Baseline bone morphometry and cellular activity modulate the degree of bone loss in the appendicular skeleton during disuse

Assessing the association between a sedentary lifestyle and prevalence of primary osteoporosis: a community-based cross-sectional study among Chinese population

OBJECTIVES

To reveal the association between a sedentary lifestyle and the prevalence of primary osteoporosis (POP).

DESIGN

A community-based cross-sectional study was conducted.

SETTING

This study was conducted in communities in Hefei city, Anhui province, China.

PARTICIPANTS

A total of 1346 residents aged 40 and above underwent POP screening via calcaneus ultrasound bone mineral density (BMD) testing and completed a questionnaire survey.

OUTCOME MEASURES

The average daily sitting time was included in the study variable and used to assess sedentary behaviour. The 15 control variables included general information, dietary information and life behaviour information. Logistic regression was used to analyse the association between the POP prevalence and study or control variables in different models.

RESULTS

1346 participants were finally included in the study. According to the 15 control variables, the crude model and 4 models were established. The analysis revealed that the average daily sitting time showed a significant correlation with the prevalence of POP in the crude model (OR=2.02, 95% CI=1.74 to 2.36, p<0.001), Model 1 (OR=2.65, 95% CI=2.21 to 3.17, p<0.001), Model 2 (OR=2.63, 95% CI=2.19 to 3.15, p<0.001), Model 3 (OR=2.62, 95% CI=2.18 to 3.15, p<0.001) and Model 4 (OR=2.58, 95% CI=2.14 to 3.11, p<0.001). Besides, gender, age and body mass index showed a significant correlation with the POP prevalence in all models.

CONCLUSIONS

This study suggests a potential association between a sedentary lifestyle and the prevalence of POP within the Chinese population. Modifying sedentary behaviours could contribute to a reduction in POP risk. However, longitudinal cohort studies are necessary to confirm this hypothesis in the future.

A mouse model of disuse osteoporosis based on a movable noninvasive 3D-printed unloading device

Objective

Disuse osteoporosis is a major type of bone loss disease characterized by regional bone loss and microstructure alterations. The condition is induced by a marked decrease in weight bearing over time, which usually occurs due to limb immobilization, therapeutic bed rest or space flight. To date, the most commonly used mouse model of disuse osteoporosis is constructed using the classical tail suspension method, which causes tail injury, movement inconvenience and mental stress. This study aimed to propose a noninvasive and effective method for the establishment of a mouse model of disuse osteoporosis and compared this method with the tail suspension method.

Methods

3D printing technology was applied to construct a movable unloading device. A movable noninvasive 3D-printed unloading device (3D-ULD) was used to unload the hindlimbs of the mice. The bone microstructure and bone volume of unloaded femurs were analysed through micro-CT and H&E staining, and von Kossa staining was performed for the detection of bone mineralization in the femurs. TRAP staining, IHC-CTSK and Q-PCR were performed for evaluation of the bone resorption ability, and double labelling, IHC-DMP1, ALP staining and Q-PCR assays were conducted to assess the osteogenic ability. The mechanical properties of disused bone were detected using the three-point bending test. The body, thymus and spleen weights of the mice were recorded, and the serum corticosterone level of the mice was assayed by enzyme-linked immunosorbent assay (ELISA).

Results

The micro-CT results showed significant trabecular bone loss, and 3D-ULD induced cortical bone loss in disused femurs as well as a decrease in the bone mineral density in the unloaded mice. TRAP staining and IHC-CTSK staining results indicated increases in the osteoclast number per bone perimeter (Oc.N/B.Pm) and the osteoclast surface per bone surface (Oc.S/BS) in the unloaded mice. The Ctsk, Trap and Mmp9 expression levels were significantly increased in the unloaded mice. Decreases in the ratio of the mineral surface to bone surface (MS/BS), mineral apposition rate (MAR) and bone formation rate per bone surface (BFR/BS) were found in unloaded mice in the 3D-ULD by double labelling. The IHC-DMP1 and ALP staining results showed decreases in the osteoblast number per bone perimeter (Ob.N/B.Pm) and osteoblast surface per bone surface (Ob. S/BS) in the mice unloaded in the 3D-ULD, and these mice also showed decreased Runx2, Alp and Dmp1 expression levels. Three-point bending test results showed that the mechanical properties were attenuated in the disused femurs of the unloaded mice. Less skin rupture and rare alterations in the thymus and spleen weights were found in the unloaded mice in the 3D-ULD. The ELISA results indicated the serum corticosterone level of the mice unloaded in the 3D-ULD was significantly lower than that of mice suspended by their tail.

Conclusion

This new disuse osteoporosis mouse model based on 3D-ULD could induce effective disuse bone loss with significantly alleviated side effects.

Translational potential of this article

This study proposes a new disuse osteoporosis mouse model based on 3D-ULD that can be used to better understand disuse bone loss in the future.

A Systematic Review of Animal Models of Disuse-Induced Bone Loss

OBJECTIVE

Several different animal models are used to study disuse-induced bone loss. This systematic review aims to give a comprehensive overview of the animal models of disuse-induced bone loss and provide a detailed narrative synthesis of each unique animal model.

METHODS

PubMed and Embase were systematically searched for animal models of disuse from inception to November 30, 2019. In addition, Google Scholar and personal file archives were searched for relevant publications not indexed in PubMed or Embase. Two reviewers independently reviewed titles and abstracts for full-text inclusion. Data were extracted using a predefined extraction scheme to ensure standardization.

RESULTS

1964 titles and abstracts were screened of which 653 full-text articles were included. The most common animal species used to model disuse were rats (59%) and mice (30%). Males (53%) where used in the majority of the studies and genetically modified animals accounted for 7%. Twelve different methods to induce disuse were identified. The most frequently used methods were hindlimb unloading (44%), neurectomy (15%), bandages and orthoses (15%), and botulinum toxin (9%). The median time of disuse was 21 days (quartiles: 14 days, 36 days) and the median number of animals per group subjected to disuse was 10 (quartiles: 7, 14). Random group allocation was reported in 43% of the studies. Fewer than 5% of the studies justified the number of animals per group by a sample size calculation to ensure adequate statistical power.

CONCLUSION

Multiple animal models of disuse-induced bone loss exist, and several species of animals have successfully been studied. The complexity of disuse-induced bone loss warrants rigid research study designs. This systematic review emphasized the need for standardization of animal disuse research and reporting.

Reversibility of marrow adipose accumulation and reduction of trabecular bone in the epiphysis of the proximal tibia

Mechanical stimuli play an important role in the homeostasis of trabecular bone and marrow adipose tissue, particularly for the weight-bearing skeleton. Prolonged immobilization and disuse have been shown to reduce trabecular bone content and increase marrow adipose tissue in the bones of lower limb joints such as the knee. However, details on the temporal response of this relationship to prolonged immobilization and its reversibility is limited. Forty rats had one knee immobilized at 45° of flexion for 2, 4, 8, or 16 weeks and subsequently remobilized for 0 or 8 weeks. The contralateral knees were used as controls. Histomorphometric measures of trabecular bone and marrow adipose tissue (MAT) areas were conducted in the epiphysis of the proximal tibia. Knee immobilization for 4, 8, and 16 weeks significantly reduced trabecular bone area by -0.125, -0.139, and -0.161 mm²/mm², respectively, with corresponding 95 % CIs of [-0.012, -0.239], [-0.006, -0.273], and [-0.101, -0.221]. MAT area significantly increased at 2 and 16 weeks by +0.008 and +0.027 mm²/mm², respectively, with 95 % CIs of [0.014, 0.002] and [0.039, 0.016]. Remobilization for 8 weeks restored trabecular bone area compared to the contralateral knee and the magnitude of change was significantly greater for 8 and 16 weeks of immobilization with effect sizes of 1.69 and 1.86, respectively. The difference in MAT area between immobilized and contralateral knees were eliminated with remobilization. These results characterize the temporal response of trabecular bone and MAT in the epiphysis of the proximal tibia to joint immobilization and remobilization.

Differing trabecular bone architecture in dinosaurs and mammals contribute to stiffness and limits on bone strain

The largest dinosaurs were enormous animals whose body mass placed massive gravitational loads on their skeleton. Previous studies investigated dinosaurian bone strength and biomechanics, but the relationships between dinosaurian trabecular bone architecture and mechanical behavior has not been studied. In this study, trabecular bone samples from the distal femur and proximal tibia of dinosaurs ranging in body mass from 23-8,000 kg were investigated. The trabecular architecture was quantified from micro-computed tomography scans and allometric scaling relationships were used to determine how the trabecular bone architectural indices changed with body mass. Trabecular bone mechanical behavior was investigated by finite element modeling. It was found that dinosaurian trabecular bone volume fraction is positively correlated with body mass similar to what is observed for extant mammalian species, while trabecular spacing, number, and connectivity density in dinosaurs is negatively correlated with body mass, exhibiting opposite behavior from extant mammals. Furthermore, it was found that trabecular bone apparent modulus is positively correlated with body mass in dinosaurian species, while no correlation was observed for mammalian species. Additionally, trabecular bone tensile and compressive principal strains were not correlated with body mass in mammalian or dinosaurian species. Trabecular bone apparent modulus was positively correlated with trabecular spacing in mammals and positively correlated with connectivity density in dinosaurs, but these differential architectural effects on trabecular bone apparent modulus limit average trabecular bone tissue strains to below 3,000 microstrain for estimated high levels of physiological loading in both mammals and dinosaurs.

Combating osteoporosis and obesity with exercise: leveraging cell mechanosensitivity

Osteoporosis, a condition of skeletal decline that undermines quality of life, is treated with pharmacological interventions that are associated with poor adherence and adverse effects. Complicating efforts to improve clinical outcomes, the incidence of obesity is increasing, predisposing the population to a range of musculoskeletal complications and metabolic disorders. Pharmacological management of obesity has yet to deliver notable reductions in weight and debilitating complications are rarely avoided. By contrast, exercise shows promise as a non-invasive and non-pharmacological method of regulating both osteoporosis and obesity. The principal components of exercise - mechanical signals - promote bone and muscle anabolism while limiting formation and expansion of fat mass. Mechanical regulation of bone and marrow fat might be achieved by regulating functions of differentiated cells in the skeletal tissue while biasing lineage selection of their common progenitors - mesenchymal stem cells. An inverse relationship between adipocyte versus osteoblast fate selection from stem cells is implicated in clinical conditions such as childhood obesity and increased marrow adiposity in type 2 diabetes mellitus, as well as contributing to skeletal frailty. Understanding how exercise-induced mechanical signals can be used to improve bone quality while decreasing fat mass and metabolic dysfunction should lead to new strategies to treat chronic diseases such as osteoporosis and obesity.

Differences in bone structure and unloading-induced bone loss between C57BL/6N and C57BL/6J mice

The C57BL/6 mouse, the most frequently utilized animal model in biomedical research, is in use as several substrains, all of which differ by a small array of genomic differences. Two of these substrains, C57BL/6J (B6J) and C57BL/6N (B6N), are commonly used but it is unclear how phenotypically similar or different they are. Here, we tested whether adolescent B6N mice have a bone phenotype and respond to the loss of weightbearing differently than B6J. At 9 weeks of age, normally ambulating B6N had lower trabecular bone volume fraction but greater bone formation rates and osteoblast surfaces than corresponding B6J. At 11 weeks of age, differences in trabecular indices persisted between the substrains but differences in cellular activity had ceased. Cortical bone indices were largely similar between the two substrains. Hindlimb unloading (HLU) induced similar degeneration of trabecular architecture and cellular activity in both substrains when comparing 11-week-old HLU mice to 11-week-old controls. However, unloaded B6N mice had smaller cortices than B6J. When comparing HLU to 9 weeks baseline control mice, deterioration in trabecular separation, osteoblast indices, and endocortical variables was significantly greater in B6N than B6J. These data indicate specific developmental differences in bone formation and morphology between B6N and B6J mice, giving rise to a differential response to mechanical unloading that may be modulated, in part, by the genes Herc2, Myo18b, and Acan. Our results emphasize that these substrains cannot be used interchangeably at least for investigations in which the phenotypic makeup and its response to extraneous stimuli are of interest.

Trabecular bone structural variation throughout the human lower limb

Trabecular bone is responsive to mechanical loading, and thus may be a useful tool for interpreting past behaviour from fossil morphology. However, the ability to meaningfully interpret variation in archaeological and hominin trabecular morphology depends on the extent to which trabecular bone properties are integrated throughout the postcranium or are locally variable in response to joint specific loading. We investigate both of these factors by comparing trabecular bone throughout the lower limb between a group of highly mobile foragers and two groups of sedentary agriculturalists. Trabecular bone structure is quantified in four volumes of interest placed within the proximal and distal joints of the femur and tibia. We determine how trabecular structures correspond to inferred behavioural differences between populations and whether the patterns are consistent throughout the limb. A significant correlation was found between inferred mobility level and trabecular bone structure in all volumes of interest along the lower limb. The greater terrestrial mobility of foragers is associated with higher bone volume fraction, and thicker and fewer trabeculae (lower connectivity density). In all populations, bone volume fraction decreases while anisotropy increases proximodistally throughout the lower limb. This observation mirrors reductions in cortical bone mass resulting from proximodistal limb tapering. The reduction in strength associated with reduced bone volume fraction may be compensated for by the increased anisotropy in the distal tibia. A similar pattern of trabecular structure is found throughout the lower limb in all populations, upon which a signal of terrestrial mobility appears to be superimposed. These results support the validity of using lower limb trabecular bone microstructure to reconstruct terrestrial mobility levels from the archaeological and fossil records. The results further indicate that care should be taken to appreciate variation resulting from differences in habitual activity when inferring behaviour from the trabecular structure of hominin fossils through comparisons with modern humans.

Microgravity Stress: Bone and Connective Tissue

The major alterations in bone and the dense connective tissues in humans and animals exposed to microgravity illustrate the dependency of these tissues' function on normal gravitational loading. Whether these alterations depend solely on the reduced mechanical loading of zero g or are compounded by fluid shifts, altered tissue blood flow, radiation exposure, and altered nutritional status is not yet well defined. Changes in the dense connective tissues and intervertebral disks are generally smaller in magnitude but occur more rapidly than those in mineralized bone with transitions to 0 g and during recovery once back to the loading provided by 1 g conditions. However, joint injuries are projected to occur much more often than the more catastrophic bone fracture during exploration class missions, so protecting the integrity of both tissues is important. This review focuses on the research performed over the last 20 years in humans and animals exposed to actual spaceflight, as well as on knowledge gained from pertinent ground-based models such as bed rest in humans and hindlimb unloading in rodents. Significant progress has been made in our understanding of the mechanisms for alterations in bone and connective tissues with exposure to microgravity, but intriguing questions remain to be solved, particularly with reference to biomedical risks associated with prolonged exploration missions.

Evaluation of Amniotic Multipotential Tissue Matrix to Augment Healing of Demineralized Bone Matrix in an Animal Calvarial Model

Amniotic multipotential tissue matrix (AmnioMTM) is a membrane material derived from placental tissues and rich in growth factors that have been reported to have potential in healing bone. This study hypothesized that demineralized bone matrix (DBM) supplemented with AmnioMTM would accelerate healing and bone formation as compared with DBM alone in a critical size (10 mm) rat calvarial bone defect model. Five DBM grafts and 5 DBM supplemented with AmnioMTM grafts were implanted in a 10-mm critical sized defect in 10 rats (1 implant per rat). After 4 weeks, animals were euthanized and defects evaluated by microCT and histology. There were no statistical differences in microCT data for mineral density, percent bone fill, or bone surface to volume ratios between groups, though the bone surface to volume ratio for the amnio-supplemented group suggested increased osteoid activity as compared with the DBM alone group. Histological data also indicated active osteoid activity and induced bone formation in the center of defects implanted with AmnioMTM supplemented graft as compared with DBM graft alone suggesting some potential osteoinductive potential. However, there was no significant difference at the mean percent of newly mineralized bone in the DBM group defect as compared with the AmnioMTM supplemented graft material. These data suggest that while bone formation was not increased at this early time point, the increased osteoid activity and the induction of new bone in the middle of the defect by the AmnioMTM indicates that further study is needed to assess its potential benefit to bone healing and regeneration.

Ovariectomized Rats with Established Osteopenia have Diminished Mesenchymal Stem Cells in the Bone Marrow and Impaired Homing, Osteoinduction and Bone Regeneration at the Fracture Site

We investigated deleterious changes that take place in mesenchymal stem cells (MSC) and its fracture healing competence in ovariectomy (Ovx)-induced osteopenia. MSC from bone marrow (BM) of ovary intact (control) and Ovx rats was isolated. (99m)Tc-HMPAO (Technitium hexamethylpropylene amine oxime) labeled MSC was systemically transplanted to rats and fracture tropism assessed by SPECT/CT. PKH26 labeled MSC (PKH26-MSC) was bound in scaffold and applied to fracture site (drill-hole in femur metaphysis). Osteoinduction was quantified by calcein binding and microcomputed tomography. Estrogen receptor (ER) antagonist, fulvestrant was used to determine ER dependence of osteo-induction by MSC. BM-MSC number was strikingly reduced and doubling time increased in Ovx rats compared to control. SPECT/CT showed reduced localization of (99m)Tc-HMPAO labeled MSC to the fracture site, 3 h post-transplantation in Ovx rats as compared with controls. Post-transplantation, Ovx MSC labeled with PKH26 (Ovx PKH26-MSC) localized less to fracture site than control PKH26-MSC. Transplantation of either control or Ovx MSC enhanced calcein binding and bone volume at the callus of control rats over placebo group however Ovx MSC had lower efficacy than control MSC. Fulvestrant blocked osteoinduction by control MSC. When scaffold bound MSC was applied to fracture, osteoinduction by Ovx PKH26-MSC was less than control PKH26-MSC. In Ovx rats, control MSC/E2 treatment but not Ovx MSC showed osteoinduction. Regenerated bone was irregularly deposited in Ovx MSC group. In conclusion, Ovx is associated with diminished BM-MSC number and its growth, and Ovx MSC displays impaired engraftment to fracture and osteoinduction besides disordered bone regeneration.

Exercise Regulation of Marrow Adipose Tissue

Despite association with low bone density and skeletal fractures, marrow adipose tissue (MAT) remains poorly understood. The marrow adipocyte originates from the mesenchymal stem cell (MSC) pool that also gives rise to osteoblasts, chondrocytes, and myocytes, among other cell types. To date, the presence of MAT has been attributed to preferential biasing of MSC into the adipocyte rather than osteoblast lineage, thus negatively impacting bone formation. Here, we focus on understanding the physiology of MAT in the setting of exercise, dietary interventions, and pharmacologic agents that alter fat metabolism. The beneficial effect of exercise on musculoskeletal strength is known: exercise induces bone formation, encourages growth of skeletally supportive tissues, inhibits bone resorption, and alters skeletal architecture through direct and indirect effects on a multiplicity of cells involved in skeletal adaptation. MAT is less well studied due to the lack of reproducible quantification techniques. In recent work, osmium-based 3D quantification shows a robust response of MAT to both dietary and exercise intervention in that MAT is elevated in response to high-fat diet and can be suppressed following daily exercise. Exercise-induced bone formation correlates with suppression of MAT, such that exercise effects might be due to either calorie expenditure from this depot or from mechanical biasing of MSC lineage away from fat and toward bone, or a combination thereof. Following treatment with the anti-diabetes drug rosiglitazone - a PPARγ-agonist known to increase MAT and fracture risk - mice demonstrate a fivefold higher femur MAT volume compared to the controls. In addition to preventing MAT accumulation in control mice, exercise intervention significantly lowers MAT accumulation in rosiglitazone-treated mice. Importantly, exercise induction of trabecular bone volume is unhindered by rosiglitazone. Thus, despite rosiglitazone augmentation of MAT, exercise significantly suppresses MAT volume and induces bone formation. That exercise can both suppress MAT volume and increase bone quantity, notwithstanding the skeletal harm induced by rosiglitazone, underscores exercise as a powerful regulator of bone remodeling, encouraging marrow stem cells toward the osteogenic lineage to fulfill an adaptive need for bone formation. Thus, exercise represents an effective strategy to mitigate the deleterious effects of overeating and iatrogenic etiologies on bone and fat.

Effects of sex and gender on adaptation to space: musculoskeletal health

There is considerable variability among individuals in musculoskeletal response to long-duration spaceflight. The specific origin of the individual variability is unknown but is almost certainly influenced by the details of other mission conditions such as individual differences in exercise countermeasures, particularly intensity of exercise, dietary intake, medication use, stress, sleep, psychological profiles, and actual mission task demands. In addition to variations in mission conditions, genetic differences may account for some aspect of individual variability. Generally, this individual variability exceeds the variability between sexes that adds to the complexity of understanding sex differences alone. Research specifically related to sex differences of the musculoskeletal system during unloading is presented and discussed.

The relationship between porosity and specific surface in human cortical bone is subject specific

A characteristic relationship for bone between bone volume fraction (BV/TV) and specific surface (BS/TV) has previously been proposed based on 2D histological measurements. This relationship has been suggested to be bone intrinsic, i.e., to not depend on bone type, bone site and health state. In these studies, only limited data comes from cortical bone. The aim of this paper was to investigate the relationship between BV/TV and BS/TV in human cortical bone using high-resolution micro-CT imaging and the correlations with subject-specific biometric data such as height, weight, age and sex. Images from femoral cortical bone samples of the Melbourne Femur Collection were obtained using synchrotron radiation micro-CT (SPring8, Japan). Sixteen bone samples from thirteen individuals were analysed in order to find bone volume fraction values ranging from 0.20 to 1. Finally, morphological models of the tissue microstructure were developed to help explain the relationship between BV/TV and BS/TV. Our experimental findings indicate that the BV/TV vs BS/TV relationship is subject specific rather than intrinsic. Sex and pore density were statistically correlated with the individual curves. However no correlation was found with body height, weight or age. Experimental cortical data points deviate from interpolating curves previously proposed in the literature. However, these curves are largely based on data points from trabecular bone samples. This finding challenges the universality of the curve: highly porous cortical bone is significantly different to trabecular bone of the same porosity. Finally, our morphological models suggest that changes in BV/TV within the same sample can be explained by an increase in pore area rather than in pore density. This is consistent with the proposed mechanisms of age-related endocortical bone loss. In addition, these morphological models highlight that the relationship between BV/TV and BS/TV is not linear at high BV/TV as suggested in the literature but is closer to a square root function.

Increasing the Number of Unloading/Reambulation Cycles does not Adversely Impact Body Composition and Lumbar Bone Mineral Density but Reduces Tissue Sensitivity

A single exposure to hindlimb unloading leads to changes in body mass, body composition and bone, but the consequences of multiple exposures are not yet understood. Within a 18wk period, adult C57BL/6 male mice were exposed to one (1x-HLU), two (2x-HLU) or three (3x-HLU) cycles of 2 wk of hindlimb unloading (HLU) followed by 4 wk of reambulation (RA), or served as ambulatory age-matched controls. In vivo µCT longitudinally tracked changes in abdominal adipose and lean tissues, lumbar vertebral apparent volumetric bone mineral density (vBMD) and upper hindlimb muscle cross-sectional area before and after the final HLU and RA cycle. Significant decreases in total adipose tissue and vertebral vBMD were observed such that all unloaded animals reached similar values after the final unloading cycle. However, the magnitude of these losses diminished in mice undergoing their 2^nd or 3^rd HLU cycle. Irrespective of the number of HLU/RA cycles, total adipose tissue and vertebral vBMD recovered and were no different from age-matched controls after the final RA period. In contrast, upper hindlimb muscle cross-sectional area was significantly lower than controls in all unloaded groups after the final RA period. These results suggest that tissues in the abdominal region are more resilient to multiple bouts of unloading and more amenable to recovery during reambulation than the peripheral musculoskeletal system.

Genetic loci that control the loss and regain of trabecular bone during unloading and reambulation

Changes in trabecular morphology during unloading and reloading are marked by large variations between individuals, implying that there is a strong genetic influence on the magnitude of the response. Here, we subjected more than 350 second-generation (BALBxC3H) 4-month-old adult female mice to 3 weeks of hindlimb unloading followed by 3 weeks of reambulation to identify the quantitative trait loci (QTLs) that define an individual's propensity to either lose trabecular bone when weight bearing is removed or to gain trabecular bone when weight bearing is reintroduced. Longitudinal in vivo micro-computed tomography (µCT) scans demonstrated that individual mice lost between 15% and 71% in trabecular bone volume fraction (BV/TV) in the distal femur during unloading (average: -43%). Changes in trabecular BV/TV during the 3-week reambulation period ranged from a continuation of bone loss (-18%) to large additions (56%) of tissue (average: +10%). During unloading, six QTLs accounted for 21% of the total variability in changes in BV/TV whereas one QTL accounted for 6% of the variability in changes in BV/TV during reambulation. QTLs were also identified for changes in trabecular architecture. Most of the QTLs defining morphologic changes during unloading or reambulation did not overlap with those QTLs identified at baseline, suggesting that these QTLs harbor genes that are specific for sensing changes in the levels of weight bearing. The lack of overlap in QTLs between unloading and reambulation also emphasizes that the genes modulating the trabecular response to unloading are distinct from those regulating tissue recovery during reloading. The identified QTLs contain the regulatory genes underlying the strong genetic regulation of trabecular bone's sensitivity to weight bearing and may help to identify individuals that are most susceptible to unloading-induced bone loss and/or the least capable of recovering.

Bilateral asymmetry in microarchitecture of trabecular bone in male C57BL/6 mouse tibia: implication for experimental sample size estimations

This study aimed to determine whether there is bilateral asymmetry between the left and right tibiae in the microarchitectural characteristics of the trabecular bone and estimate a moderate sample size for detections of such bilateral differences. The left and right tibiae of 20 C57BL/6 mice (12 weeks old) were scanned by micro-computed tomography, and the structural parameters of the trabecular bone were measured. There were significant differences in the structural parameters between the left and right tibiae (p<0.05); specifically, the microarchitecture was greater in quantity and quality in the left tibia than in the right tibia. Moreover, the percent of bilateral differences between the left and right tibiae ranged from 2.26% to 22.52%. For most of the structural parameters except for trabecular bone thickness (Tb.Th) and trabecular bone separation (Tb.Sp), the 20 mice involved in this study were enough to detect differences. These results show that the directional left-right asymmetry in the microarchitecture of the trabecular bone may exist despite the samples being from an inbred strain. Furthermore, we estimated a sample size for detections of such differences between the left and right tibiae.

Contributions of severe burn and disuse to bone structure and strength in rats

Burn and disuse results in metabolic and bone changes associated with substantial and sustained bone loss. Such loss can lead to an increased fracture incidence and osteopenia. We studied the independent effects of burn and disuse on bone morphology, composition and strength, and microstructure of the bone alterations 14days after injury. Sprague-Dawley rats were randomized into four groups: Sham/Ambulatory (SA), Burn/Ambulatory (BA), Sham/Hindlimb Unloaded (SH) and Burn/Hindlimb Unloaded (BH). Burn groups received a 40% total body surface area full-thickness scald burn. Disuse by hindlimb unloading was initiated immediately following injury. Bone turnover was determined in plasma and urine. Femur biomechanical parameters were measured by three-point bending tests and bone microarchitecture was determined by micro-computed tomography (uCT). On day 14, a significant reduction in body mass was observed as a result of burn, disuse and a combination of both. In terms of bone health, disuse alone and in combination affected femur weight, length and bone mineral content. Bending failure energy, an index of femur strength, was significantly reduced in all groups and maximum bending stress was lower when burn and disuse were combined. Osteocalcin was reduced in BA compared to the other groups, indicating influence of burn. The reductions observed in femur weight, BMC, biomechanical parameters and indices of bone formation are primarily responses to the combination of burn and disuse. These results offer insight into bone degradation following severe injury and disuse.

Low-level laser therapy using the minimally invasive laser needle system on osteoporotic bone in ovariectomized mice

This study tested the effectiveness of low-level laser therapy (LLLT) in preventing and/or treating osteoporotic trabecular bone. Mice were ovariectomized (OVX) to induce osteoporotic bone loss. The tibiae of eight OVX mice were treated for 5 days each week for 2 weeks by LLLT (660 nm, 3 J) using a minimally invasive laser needle system (MILNS) which is designed to minimize loss of laser energy before reaching bone (LASER group). Another eight mice received a sham treatment (SHAM group). Structural parameters of trabecular bone were measured with in vivo micro-computed tomography images before and after laser treatment. After LLLT for 2 weeks, the percentage reduction (%R) was significantly lower in BV/TV (bone volume fraction) and Tb.N (trabecular number, p<0.05 and p<0.05) and significant higher in Tb.Sp (trabecular separation) and SMI (structure model index, p<0.05 and p<0.05) than in the SHAM group. The %R in BV/TV at sites directly treated by LLLT was significantly lower in the LASER group than the SHAM group (p<0.05, p<0.05). These results indicated that LLLT using MILNS may be effective for preventing and/or treating trabecular bone loss and the effect may be site-dependent in the same bone.

Effect of proton irradiation followed by hindlimb unloading on bone in mature mice: a model of long-duration spaceflight

Bone loss associated with microgravity unloading is well documented; however, the effects of spaceflight-relevant types and doses of radiation on the skeletal system are not well defined. In addition, the combined effect of unloading and radiation has not received much attention. In the present study, we investigated the effect of proton irradiation followed by mechanical unloading via hindlimb suspension (HLS) in mice. Sixteen-week-old female C57BL/6 mice were either exposed to 1 Gy of protons or a sham irradiation procedure (n=30/group). One day later, half of the mice in each group were subjected to four weeks of HLS or normal loading conditions. Radiation treatment alone (IRR) resulted in approximately 20% loss of trabecular bone volume fraction (BV/TV) in the tibia and femur, with no effect in the cortical bone compartment. Conversely, unloading induced substantially greater loss of both trabecular bone (60-70% loss of BV/TV) and cortical bone (approximately 20% loss of cortical bone volume) in both the tibia and femur, with corresponding decreases in cortical bone strength. Histological analyses and serum chemistry data demonstrated increased levels of osteoclast-mediated bone resorption in unloaded mice, but not IRR. HLS+IRR mice generally experienced greater loss of trabecular bone volume fraction, connectivity density, and trabecular number than either unloading or irradiation alone. Although the duration of unloading may have masked certain effects, the skeletal response to irradiation and unloading appears to be additive for certain parameters. Appropriate modeling of the environmental challenges of long duration spaceflight will allow for a better understanding of the underlying mechanisms mediating spaceflight-associated bone loss and for the development of effective countermeasures.

Multiple exposures to unloading decrease bone's responsivity but compound skeletal losses in C57BL/6 mice

A single exposure to mechanical unloading can result in significant bone loss, but the consequences of multiple exposures are largely unknown. Within a 18-wk period, adult C57BL/6 male mice were exposed to 2 wk of hindlimb unloading (HLU) followed by 4 wk of reambulation (RA) once (1x-HLU), twice (2x-HLU), or three times (3x-HLU), or served as ambulatory age-matched controls. In vivo μCT longitudinally tracked changes in trabecular and cortical compartments of the femur. Normally ambulating control mice experienced significant age-related loss in trabecular bone volume fraction throughout the course of the experiment. This loss was compounded by HLU with 2x- and 3x-HLU mice experiencing a 27% and 24% greater reduction in trabecular bone and a 60% and 63% inhibition of age-related trabecular thickening. The recovery of cortical bone was also incomplete during each 4-wk RA period and, at completion of the experiment, cortical area in 3x-HLU mice was 5% smaller than in control and 1x-HLU. When eliminating age as a confounding variable, comparison between individual HLU/RA cycles showed that the magnitude of the response diminished during subsequent exposures. The extent of trabecular thinning in mice unloaded for the first time was 1.6-fold greater than the second time and nearly twofold greater than the third time. Similarly, the increase in trabecular thickness during the first RA cycle was twofold greater than during the second and third RA cycle. Together, our data demonstrate that even though multiple exposures to mechanical unloading are more detrimental than a single unloading period, bone's mechanosensitivity is reduced with consecutive unloading/reambulation cycles.

Trabecular bone scales allometrically in mammals and birds

Many bones are supported internally by a latticework of trabeculae. Scaling of whole bone length and diameter has been extensively investigated, but scaling of the trabecular network is not well characterized. We analysed trabecular geometry in the femora of 90 terrestrial mammalian and avian species with body masses ranging from 3 g to 3400 kg. We found that bone volume fraction does not scale substantially with animal size, while trabeculae in larger animals' femora are thicker, further apart and fewer per unit volume than in smaller animals. Finite element modelling indicates that trabecular scaling does not alter the bulk stiffness of trabecular bone, but does alter strain within trabeculae under equal applied loads. Allometry of bone's trabecular tissue may contribute to the skeleton's ability to withstand load, without incurring the physiological or mechanical costs of increasing bone mass.

Transient muscle paralysis disrupts bone homeostasis by rapid degradation of bone morphology

We have previously shown that transient paralysis of murine hindlimb muscles causes profound degradation of both trabecular and cortical bone in the adjacent skeleton within 3 weeks. Morphologically, the acute loss of bone tissue appeared to arise primarily due to osteoclastic bone resorption. Given that the loss of muscle function in this model is transient, we speculated that the stimulus for osteoclastic activation would be rapid and morphologic evidence of bone resorption would appear before 21 days. We therefore utilized high-resolution in vivo serial micro-CT to assess longitudinal alterations in lower hindlimb muscle volume, proximal tibia trabecular, and tibia mid-diaphysis cortical bone morphology in 16-week-old female C57 mice following transient calf paralysis from a single injection of botulinum toxin A (BtA; 2U/100 g body weight). In an acute study, we evaluated muscle and bone alterations at days 0, 3, 5, and 12 following transient calf paralysis. In a chronic study, following day 0 imaging, we assessed the recovery of these tissues following the maximum observed trabecular degradation (day 12) through day 84 post-paralysis. The time course and degree of recovery of muscle, trabecular, and cortical bone varied substantially. Significant atrophy of lower limb muscle was evident by day 5 of paralysis, maximal at day 28 (-34.1+/-0.9%) and partially recovered by day 84. Trabecular degradation within the proximal tibia metaphysis occurred more rapidly, with significant reduction in BV/TV by day 3, maximal loss at day 12 (-76.8+/-2.9%) with only limited recovery by day 84 (-51.7+/-5.1% vs. day 0). Significant cortical bone volume degradation at the tibia mid-diaphysis was first identified at day 12, was maximal at day 28 (-9.6+/-1.2%), but completely recovered by day 84. The timing, magnitude, and morphology of the observed bone erosion induced by transient muscle paralysis were consistent with a rapid recruitment and prolific activation of osteoclastic resorption. In a broader context, understanding how brief paralysis of a single muscle group can precipitate such rapid and profound bone resorption in an adjacent bone is likely to provide new insight into how normal muscle function modulates bone homeostasis.

Anatomic variations of the lacunar-canalicular system influence solute transport in bone

Solute transport in the lacunar-canalicular system (LCS) is essential for bone metabolism and mechanotransduction. Using the technique of fluorescence recovery after photobleaching (FRAP) we have been quantifying solute transport in the LCS of murine long bone as a function of loading parameters and molecular size. However, the influence of LCS anatomy, which varies among animal species, bone type and location, age and health condition, is not well understood. In this study, we developed a mathematical model to simulate solute convection in the LCS during a FRAP experiment under a physiological cyclic flow. We found that the transport rate (the reciprocal time constant for refilling the photobleached lacuna) increased linearly with canalicular number and decreased with canalicular length for both diffusion and convection. As a result, the transport enhancement of convection over diffusion was much less sensitive to the variations associated with chick, mouse, rabbit, bovine, dog, horse, and human LCS anatomy, when compared with the rates of diffusion or convection alone. Canalicular density did not affect transport enhancement, while solute size and the lacunar density had more complicated, non-linear effects. This parametric study suggests that solute transport could be altered by varying LCS parameters, and that the anatomical details of the LCS need systemic examination to further understand the etiology of aged and osteoporotic bones.

Regulation of mechanical signals in bone

OBJECTIVES

Response of the skeleton to application and removal of specific mechanical signals is discussed. Anabolic effects of high-frequency, low-magnitude vibrations, a mechanical intervention with a favorable safety profile, as well as the modulation of bone loss by genetic and epigenetic factors during disuse are highlighted.

METHODS

Review.

RESULTS

Bone responds to a great variety of mechanical signals and both high- and low-magnitude stimuli can be sensed by the skeleton. The ability of physical signals to influence bone morphology is strongly dependent on the signal's magnitude, frequency, and duration. Loading protocols at high signal frequencies (vibrations) allow a dramatic reduction in the magnitude of the signal. In the axial skeleton, these signals can be anabolic and anti-catabolic and increase the structural strength of the tissue. They further have shown potential in maxillofacial applications to accelerate the regeneration of bone within defects. Bone's sensitivity to the application and removal of mechanical signals is heavily under the control of the genome. Bone loss modulated by the removal of weight-bearing from the skeleton is profoundly influenced by factors such as genetics, gender, and baseline morphology.

CONCLUSIONS

Adaptation of bone to functional challenges is complex but it is clear that more is not necessarily better and that even very low-magnitude mechanical signals can be anabolic. The development of effective biomechanical interventions in areas such as orthodontics, craniofacial repair, or osteoporosis will require the identification of the specific components of bone's mechanical environment that are anabolic, catabolic, or anti-catabolic.

Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength

Reduced skeletal loading typically leads to bone loss because bone formation and bone resorption become unbalanced. Hibernation is a natural model of musculoskeletal disuse because hibernating animals greatly reduce weight-bearing activity, and therefore, they would be expected to lose bone. Some evidence suggests that small mammals like ground squirrels, bats, and hamsters do lose bone during hibernation, but the mechanism of bone loss is unclear. In contrast, hibernating bears maintain balanced bone remodeling and preserve bone structure and strength. Differences in the skeletal responses of bears and smaller mammals to hibernation may be due to differences in their hibernation patterns; smaller mammals may excrete calcium liberated from bone during periodic arousals throughout hibernation, leading to progressive bone loss over time, whereas bears may have evolved more sophisticated physiological processes to recycle calcium, prevent hypercalcemia, and maintain bone integrity. Investigating the roles of neural and hormonal control of bear bone metabolism could give valuable insight into translating the mechanisms that prevent disuse-induced bone loss in bears into novel therapies for treating osteoporosis.