Effect of clodronate on fracture healing in denervated rats

The effect of clodronate on healing of the fracture of osteopenic bone was studied in rats. A total of 165 female rats (14 ± 1 weeks, 216 ± 2 g) were divided into five fracture groups (n = 30), and a neurectomized group (n = 15). Osteopenia (op) was induced by right sciatic neurectomy 4 weeks before the fracture. Nonosteopenic (nop) rats were not operated. A closed prepinned diaphyseal fracture of the right femur was done by three-point bending method both to op and nop rats, and the left femur served as an unoperated control. All the fracture groups were divided into treatment (clodronate 10 mg/kg/day sc) and control (saline sc) groups, and the administration was continued throughout the study. The op rats were killed 2, 4, 8, and 12 weeks and nop rats 8 weeks after the fracture. Fracture healing was examined by x-ray and bone-bending strength. Neurectomy reduced bone strength (p < 0.01) at 4 weeks. Clodronate did not affect the bending strength of healing callus of op rats at 2, 4, 8, or 12 weeks after fracture, but reduced the strength of healing callus in nop rats (p < 0.05) at 8 weeks. Radiologic callus width increased in clodronate-treated groups both in op (8 and 12 weeks, p < 0.001) and nop rats (8 weeks, p < 0.05) when compared with saline-treated groups. Clodronate did not affect normal bone strength.

In conclusion, clodronate did not affect the bending strength of op fracture nor the strength of the control bones. The remodeling of the fracture was delayed with clodronate.     

Matrix Metalloproteinase–Driven Endochondral Fracture Union Proceeds Independently of Osteoclast Activity

As new insights into the complexities of endochondral fracture repair emerge, the temporal role of osteoclast activity remains ambiguous. With numerous antiresorptive agents available to treat bone disease, understanding their impact on bone repair is vital. Further, in light of recent work suggesting osteoclast activity may not be necessary during early endochondral fracture union, we hypothesize instead a pivotal role of matrix metalloproteinase (MMP) secreting cells in driving this process. Although the role of MMPs in fracture healing has been examined, no directly comparative experiments exist. We examined a number of antiresorptive treatments to either block osteoclast activity, including the potent bisphosphonates zoledronic acid (ZA) and clodronate (CLOD), which work via differing mechanisms, or antagonize osteoclastogenesis with recombinant OPG (HuOPG‐Fc), comparing these directly to an inhibitor of MMP activity (MMI270). Endochondral ossification to union occurred normally in all antiresorptive groups. In contrast, MMP inhibition greatly impaired endochondral union, significantly delaying cartilage callus removal. MMP inhibition also produced smaller, denser hard calluses. Hard callus remodeling was, as expected, delayed with ZA, CLOD, and OPG treatment at 4 and 6 weeks, resulting in larger, more mineralized calluses at 6 weeks. As a result of reduced hard callus turnover, bone formation was reduced with antiresorptive agents at these time points. These results confirm that the achievement of endochondral fracture union occurs independently of osteoclast activity. Alternatively, MMP secretion by invading cells is obligatory to endochondral union. This study provides new insight into cellular contributions to bone repair and may abate concerns regarding antiresorptive therapies impeding initial fracture union.     

Effects of receptor activator of NFkappaB (RANK) signaling blockade on fracture healing.

As dominant regulators of osteoclastogenesis and bone resorption, receptor activator of NFkappaB (RANK), receptor activator of NFkappaB ligand, and OPG have been identified as ideal drug targets for the treatment of metabolic bone disease. One concern regarding the therapeutic use of RANK signaling inhibitors is their effect on fracture healing. Therefore we tested if uncoupling and osteoclast depletion via RANK blockade affects callus formation, maturation and matrix remodeling, as well as union rates in a mouse tibia fracture model. Low dose (1 mg/kg i.p.) RANK:Fc therapy had no effect on callus formation, matrix maturation and remodeling, and resulted in 100% bony union by day 28. High dose RANK:Fc treatment (10 mg/kg i.p.) effectively eliminated osteoclasts at the fracture site on day 14, with no significant effects on fracture healing. When therapy was discontinued, normal numbers of osteoclasts were observed at the fracture site by day 28. However, continuous therapy resulted in a large osteopetrotic callus consisting of both mineralized and unmineralized matrix that was void of osteoclasts, but bony union was unaffected at day 28. We also evaluated this process in the complete absence of RANK signaling using RANK -/- mice. These animals exhibited significant radiographic and histologic evidence of callus formation, indicating that RANK signaling is not required for fracture callus formation. However, only 33% of RANK -/- animals formed bony unions compared to 100% of the osteopetrotic control mice. This defect was most likely a result of decreased blood flow, as evidenced by fewer blood vessels in the RANK -/- animals. Together, these data imply that osteoclast depletion via inhibition of RANK signaling is a viable option for the treatment of pathological bone loss since no adverse effects on fracture healing are observed when therapy is discontinued.     

Long-term administration of clodronate does not prevent fracture healing in rats

Clinicians have been concerned that fractures do not heal properly in individuals exposed to bisphosphonate treatment, a treatment that strongly affects bone metabolism. The current study attempted to clarify the long-term effects of clodronate (dichloromethylene bisphosphonate) treatment on fracture healing in growing rats. Clodronate was administered subcutaneously twice a week in a dose of 2 mg/kg or 10 mg/kg. Physiologic saline served as a control. After 24 weeks of treatment, the tibiae were fractured, and the treatment was continued for another 4 weeks and 8 weeks. At both end points the cross-sectional areas of the callus, measured by peripheral quantitative computed tomography, were greater in the clodronate-treated rats than in controls, but there were no significant differences in bone mineral density. There were no significant differences between treatments in radiologic healing, histomorphometry, or in mechanical failure load of the callus with the exception of increased tensile stiffness at a dose of 2 mg/kg at 4 weeks. Clodronate treatment does not seem to prolong the fracture healing process, even when administered on a long-term basis before the fracture. Clodronate increases the size of the callus, but has only a minor effect on its biomechanical properties. The current results suggest that long-term clodronate treatment does not inhibit fracture healing.     

Expression of metalloproteinase-13 (Collagenase-3) is induced during fracture healing in mice.

In fracture healing, a large amount of cartilage is formed, then rapidly replaced by osseous tissue. This process requires the transition of extracellular matrix component from type II to type I collagen. We investigated the expression of matrix metalloproteinase-13 (MMP-13), which has a high potential to cleave type II as well as type I collagen, during fracture repair in mouse ribs. In situ hybridization demonstrated that MMP-13 mRNA was present throughout the healing process. It was detected in the cells of the periosteum at day 1. As fracture callus grew, strong MMP-13 mRNA signals were detected in cells of the cartilaginous callus. In the reparative and remodeling phases, both hypertrophic chondrocytes and immature osteoblastic cells in the fracture callus expressed MMP-13 mRNA strongly. These cells were located adjacent to tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts at the sites of cartilage/bone transition. In osteoclasts, MMP-13 expression was not detected. The level of MMP-13 mRNA peaked at day 14 postfracture by northern blotting. Immunohistochemical staining showed that MMP-13 was detected primarily in hypertrophic chondrocytes. These results indicate that MMP-13 is induced during fracture healing. The site- and cell-specific expression of MMP-13 and its enzymatic property suggest that MMP-13 initiates the degradation of cartilage matrix, resulting in resorption and remodeling of the callus. In conclusion, MMP-13 plays an important role in the healing process of fractured bone in mice.     

Urokinase plasminogen activator gene deficiency inhibits fracture cartilage remodeling

Urokinase plasminogen activator (uPA) regulates a proteolytic cascade of extracellular matrix degradation that functions in tissue development and tissue repair. The development and remodeling of the skeletal extracellular matrix during wound healing suggests that uPA might regulate bone development and repair. To determine whether uPA functions regulate bone development and repair, we examined the basal skeletal phenotype and endochondral bone fracture repair in uPA-deficient mice. The skeletal phenotype of uPA knockout mice was compared with that of control mice under basal conditions by dual-energy X-ray absorptiometry and micro-CT analysis, and during femur fracture repair by micro-CT and histological examination of the fracture callus. No effects of uPA gene deficiency were observed in the basal skeletal phenotype of the whole body or the femur. However, uPA gene deficiency resulted in increased fracture callus cartilage abundance during femur fracture repair at 14 days healing. The increase in cartilage corresponded to reduced tartrate-resistant acid phosphatase (TRAP) staining for osteoclasts in the uPA knockout fracture callus at this time, consistent with impaired osteoclast-mediated remodeling of the fracture cartilage. CD31 staining was reduced in the knockout fracture tissues at this time, suggesting that angiogenesis was also reduced. Osteoclasts also colocalized with CD31 expression in the endothelial cells of the fracture tissues during callus remodeling. These results indicate that uPA promotes remodeling of the fracture cartilage by osteoclasts that are associated with angiogenesis and suggest that uPA promotes angiogenesis and remodeling of the fracture cartilage at this time of bone fracture repair.     

Diminished bone formation during diabetic fracture healing is related to the premature resorption of cartilage associated with increased osteoclast activity.

Histological and molecular analysis of fracture healing in normal and diabetic animals showed significantly enhanced removal of cartilage in diabetic animals. Increased cartilage turnover was associated with elevated osteoclast numbers, a higher expression of genes that promote osteoclastogenesis, and diminished primary bone formation. INTRODUCTION: Diminished bone formation, an increased incidence of nonunions, and delayed fracture healing have been observed in animal models and in patients with diabetes. Fracture healing is characterized by the formation of a stabilizing callus in which cartilage is formed and then resorbed and replaced by bone. To gain insight into how diabetes affects fracture healing, studies were carried out focusing on the impact of diabetes on the transition from cartilage to bone. MATERIALS AND METHODS: A low-dose treatment protocol of streptozotocin in CD-1 mice was used to induce a type 1 diabetic condition. After mice were hyperglycemic for 3 weeks, controlled closed simple transverse fractures of the tibia were induced and fixed by intramedullary pins. Histomorphometric analysis of the tibias obtained 12, 16, and 22 days after fracture was performed across the fracture callus at 0.5 mm proximal and distal increments using computer-assisted image analysis. Another group of 16-day samples were examined by microCT. RNA was isolated from a separate set of animals, and the expression of genes that reflect the formation and removal of cartilage and bone was measured by real-time PCR. RESULTS: Molecular analysis of collagen types II and X mRNA expression showed that cartilage formation was the same during the initial period of callus formation. Histomorphometric analysis of day 12 fracture calluses showed that callus size and cartilage area were also similar in normoglycemic and diabetic mice. In contrast, on day 16, callus size, cartilage tissue, and new bone area were 2.0-, 4.4-, and 1.5-fold larger, respectively, in the normoglycemic compared with the diabetic group (p < 0.05). Analysis of microCT images indicated that the bone volume in the normoglycemic animals was 38% larger than in diabetic animals. There were 78% more osteoclasts in the diabetic group compared with the normoglycemic group (p < 0.05) on day 16, consistent with the reduction in cartilage. Real-time PCR showed significantly elevated levels of mRNA expression for TNF-alpha, macrophage-colony stimulating factor, RANKL, and vascular endothelial growth factor-A in the diabetic group. Similarly, the mRNA encoding ADAMTS 4 and 5, major aggrecanases that degrade cartilage, was also elevated in diabetic animals. CONCLUSIONS: These results suggest that impaired fracture healing in diabetes is characterized by increased rates of cartilage resorption. This premature loss of cartilage leads to a reduction in callus size and contributes to decreased bone formation and mechanical strength frequently reported in diabetic fracture healing.     

Stimulation of angiogenesis by cilostazol accelerates fracture healing in mice

Cilostazol, a selective phosphodiesterase‐3 inhibitor, is known to control cyclic adenosine monophosphate (c‐AMP) and to stimulate angiogenesis through upregulation of pro‐angiogenic factors. There is no information, however, whether cilostazol affects fracture healing. We, therefore, studied the effect of cilostazol on callus formation and biomechanics during fracture repair. Bone healing was analyzed in a murine femur fracture stabilized with an intramedullary screw. Radiological, biomechanical, histomorphometric, histochemical, and protein biochemical analyses were performed at 2 and 5 weeks after fracture. Twenty‐five mice received 30 mg/kg body weight cilostazol p.o. daily. Controls (n = 24) received equivalent amounts of vehicle. In cilostazol‐treated animals radiological analysis at 2 weeks showed an improved healing with an accelerated osseous bridging compared to controls. This was associated with a significantly higher amount of bony tissue and a smaller amount of cartilage tissue within the callus. Western blot analysis showed a higher expression of cysteine‐rich protein 61 (CYR61), bone morphogenetic protein (BMP)‐4, and receptor activator of NF‐kappaB ligand (RANKL). At 5 weeks, improved fracture healing after cilostazol treatment was indicated by biomechanical analyses, demonstrating a significant higher bending stiffness compared to controls. Thus, cilostazol improves fracture healing by accelerating both bone formation and callus remodeling. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1880–1887, 2015.     

Dissociation of angiogenesis and osteoclastogenesis during endochondral bone formation in neonatal mice.

Invasion of the mineralized matrix by endothelial cells and osteoclasts is a key event in endochondral bone formation. To examine the putative role of osteoclast activity in the angiogenic process, we used two in vivo models of suppressed bone resorption: mice treated with the bisphosphonate clodronate and in osteoclast-deficient, osteopetrotic mice. Angiogenesis was assessed in caudal vertebrae of these neonatal mice. This model enables us to study the interaction between osteoclasts and endothelial cells during endochondral bone formation. In control conditions, sinusoid-like structures were detected in the vicinity of tartrate resistance acid phosphatase positive (TRAcP+) osteoclasts. Treatment with clodronate completely abolished osteoclastic bone resorption, whereas angiogenesis remained unaffected. In line with these observations, in the osteopetrotic mouse mutants c-fos knockout mice and op/op mice, capillaries invaded the calcified cartilage in the absence of osteoclasts. In conclusion, our data strongly suggest that during endochondral bone formation, vascular invasion can occur in the absence of osteo(chondro)clastic resorption. In addition, bisphosphonates show no apparent effect on angiogenesis in this in vivo model. These findings may have important clinical implications in the management of skeletal disorders such as metastatic bone disease, in which both osteoclastic bone resorption and angiogenesis contribute to tumor growth. On the other hand, our results confirm that bisphosphonates can be used safely in the treatment of disorders that affect the growing skeleton, such as in juvenile osteoporosis.     

Characterization of the bone phenotype and fracture repair in osteopetrotic Incisors absent rats

Osteopetrotic patients possess a genetic condition that leads to a deficiency in osteoclast number or function. Patients have a high bone density and suffer from an increased risk of fracture. The lack of normal osteoclast activity has the potential to impede repair by complicating orthopedic fixation and/or by affecting the biology of fracture healing. The naturally occurring incisors absent (ia/ia) rat was adopted as a rodent model of congenital osteopetrosis. A detailed phenotypic analysis of the ia/ia rat indicated that some functional recovery occurred between 7 and 9 weeks. Consequently a fracture repair study was undertaken using 5‐week‐old rats. Closed femoral fractures were generated in ia/ia rats and control ia/+ and +/+ rats using an Einhorn apparatus. Fracture healing was examined radiologically and histologically at 1–3 weeks. No difference was seen in bridging between ia/ia and control rats at any time point. The ia/ia rats showed no delay in cartilage removal but showed a significant delay in hard callus remodeling. This is consistent with an essential role for osteoclasts in only the latter stages of endochondral bone repair. This delay in hard callus remodeling was offset by an increase in moment of inertia. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:726–733, 2011     

Healing of a tibial double osteotomy is modified by clodronate administration

We studied the healing of tibial double osteotomies in rats during clodronate administration 10 mg/kg daily subcutaneously for 6 weeks. Clodronate treatment did not inhibit the endochondral bone formation in callus or in the epiphyseal plate. The differentiation of osteoblasts into mature osteocytes was disturbed, and the bone structure around the osteocytes was not normal. In the clodronatetreated group, the amount of newly formed bone matrix seemed increased, but mineralization of the bone matrix was delayed. The bundle bone trabeculae in callus were not so spatially well organized as in the controls. No delay in the union of the bone segment by new bone was seen during clodronate administration.     

Endostatin Inhibits Callus Remodeling during Fracture Healing in Mice

Information on the impact of endogenous anti‐angiogenic factors on bone repair is limited. The hypothesis of the present study was endostatin, an endogenous inhibitor of angiogenesis, disturbs fracture healing. We evaluated this hypothesis in a closed femoral fracture model studying two groups of mice, one that was treated by a daily injection of 10 µg recombinant endostatin subcutaneously (n = 38) and a second one that received the vehicle for control (n = 37). Histomorphometric analysis showed a significantly increased callus formation in endostatin‐treated animals at 2 and 5 weeks post‐fracture. This was associated with a significantly higher callus tissue fraction of cartilage and fibrous tissue at 2 weeks and a significantly higher fraction of bone at 5 weeks post‐fracture. Biomechanical testing revealed a significantly higher torsional stiffness in the endostatin group at 2 weeks. For both groups, we could demonstrate the expression of the endostatin receptor unit integrin alpha5 in endothelial cells, osteoblasts, osteoclasts, and chondrocytes at 2 weeks. Immunohistochemical fluorescence staining of CD31 showed a lower number of blood vessels in endostatin‐treated animals compared to controls. The results of the present study indicate endostatin promotes soft callus formation but inhibits callus remodeling during fracture healing most probably by an inhibition of angiogenesis. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:1579–1584, 2013.     

Traumatic late dissociation of the polyethylene articulating surface in a total knee arthroplasty--a case report

Clodronate was administered daily 28 days before and after an experimental tibial fracture in 35 male rats, and the effect on fracture healing and posttraumatic bone loss was studied. 5 groups were tested. The clodronate/clodronate group received clodronate in daily doses of 10 mg/ kg body weight for 28 days before being subjected to a standardized fracture of the right tibia, and during the fracture healing period of 28 days. The clodronate/ saline group received clodronate before fracture and saline during the healing period. The saline/ clodronate group received saline before and clodronate after fracture. The saline/saline group received saline only, while the control group served as unfractured, untreated controls. After 28 days of fracture healing, the tibias were evaluated with dual energy x-ray absorptiometry, and tested mechanically in a 3-point ventral bending test. Bone mineral content and bone mineral density were approximately 30% higher in the groups receiving clodronate during the experiment, compared to the untreated groups. The weight and cross-sectional area of the fracture callus were equal in all groups. Whether clodronate was administered before the fracture, after the fracture or both, did not affect the bone mineral. Ultimate bending moment, energy absorption, stiffness and deflection were not significantly different between the groups. Our findings suggest that clodronate increases bone mineral both when given before and after a tibial shaft fracture, without affecting fracture healing at 28 days.     

Clodronate increases the calcium content in fracture callus

Summary Eighty-eight rats underwent intramedullary pin fixation and fracture of both tibiae. Half of the animals were given clodronate 50 mg/kg s. c. weekly. Clodronate treatment did not affect the growth of fibrocartilage or the endochondral and membranous new bone formation. The regaining of tensile load capacity of fractured bone remained unaffected by the drug. Calluses were remodeled to lamellar bone in both groups. However, although the total area invaded by mineralized tissue in callus remained unaffected by the drug, the areas of hematopoietic bone marrow tissue within mineralized callus were observed to be markedly smaller in clodronate-treated animals than in controls. The calluses in the clodronate group were significantly heavier and contained more calcium at 2 months after fracture than those in the controls.     

Low‐magnitude high‐frequency vibration (LMHFV) enhances bone remodeling in osteoporotic rat femoral fracture healing

Low‐magnitude high‐frequency vibration (LMHFV) (35 Hz, 0.3 g) accelerates fracture healing by enhancing callus formation and mineralization for both normal and osteoporotic rats in our previous studies. 1 We hypothesized that LMHFV enhances fracture healing through bone remodeling. Ibandronate was used to suppress LMHFV‐stimulated bone remodeling and changes in remodeling were investigated to verify our hypothesis. Closed femoral fractures were created in 80 osteoporotic female Sprague–Dawley rats. The rats were randomly assigned into control (CG), LMHFV (VG) (20 min/day, 5 days/week), ibandronate (BG) (7 µg/kg/week), or LMHFV + ibandronate (VBG) for a treatment duration of 2, 4, 6, or 8 weeks. Blood was taken and the femora were harvested for histological and radiological analyses. VG had the fastest drop in callus area (CA) and width (CW), and bone volume to tissue volume ratio (BV/TV); whereas, a plateaued trend in BG and VBG was observed. The fastest callus reduction, highest mineral apposition rate at week 6, and increased serum concentration of osteocalcin and TRAP5b in VG suggested enhanced remodeling. LMHFV partially reversed the inhibition of bone remodeling by ibandronate suggested LMHFV had an opposite effect on bone remodeling to ibandronate. In conclusion, LMHFV accelerated fracture healing by enhancing bone remodeling and the administration of ibandronate can impair this enhancement. LMHFV has great potential in improving fracture outcome clinically. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:746–752, 2011     

Systemic treatment with the sphingosine‐1‐phosphate analog FTY720 does not improve fracture healing in mice

Sphingosine‐1‐phosphate (S1P) has recently been recognized as a crucial coupling molecule of osteoclast and osteoblast activity provoking osteoanabolic effects. Targeting S1P receptors could, therefore, be a potential strategy to support bone formation in osteopenic diseases or in fracture repair. Here we investigated whether systemic treatment with the S1P analog FTY720 (Fingolimod) could improve fracture healing. Twelve‐week‐old, female C57BL/6 mice received an osteotomy of the femur, which was stabilized using an external fixator. The mice received a daily subcutaneous injection of either FTY720 (6 mg/kg) or vehicle from the third postoperative day. Fracture healing was evaluated after 10 and 21 days using biomechanical testing, µ‐computed tomography, and histomorphometry. Because FTY720 is supposed to influence osteoclast recruitment, osteoclasts were identified in the fracture callus by staining for tartrate resistant acid phosphatase (TRAP). There were no significant differences in callus mechanical properties, tissue composition and osteoclast number between the groups, suggesting that systemically applied FTY720 did not influence bone regeneration in this model of regular fracture healing. Even if further studies should test the potency of FTY720 under unfavorable healing conditions, we conclude that the effect of systemically applied FTY720 on fracture healing might be inferior compared to other anabolic treatments. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:1845–1850, 2013     

Effect of osteoporosis medications on fracture healing

Antiosteoporotic medications are often used to concurrently treat a patient’s fragility fractures and underlying osteoporosis. This review evaluates the existing literature from animal and clinical models to determine these drugs’ effects on fracture healing. The data suggest that these medications may enhance bone healing, yet more thorough prospective studies are warranted. Pharmacologic agents that influence bone remodeling are an essential component of osteoporosis management. Because many patients are first diagnosed with osteoporosis when presenting with a fragility fracture, it is critical to understand how osteoporotic medications influence fracture healing. Vitamin D and its analogs are essential for the mineralization of the callus and may also play a role in callus formation and remodeling that enhances biomechanical strength. In animal models, antiresorptive medications, including bisphosphonates, denosumab, calcitonin, estrogen, and raloxifene, do not impede endochondral fracture healing but may delay repair due to impaired remodeling. Although bisphosphonates and denosumab delay callus remodeling, they increase callus volume and result in unaltered biomechanical properties. Calcitonin increases cartilage formation and callus maturation, resulting in improved biomechanical properties. Parathyroid hormone, an anabolic agent, has demonstrated promise in animal models, resulting in accelerated healing with increased callus volume and density, more rapid remodeling to mature bone, and improved biomechanical properties. Clinical data with parathyroid hormone have demonstrated enhanced healing in distal radius and pelvic fractures as well as postoperatively following spine surgery. Strontium ranelate, which may have both antiresorptive and anabolic properties, affects fracture healing differently in normal and osteoporotic bone. While there is no effect in normal bone, in osteoporotic bone, strontium ranelate increases callus bone formation, maturity, and mineralization; forms greater and denser trabeculae; and improves biomechanical properties. Further clinical studies with these medications are needed to fully understand their effects on fracture healing in order to simultaneously treat fragility fractures and underlying osteoporosis.     

EP1−/− mice have enhanced osteoblast differentiation and accelerated fracture repair

As a downstream product of cyclooxygenase 2 (COX‐2), prostaglandin E₂ (PGE₂) plays a crucial role in the regulation of bone formation. It has four different receptor subtypes (EP1 through EP4), each of which exerts different effects in bone. EP2 and EP4 induce bone formation through the protein kinase A (PKA) pathway, whereas EP3 inhibits bone formation in vitro. However, the effect of EP1 receptor signaling during bone formation remains unclear. Closed, stabilized femoral fractures were created in mice with EP1 receptor loss of function at 10 weeks of age. Healing was evaluated by radiographic imaging, histology, gene expression studies, micro–computed tomographic (µCT), and biomechanical measures. EP1^?/? mouse fractures have increased formation of cartilage, increased fracture callus, and more rapid completion of endochondral ossification. The fractures heal faster and with earlier fracture callus mineralization with an altered expression of genes involved in bone repair and remodeling. Fractures in EP1^?/? mice also had an earlier appearance of tartrate‐resistant acid phosphatase (TRAcP)–positive osteoclasts, accelerated bone remodeling, and an earlier return to normal bone morphometry. EP1^?/? mesenchymal progenitor cells isolated from bone marrow have higher osteoblast differentiation capacity and accelerated bone nodule formation and mineralization in vitro. Loss of the EP1 receptor did not affect EP2 or EP4 signaling, suggesting that EP1 and its downstream signaling targets directly regulate fracture healing. We show that unlike the PGE₂ receptors EP2 and EP4, the EP1 receptor is a negative regulator that acts at multiple stages of the fracture healing process. Inhibition of EP1 signaling is a potential means to enhance fracture healing. © 2011 American Society for Bone and Mineral Research.