A murine model of distraction osteogenesis

A 4D time-lapse morphometry method to quantify bone formation and resorption during distraction osteogenesis

Distraction osteogenesis (DO) is widely utilized for treating limb length discrepancy, nonunion, bone deformities and defects. This study sought to develop a 4D time-lapse morphometry method to quantify bone formation and resorption in mouse femur during DO based on image registration of longitudinal in vivo micro-CT scans. Female C57BL/6 mice (n = 7) underwent osteotomy, followed by 5 days of latency, 10 days of distraction and 35 days of consolidation. The mice were scanned with micro-CT at Days 5, 15, 25, 35, 45, and 50. Histological sectioning and Movat Pentachrome straining were performed at Day 50. After registration of two consecutive micro-CT images of the same bone (day x and day y), the spatially- and temporally-linked sequences of formation, resorption and quiescent bones at the distraction gap were identified and bone formation and resorption rates (BFRdayx-y and BRRdayx-y) were calculated. The overall percentage error of the registration method was 2.98% ± 0.89% and there was a strong correlation between histologically-measured bone area fraction and micro-CT-determined bone volume fraction at Day 50 (r = 0.89, p < 0.05). The 4D time-lapse morphometry indicated a rapid bone formation during the first 10 days of the consolidation phase (BFRday15-25 = 0.14 ± 0.05 mm3/day), followed by callus reshaping via equivalent bone formation and resorption rates. The 4D time-lapse morphometry method developed in this study allows for a continuous quantitative monitoring of the dynamic process of bone formation and resorption following distraction, which may offer a better understanding of the mechanism for mechano-regulated bone regeneration and aid for development of new treatment strategies of DO.

Creation of a Tibia Extension Model With a Perforated Ilizarov Ring System: An Experimental Study in Rats

Distraction osteogenesis is a valuable clinical technique used to address length discrepancies in long bone deformities. This procedure involves performing an osteotomy at an appropriate site in the bone and correcting the deformity through an extension system. This research aims to investigate the efficacy of a newly developed device for use in rat tibias and to provide an alternative to existing devices used in animal experiments. A total of 16 male Wistar-Albino rats, each weighing approximately 300-350 grams and aged 20-28 weeks, were used in the study. On the first day, a fixator was applied to the right tibias of the rats, and Ilizarov osteotomy was performed. Distraction of the tibia commenced on the Day 7. During the distraction phase, which lasted seven days, tibia lengthening was performed twice daily at 08:00 and 16:00, with each session involving a distraction of 0.25 mm. After a 14-day waiting period, evaluations were conducted on the 28th, 35th, 42nd, and 49th days post-surgery. Following these assessments, the rats were evaluated with X-rays and subsequently sacrificed. The distraction procedures proceeded largely without issues in the rats. Union was observed during the follow-up after distraction. In the initial postoperative X-ray of one rat, no problems with the reduction of the osteotomy were detected. To verify the functionality of the system, acute distraction was tested in one rat model, and successful elongation was achieved. However, one rat experienced circulatory disturbances post-operation, with the extremity showing an ecchymotic appearance. The extremity returned to its normal state during follow-up. Infection occurred in three rats. No postoperative antibiotic therapy was administered to any of the rat models. During follow-up, the infections resolved with regular dressing changes. Due to fewer complications and improved radiological imaging with the extension models performed in the metaphyseal region of rat tibias, our system could be utilized in future fracture model applications or distraction osteogenesis studies involving rat models. We believe it could serve as an alternative to other models for creating extension models at a lower cost.

Development of a standardized and reproducible murine femoral distraction osteogenesis model

OBJECTIVE

Distraction osteogenesis (DO) has been widely used to treat bone defects as its effectiveness in bone regeneration. Currently, distraction devices for establishing DO models are mainly developed for rats or large animals. However, a mouse DO model is in great need for in-depth mechanistic investigations using various transgenic mice. The current study reports the development of a reproducible murine DO model.

METHODS

A mini-titanium lengthener was designed and fabricated. The mini-lengthener was applied on the murine femur with four threaded pins using a designed clamp as the drilling and insertion guide. After transverse osteotomy using a Gigli saw, and after 5 days of latency, DO procedures started at 0.3 mm/day for 10 days, and the consolidation period was left for 28 days. The bone formation was monitored by radiography and histology. Potential effects on animal locomotion during DO were also measured by behavior tests.

RESULTS

Separated bone segments maintained good alignment during the entire DO phases. New bone formation was found as early as the end of the distraction phase. Active bone remodeling was found between the separated bone segments at late distraction and early consolidation phases. At the mature consolidation phase, bone remodeling was mainly observed in the contact cortical bone. Mice underwent DO procedure did not have significant impairment in their locomotion.

CONCLUSION

We have successfully developed a murine femoral DO model, which may be used to study the biological processes of DO. We also developed the mini-lengthener and the guide clamp to ensure the standardization and reproducibility of the mouse DO model.The translational potential of this article: Current study reports the development of a murine femoral DO model. A well-established murine DO model will facilitate further investigations of the biological mechanisms of DO in various transgenic and normal mice.

Chondromodulin is necessary for cartilage callus distraction in mice

Chondromodulin (Cnmd) is a glycoprotein known to stimulate chondrocyte growth. We examined in this study the expression and functional role of Cnmd during distraction osteogenesis that is modulated by mechanical forces. The right tibiae of the mice were separated by osteotomy and subjected to slow progressive distraction using an external fixator. In situ hybridization and immunohistochemical analyses of the lengthened segment revealed that Cnmd mRNA and its protein in wild-type mice were localized in the cartilage callus, which was initially generated in the lag phase and was lengthened gradually during the distraction phase. In Cnmd null (Cnmd-/-) mice, less cartilage callus was observed, and the distraction gap was filled by fibrous tissues. Additionally, radiological and histological investigations demonstrated delayed bone consolidation and remodeling of the lengthened segment in Cnmd-/- mice. Eventually, Cnmd deficiency caused a one-week delay in the peak expression of VEGF, MMP2, and MMP9 genes and the subsequent angiogenesis and osteoclastogenesis. We conclude that Cnmd is necessary for cartilage callus distraction.

In vivo and in silico monitoring bone regeneration during distraction osteogenesis of the mouse femur

BACKGROUND AND OBJECTIVE

Distraction osteogenesis (DO) is a mechanobiological process of producing new bone by gradual and controlled distraction of the surgically separated bone segments. Mice have been increasingly used to study the role of relevant biological factors in regulating bone regeneration during DO. However, there remains a lack of in silico DO models and related mechano-regulatory tissue differentiation algorithms for mouse bone. This study sought to establish an in silico model based on in vivo experimental data to simulate the bone regeneration process during DO of the mouse femur.

METHODS

In vivo micro-CT, including time-lapse morphometry was performed to monitor the bone regeneration in the distraction gap. A 2D axisymmetric finite element model, with a geometry originating from the experimental data, was created. Bone regeneration was simulated with a fuzzy logic-based two-stage (distraction and consolidation) mechano-regulatory tissue differentiation algorithm, which was adjusted from that used for fracture healing based on our in vivo experimental data. The predictive potential of the model was further tested with varied distraction frequencies and distraction rates.

RESULTS

The computational simulations showed similar bone regeneration patterns to those observed in the micro-CT data from the experiment throughout the DO process. This consisted of rapid bone formation during the first 10 days of the consolidation phase, followed by callus reshaping via bone remodeling. In addition, the computational model predicted a faster and more robust bone healing response as the model's distraction frequency was increased, whereas higher or lower distraction rates were not conducive to healing.

CONCLUSIONS

This in silico model could be used to investigate basic mechanobiological mechanisms involved in bone regeneration or to optimize DO strategies for potential clinical applications.

Overexpressed osteoactivin reduced osteoclastic callus resorption during distraction osteogenesis in mice

Distraction osteogenesis is a widely used surgical technique to treat bone deformity and shortening. Several biological treatments have been studied to enhance bone formation during distraction osteogenesis in animals. However, role of osteoactivin in the osseous tissues during distraction osteogenesis remains poorly understood. In this animal experimental study, we investigated the spatiotemporal expression of osteoactivin by immunohistochemistry and real-time PCR using a mouse model for tibial lengthening. Furthermore, to address the role of osteoactivin in bone lengthening, we subjected the osteoactivin-transgenic mice to distraction osteogenesis model. During the lag phase, the fibroblast-like cells (possible progenitors of the osteoblasts or chondrocytes), which mainly express osteoactivin, were infiltrated into the osteotomy site. Osteoactivin was ubiquitously expressed in the lengthened segment during the distraction and consolidation phases. Consistent with the immunohistochemical analysis, the levels of the osteoactivin transcripts in the tibias were significantly increased throughout the distraction osteogenesis process. The bone mineral content in the osteoactivin-transgenic mice calculated using peripheral quantitative computed tomography was also significantly increased at the remodeling zone. The histomorphometric analysis revealed that newly formed callus resorption in the remodeling zone was significantly reduced but bone formation was not altered in the osteoactivin-transgenic mice. We conclude that osteoactivin functions as an inhibitor of callus resorption during the consolidation phase of distraction osteogenesis.

An Efficient and Reproducible Protocol for Distraction Osteogenesis in a Rat Model Leading to a Functional Regenerated Femur

This protocol describes the use of a newly developed external fixator for distraction osteogenesis in a rat femoral model. Distraction osteogenesis (DO) is a surgical technique leading to bone regeneration after an osteotomy. The osteotomized extremities are moved away from each other by gradual distraction to reach the desired elongation. This procedure is widely used in humans for lower and upper limb lengthening, treatment after a bone nonunion, or the regeneration of a bone defect following surgery for bone tumor excision, as well as in maxillofacial reconstruction. Only a few studies clearly demonstrate the efficiency of their protocol in obtaining a functional regenerated bone, i.e., bone that will support physiological weight-bearing without fracture after removal of the external fixator. Moreover, protocols for DO vary and reproducibility is limited by lack of information, making comparison between studies difficult. The aim of this study was to develop a reproducible protocol comprising an appropriate external fixator design for rat limb lengthening, with a detailed surgical technique that permits physiological weight-bearing by the animal after removal of the external fixator.

Bilateral Continuous Automated Distraction Osteogenesis: Proof of Principle

The purpose of this study was to demonstrate that automated, continuous, curvilinear distraction osteogenesis (DO) in a minipig model is effective when performed bilaterally, at rates up to 3 mm/day, to achieve clinically relevant lengthening. A Yucatan minipig in the mixed dentition phase underwent bilaterally, at a continuous DO at a rate of 2 mm/day at the center of rotation; 1.0 and 3.0 mm/day at the superior and inferior regions, respectively. The distraction period was 13 days with no latency period. Vector and rate of distraction were remotely monitored without radiographs, using the device sensor. After fixation and euthanasia, the mandible and digastric muscles were harvested. The ex vivo appearance, stability, and radiodensity of the regenerate were evaluated using a semiquantitative scale. Percent surface area (PSA) occupied by bone, fibrous tissue, cartilage, and hematoma were calculated using histomorphometrics. The effects of DO on the digastric muscles and mandibular condyles were assessed via microscopy, and degenerative changes were quantified. The animal was distracted to 21 mm and 24 mm on the right and left sides, respectively. Clinical appearance, stability, and radiodensity were scored as "3" bilaterally indicating osseous union. The total PSA occupied by bone (right = 75.53 ± 2.19%; left PSA = 73.11 ± 2.18%) approached that of an unoperated mandible (84.67 ± 0.86%). Digastric muscles and condyles showed negligible degenerative or abnormal histologic changes. This proof of principle study is the first report of osseous healing with no ill-effect on associated soft tissue and the mandibular condyle using bilateral, automated, continuous, and curvilinear DO at rates up to 3 mm/day. The model approximates potential human application of continuous automated distraction with a semiburied device.

Conditioned media from hypoxic-cultured human dental pulp cells promotes bone healing during distraction osteogenesis

Distraction osteogenesis (DO) is a surgical procedure used to correct various skeletal disorders. Improving the technique by reducing the healing time would be of clinical relevance. The aim of this study was to determine the angiogenic and regenerative potential of conditioned media (CMs) collected from human dental pulp cells (hDPCs) grown under different culture conditions. CM collected from cells under hypoxia was used to improve bone healing and the DO procedure in vivo. The angiogenic potentials of CMs collected from hDPCs grown under normoxic (−Nor) and hypoxic (−Hyp) conditions were evaluated by quantitative PCR (VEGF‐A, angiopoietin‐1, angiopoietin‐2, interleukin‐6 (IL‐6) and CXCL12), ELISA assays (VEGF‐A, Ang‐2), tube‐formation and wound‐healing assays, using human umbilical vein endothelial cells. The results demonstrated that hypoxic CM had significantly higher angiogenic potential than normoxic CM. Human fetal osteoblasts (hFOBs) were exposed to CM, followed by alizarin red staining, to assess the osteogenic potential. It was found that CM did not enhance the mineralization capacity of hFOBs. DO was performed in the tibiae of 30 mice, followed by a local injection of 20 µl CM (CM–Nor and CM–Hyp groups) or serum‐free DMEM (control group) into the distraction zone every second day. The mice were sacrificed at days 13 and 27. The CM–Hyp treatment revealed a higher X‐ray density than the control group (p < 0.05). Our study suggests that the angiogenic effect promoted by hypoxic culture conditions is dependent on VEGF‐A and Ang‐2 released from hDPCs. Furthermore, CM–Hyp treatment may thus improve the DO procedure, accelerating bone healing. © 2015 The Authors. Journal of Tissue Engineering and Regenerative Medicine published by John Wiley & Sons, Ltd.

Endochondral fracture healing with external fixation in the Sost knockout mouse results in earlier fibrocartilage callus removal and increased bone volume fraction and strength

Sclerostin deficiency, via genetic knockout or anti-Sclerostin antibody treatment, has been shown to cause increased bone volume, density and strength of calluses following endochondral bone healing. However, there is limited data on the effect of Sclerostin deficiency on the formative early stage of fibrocartilage (non-bony tissue) formation and removal. In this study we extensively investigate the early fibrocartilage callus. Closed tibial fractures were performed on Sost(-/-) mice and age-matched wild type (C57Bl/6J) controls and assessed at multiple early time points (7, 10 and 14days), as well as at 28days post-fracture after bony union. External fixation was utilized, avoiding internal pinning and minimizing differences in stability stiffness, a variable that has confounded previous research in this area. Normal endochondral ossification progressed in wild type and Sost(-/-) mice with equivalent volumes of fibrocartilage formed at early day 7 and day 10 time points, and bony union in both genotypes by day 28. There were no significant differences in rate of bony union; however there were significant increases in fibrocartilage removal from the Sost(-/-) fracture calluses at day 14 suggesting earlier progression of endochondral healing. Earlier bone formation was seen in Sost(-/-) calluses over wild type with greater bone volume at day 10 (221%, p<0.01). The resultant Sost(-/-) united bony calluses at day 28 had increased bone volume fraction compared to wild type calluses (24%, p<0.05), and the strength of the fractured Sost(-/-) tibiae was greater than that that of wild type fractured tibiae. In summary, bony union was not altered by Sclerostin deficiency in externally-fixed closed tibial fractures, but fibrocartilage removal was enhanced and the resultant united bony calluses had increased bone fraction and increased strength.

Induction of fully stabilized cortical bone defects to study intramembranous bone regeneration

Bone is a regenerative tissue with an innate ability to self-remodel in response to environmental stimuli and the need to repair damage. Rodent models of fracture healing, and in particular genetic mouse models, can be used to study the contributions of specific molecular switches to skeletal repair, as well as to recreate and exacerbate biological development and repair mechanisms in postnatal skeletons. Here, we describe methodology for producing fully stabilized, single-cortex defects in mouse femurs to study mechanisms of intramembranous bone regeneration.

New bone formation enhanced by ADSCs overexpressing hRunx2 during mandibular distraction osteogenesis in osteoporotic rabbits

Promoting new bone formation during distraction osteogenesis (DO) in elderly patients with osteoporosis is still a challenge. In this study, we investigated the effect of gene therapy using local Runt-related gene 2 on new bone formation during osteoporotic mandibular DO in rabbits. First, we successfully established a mandibular osteoporotic animal model by ovariectomizing rabbits. Second, the right mandibles of the osteoporotic rabbits were distracted after corticotomy. The distraction gap of the rabbits in Group A2 and B2 were injected with Adv-hRunx2-GFP-transfected adipose-derived stromal cells (ADSCs) and Adv-GFP-transfected ADSCs, respectively. Rabbits in Groups C2 (ovariectomized control) and D2 (sham surgery control) were injected with physiologic saline. New-generation bone tissue in the distraction gap was analyzed via plain radiographic examinations, micro-computed tomography, histological examinations, and biomechanical testing at weeks 3, 6, and 9 of the consolidation period. Results of above examinations showed that no ideal new bone formation was observed in Groups B2 and C2, but obvious ideal new bone formation was observed in Group A2 and D2. The results suggested that gene therapy using rhRunx2-modified ADSCs promoted new bone formation during osteoporotic mandibular DO and effectively compensated for the detrimental effects of systemic osteoporosis on new bone formation.

Stem cell-conditioned medium accelerates distraction osteogenesis through multiple regenerative mechanisms

Distraction osteogenesis (DO) successfully induces large-scale skeletal tissue regeneration, but it involves an undesirably long treatment period. A high-speed DO mouse model (H-DO) with a distraction speed twice that of a control DO model failed to generate new bone callus in the distraction gap. Here we demonstrate that the local administration of serum-free conditioned medium from human mesenchymal stem cells (MSC-CM) accelerated callus formation in the mouse H-DO model. Secretomic analysis identified factors contained in MSC-CM that recruit murine bone marrow stromal cells (mBMSCs) and endothelial cells/endothelial progenitor cells (EC/EPCs), inhibit inflammation and apoptosis, and promote osteoblast differentiation, angiogenesis, and cell proliferation. Functional assays identified MCP-1/-3 and IL-3/-6 as essential factors in recruiting mBMSCs and EC/EPCs. IL-3/-6 also enhanced the osteogenic differentiation of mBMSCs. MSC-CM that had been depleted of MCP-1/-3 failed to recruit mBMSCs, and consequently failed to promote callus formation. Taken together, our data suggest that MSCs produce a broad repertoire of trophic factors with tissue-regenerative activities that accelerate healing in the DO process.

Skeletal and soft tissue response to automated, continuous, curvilinear distraction osteogenesis

PURPOSE

To document the bone formation and soft tissue changes in response to automated, continuous, curvilinear distraction osteogenesis (DO) at rates greater than 1 mm/day in a minipig model.

MATERIALS AND METHODS

Two groups of Yucatan minipigs underwent automated, continuous, curvilinear DO of the right mandible: group A, 1.5 mm/day (n = 5); and group B, 3.0 mm/day (n = 5). Each minipig underwent 12 mm of distraction followed by 24 days of fixation. The distracted and contralateral mandibles were harvested at the end of fixation. The percentage of surface area (PSA) of the regenerate occupied by bone, fibrous tissue, cartilage, and hematoma was determined using computerized histomorphometric analysis. The control groups consisted of DO wounds distracted discontinuously at 1 mm/day and the nonoperated contralateral mandible. The ipsilateral and contralateral digastric muscles were harvested and stained for proliferating cell nuclear antigen (PCNA), myogenic differentiation-1 (MyoD), and paired Box 7 protein (PAX7).

RESULTS

All 10 minipigs completed the distraction and fixation period. The PSA occupied by bone was similar for groups A (PSA 64.36% ± 5.87%) and B (PSA 63.83% ± 3.37%) and the control group (1 mm/day; PSA 64.89% ± 0.56%) but was less than that on the nonoperated side (PSA 84.67% ± 0.86%). The PSA occupied by cartilage and hematoma in all groups was minimal (<1.1%). The digastric muscles had no abnormal tissue or inflammation, and PAX7, MyoD, and PCNA expression had returned to the baseline levels.

CONCLUSIONS

The results of the present study indicate that bone formation in response to automated, continuous, and curvilinear DO at a rate of 1.5 and 3.0 mm/day is nearly identical to that with discontinuous DO at 1 mm/day. In addition, no deleterious effects were found on the digastric muscles.

The effect of heparan sulfate application on bone formation during distraction osteogenesis

Bone morphogenetic proteins (BMPs) are recognized for their ability to induce bone formation in vivo and in vitro. Their osteogenic and osteoinductive properties are tightly regulated by the secretion of specific BMP antagonists, which have been shown to physically bind and sometimes be blocked by the extracellular proteoglycan heparan sulphate side chains (from hereon referred to as HS). The purpose of this study was to investigate if local application of 5 µg of HS proteoglycan to a bone regenerate site in a mouse model of distraction osteogenesis (DO) can accelerate bone healing and affect the expression of key members of the BMP signaling pathway. DO was performed on the right tibia of 115 adult male wild-type mice. At mid-distraction (day 11), half the group was injected locally with 5 µg of HS, while the other half was injected with saline. The mice were sacrificed at 2 time-points: mid-consolidation (34 days) and full consolidation (51 days). The distracted tibial zone was then collected for analysis by μCT, radiology, biomechanical testing, immunohistochemistry, and histology. While μCT data showed no statistically significant difference in bone formation, the results of biomechanical testing in stiffness and ultimate force were significantly lower in the HS-injected bones at 51 days, compared to controls. Immunohistochemistry results also suggested a decrease in expression of several key members of the BMP signaling pathway at 34 days. Furthermore, wound dehiscence and infection rates were significantly elevated in the HS group compared to the controls, which resulted in a higher rate of euthanasia in the treatment group. Our findings demonstrate that exogenous application of 5 µg of HS in the distracted gap of a murine model had a negative impact on bone and wound healing.

Angiogenesis and myogenesis in mouse tibialis anterior muscles during distraction osteogenesis: VEGF, its receptors, and myogenin genes expression

Angiogenesis and myogenesis occur in the surrounding skeletal muscles following distraction osteogenesis, but their molecular mechanisms remain unclear. The present study investigated morphological features of lengthened muscles and the time course change of vascular endothelial growth factor (VEGF), its receptors (VEGFR-1 and VEGFR-2) and myogenin gene expression profiles related to angiogenesis and myogenesis in tibialis anterior (TA) muscles with a mouse model of distraction osteogenesis, which involves 1 week of waiting period (latency phase), 2 weeks of intermittent distraction (distraction phase), and 5 weeks of remodeling period (consolidation phase). Macroscopic findings showed that lengthened TA muscles increased to approximately 42% longer and 10% heavier at the end of the process when compared to pre-surgery. During the distraction phase, VEGF and its receptors were induced in the vascular endothelial cells, myogenin-positive satellite cells and myocytes, and subsequently, capillary progression and myogenesis were increased. Real-time RT-PCR showed that Vegf, Vegfr-1, Vegfr-2, and myogenin genes expression was enhanced during the muscle lengthening. Vegf and Vegfr-1 were upregulated following the recession of angiogenesis at the consolidation phase. We conclude that upregulation of VEGF and its receptors by mechanical tension-stress could be involved in the process of angiogenesis and myogenesis in lengthened muscles.

Stromal cell-derived factor-1 enhances distraction osteogenesis-mediated skeletal tissue regeneration through the recruitment of endothelial precursors

Distraction osteogenesis (DO) is a unique therapy that induces skeletal tissue regeneration without stem/progenitor cell transplantation. Although the self-regeneration property of DO provides many clinical benefits, the long treatment period required is a major drawback. A high-speed DO mouse model (H-DO), in which the distraction was done two times faster than in control DO (C-DO) mice, failed to generate new bone callus in the DO gap. We found that this was caused by the unsuccessful recruitment of bone marrow endothelial cells (BM-ECs)/endothelial progenitor cells (EPCs) into the gap. We then tested the ability of a local application of stromal cell-derived factor-1 (SDF-1), a major chemo-attractant for BM-ECs/EPCs, to accelerate the bone regeneration in H-DO. Our data showed that, in H-DO, SDF-1 induced callus formation in the gap through the recruitment of BM-ECs/EPCs, the maturation of neo-blood vessels, and increased blood flow. These results indicate that the active recruitment of endogenous BM-ECs/EPCs may provide a substantial clinical benefit for shortening the treatment period of DO.

Murine model of oligotrophic tibial nonunion

OBJECTIVES

This study aimed to use modified distraction osteogenesis techniques to develop a reliable mouse fracture nonunion model with an oligotrophic phenotype.

METHODS

Twenty-six 10- to 14-week-old C57BL/6 male mice underwent a proximal diaphyseal tibial osteotomy with a 2-mm bone resection. An external fixation device was applied to the tibia using cerclage wires. A total of 2.25 mm of distraction was applied over 3 days, resulting in an average distraction gap of 4.28 mm. Plain radiographs were taken at regular intervals until euthanasia at 7 (n = 9), 10 (n = 13), or 12 (n = 4) weeks. After euthanasia, all samples were fixed in formalin, scanned with microcomputed tomography, decalcified in formic acid, prepared in paraffin, and stained with Alcian blue/Mayer's hematoxylin.

RESULTS

In the distraction groups, five mice were prematurely euthanized as a result of wound complications stemming from loss of distal fixation. Of the remaining 21, two healed, resulting in a 90% nonunion rate. These nonunions radiographically resembled clinical nonunions with tapered, cone-like fracture ends and histologically demonstrated evidence of attempted healing as seen with cartilage capping. Additionally, the plain radiographic appearance of those nonunions from mice euthanized at 10 and 12 weeks did not change over the final 4 to 6 weeks.

CONCLUSIONS

The use of 2-mm tibial resection osteotomy with 2-mm distraction provides a predictable model for fracture nonunion in mice with the oligotrophic phenotype closely resembling the clinical correlate. This model offers a promising means for characterization of the molecular events that occur during the development of fracture nonunion and for evaluation of noninvasive methods of nonunion rescue.

Serial histologic and immunohistochemical changes in anterior digastric myocytes in response to distraction osteogenesis

PURPOSE

To document histologic and immunohistochemical changes in the anterior digastric muscle during distraction osteogenesis (DO).

MATERIALS AND METHODS

Nineteen Yucatan minipigs with mixed dentition were used for these experiments. Group A (n = 16) underwent unilateral mandibular distraction at a rate of 1 mm/day (no latency) for 12 days. Animals were killed at mid-DO (n = 5), end-DO (n = 5), mid-fixation (n = 4), and end-fixation (n = 2). Group B (n = 2) underwent acute 12-mm advancement, and group C (n = 1) dissection and osteotomy. Animals from groups B and C were killed at the end-DO time point. Digastric muscles from treatment and contralateral sides of all animals were harvested and embedded in paraffin. Specimens were stained with hematoxylin/eosin or immunohistochemically for proliferating cell nuclear antigen (PCNA; total cell proliferation), paired Box-7 gene protein (Pax7; satellite cells), or myogenic differentiation 1 protein (MyoD; differentiating myoblasts). Descriptive and bivariate statistics were computed to compare groups (P ≤ .05 statistically significant).

RESULTS

All animals survived the operation and observation period; there were no device failures. Two animals (1 at mid-DO, 1 at mid-fixation) were eliminated from the study because of postoperative infection. There was minimal digastric inflammation, fibrosis, and muscle fiber size variability during active DO. Immunohistochemical analysis showed statistically significant increases in PCNA (cellular proliferation), Pax7 (satellite cells), and MyoD (differentiating myoblasts) positive nuclei in digastrics at mid-DO and end-DO.

CONCLUSIONS

Results of this study indicate that there are minimal pathologic changes but significant increases in PCNA, Pax7, and MyoD positive nuclei during active distraction. This supports the hypothesis that the digastric muscle response to DO consists of proliferation and hypertrophy.

Histomorphometric analysis of the porcine mandibular distraction wound

PURPOSE

To analyze the sequence of histomorphometric changes in the regenerate during distraction osteogenesis (DO) of the minipig mandible.

MATERIALS AND METHODS

A total of 16 minipigs underwent unilateral mandibular DO using a protocol of 0-day latency and a 1-mm/day rate for 12 days, and 24 days of fixation. The mandibles were harvested at mid-DO, end-DO, mid-fixation, and end-fixation. An additional 2 minipigs underwent acute lengthening, and 1 sham control was included. Serial gross examinations and plain radiographs were performed before paraffin embedding. The sections were stained with hematoxylin-eosin or hematoxylin/alcian blue/sirius red stain. Histomorphometric analysis was performed to determine the percentage of surface area (PSA) occupied by hematoma, fibrous tissue, cartilage, and bone.

RESULTS

All 19 minipigs survived the operation, and 17 survived the observation period; 2 were killed because of infection (mid-DO, n = 1 and end-fixation, n = 1). No device failures occurred. Of the 17 specimens, 4 were at mid-DO, 4 at end-DO, 4 at mid-fixation, and 2 at end-fixation; 2 were in the acute lengthening group, and 1 was the sham control. Hematoma was present only at mid-DO (16.61 +/- 8.07 PSA) and end-DO (1.17 +/- 2.33 PSA). Fibrous tissue decreased from mid-DO (53.12 +/- 8.59 PSA) to end-fixation (25.00 +/- 0.83 PSA). Cartilage was present in end-DO (1.72 +/- 2.71 PSA), mid-fixation (5.82 +/- 6.64 PSA), and acute lengthening (1.43 +/- 0.95 PSA). Bone increased from mid-DO (25.18 +/- 0.99 PSA) to end-fixation (64.89 +/- 0.79 PSA) and occurred earlier in the superior and middle thirds of the wounds. Periosteal bone formation predominated over endosteal bone formation early in distraction.

CONCLUSION

The results of the present study indicate that bone formation in this model consists of both intramembranous and endochondral components, with intramembranous osteogenesis predominating. Bone formation occurred earlier in the superior/middle portions of the wound, possibly owing to osteoinductive properties of developing tooth buds and the inferior alveolar nerve, respectively.

Restoration of regenerative osteoblastogenesis in aged mice: modulation of TNF

Skeletal changes accompanying aging are associated with both increased risk of fractures and impaired fracture healing, which, in turn, is due to compromised bone regeneration potential. These changes are associated with increased serum levels of selected proinflammatory cytokines, e.g., tumor necrosis factor alpha (TNF-alpha). We have used a unique model of bone regeneration to demonstrate (1) that aged-related deficits in direct bone formation can be restored to young mice by treatment with TNF blockers and (2) that the cyclin-dependent kinase inhibitor p21 is a candidate for mediation of the osteoinhibitory effects of TNF. It has been hypothesized recently that TNF antagonists may represent novel anabolic agents, and we believe that the data presented here represent a successful test of this hypothesis.

Short-term exercise in mice increases tibial post-yield mechanical properties while two weeks of latency following exercise increases tissue-level strength

We have previously shown that exercise during growth increases post-yield deformation in C57BL6/129 (B6;129) male tibiae at the expense of reduced pre-yield deformation and structural and tissue strength. Other research in the literature indicates that increased mineral content, cross-sectional geometry and structural strength due to exercise can be maintained or increased after exercise ends for as long as 14 weeks. It was therefore hypothesized that after our exercise protocol ended, effects of exercise on mechanical properties would persist, resulting in increased post-yield behavior and rescued strength versus age-matched control mice. Beginning at 8 weeks of age, exercise consisted of running on a treadmill (30 min/day, 12 m/min, 5 degrees incline) for 21 consecutive days. At the end of running and 2 weeks later, in the cortical bone of the tibial mid-diaphyses of B6;129 male mice, changes due to exercise and latency following exercise were assayed by mechanical tests and analyses of cross-sectional geometry. Exercise increased structural post-yield deformation compared with weight-matched control mice, without changes in bone size or shape, suggesting that exercised-induced changes in pre-existing bone quality were responsible. Over the 2-week latency period, no growth-related changes were noted in control mice, but exercise-induced changes resulted in increased tissue stiffness and strength versus mice sacrificed immediately after exercise ended. Our data indicate that periods of exercise followed by latency can alter strength, stiffness, and ductility of bone independent of changes in size or shape, suggesting that exercise may be a practical way to increase the quality of the bone extracellular matrix.

Altered fracture callus formation in chondromodulin-I deficient mice

Chondromodulin-I (Chm-I) is a glycoprotein that stimulates the growth of chondrocytes and inhibits angiogenesis in vitro. Mice lacking the Chm1 gene show abnormal bone metabolism and pathological angiogenesis in cardiac valves in the mature stage although they develop normally without aberrations in endochondral bone formation during embryogenesis or in cartilage development during growth. These findings indicate that Chm-I is critical under conditions of stress such as bone repair through endochondral ossification of a fracture callus. We carried out the present study to examine the expression and role of Chm-I in bone repair using a stabilized tibial fracture model, and compared fracture healing in Chm1 knockout (Chm1(-/-)) mice with that in wild-type mice. Chm-I mRNA and protein localized in the external cartilaginous callus in the reparative phase of fracture healing. Radiological examination showed a delayed union in Chm1(-/-) mice although the fracture site was covered with both external and internal calluses. Chm1 null mutation reduced external cartilaginous callus formation as judged by marked decrease of type X collagen alpha 1 (Col10a1) expression and the total amount of cartilage matrix. Interestingly, the majority of chondrocytes in the periosteal callus failed to differentiate into mature chondrocytes in Chm1(-/-) mice, while the hypertrophic maturation of chondrocytes between the cortices was not affected. These results suggest that Chm-I is involved in hypertrophic maturation of periosteal chondrocytes. Although a direct effect of Chm-I on bones is still unclear, bony callus formation was increased while external cartilaginous callus decreased in Chm1(-/-) mice. We conclude that in the absence of Chm1, predominant primary bone healing occurs due to an indirect effect induced by reduction of cartilaginous callus rather than to a direct effect on osteogenic function, resulting in a delayed union.

Development of a femoral non-union model in the mouse

OBJECTIVES

Advancements in our knowledge of fracture healing have occurred in large part by the understanding of this process on a microscopic level. The ability to develop experimental non-union models in animals will assist in the investigation of this problem and are likely to lead to novel treatments. We report on a technique for developing experimental non-unions in mice.

METHODS

Femoral fractures were created in 48 CD1 mice, 24 mice underwent standard closed femoral fractures, and 24 mice underwent creation of a femoral non-union through an open osteotomy and fracture devascularisation method. All fractures were subsequently rodded. Histological examinations of the fractures were then conducted at eight time points post-operatively.

RESULTS

The control group showed normal fracture healing with histological evidence of bony fracture bridging by 28 days and mature bony remodelling at 63 days. The non-union group showed delayed fracture healing at all time points and no evidence of bony healing at 63 days.

CONCLUSION

This is the first report of a reliable method to develop fracture non-union in mice. We believe this technique will be critical to further the investigation of fracture non-union in normal mice and provides the great advantage of using the plethora of transgenic and knockout mouse models to analyse non-union at the cell and molecular level.

Characterizing the BMP pathway in a wild type mouse model of distraction osteogenesis

Distraction osteogenesis (DO) is a well established surgical technique for limb lengthening and replacement of bone loss due to trauma, infection or malignancies. Although the technique is widely used, one of its limitations is the long period of time required for the newly formed bone to consolidate. We have previously shown that exogenous application of bone morphogenetic proteins (BMPs) can increase bone formation during DO, however, exogenous BMPs have many drawbacks. An alternative method for accelerating the rate of bone formation may be to modulate the intrinsic BMP signaling pathway. The aim of the current study was to analyze the expression of various genes involved in the BMP pathway at various time periods during DO in order to identify potential targets for therapeutic manipulation. DO was applied to the right tibia of 80 adult wild type mice. Distraction began after a latency period of 5 days at a rate of 0.2 mm/12 h for 2 weeks. Mice were sacrificed in groups of 12 at the following times post surgery: day 5 (latency), days 11 and 17 (distraction) and days 34 and 51 (consolidation). Specimens were examined using radiology, microCT, histology, RT(2)PCR, immunohistochemistry and Western analysis. Genes involved in the BMP pathway including the BMP ligands, receptors, antagonists and downstream effectors were examined. A significant upregulation of BMPs 2, 4 and 6 was observed using both PCR and immunohistochemistry during the distraction phase. The expression of BMP7 remained constant throughout the distraction and consolidation process. Surprisingly, the only receptors which were upregulated significantly were the Activin Receptor Type 1 (ActR1) during distraction and Activin Receptor Type 2b (ActR2b) during consolidation. Most interestingly, simultaneously with the ligands, an increase in the expression of the antagonists, Noggin, Chordin, Inhibin and BMP3 was observed. This study provides a clearer understanding of expression patterns during DO, which is a valuable resource for finding therapeutic options to stimulate bone formation. The results suggest that blocking BMP inhibitors may be a possible method for increasing the function of intrinsic growth factors involved in bone regeneration.

Bone formation during distraction osteogenesis is dependent on both VEGFR1 and VEGFR2 signaling

INTRODUCTION

Distraction osteogenesis (DO) is characterized by the induction of highly vascularized new bone formation through an intramembranous process largely devoid of the formation of cartilage.

MATERIALS AND METHODS

To test the hypothesis that DO is strictly dependent on vascualrization, we inhibited vascular endothelial growth factor (VEGF) activity by antibody blockade of both receptors VEGFR1 (Flt-1) and VEGFR2 (Flk-1) or only VEGFR2 (Flk-1) in a previously developed murine tibia DO model. During normal DO, VEGFR1 (Flt-1), VEGFR2 (Flk-1), VEGFR3 (Flt4) and all four VEGF ligand (A, B, C, and D) mRNAs are induced.

RESULTS

The expression of mRNA for the receptors generally paralleled those of the ligands during the period of active distraction. Bone formation, as assessed by muCT, showed a significant decrease with the double antibody treatment and a smaller decrease with single antibody treatment. Vessel volume, number, and connectivity showed progressive and significant inhibition in all of these of parameters between the single and double antibody blockade. Molecular analysis showed significant inhibition in skeletal cell development with the single and double antibody blockade of both VEGFR1 and 2. Interestingly, the single antibody treatment led to selective early development of chondrogenesis, whereas the double antibody treatment led to a failure of both osteogenesis and chondrogenesis.

CONCLUSIONS

Both VEGFR1 and VEGFR2 are functionally essential in blood vessel and bone formation during DO and are needed to promote osteogenic over chondrogenic lineage progression.

Mechanical force-induced midpalatal suture remodeling in mice

Mechanical stress is an important epigenetic factor for regulating skeletal remodeling, and application of force can lead to remodeling of both bone and cartilage. Chondrocytes, osteoblasts and osteoclasts all participate and interact with each other in this remodeling process. To study cellular responses to mechanical stimuli in a system that can be genetically manipulated, we used mouse midpalatal suture expansion in vivo. Six-week-old male C57BL/6 mice were subjected to palatal suture expansion by opening loops with an initial force of 0.56 N for the periods of 1, 3, 5, 7, 14 or 28 days. Periosteal cells in expanding sutures showed increased proliferation, with Ki67-positive cells representing 1.8+/-0.1% to 4.5+/-0.4% of total suture cells in control groups and 12.0+/-2.6% to 19.9+/-1.2% in experimental/expansion groups (p<0.05). Starting at day 1, cells expressing alkaline phosphatase and type I collagen were seen. New cartilage and bone formation was observed at the oral edges of the palatal bones at day 7; at the nasal edges only bone formation without cartilage appeared to occur. An increase in osteoclast numbers suggested increased bone remodeling, ranging from 60 to 160% throughout the experimental period. Decreased Saffranin O staining after day 3 suggested decreased proteoglycan content in the secondary cartilage. Micro-CT showed a significant increase in maxillary width at days 14 and 28 (from 2334+/-4 microm to 2485+/-3 microm at day 14 and from 2383+/-5 microm to 2574+/-7 microm at day 28, p<0.001). The suture width was increased at days 14 and 28, except in the oral third region at day 28 (from 48+/-5 microm to 36+/-4 microm, p<0.05). Bone volume/total volume was significantly reduced at days 14 and 28 (50.2+/-0.7% vs. 68.0+/-3.7% and 56.5+/-1.0% vs. 60.9+/-1.3%, respectively, p<0.05), indicative of increased bone marrow space. These findings demonstrate that expansion forces across the midpalatal suture promote bone resorption through activation of osteoclasts and bone and cartilage formation via increased proliferation and differentiation of periosteal cells. Mouse midpalatal suture expansion would be useful in further studies of the ability of mineralized tissues to respond to mechanical stimulation.

Histomorphometrical analysis of new bone obtained by distraction osteogenesis and osteogenesis by periosteal distraction in rabbits

The aim of this study was to compare histomorphometrically the new bone tissue obtained using two different distraction methods, and evaluate these two methods in terms of their advantages and disadvantages. In 36 New Zealand adult male rabbits, divided into two groups, a gradual distraction was performed using a device placed on the lateral surface of the mandibular corpus. In one group osteotomy was not performed and osteogenesis by periosteal distraction (OPD) only was used. In the other group, conventional distraction osteogenesis (DO) was performed. After a 7-day latent phase, the same distraction protocol was applied to both groups. Each group of rabbits was further divided into three sub-groups killed on the 15th, 30th and 60th days of the consolidation period, and histological analysis was performed. The mean extent of newly formed bone tissue was 14.4 mm2 in the OPD groups and 25.4 mm2 in the DO groups. When compared statistically, there were significant differences between all the DO and OPD sub-groups. The newly formed bone tissue obtained by OPD was rich in interstitial fatty tissue. These results indicate that bone tissue newly formed by OPD is not suitable for occlusal forces.

A novel mouse model for the study of the inhibitory effects of chronic ethanol exposure on direct bone formation

Excessive alcohol consumption has been reported to interfere with human bone homeostasis and repair in multiple ways. Previous studies have demonstrated that chronic ethanol exposure in the rat via an intragastric dietary delivery system inhibits direct bone formation during distraction osteogenesis (DO, limb lengthening). The opportunity to extend the rat ethanol studies to mice is now possible due to the development of mouse models of DO. This study employed a novel combination of liquid ethanol diet delivery and a murine DO model to test the hypothesis that chronic ethanol exposure would result in deficits in direct bone formation during DO in contrast to the pair-fed controls. Twenty-eight 12-month-old C57BL/6 male mice were acclimated to the Lieber-DeCarli liquid control diet #710027 (Dyets Inc.) over a 1-week period. The mice were separated into two diet groups (n=14/group): pair-fed control and ethanol (diet #710260). After being on diet for 82 days, all mice underwent placement of an external fixator and osteotomy on the left tibia. Following a 6-day latency period, distraction began at a rate of 0.075 mm twice a day (b.i.d.) for 14 days. The weight changes were equivalent for both groups. The hypothesis that chronic ethanol exposure would inhibit direct bone formation and produce skeletal toxicity was supported by radiographic (P=.011) and histologic (P=.002) analyses of the % new bone formation in the DO gaps, by peripheral quantitative computed tomography analysis of the total volumetric bone mineral density of the contralateral proximal tibias (P<.001) and contralateral femoral necks (P=.012), by three-point bending on the contralateral tibias (P<.001 energy to break), by pin site bone formation measures (P<.001), and by ethanol-associated increased adipocyte area (adjacent to the gap) percentages (P<.002). We conclude that this model can be used to study the mechanisms underlying inhibition of bone formation by chronic ethanol exposure and to test preclinical interventions.

Histology of the porcine mandibular distraction wound

The purpose of this study was to document the progression and pattern of endosteal bone formation in a porcine mandibular distraction wound. Bone formation was assessed in a 0-day latency model (n=24 pigs) using distraction rates of 1, 2, or 4 mm/day to create a 12-mm gap. Macro-radiographs and sagittal histologic sections, from the center of the mandible, were evaluated by computer morphometrics (% bone fill) and by a semi-quantitative bone formation score. Mean percent area of new bone was 12.4% (0-25.9%), 7.5% (0-21.3%) and 3.8% (0-10.5%) in mandibles distracted at 1, 2 or 4 mm/day respectively. At all time points, percent area of new bone was highest in mandibles distracted at 1 mm/day. Bone was deposited from the margins of the osteotomy toward the center of the wound and occurred first around the inferior alveolar canal and tooth bud regions. New bone formed by intramembranous ossification alone. The results of this experiment document the contribution of endosteal bone formation in this model of distraction osteogenesis.

Distraction osteogenesis in the Cbfa-1+/- mouse

Distraction osteogenesis involves division of a bone and gradually pulling the bone ends apart. This delivers mechanical stimulation to mesenchymal cells in the distraction gap, where new bone is regenerated predominantly by intramembranous ossification. The transcription factor Cbfa1 has been reported to be essential for the differentiation of mesenchymal cells to osteoblasts. In homozygous Cbfa1 knockout mice, both intramembranous and endochondral ossification mechanisms are blocked and no bone formation occurs. In heterozygous Cbfa1 knockout mice, only the cranial bones and the clavicles, which form through intramembranous ossification, fail to develop properly as in the human condition of cleidocranialdysostosis. It has been suggested, therefore, that intramembranous ossification is affected by the absence of one of the paired Cbfa1 genes. We have assessed the potential for intramembranous ossification following distraction osteogenesis in heterozygous Cbfa1 knockout mice. Fourteen skeletally mature male heterozygous mice were used, together with 10 wild-type controls. The tibia was distracted by 0.25 mm twice a day (0.5 mm/day) for 10 days using the half-ring type fixator. Nine mice were kept for a further 28 days to observe the consolidation phase. In four out of five mice of the heterozygous group and in all three wild-type mice, bony fusion within the distraction gap was observed on radiographs. Histological findings were almost the same in the two groups at various stages of the procedure and intramembranous ossification was predominant in both the groups. Despite the inhibition of intramembranous ossification during the foetal development of Cbfa1+/- mice, distraction osteogenesis was as successful as in control mice.

Creation and characterization of a mouse model of mandibular distraction osteogenesis

While the histological and ultrastructural changes associated with distraction osteogenesis have been extensively characterized using various animal models, the molecular mechanisms governing this technique remain poorly understood. In the current study, for the first time, we describe a mouse mandibular distraction osteogenesis model. Development of this model will allow assessment of factors involved in normal vs. abnormal healing (especially in non-unions) of craniofacial skeletal elements. Complete osteotomies were created on the right hemimandibles of 51 adult male CD-1 mice and customized distraction devices attached. Thirty-three animals underwent gradual distraction (5 days latency, distraction at 0.2 mm BID x 8 days, 28 days consolidation), while the remaining 18 mice underwent acute lengthening (immediate distraction to 3.2 mm) at the time of surgery. Mandibles were harvested at time points corresponding to the latent (POD 5), distraction (POD 9, 13), and consolidation (POD 28, 41) periods and processed for histological or quantitative real-time RT-PCR analysis. Specimens from each group were processed for microCT analysis. Histological and radiological data demonstrated that all mandibles undergoing gradual distraction achieved complete bony union by the end of consolidation, while those undergoing acute lengthening formed a fibrous non-union. Quantitative real-time RT-PCR demonstrated upregulation of mRNA for VEGF, FGF-2, collagen I, and osteopontin during gradual distraction but not during acute lengthening. These data validate our novel mouse mandibular distraction model and demonstrate its utility in elucidating the molecular mechanisms regulating bone formation during distraction osteogenesis as compared to those that are expressed during the formation of fibrous non-unions.

The role of angiogenesis in a murine tibial model of distraction osteogenesis

Distraction osteogenesis (DO) is one of the most dramatic in vivo applications of mechanical stimulation as a means of inducing bone regeneration. A simple and reproducible murine model of tibia distraction osteogenesis was developed using a monolateral fixator. Bone formation was assessed histologically over a 35-day time course. The steady state expression of a broad family of angiogenesis-associated genes was assessed by microarray hybridization analyses over the same time course, while the immediate gene response that was induced during each cycle of distraction was assessed at 30 min and 8 h after the first and last rounds of activation of the fixator. Distraction osteogenesis promoted new bone formation primarily through an intramembranous process with maximal osteogenesis during the active distraction period. Histological analysis also showed that dense cortical bone continued to be formed, during the consolidation phase, for 2 weeks after distraction ended. The analysis of steady state mRNA expression levels over the time course of DO showed that VEGF-A and neuropilin, an alternate receptor for VEGF-A, both angiopoietin (Ang) 1 and 2 factors, and the Ang receptor Tie2 were the critical angiogenic factors during DO. A key transcriptional regulator of many of the angiogenic factors, hypoxia-induced factor1alpha (Hif-1a), the FGF binding protein pleiotropin/OSF1, and multiple MMP(s), were also induced during the active distraction period. Examination of the expression of angiogenic factors that were induced after each cycle of activation, demonstrated that Hif-1a, Nrp1, and VEGF-A were all cyclically induced after each increment of distraction. These results suggest that these factors are early mediators that are produced by distraction and contribute toward the processes that promote bone formation. These experiments represent the first step in defining the molecular mechanisms that regulate skeletal regeneration and the functional relationship between angiogenesis and osteogenesis during distraction osteogenesis.

Skeletal toxicity associated with chronic ethanol exposure in a rat model using total enteral nutrition

Chronic alcohol abuse decreases bone mass, inhibits osteoblast differentiation and function, increases fracture incidence, and delays fracture healing. Four studies were designed to use intragastric ethanol delivery as part of a total enteral nutrition (TEN) system to determine the negative systemic effects of chronic ethanol on 1) the rat skeleton and 2) local rapid bone formation during limb lengthening (distraction osteogenesis, DO). In study 1, three-point bending tests demonstrated that after 75 days of ethanol exposure, the tibiae had significantly lower load to failure versus control diet (p = 0.0006) or ad libitum chow-fed rats (p = 0.0029). Study 2 examined alcohol's effects on the density and cross-sectional area of the proximal tibial metaphysis using peripheral quantitative computed tomography and found that after 25 days of ethanol exposure the trabecular volumetric bone mineral density (p = 0.011) and cortical cross-sectional area (p = 0.011) were lower compared with controls. In study 3, a comparison of distracted tibial radiographs and histological sections demonstrated ethanol-related decreases in both gap mineralization (p = 0.03) and bone column formation (p = 0.01). Histological comparisons in study 4 reproduced the ethanol-related deficits in new bone formation during DO (p = 0.001). These results indicate that the TEN system is a viable model to study ethanol's effects on the skeleton and that chronic ethanol delivery via TEN decreases trabecular bone density, cortical area, and mature bone strength. Also, the DO studies demonstrate, for the first time, that chronic ethanol inhibits rapid bone formation during limb lengthening.