Quantitative Assessment of Bone Tissue Mineralization with Polychromatic Micro-Computed Tomography

Micro-computed tomography (muCT) has become an important tool for morphological characterization of cortical and trabecular bone. Quantitative assessment of bone tissue mineral density (TMD) from muCT images may be possible; however, the methods for calibration and accuracy have not been thoroughly evaluated. This study investigated hydroxyapatite (HA) phantom sampling limitations, short-term reproducibility of phantom measurements, and accuracy of TMD measurements by correlation to ash density. Additionally, the performance of a global and a local threshold for determining TMD was tested. The full length of a commercial density phantom was imaged by muCT, and mean calibration parameters were determined for a volume of interest (VOI) at 10 random positions along the longitudinal axis. Ten different VOI lengths were used (0.9-13 mm). The root mean square error (RMSE) was calculated for each scan length. Short-term reproducibility was assessed by five repeat phantom measurements for three source voltage settings. Accuracy was evaluated by imaging rat cortical bone (n = 16) and bovine trabecular bone (n = 15), followed by ash gravimetry. Phantom heterogeneity was associated with <0.5% RMSE. The coefficient of variation for five repeat measurements was generally <0.25% across all energies and phantom densities. Bone mineral content was strongly correlated to ash weight (R (2) = 1.00 for both specimen groups and both threshold methods). Ash density was well correlated for the trabecular bone specimens (R (2) > 0.80). In cortical bone specimens, the correlation was somewhat weaker when a global threshold was applied (R (2) = 0.67) compared to the local threshold method (R (2) = 0.78).     

Polyethylene Particle-Induced Bone Resorption in Substance P-Deficient Mice

Aseptic loosening is the major cause of total joint replacement failure. Substance P (SP) is a neurotransmitter richly distributed in sensory nerve fibers, bone, and bone-related tissue. The purpose of this study was to investigate the potential impact of SP on bone metabolism in polyethylene particle-induced osteolysis. We utilized the murine calvarial osteolysis model based on ultrahigh molecular weight polyethylene (UHMWPE) particles in 14 wild-type mice (C57BL/J6) and 14 SP-deficient mice. Group 1 (C57BL/J 6) and group 3 (SP-knockout) received sham surgery, and group 2 (C57BL/J6) and group 4 (SP-knockout) were treated with polyethylene particles. Analytical methods included three-dimensional micro-computed tomographic (micro-CT) analysis and histomorphometry. Bone resorption was measured within the midline suture. The number of osteoclasts was determined by counting the tartrate-resistant acid phosphatase-positive cells. UHMWPE-particle treated SP-deficient mice showed significantly reduced osteolysis compared to wild-type mice, as confirmed by histomorphometry (P < 0.001) and micro-CT (P = 0.035). Osteoclast numbers were significantly reduced in groups 3 and 4 compared to groups 1 and 2 (P < 0.001). Unexpectedly, SP-deficient mice (group 3) showed a significantly increased absolute bone mass compared to wild-type mice (group 1) (P = 0.02). The findings of our murine calvaria model lead to the assumption that SP is a promoter in particle-induced osteolysis. The pathophysiology of aseptic loosening is complex, and neuropeptides are not solely responsible for the progress of implant loosening; however, we conclude that there could be coherence between neurotransmitters and particle-induced osteolysis in patients with aseptic loosening.     

Three-Dimensional Surface Deviation Maps for Analysis of Retrieved Polyethylene Acetabular Liners Using Micro-Computed Tomography

Micro-computed tomography (micro-CT) has previously been validated for measuring wear volume in polyethylene acetabular liners. The creation of 3-dimensional (3D) articular and backside surface deviation maps using micro-CT is described. An acetabular liner was retrieved after 16.7 years of implantation and scanned with micro-CT along with a new, never-implanted liner of the same size and type. The liner surface geometries were reconstructed and co-aligned. A 3D comparison and cross-sectional analysis was performed. Maximum 3D deviation of the articular surface was −2.48 ± 0.02 mm, with maximum backside deviation of 0.46 ± 0.02 mm. Micro-CT can measure surface deviation and therefore calculate the volume of wear plus creep of retrieved acetabular liners, and may be applicable for wear simulator studies and analyzing other polyethylene components including tibial inserts.     

Quantitative Micro-Computed Tomography Imaging of Vascular Dysfunction in Progressive Kidney Diseases

Progressive kidney diseases and renal fibrosis are associated with endothelial injury and capillary rarefaction. However, our understanding of these processes has been hampered by the lack of tools enabling the quantitative and noninvasive monitoring of vessel functionality. Here, we used micro-computed tomography (µCT) for anatomical and functional imaging of vascular alterations in three murine models with distinct mechanisms of progressive kidney injury: ischemia-reperfusion (I/R, days 1–56), unilateral ureteral obstruction (UUO, days 1–10), and Alport mice (6–8 weeks old). Contrast-enhanced in vivoµCT enabled robust, noninvasive, and longitudinal monitoring of vessel functionality and revealed a progressive decline of the renal relative blood volume in all models. This reduction ranged from −20% in early disease stages to −61% in late disease stages and preceded fibrosis. Upon Microfil perfusion, high-resolution ex vivoµCT allowed quantitative analyses of three-dimensional vascular networks in all three models. These analyses revealed significant and previously unrecognized alterations of preglomerular arteries: a reduction in vessel diameter, a prominent reduction in vessel branching, and increased vessel tortuosity. In summary, using µCT methodology, we revealed insights into macro-to-microvascular alterations in progressive renal disease and provide a platform that may serve as the basis to evaluate vascular therapeutics in renal disease.     

An Automated Algorithm to Detect the Trabecular-Cortical Bone Interface in Micro-Computed Tomographic Images

Micro-computed tomography (microCT) has become a standard tool for the evaluation of bone morphology in preclinical studies. Unfortunately, the user-dependent definition of contour lines that separate trabecular from cortical bone is not only extremely time-consuming but may also represent a source of data bias and increased variability. Here, an automated image segmentation technique was developed and tested over a large range of bone phenotypes. The principal steps of the algorithm involve blurring, segmentation at different thresholds, and volumetric component labeling to first identify the periosteal edge and then create a cortical mask, the inner edge of which defines the trabecular-cortical interface. The algorithm was tested against (1) eight skilled microCT operators who manually defined the trabecular bone within the distal femur of four adult mice as well as (2) contour lines drawn by a single user in femurs from 71 rodents. Across the four femurs, the coefficient of variation between users was 9% for bone volume fraction, 13% for connectivity density, and 3% for trabecular thickness. Morphometric data produced by the algorithm were within 2% of the mean values of the eight operators. Across the 71 femurs, the slope and intercept of the regressions between morphometric automatic and user data were, with the exception of trabecular thickness, not significantly different from 1 and 0, respectively. Because of the excellent match with the current gold-standard technique, this algorithm may present a valuable tool for the standardized and automated evaluation of bone morphology without the time-consuming task of drawing contour lines.     

Role of polyethylene particles in peri-prosthetic osteolysis: A review

There is convincing evidence that particles produced by the wear of joint prostheses are causal in the peri-prosthetic loss of bone, or osteolysis, which, if it progresses, leads to the phenomenon of aseptic loosening. It is important to fully understand the biology of this bone loss because it threatens prosthesis survival, and loosened implants can result in peri-prosthetic fracture, which is disastrous for the patient and presents a difficult surgical scenario. The focus of this review is the bioactivity of polyethylene (PE) particles, since there is evidence that these are major players in the development and progression of osteolysis around prostheses which use PE as the bearing surface. The review describes the biological consequences of interaction of PE particles with macrophages, osteoclasts and cells of the osteoblast lineage, including osteocytes. It explores the possible cellular mechanisms of action of PE and seeks to use the findings to date to propose potential non-surgical treatments for osteolysis. In particular, a non-surgical approach is likely to be applicable to implants containing newer, highly cross-linked PEs (HXLPEs), for which osteolysis seems to occur with much reduced PE wear compared with conventional PEs. The caveat here is that we know little as yet about the bioactivity of HXLPE particles and addressing this constitutes our next challenge.     

Parthenolide inhibits polyethylene particle–induced mouse calvarial osteolysis in vivo

Background

Periprosthetic osteolysis and aseptic loosening (AL) after joint arthroplasty are serious problems encountered after an implant surgery. AL is possibly caused by osteolysis or local bone resorption induced by implant-derived wear particles. However, effective treatments for osteoclastic bone resorption and AL mediated by wear particles have not been developed except surgical revision. Therefore, a new strategy should be developed to improve osteolysis associated with AL via pharmacologic intervention.

Materials and methods

The effects of parthenolide (PTN), a nuclear factor-kappa B inhibitor and sesquiterpene lactone, on polyethylene particle–induced osteolysis in vivo were investigated using a mouse calvarial model. Bone volume/tissue volume (BV/TV, %), bone surface/bone volume (BS/BV, 1/mm), osteoclast number per bone perimeter (N.Oc/B.Pm, /mm), and eroded surface per bone surface (ES/BS, %) were determined by micro-computed tomography and histologic analyses.

Results

Severe bone resorption and rapid osteoclast formation were found in the cranium of the subjects after polyethylene particles were implanted. ES/BS (P < 0.001), N.Oc/B.Pm (group III, P < 0.05; group IV, P < 0.001), and BS/BV (P < 0.001) increased compared with those in group II; BS/BV (P < 0.001) decreased in group II but was improved in groups III and IV, which were treated with PTN. No significant difference in these parameters was observed among groups I, III, and IV.

Conclusions

PTN possibly elicited therapeutic effects on osteolysis induced by wear particles, indicating that PTN could be used as a therapeutic agent of AL induced by wear particles.     

髋臼骨折的三维CT重建

本文报告12例髋臼骨折行三维CT重建的临床应用,三维CT对复杂的髋曰骨折能明确骨折移位、粉碎程度,能显示整个骨盆的轮廓及立体感,对术前骨折的评估、分类、指导手术入路有一定的临床意义。     

唑来膦酸钠防治假体周围骨溶解的实验研究

目的:观察唑来膦酸钠对磨损颗粒诱导的大鼠假体周围骨溶解的影响及其作用机制。方法:选用30只成年雄性SD大鼠,体重250～300 g,随机分3组,每组10只,分别为空白对照组、模型对照组和唑来磷酸钠组。空白对照组不作任何处理;模型对照组及唑来磷酸钠组行右侧股骨植入聚乙烯颗粒和钛棒制备聚乙烯颗粒诱导假体周围骨溶解大鼠模型,术后唑来膦酸钠组每周皮下注射唑来磷酸钠0.1 mg/kg,连续用药8周后取血、处死并采集右侧股骨标本。测定各组股骨骨密度(BMD)、IL-1β、IL-6、TNF-α及血清TRAP5b、CTX-Ⅰ的含量。结果:与模型对照组比较:唑来膦酸钠组大鼠股骨骨密度增高,IL-1β、IL-6、TNF-α含量均下降;唑来膦酸钠组大鼠血清TRACP5b、CTX-Ⅰ水平降低。结论:唑来膦酸钠组能够有效抑制聚乙烯颗粒诱导的大鼠假体周围骨溶解,可能是通过抑制破骨细胞的活性及IL-1β、IL-6、TNF-α等细胞因子的表达实现,为临床防治人工关节假体周围骨溶解提供理论基础。     

Mechanical loading enhances the anabolic effects of intermittent parathyroid hormone (1-34) on trabecular and cortical bone in mice

The separate and combined effects of intermittent parathyroid hormone (iPTH) (1-34) and mechanical loading were assessed at trabecular and cortical sites of mouse long bones. Female C57BL/6 mice from 13 to 19 weeks of age were given daily injections of vehicle or PTH (1-34) at low (20 microg/kg/day), medium (40 microg/kg/day) or high (80 microg/kg/day) dose. For three alternate days per week during the last two weeks of this treatment, the tibiae and ulnae on one side were subjected to a single period of non-invasive, dynamic axial loading (40 cycles at 10 Hz with 10-second intervals between each cycle). Two levels of peak load were used; one sufficient to engender an osteogenic response, and the other insufficient to do so. The whole tibiae and ulnae were analyzed post-mortem by micro-computed tomography with a resolution of 5 microm. Treatment with iPTH (1-34) modified bone structure in a dose- and time-dependent manner, which was particularly evident in the trabecular region of the proximal tibia. In the tibia, loading at a level sufficient by itself to stimulate osteogenesis produced an osteogenic response in the low-dose iPTH (1-34)-treated trabecular bone and in the proximal and middle cortical bone treated with all doses of iPTH (1-34). In the ulna, loading at a level that did not by itself stimulate osteogenesis was osteogenic at the distal site when combined with high-dose iPTH (1-34). At both levels of loading, there were synergistic effects in cortical bone volume of the proximal tibia and distal ulna between loading and high-dose iPTH (1-34). Images of fluorescently labelled bones confirmed that such synergism resulted from increases in both endosteal and periosteal bone formation. No woven bone was induced by iPTH (1-34) or either level of loading alone, whereas the combination of iPTH (1-34) and the "sufficient" level of loading stimulated woven bone formation on endosteal and periosteal surfaces of the proximal cortex in the tibiae. Together, these data suggest that in female C57BL/6 mice, under some but not all circumstances, mechanical loading exerts an osteogenic response with iPTH (1-34) in trabecular and cortical bone.     

病灶清除颗粒骨植骨治疗全髋关节置换术后髋臼周围骨溶解

目的 探讨全髋关节置换（THA)术后髋臼周围骨溶解的治疗方法及疗效.方法 对24例(24髋)THA术后髋臼周围骨溶解行骨溶解病灶清除、同种异体颗粒骨植骨、更换高交联聚乙烯内衬和股骨头假体.14例（14髋）髋臼杯稳定保留金属臼杯;2例（2髋）髋臼杯稳定卡环损坏,被迫取出稳定臼杯,行非骨水泥型髋臼杯翻修术;8例（8髋）髋臼杯松动行髋臼翻修术.采用Harris评分评价髋关节功能.通过X线片观察假体是否松动和移植骨愈合情况.结果 23例获得随访,1例失访,平均随访16个月（10～45个月）.Harris评分由术前(58±23)分（17～86分）,提高到末次随访时(92±12)分（80～98分）,差异有统计学意义(P＜0.05).术后无感染、脱位、静脉血栓形成等.X线片显示髋臼假体无松动和异位骨化,可见移植骨-宿主骨交界处有连续性骨小梁通过.结论 保留髋臼杯积极进行骨溶解病灶清除,同种异体颗粒骨植骨,更换聚乙烯内衬和股骨头假体可阻断骨溶解的进展;髋臼翻修,同种异体颗粒骨植骨,短期随访示移植骨愈合、髋臼杯稳定,临床效果满意.     

Comparison of Quantitative Cancellous Bone Connectivity Analyses at Two- and Three-Dimensional Levels in Dialysis Patients

Assessment of cancellous bone connectivity has the potential to aid in predicting fracture risk. Today, cancellous bone connectivity is generally assessed using bone sections obtained from biopsy. However, how reliably such two-dimensional (2-D) analyses visualize the 3-D properties has not been evaluated. Biopsied iliac bone samples were obtained from 47 chronic hemodialysis patients. Bone samples were observed using a microfocus X-ray computed tomography (μCT) system en bloc, and the cancellous bone microstructure was quantitatively assessed at both the 2- and 3-D levels. Cancellous bone microarchitecture was successfully reconstructed from the data obtained by the μCT system. Most of the results from node-strut analysis (NSA) revealed no statistically significant correlations between the 2- and 3-D analyses, with the exception that the number of nodes (N.Nd/TV) showed a mild but significant correlation. In contrast, the marrow space star volumes (V*m) of the 2- and 3-D analyses were highly correlated. NSA parameters including N.Nd/TV showed significant correlations with V*m at the 3-D level. In conclusion, V*m values were similar in the 2- and 3-D analyses, while most of the 2-D NSA parameters did not reflect the 3-D ones. Since V*m and most of the NSA parameters were correlated in the 3-D analyses, 2-D NSA would seem to have serious limitations for the assessment of cancellous bone microstructural properties. Further studies will thus be needed to establish appropriate methods for assessing cancellous bone connectivity in clinical practice.     

Three-Dimensional Computerized Tomography in the Presurgical Evaluation of Chiari Malformations

Summary Malformations of the cranio-cervical junction represent a complex entity, comprising neuro-meningeal and bone anomalies. Malformations of the central nervous system are nowadays easily explored using magnetic resonance imaging (MRI), while osseous malformations are classically assessed with standard radiographic techniques, which can give only incomplete information, and are often difficult to realize and analyse. We report a retrospective study of 10 patients with a Chiari malformation operated on in our department at La Salpétrière hospital, between July 1995 and December 1996 (1 man, 9 women; mean of age: 35 years; postoperative median follow-up: 6 months,; 70% of improvement and 20% of stabilisation), and we underline the interest of the systematic pre-operative realization of a 3D CT and angio-CT studies of the cranio-cervical junction. This examination offers advantages of: ?– a precise analysis of the more complex osseous malformations; ?– a study of the relationships between vascular and bony structures; ?– a study of the relationships between neuraxis and spinal and cranial structures; ?– and an optimal planning of the surgical procedure, adapted to the anatomical particularities of the patient.     

Analysis of Trabecular Bone Structure with Multidetector Spiral Computed Tomography in a Simulated Soft-Tissue Environment

We investigated the influence of soft tissue (ST) on image quality by high-resolution multidetector computed tomography (MDCT) scans and assessed the effect of surrounding ST on the quantification of trabecular bone structure. Eight bone cores obtained from human proximal femoral heads discarded during hip replacement surgery were scanned with micro-computed tomography (μCT) as well as with MDCT both without (w/o) and with (w) simulated surrounding ST, where a phantom imitated a human torso. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured in all scans. Apparent trabecular bone structure parameters were calculated and compared to similar parameters obtained in coregistered sections of the μCT scans. Residual errors were calculated as root-mean-square (RMS) errors relative to the μCT measurements. Compared to μCT results, trabecular structure parameters were overestimated by MDCT both w and w/o ST. SNR and CNR were significantly higher in the scans w/o ST. Significant correlations between μCT and MDCT results were found for bone fraction (r = 0.90 w/o ST, r = 0.84 w ST), trabecular number, and separation. RMS ranged from 10% to 15% for MDCT w/o ST and from 10% to 17% for MDCT w ST. Only bone fraction showed significantly different RMS and correlations for scans w/o vs. w ST (P < 0.05). This study showed that MDCT is able to visualize trabecular bone structure in an in vivo-like setting at skeletal sites within the torso such as the proximal femur. Even though ST scatter compromises image quality substantially, the major characteristics of the trabecular network can still be appreciated and quantified. Funding source Seed grant by the Department of Radiology, University of California, San Francisco, CA, USA     

Glucocorticoid-induced osteopenia in the mouse as assessed by histomorphometry,microcomputed tomography,and biochemical markers

Glucocorticoids are potent anti-inflammatory molecules used in the treatment of asthma, rheumatoid arthritis, inflammatory bowel disease, and other inflammatory and dermatological diseases, as well as in posttransplantation immunotherapy. Although glucocorticoids have been prescribed for many years, their potential side effects, when administered orally, can prevent their long-term use. The most serious side effect observed in the clinic is glucocorticoid-induced osteoporosis (GIOP). To develop a small animal model to characterize glucocorticoid-induced bone loss, we carried out a series of experiments using BALB/c mice given daily intraperitoneal doses of the synthetic glucocorticoid, dexamethasone. Following dexamethasone treatment, the mice became osteopenic, with highly significant decreases in bone formation rate and mineral apposition rate, as assessed by standard histomorphometry. Moreover, 3 week treatment with dexamethasone resulted in a decrease in trabecular thickness and trabecular number with an increase in surface-to-volume ratio of trabeculae in the distal femur, as measured using microcomputed tomography (micro-CT). The serum bone formation marker, osteocalcin, was dose-dependently decreased in all mice treated with dexamethasone and showed a parallel extent of regulation to the bone formation rate changes. In addition, serum levels of leptin, recently identified as playing a role in the regulation of bone mass, increased following dexamethasone treatment. BALB/c mice therefore represent a useful model system in which the detrimental effects of glucocorticoids on bone can be studied.     

Inflammatory Response with Osteolysis Related to a Bioabsorbable Anchor in the Finger: a Case Report

Soft tissue fixation of ligaments and tendons in the hand can be achieved by the use of metal or bioabsorbable suture anchors. Advantages of bioabsorbable suture anchors include lack of interference in magnetic resonance imaging, resorption of anchor, replacement by bone, and no need for hardware removal. However, complications of these bioabsorbable implants include inflammatory response to the material use. We present what we believe to be the first case in the hand of a poly(l-lactide-co-d,l-lacitide) suture anchor causing an inflammatory response leading to significant osteolysis 4 months postoperatively after repair of a ring finger flexor digitorum profundus avulsion. Exploration of the distal phalanx revealed an intact implant and repair, no signs of infection, and an extensive bone defect. Pathology showed chronically inflamed tissue. This case has led us to reconsider the use of bioabsorbable anchor sutures in the hand. Further research is necessary to better define the contraindications to bioabsorbable suture anchor use in the hand.     

Mechanical Properties of Ewe Vertebral Cancellous Bone Compared With Histomorphometry and High-Resolution Computed Tomography Parameters

The goal of the present study was to determine if a high-resolution computed tomography (HRCT) system with 150 μm resolution was sufficient to predict mechanical properties in ewe lumbar vertebrae. To answer this question, we used a triangular comparison between: HRCT; biomechanics (compression and shear tests); and histomorphometry, which was the reference method for the measurements of morphometric parameters. Two dissected lumbar vertebrae (L-4 and L-5) from 32 ewes were used. Both compressive and shear properties correlated significantly with amount of bone and structural parameters evaluated by histomorphometry (bone volume/tissue volume, trabecular thickness, trabecular separation), but no significant correlation was found with the trabecular number. With our shear test involving the trabecular architecture itself more significant correlations were found with the node-strut analysis parameters than from the compressive test. Significant correlations were also found between HRCT and histological parameters (bone volume/tissue volume, bone surface/bone volume, trabecular separation, trabecular number, total strut length, number of nodes, and number of termini). Correlations between HRCT structural parameters and mechanical properties on L-4 were of the same magnitude as the correlations between the histomorphometric structural parameters and mechanical results on L-5 but with the remarkable advantage that HRCT is a noninvasive method. In spite of the resolution (150 μm) of our HRCT system, which entailed mainly an enlargement of the thinnest trabeculae or their loss during the segmentation process, we obtained coherent relationships between mechanical and tomographic parameters. The thinnest trabeculae probably had little effect on the mechanical strength. Also, this type of resolution allows us to consider the possibility of perfecting an in vivo HRCT system. However, physical density and bone mineral density correlated much better with strength than either classical histomorphometric or tomographic parameters. The current conclusion is fairly negative with respect to the ability of HRCT to assess mechanical properties nondestructively as compared with dual-energy X-ray absorptiometry. But, the noninvasive nature of the imaging modality and the capacity for three-dimensional imaging at arbitrary orientation make HRCT a promising tool in the quantitative assessment of cancellous architecture.     

The effectiveness of polyethylene versus titanium particles in inducing osteolysis in vivo.

Bearing surface wear and periprosthetic osteolysis due to wear particles are among the most common reasons for joint replacement failure. A murine calvarial model of wear particle-induced osteolysis has been used to identify different biologic factors associated with this problem and to test nonsurgical methods of modulating the host response to particulate debris. This model has utilized titanium particles, however, in clinical practice the most common source of particulate debris is polyethylene particles from bearing surface wear. We now report a calvarial model of wear particle-induced osteolysis based on commercially available polyethylene particles. We found that compared to sham surgery osteoclast recruitment and bone resorption can be induced by introduction of the titanium particles or polyethylene particles. However, bone resorption was significantly higher with polyethylene particles compared to titanium particles (p=0.02). We consider the polyethylene based murine calvarial model of wear particle-induced osteolysis a reliable and clinically relevant tool to understand the host factors and potential pharmacologic interventions that can influence wear debris generated osteolysis. This model might serve as an extension of the well-established titanium based bone resorption model.     

Additional Weight Bearing during Exercise and Estrogen in the Rat: The Effect on Bone Mass, Turnover, and Structure

Mechanical loading and estrogen play important roles in bone homeostasis. The aim of this study was to evaluate the effects of mechanical loading on trabecular bone in the proximal femur of ovariectomized rats. We hypothesized that mechanical loading suppresses bone resorption and increases bone formation, which differs from the suppressive effects of estrogen on both resorption and formation. Furthermore, we expected to find changes in trabecular architecture elicited by the effects of mechanical loading and estrogen deficiency. Sixty female Wistar rats, 12 weeks old, were assigned to either the sedentary groups sham surgery (SED), ovariectomy (SED+OVX), and ovariectomy with estrogen replacement (SED+OVX+E2) or to the exercise groups EX, EX+OVX, EX+OVX+E2. Following ovariectomy, 5 microg 17beta-estradiol was given once weekly to the estrogen replacement groups. Exercise consisted of running with a backpack (load +/-20% of body weight) for 15 minutes/day, 5 days/week, for 19 weeks. Dual-energy X-ray absorptiometry (DXA) scans were performed before (T0), during (T6), and after (T19) the exercise period to obtain bone mineral content (BMC) and bone mineral density (BMD) data. After the exercise program, all rats were killed and right and left femora were dissected and prepared for micro-CT scanning and histomorphometric analysis of the proximal femoral metaphysis. After 19 weeks, increases in BMC (P = 0.010) and BMD (P = 0.031) were significant. At T19, mechanical loading had a significant effect on BMC (P = 0.025) and BMD (P = 0.010), and an interaction between mechanical loading and estrogen (P = 0.023) was observed. Bone volume and trabecular number decreased significantly after ovariectomy, while trabecular separation, mineralizing surface, bone formation rate, osteoclast surface, degree of anisotropy, and structure model index increased significantly after ovariectomy (P < 0.05). Trabecular bone turnover and structural parameters in the proximal femur were not affected by exercise. Estrogen deficiency resulted in a less dense and more oriented trabecular bone structure with increased marrow cavity and a decreased number of trabeculae. In conclusion, mechanical loading has beneficial effects on BMC and BMD of the ovariectomized rat. This indicates that the load in the backpack was high enough to elicit an osteogenic response sufficient to compensate for the ovariectomy-induced bone loss. The results confirm that estrogen suppresses both bone resorption and bone formation in the proximal metaphysis in the femoral head of our rat-with-backpack model. The effects of mechanical loading on the trabecular bone of the femoral head were not significant. This study suggests that the effect of mechanical loading in the rat-with-backpack model mainly occurs at cortical bone sites.