Development of a locking femur nail for mice

Intramedullary implant stability affects patterns of fracture healing in mice with morphologically different bone phenotypes

Almost all prior mouse fracture healing models have used needles or K-wires for fixation, unwittingly providing inadequate mechanical stability during the healing process. Our contention is that the reported outcomes have predominantly reflected this instability, rather than the impact of diverse biological conditions, pharmacologic interventions, exogenous growth factors, or genetic considerations. This important issue becomes obvious upon a critical review of the literature. Therefore, the primary aim of this study was to demonstrate the significance of mouse-specific implants designed to provide both axial and torsional stability (Screw and IM Nail) compared to conventional pins (Needle and K-wires), even when used in mice with differently sized marrow canals and diverse genetic backgrounds. B6 (large medullary canal), DBA, and C3H (smaller medullary canals) mice were employed, all of which have different bone morphologies. Closed femoral fractures were created and stabilized with intramedullary implants that provide different mechanical conditions during the healing process. The most important finding of this study was that appropriately designed mouse-specific implants, providing both axial and torsional stability, had the greatest influence on bone healing outcomes regardless of the different bone morphologies encountered. For instance, unstable implants in the B6 strain (largest medullary canal) resulted in significantly greater callus, with a fracture region mainly comprising trabecular bone along with the presence of cartilage 28 days after surgery. The DBA and C3H strains (with smaller medullary canals) instead formed significantly less callus, and only had a small amount of intracortical trabeculation remaining. Moreover, with more stable fracture fixation a higher BV/TV was observed and cortices were largely restored to their original dimensions and structure, indicating an accelerated healing and remodeling process. These observations reveal that the diaphyseal cortical thickness, influenced by the genetic background of each strain, played a pivotal role in determining the amount of bone formation in response to the fracture. These findings are highly important, indicating the rate and type of tissue formed is a direct result of mechanical instability, and this most likely would mask the true contribution of the tested genes, genetic backgrounds, or various therapeutic agents administered during the bone healing process.

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

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

Dynamic in vivo monitoring of fracture healing process in response to magnesium implant with multimodal imaging: Pilot longitudinal study in a rat external fixation model

Rodent models are commonly used in pre-clinical research of magnesium (Mg) -based and other types of biomaterials for fracture treatment. Most studies selected unstable fixation methods, and there is a...

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

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

A novel MRI compatible mouse fracture model to characterize and monitor bone regeneration and tissue composition

Over the last years, murine in vivo magnetic resonance imaging (MRI) contributed to a new understanding of tissue composition, regeneration and diseases. Due to artefacts generated by the currently used metal implants, MRI is limited in fracture healing research so far. In this study, we investigated a novel MRI-compatible, ceramic intramedullary fracture implant during bone regeneration in mice. Three-point-bending revealed a higher stiffness of the ceramic material compared to the metal implants. Electron microscopy displayed a rough surface of the ceramic implant that was comparable to standard metal devices and allowed cell attachment and growth of osteoblastic cells. MicroCT-imaging illustrated the development of the callus around the fracture site indicating a regular progressing healing process when using the novel implant. In MRI, different callus tissues and the implant could clearly be distinguished from each other without any artefacts. Monitoring fracture healing using MRI-compatible implants will improve our knowledge of callus tissue regeneration by 3D insights longitudinal in the same living organism, which might also help to reduce the consumption of animals for future fracture healing studies, significantly. Finally, this study may be translated into clinical application to improve our knowledge about human bone regeneration.

A comprehensive review of mouse diaphyseal femur fracture models

Complications related to treatment of long bone fractures still stand as a major challenge for orthopaedic surgeons. Elucidation of the mechanisms of bone healing and development, and the subsequent alteration of these mechanisms to improve outcomes, typically requires animal models as an intermediary between in vitro and human clinical studies. Murine models are some of the most commonly used in translational research, and mouse fracture models are particularly diverse, offering a wide variety of customization with distinct benefits and limitations depending on the study. This review critically examines three common femur fracture models in the mouse, namely cortical hole, 3-point fracture (Einhorn), and segmental bone defect. We lay out the general procedure for execution of each model, evaluate the practical implications and important advantages/disadvantages of each and describe recent innovations. Furthermore, we explore the applications that each model is best adapted for in the context of the current state of murine orthopaedic research.

Impact of the Sensory and Sympathetic Nervous System on Fracture Healing in Ovariectomized Mice

The peripheral nervous system modulates bone repair under physiological and pathophysiological conditions. Previously, we reported an essential role for sensory neuropeptide substance P (SP) and sympathetic nerve fibers (SNF) for proper fracture healing and bone structure in a murine tibial fracture model. A similar distortion of bone microarchitecture has been described for mice lacking the sensory neuropeptide α-calcitonin gene-related peptide (α-CGRP). Here, we hypothesize that loss of SP, α-CGRP, and SNF modulates inflammatory and pain-related processes and also affects bone regeneration during fracture healing under postmenopausal conditions. Intramedullary fixed femoral fractures were set to 28 days after bilateral ovariectomy (OVX) in female wild type (WT), SP-, α-CGRP-deficient, and sympathectomized (SYX) mice. Locomotion, paw withdrawal threshold, fracture callus maturation and numbers of TRAP-, CD4-, CD8-, F4/80-, iNos-, and Arg1-positive cells within the callus were analyzed. Nightly locomotion was reduced in unfractured SP-deficient and SYX mice after fracture. Resistance to pressure was increased for the fractured leg in SP-deficient mice during the later stages of fracture healing, but was decreased in α-CGRP-deficient mice. Hypertrophic cartilage area was increased nine days after fracture in SP-deficient mice. Bony callus maturation was delayed in SYX mice during the later healing stages. In addition, the number of CD 4-positive cells was reduced after five days and the number of CD 8-positive cells was additionally reduced after 21 days in SYX mice. The number of Arg1-positive M2 macrophages was higher in α-CGRP-deficient mice five days after fracture. The alkaline phosphatase level was increased in SYX mice 16 days after fracture. Absence of α-CGRP appears to promote M2 macrophage polarization and reduces the pain threshold, but has no effect on callus tissue maturation. Absence of SP reduces locomotion, increases the pain-threshold, and accelerates hypertrophic callus tissue remodeling. Destruction of SNF reduces locomotion after fracture and influences bony callus tissue remodeling during the later stages of fracture repair, whereas pain-related processes are not affected.

Development of a novel murine femur fracture and fixation model

BACKGROUND

Animal models have been used for decades to simulate human fractures in the laboratory setting. Fracture models in mice are attractive because they offer a high volume, relatively low-cost method of investigating fracture healing characteristics. We report on the development of a novel murine femur fracture model that is rapid, reproducible and inexpensive.

METHODS

As part of a pilot study to investigate the effects of smoking on fracture healing, fifteen 35-43 g twelve-week old female CD-1 mice underwent a novel surgical protocol using direct visualization of femur fracture creation and fixation. Following surgery, mice were sacrificed at 14 days, 28 days and 42 days. After sacrifice, the femora were analyzed using MicroCT and histology to evaluate progression of healing.

RESULTS

Of the 14 mice that survived the surgical procedure (one succumbed to a complication of anesthesia), two lost reduction and did not heal. Histology demonstrated at 14 days 44.1% (SD±2.9%) of callus composed of cartilage. At 28 days there was 19.0% (SD±3.4%) of callus composed of cartilage. At 42 days there was 8.4% (SD±2.6%) callus composed of cartilage (p < 0.005). MicroCT demonstrated that from 14 to 42 days the average callus volume decreased from 101.6 mm3 to 68.2 mm3 while the relative bone volume of callus increased from 14 to 42 days (15%-31%) (p = 0.068).

CONCLUSIONS

Our novel fracture and fixation model is an effective, rapid, reproducible and inexpensive method to simulate a fracture in a laboratory setting. Additionally, our model reliably creates a reproducible progression of radiographic and histological bone healing.

Genetic variation in mice affects closed femoral fracture pattern outcomes

The purpose of this study was to determine whether differences in structural and material properties of bone between different mouse strains influence the fracture patterns produced under experimental fracture conditions. Femurs of C57BL/6 (B6), C3H/HeJ (C3H), and DBA/2 (DBA) strains were evaluated using micro-computed tomography (μCT), measurements derived from radiographic images and mechanical testing to determine differences in the geometry and mechanical properties. A fracture device was used to create femoral fractures on freshly sacrificed animals using a range of kinetic energies (∼20-80mJ) which were classified as transverse, oblique, or comminuted. B6 femurs had the lowest bone volume/total volume (BV/TV) and bone mineral density (BMD), thinnest cortex, and had the most variable fracture patterns, with 77.5% transverse, 15% oblique, and 7.5% comminuted fractures. In contrast, C3H had the highest BV/TV, BMD, and thickest cortices, resulting in 97.5% transverse, 2.5% oblique, and 0% comminuted fractures. DBA had an intermediate BV/TV and thickness of cortices, with BMD similar to C3H, resulting in 92.9% transverse, 7.1% oblique, and 0% comminuted fractures. A binomial logistic regression confirmed that bone morphometry was the single strongest predictor of the resulting fracture pattern. This study demonstrated that the reproducibility of closed transverse femoral fractures was most influenced by the structural and material properties of the bone characteristics in each strain, rather than the kinetic energy or body weight of the mice. This was evidenced through geometric analysis of X-ray and μCT data, and further supported by the bone mineral density measurements from each strain, derived from μCT. Furthermore, this study also demonstrated that the use of lower kinetic energies was more than sufficient to reproducibly create transverse fractures, and to avoid severe tissue trauma. The creation of reproducible fracture patterns is important as this often dictates the outcomes of fracture healing, and those studies that do not control this potential variability could lead to a false interpretation of the results.

Development of a novel murine delayed secondary fracture healing in vivo model using periosteal cauterization

INTRODUCTION

Delayed union and nonunion development remain a major clinical problematic complication during fracture healing, with partially unclear pathophysiology. Incidences range from 5 to 40% in high-risk patients, such as patients with periosteal damage. The periosteum is essential in adequate fracture healing, especially during soft callus formation. In this study, we hypothesize that inducing periosteal damage in a murine bone healing model will result in a novel delayed union model.

MATERIALS AND METHODS

A mid-shaft femoral non-critically sized osteotomy was created in skeletally mature C57BL/6 mice and stabilized with a bridging plate. In half of the mice, a thin band of periosteum adjacent to the osteotomy was cauterized. Over 42 days of healing, radiographic, biomechanical, micro-computed tomography and histological analysis was performed to assess the degree of fracture healing.

RESULTS

Analysis showed complete secondary fracture healing in the control group without periosteal injury. Whereas the periosteal injury group demonstrated less than half as much maximum callus volume (p < 0.05) and bridging, recovery of stiffness and temporal expression of callus growth and remodelling was delayed by 7-15 days.

CONCLUSION

This paper introduces a novel mouse model of delayed union without a critically sized defect and with standardized biomechanical conditions, which enables further investigation into the molecular biological, biomechanical, and biochemical processes involved in (delayed) fracture healing and nonunion development. This model provides a continuum between normal fracture healing and the development of nonunions.

Strontium Ranelate Combined with Insulin Is as Beneficial as Insulin Alone in Treatment of Fracture Healing in Ovariectomized Diabetic Rats

Background

Type 2 diabetes mellitus (T2DM) and estrogen deficiency both predispose fracture patients to increased risk of delayed union or nonunion. The present study investigated the effects of strontium ranelate (SR) on fracture healing in ovariectomized (OVX) diabetic rats.

Material/Methods

A mid-shaft fracture was established in female normal control (CF), diabetic (DF), and OVX diabetic (DOF) rats. Treated DOF rats received either insulin alone (DOFI) or combined with SR (DOFIS). All rats were euthanized at 2 or 3 weeks after fracture. Fracture healing was evaluated using radiological, histological, immunohistochemical, and micro-computed tomography analyses.

Results

At 3 weeks after fracture, radiological and histological evaluations demonstrated delayed fracture healing in the DF group compared with the CF group, which was exacerbated by OVX, as indicated by the significantly lower X-ray score, BMD, BV/TV, and Md.Ar/Ps.Cl.Ar, and the markedly decreased OCN and Col I expression in the DOF group. All these changes were prevented by insulin alone or combined with SR treatment. In comparison with the DOFI group, DOFIS rats displayed markedly higher OCN expression at 2 weeks after fracture and Col I expression at 2 and 3 weeks after fracture.

Conclusions

These results demonstrated delayed fracture healing with preexisting estrogen deficiency and T2DM. While insulin alone and combined with SR were both effective in promoting bone fracture healing in this model, their combined treatment showed significant improvement in promoting osteogenic marker expression, but not of the radiological appearance, compared with insulin alone.

The Generation of Closed Femoral Fractures in Mice: A Model to Study Bone Healing

Bone fractures impose a tremendous socio-economic burden on patients, in addition to significantly affecting their quality of life. Therapeutic strategies that promote efficient bone healing are non-existent and in high demand. Effective and reproducible animal models of fractures healing are needed to understand the complex biological processes associated with bone regeneration. Many animal models of fracture healing have been generated over the years; however, murine fracture models have recently emerged as powerful tools to study bone healing. A variety of open and closed models have been developed, but the closed femoral fracture model stands out as a simple method for generating rapid and reproducible results in a physiologically relevant manner. The goal of this surgical protocol is to generate unilateral closed femoral fractures in mice and facilitate a post-fracture stabilization of the femur by inserting an intramedullary steel rod. Although devices such as a nail or a screw offer greater axial and rotational stability, the use of an intramedullary rod provides a sufficient stabilization for consistent healing outcomes without producing new defects in the bone tissue or damaging nearby soft tissue. Radiographic imaging is used to monitor the progression of callus formation, bony union, and subsequent remodeling of the bony callus. Bone healing outcomes are typically associated with the strength of the healed bone and measured with torsional testing. Still, understanding the early cellular and molecular events associated with fracture repair is critical in the study of bone tissue regeneration. The closed femoral fracture model in mice with intramedullary fixation serves as an attractive platform to study bone fracture healing and evaluate therapeutic strategies to accelerate healing.

A Minimally Invasive Model to Analyze Endochondral Fracture Healing in Mice Under Standardized Biomechanical Conditions

Bone healing models are necessary to analyze the complex mechanisms of fracture healing to improve clinical fracture treatment. During the last decade, an increased use of mouse models in orthopedic research was noted, most probably because mouse models offer a large number of genetically-modified strains and special antibodies for the analysis of molecular mechanisms of fracture healing. To control the biomechanical conditions, well-characterized osteosynthesis techniques are mandatory, also in mice. Here, we report on the design and use of a closed bone healing model to stabilize femur fractures in mice. The intramedullary screw, made of medical-grade stainless steel, provides through fracture compression an axial and rotational stability compared to the mostly used simple intramedullary pins, which show a complete lack of axial and rotational stability. The stability achieved by the intramedullary screw allows the analysis of endochondral healing. A large amount of callus tissue, received after stabilization with the screw, offers ideal conditions to harvest tissue for biochemical and molecular analyses. A further advantage of the use of the screw is the fact that the screw can be inserted into the femur with a minimally invasive technique without inducing damage to the soft tissue. In conclusion, the screw is a unique implant that can ideally be used in closed fracture healing models offering standardized biomechanical conditions.

Osteotomy models - the current status on pain scoring and management in small rodents

Fracture healing is a complex regeneration process which produces new bone tissue without scar formation. However, fracture healing disorders occur in approximately 10% of human patients and cause severe pain and reduced quality of life. Recently, the development of more standardized, sophisticated and commercially available osteosynthesis techniques reflecting clinical approaches has increased the use of small rodents such as rats and mice in bone healing research dramatically. Nevertheless, there is no standard for pain assessment, especially in these species, and consequently limited information regarding the welfare aspects of osteotomy models. Moreover, the selection of analgesics is restricted for osteotomy models since non-steroidal anti-inflammatory drugs (NSAIDs) are known to affect the initial, inflammatory phase of bone healing. Therefore, opioids such as buprenorphine and tramadol are often used. However, dosage data in the literature are varied. Within this review, we clarify the background of osteotomy models, explain the current status and challenges of animal welfare assessment, and provide an example score sheet including model specific parameters. Furthermore, we summarize current refinement options and present a brief outlook on further 3R research.

A new multiple trauma model of the mouse

Background

Blunt trauma is the most frequent mechanism of injury in multiple trauma, commonly resulting from road traffic collisions or falls. Two of the most frequent injuries in patients with multiple trauma are chest trauma and extremity fracture. Several trauma mouse models combine chest trauma and head injury, but no trauma mouse model to date includes the combination of long bone fractures and chest trauma. Outcome is essentially determined by the combination of these injuries. In this study, we attempted to establish a reproducible novel multiple trauma model in mice that combines blunt trauma, major injuries and simple practicability.

Methods

Ninety-six male C57BL/6 N mice (n = 8/group) were subjected to trauma for isolated femur fracture and a combination of femur fracture and chest injury. Serum samples of mice were obtained by heart puncture at defined time points of 0 h (hour), 6 h, 12 h, 24 h, 3 d (days), and 7 d.

Results

A tendency toward reduced weight and temperature was observed at 24 h after chest trauma and femur fracture. Blood analyses revealed a decrease in hemoglobin during the first 24 h after trauma. Some animals were killed by heart puncture immediately after chest contusion; these animals showed the most severe lung contusion and hemorrhage. The extent of structural lung injury varied in different mice but was evident in all animals. Representative H&E-stained (Haematoxylin and Eosin-stained) paraffin lung sections of mice with multiple trauma revealed hemorrhage and an inflammatory immune response. Plasma samples of mice with chest trauma and femur fracture showed an up-regulation of IL-1β (Interleukin-1β), IL-6, IL-10, IL-12p70 and TNF-α (Tumor necrosis factor- α) compared with the control group. Mice with femur fracture and chest trauma showed a significant up-regulation of IL-6 compared to group with isolated femur fracture.

Conclusions

The multiple trauma mouse model comprising chest trauma and femur fracture enables many analogies to clinical cases of multiple trauma in humans and demonstrates associated characteristic clinical and pathophysiological changes. This model is easy to perform, is economical and can be used for further research examining specific immunological questions.

Parathyroid hormone (1-34) promotes fracture healing in ovariectomized rats with type 2 diabetes mellitus

Ovariectomized (OVX) rats with type 2 diabetes mellitus (T2DM) with femur fracture received vehicle, insulin, or insulin plus parathyroid hormone (PTH) treatment for 2 and 3 weeks. Radiography, histomorphometry, histology, and immunohistochemistry in callus were evaluated.

INTRODUCTION

Reports about effects of PTH plus insulin on callus formation of osteoporotic fracture with T2DM were limited. This study was designed to investigate the effects of the combination of PTH and insulin on fracture healing in OVX rats with T2DM.

METHODS

Two-month-old female rats were randomly divided into five groups: normal fracture (F), OVX fracture (OF), T2DM + OVX fracture (DOF), insulin-treated (2-4 u/daylight, 4-6 u/night, DOFI), and treated with insulin and PTH (50 μg/kg/day, 5 days/week, DOFIP). A closed mid-shaft fracture was established in the right femurs of all rats after 6 weeks of OVX. Rats were euthanized at 2 and 3 weeks post-fracture according to the time schedule, respectively.

RESULTS

The administration of insulin alone or insulin combined with PTH significantly increased mineralized bone volume fraction (BV/TV) and connectivity density (Conn.D) compared with those of the DOF group at 3 weeks post-fracture and also increased cartilaginous callus area ratio in the DOFI and DOFIP groups at 2 weeks and bony callus area ratio in the DOFIP groups at both the 2 and 3 weeks post-fracture.

CONCLUSIONS

OVX rats with T2DM exhibited a marked delay in the fracture healing process; insulin treatment ameliorated these effects, and the healing process was enhanced following treatment with a combination of insulin and PTH.

Murine Models of Sepsis and Trauma: Can We Bridge the Gap?

Sepsis and trauma are both leading causes of death in the United States and represent major public health challenges. Murine models have largely been used in sepsis and trauma research to better understand the pathophysiological changes that occur after an insult and to develop potential life-saving therapeutic agents. Mice are favorable subjects for this type of research given the variety of readily available strains including inbred, outbred, and transgenic strains. In addition, they are relatively easy to maintain and have a high fecundity. However, pharmacological therapies demonstrating promise in preclinical mouse models of sepsis and trauma often fail to demonstrate similar efficacy in human clinical trials, prompting considerable criticism surrounding the capacity of murine models to recapitulate complex human diseases like sepsis and traumatic injury. Fundamental differences between the two species include, but are not limited to, the divergence of the transcriptomic response, the mismatch of temporal response patterns, differences in both innate and adaptive immunity, and heterogeneity within the human population in comparison to the homogeneity of highly inbred mouse strains. Given the ongoing controversy, this narrative review aims to not only highlight the historical importance of the mouse as an animal research model but also highlight the current benefits and limitations of the model as it pertains to sepsis and trauma. Lastly, this review will propose future directions that may promote further use of the model.

Fracture healing in osteoporotic bone

As the world population rises, osteoporotic fracture is an emerging global threat to the well-being of elderly patients. The process of fracture healing by intramembranous ossification or/and endochondral ossification involve many well-orchestrated events including the signaling, recruitment and differentiation of mesenchymal stem cells (MSCs) during the early phase; formation of a hard callus and extracellular matrix, angiogenesis and revascularization during the mid-phase; and finally callus remodeling at the late phase of fracture healing. Through clinical and animal research, many of these factors are shown to be impaired in osteoporotic bone. Animal studies related to post-menopausal estrogen deficient osteoporosis (type I) have shown healing to be prolonged with decreased levels of MSCs and decreased levels of angiogenesis. Moreover, the expression of estrogen receptor (ER) was shown to be delayed in ovariectomy-induced osteoporotic fracture. This might be related to the observed difference in mechanical sensitivity between normal and osteoporotic bones, which requires further experiments to elucidate. In mice fracture models related to senile osteoporosis (type II), it was observed that chondrocyte and osteoblast differentiation were impaired; and that transplantation of juvenile bone marrow would result in enhanced callus formation. Other factors related to angiogenesis and vasculogenesis have also been noted to be impaired in aged models, affecting the degradation of cartilaginous matrixes and vascular invasion; the result is changes in matrix composition and growth factors concentrations that ultimately impairs healing during age-related osteoporosis. Most osteoporotic related fractures occur at metaphyseal sites clinically, and reports have indicated that differences exist between diaphyseal and metaphyseal fractures. An animal model that satisfies three main criteria (metaphyseal region, plate fixation, osteoporosis) is suggested for future research for more comprehensive understanding of the impairment in osteoporotic fractures. Therefore, a metaphyseal fracture or osteotomy that achieves complete discontinuity fixed with metal implants is suggested on ovariectomized aged rodent models.

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.

Rapamycin-induced autophagy activity promotes bone fracture healing in rats

Autophagy is a crucial mediating process for normal bone cell function and metabolism in physiology or pathology. Rapamycin has been demonstrated to induce the autophagy pathway by inhibiting the mammalian target of rapamycin (mTOR) pathway. However, the contribution of autophagy in orthopedic diseases is rarely reported. The aim of the present study was to evaluate the capacity of pharmacologically induced autophagy to modify disease function in a rat model of bone fracture. A femur fracture model was established via surgery in adult male Sprague-Dawley rats. Rapamycin (n=63 rats) or dimethyl sulfoxide (DMSO) vehicle control (n=63 rats) was administered intraperitoneally for 2, 4 and 6 weeks, and 21 randomly selected rats were sacrificed in each group at each time point. X-ray micro-computed tomography and hematoxylin and eosin staining were used to evaluate the extent of fracture healing in each group. The effects of rapamycin on autophagy, mTOR signaling and the expression levels of vascular endothelial growth factor (VEGF) and proliferating cell nuclear antigen (PCNA) were analyzed using immunohistochemistry, immunofluorescence staining and western blot analysis. Rapamycin affected the mTOR signaling pathway in rats following fracture, as indicated by the inhibition of the phosphorylation of ribosomal protein S6, a target of mTOR, and activation of microtubule-associated protein 2 light chain 3, a key marker of autophagy. Histomorphometry and image examination indicated that the number of osteoblasts in each section was significantly (P<0.01) increased in the rapamycin group compared with the control group, and this was associated with a significant (P<0.05) increase in mineralized callus fraction. Furthermore, rapamycin treatment increased the expression levels of VEGF and PCNA in the rat callus tissue. These results suggest that rapamycin may serve a beneficial function in fracture healing, and that the underlying mechanism may involve the activation of autophagy.

Absence of substance P and the sympathetic nervous system impact on bone structure and chondrocyte differentiation in an adult model of endochondral ossification

OBJECTIVE

Sensory and sympathetic nerve fibers (SNF) innervate bone and epiphyseal growth plate. The role of neuronal signals for proper endochondral ossification during skeletal growth is mostly unknown. Here, we investigated the impact of the absence of sensory neurotransmitter substance P (SP) and the removal of SNF on callus differentiation, a model for endochondral ossification in adult animals, and on bone formation.

METHODS

In order to generate callus, tibia fractures were set in the left hind leg of wild type (WT), tachykinin 1-deficient (Tac1-/-) mice (no SP) and animals without SNF. Locomotion was tested in healthy animals and touch sensibility was determined early after fracture. Callus tissue was prepared for immunofluorescence staining for SP, neurokinin1-receptor (NK1R), tyrosine-hydroxylase (TH) and adrenergic receptors α1, α2 and β2. At the fracture site, osteoclasts were stained for TRAP, osteoblasts were stained for RUNX2, and histomorphometric analysis of callus tissue composition was performed. Primary murine bone marrow derived macrophages (BMM), osteoclasts, and osteoblasts were tested for differentiation, activity, proliferation and apoptosis in vitro. Femoral fractures were set in the left hind leg of all the three groups for mechanical testing and μCT-analysis.

RESULTS

Callus cells stained positive for SP, NK1R, α1d- and α2b adrenoceptors and remained β2-adrenoceptor and TH-negative. Absence of SP and SNF did not change the general locomotion but reduces touch sensitivity after fracture. In mice without SNF, we detected more mesenchymal callus tissue and less cartilaginous tissue 5 days after fracture. At day 13 past fracture, we observed a decrease of the area covered by hypertrophic chondrocytes in Tac1-/- mice and mice without SNF, a lower number of osteoblasts in Tac1-/- mice and an increase of osteoclasts in mineralized callus tissue in mice without SNF. Apoptosis rate and activity of osteoclasts and osteoblasts isolated from Tac1-/- and sympathectomized mice were partly altered in vitro. Mechanical testing of fractured- and contralateral legs 21 days after fracture, revealed an overall reduced mechanical bone quality in Tac1-/- mice and mice without SNF. μCT-analysis revealed clear structural alteration in contralateral and fractured legs proximal of the fracture site with respect to trabecular parameters, bone mass and connectivity density. Notably, structural parameters are altered in fractured legs when related to unfractured legs in WT but not in mice without SP and SNF.

CONCLUSION

The absence of SP and SNF reduces pain sensitivity and mechanical stability of the bone in general. The micro-architecture of the bone is profoundly impaired in the absence of intact SNF with a less drastic effect in SP-deficient mice. Both sympathetic and sensory neurotransmitters are indispensable for proper callus differentiation. Importantly, the absence of SP reduces bone formation rate whereas the absence of SNF induces bone resorption rate. Notably, fracture chondrocytes produce SP and its receptor NK1 and are positive for α-adrenoceptors indicating an endogenous callus signaling loop. We propose that sensory and sympathetic neurotransmitters have crucial trophic effects which are essential for proper bone formation in addition to their classical neurological actions.

Systemic treatment with telmisartan improves femur fracture healing in mice

Recent clinical studies indicated that angiotensin receptor blockers (ARBs) would decrease the risk of bone fractures in the elderly populations. There is little known about the role of the ARBs in the process of fracture healing. The purpose of the present study was to verify the hypothesis that systemic treatment with telmisartan has the ability to promote fracture healing. In this study, femur fractures were produced in 96 mature male BALB/c mice. Animals were treated with the ARBs telmisartan or vehicle. Fracture healing was analysed after 2, 5 and 10 weeks postoperatively using X-ray, biomechanical testing, histomorphometry, immunohistochemistry and micro-computed tomography (micro-CT). Radiological analysis showed the diameter of the callus in the telmisartan treated animals was significantly increased when compared with that of vehicle treated controls after two weeks of fracture healing. The radiologically observed promotion of callus formation was confirmed by histomorphometric analyses, which revealed a significantly increased amount of bone formation when compared with vehicle-treated controls. Biomechanical testing further showed a significantly greater peak torque at failure, and a higher torsional stiffness in telmisartan-treated animals compared with controls. There was an increased fraction of PCNA-positive cells and VEGF-positive cells in telmisartan-treated group compared with vehicle-treated controls. From the three-dimensional reconstruction of the bony callus, telmisartan-treated group significantly increased the values of BV/TV by 21.7% and CsAr by 26.0% compared to the vehicle-treated controls at 5 weeks post-fracture. In summary, we demonstrate in the current study that telmisartan could promote fracture healing in a mice model via increasing mechanical strength and improving microstructure. The most mechanism is probably by an increase of cell proliferation and neovascularization associated with a decreased VEGF expression in hypertrophic chondrocytes.

Modelling for conflict: the legacy of ballistic research and current extremity in vivo modelling

Extremity ballistic injury is unique and the literature intended to guide its management is commonly misinterpreted. In order to care for those injured in conflict and conduct appropriate research, clinicians must be able to identify key in vivo studies, understand their weaknesses and desist the propagation of miscited and misunderstood ballistic dogma. This review provides the only inclusive critical overview of key studies of relevance to military extremity injury. In addition, the non-ballistic studies of limb injury, stabilisation and contamination that will form the basis from which future small animal extremity studies are constructed are presented. With an awareness of the legacy of military wound models and an insight into available generic models of extremity injury and contamination, research teams are well placed to optimise future military extremity injury management.

A new metaphyseal bone defect model in osteoporotic rats to study biomaterials for the enhancement of bone healing in osteoporotic fractures

The intention of this study was to establish a new critical size animal model that represents clinically relevant situations with osteoporotic bone status and internally fixated metaphyseal defect fractures in which biomaterials for the enhancement of fracture healing in osteoporotic fracture defects can be studied. Twenty-eight rats were ovariectomized (OVX) and treated with a calcium-, phosphorus-, vitamin D3-, soy- and phytoestrogen-free diet. After 3months Dual-energy X-ray absorptiometry measurements showed statistically significant reductions in bone mineral density of the spine of -25.9% and of the femur of -21.3% of the OVX rats compared with controls, confirming osteoporosis in the OVX rats. The OVX rats then underwent either 3 or 5mm wedge-shaped osteotomy of the distal metaphyseal area of the femur that was internally stabilized with a T-shaped mini-plate. After 42days biomechanical testing yielded completely unstable conditions in the 5mm defect femora (bending stiffness 0Nmm(-2)) and a bending stiffness of 12,500Nmm(-2) in the 3mm defects, which showed the beginning of fracture consolidation. Micro-computed tomography showed statistically significant more new bone formation in the 3mm defects (4.83±0.37mm(2)), with bridging of the initial fracture defect area, compared with the 5mm defects (2.68±0.34mm(2)), in which no bridging of the initial defect was found. These results were confirmed by histology. In conclusion, the 5mm defect can be considered as a critical size defect model in which biomaterials can be tested.

A murine femoral segmental defect model for bone tissue engineering using a novel rigid internal fixation system

BACKGROUND

As a model animal, the mouse has already been widely used in bone-related research. However, there is a lack of ideal long bone segmental defect mouse model. Since external fixation has disadvantages of heavy weight, penetrating the skin, and hampering mobility, an internal fixation device is probably more preferable to maintain the segmental bone defect. The aim of this study was to establish a simple, reproducible, and standardized murine critical-size defect model through designing an internal fixation system, verifying its adaptability, and investigating the critical size of femoral segmental defect.

METHODS

By utilizing computer-aided measuring and processing system, anatomical data of adult C57BL/6 mouse femur was obtained, and a plate-bolts system was designed for rigid fixation. The plate and screws were fixed in 67 mice and 1.5 or 2.0 mm defect gaps were created in the femoral midshaft. Compression and three-point bending of bone-implant construct were tested in mice at 0, 2, 5, and 12 wk postoperative to test the biomechanical stability. X-ray, micro-computed tomography, and histology were used to investigate the defect healing process.

RESULTS

The plate- and screws-fitted mouse femur and unilateral or bilateral operation had seemingly no adverse impact on the mouse in general. Mechanical tests indicated that there were no significant differences between the bone-implant construct and intact femur in compression and three-point bending loading. Micro-computed tomography scanning showed the bone mineral density had not been affected by the implantation of fixation device. There was no union of the 2.0 mm segmental defect in 12-wk period.

CONCLUSION

Using the specifically designed rigid internal fixation device, a segmental defect size of 2.0 mm in C57BL/6 mouse femur will show nonunion and can serve as a critical defect size for bone tissue engineering and bone regeneration research.

A novel murine femoral segmental critical-sized defect model stabilized by plate osteosynthesis for bone tissue engineering purposes

Mouse models are invaluable tools for mechanistic and efficacy studies of the healing process of large bone defects resulting in atrophic nonunions, a severe medical problem and a financial health-care-related burden. Models of atrophic nonunions are usually achieved by providing a highly stable biomechanical environment. For this purpose, external fixators have been investigated, but plate osteosynthesis, despite its high clinical relevance, has not yet been considered in mice. We hereby proposed and investigated the use of an internal osteosynthesis for stabilizing large bone defects. To this aim, a 3.5-mm-long segmental bone defect was induced in the mid-shaft of the femur using a Gigli saw and a jig. Bone fixation was performed using a titanium microlocking plate with four locking screws. The bone defect was either left empty or filled with a syngenic bone graft or filled with a coralline scaffold. Healing was monitored using radiographs. The healing process was further assessed using microcomputed tomography and histology 10 weeks after surgery. With the exception of one mouse that died during the surgical procedure, no complications were observed. A stable and reproducible bone fixation as well as a reproducible fixation of the implanted materials with full weight bearing was obtained in all animals tested. Nonunion was consistently observed in the group in which the defects were left empty. Bone union was obtained with the syngenic bone grafts, providing evidence that, although such defects were of critical size, bone healing was possible when the gold-standard material was used to fill the defect. Although new bone formation was greater in the coralline scaffold group than in the left-empty animal group, it remained limited and localized close to the bony edges, a consequence of the critical size of such bone defect. Our study established a reproducible, clinically relevant, femoral, atrophic nonunion, critical-sized defect, low morbidity mouse model. The present study was successful in designing and testing in a small animal model, a novel surgical method for the assessment of bone repair; this model has the potential to facilitate investigations of the molecular and cellular events involved in bone regeneration in load-bearing, segmental-bone defects.

In vivo models of bone repair

This review is aimed at clinicians appraising preclinical trauma studies and researchers investigating compromised bone healing or novel treatments for fractures. It categorises the clinical scenarios of poor healing of fractures and attempts to match them with the appropriate animal models in the literature.

We performed an extensive literature search of animal models of long bone fracture repair/nonunion and grouped the resulting studies according to the clinical scenario they were attempting to reflect; we then scrutinised them for their reliability and accuracy in reproducing that clinical scenario.

Models for normal fracture repair (primary and secondary), delayed union, nonunion (atrophic and hypertrophic), segmental defects and fractures at risk of impaired healing were identified. Their accuracy in reflecting the clinical scenario ranged greatly and the reliability of reproducing the scenario ranged from 100% to 40%.

It is vital to know the limitations and success of each model when considering its application.

Increased exercise after stable closed fracture fixation does not affect fracture healing in mice

PURPOSE

The aim of the present study was to evaluate the systemic biological effect of increased exercise on bone repair after stable fracture fixation.

METHODS

Two groups of SKH-1h mice were studied. Animals of the first group (n=36) were housed in cages supplied with a running wheel, while mice of the second group (n=37) were housed in standard cages for control. Using a closed femur fracture model, bone repair was analysed by histomorphometry and biomechanical testing at 2 and 5 weeks. At 2 weeks, we additionally evaluated the expression of the proliferation marker PCNA (proliferating cell nuclear antigen) and the angiogenic and osteogenic growth factor VEGF (vascular endothelial growth factor). To standardise the mechanical conditions in the fracture gap, we used an intramedullary compression screw for stable fracture fixation.

RESULTS

Each mouse of the exercise group run a mean total distance of 23.5 km after 2 weeks and 104.3 km after 5 weeks. Histomorphometric analysis of the size and tissue composition of the callus could not reveal significant differences between mice undergoing exercise and controls. Accordingly, biomechanical testing showed a comparable torsional stiffness, peak rotation angle, and load at failure of the healing bones in the two groups. The expression of PCNA and VEGF did also not differ between mice of the exercise group and controls.

CONCLUSION

We conclude that increased exercise does not affect bone repair after stable fracture fixation.

Small animal bone healing models: standards, tips, and pitfalls results of a consensus meeting

Small animal fracture models have gained increasing interest in fracture healing studies. To achieve standardized and defined study conditions, various variables must be carefully controlled when designing fracture healing experiments in mice or rats. The strain, age and sex of the animals may influence the process of fracture healing. Furthermore, the choice of the fracture fixation technique depends on the questions addressed, whereby intra- and extramedullary implants as well as open and closed surgical approaches may be considered. During the last few years, a variety of different, highly sophisticated implants for fracture fixation in small animals have been developed. Rigid fixation with locking plates or external fixators results in predominantly intramembranous healing in both mice and rats. Locking plates, external fixators, intramedullary screws, the locking nail and the pin-clip device allow different degrees of stability resulting in various amounts of endochondral and intramembranous healing. The use of common pins that do not provide rotational and axial stability during fracture stabilization should be discouraged in the future. Analyses should include at least biomechanical and histological evaluations, even if the focus of the study is directed towards the elucidation of molecular mechanisms of fracture healing using the largely available spectrum of antibodies and gene-targeted animals to study molecular mechanisms of fracture healing. This review discusses distinct requirements for the experimental setups as well as the advantages and pitfalls of the different fixation techniques in rats and mice.

Differential fracture healing resulting from fixation stiffness variability: a mouse model

BACKGROUND

The mechanisms underlying the interaction between the local mechanical environment and fracture healing are not known. We developed a mouse femoral fracture model with implants of different stiffness, and hypothesized that differential fracture healing would result.

METHODS

Femoral shaft fractures were created in 70 mice, and were treated with an intramedullary nail made of either tungsten (Young's modulus = 410 GPa) or aluminium (Young's modulus = 70 GPa). Mice were then sacrificed at 2 or 5 weeks. Fracture calluses were analyzed using standard microCT, histological, and biomechanical methods.

RESULTS

At 2 weeks, callus volume was significantly greater in the aluminium group than in the tungsten group (61.2 vs. 40.5 mm(3), p = 0.016), yet bone volume within the calluses was no different between the groups (13.2 vs. 12.3 mm(3)). Calluses from the tungsten group were stiffer on mechanical testing (18.7 vs. 9.7 N/mm, p = 0.01). The percent cartilage in the callus was 31.6% in the aluminium group and 22.9% in the tungsten group (p = 0.40). At 5 weeks, there were no differences between any of the healed femora.

CONCLUSIONS

In this study, fracture implants of different stiffness led to different fracture healing in this mouse fracture model. Fractures treated with a stiffer implant had more advanced healing at 2 weeks, but still healed by callus formation. Although this concept has been well documented previously, this particular model could be a valuable research tool to study the healing consequences of altered fixation stiffness, which may provide insight into the pathogenesis and ideal treatment of fractures and non-unions.

Pseudofracture: an acute peripheral tissue trauma model

Following trauma there is an early hyper-reactive inflammatory response that can lead to multiple organ dysfunction and high mortality in trauma patients; this response is often accompanied by a delayed immunosuppression that adds the clinical complications of infection and can also increase mortality. Many studies have begun to assess these changes in the reactivity of the immune system following trauma. Immunologic studies are greatly supported through the wide variety of transgenic and knockout mice available for in vivo modeling; these strains aid in detailed investigations to assess the molecular pathways involved in the immunologic responses. The challenge in experimental murine trauma modeling is long term investigation, as fracture fixation techniques in mice, can be complex and not easily reproducible. This pseudofracture model, an easily reproduced trauma model, overcomes these difficulties by immunologically mimicking an extremity fracture environment, while allowing freedom of movement in the animals and long term survival without the continual, prolonged use of anaesthesia. The intent is to recreate the features of long bone fracture; injured muscle and soft tissue are exposed to damaged bone and bone marrow without breaking the native bone. The pseudofracture model consists of two parts: a bilateral muscle crush injury to the hindlimbs, followed by injection of a bone solution into these injured muscles. The bone solution is prepared by harvesting the long bones from both hindlimbs of an age- and weight-matched syngeneic donor. These bones are then crushed and resuspended in phosphate buffered saline to create the bone solution. Bilateral femur fracture is a commonly used and well-established model of extremity trauma, and was the comparative model during the development of the pseudofracture model. Among the variety of available fracture models, we chose to use a closed method of fracture with soft tissue injury as our comparison to the pseudofracture, as we wanted a sterile yet proportionally severe peripheral tissue trauma model. Hemorrhagic shock is a common finding in the setting of severe trauma, and the global hypoperfusion adds a very relevant element to a trauma model. The pseudofracture model can be easily combined with a hemorrhagic shock model for a multiple trauma model of high severity.

Low dose erythropoietin stimulates bone healing in mice

Beyond its classical role in regulation of erythropoiesis, erythropoietin (EPO) has been shown to exert protective and regenerative actions in a variety of non-hematopoietic tissues. However, little is known about potential actions in bone regeneration. To analyze fracture healing in mice, a femoral 0.25 mm osteotomy gap was stabilized with a pin-clip technique. Animals were treated with 500 U EPO/kg bw per day or with vehicle only. After 2 and 5 weeks, fracture healing was analyzed biomechanically, radiologically and histologically. Expression of PCNA and NFκB was examined by Western blot analysis. Vascularization was analyzed by immunohistochemical staining of PECAM-1. Circulating endothelial progenitor cells were measured by flow-cytometry. Herein, we demonstrate that EPO-treatment significantly accelerates bone healing in mice. This is indicated by a significantly greater biomechanical stiffness and a higher radiological density of the periosteal callus at 2 and 5 weeks after fracture and stabilization. Histological analysis demonstrated significantly more bone and less cartilage and fibrous tissue in the periosteal callus. Endosteal vascularization was significantly increased in EPO-treated animals when compared to controls. The number of circulating endothelial progenitor cells was significantly greater in EPO-treated animals. The herein shown acceleration of healing by EPO may represent a promising novel treatment strategy for fractures with delayed healing and non-union formation.

Experimental trauma models: an update

Treatment of polytrauma patients remains a medical as well as socioeconomic challenge. Although diagnostics and therapy improved during the last decades, multiple injuries are still the major cause of fatalities in patients below 45 years of age. Organ dysfunction and organ failure are major complications in patients with major injuries and contribute to mortality during the clinical course. Profound understanding of the systemic pathophysiological response is crucial for innovative therapeutic approaches. Therefore, experimental studies in various animal models are necessary. This review is aimed at providing detailed information of common trauma models in small as well as in large animals.

Fracture healing in mice under controlled rigid and flexible conditions using an adjustable external fixator

Mice are increasingly used to investigate mechanobiology in fracture healing. The need exists for standardized models allowing for adjustment of the mechanical conditions in the fracture gap. We introduced such a model using rigid and flexible external fixators with considerably different stiffness (axial stiffnesses of 18.1 and 0.82 N/mm, respectively). Both fixators were used to stabilize a 0.5 mm osteotomy gap in the femur of C57BL/6 mice (each n = 8). Three-point bending tests, µCT, and histomorphometry demonstrated a different healing pattern after 21 days. Both fixations induced callus formation with a mixture of intramembranous and enchondral ossification. Under flexible conditions, the bending stiffness of the callus was significantly reduced, and a larger but qualitatively inferior callus with a significantly lower fraction of bone but a higher fraction of cartilage and soft tissue was formed. Monitoring of the animal movement and the ground reaction forces demonstrated physiological loading with no significant differences between the groups, suggesting that the differences in healing were not based on a different loading behavior. In summary, flexible external fracture fixation of the mouse femur led to delayed fracture healing in comparison to a more rigid situation.

Healing of non-displaced fractures produced by fatigue loading of the mouse ulna

We developed a fatigue loading protocol in mice to produce a non-displaced ulnar fracture in vivo, and characterized the early healing response. Using adult (5 month) C57Bl/6 mice, we first determined that cyclic compression of the forelimb under load-control leads to increasing applied displacement and, eventually, complete fracture. We then subjected the right forelimbs of 80 mice to cyclic loading (2 Hz; peak force approximately 4N) and limited the displacement increase to 0.75 mm (60% of the average displacement increase at complete fracture). This fatigue protocol created a partial, non-displaced fracture through the medial cortex near the ulnar mid-shaft, and reduced ulnar strength and stiffness by >50%. Within 1 day, there was significant upregulation of genes related to hypoxia (Hif1a) and osteogenesis (Bmp2, Bsp) in loaded ulnae compared to non-loaded, contralateral controls. The gene expression response peaked in magnitude near day 7 (e.g., Osx upregulated 8-fold), and included upregulation of FGF-family genes (e.g., Fgfr3 up 6-fold). Histologically, a localized periosteal response was seen at the site of the fracture; by day 7 there was abundant periosteal woven bone surrounding a region of cartilage. From days 7 to 14, the woven bone became denser but did not increase in area. By day 14, the woven-bone response resulted in complete recovery of ulnar strength and stiffness, restoring mechanical properties to normal levels. In the future, the fatigue loading approach can be used create non-displaced bone fractures in transgenic and knockout mice to study the mechanisms by which the skeleton rapidly repairs damage.

Inhibition of angiotensin-converting enzyme stimulates fracture healing and periosteal callus formation - role of a local renin-angiotensin system

BACKGROUND AND PURPOSE

The renin-angiotensin system (RAS) regulates blood pressure and electrolyte homeostasis. In addition, 'local' tissue-specific RAS have been identified, regulating regeneration, cell growth, apoptosis, inflammation and angiogenesis. Although components of the RAS are expressed in osteoblasts and osteoclasts, a local RAS in bone has not yet been described and there is no information on whether the RAS is involved in fracture healing. Therefore, we studied the expression and function of the key RAS component, angiotensin-converting enzyme (ACE), during fracture healing.

EXPERIMENTAL APPROACH

In a murine femur fracture model, animals were treated with the ACE inhibitor perindopril or vehicle only. Fracture healing was analysed after 2, 5 and 10 weeks using X-ray, micro-CT, histomorphometry, immunohistochemistry, Western blotting and biomechanical testing.

KEY RESULTS

ACE was expressed in osteoblasts and hypertrophic chondrocytes in the periosteal callus during fracture healing, accompanied by expression of the angiotensin type-1 and type-2 receptors. Perindopril treatment reduced blood pressure and bone mineral density in unfractured femora. However, it improved periosteal callus formation, bone bridging of the fracture gap and torsional stiffness. ACE inhibition did not affect cell proliferation, but reduced apoptotic cell death. After 10 week treatment, a smaller callus diameter and bone volume after perindopril treatment indicated an advanced stage of bone remodelling.

CONCLUSIONS

Our study provides evidence for a local RAS in bone that influenced the process of fracture healing. We show for the first time that inhibition of ACE is capable of accelerating bone healing and remodelling.

An internal locking plate to study intramembranous bone healing in a mouse femur fracture model

In most murine fracture models, the femur is stabilized by an intramedullary implant and heals predominantly through endochondral ossification. The aim of the present study was to establish a mouse model in which fractures heal intra-membraneously. Femur fractures of 16 SKH-mice were stabilized by an internal locking plate. Femur fractures of another 16 animals were stabilized by an intramedullary screw. Bone repair was analyzed by radiographic, biomechanical, and histological methods. At 2 weeks, histological analysis showed a significantly smaller callus diameter and callus area after locking plate fixation. Cartilage formation within the callus could only be observed after screw fixation, but not after fracture stabilization with the locking plate. Radiological and biomechanical analysis after 2 and 5 weeks showed a significantly improved healing and a higher bending stiffness of fractures stabilized by the locking plate. Fractures stabilized by the locking plate healed exclusively by intramembranous ossification, which is most probably a result of the anatomical reduction and stable fixation. The fractures that healed by intramembranous ossification showed an increased stiffness compared to fractures that healed by endochondral ossification. This model may be used to study molecular mechanisms of intramembranous bone healing.

Ex vivo analysis of rotational stiffness of different osteosynthesis techniques in mouse femur fracture

The various molecular mechanisms of cell regeneration and tissue healing can best be studied in mouse models with the availability of a wide range of monoclonal antibodies and gene-targeted animals. The influence of the mechanical stability of individual stabilization techniques on the molecular mechanisms of fracture healing has not been completely elucidated yet. Although during recent years several osteosynthesis techniques have been introduced in mouse fracture models, no comparative study on fracture stabilization is available yet. We therefore analyzed herein in a standardized ex vivo setup the rotational stiffness of seven different osteosynthesis techniques using osteotomized right cadaver femora of CD-1 mice. Uninjured femora without osteotomy served as controls. Femur stabilization with a locking plate or an external fixator resulted in a rotational stiffness almost similar to the intact femur. The use of a "pin-clip" device, a "locking nail," a "mouse nail," or an "intramedullary screw" produced a lower torsional stiffness, which, however, was still significantly higher than that achieved with the widely applied conventional pin. By the use of the presented data a more specific choice of stabilization technique will be possible according to the various questions concerning molecular aspects in fracture healing.

Remodeling of fracture callus in mice is consistent with mechanical loading and bone remodeling theory

During the remodeling phase of fracture healing in mice, the callus gradually transforms into a double cortex, which thereafter merges into one cortex. In large animals, a double cortex normally does not form. We investigated whether these patterns of remodeling of the fracture callus in mice can be explained by mechanical loading. Morphologies of fractures after 21, 28, and 42 days of healing were determined from an in vivo mid-diaphyseal femoral osteotomy healing experiment in mice. Bone density distributions from microCT at 21 days were converted into adaptive finite element models. To assess the effect of loading mode on bone remodeling, a well-established remodeling algorithm was used to examine the effect of axial force or bending moment on bone structure. All simulations predicted that under axial loading, the callus remodeled to form a single cortex. When a bending moment was applied, dual concentric cortices developed in all simulations, corresponding well to the progression of remodeling observed experimentally and resulting in quantitatively comparable callus areas of woven and lamellar bone. Effects of biological differences between species or other reasons cannot be excluded, but this study demonstrates how a difference in loading mode could explain the differences between the remodeling phase in small rodents and larger mammals.

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.

Rapamycin affects early fracture healing in mice

BACKGROUND AND PURPOSE

The immunosuppressive drug rapamycin (RAPA) prevents rejection in organ transplantation by inhibiting interleukin-2-stimulated T-cell division. Rapamycin has also been suggested to possess strong anti-angiogenic activities linked to a decrease in production of vascular endothelial growth factor (VEGF). Angiogenesis and VEGF are thought to play a crucial role in fracture healing and as osteoporotic and traumatic fractures are common complications in immunosuppressed, organ transplantation patients, we conducted this study to analyze the effect of rapamycin treatment on bone repair.

EXPERIMENTAL APPROACH

We investigated the effect of rapamycin treatment on bone repair in a murine closed femur fracture model using radiological, histomorphometric, immunohistochemical, biomechanical and protein biochemical analyses.

KEY RESULTS

X-ray analyses demonstrated that rapamycin treatment inhibits callus formation after two weeks of fracture healing. The radiologically observed lack of callus formation was confirmed by histomorphometric analyses, which revealed a significantly diminished callus size and a reduced amount of bone formation when compared with vehicle-treated controls. Biomechanical testing further demonstrated that rapamycin significantly reduces torsional stiffness of the callus. Interestingly, this effect was associated with decreased vessel formation; a diminished proliferation of osteoblasts, endothelial cells and periosteal cells; and a reduced VEGF expression in hypertrophic chondrocytes. After five weeks treatment, however, the negative impact of rapamycin on fracture healing was overcome.

CONCLUSIONS AND IMPLICATIONS

Thus, rapamycin initially delays fracture healing, most probably by inhibiting cell proliferation and neovascularization in the callus. These undesirable effects should be considered when rapamycin is administered to patients sustaining bone fractures.

An improved murine femur fracture device for bone healing studies

Murine models are commonly used to investigate bone healing and test new treatments before human trials. Our objective was to design an improved murine femur fracture device and determine optimal mass and velocity settings for maximal likelihood of transverse fracture. Fracture reproducibility was maximized using an adjustable kinetic energy level, a novel mouse positioning system and an electromagnet striker release assembly. Sixty wild-type mice of 8-12-week-old male and female with a weight of 26.4+/-6.1g were subjected to an experimental postmortem fracture in the left and right femur (n=120) using variable kinetic energy inputs. A best-fit prediction equation for transverse fracture was developed using multivariate linear regression. Transverse fracture was shown to correlate most highly with kinetic energy with a maximum likelihood at mv2=292 where m is mass (g) and v is velocity (m/s). Model validation with a group of 134 anesthetized C57BL/6 mice resulted in a favorable transverse fracture rate of 85.8%. Simple modifications to existing fracture devices can improve accuracy and reproducibility. The results may assist researchers studying the effects of genetic modifications and novel treatments on boney healing in murine femur fracture models. Maintaining kinetic energy parameters within suggested ranges may also aid in ensuring accuracy and reproducibility.

Development of a stable closed femoral fracture model in mice

BACKGROUND

Mice have become of increasing interest as experimental model for fracture studies. Due to their small size, most studies use simple pins for fracture stabilization, although insufficient rigidity of fixation critically affects fracture healing. Herein, we studied whether longitudinal fracture compression by an intramedullary screw represents a standardized, stable osteosynthesis technique in mice, and whether it may accelerate fracture healing.

MATERIALS AND METHODS

A micro-screw (MouseScrew) was constructed, allowing closed fracture stabilization without traumatizing surgery. Fracture stabilization was achieved by longitudinal compression, which was confirmed by biomechanical testing of osteotomized cadaver femora. Bone repair was analyzed histomorphometrically at 2 and 5 wk after surgery.

RESULTS

Ex vivo analyses showed a significantly increased rotational and axial stiffness after screw stabilization (n = 8 each) compared with stabilization techniques using a conventional pin (n = 8 each) or a locking nail (n = 8 each). In the in vivo setting, 2 wk of screw stabilization (n = 8) demonstrated a significantly decreased fibrous tissue formation and an increased cartilage production compared with fractures stabilized by the locking nail (n = 8). After 5 wk callus consisted exclusively of bone in all animals studied without differences between the two stabilization techniques (n = 8 each).

CONCLUSIONS

Because prolonged fibrous tissue formation indicates delayed fracture healing, we conclude that the increased stability of the fracture by the use of our newly developed MouseScrew accelerates initial bone repair. Further, this fracture model may represent an ideal tool to study bone repair in mice under conditions of stable fixation.