Three-dimensional reconstruction of fracture callus morphogenesis

Does cefuroxime alter fracture healing in vivo? A micro-computertomographic, biomechanical, and histomorphometric evaluation using a rat fracture model

Cefuroxime is widely used for antibiotic prophylaxis in orthopedic surgery. However, a recent study indicated a dose-dependent reduction in osteoblast function in vitro. Nevertheless, cell culture might not sufficiently imitate the complex process of bone remodeling. As data concerning possible in vivo interactions of cefuroxime on fracture healing are completely missing, we investigated the following hypothesis: Does Cefuroxime impair bone healing in vivo? Therefore, 34 male Wistar rats were randomised to cefuroxime-treated or control groups, a Kirschner wire was inserted into right femora and closed transverse fractures were produced. Twenty-one days later, the structural properties of the fracture callus in the early fracture healing phase were evaluated via a combination of micro-CT (μCT), biomechanics and histology. µCT demonstrated similar values in the cefuroxime and control group (e.g., callus volume: 67.19 ± 14.90 mm³ vs. 55.35 ± 6.74 mm³ , p = 0.12; density: 635.48 ± 14.81 mg HA/cm³ vs. 647.87 ± 13.01 mg HA/cm³ , p = 0.16). Biomechanically, similar values were again determined between the groups, in terms of both maximum load (77.65 ± 41.82 vs. 78.54 ± 20.52, p = 0.95) and stiffness (122.44 ± 81.16 vs. 123.74 ± 60.08, p = 0.97). Histological findings were consistent with the radiographic results. Thus, no relevant differences between the cefuroxime and control groups could be found and the reported negative effects on osteoblasts in vitro were not confirmed in vivo by using standard concentrations of cefuroxime. In conclusion, cefuroxime can reasonably be recommended in a clinical setting as an antibiotic therapy when fracture healing is involved. However, supraphysiological doses were not evaluated, which may be present when cefuroxime is used as an additive to bone cement and released over time. Therefore, future studies should evaluate the in vivo effects of prolonged high cefuroxime doses on implant incorporation. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2282-2291, 2017.

Ovariectomy-Induced Osteopenia Influences the Middle and Late Periods of Bone Healing in a Mouse Femoral Osteotomy Model

OBJECTIVE

It is known that bone healing is delayed in the presence of osteoporosis in humans. However, due to the complexities of the healing of osteoporotic fractures, animal models may be more appropriate for studying the effects of osteoporosis in more detail and for testing drugs on the fracture repair process. The purpose of this study was to investigate the influence of ovariectomy-induced osteopenia in bone healing in an open femoral osteotomy model, and to test the feasibility of this model for evaluating the healing process under osteopenic conditions.

METHODS

Ovariectomized (OVX) mouse models were employed to assess the effects of osteopenia on fracture healing, A mid-shaft femur osteotomy model was also established 3 weeks after ovariectomy as an osteopenic fracture group (OVX group). Femurs were then harvested at 2 weeks and 6 weeks after fracture for X-ray radiography, micro-computed tomography (micro-CT), histology, and biomechanical analysis. A sham-operated group (sham group) was used for comparison.

RESULTS

The OVX mice had significantly lower bone volume density (BVF), volumetric bone mineral density (vBMD), and tissue mineral density (TMD) in the fracture calluses at 6 weeks (p<0.05), and similar trend was observed in 2 weeks. Additionally, larger calluses in OVX animals were observed via micro-CT and X-ray, but these did not result in better healing outcomes, as determined by biomechanical test at 6 weeks. Histological images of the healing fractures in the OVX mice found hastening of broken end resorption and delay of hard callus remodeling. The impaired biomechanical measurements in the OVX group (p<0.05) were consistent with micro-CT measurements and radiographic scoring, which also indicated delay in fracture healing of the OVX group.

CONCLUSIONS

This study provided evidence that ovariectomy-induced osteopenia impair the middle and late bone healing process. These data also supported the validity of the mouse femoral osteotomy model in evaluating the process of bone healing under osteopenic conditions.

Stimulation of Bone Repair with Ultrasound

This chapter reviews the different options available for the use of ultrasound in the enhancement of fracture healing or in the reactivation of a failed healing process: LIPUS, shock waves and ultrasound-mediated delivery of bioactive molecules, such as growth factors or plasmids. The main emphasis is on LIPUS, or Low Intensity Pulsed Ultrasound, the most widespread and studied technique. LIPUS has pronounced bioeffects on tissue regeneration, while employing intensities within a diagnostic range. The biological response to LIPUS is complex as the response of numerous cell types to this stimulus involves several pathways. Known to-date mechanotransduction pathways involved in cell responses include MAPK and other kinases signaling pathways, gap-junctional intercellular communication, up-regulation and clustering of integrins, involvement of the COX-2/PGE2 and iNOS/NO pathways, and activation of the ATI mechanoreceptor. Mechanisms at the origin of LIPUS biological effects remain intriguing, and analysis is hampered by the diversity of experimental systems used in-vitro. Data point to clear evidence that bioeffects can be modulated by direct and indirect mechanical effects, like acoustic radiation force, acoustic streaming, propagation of surface waves, heat, fluid-flow induced circulation and redistribution of nutrients, oxygen and signaling molecules. One of the future engineering challenge is therefore the design of dedicated experimental set-ups allowing control of these different mechanical phenomena, and to relate them to biological responses. Then, the derivation of an 'acoustic dose' and the cross-calibration of the different experimental systems will be possible. Despite this imperfect knowledge of LIPUS biophysics, the clinical evidence, although most often of low quality, speaks in favor of the clinical use of LIPUS, when the economics of nonunion and the absence of toxicity of this ultrasound technology are taken into account.

Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing

Uncovering the mechanisms of the sensitivity of bone healing to mechanical factors is critical for understanding the basic biology and mechanobiology of the skeleton, as well as for enhancing clinical treatment of bone injuries. This study refined an experimental method of measuring the strain microenvironment at the site of a bone injury during bone healing. This method used a rat model in which a well-controlled bending motion was applied to an osteotomy to induce the formation of pseudarthrosis that is composed of a range of skeletal tissues, including woven bone, cartilage, fibrocartilage, fibrous tissue, and clot tissue. The goal of this study was to identify both the features of the strain microenvironment associated with formation of these different tissues and the expression of proteins frequently implicated in sensing and transducing mechanical cues. By pairing the strain measurements with histological analyses that identified the regions in which each tissue type formed, we found that formation of the different tissue types occurs in distinct strain microenvironments and that the type of tissue formed is correlated most strongly to the local magnitudes of extensional and shear strains. Weaker correlations were found for dilatation. Immunohistochemical analyses of focal adhesion kinase and rho family proteins RhoA and CDC42 revealed differences within the cartilaginous tissues in the calluses from the pseudarthrosis model as compared to fracture calluses undergoing normal endochondral bone repair. These findings suggest the involvement of these proteins in the way by which mechanical stimuli modulate the process of cartilage formation during bone healing.

Loss of Gi G-Protein-Coupled Receptor Signaling in Osteoblasts Accelerates Bone Fracture Healing

G-protein-coupled receptors (GPCRs) are key regulators of skeletal homeostasis and are likely important in fracture healing. Because GPCRs can activate multiple signaling pathways simultaneously, we used targeted disruption of G(i) -GPCR or activation of G(s) -GPCR pathways to test how each pathway functions in the skeleton. We previously demonstrated that blockade of G(i) signaling by pertussis toxin (PTX) transgene expression in maturing osteoblastic cells enhanced cortical and trabecular bone formation and prevented age-related bone loss in female mice. In addition, activation of G(s) signaling by expressing the G(s) -coupled engineered receptor Rs1 in maturing osteoblastic cells induced massive trabecular bone formation but cortical bone loss. Here, we test our hypothesis that the G(i) and G(s) pathways also have distinct functions in fracture repair. We applied closed, nonstabilized tibial fractures to mice in which endogenous G(i) signaling was inhibited by PTX, or to mice with activated G(s) signaling mediated by Rs1. Blockade of endogenous G(i) resulted in a smaller callus but increased bone formation in both young and old mice. PTX treatment decreased expression of Dkk1 and increased Lef1 mRNAs during fracture healing, suggesting a role for endogenous G(i) signaling in maintaining Dkk1 expression and suppressing Wnt signaling. In contrast, adult mice with activated Gs signaling showed a slight increase in the initial callus size with increased callus bone formation. These results show that G(i) blockade and G(s) activation of the same osteoblastic lineage cell can induce different biological responses during fracture healing. Our findings also show that manipulating the GPCR/cAMP signaling pathway by selective timing of G(s) and G(i) -GPCR activation may be important for optimizing fracture repair.

Intramembranous bone regeneration differs among common inbred mouse strains following marrow ablation

Various intact and post-injury bone phenotypes are heritable traits. In this study, we sought to determine if intramembranous bone regeneration following marrow ablation differed among common inbred mouse strains and to identify how early the differences appear. We found a ∼four-fold difference in the regenerated bone volume 21 days after marrow ablation in females from four inbred mouse strains: FVB/N (15.7 ± 8.1%, mean and standard deviation), C3H/He (15.5 ± 4.2%), C57BL/6 (12.2 ± 5.2%), and BALB/c (4.0 ± 4.4%); with BALB/c different from FVB/N (p = 0.007) and C3H/He (p = 0.002). A second experiment showed that FVB/N compared to BALB/c mice had more regenerated bone 7 and 14 days after ablation (p < 0.001), while at 21 days FVB/N mice had a greater fraction of mineralizing surface (p = 0.008) without a difference in mineral apposition rate. Thus, differences among strains are evident early during intramembranous bone regeneration following marrow ablation and appear to be associated with differences in osteogenic cell recruitment, but not osteoblast activity. The amount of regenerating bone was not correlated with other heritable traits such as the intact bone phenotype or soft tissue wound healing, suggesting that there may be independent genetic pathways for these traits.

Fracture bone healing and biodegradation of AZ31 implant in rats

The ideal temporary implant should offer enough mechanical support to allow healing of the fracture and then biodegrade and be resorbed by metabolic mechanisms without causing any toxic effect. The aim of this research has been to simultaneously study in situ bone healing and the biodegradation of AZ31 Mg alloy as an osteosynthesis material. The in vivo study was carried out in AZ31 implants with and without Mg-fluoride coating inserted in un-fractured and fractured femurs of Wistar rats for long experimentation time, from 1 to 13 months, by means of computed tomography, histological and histomorphometric analysis. Tomography analysis showed the bone healing and biodegradation of AZ31 implants. The fracture is healed in 100% of the animals, and AZ31 maintains its mechanical integrity throughout the healing process. Biodegradation was monitored, quantifying the evolution of gas over time by 3D composition of tomography images. In all the studied groups, gas pockets disappear with time as a result of the diffusion process through soft tissues. Histomorphometric studies reveal that after 13 months the 46.32% of AZ31 alloy has been resorbed. The resorption of the coated and uncoated AZ31 implants inserted in fractured femurs after 1, 9 and 13 months does not have statistically significant differences. There is a balance between the biodegradation of AZ31 and bone healing which allows the use of AZ31 to be proposed as an osteosynthesis material.

Mouse models of bone healing: fracture, marrow ablation, and distraction osteogenesis

Three commonly used murine surgical models of bone healing [closed fracture with intramedullary fixation, distraction osteogenesis (DO), and marrow ablation by reaming] are presented. Detailed surgical protocols for each model are outlined. The nature of the regenerative processes and the types of research questions that may be addressed with these models are briefly outlined. The relative strengths and weaknesses of these models are compared to a number of other surgical models that are used to address similar research questions.

Overview of biological mechanisms and applications of three murine models of bone repair: closed fracture with intramedullary fixation, distraction osteogenesis, and marrow ablation by reaming

Fractures are one of the most common large-organ, traumatic injuries in humans, and osteoporosis-related fractures are the fastest growing health care problem of aging. Elective orthopedic surgeries of the bones and joints also represent some of most common forms of elective surgeries performed. Optimal repair of skeletal tissues is necessary for successful outcomes of these many different orthopedic surgical treatments. Research focused on post-natal skeletal repair is therefore of immense clinical importance and of particular relevance in situations in which bone tissue healing is compromised due to the extent of tissue trauma or specific medical co-morbidities. Three commonly used murine surgical models of bone healing, closed fracture with intramedullary fixation, distraction osteogenesis (DO), and marrow ablation by reaming, are presented. The biological aspects of these models are contrasted and the types of research questions that may be addressed with these models are presented.

Diabetes reduces mesenchymal stem cells in fracture healing through a TNFα-mediated mechanism

AIMS/HYPOTHESIS

Diabetes interferes with bone formation and impairs fracture healing, an important complication in humans and animal models. The aim of this study was to examine the impact of diabetes on mesenchymal stem cells (MSCs) during fracture repair.

METHODS

Fracture of the long bones was induced in a streptozotocin-induced type 1 diabetic mouse model with or without insulin or a specific TNFα inhibitor, pegsunercept. MSCs were detected with cluster designation-271 (also known as p75 neurotrophin receptor) or stem cell antigen-1 (Sca-1) antibodies in areas of new endochondral bone formation in the calluses. MSC apoptosis was measured by TUNEL assay and proliferation was measured by Ki67 antibody. In vitro apoptosis and proliferation were examined in C3H10T1/2 and human-bone-marrow-derived MSCs following transfection with FOXO1 small interfering (si)RNA.

RESULTS

Diabetes significantly increased TNFα levels and reduced MSC numbers in new bone area. MSC numbers were restored to normal levels with insulin or pegsunercept treatment. Inhibition of TNFα significantly reduced MSC loss by increasing MSC proliferation and decreasing MSC apoptosis in diabetic animals, but had no effect on MSCs in normoglycaemic animals. In vitro experiments established that TNFα alone was sufficient to induce apoptosis and inhibit proliferation of MSCs. Furthermore, silencing forkhead box protein O1 (FOXO1) prevented TNFα-induced MSC apoptosis and reduced proliferation by regulating apoptotic and cell cycle genes.

CONCLUSIONS/INTERPRETATION

Diabetes-enhanced TNFα significantly reduced MSC numbers in new bone areas during fracture healing. Mechanistically, diabetes-enhanced TNFα reduced MSC proliferation and increased MSC apoptosis. Reducing the activity of TNFα in vivo may help to preserve endogenous MSCs and maximise regenerative potential in diabetic patients.

Early stages of bone fracture healing: formation of a fibrin-collagen scaffold in the fracture hematoma

This work is concerned with the sequence of events taking place during the first stages of bone fracture healing, from bone breakup until the formation of early fibrous callus (EFC). The latter provides a scaffold over which subsequent remodeling processes will eventually result in successful bone repair. Specifically, some mathematical models are proposed to estimate the time required for (1) the formation immediately after fracture of a fibrin clot, described in terms of a phase transition in a polymerization process, and (2) the onset of EFC which is produced when fibroblasts arising from differentiation of chemotactically recruited mesenchymal stem cells remodel a previous fibrin clot by releasing a collagen matrix over it. An attempt has been made to keep models as simple as possible, so that a explicit dependence of the estimates obtained on relevant biochemical parameters involved is obtained.

Overview of skeletal repair (fracture healing and its assessment)

The study of postnatal skeletal repair is of immense clinical interest. Optimal repair of skeletal tissue is necessary in all varieties of elective and reparative orthopedic surgical treatments. However, the repair of fractures is unique in this context in that fractures are one of the most common traumas that humans experience and are the end-point manifestation of osteoporosis, the most common chronic disease of aging. In the first part of this introduction the basic biology of fracture healing is presented. The second part discusses the primary methodological approaches that are used to examine repair of skeletal hard tissue and specific considerations for choosing among and implementing these approaches.

Initiation and early control of tissue regeneration - bone healing as a model system for tissue regeneration

INTRODUCTION

Tissue regeneration in itself is a fascinating process that promises repeated renewal of tissue and organs.

AREAS COVERED

This article aims to illustrate the different strategies available to control tissue regeneration at a very early stage, using bone as an exemplary tissue. The aspects of a controlled inflammatory cascade to achieve a balanced immune response, cell therapeutic approaches for improved tissue formation and angiogenesis, guiding the organization of newly formed extracellular matrix by biomaterials, the relevance of mechanical signals for tissue regeneration processes, and the chances and limitations of growth factor treatments are discussed.

EXPERT OPINION

The currently available knowledge is reviewed and perspectives for potential new targets are given. This is done under the assumption that early identification of risk patients as well as the application of early intervention strategies is possible.

Stimulation of bone repair with ultrasound: a review of the possible mechanic effects

In vivo and in vitro studies have demonstrated the positive role that ultrasound can play in the enhancement of fracture healing or in the reactivation of a failed healing process. We review the several options available for the use of ultrasound in this context, either to induce a direct physical effect (LIPUS, shock waves), to deliver bioactive molecules such as growth factors, or to transfect cells with osteogenic plasmids; with a main focus on LIPUS (or Low Intensity Pulsed Ultrasound) as it is the most widespread and studied technique. The biological response to LIPUS is complex as numerous cell types respond to this stimulus involving several pathways. Known to-date mechanotransduction pathways involved in cell responses include MAPK and other kinases signaling pathways, gap-junctional intercellular communication, up-regulation and clustering of integrins, involvement of the COX-2/PGE2, iNOS/NO pathways and activation of ATI mechanoreceptor. The mechanisms by which ultrasound can trigger these effects remain intriguing. Possible mechanisms include direct and indirect mechanical effects like acoustic radiation force, acoustic streaming, and propagation of surface waves, fluid-flow induced circulation and redistribution of nutrients, oxygen and signaling molecules. Effects caused by the transformation of acoustic wave energy into heat can usually be neglected, but heating of the transducer may have a potential impact on the stimulation in some in-vitro systems, depending on the coupling conditions. Cavitation cannot occur at the pressure levels delivered by LIPUS. In-vitro studies, although not appropriate to identify the overall biological effects, are of great interest to study specific mechanisms of action. The diversity of current experimental set-ups however renders this analysis very complex, as phenomena such as transducer heating, inhomogeneities of the sound intensity in the near field, resonances in the transmission and reflection through the culture dish walls and the formation of standing waves will greatly affect the local type and amplitude of the stimulus exerted on the cells. A future engineering challenge is therefore the design of dedicated experimental set-ups, in which the different mechanical phenomena induced by ultrasound can be controlled. This is a prerequisite to evaluate the biological effects of the different phenomena with respect to particular parameters, like intensity, frequency, or duty cycle. By relating the variations of these parameters to the induced physical effects and to the biological responses, it will become possible to derive an 'acoustic dose' and propose a quantification and cross-calibration of the different experimental systems. Improvements in bone healing management will probably also come from a combination of ultrasound with a 'biologic' components, e.g. growth factors, scaffolds, gene therapies, or drug delivery vehicles, the effects of which being potentiated by the ultrasound.

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.

Effect of perioperative buccal fracture of the proximal segment on postoperative stability after sagittal split ramus osteotomy

Objectives

Buccal fracture of the mandibular proximal bone segment during bilateral sagittal split ramus osteotomy (SSRO) reduces the postoperative stability. The primary aim of this study is to evaluate the effect of this type of fracture on bone healing and postoperative stability after mandibular setback surgery.

Materials and Methods

Ten patients who experienced buccal fracture during SSRO for mandibular setback movement were evaluated. We measured the amount of bone generation on a computed tomography scan, using an image analysis program, and compared the buccal fracture side to the opposite side in each patient. To investigate the effect on postoperative stability, we measured the postoperative relapse in lateral cephalograms, immediately following and six months after the surgery. The control group consisted of ten randomly-selected patients having a similar amount of set-back without buccal fracture.

Results

Less bone generation was observed on the buccal fracture side compared with the opposite side (P<0.05). However, there was no significant difference in anterior-posterior postoperative relapse between the group with buccal fracture and the control group. The increased mandibular plane angle and anterior facial height after the surgery in the group with buccal fracture manifested as a postoperative clockwise rotation of the mandible.

Conclusion

Bone generation was delayed compared to the opposite side. However, postoperative stability in the anterior-posterior direction could be maintained with rigid fixation.

Generation of closed transverse fractures in small animals

The most common procedure that has been developed for use in rats and mice to model fracture healing is described. The nature of the regenerative processes that may be assessed and the types of research questions that may be addressed with this model are briefly outlined. The detailed surgical protocol to generate closed simple transverse fractures is presented, and general considerations when setting up an experiment using this model are described.

Deferoxamine restores callus size, mineralization, and mechanical strength in fracture healing after radiotherapy

BACKGROUND

Therapeutic augmentation of fracture-site angiogenesis with deferoxamine has proven to increase vascularity, callus size, and mineralization in long-bone fracture models. The authors posit that the addition of deferoxamine would enhance pathologic fracture healing in the setting of radiotherapy in a model where nonunions are the most common outcome.

METHODS

Thirty-five Sprague-Dawley rats were divided into three groups. Fracture, irradiated fracture, and irradiated fracture plus deferoxamine. The irradiated fracture and irradiated fracture plus deferoxamine groups received a human equivalent dose of radiotherapy [7 Gy/day for 5 days, (35 Gy)] 2 weeks before mandibular osteotomy and external fixation. The irradiated fracture plus deferoxamine group received injections of deferoxamine into the fracture callus after surgery. After a 40-day healing period, mandibles were dissected, clinically assessed for bony union, imaged with micro-computed tomography, and tension tested to failure.

RESULTS

Compared with irradiated fractures, metrics of callus size, mineralization, and strength in deferoxamine-treated mandibles were significantly increased. These metrics were restored to a level demonstrating no statistical difference from control fractures. In addition, the authors observed an increased rate of achieving bony unions in the irradiated fracture plus deferoxamine-treated group when compared with irradiated fracture (67 percent and 20 percent, respectively).

CONCLUSIONS

The authors' data demonstrate nearly total restoration of callus size, mineralization, and biomechanical strength, and a threefold increase in the rate of union with the use of deferoxamine. The authors' results suggest that the administration of deferoxamine may have the potential for clinical translation as a new treatment paradigm for radiation-induced pathologic fractures.

Enhanced reconstruction of long bone architecture by a growth factor mutant combining positive features of GDF-5 and BMP-2

Non healing bone defects remain a worldwide health problem and still only few osteoinductive growth factors are available for clinical use in bone regeneration. By introducing BMP-2 residues into growth and differentiation factor (GDF)-5 we recently produced a mutant GDF-5 protein BB-1 which enhanced heterotopic bone formation in mice. Designed to combine positive features of GDF-5 and BMP-2, we suspected that this new growth factor variant may improve long bone healing compared to the parent molecules and intended to unravel functional mechanisms behind its action. BB-1 acquired an increased binding affinity to the BMP-IA receptor, mediated enhanced osteogenic induction of human mesenchymal stem cells versus GDF-5 and higher VEGF secretion than BMP-2 in vitro. Rabbit radius defects treated with a BB-1-coated collagen carrier healed earlier and with increased bone volume compared to BMP-2 and GDF-5 according to in vivo micro-CT follow-up. While BMP-2 callus often remained spongy, BB-1 supported earlier corticalis and marrow cavity formation, showing no pseudojoint persistence like with GDF-5. Thus, by combining positive angiogenic and osteogenic features of GDF-5 and BMP-2, only BB-1 restored a natural bone architecture within 12 weeks, rendering this promising growth factor variant especially promising for long bone regeneration.

Assessment of contrast-enhanced computed tomography for imaging of cartilage during fracture healing

Assessment of the early stages of fracture healing via X-rays and computed tomography is limited by the low radio-opacity of cartilage. We validated a method of contrast-enhanced computed tomography (CECT) for non-destructive identification of cartilage within a healing fracture callus. Closed, stabilized fractures in femora of C57BL/6 mice were harvested on post-operative day 9.5 and imaged ex vivo with micro-computed tomography (µCT) before and after incubation in a cationic contrast agent that preferentially accumulates in cartilage due to the high concentration of sulfated glycosaminoglycans in the tissue. Co-registration of the pre- and post-incubation images, followed by image subtraction, enabled two- and three-dimensional delineation of mineralized tissue, soft callus, and cartilage. The areas of cartilage and callus identified with CECT were compared to those identified with the gold-standard method of histomorphometry. No difference was found between the areas of cartilage measured by the two methods (p = 0.999). Callus area measured by CECT was smaller than, but strongly predictive of (R(2) = 0.80, p < 0.001), the corresponding histomorphometric measurements. CECT also enabled qualitative identification of mineralized cartilage. These findings indicate that the CECT method provides accurate, quantitative, and non-destructive visualization of the shape and composition of the fracture callus, even during the early stages of repair when little mineralized tissue is present. The non-destructive nature of this method would allow subsequent analyses, such as mechanical testing, to be performed on the callus, thus enabling higher-throughput, comprehensive investigations of bone healing.

Chemokine expression is upregulated in chondrocytes in diabetic fracture healing

Chemokines are thought to play an important role in several aspects of bone metabolism including the recruitment of leukocytes and the formation of osteoclasts. We investigated the impact of diabetes on chemokine expression in normal and diabetic fracture healing. Fracture of the femur was performed in streptozotocin-induced diabetic and matched normoglycemic control mice. Microarray analysis was carried out and chemokine mRNA levels in vivo were assessed. CCL4 were examined in fracture calluses by immunohistochemistry and the role of TNF in diabetes-enhanced expression was investigated by treatment of animals with the TNF-specific inhibitor, pegsunercept. In vitro studies were conducted with ATDC5 chondrocytes. Diabetes significantly upregulated mRNA levels of several chemokines in vivo including CCL4, CCL8, CCL6, CCL11, CCL20, CCL24, CXCL2, CXCL5 and chemokine receptors CCR5 and CXCR4. Chondrocytes were identified as a significant source of CCL4 and its expression in diabetic fractures was dependent on TNF (P<0.05). TNF-α significantly increased mRNA levels of several chemokines in vitro which were knocked down with FOXO1 siRNA (P<0.05). CCL4 expression at the mRNA and proteins levels was induced by FOXO1 over-expression and reduced by FOXO1 knockdown. The current studies point to the importance of TNF-α as a mechanism for diabetes enhanced chemokine expression by chondrocytes, which may contribute to the accelerated loss of cartilage observed in diabetic fracture healing. Moreover, in vitro results point to FOXO1 as a potentially important transcription factor in mediating this effect.

Nanofiber-based delivery of bioactive agents and stem cells to bone sites

Biodegradable nanofibers are important scaffolding materials for bone regeneration. In this paper, the basic concepts and recent advances of self-assembly, electrospinning and thermally induced phase separation techniques that are widely used to generate nanofibrous scaffolds are reviewed. In addition, surface functionalization and bioactive factor delivery within these nanofibrous scaffolds to enhance bone regeneration are also discussed. Moreover, recent progresses in applying these nanofiber-based scaffolds to deliver stem cells for bone regeneration are presented. Along with the significant advances, challenges and obstacles in the field as well as the future perspective are discussed.

Vascular tissues are a primary source of BMP2 expression during bone formation induced by distraction osteogenesis

Prior studies showed that bone regeneration during distraction osteogenesis (DO) was dependent on vascular tissue development and that inhibition of VEGFR signaling diminished the expression of BMP2. A combination of micro-computed tomography (μCT) analysis of vascular and skeletal tissues, immunohistological and histological analysis of transgenic mice containing a BAC transgene in which β-galactosidase had been inserted into the coding region of BMP2 and qRT-PCR analysis, was used to examine how the spatial temporal expression of the morphogenetic signals that drive skeletal and vascular tissue development is coordinated during DO. These results showed that BMP2 expression was induced in smooth muscle and vascular endothelial cells of arteries and veins, capillary endothelial cells, hypertrophic chondrocytes and osteocytes. BMP2 was not expressed by lymphatic vessels or macrophages. Separate peaks of BMP2 mRNA expression were induced in the surrounding muscular tissues and the distraction gap and corresponded first with large vessel collateralization and arteriole remodeling followed by periods of angiogenesis in the gap region. Immunohistological and qRT-PCR analysis of VEGF receptors and ligands showed that mesenchymal cells, lining cells and chondrocytes, expressed VEGFA, although PlGF expression was only seen in mesenchymal cells within the gap region. On the other hand VEGFR2 appeared to be predominantly expressed by vascular endothelial and hematopoietic cells. These results suggest that bone and vascular tissue formation is coordinated via a mutually supporting set of paracrine loops in which blood vessels primarily synthesize the morphogens that promote bone formation while mesenchymal cells primarily synthesize the morphogens that promote vascular tissue formation.

Drugs for bone healing

INTRODUCTION

The biological process of fracture healing is complex with influences that are both patient-dependent and related to the trauma experienced and stability of the fracture. Fracture healing complications negatively affect the patient's quality of life, even more when fractures occur in the elderly osteoporotic patients.

AREAS COVERED

In the polytherapy for bone regeneration, a high success rate was obtained with the use of growth factors, osteogenic cells, and osteoconductive factors. There have been high expectations that treatment with drugs active on bone remodeling would be efficient for acceleration of fracture healing. A literature search was undertaken using wording like "drug or pharmacology of fracture healing." This report will review the systemic pharmacological agents for which clinical trials documenting their efficacy on bone healing have been carried out or are underway.

EXPERT OPINION

At present the use of systemic pharmacological agents to enhance fracture healing in the clinical setting is still controversial. However, future clinical trials will offer the possibility to obtain data that will make possible the registration of a drug as a "healer."

Urine matrix metalloproteinases (MMPs) as biomarkers for the progression of fracture healing

Whilst the majority of fractures heal normally, it is estimated that ∼10% of fractures exhibit some level of delayed or impaired healing. Although radiography is the primary diagnostic tool to assess the progression of fracture healing, radiographic features only qualitatively correlate with tissue level increases in mineral content and do not quantitatively measure underlying biological processes that are associated with the progression of healing. Specific metaloproteinases have been shown to be essential to processes of both angiogenesis and mineralised cartilage resorption and bone remodelling at different phases of fracture healing. The aim of this study was to determine the potential of using a simple urine based assay of the activity of two MMPs as a means of assessing the biological progression of fracture healing through the endochondral phase of healing. Using a standard mid-diaphyseal murine model of femoral fracture, MMP9 and MMP13 proteins and enzymatic activity levels were quantified in the urine of mice across the time-course of fracture healing and compared to the mRNA and protein expression profiles in the calluses. Both urinary MMP9 and MMP13 protein and enzymatic activity levels, assessed by Western blot, zymogram and specific MMP fluorometric substrate assays, corresponded to mRNA expression and immunohistologic assays of the proteins within callus tissues. These studies suggest that urinary levels of MMP9 and MMP13 may have potential as metabolic markers to monitor the progression of fracture healing.

Decreased autocrine EGFR signaling in metastatic breast cancer cells inhibits tumor growth in bone and mammary fat pad

Breast cancer metastasis to bone triggers a vicious cycle of tumor growth linked to osteolysis. Breast cancer cells and osteoblasts express the epidermal growth factor receptor (EGFR) and produce ErbB family ligands, suggesting participation of these growth factors in autocrine and paracrine signaling within the bone microenvironment. EGFR ligand expression was profiled in the bone metastatic MDA-MB-231 cells (MDA-231), and agonist-induced signaling was examined in both breast cancer and osteoblast-like cells. Both paracrine and autocrine EGFR signaling were inhibited with a neutralizing amphiregulin antibody, PAR34, whereas shRNA to the EGFR was used to specifically block autocrine signaling in MDA-231 cells. The impact of these was evaluated with proliferation, migration and gene expression assays. Breast cancer metastasis to bone was modeled in female athymic nude mice with intratibial inoculation of MDA-231 cells, and cancer cell-bone marrow co-cultures. EGFR knockdown, but not PAR34 treatment, decreased osteoclasts formed in vitro (p<0.01), reduced osteolytic lesion tumor volume (p<0.01), increased survivorship in vivo (p<0.001), and resulted in decreased MDA-231 growth in the fat pad (p<0.01). Fat pad shEGFR-MDA-231 tumors produced in nude mice had increased necrotic areas and decreased CD31-positive vasculature. shEGFR-MDA-231 cells also produced decreased levels of the proangiogenic molecules macrophage colony stimulating factor-1 (MCSF-1) and matrix metalloproteinase 9 (MMP9), both of which were decreased by EGFR inhibitors in a panel of EGFR-positive breast cancer cells. Thus, inhibiting autocrine EGFR signaling in breast cancer cells may provide a means for reducing paracrine factor production that facilitates microenvironment support in the bone and mammary gland.

MC3T3-E1 osteoprogenitor cells systemically migrate to a bone defect and enhance bone healing

Although iliac crest autologous bone graft remains the gold standard for treatment of bone defects, delayed- and nonunions, and arthrodeses, several alternative strategies have been attempted, including the use of mesenchymal stem cells. Whether cells from the osteoblast lineage demonstrate systemic recruitment to an acute bone defect or fracture, and whether these cells directly participate in bone healing is controversial. This study tests two hypotheses: (1) that exogenous murine MC3T3-E1 osteoprogenitor cells with a high propensity for osteoblast differentiation are able to systemically migrate to a bone defect and (2) that the migrated MC3T3-E1 cells enhance bone healing. Two groups of nude mice were used; a bone defect was drilled in the left femoral shaft in both groups. MC3T3-E1 were used as reporter cells and injected in the left ventricle of the heart, to avoid sequestration in the lungs. Injection of saline served as a control. We used bioluminescence and microCT to assay cell recruitment and bone mineral density (BMD). Immunohistochemical staining was used to confirm the migration of reporter cells. MC3T3-E1 cells were found to systemically migrate to the bone defect. Further, BMD at the defect was significantly increased when cells were injected. Systemic cell therapy using osteoprogenitor cells may be a potential strategy to enhance bone healing.

Employing the biology of successful fracture repair to heal critical size bone defects

Bone has the natural ability to remodel and repair. Fractures and small noncritical size bone defects undergo regenerative healing via coordinated concurrent development of skeletal and vascular elements in a soft cartilage callus environment. Within this environment bone regeneration recapitulates many of the same cellular and molecular mechanisms that form embryonic bone. Angiogenesis is intimately involved with embryonic bone formation and with both endochondral and intramembranous bone formation in differentiated bone. During bone regeneration osteogenic cells are first associated with vascular tissue in the adjacent periosteal space or the adjacent injured marrow cavity that houses endosteal blood vessels. Critical size bone defects cannot heal without the assistance of therapeutic aids or materials designed to encourage bone regeneration. We discuss the prospects for using synthetic hydrogels in a bioengineering approach to repair critical size bone defects. Hydrogel scaffolds can be designed and fabricated to potentially trigger the same bone morphogenetic cascade that heals bone fractures and noncritical size defects naturally. Lastly, we introduce adult Xenopus laevis hind limb as a novel small animal model system for bone regeneration research. Xenopus hind limbs have been used successfully to screen promising scaffolds designed to heal critical size bone defects.

The transcriptome of fracture healing defines mechanisms of coordination of skeletal and vascular development during endochondral bone formation

Fractures initiate one round of endochondral bone formation in which callus cells differentiate in a synchronous manner that temporally phenocopies the spatial variation of endochondral development of a growth plate. During fracture healing C57BL/6J (B6) mice initiate chondrogenesis earlier and develop more cartilage than bone, whereas C3H/HeJ (C3H) mice initiate osteogenesis earlier and develop more bone than cartilage. Comparison of the transcriptomes of fracture healing in these strains of mice identified the genes that showed differences in timing and quantitative expression and encode for the variations in endochondral bone development of the two mouse strains. The complement of strain-dependent differences in gene expression was specifically associated with ontologies related to both skeletal and vascular formation. Moreover, the differences in gene expression associated with vascular tissue formation during fracture healing were correlated with the underlying differences in development and function of the cardiovascular systems of these two strains of mice. Significant differences in gene expression associated with bone morphogenetic protein/transforming growth factor β (BMP/TGF-β) signal-transduction pathways were identified between the two strains, and a network of differentially expressed genes specific to the MAP kinase cascade was further defined as a subset of the genes of the BMP/TGF-β pathways. Other signal-transduction pathways that showed significant strain-specific differences in gene expression included the RXR/PPAR and G protein-related pathways. These data identify how bone and vascular regeneration are coordinated through expression of common sets of transcription and morphogenetic factors and suggest that there is heritable linkage between vascular and skeletal tissue development during postnatal regeneration.

MRT letter: Contrast-enhanced computed tomographic imaging of soft callus formation in fracture healing

Formation of a cartilaginous soft callus at the site of a bone fracture is a pivotal stage in the healing process. Noninvasive, or even nondestructive, imaging of soft callus formation can be an important tool in experimental and pre-clinical studies of fracture repair. However, the low X-ray attenuation of cartilage renders the soft callus nearly invisible in radiographs. This study utilized a recently developed, cationic, iodinated contrast agent in conjunction with micro-computed tomography to identify cartilage in fracture calluses in the femora of C57BL/6J and C3H/HeJ mice. Fracture calluses were scanned before and after incubation in the contrast agent. The set of pre-incubation images was registered against and then subtracted from the set of post-incubation images, resulting in a three-dimensional map of the locations of cartilage in the callus, as labeled by the contrast agent. This map was then compared to histology from a previous study. The results showed that the locations where the contrast agent collected in relatively high concentrations were similar to those of the cartilage. The contrast agent also identified a significant difference between the two strains of mice in the percentage of the callus occupied by cartilage, indicating that this method of contrast-enhanced computed tomography may be an effective technique for nondestructive, early evaluation of fracture healing.

Three-dimensional reconstructions of intrahepatic bile duct tubulogenesis in human liver

Background

During liver development, intrahepatic bile ducts are thought to arise by a unique asymmetric mode of cholangiocyte tubulogenesis characterized by a series of remodeling stages. Moreover, in liver diseases, cells lining the Canals of Hering can proliferate and generate new hepatic tissue. The aim of this study was to develop protocols for three-dimensional visualization of protein expression, hepatic portal structures and human hepatic cholangiocyte tubulogenesis.

Results

Protocols were developed to digitally visualize portal vessel branching and protein expression of hepatic cell lineage and extracellular matrix deposition markers in three dimensions. Samples from human prenatal livers ranging from 7 weeks + 2 days to 15½ weeks post conception as well as adult normal and acetaminophen intoxicated liver were used. The markers included cytokeratins (CK) 7 and 19, the epithelial cell adhesion molecule (EpCAM), hepatocyte paraffin 1 (HepPar1), sex determining region Y (SRY)-box 9 (SOX9), laminin, nestin, and aquaporin 1 (AQP1).

Digital three-dimensional reconstructions using CK19 as a single marker protein disclosed a fine network of CK19 positive cells in the biliary tree in normal liver and in the extensive ductular reactions originating from intrahepatic bile ducts and branching into the parenchyma of the acetaminophen intoxicated liver. In the developing human liver, three-dimensional reconstructions using multiple marker proteins confirmed that the human intrahepatic biliary tree forms through several developmental stages involving an initial transition of primitive hepatocytes into cholangiocytes shaping the ductal plate followed by a process of maturation and remodeling where the intrahepatic biliary tree develops through an asymmetrical form of cholangiocyte tubulogenesis.

Conclusions

The developed protocols provide a novel and sophisticated three-dimensional visualization of vessels and protein expression in human liver during development and disease.

The biology of fracture healing

The biology of fracture healing is a complex biological process that follows specific regenerative patterns and involves changes in the expression of several thousand genes. Although there is still much to be learned to fully comprehend the pathways of bone regeneration, the over-all pathways of both the anatomical and biochemical events have been thoroughly investigated. These efforts have provided a general understanding of how fracture healing occurs. Following the initial trauma, bone heals by either direct intramembranous or indirect fracture healing, which consists of both intramembranous and endochondral bone formation. The most common pathway is indirect healing, since direct bone healing requires an anatomical reduction and rigidly stable conditions, commonly only obtained by open reduction and internal fixation. However, when such conditions are achieved, the direct healing cascade allows the bone structure to immediately regenerate anatomical lamellar bone and the Haversian systems without any remodelling steps necessary. In all other non-stable conditions, bone healing follows a specific biological pathway. It involves an acute inflammatory response including the production and release of several important molecules, and the recruitment of mesenchymal stem cells in order to generate a primary cartilaginous callus. This primary callus later undergoes revascularisation and calcification, and is finally remodelled to fully restore a normal bone structure. In this article we summarise the basic biology of fracture healing.

Bone fracture and bone fracture repair

Fracture healing is a multistage repair process that involves complex, well-orchestrated steps initiated in response to tissue injury. The early upregulation of IL-6, osteoprotegerin (OPG), VEGF, and BMPs indicates a central role for these factors in the initiation of cartilage and periosteal woven bone formation. In both callus fracture repair and stress fracture repair, the RANKL/OPG ratio is initially reduced, but peaks earlier in stress fracture healing than callus fracture healing. Though the understanding of the biological processes and molecular signals that coordinate fracture repair has advanced, the cause of variability observed in fracture repair is poorly understood.

The spatio-temporal arrangement of different tissues during bone healing as a result of simple mechanobiological rules

During secondary bone healing, different tissue types are formed within the fracture callus depending on the local mechanical and biological environment. Our aim was to understand the temporal succession of these tissue patterns for a normal bone healing progression by means of a basic mechanobiological model. The experimental data stemmed from an extensive, previously published animal experiment on sheep with a 3 mm tibial osteotomy. Using recent experimental data, the development of the hard callus was modelled as a porous material with increasing stiffness and decreasing porosity. A basic phenomenological model was employed with a small number of simulation parameters, which allowed comprehensive parameter studies. The model distinguished between the formation of new bone via endochondral and intramembranous ossification. To evaluate the outcome of the computer simulations, the tissue images of the simulations were compared with experimentally derived tissue images for a normal healing progression in sheep. Parameter studies of the threshold values for the regulation of tissue formation were performed, and the source of the biological stimulation (comprising e.g. stem cells) was varied. It was found that the formation of the hard callus could be reproduced in silico for a wide range of threshold values. However, the bridging of the fracture gap by cartilage on the periosteal side was observed only (i) for a rather specific choice of the threshold values for tissue differentiation and (ii) when assuming a strong source of biological stimulation at the periosteum.

Mechanobiology of bone healing and regeneration: in vivo models

Mechanical boundary conditions are well known to influence the regeneration of bone and mechanobiology is the study of how mechanical or physical stimuli regulate biological processes. In vivo models have been applied over many years to investigate the effects of mechanics on bone healing. Early models have focused on the influence of mechanical stability on healing outcome, with an interest in parameters such as the magnitude of interfragmentary movement, the rate and timing of application of micromotion and the number of loading cycles. As measurement techniques have been refined, there has been a shift in orders of magnitude from investigations targeted at the organ level to those targeted at the tissue level and beyond. An understanding of how mechanics influences tissue differentiation during repair and regeneration crucially requires spatial and temporal knowledge of both the local mechanical environment in the healing tissue and a characterization of the tissues formed over the course of regeneration. Owing to limitations in the techniques available to measure the local mechanical conditions during repair directly, simulation approaches, such as the finite element method, are an integral part of the mechanobiologist's toolkit, while histology remains the gold standard in the characterization of the tissue formed. However, with rapid advances occurring in imaging modalities and methods to characterize tissue properties, new opportunities exist to better understand the role of mechanics in the biology of bone regeneration. Combined with developments in molecular biology, mechanobiology has the potential to offer exciting, new regenerative treatments for bone healing.

NELL1 promotes high-quality bone regeneration in rat femoral distraction osteogenesis model

NELL1 (NEL-like molecule-1; NEL [a protein strongly expressed in neural tissue encoding epidermal growth factor like domain]) is a cranisynostosis-associated molecule directly regulated by Runx2, the master molecule in controlling osteoblastic differentiation. NELL1 has exhibited potent osteoinductive activity for bone regeneration in several animal models. However, its capacity for promoting repair of long-bone defects remains unknown. In this study, we investigated the osteogenic effects of NELL1 on femoral distraction osteogenesis using adenoviral gene delivery and multiple approaches of in vivo analysis. Thirty Sprague-Dawley (SD) rats were randomly assigned to 3 groups for treatment (n=10 each): adenovirus-green fluorescent protein (Ad-GFP)-NELL1 or Ad-GFP at 1×10⁹ plaque-forming units/ml diluted in saline, or saline alone. The femoral distraction was at a speed of 0.25 mm every 12h for 14 days, and a single injection of Ad-GFP-NELL1 or Ad-GFP was given at the mid-distraction period. The effective NELL1 delivery in vivo after Ad-GFP-NELL1 injection was evaluated by optical imaging. The bone regeneration was assessed quantitatively at days 21, 28, 42, and 56 by live 3-D micro-computed tomography (micro-CT), and animals were sacrificed at day 56 for biomechanical testing and histological analysis. Exogenous NELL1 was expressed in the distracted gap for at least 14 days after Ad-GFP-NELL1 transfection. The bone union rate in the distracted gap was significantly higher with Ad-GFP-NELL1 than with Ad-GFP (9/9 vs. 4/9 rats) or saline alone (5/9 rats) at day 56. The serial 3-D micro-CT images and quantitation obtained with the development and application of radiolucent external fixators showed less callus but more mature cortical bones formed with Ad-GFP-NELL1 than with Ad-GFP transfection and saline administration during distraction osteogenesis. The biomechanical properties of femur samples with Ad-GFP-NELL1 transfection were better than samples with Ad-GFP transfection or saline treatment, and were similar with unoperated femurs. Histology revealed cartilaginous tissues in the middle of distraction gaps with Ad-GFP transfection and saline treatment but only bony bridges with Ad-GFP-NELL1 transfection at the final time point (day 56). Coincidently, the expression of Runx2, BMP2, and BMP7 did not differ among groups at day 56, whereas the expression of osteocalcin and osteopontin was slightly higher with Ad-GFP-NELL1 transfection. Thus, sustained Ad-NELL1 protein delivery into a local area of a rat femoral distraction osteogenesis model remarkably improved regeneration of good-quality bones and accelerated bone union at a high rate. Acquiring serial micro-CT data during rat femoral distraction osteogenesis and regional adenovirus delivery of NELL1 may facilitate future in vivo studies.

Recent advances in the use of serological bone formation markers to monitor callus development and fracture healing

The failure of an osseous fracture to heal, or the development of a nonunion, is common; however, current diagnostic measures lack the capability of early and reliable detection of such events. Analyses of radiographic imaging and clinical examination, in combination, remain the gold standard for diagnosis; however, these methods are not reliable for early detection. Delayed diagnosis of a nonunion is costly from both the patient and treatment standpoints. In response, repeated efforts have been made to identify bone metabolic markers as diagnostic or prognostic tools for monitoring bone healing. Thus far, the evidence regarding a correlation between the kinetics of most bone metabolic markers and nonunion is very limited. With the aim of classifying the role of biological pathways of bone metabolism and of understanding bone conditions in the development of osteoporosis, advances have been made in our knowledge of the molecular basis of bone remodeling, fracture healing, and its failure. Procollagen type I amino-terminal propeptide has been shown to be a reliable bone formation marker in osteoporosis therapy and its kinetics during fracture healing has been recently described. In this article, we suggest that procollagen type I amino-terminal propeptide presents a good opportunity for early detection of nonunion. We also review the role and potential of serum PINP, as well as other markers, as indications of fracture healing.

Lysyl oxidase-like-2 (LOXL2) is a major isoform in chondrocytes and is critically required for differentiation

The lysyl oxidase family is made up of five members: lysyl oxidase (LOX) and lysyl oxidase-like 1-4 (LOXL1-LOXL4). All members share conserved C-terminal catalytic domains that provide for lysyl oxidase or lysyl oxidase-like enzyme activity; and more divergent propeptide regions. LOX family enzyme activities catalyze the final enzymatic conversion required for the formation of normal biosynthetic collagen and elastin cross-links. The importance of lysyl oxidase enzyme activity to normal bone development has long been appreciated, but regulation and roles for specific LOX isoforms in bone formation in vivo is largely unexplored. Fracture healing recapitulates aspects of endochondral bone development. The present study first investigated the expression of all LOX isoforms in fracture healing. A remarkable coincidence of LOXL2 expression with the chondrogenic phase of fracture healing was found, prompting more detailed analyses of LOXL2 expression in normal growth plates, and LOXL2 expression and function in developing ATDC5 chondrogenic cells. Data show that LOXL2 is expressed by pre-hypertrophic and hypertrophic chondrocytes in vivo, and that LOXL2 expression is regulated in vitro as a function of chondrocyte differentiation. Moreover, LOXL2 knockdown studies in vitro show that LOXL2 expression is required for ATDC5 chondrocyte cell line differentiation through regulation of SNAIL and SOX9, important transcription factors that control chondrocyte differentiation. Taken together, data provide evidence that LOXL2, like LOX, is a multifunctional protein. LOXL2 promotes chondrocyte differentiation by mechanisms that are likely to include roles as both a regulator and an effector of chondrocyte differentiation.

Temporal tissue patterns in bone healing of sheep

Secondary fracture healing in long bones leads to the successive formation of intricate patterns of tissues in the newly formed callus. The main aim of this work was to quantitatively describe the topology of these tissue patterns at different stages of the healing process and to generate averaged images of tissue distribution. This averaging procedure was based on stained histological sections (2, 3, 6, and 9 weeks post-operatively) of 64 sheep with a 3 mm tibial mid-shaft osteotomy, stabilized either with a rigid or a semi-rigid external fixator. Before averaging, histological images were sorted for topology according to six identified tissue patterns. The averaged images were obtained for both fixation types and the lateral and medial side separately. For each case, the result of the averaging procedure was a collection of six images characterizing quantitatively the progression of the healing process. In addition, quantified descriptions of the newly formed cartilage and the bone area fractions (BA/TA) of the bony callus are presented. For all cases, a linear increase in the BA/TA of the bony callus was observed. The slope was greatest in the case of the most rigid stabilization and lowest in the case of the least stiff. This topological description of the progression of bone healing will allow quantitative validation (or falsification) of current mechano-biological theories.

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.

Genetic variation in the structural pattern of osteoclast activity during post-natal growth of mouse femora

While the spatial activity of osteoblasts has been associated with modeling of bones during development, few studies have examined if variation in the spatial activity of osteoclasts also contributes to the morphogenesis of skeletal tissues. We examined this question by histomorphometric analysis and reconstructing the three-dimensional spatial distribution of osteoclasts in the femora of three inbred strains of male mice (A/J, C57BL/6J [B6], and C3H/HeJ [C3H]) that have differing skeletal, structural, and material properties. Our data show that total osteoclast surface area and osteoclast numbers are related to the overall bone density, but not related to the development of bone diameter or overall cortical area. The analysis of the spatial distribution of the osteoclasts showed that the asymmetrical mid-diaphyseal distribution of osteoclasts in A/J and B6 compared to the more uniform distribution of these cells around the circumference in the C3H mice was consistent with the more ellipsoid shape of A/J and B6 femora compared to the more circular mid-diaphyseal shape of the femora in the C3H mice. The statistically 2- to 3-fold fewer cells on the periosteal surface in the C3H compared to either the B6 or A/J mice is also consistent with the greater cortical thickness that is seen for the C3H mice compared to either B6 or A/J strains. In vitro studies of osteoclastogenesis and the expression of numerous phenotypic properties of osteoclasts prepared from the three strains of mice showed that A/J and B6 mice developed statistically greater numbers of tartrate resistant acid phosphatase (TRAP) positive cells and expressed statistically higher levels of multiple mRNAs that are unique to differentiated osteoclasts than those isolated from the C3H strain. In summary, the 3D reconstructions and histomorphometric analysis suggest that genetic differences lead to spatial variation in the distribution of osteoclasts. These variations in spatial distribution of osteoclasts in turn contribute in part to the development of the structural variations of the femora that are seen in the three strains of mice. In vitro studies suggest that intrinsic genetic variation in osteoclastogenesis and their phenotypic expression may contribute to the differences in their functional activities that give rise to the unique spatial distributions of these cells in bones.

Locked plating of distal femur fractures leads to inconsistent and asymmetric callus formation

OBJECTIVES

Locked plating constructs may be too stiff to reliably promote secondary bone healing. This study used a novel imaging technique to quantify periosteal callus formation of distal femur fractures stabilized with locking plates. It investigated the effects of cortex-to-plate distance, bridging span, and implant material on periosteal callus formation.

DESIGN

Retrospective cohort study.

SETTING

One Level I and one Level II trauma center.

PATIENTS

Sixty-four consecutive patients with distal femur fractures (AO types 32A, 33A-C) stabilized with periarticular locking plates.

INTERVENTION

Osteosynthesis using indirect reduction and bridge plating with periarticular locking plates.

MAIN OUTCOME MEASUREMENT

Periosteal callus size on lateral and anteroposterior radiographs.

RESULTS

Callus size varied from 0 to 650 mm2. Deficient callus (20 mm2 or less) formed in 52%, 47%, and 37% of fractures at 6, 12, and 24 weeks postsurgery, respectively. Callus formation was asymmetric, whereby the medial cortex had on average 64% more callus (P=0.001) than the anterior or posterior cortices. A longer bridge span correlated minimally with an increased callus size at Week 6 (P=0.02), but no correlation was found at Weeks 12 and 24 postsurgery. Compared with stainless steel plates, titanium plates had 76%, 71%, and 56% more callus at Week 6 (P=0.04), Week 12 (P=0.03), and Week 24 (P=0.09), respectively.

CONCLUSIONS

Stabilization of distal femur fractures with periarticular locking plates can cause inconsistent and asymmetric formation of periosteal callus. A larger bridge span only minimally improves callus formation. The more flexible titanium plates enhanced callus formation compared with stainless steel plates.

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.