth extraction. Five craniomaxillary tissue specimens were dissected at the following time points: at 12 h, days 1, 3, 5, 7 and 10 after surgery. The immunohistochemical expression of both proliferating cell nuclear antigen (PCNA) and the Ki67 counterpart of rodent (MIB5) for proliferative activity and both the TUNEL reaction and the immunohistochemical expression of single-strand DNA for apoptosis were evaluated using 6-mum-thick serial paraffin sections, which were prepared in the coronal plane. The positive cell counts in the socket were converted into the cell numbers per mm(2) as either the proliferative index (PI) or the apoptotic index (AI). The PI and the AI showed maximum levels at 5 days and 12 h after extraction, respectively. The proliferative activity in the early stages of extraction wound healing is initially distributed in the remaining periodontium with load-induced apoptosis, next in the proliferative fibrous tissue, and then around the trabeculae of the new bone marking its peak.

Molecular aspects of fracture healing: which are the important molecules?

Fracture healing is a complex physiological process involving a coordinated interaction of hematopoietic and immune cells within the bone marrow, in conjunction with vascular and skeletal cell precursors. Multiple factors regulate this cascade of molecular events, which affects different stages in the osteoblast and chondroblast lineage during processes such as migration, proliferation, chemotaxis, differentiation, inhibition, and extracellular protein synthesis. A clear understanding of the cellular and molecular pathways in fracture healing is not only critical for advancing fracture treatment, but it may also enhance further our knowledge of the mechanisms involved within skeletal growth and repair, as well as the mechanisms of aging. An overview of the important molecules involved in fracture healing, including osteogenic autocoids and inhibitory molecules, and their interactions and possible mechanisms of synergy during the healing process is presented in this article.

Erythropoietin (EPO) — EPO-receptor signaling improves early endochondral ossification and mechanical strength in fracture healing

Beyond its role in the regulation of red blood cell proliferation, the glycoprotein erythropoietin (EPO) has been shown to promote cell regeneration and angiogenesis in a variety of different tissues. In addition, EPO has been indicated to share significant functional and structural homologies with the vascular endothelial growth factor (VEGF), a cytokine essential in the process of fracture healing. However, there is complete lack of information on the action of EPO in bone repair and fracture healing. Therefore, we investigated the effect of EPO treatment on bone healing in a murine closed femur fracture model using radiological, histomorphometric, immunohistochemical, biomechanical and protein biochemical analysis. Thirty-six SKH1-hr mice were treated with daily i.p. injections of 5000 U/kg EPO from day 1 before fracture until day 4 after fracture. Controls received equivalent amounts of the vehicle. After 2 weeks of fracture healing, we could demonstrate expression of the EPO-receptor (EPOR) in terminally differentiating chondrocytes within the callus. At this time point EPO-treated animals showed a higher torsional stiffness (biomechanical analysis: 39.6+/-19.4% of the contralateral unfractured femur) and an increased callus density (X-ray analysis (callus density/spongiosa density): 110.5+/-7.1%) when compared to vehicle-treated controls (14.3+/-8.2% and 105.9+/-6.6%; p<0.05). Accordingly, the histomorphometric examination revealed an increased fraction of mineralized bone and osteoid (33.0+/-3.0% versus 28.5+/-3.6%; p<0.05). Of interest, this early effect of the initial 6-day EPO treatment had vanished at 5 weeks after fracture. We conclude that EPO-EPOR signaling is involved in the process of early endochondral ossification, enhancing the transition of soft callus to hard callus.

The enhancement of bone regeneration by ultrasound

Millions of fractures occur every year worldwide, with nearly 6.2 million fractures reported annually in the United States alone. Even though treatment methods have improved over the last few decades, 5-10% of fractures still show delayed healing. A significant subpopulation of these delayed healings do not heal by nine months and are thus termed non-unions. Experimental studies have shown some evidence that low intensity pulsed ultrasound stimulation (LIPUS) results in enhanced bone regeneration during fracture healing and callus distraction. LIPUS treatment has led to increased callus area and accelerated return of bone strength following fracture. Histological studies suggest that LIPUS influences all major cell types involved in bone healing, including osteoblasts, osteoclasts, chondrocytes and mesenchymal stem cells. The affect of LIPUS seems to be limited to cells in soft tissue, whereas cells in calcified bone seem not to be effected. In vitro cell culture studies as well as tissue culture studies have shown some effects on cell differentiation and protein synthesis. Even though the energy used by LIPUS treatment is extremely low, the effects are evident. The most probable source of the therapeutic benefits observed with LIPUS treatment involves nonthermal mechanisms that influence cell membrane permeability and increase cellular activity. Despite clinical and experimental studies demonstrating the enhancing effect of LIPUS on bone regeneration, the biophysical mechanisms involved in the complex fracture healing process remain unclear and requires further research.

Smoking delays chondrogenesis in a mouse model of closed tibial fracture healing

Smoking delays the healing process and increases morbidity associated with many common musculoskeletal disorders, including long bone fracture. In the current study, a murine model of tibial fracture healing was used to test the hypothesis that smoking delays chondrogenesis after fracture. Mice were divided into two groups, a nonsmoking control group and a group exposed to cigarette smoke for 1 month prior to surgical tibial fracture. Mice were euthanized at 7, 14, and 28 days after surgery. The outcomes measured were immunohistochemical staining for type II collagen protein expression as a marker of cartilage matrix and proliferating cell nuclear antigen (PCNA) staining to measure proliferation at the site of injury. Toluidine blue staining and histomorphometry were used to quantify areas of cartilaginous and noncartilaginous fracture callus. Radiographs were analyzed for evidence of remodeling after injury. At day 7 after injury, mice exposed to cigarette smoke had a smaller fracture callus with less cartilage matrix compared to controls. Proliferation was present at high levels in both groups at this time point, but proliferating cells had a more immature morphology in the smoking group. At day 14, chondrogenesis was more active in smokers compared to controls, while a higher percentage of bone was present in the control animals. At day 28, X-ray analysis revealed a larger fracture callus remaining in the smoking animals. Together, these findings show that the chondrogenic phase of tibial fracture healing is delayed by smoking. This study represents, to our knowledge, the first analysis of molecular and cellular mechanisms of healing in a smoking mouse fracture model.

Current concepts of molecular aspects of bone healing

Fracture healing is a complex physiological process. It involves the coordinated participation of haematopoietic and immune cells within the bone marrow in conjunction with vascular and skeletal cell precursors, including mesenchymal stem cells (MSCs) that are recruited from the surrounding tissues and the circulation. Multiple factors regulate this cascade of molecular events by affecting different sites in the osteoblast and chondroblast lineage through various processes such as migration, proliferation, chemotaxis, differentiation, inhibition, and extracellular protein synthesis. An understanding of the fracture healing cellular and molecular pathways is not only critical for the future advancement of fracture treatment, but it may also be informative to our further understanding of the mechanisms of skeletal growth and repair as well as the mechanisms of aging.

The science of fracture healing

Fracture healing is a unique repair process in which the events of endochondral and intramembranous bone formation follow a specific time sequence. There are many ways in which these events can be disrupted, leading to unsuccessful fracture healing. This article summarizes the series of cellular and molecular events that take place in the process of endochondral fracture healing, based on observations obtained from an animal model of a femoral fracture.

Osteocyte apoptosis and osteoclast presence in chicken radii 0-4 days following osteotomy

Osteocyte apoptosis caused by load-induced microdamage is followed by osteoclastic bone remodeling, and a causal link between apoptosis and repair has been suggested. The objectives of the present study were to use a chick model to examine the incidence of osteocyte apoptosis and the presence of osteoclasts during the first 96 hours following an osteotomy, prior to extensive callus mineralization. Osteotomies were performed on the right radii of 24 chicks at 23-24 days of age. The left radii served as controls. Radii were collected and processed at six time points following surgery (0, 12, 24, 48, 72, and 96 hours). Decalcified bone tissue sections were stained either for apoptosis using a modified TUNEL procedure or for tartrate-resistant acid phosphatase to identify osteoclasts in the intracortical and periosteal envelopes. The percentage of apoptotic osteocytes, as well as osteoclast counts (n/mm or n/mm2) were quantified in four regions (0-1, 1-2, 2-4, and 4-8 mm from the site of the osteotomy; regions 1-4, respectively) in the osteotomized radii and in the same measured areas in the control radii. Data for osteocyte apoptosis and osteoclasts in the control limb were subtracted from the osteotomized limb data to identify differences due to surgical influence. The incidence of osteocyte apoptosis was significantly higher at 12, 24, 48, and 72 hours versus 0 hours following osteotomy, and the response was highest in region 1; however, there was no interaction between time and region. Intracortical osteoclast counts (n/mm2) were elevated after 48 hours, and the response was similar in all regions. The data demonstrate that osteocyte apoptosis occurs within 24 hours in response to an osteotomy and temporally precedes an increase in osteoclast presence. Hence, osteocyte apoptosis may play a role in signaling during the bone healing process.

Apoptosis in the regenerate produced by mandibular osteodistraction in the mature rat*

AUTHORS: Liu ZJ, Anderson MW, Gu GM, King GJ.

OBJECTIVE

Little is known about the contribution of apoptosis to the formation of the regenerate in distraction osteogenesis. This study investigated the role of apoptosis in the regulation of osteogenesis in relation to mandibular distraction rate and recovery time.

METHODS

One hundred and twenty-nine 3-month rats received unilateral mandibular ramus osteotomy and distraction device placement. After 3-day latency, these were distracted for 5 days. The slow group was distracted a total of 1 mm (0.2 mm/day), the moderate 2 mm (0.4 mm/day), the rapid 3 mm (0.6 mm/day), and the sham was not distracted (0.0 mm/day). Eight to nine rats from each group were killed at 6 (mid-distraction), 10 (early-consolidation), 24 (mid-consolidation) and 38 (late-consolidation) days following device placement. Baseline data were obtained from an additional eight rats killed at day 3 (end of latency). Sagittal sections (7 mum) of the harvested hemi-mandibles were embedded in paraffin, double-stained with the DeadEnd(TM) Colorimetric TUNEL system for apoptotic cells and tartrate-resistant acid phosphatase for osteoclasts. Cell counts of apoptotic cells and osteoclasts (apoptotic or otherwise) were preformed at 40x magnification using a calibrated grid at the middle regions of the caudal, central and rostral parts of the distraction gap. Counting reproducibility was verified as <13%.

RESULTS

Counts from all three regions were combined because no significant regional difference was found. The majority of the fibrous matrix regenerate was seen at days 6 and 10 while osseous regenerate occurred mainly at days 24 and 38. Significantly higher levels of apoptotic activity were seen at day 24. Apoptotic osteoclasts also peaked at day 24. Apoptotic cell numbers in the slow and moderate groups most closely followed the pattern of the sham in which the apoptosis activity peaked at days 10 or 24. However, those in the rapid group showed delayed peaks at days 24 or 38.

CONCLUSIONS

Chalmers Street, Surry Hill, NSW 2010, Australia. gshe6437@mail.usyd.edu.au The transition from fibrous to osseous regenerate during mandibular distraction osteogenesis is accompanied by an increase in cell clearance via apoptosis. A slow to moderate distraction rate allows for the most typical pattern of bone healing while a rapid rate prolongs the healing process.

Recruitment of osteoclast precursors by stromal cell derived factor-1 (SDF-1) in giant cell tumor of bone

Giant cell tumor (GCT) of bone is a unique bone lesion that is characterized by an excessive number of multinucleated osteoclasts. GCT consists of neoplastic stromal cells, multinucleated osteoclasts and their precursors, thus serving as a naturally occurring human disease model for the study of osteoclastogenesis. It still remains unclear how stromal cells of GCT recruit osteoclast precursors. In the present study, we characterized the cellular components of GCT and confirmed the presence of CD14(+)-monocytes/CD68(+)-macrophages and CD34(+)-hematopoetic stem cells that express CXCR4, a specific receptor for SDF-1; SDF-1 gene expression and presence of SDF-1 protein were confirmed by real time RT-PCR, in situ hybridization, and immunohistochemistry in the GCT tissue and cultured cells. SDF-1 was present at 25-50 ng/ml in the conditioned media from the GCT cultures, which is in the range of physiological chemotactic concentration. Migration of osteoclast precursors was 2.5-fold higher in response to GCT conditioned media compared to the control media; and migration was inhibited by an average of 36% with anti-SDF-1 neutralizing antibody or competing recombinant SDF-1. These results suggest that SDF-1 is one of the significant chemoattractant factors involved in the recruitment of hematopoietic osteoclast precursor cells during tumor-induced osteoclastogenesis.

Transient retention of endochondral cartilaginous matrix with bisphosphonate treatment in a long-term rabbit model of distraction osteogenesis

Bisphosphonates induce major increases in strength of callus in distraction osteogenesis in the short term. Poor understanding of the underlying mechanism, however, raises concerns about long-term consequences. In this long-term study in 32 rabbits, zoledronic acid transiently increased trabeculae by delayed temporal progression of endochondral bone remodeling but did not prevent radiographic completion of bone repair.

INTRODUCTION

We hypothesized that bisphosphonate inhibition of osteoclast-mediated resorption would retain bone during repair, producing a larger callus in the short term. However, if remodeling was not restored, completion of the bone repair process in the long term could be jeopardized.

MATERIALS AND METHODS

Juvenile rabbits underwent right tibial osteotomy and 2 weeks of distraction, followed by a period of consolidation. Animals received saline (controls) or zoledronic acid (ZA; 0.1 mg/kg at surgery and again 2 weeks later), and distracted tibias were examined by radiograph, DXA, histology, and histomorphometry at 2, 4, 6, 18, and 44 weeks after surgery.

RESULTS

Regenerated bone in ZA-treated animals was denser than controls on radiographs at 6 weeks and had more distinct radiodense trabeculae and retention of original cortices at 18 weeks. By 44 weeks, controls and ZA-treated animals were radiographically healed and indistinguishable. Regenerate BMD and BMC increased between 2 and 4 weeks in all animals, with a greater effect in ZA. At 6 weeks, BMD and BMC in ZA-treated animals were 1.6- and 2-fold greater, respectively, than controls (p < 0.01). From 6 to 44 weeks, the control values gradually increased and approached the ZA-treated values. Regenerate bone volume and trabecular number by histomorphometry were from 1.6- to 2-fold greater in ZA-treated animals at 6 and 18 weeks (p < 0.05). Endochondral cartilaginous matrix volume was up to 2.4-fold greater in ZA-treated animals at 2 and 4 weeks (p < 0.05). TRACP+ cells in ZA-treated animals were larger with more nuclei. Mineral apposition rate and osteoblast number and surface were lower in ZA-treated animals at 6 weeks (p < 0.01) but not at later times.

CONCLUSIONS

Disruption of TRACP+ cell function by ZA during bone regeneration seems to lead to an accretion of cancellous bone built on a larger endochondral cartilaginous matrix and increased bone mass, consistent with reported increases in short-term callus strength. This increase in bone mass, caused by a delay in remodeling, provided a transient advantage without preventing radiographic completion of the bone repair process in the long term. Noncontinuous treatment with nitrogen-containing bisphosphonates thus can have short-term beneficial effects without preventing long-term bone repair.

Matrix metalloproteinase-9 expression, tartrate-resistant acid phosphatase activity, and DNA fragmentation in vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones

Vascular and cellular invasion into cartilage are essential for endochondral ossification. Recently it has been shown that matrix metalloproteinase-9 (MMP-9)/gelatinase B is a key regulator of growth plate angiogenesis and apoptosis of hypertrophic chondrocytes. To study vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones, precursor femurs from 13- to 16-day-old murine embryos were sectioned. Tartrate-resistant acid phosphatase (TRAP) activity, in situ hybridization for matrix metalloproteinase-9 (MMP-9), immunostaining for CD31, and in situ detection of apoptosis (TUNEL) were studied. TRAP activity, MMP-9 mRNA, and CD31 expression were initially detected in the intertrabecular spaces of the perichondral collar, and then in cells migrating into the cartilage. The first cells involved in the primary invasion into cartilage were CD31-positive vascular endothelial cells and MMP-9-positive cells, followed by TRAP-positive cells. At the cartilage-marrow interface, CD31-positive vascular endothelial cells and MMP-9-positive cells were predominant. These results suggest that MMP-9-positive cells cooperate with vascular endothelial cells in cartilage angiogenesis. TUNEL-positive staining was detected on chondrocytes attached to the inner surface of the perichondral collar, and also detected in the area where cartilage was removed. These results suggest that chondrocytes separated from the cartilage matrix may undergo apoptosis.

Long-term intermittent shear deformation improves the quality of cartilaginous tissue formed in vitro

The formation of cartilaginous tissue in vitro is a promising alternative to repair damaged articular cartilage. However, recent attempts to tissue-engineer articular cartilage that has similar properties to the native tissue have proven to be difficult. The in vitro-formed cartilaginous tissue typically has a similar proteoglycan content to native cartilage, but has a reduced collagen content and only a fraction of the mechanical properties. In this study, we investigated whether the intermittent application of cyclic shearing forces during tissue formation would improve the tissue quality. Chondrocyte cultures were stimulated at a 2% shear strain amplitude at a frequency of 1 Hz for 400 cycles every 2nd day. At one week, both collagen and proteoglycan synthesis increased (23+/-6% and 20+/-6%, respectively) over the unstimulated, static controls. At four weeks, an increased amount of tissue formed (stimulated: 1.85+/-0.08, unstimulated: 1.58+/-0.07 mg dry wt.). This tissue contained approximately 40% more collagen (stimulated: 511+/-23, unstimulated: 367+/-24 microg/construct) and 35% more proteoglycans (stimulated: 376+/-21, unstimulated: 279+/-26 microg/construct). Tissues that formed in the presence of shearing forces also displayed a 3-fold increase in compressive load-bearing capacity (stimulated: 16+/-5, unstimulated: 5+/-1 kPa max. equilibrium stress) and a 6-fold increase in stiffness (stimulated: 112+/-36, unstimulated: 20+/-6 kPa max. equilibrium modulus) compared to the static controls. These results demonstrate that intermittent application of dynamic shearing forces over a four-week period improves the quality of cartilaginous tissue formed in vitro. Interestingly, low amplitudes of shear stimulation for short periods of time (6 min of stimulation applied every 2nd day) produced these changes.

Fracture healing: bone healing, fracture management, and current concepts related to the hand

Bones fracture frequently and often result in significant impairments, functional limitations, and disabilities, especially when the hand is involved. When fractures occur, there is a disruption of the skeletal tissue organization and a loss of mechanical integrity. The goal of fracture healing is to regenerate mineralized tissue in the fracture area and restore mechanical strength to the bone. Of equal importance is the reconstitution of the normal soft tissue gliding and movement about the fracture site. This article briefly reviews the history of fracture healing and the advances in mechanics and cellular and molecular biology, which should help the reader better understand the current mechanisms related to bone healing (primarily and secondarily). Fracture fixation modes also are described along with the temporal sequencing as to when to protect or move the fractured region.

The use of specific chondrocyte populations to modulate the properties of tissue-engineered cartilage

Tissue engineering of articular cartilage is a promising alternative to the conventional approaches for cartilage repair. However, recent attempts to develop articular cartilage in vitro have proven to be difficult. The tissue formed in vitro may not accumulate enough extracellular matrix, and the resulting mechanical properties are only a fraction of the native tissue. We investigated whether using specific populations of chondrocytes would improve the properties of the cartilaginous tissue that was generated in vitro. Full-thickness (FT), mid-and-deep zone (MD), and deep-zone (DEEP) chondrocytes were isolated, placed on the surface of porous ceramic substrates and maintained in culture for eight weeks. Tissue developed from DEEP chondrocytes was thicker (FT: 0.94+/-0.03, MD: 0.88+/-0.04, DEEP: 2.4+/-0.1 mm) and had accumulated larger amounts of extracellular matrix (FT: 1.61+/-0.05, MD: 1.5+/-0.1, DEEP: 3.8+/-0.2 mg dry weight) than the tissues formed by the FT and MD chondrocytes. The tissue formed by the FT chondrocytes accumulated the greatest amount of collagen (FT: 211+/-14, MD: 185+/-8, DEEP: 178+/-5 microg/mg dry weight) whereas the tissue formed by the MD chondrocytes accumulated significantly more proteoglycans (FT: 198+/-10, MD: 265+/-10, DEEP: 215+/-5 microg/mg dry weight). Interestingly, MD chondrocytes produced tissue that had compressive mechanical properties up to four times greater than the cartilaginous tissues formed by cells from either the FT or DEEP of cartilage. Thus, a combined population of intermediate and DEEP chondrocytes might be more suitable for the tissue engineering of articular cartilage.

The fate of soft callus chondrocytes during long bone fracture repair

Following fracture, the cartilaginous tissue of the soft callus is eventually replaced by bone. Removal of the cartilage is a critical part of the bone healing process but information concerning the changes in chondrocytes during this process is sparse. The aim of the study was to investigate the fate of chondrocytes in the soft callus during the bone repair process using a rabbit tibial fracture model. Fracture tissue was processed for collagen I-III and keratan sulphate immunohistochemistry to study changes in matrix composition and the TUNEL technique (terminal deoxynucleotidyl transferase medicated dUTP nick-end labelling) to identify death of soft callus chondrocytes. Transmission electron microscopy (TEM) was also carried out to investigate the ultrastructure of chondrocytes within the soft callus. Results showed that the size of the cartilage area decreased over time and that cartilage matrix was replaced with new matrix rich in collagen I and III. Chondrocytes became engulfed in the new matrix and appeared to stop producing cartilage matrix. Chondrocyte cell death was seen at the border of the soft callus, just within the newly produced matrix. TEM revealed that these dying/dead cells were not typically apoptotic in appearance. In conclusion, results indicate that chondrocytes of the soft callus die as a result of the progressive production of bone matrix which eventually engulfs them and leads to the remodelling of the area and eventual bone repair.

Rapid new bone tissue remodeling during distraction osteogenesis is associated with apoptosis

During the process of distraction osteogenesis new bone forms and undergoes rapid remodeling. Apoptosis may be one of the regulatory mechanisms governing the removal of the redundant callus during distraction osteogenesis. A rabbit tibial lengthening model was used and lengthened at 0.7 mm/day for 3 weeks. The regenerating tissues from the distraction gap were examined for apoptotic changes by transmission electron microscopy (TEM) and the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Osteoclastic bone resorption activities were demonstrated by tartrate resistant acid phosphatase (TRAP) staining. The apoptotic cells were mainly present in the transitional regions between the fibrous tissue and the new bone in the mineralization front, and close to or on the new bone surfaces near the center of the regenerate. The TUNEL labeling was greatly reduced in the mature bone near the osteotomied bone ends. TEM examination confirmed the presence of cells with apoptotic changes at various regions of the regenerate. TRAP staining revealed that osteoclastic bone resorption activities in the regenerate were in a similar pattern of distribution to those of the TUNEL labeling. The localization of apoptotic cells at the different regions of the regenerate, accompanied by the osteoclast activities, suggest that apoptosis is closely related to bone formation and remodeling during distraction osteogenesis.

Expression of smooth muscle actin in connective tissue cells participating in fracture healing in a murine model

The role of alpha-smooth muscle actin (SMA)-expressing fibroblasts in the contraction of skin wounds has been known for three decades. Recent studies have demonstrated that osteoblasts can also express the gene for this contractile muscle actin isoform and can contract a collagen-glycosaminoglycan analog of extracellular matrix in vitro. These findings provided rationale for the hypothesis that SMA-expressing cells contribute to fracture healing by drawing the bone ends together. To begin to test this hypothesis, immunohistochemistry was employed to evaluate the distribution of connective tissue cells expressing SMA in a mouse model of successful fracture healing. The results demonstrated that the majority of the cells comprising the mesenchymal tissue interposed between the fracture ends contained SMA after 7 and 21 days, supporting the working hypothesis. Most of the osteoblasts lining the surfaces of newly forming bone and the chondrocytes comprising the cartilaginous callus also expressed this contractile actin isoform. The maximal SMA expression extended from 7 to 21 days postfracture. The finding of high levels of SMA expression in connective tissue cells participating in fracture healing suggests that SMA-enabled contraction may be playing a role in the healing process. These results warrant further study of the specific SMA-dependent cell behavior.