Quantification of skeletal blood flow and fluoride metabolism in rats using PET in a pre-clinical stress fracture model

Patients with unilateral patellofemoral pain have altered bone turnover in the painful knee compared to the pain-free knee at rest and after acute knee loading

Objective

The objective of this study was to investigate subchondral bone turnover at rest and after acute loading using Fluorine-18-labeled sodium fluoride (Na[18F]F) Positron Emission Tomography (PET), in patients with unilateral PFP.

Design

Twenty-seven patients with unilateral PFP were recruited from the Institute of Sports Medicine Copenhagen. Participants underwent Na[18F]F-PET imaging before and after a bout of single-leg squats. Bone turnover measures, including mean and maximal standardized uptake value (SUVmean and SUVmax), rate of bone perfusion (K1), rate of tracer uptake into bone (Ki), and extraction fraction of tracer absorbed into bone mineral were assessed for patella and trochlea.

Results

At rest, the painful knees showed lower SUVmax, K1, and Ki compared to the pain-free knees in the superficial part of the patella. No significant differences were found in the profound part of the patella or trochlea at rest. Following knee loading, the acute increases in SUVmean, SUVmax, Ki and blood flow were reduced in the superficial patella of the painful knees compared to the pain-free knees. In the trochlea, painful knees showed larger increases in SUVmean and Ki in the lateral part, whereas the medial part showed greater increases in K1, Ki, and a larger decrease in extraction fraction after loading.

Conclusion

Patella displayed decreased bone metabolism at rest and reduced response to loading in the painful versus pain-free knees. Trochlea in the painful knees showed significantly larger increases in subchondral bone metabolism following knee loading compared to the pain-free knees. These novel findings highlight potential differences in bone turnover between the patellar and trochlear regions.

Biologic therapies in stress fractures: Current concepts

Stress fractures, a common overuse injury in physically active individuals, present a significant challenge for athletes and military personnel. Patients who sustain stress fractures have demanding training regimes where periods of rest and immobilisation have unacceptable negative consequences on sports goals and finances. Aside from being an overuse injury, there are various contributing risk factors that put certain individuals at risk of a stress fracture. The main two being nutritional deficiencies and hormonal variations, which have significant effects on bone metabolism and turnover. Historically, treatment of stress fractures focused on conservative strategies such as rest and immobilisation. Calcium and vitamin D deficiencies have been closely linked to stress fractures and so over time supplementation has also played a role in treatment. With the introduction of biologics into orthopaedics, newer treatment strategies have been applied to accelerate fracture healing and perhaps improve fracture callus quality. If such therapies can reduce time spent away from sport and activity, it would be ideal for treating stress fractures. This article aims to offer insights into the evolving landscape of stress fracture management. It investigates the pre-clinical evidence and available published clinical applications. Though fracture healing is well understood, the role of biologics for fracture healing is still indeterminate. Available literature for the use of biologic therapies in stress fractures are restricted and most reports have used biologics as a supplement to surgical fixation in subjects in studies that lack control groups. Randomised control trials have been proposed and registered by a few groups, with results awaited. Assessing individuals for risk factors, addressing hormonal imbalances and nutritional deficiencies seems like an effective approach to addressing the burden of stress fractures. We await better designed trials and studies to accurately determine the clinical benefit of adding biologics to the management of these injuries.

Imaging of joint response to exercise with MRI and PET

Imaging of the joint in response to loading stress may provide additional measures of joint structure and function beyond conventional, static imaging studies. Exercise such as running, stair climbing, and squatting allows evaluation of the joint response to larger loading forces than during weight bearing. Quantitative MRI (qMRI) may assess properties of cartilage and meniscus hydration and organization in vivo that have been investigated to assess the functional response of these tissues to physiological stress. [18F]sodium fluoride ([18F]NaF) interrogates areas of newly mineralizing bone and provides an opportunity to study bone physiology, including perfusion and mineralization rate, as a measure of joint loading stress. In this review article, methods utilizing quantitative MRI, PET, and hybrid PET-MRI systems for assessment of the joint response to loading from exercise in vivo are examined. Both methodology and results of various studies performed are outlined and discussed. Lastly, the technical considerations, challenges, and future opportunities for these approaches are addressed.

[18F]Sodium fluoride PET-MRI detects increased metabolic bone response to whole-joint loading stress in osteoarthritic knees

OBJECTIVE

Altered joint function is a hallmark of osteoarthritis (OA). Imaging techniques for joint function are limited, but [18F]sodium fluoride (NaF) PET-MRI may assess the acute joint response to loading stresses. [18F]NaF PET-MRI was used to study the acute joint response to exercise in OA knees, and compare relationships between regions of increased uptake after loading and structural OA progression two years later.

METHODS

In this prospective study, 10 participants with knee OA (59 ± 8 years; 8 female) were scanned twice consecutively using a PET-MR system and performed a one-legged squat exercise between scans. Changes in tracer uptake measures in 9 bone regions were compared between knees that did and did not exercise with a mixed-effects model. Areas of focally large changes in uptake between scans (ROIfocal, ΔSUVmax > 3) were identified and the presence of structural MRI features was noted. Five participants returned two years later to assess structural change on MRI.

RESULTS

There was a significant increase in [18F]NaF uptake in OA exercised knees (SUV P < 0.001, KiP = 0.002, K1P < 0.001) that differed by bone region.

CONCLUSION

There were regional differences in the acute bone metabolic response to exercise and areas of focally large changes in the metabolic bone response that might be representative of whole-joint dysfunction.

Similarities Between Disuse and Age-Induced Bone Loss

Disuse and aging are known risk factors associated with low bone mass and quality deterioration, resulting in increased fracture risk. Indeed, current and emerging evidence implicate a large number of shared skeletal manifestations between disuse and aging scenarios. This review provides a detailed overview of current preclinical models of musculoskeletal disuse and the clinical scenarios they seek to recapitulate. We also explore and summarize the major similarities between bone loss after extreme disuse and advanced aging at multiple length scales, including at the organ/tissue, cellular, and molecular level. Specifically, shared structural and material alterations of bone loss are presented between disuse and aging, including preferential loss of bone at cancellous sites, cortical thinning, and loss of bone strength due to enhanced fragility. At the cellular level bone loss is accompanied, during disuse and aging, by increased bone resorption, decreased formation, and enhanced adipogenesis due to altered gap junction intercellular communication, WNT/β-catenin and RANKL/OPG signaling. Major differences between extreme short-term disuse and aging are discussed, including anatomical specificity, differences in bone turnover rates, periosteal modeling, and the influence of subject sex and genetic variability. The examination also identifies potential shared mechanisms underlying bone loss in aging and disuse that warrant further study such as collagen cross-linking, advanced glycation end products/receptor for advanced glycation end products (AGE-RAGE) signaling, reactive oxygen species (ROS) and nuclear factor κB (NF-κB) signaling, cellular senescence, and altered lacunar-canalicular connectivity (mechanosensation). Understanding the shared structural alterations, changes in bone cell function, and molecular mechanisms common to both extreme disuse and aging are paramount to discovering therapies to combat both age-related and disuse-induced osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Minimally invasive laser Doppler flowmetry is suitable for serial bone perfusion measurements in mice

In vivo laser Doppler flowmetry (LDF) has previously been used to quantify blood perfusion accurately at a single timepoint in the murine tibial metaphysis. However, this procedure entailed substantial disruption to soft tissues overlying the bone and caused notable localized inflammation for several weeks after the procedure, impeding serial measurements in the same mouse. In this study, we tested a less invasive technique to measure perfusion in the tibia with LDF and determined that it can be used serially in the same mouse without causing signs of inflammation or gait perturbations. Twenty 14-week-old C57Bl/6J mice were evenly divided into groups that either had daily treadmill exercise or remained sedentary. Within these activity groups, mice were evenly subdivided into groups that received LDF measurements either weekly or only once at the study endpoint. Bone perfusion was measured with LDF in the anteromedial region of the right tibial metaphysis. Serum concentrations of interleukin 6, incision site wound area, and interlimb coordination during gait were measured weekly for four weeks. Tibial perfusion did not differ significantly between exercise and sedentary groups within the weekly or endpoint-only LDF groups at any timepoint. Perfusion was significantly increased in the third week in the weekly LDF group relative to measurements in the second and fourth weeks. Ligation of the femoral artery caused consistent, rapid reductions in tibial perfusion, validating that LDF is sensitive to changes in tibial blood supply. Weekly LDF procedures did not adversely affect gait, as interlimb coordination during treadmill locomotion was similar between weekly and endpoint-only LDF groups at every timepoint. Images of the incision site show wound closure within one week, and serum concentrations of interleukin 6 were not significantly different between weekly and endpoint-only groups. Together, these findings demonstrate that our minimally invasive LDF technique is suitable for serial in vivo measurements of intraosseous blood perfusion without inducing localized inflammation or negatively affecting gait patterns in mice.

Assessment of acute bone loading in humans using [18F]NaF PET/MRI

PURPOSE

The acute effect of loading on bone tissue and physiology can offer important information with regard to joint function in diseases such as osteoarthritis. Imaging studies using [¹⁸F]-sodium fluoride ([¹⁸F]NaF) have found changes in tracer kinetics in animals after subjecting bones to strain, indicating an acute physiological response. The aim of this study is to measure acute changes in NaF uptake in human bone due to exercise-induced loading.

METHODS

Twelve healthy subjects underwent two consecutive 50-min [¹⁸F]NaF PET/MRI examinations of the knees, one baseline followed by one post-exercise scan. Quantification of tracer kinetics was performed using an image-derived input function from the popliteal artery. For both scans, kinetic parameters of K_i^NLR, K₁, k₂, k₃, and blood volume were mapped parametrically using nonlinear regression with the Hawkins model. The kinetic parameters along with mean SUV and SUV_max were compared between the pre- and post-exercise examinations. Differences in response to exercise were analysed between bone tissue types (subchondral, cortical, and trabecular bone) and between regional subsections of knee subchondral bone.

RESULTS

Exercise induced a significant (p < <0.001) increase in [¹⁸F]NaF uptake in all bone tissues in both knees, with mean SUV increases ranging from 47% in trabecular bone tissue to 131% in subchondral bone tissue. Kinetic parameters involving vascularization (K₁ and blood volume) increased, whereas the NaF extraction fraction [k₃/(k₂ + k₃)] was reduced.

CONCLUSIONS

Bone loading induces an acute response in bone physiology as quantified by [¹⁸F]NaF PET kinetics. Dynamic imaging after bone loading using [¹⁸F]NaF PET is a promising diagnostic tool in bone physiology and imaging of biomechanics.

Kinetic [18F]-Fluoride of the Knee in Normal Volunteers

PURPOSE

[F]-sodium fluoride ([F]NaF) is a well-established bone-seeking agent that has shown promise to assess bone turnover in a variety of disorders, but its distribution in healthy knee joints has not been explored. This study aimed to investigate parametric values for [F]NaF uptake in various bone tissues types of the knee and their spatial distributions.

METHODS

Twelve healthy subjects were hand-injected with 92.5 MBq of [F]NaF and scanned on a 3-T PET/MRI system. Listmode PET data for both knees were acquired for 50 minutes from injection simultaneously with MRI Dixon and angiography data. The image-derived input function was determined from the popliteal artery. Using the Hawkins model, Patlak analysis was performed to obtain Ki (Ki) values and nonlinear regression analysis to obtain Ki, K1, k3/(k2 + k3), and blood volume. Comparisons for the measured kinetic parameters, SUV, and SUVmax were made between tissue types (subchondral, cortical, and trabecular bone) and between regional subsections of subchondral bone.

RESULTS

Cortical bone had the highest [F]NaF uptake differing significantly in all measured parameters when compared with trabecular bone and significantly higher SUVmax and K1 than subchondral bone. Subchondral bone also had significantly higher SUV, SUVmax, and Ki than trabecular bone tissue. Regional differences were observed in K1 and k3/(k2 + k3) values.

CONCLUSIONS

Quantitative [F]NaF PET is sensitive to variations in bone vascularization and metabolism in the knee joint.

Design, Synthesis, and In Vitro and In Vivo Evaluation of an (18)F-Labeled Sphingosine 1-Phosphate Receptor 1 (S1P1) PET Tracer

Sphingosine 1-phosphate receptor 1 (S1P1) plays a pivotal signaling role in inflammatory response; because S1P1 modulation has been identified as a therapeutic target for various diseases, a PET tracer for S1P1 would be a useful tool. Fourteen fluorine-containing analogues of S1P ligands were synthesized and their in vitro binding potency measured; four had high potency and selectivity for S1P1 (S1P1 IC50 < 10 nM, >100-fold selectivity for S1P1 over S1P2 and S1P3). The most potent ligand, 28c (IC50 = 2.63 nM for S1P1) was (18)F-labeled and evaluated in a mouse model of LPS-induced acute liver injury to determine its S1P1-binding specificity. The results from biodistribution, autoradiography, and microPET imaging showed higher [(18)F]28c accumulation in the liver of LPS-treated mice than controls. Increased expression of S1P1 in the LPS model was confirmed by immunohistochemical analysis (IHC). These data suggest that [(18)F]28c is a S1P1 PET tracer with high potential for imaging S1P1 in vivo.

Correlation between mechanical stress by finite element analysis and 18F-fluoride PET uptake in hip osteoarthritis patients

18F-fluoride positron emission tomography (18F-fluoride PET) is a functional imaging modality used primarily to detect increased bone metabolism. Increased 18F-fluoride PET uptake suggests an association between increased bone metabolism and load stress at the subchondral level. This study therefore examined the relationship between equivalent stress distribution calculated by finite element analysis and 18F-fluoride PET uptake in patients with hip osteoarthritis. The study examined 34 hips of 17 patients who presented to our clinic with hip pain, and were diagnosed with osteoarthritis or pre-osteoarthritis. The hips with trauma, infection, or bone metastasis of cancer were excluded. Three-dimensional models of each hip were created from computed tomography data to calculate the maximum equivalent stress by finite element analysis, which was compared with the maximum standardized uptake value (SUVmax) examined by 18F-fluoride PET. The SUVmax and equivalent stress were correlated (Spearman's rank correlation coefficient ρ=0.752), and higher equivalent stress values were noted in higher SUVmax patients. The correlation between SUVmax and maximum equivalent stress in osteoarthritic hips suggests the possibility that 18F-fluoride PET detect increased bone metabolism at sites of stress concentration. This study demonstrates the correlation between mechanical stress and bone remodeling acceleration in hip osteoarthritis.

Antagonizing the αv β3 integrin inhibits angiogenesis and impairs woven but not lamellar bone formation induced by mechanical loading

Angiogenesis and osteogenesis are critically linked, although the role of angiogenesis is not well understood in osteogenic mechanical loading. In this study, either damaging or non-damaging cyclic axial compression was used to generate woven bone formation (WBF) or lamellar bone formation (LBF), respectively, at the mid-diaphysis of the adult rat forelimb. αv β3 integrin-targeted nanoparticles or vehicle was injected intravenously after mechanical loading. β3 integrin subunit expression on vasculature was maximal 7 days after damaging mechanical loading, but was still robustly expressed 14 days after loading. Accordingly, targeted nanoparticle delivery in WBF-loaded limbs was increased compared with non-loaded limbs. Vascularity was dramatically increased after WBF loading (+700% on day 14) and modestly increased after LBF loading (+50% on day 14). This increase in vascularity was inhibited by nanoparticle treatment in both WBF- and LBF-loaded limbs at days 7 and 14 after loading. Decreased vascularity led to diminished woven, but not lamellar, bone formation. Decreased woven bone formation resulted in impaired structural properties of the skeletal repair, particularly in post-yield behavior. These results demonstrate that αv β3 integrin-mediated angiogenesis is critical for recovering fracture resistance after bone injury but is not required for bone modeling after modest mechanical strain. © 2014 American Society for Bone and Mineral Research.

18 F-Fluoride positron emission tomography/computed tomography for noninvasive in vivo quantification of pathophysiological bone metabolism in experimental murine arthritis

Introduction

Evaluation of disease severity in experimental models of rheumatoid arthritis is inevitably associated with assessment of structural bone damage. A noninvasive imaging technology allowing objective quantification of pathophysiological alterations of bone structure in rodents could substantially extend the methods used to date in preclinical arthritis research for staging of autoimmune disease severity or efficacy of therapeutical intervention. Sodium ¹⁸ F-fluoride (¹⁸ F-NaF) is a bone-seeking tracer well-suited for molecular imaging. Therefore, we systematically examined the use of ¹⁸ F-NaF positron emission tomography/computed tomography (PET/CT) in mice with glucose-6-phosphate isomerase (G6PI)–induced arthritis for quantification of pathological bone metabolism.

Methods

F-fluoride was injected into mice before disease onset and at various time points of progressing experimental arthritis. Radioisotope accumulation in joints in the fore- and hindpaws was analyzed by PET measurements. For validation of bone metabolism quantified by ¹⁸ F-fluoride PET, bone surface parameters of high-resolution μCT measurements were used.

Results

Before clinical arthritis onset, no distinct accumulation of ¹⁸ F-fluoride was detectable in the fore- and hindlimbs of mice immunized with G6PI. In the course of experimental autoimmune disease, ¹⁸ F-fluoride bone uptake was increased at sites of enhanced bone metabolism caused by pathophysiological processes of autoimmune disease. Moreover, ¹⁸ F-fluoride signaling at different stages of G6PI-induced arthritis was significantly correlated with the degree of bone destruction. CT enabled identification of exact localization of ¹⁸ F-fluoride signaling in bone and soft tissue.

Conclusions

The results of this study suggest that small-animal PET/CT using ¹⁸ F-fluoride as a tracer is a feasible method for quantitative assessment of pathophysiological bone metabolism in experimental arthritis. Furthermore, the possibility to perform repeated noninvasive measurements in vivo allows longitudinal study of therapeutical intervention monitoring.

Quantification of 18F-Fluoride Kinetics: Evaluation of Simplified Methods

(18)F-fluoride PET is a promising noninvasive method for measuring bone metabolism and bone blood flow. The purpose of this study was to assess the performance of various clinically useful simplified methods by comparing them with full kinetic analysis. In addition, the validity of deriving bone blood flow from K1 of (18)F-fluoride was investigated using (15)O-H2O as a reference.

METHODS

Twenty-two adults (mean age ± SD, 44.8 ± 25.2 y), including 16 patients scheduled for bone surgery and 6 healthy volunteers, were studied. All patients underwent dynamic (15)O-H2O and (18)F-fluoride scans before surgery. Ten of these patients had serial PET measurements before and at 2 time points after local bone surgery. During all PET scans, arterial blood was monitored continuously. (18)F-fluoride data were analyzed using nonlinear regression (NLR) and several simplified methods (Patlak and standardized uptake value [SUV]). SUV was evaluated for different time intervals after injection and after normalizing to body weight, lean body mass, and body surface area, and simplified measurements were compared with NLR results. In addition, changes in SUV and Patlak-derived fluoride influx rate (Ki) after surgery were compared with corresponding changes in NLR-derived Ki. Finally, (18)F-fluoride K1 was compared with bone blood flow derived from (15)O-H2O data, using the standard single-tissue-compartment model.

RESULTS

K1 of (18)F-fluoride correlated with measured blood flow, but the correlation coefficient was relatively low (r = 0.35, P < 0.001). NLR resulted in a mean Ki of 0.0160 ± 0.0122, whereas Patlak analysis, for the interval 10-60 min after injection, resulted in an almost-identical mean Ki of 0.0161 ± 0.0117. The Patlak-derived Ki, for 10-60 min after injection, showed a high correlation with the NLR-derived Ki (r = 0.976). The highest correlation between Ki and lean body mass-normalized SUV was found for the interval 50-60 min (r = 0.958). Finally, changes in SUV correlated significantly with those in Ki (r = 0.97).

CONCLUSION

The present data support the use of both Patlak and SUV for assessing fluoride kinetics in humans. However, (18)F-fluoride PET has only limited accuracy in monitoring bone blood flow.

Skeletal Blood Flow in Bone Repair and Maintenance

Bone is a highly vascularized tissue, although this aspect of bone is often overlooked. In this article, the importance of blood flow in bone repair and regeneration will be reviewed. First, the skeletal vascular anatomy, with an emphasis on long bones, the distinct mechanisms for vascularizing bone tissue, and methods for remodeling existing vasculature are discussed. Next, techniques for quantifying bone blood flow are briefly summarized. Finally, the body of experimental work that demonstrates the role of bone blood flow in fracture healing, distraction osteogenesis, osteoporosis, disuse osteopenia, and bone grafting is examined. These results illustrate that adequate bone blood flow is an important clinical consideration, particularly during bone regeneration and in at-risk patient groups.

Nitric oxide-mediated vasodilation increases blood flow during the early stages of stress fracture healing

Despite the strong connection between angiogenesis and osteogenesis in skeletal repair conditions such as fracture and distraction osteogenesis, little is known about the vascular requirements for bone formation after repetitive mechanical loading. Here, established protocols of damaging (stress fracture) and nondamaging (physiological) forelimb loading in the adult rat were used to stimulate either woven or lamellar bone formation, respectively. Positron emission tomography was used to evaluate blood flow and fluoride kinetics at the site of bone formation. In the group that received damaging mechanical loading leading to woven bone formation (WBF), (15)O water (blood) flow rate was significantly increased on day 0 and remained elevated 14 days after loading, whereas (18)F fluoride uptake peaked 7 days after loading. In the group that received nondamaging mechanical loading leading to lamellar bone formation (LBF), (15)O water and (18)F fluoride flow rates in loaded limbs were not significantly different from nonloaded limbs at any time point. The early increase in blood flow rate after WBF loading was associated with local vasodilation. In addition, Nos2 expression in mast cells was increased in WBF-, but not LBF-, loaded limbs. The nitric oxide (NO) synthase inhibitor N(ω)-nitro-l-arginine methyl ester was used to suppress NO generation, resulting in significant decreases in early blood flow rate and bone formation after WBF loading. These results demonstrate that NO-mediated vasodilation is a key feature of the normal response to stress fracture and precedes woven bone formation. Therefore, patients with impaired vascular function may heal stress fractures more slowly than expected.

Validated Laser Doppler protocol for measurement of mouse bone blood perfusion - response to age or ovariectomy differs with genetic background

The physiological role of bone vascularization in bone metabolism begins to be understood; however, its involvement in pathological situations remains poorly explored. Bone blood supply depends on both vascular density and blood flow. However, in mice, the specific evaluation of perfusion in bone suffers from a lack of easy-handling measurement tools. In the present study, we first developed a Laser Doppler Perfusion Measurement (LDPM) protocol in mouse tibia, which we validated with ex vivo and in vivo experiments. Then we carried out a study associating both structural (vascular quantitative histomorphometry) and functional (LDPM) approaches. We studied the effects of aging in 4, 7 and 17 month-old male mice and the early effects of ovariectomy in 4 month-old females. Both studies were carried out in inbred mice (C57BL/6) and in mice of mixed background (129sv/CD1). The significant differences we observed between strains in unchallenged 4 month-old animals concerned both perfusion and vascular density and depended on gender. Additionally, the age-related bone loss observed in male mice was not temporally associated with vascular changes in either strain. Between 7 and 17 months, we did not find any decrease in bone vascular density or perfusion. In contrast, ovariectomy triggered early vascular structural and functional adaptations which differed between genetic backgrounds. We observed that bone vessel density did not generally account for bone perfusion levels. In conclusion, we describe here a LDPM-based experimental protocol which provides a reproducible quantitative evaluation of bone perfusion in mouse tibia, hence allowing intergroup comparisons. This integrative structural and functional approach of bone vascularization showed that bone vascular adaptation occurs during aging or after ovariectomy and is affected by the genetic background.

Angiogenesis is required for stress fracture healing in rats

Although angiogenesis and osteogenesis are critically linked, the importance of angiogenesis for stress fracture healing is unknown. In this study, mechanical loading was used to create a non-displaced stress fracture in the adult rat forelimb. Fumagillin, an anti-angiogenic agent, was used as the water soluble analogue TNP-470 (25mg/kg) as well as incorporated into lipid-encapsulated α(v)β(3) integrin targeted nanoparticles (0.25mg/kg). In the first experiment, TNP-470 was administered daily for 5 days following mechanical loading, and changes in gene expression, vascularity, and woven bone formation were quantified. Although no changes in vascularity were detected 3 days after loading, treatment-related downregulation of angiogenic (Pecam1) and osteogenic (Bsp, Osx) genes was observed at this early time point. On day 7, microCT imaging of loaded limbs revealed diminished woven bone formation in treated limbs compared to vehicle treated limbs. In the second experiment, α(v)β(3) integrin targeted fumagillin nanoparticles were administered as before, albeit with a 100-fold lower dose, and changes in vascularity and woven bone formation were determined. There were no treatment-related changes in vessel count or volume 3 days after loading, although fewer angiogenic (CD105 positive) blood vessels were present in treated limbs compared to vehicle treated limbs. This result manifested on day 7 as a reduction in total vascularity, as measured by histology (vessel count) and microCT (vessel volume). Similar to the first experiment, treated limbs had diminished woven bone formation on day 7 compared to vehicle treated limbs. These results indicate that angiogenesis is required for stress fracture healing, and may have implications for inducing rapid repair of stress fractures.