Predicting mouse vertebra strength with micro-computed tomography-derived finite element analysis

Radiation-induced changes in load-sharing and structure-function behavior in murine lumbar vertebrae

In this study, we used micro-CT-based finite element analysis to investigate the biomechanical effects of radiation on the microstructure and mechanical function of murine lumbar vertebrae. Specifically, we evaluated vertebral microstructure, whole-bone stiffness, and cortical-trabecular load sharing in the L5 vertebral body of mice exposed to ionizing radiation 11 days post exposure (5 Gy total dose; n = 13) and controls (n = 14). Our findings revealed the irradiated group exhibited reduced trabecular bone volume and microstructure (p < 0.001) compared to controls, while cortical bone volume remained unchanged (p = 0.91). Axially compressive loads in the irradiated group were diverted from the trabecular centrum and into the vertebral cortex, as evidenced by a higher cortical load-fraction (p = 0.02) and a higher proportion of cortical tissue at risk of initial failure (p < 0.01). Whole-bone stiffness was lower in the irradiated group compared to the controls, though the difference was small and non-significant (2045 ± 142 vs. 2185 ± 225 vs. N/mm, irradiated vs. control, p = 0.07). Additionally, the structure-function relationship between trabecular bone volume and trabecular load fraction differed between groups (p = 0.03), indicating a less biomechanically efficient trabecular network in the irradiated group. We conclude that radiation can decrease trabecular bone volume and result in a less biomechanically efficient trabecular structure, leading to increased reliance on the vertebral cortex to resist axially compressive loads. These findings offer biomechanical insight into the effects of radiation on structure-function behavior in murine lumbar vertebrae independent of possible tissue-level material effects.

Long-term whole-body vibration induces degeneration of intervertebral disc and facet joint in a bipedal mouse model

Background: Whole body vibration (WBV) has been used to treat various musculoskeletal diseases in recent years. However, there is limited knowledge about its effects on the lumbar segments in upright posture mice. This study was performed to investigate the effects of axial Whole body vibration on the intervertebral disc (IVD) and facet joint (FJ) in a novel bipedal mouse model.

Methods: Six-week-old male mice were divided into control, bipedal, and bipedal + vibration groups. Taking advantage of the hydrophobia of mice, mice in the bipedal and bipedal + vibration groups were placed in a limited water container and were thus built standing posture for a long time. The standing posture was conducted twice a day for a total of 6 hours per day, 7 days per week. Whole body vibration was conducted during the first stage of bipedal building for 30 min per day (45 Hz with peak acceleration at 0.3 g). The mice of the control group were placed in a water-free container. At the 10th-week after experimentation, intervertebral disc and facet joint were examined by micro-computed tomography (micro-CT), histologic staining, and immunohistochemistry (IHC), and gene expression was quantified using real-time polymerase chain reaction. Further, a finite element (FE) model was built based on the micro-CT, and dynamic Whole body vibration was loaded on the spine model at 10, 20, and 45 Hz.

Results: Following 10 weeks of model building, intervertebral disc showed histological markers of degeneration, such as disorders of annulus fibrosus and increased cell death. Catabolism genes’ expression, such as Mmp13, and Adamts 4/5, were enhanced in the bipedal groups, and Whole body vibration promoted these catabolism genes’ expression. Examination of the facet joint after 10 weeks of bipedal with/without Whole body vibration loading revealed rough surface and hypertrophic changes at the facet joint cartilage resembling osteoarthritis. Moreover, immunohistochemistry results demonstrated that the protein level of hypertrophic markers (Mmp13 and Collagen X) were increased by long-durationstanding posture, and Whole body vibration also accelerated the degenerative changes of facet joint induced by bipedal postures. No changes in the anabolism of intervertebral disc and facet joint were observed in the present study. Furthermore, finite element analysis revealed that a larger frequency of Whole body vibration loading conditions induced higher Von Mises stresses on intervertebral disc, contact force, and displacement on facet joint.

Conclusion: The present study revealed significant damage effects of Whole body vibration on intervertebral disc and facet joint in a bipedal mouse model. These findings suggested the need for further studies of the effects of Whole body vibration on lumbar segments of humans.

Combination treatment with ibandronate and eldecalcitol prevents osteoporotic bone loss and deterioration of bone quality characterized by nano-arrangement of the collagen/apatite in an ovariectomized aged rat model

Combination therapy with bisphosphonates and vitamin D₃ analogs has been frequently used for the treatment of osteoporosis. However, its effects on bone anisotropies, such as orientations of collagen and apatite at the nanometer-scale, which is a promising bone quality index, and its trabecular architecture at the micrometer scale, are not well understood despite its important mechanical properties and its role in fracture risk. In the present study, we analyzed the effects of ibandronate (IBN), eldecalcitol (ELD), and their combination on the collagen/apatite orientation and trabecular architectural anisotropy using an estrogen-deficiency-induced osteoporotic rat model. Estrogen deficiency caused by ovariectomy (OVX) excessively increased the degree of collagen/apatite orientation or trabecular architectural anisotropy along the craniocaudal axis in the lumbar vertebra compared to that of the sham-operated group. The craniocaudal axis corresponds to the direction of principal stress in the spine. The excessive material anisotropy in the craniocaudal axis contributed to the enhanced Young's modulus, which may compensate for the reduced mechanical resistance by bone loss to some extent. The solo administration of IBN and ELD prevented the reduction of bone fraction (BV/TV) determined by μ-CT, and combination therapy showed the highest efficacy in BV/TV gain. Furthermore, the solo administration and combination treatment significantly decreased the degree of collagen/apatite orientation to the sham level. Based on the results of bone mass and collagen/apatite orientation, combination treatment is an effective strategy. This is the first report to demonstrate the efficacy of IBN, ELD, and combination treatment with IBN and ELD relative to the bone micro-architectural anisotropy characterized by collagen/apatite orientation.

Non-invasive prediction of the mouse tibia mechanical properties from microCT images: comparison between different finite element models

New treatments for bone diseases require testing in animal models before clinical translation, and the mouse tibia is among the most common models. In vivo micro-Computed Tomography (microCT)-based micro-Finite Element (microFE) models can be used for predicting the bone strength non-invasively, after proper validation against experimental data. Different modelling techniques can be used to estimate the bone properties, and the accuracy associated with each is unclear. The aim of this study was to evaluate the ability of different microCT-based microFE models to predict the mechanical properties of the mouse tibia under compressive load. Twenty tibiae were microCT scanned at 10.4 µm voxel size and subsequently compressed at 0.03 mm/s until failure. Stiffness and failure load were measured from the load-displacement curves. Different microFE models were generated from each microCT image, with hexahedral or tetrahedral mesh, and homogeneous or heterogeneous material properties. Prediction accuracy was comparable among models. The best correlations between experimental and predicted mechanical properties, as well as lower errors, were obtained for hexahedral models with homogeneous material properties. Experimental stiffness and predicted stiffness were reasonably well correlated (R2 = 0.53-0.65, average error of 13-17%). A lower correlation was found for failure load (R2 = 0.21-0.48, average error of 9-15%). Experimental and predicted mechanical properties normalized by the total bone mass were strongly correlated (R2 = 0.75-0.80 for stiffness, R2 = 0.55-0.81 for failure load). In conclusion, hexahedral models with homogeneous material properties based on in vivo microCT images were shown to best predict the mechanical properties of the mouse tibia.

Abaloparatide treatment increases bone formation, bone density and bone strength without increasing bone resorption in a rat model of hindlimb unloading

Disuse osteoporosis can result from prolonged bed rest, paralysis, casts, braces, fractures and other conditions. Abaloparatide (ABL) is a PTHrP analog that increases bone density and strength by stimulating osteogenesis with limited effects on bone resorption. We examined skeletal responses to abaloparatide in young adult male rats with normal weight-bearing and with hindlimb unloading via a pelvic harness. Rats were allocated to four groups (10-12 per group): normal weight-bearing plus vehicle treatment (CON-VEH), normal weight-bearing plus ABL treatment (CON-ABL), hindlimb-unloading plus vehicle (HLU-VEH), or hindlimb-unloading plus ABL (HLU-ABL). Rats received ABL (25 μg/kg/day, s.c.) or vehicle throughout the 28-day unloading period and were then sacrificed, at which time HLU-VEH rats exhibited reduced bone formation and significant deficits in tibial, femoral, and vertebral bone mass compared with CON-VEH. ABL treatment increased serum osteocalcin in CON and HLU animals while having no effect on the osteoclast marker TRACP-5b. Longitudinal peripheral quantitative computed tomography (pQCT) indicated that ABL increased trabecular and cortical bone mass in the tibia. ABL was also associated with improved trabecular and cortical bone mass and architectural parameters at the femur, tibia, and vertebrae by μCT. Tibial histomorphometry indicated increased trabecular and endocortical bone formation with HLU-ABL versus HLU-VEH and with CON-ABL versus CON-VEH, and ABL was also associated with lower trabecular and endocortical osteoclast surfaces. Vertebral finite element analysis indicated higher ultimate load and stiffness for CON-ABL versus CON-VEH and for HLU-ABL versus HLU-VEH. In summary, ABL was associated with improved trabecular and cortical bone density and architecture in normal weight-bearing and hindlimb-unloaded rats, with higher bone formation and no difference in bone resorption. ABL was also associated with improved bone biomechanical parameters. These results provide rationale for investigating the ability of abaloparatide to prevent or treat disuse osteoporosis in humans.

Optimization of the failure criterion in micro-Finite Element models of the mouse tibia for the non-invasive prediction of its failure load in preclinical applications

New treatments against osteoporosis require testing in animal models and the mouse tibia is among the most common studied anatomical sites. In vivo micro-Computed Tomography (microCT) based micro-Finite Element (microFE) models can be used for predicting the bone strength non-invasively, after proper validation against experiments. The aim of this study was to evaluate the ability of different microCT-based bone parameters and microFE models to predict tibial structural mechanical properties in compression. Twenty tibiae were scanned at 10.4 μm voxel size and subsequently tested in uniaxial compression at 0.03 mm/s until failure. Stiffness and failure load were measured from the load-displacement curves. Standard morphometric parameters were measured from the microCT images. The spatial distribution of bone mineral content (BMC) was evaluated by dividing the tibia into 40 regions. MicroFE models were generated by converting each microCT image into a voxel-based mesh with homogeneous isotropic material properties. Failure load was estimated by using different failure criteria, and the optimized parameters were selected by minimising the errors with respect to experimental measurements. Experimental and predicted stiffness were moderately correlated (R² = 0.65, error = 14% ± 8%). Normalized failure load was best predicted by microFE models (R² = 0.81, error = 9% ± 6%). Failure load was not correlated to the morphometric parameters and weakly correlated with some geometrical parameters (R² < 0.37). In conclusion, microFE models can improve the current estimation of the mouse tibia structural properties and in this study an optimal failure criterion has been defined. Since it is a non-invasive method, this approach can be applied longitudinally for evaluating temporal changes in the bone strength.

The BALB/c mouse as a preclinical model of the age-related deterioration in the lumbar vertebra

The likelihood of experiencing an osteoporotic fracture of one or more vertebral bodies increases with age, and this increase is not solely due to sex steroid deficiency. For the purpose of assessing the effectiveness of novel therapeutic strategies in the prevention of vertebral fractures among the elderly, we hypothesized that the BALB/c mouse model of aging phenocopies the age-related decrease in human VB strength. To test this hypothesis, we assessed the age-related changes in trabecular architecture of the L6 VB, with respect to those in the distal femur metaphysis, between 6-mo. (young adulthood, n = 20/sex) and 20-mo. of age (old age, n = 18/sex) and then determined how well the architectural characteristics, volumetric bone mineral density (vBMD), and predicted failure force from μCT-derived finite element analysis (μFEA) with linear elastic failure criteria explained the age-related variance in VB strength, which was the ultimate force during quasi-static loading of the VB in compression. While there was a pronounced age-related deterioration in trabecular architecture in the distal femur metaphysis of female and male BALB/c mice, the decrease in trabecular bone volume fraction and trabecular number between 6-mo. and 20-mo. of age occurred in male mice, but not in female mice. As such, the VB strength was lower with age in males only. Nonetheless, BV/TV and volumetric bone mineral density (vBMD) positively correlated with the ultimate compressive force of the L6 VB for both females and males. Whether using a fixed homogeneous distribution of tissue modulus (E_t = 18 GPa) or a heterogeneous distribution of E_t based on a positive relationship with TMD, the predicted failure force of the VB was not independent of age, thereby suggesting linear μFEA may not be a suitable replacement for mechanical-based measurements of strength with respect to age-related changes. Overall, the BALB/c mouse model of aging mimics the age-related in decline in human VB strength when comparing 6-mo. and 20-mo. old male mice. The decrease in VB strength in female mice may occur over a different age range.

In silico clinical trials: concepts and early adoptions

Innovations in information and communication technology infuse all branches of science, including life sciences. Nevertheless, healthcare is historically slow in adopting technological innovation, compared with other industrial sectors. In recent years, new approaches in modelling and simulation have started to provide important insights in biomedicine, opening the way for their potential use in the reduction, refinement and partial substitution of both animal and human experimentation. In light of this evidence, the European Parliament and the United States Congress made similar recommendations to their respective regulators to allow wider use of modelling and simulation within the regulatory process. In the context of in silico medicine, the term 'in silico clinical trials' refers to the development of patient-specific models to form virtual cohorts for testing the safety and/or efficacy of new drugs and of new medical devices. Moreover, it could be envisaged that a virtual set of patients could complement a clinical trial (reducing the number of enrolled patients and improving statistical significance), and/or advise clinical decisions. This article will review the current state of in silico clinical trials and outline directions for a full-scale adoption of patient-specific modelling and simulation in the regulatory evaluation of biomedical products. In particular, we will focus on the development of vaccine therapies, which represents, in our opinion, an ideal target for this innovative approach.

Finite element analysis of bone strength in osteogenesis imperfecta

As a dedicated experimentalist, John Currey praised the high potential of finite element (FE) analysis but also recognized its critical limitations. The application of the FE methodology to bone tissue is reviewed in the light of his enthusiastic and colorful statements. In the past decades, FE analysis contributed substantially to the understanding of structure-function properties in the hierarchical organization of bone and to the simulation of bone adaptation. The systematic experimental validation of FE analysis of bone strength in anatomical locations at risk of fracture led to its application in clinical studies to evaluate efficacy of antiresorptive or anabolic treatment of bone fragility. Beyond the successful analyses of healthy or osteoporotic bone, FE analysis becomes increasingly involved in the investigation of other fragility-related bone diseases. The case of osteogenesis imperfecta (OI) is exposed, the multiscale alterations of the bone tissue and the effect of treatment summarized. A few FE analyses attempting to answer open questions in OI are then reported. An original study is finally presented that explored the structural properties of the Brtl/+ murine model of OI type IV subjected to sclerostin neutralizing antibody treatment using microFE analysis. The use of identical material properties in the four-point bending FE simulations of the femora reproduced not only the experimental values but also the statistical comparisons examining the effect of disease and treatment. Further efforts are needed to build upon the extraordinary legacy of John Currey and clarify the impact of different bone diseases on the hierarchical mechanical properties of bone.

A novel technique with reduced computed tomography exposure to predict vertebral compression fracture: a finite element study based on rat vertebrae

Vertebral compression fractures are a significant clinical issue with an annual incidence of approximately 750,000 cases in the USA alone. Mechanical properties of vertebrae are successfully evaluated through finite element (FE) models based on vertebrae CT. However, clinical drawbacks associated to radiation transmission encouraged to explore the possibility to use selected or reduced portions of the vertebra. The objective of our study was to develop a new procedure to predict vertebral compression fracture from sub-volumes. We reconstructed rat vertebras from micro-CT of thoracic and lumbar groups. Each vertebra was partitioned into three sub-volumes of different axial thickness. FE simulating compression tests were performed on each model to evaluate their failure load and stiffness. Using a power function, a high correlation was found for stiffness and strength. The sub-volume with three fifths thickness had a failure load of 180.7 ± 19.2 N for thoracic and of 209.5 ± 27.4 N for the lumbar vertebra. These values were not significantly different from the values found for the entire vertebra (p > 0.05). Based on our findings, failure loads and stiffnesses obtained with reduced CT scans can be successfully used to predict full vertebral failure. This sub-region analysis and power relationship suggests that one can limit radiation exposure to patients when bone characterization is needed. Graphical abstract Estimated mechanical properties in relation to the extent of the computed tomography reconstruction.

Ciliary parathyroid hormone signaling activates transforming growth factor-β to maintain intervertebral disc homeostasis during aging

Degenerative disc disease (DDD) is associated with intervertebral disc degeneration of spinal instability. Here, we report that the cilia of nucleus pulposus (NP) cells mediate mechanotransduction to maintain anabolic activity in the discs. We found that mechanical stress promotes transport of parathyroid hormone 1 receptor (PTH1R) to the cilia and enhances parathyroid hormone (PTH) signaling in NP cells. PTH induces transcription of integrin α_vβ₆ to activate the transforming growth factor (TGF)-β-connective tissue growth factor (CCN2)-matrix proteins signaling cascade. Intermittent injection of PTH (iPTH) effectively attenuates disc degeneration of aged mice by direct signaling through NP cells, specifically improving intervertebral disc height and volume by increasing levels of TGF-β activity, CCN2, and aggrecan. PTH1R is expressed in both mouse and human NP cells. Importantly, knockout PTH1R or cilia in the NP cells results in significant disc degeneration and blunts the effect of PTH on attenuation of aged discs. Thus, mechanical stress-induced transport of PTH1R to the cilia enhances PTH signaling, which helps maintain intervertebral disc homeostasis, particularly during aging, indicating therapeutic potential of iPTH for DDD.

Sensory structures found in the jelly-like space between spinal discs release a hormone that helps preserve back health in aging mice. Xu Cao from Johns Hopkins University in Baltimore, Maryland, USA, and colleagues observed that levels of a critical growth factor declined in the space between adjacent vertebrae as mice aged, and that injecting a naturally occurring hormone that activates this growth factor could attenuate disc degeneration in older animals. The researchers showed, in response to mechanical stresses, receptor proteins that respond to this hormone relocate themselves to particular sensory organelles known as cilia that found within cells of the intervertebral core. That results in elevated hormone signaling—and drugs designed to have the same effect could help treat degenerative disc disease, one of the most common causes of chronic back pain.

Bone Toolbox: Biomarkers, Imaging Tools, Biomechanics, and Histomorphometry

Bone is a unique tissue with turnover, metabolic, and cellular activities that vary through development to aging and with a mineralized matrix in which the current state and the history of a bone coexist. Qualitative histopathology often lacks sensitivity to detect changes in bone formation, mineralization and resorption, which often requires chronic dosing to result in structural changes such as variation in bone mass and geometry. A large panel of modalities can be used to fully analyze the health of the skeleton, including biomarker evaluation in serum or urine, imaging techniques ranging from radiology to computed tomography, biomechanical testing, and undecalcified tissue processing with bone histomorphometry. The use of clinically relevant biomarkers provides an important noninvasive, sensitive, rapid, and real-time tool to monitor bone activity at the whole skeleton level when conducting safety assessments in a preclinical setting. Imaging modalities also allow in vivo longitudinal assessments with a powerful, noninvasive and clinically translatable tools to monitor drug effects. Different imaging modalities are used in the preclinical studies to evaluate the bone tissues: standard radiography, dual-energy X-ray absorptiometry, peripheral quantitative computed tomography (pQCT), micro-computed tomography, and high-resolution pQCT. Bone histomorphometry is an important tool that provides sensitive evaluation to detect effects of test articles on bone resorption, formation, mineralization, remodeling rates and growth to address a potential target- or class-related theoretical bone liability. Ultimately, the measurement of bone mechanical properties in pharmaceutical testing is critical to understand the potential effects of that pharmaceutical on bone health and fracture risk. Important considerations are required for including these different techniques in toxicology rodents and nonrodent studies, to actually integrate these into safety assessment.

In Vivo 3D Histomorphometry Quantifies Bone Apposition and Skeletal Progenitor Cell Differentiation

Histomorphometry and Micro-CT are commonly used to assess bone remodeling and bone microarchitecture. These approaches typically require separate cohorts of animals to analyze 3D morphological changes and involve time-consuming immunohistochemistry preparation. Intravital Microscopy (IVM) in combination with mouse genetics may represent an attractive option to obtain bone architectural measurements while performing longitudinal monitoring of dynamic cellular processes in vivo. In this study we utilized two-photon, multicolor fluorescence IVM together with a lineage tracing reporter mouse model to image skeletal stem cells (SSCs) in their calvarial suture niche and analyze their differentiation fate after stimulation with an agonist of the canonical Wnt pathway (recombinant Wnt3a). Our in vivo histomorphometry analyses of bone formation, suture volume, and cellular dynamics showed that recombinant Wnt3a induces new bone formation, differentiation and incorporation of SSCs progeny into newly forming bone. IVM technology can therefore provide additional dynamic 3D information to the traditional static 2D histomorphometry.

Combined treatment with a transforming growth factor beta inhibitor (1D11) and bortezomib improves bone architecture in a mouse model of myeloma-induced bone disease

Multiple myeloma (MM) patients frequently develop tumor-induced bone destruction, yet no therapy completely eliminates the tumor or fully reverses bone loss. Transforming growth factor-β (TGF-β) activity often contributes to tumor-induced bone disease, and pre-clinical studies have indicated that TGF-β inhibition improves bone volume and reduces tumor growth in bone metastatic breast cancer. We hypothesized that inhibition of TGF-β signaling also reduces tumor growth, increases bone volume, and improves vertebral body strength in MM-bearing mice. We treated myeloma tumor-bearing (immunocompetent KaLwRij and immunocompromised Rag2-/-) mice with a TGF-β inhibitory (1D11) or control (13C4) antibody, with or without the anti-myeloma drug bortezomib, for 4weeks after inoculation of murine 5TGM1 MM cells. TGF-β inhibition increased trabecular bone volume, improved trabecular architecture, increased tissue mineral density of the trabeculae as assessed by ex vivo micro-computed tomography, and was associated with significantly greater vertebral body strength in biomechanical compression tests. Serum monoclonal paraprotein titers and spleen weights showed that 1D11 monotherapy did not reduce overall MM tumor burden. Combination therapy with 1D11 and bortezomib increased vertebral body strength, reduced tumor burden, and reduced cortical lesions in the femoral metaphysis, although it did not significantly improve cortical bone strength in three-point bending tests of the mid-shaft femur. Overall, our data provides rationale for evaluating inhibition of TGF-β signaling in combination with existing anti-myeloma agents as a potential therapeutic strategy to improve outcomes in patients with myeloma bone disease.

Age-related changes in the fracture resistance of male Fischer F344 rat bone

In addition to the loss in bone volume that occurs with age, there is a decline in material properties. To test new therapies or diagnostic tools that target such properties as material strength and toughness, a pre-clinical model of aging would be useful in which changes in bone are similar to those that occur with aging in humans. Toward that end, we hypothesized that similar to human bone, the estimated toughness and material strength of cortical bone at the apparent-level decreases with age in the male Fischer F344 rat. In addition, we tested whether the known decline in trabecular architecture in rats translated to an age-related decrease in vertebra (VB) strength and whether non-X-ray techniques could quantify tissue changes at micron and sub-micron length scales. Bones were harvested from 6-, 12-, and 24-month (mo.) old rats (n=12 per age). Despite a loss in trabecular bone with age, VB compressive strength was similar among the age groups. Similarly, whole-bone strength (peak force) in bending was maintained (femur) or increased (radius) with aging. There was though an age-related decrease in post-yield toughness (radius) and bending strength (femur). The ability to resist crack initiation was actually higher for the 12-mo. and 24-mo. than for 6-mo. rats (notch femur), but the estimated work to propagate the crack was less for the aged bone. For the femur diaphysis region, porosity increased while bound water decreased with age. For the radius diaphysis, there was an age-related increase in non-enzymatic and mature enzymatic collagen crosslinks. Raman spectroscopy analysis of embedded cross-sections of the tibia mid-shaft detected an increase in carbonate subsitution with advanced aging for both inner and outer tissue.

Fibrinolysis is essential for fracture repair and prevention of heterotopic ossification

Bone formation during fracture repair inevitably initiates within or around extravascular deposits of a fibrin-rich matrix. In addition to a central role in hemostasis, fibrin is thought to enhance bone repair by supporting inflammatory and mesenchymal progenitor egress into the zone of injury. However, given that a failure of efficient fibrin clearance can impede normal wound repair, the precise contribution of fibrin to bone fracture repair, whether supportive or detrimental, is unknown. Here, we employed mice with genetically and pharmacologically imposed deficits in the fibrin precursor fibrinogen and fibrin-degrading plasminogen to explore the hypothesis that fibrin is vital to the initiation of fracture repair, but impaired fibrin clearance results in derangements in bone fracture repair. In contrast to our hypothesis, fibrin was entirely dispensable for long-bone fracture repair, as healing fractures in fibrinogen-deficient mice were indistinguishable from those in control animals. However, failure to clear fibrin from the fracture site in plasminogen-deficient mice severely impaired fracture vascularization, precluded bone union, and resulted in robust heterotopic ossification. Pharmacological fibrinogen depletion in plasminogen-deficient animals restored a normal pattern of fracture repair and substantially limited heterotopic ossification. Fibrin is therefore not essential for fracture repair, but inefficient fibrinolysis decreases endochondral angiogenesis and ossification, thereby inhibiting fracture repair.