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Bone mineral density (BMD) is heavily relied upon to reflect structural changes affecting hip strength and fracture risk. Strong correlations between BMD and strength are needed to provide confidence that structural changes are reflected in BMD and, in turn, strength. This study investigated how variation in bone structure gives rise to variation in BMD and strength and tested whether these associations differ with external bone size. Cadaveric proximal femurs ( n  = 30, White women, 36-89+ years) were imaged using nanocomputed tomography (nano-CT) and loaded in a sideways fall configuration to assess bone strength and brittleness. Bone voxels within the nano-CT images were projected onto a plane to create pseudo dual-energy X-ray absorptiometry (pseudo-DXA) images consistent with a clinical DXA scan. A validation study using 19 samples confirmed pseudo-DXA measures correlated significantly with those measured from a commercially available DXA system, including bone mineral content (BMC) ( R 2  = 0.95), area ( R 2  = 0.58), and BMD ( R 2  = 0.92). BMD-strength associations were conducted using multivariate linear regression analyses with the samples divided into narrow and wide groups by pseudo-DXA area. Nearly 80% of the variation in strength was explained by age, body weight, and pseudo-DXA BMD for the narrow subgroup. Including additional structural or density distribution information in regression models only modestly improved the correlations. In contrast, age, body weight, and pseudo-DXA BMD explained only half of the variation in strength for the wide subgroup. Including bone density distribution or structural details did not improve the correlations, but including post-yield deflection (PYD), a measure of bone material brittleness, did increase the coefficient of determination to more than 70% for the wide subgroup. This outcome suggested material level effects play an important role in the strength of wide femoral necks. Thus, the associations among structure, BMD, and strength differed with external bone size, providing evidence that structure-function relationships may be improved by judiciously sorting study cohorts into subgroups. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research., Competing Interests: All authors state they have no conflicts of interest., (© 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.)

Given prior work showing associations between remodeling and external bone size, we tested the hypothesis that wide bones would show a greater negative correlation between whole-bone strength and age compared with narrow bones. Cadaveric male radii (n = 37 pairs, 18 to 89 years old) were evaluated biomechanically, and samples were sorted into narrow and wide subgroups using height-adjusted robustness (total area/bone length). Strength was 54% greater (p < 0.0001) in wide compared with narrow radii for young adults (<40 years old). However, the greater strength of young-adult wide radii was not observed for older wide radii, as the wide (R 2  = 0.565, p = 0.001), but not narrow (R 2  = 0.0004, p = 0.944) subgroup showed a significant negative correlation between strength and age. Significant positive correlations between age and robustness (R 2  = 0.269, p = 0.048), cortical area (Ct.Ar; R 2  = 0.356, p = 0.019), and the mineral/matrix ratio (MMR; R 2  = 0.293, p = 0.037) were observed for narrow, but not wide radii (robustness: R 2  = 0.015, p = 0.217; Ct.Ar: R 2  = 0.095, p = 0.245; MMR: R 2  = 0.086, p = 0.271). Porosity increased with age for the narrow (R 2  = 0.556, p = 0.001) and wide (R 2  = 0.321, p = 0.022) subgroups. The wide subgroup (p < 0.0001) showed a significantly greater elevation of a new measure called the Cortical Pore Score, which quantifies the cumulative effect of pore size and location, indicating that porosity had a more deleterious effect on strength for wide compared with narrow radii. Thus, the divergent strength-age regressions implied that narrow radii maintained a low strength with aging by increasing external size and mineral content to mechanically offset increases in porosity. In contrast, the significant negative strength-age correlation for wide radii implied that the deleterious effect of greater porosity further from the centroid was not offset by changes in outer bone size or mineral content. Thus, the low strength of elderly male radii arose through different biomechanical mechanisms. Consideration of different strength-age regressions (trajectories) may inform clinical decisions on how best to treat individuals to reduce fracture risk. © 2019 American Society for Bone and Mineral Research., (© 2019 American Society for Bone and Mineral Research.)

The phenotypic response of bones differing in morphological, compositional, and mechanical traits to an increase in loading during growth is not well understood. We tested whether bones of two inbred mouse strains that assemble differing sets of traits to achieve mechanical homeostasis at adulthood would show divergent responses to voluntary cage-wheel running. Female A/J and C57BL6/J (B6) 4-week-old mice were provided unrestricted access to a standard cage-wheel for 4 weeks. A/J mice have narrow and highly mineralized femora and B6 mice have wide and less mineralized femora. Both strains averaged 2 to 9.5 km of running per day, with the average-distance run between strains not significantly different ( p  = 0.133). Exercised A/J femora showed an anabolic response to exercise with the diaphyses showing a 2.8% greater total area (Tt.Ar, p  = 0.06) and 4.7% greater cortical area (Ct.Ar, p  = 0.012) compared to controls. In contrast, exercised B6 femora showed a 6.2% ( p  < 0.001) decrease in Tt.Ar ( p  < 0.001) and a 6.7% decrease in Ct.Ar ( p  = 0.133) compared to controls, with the femora showing significant marrow infilling ( p  = 0.002). These divergent morphological responses to exercise, which did not depend on the daily distance run, translated to a 7.9% ( p  = 0.001) higher maximum load (ML) for exercised A/J femora but no change in ML for exercised B6 femora compared to controls. A consistent response was observed for the humeri but not the vertebral bodies. This differential outcome to exercise has not been previously observed in isolated loading or forced treadmill running regimes. Our findings suggest there are critical factors involved in the metabolic response to exercise during growth that require further consideration to understand how genotype, exercise, bone morphology, and whole-bone strength interact during growth. © 2018 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

The impact of adult bone traits on changes in bone structure and mass during aging is not well understood. Having shown that intracortical remodeling correlates with external size of adult long bones led us to hypothesize that age-related changes in bone traits also depend on external bone size. We analyzed hip dual-energy X-ray absorptiometry images acquired longitudinally over 14 years for 198 midlife women transitioning through menopause. The 14-year change in bone mineral content (BMC, R 2  = 0.03, p = 0.015) and bone area (R 2  = 0.13, p = 0.001), but not areal bone mineral density (aBMD, R 2  = 0.00, p = 0.931) correlated negatively with baseline femoral neck external size, adjusted for body size using the residuals from a linear regression between baseline bone area and height. The dependence of the 14-year changes in BMC and bone area on baseline bone area remained significant after adjusting for race/ethnicity, postmenopausal hormone use, the 14-year change in weight, and baseline aBMD, weight, height, and age. Women were sorted into tertiles using the baseline bone area-height residuals. The 14-year change in BMC (p = 0.009) and bone area (p = 0.001) but not aBMD (p = 0.788) differed across the tertiles. This suggested that women showed similar changes in aBMD for different structural and biological reasons: women with narrow femoral necks showed smaller changes in BMC but greater increases in bone area compared to women with wide femoral necks who showed greater losses in BMC but without large compensatory increases in bone area. This finding is opposite to expectations that periosteal expansion acts to mechanically offset bone loss. Thus, changes in femoral neck structure and mass during menopause vary widely among women and are predicted by baseline external bone size but not aBMD. How these different structural and mass changes affect individual strength-decline trajectories remains to be determined. © 2017 American Society for Bone and Mineral Research., (© 2017 American Society for Bone and Mineral Research.)

Previously, we showed that cortical mineralization is coordinately adjusted to mechanically offset external bone size differences between A/J (narrow) and C57BL/6J (wide) mouse femora to achieve whole bone strength equivalence at adulthood. The identity of the genes and their interactions that are responsible for establishing this homeostatic state (ie, canalization) remain unknown. We hypothesize that these inbred strains, whose interindividual differences in bone structure and material properties mimic that observed among humans, achieve functional homeostasis by differentially adjusting key molecular pathways regulating external bone size and mineralization throughout growth. The cortices of A/J and C57BL/6J male mouse femora were phenotyped and gene expression levels were assessed across growth (ie, ages 2, 4, 6, 8, 12, 16 weeks). A difference in total cross-sectional area (p < 0.01) and cortical tissue mineral density were apparent between mouse strains by age 2 weeks and maintained at adulthood (p < 0.01). These phenotypic dissimilarities corresponded to gene expression level differences among key regulatory pathways throughout growth. A/J mice had a 1.55- to 7.65-fold greater expression among genes inhibitory to Wnt pathway induction, whereas genes involved in cortical mineralization were largely upregulated 1.50- to 3.77-fold to compensate for their narrow diaphysis. Additionally, both mouse strains showed an upregulation among Wnt pathway antagonists corresponding to the onset of adult ambulation (ie, increased physiological loads). This contrasts with other studies showing an increase in Wnt pathway activation after functionally isolated, experimental in vivo loading regimens. A/J and C57BL/6J long bones provide a model to develop a systems-based approach to identify individual genes and the gene-gene interactions that contribute to trait differences between the strains while being involved in the process by which these traits are coordinately adjusted to establish similar levels of mechanical function, thus providing insight into the process of canalization. © 2017 American Society for Bone and Mineral Research., (© 2017 American Society for Bone and Mineral Research.)

A major unmet challenge in developing preventative treatment programs for osteoporosis is that the optimal timing of treatment remains unknown. In this commentary we make the argument that the menopausal transition (MT) is a critical period in a woman's life for bone health, and that efforts aimed at reducing fracture risk later in life may benefit greatly from strategies that treat women earlier with the intent of keeping bones strong as long as possible. Bone strength is an important parameter to monitor during the MT because engineering principles can be applied to differentiate those women that maintain bone strength from those women that lose bone strength and are in need of early treatment. It is critical to understand the underlying mechanistic causes for reduced strength to inform treatment strategies. Combining measures of strength with data on how bone structure changes during the MT may help differentiate whether a woman is losing strength because of excessive bone resorption, insufficient compensatory bone formation, trabeculae loss, or some combination of these factors. Each of these biomechanical mechanisms may require a different treatment strategy to keep bones strong. The technologies that enable physicians to differentially diagnose and treat women in a preventive manner, however, have lagged behind the development of prophylactic treatments for osteoporosis. To take advantage of these treatment options, advances in preventive treatment strategies for osteoporosis may require developing new technologies with imaging resolutions that match the pace by which bone changes during the MT and supplementing a woman's bone mineral density (BMD)-status with information from engineering-based analyses that reveal the structural and material changes responsible for the decline in bone strength during the menopausal transition.

Background: The risk of fragility fractures in the United States is approximately 2.5 times greater among black and white women compared with their male counterparts. On average, men of both ethnicities have wider bones of greater cortical mass compared with the narrower bones of lower cortical mass among women. However, it remains uncertain whether the low cortical area observed in the long bones of women is consistent with their narrower bone diameter or if their cortical area is reduced beyond that which is expected for the sex differences in body size and external bone size., Questions/purposes: We asked (1) do black and white women consistently have narrower bones of less strength across long bones compared with black and white men; and (2) do all long bones of black and white women have reduced cortical area compared with black and white men?, Methods: Peripheral quantitative CT was used to quantify bone strength and cross-sectional morphology from the major long bones of 125 white and 115 black adult men and women (20-35 years of age). Regression analyses were used to test for differences in bone strength and cortical area after for adjusting for either body size, bone size, or both., Results: After adjusting bone strength for body size, regression analyses showed that black women had lower bone strength compared with black men (women: mean=298.7-25,522 mg HA mm4, 95% confidence interval [CI], 270-27,692 mg HA mm4; men: mean = 381.6-30,945 mg HA mm4, 95% CI, 358.2-32,853 mg HA mm4; percent difference=12%-38%, p=0.06-0.0001). Similarly, white women also had lower bone strength compared with white men (women: mean=229.5-22,892 mg HA mm4, 95% CI, 209.3-24,539 mg HA mm4; men: mean=314.3-29,986 mg HA mm4, 95% CI, 297.3-31,331 mg HA mm4; percent difference=27%-49%, p=0.0001). All long bones of women for both ethnicities showed lower cortical area compared with men. After accounting for both body size and external bone size, black women (women: mean=43.25-357.70 mm2, 95% CI, 41.45-367.52 mm2; men: mean=48.06-400.10 mm2, 95% CI, 46.67-408.72; percent difference=6%-25%, p=0.02-0.0001) and white women (women: mean=38.53-350.10 mm2, 95% CI, 36.99-359.80 mm2; men: mean=42.06-394.30 mm2, 95% CI, 40.95-402.10 mm2; percent difference=6%-22%, p=0.02-0.0001) were shown to have lower cortical area than their male counterparts. Therefore, the long bones of women are not only more slender than those of men, but also show a reduced cortical area that is 6% to 25% greater than expected for their external size, depending on the bone being considered., Conclusions: The long bones of females are not just a more slender version of male long bones. Women have less cortical area than expected for their body size and bone size, which in part explains their reduced bone strength when compared with the more robust bones of men., Clinical Relevance: The outcome of this assessment may be clinically important for the development of diagnostics and treatment regimens used to combat fractures. Future work should look at how the relationship among parameters reported here translates to the more fracture-prone metaphyseal regions.

Background: The twofold greater lifetime risk of fracturing a bone for white women compared with white men and black women has been attributed in part to differences in how the skeletal system accumulates bone mass during growth. On average, women build more slender long bones with less cortical area compared with men. Although slender bones are known to have a naturally lower cortical area compared with wider bones, it remains unclear whether the relatively lower cortical area of women is consistent with their increased slenderness or is reduced beyond that expected for the sex-specific differences in bone size and body size. Whether this sexual dimorphism is consistent with ethnic background and is recapitulated in the widely used mouse model also remains unclear., Questions/purposes: We asked (1) do black women build bones with reduced cortical area compared with black men; (2) do white women build bones with reduced cortical area compared with white men; and (3) do female mice build bones with reduced cortical area compared with male mice?, Methods: Bone strength and cross-sectional morphology of adult human and mouse bone were calculated from quantitative CT images of the femoral midshaft. The data were tested for normality and regression analyses were used to test for differences in cortical area between men and women after adjusting for body size and bone size by general linear model (GLM)., Results: Linear regression analysis showed that the femurs of black women had 11% lower cortical area compared with those of black men after adjusting for body size and bone size (women: mean=357.7 mm2; 95% confidence interval [CI], 347.9-367.5 mm2; men: mean=400.1 mm2; 95% CI, 391.5-408.7 mm2; effect size=1.2; p<0.001, GLM). Likewise, the femurs of white women had 12% less cortical area compared with those of white men after adjusting for body size and bone size (women: mean=350.1 mm2; 95% CI, 340.4-359.8 mm2; men: mean=394.3 mm2; 95% CI, 386.5-402.1 mm2; effect size=1.3; p<0.001, GLM). In contrast, female and male femora from recombinant inbred mouse strains showed the opposite trend; femurs from female mice had a 4% larger cortical area compared with those of male mice after adjusting for body size and bone size (female: mean=0.73 mm2; 95% CI, 0.71-0.74 mm2; male: mean=0.70 mm2; 95% CI, 0.68-0.71 mm2; effect size=0.74; p=0.04, GLM)., Conclusions: Female femurs are not simply a more slender version of male femurs. Women acquire substantially less mass (cortical area) for their body size and bone size compared with men. Our analysis questions whether mouse long bone is a suitable model to study human sexual dimorphism., Clinical Relevance: Identifying differences in the way bones are constructed may be clinically important for developing sex-specific diagnostics and treatment strategies to reduce fragility fractures.

Advances in computed tomography (CT) imaging are opening new avenues toward more precise characterization and quantification of connective tissue microarchitecture. In the last two decades, micro-computed tomography (microCT) has significantly augmented destructive methods for the 3D micro-analysis of tissue structure, primarily in the bone research field. Recently, microCT has been employed in combination with contrast agents to generate contrast-enhanced images of soft tissues that are otherwise difficult to visualize due to their native radiodensity. More recent advances in CT technology have enabled ultra-high resolution imaging by utilizing a more powerful nano-focused X-ray source, such as that found in nano-computed tomography (nanoCT) systems. NanoCT imaging has facilitated the expansion of musculoskeletal research by reducing acquisition time and significantly expanding the range of samples that can be imaged in terms of size, age and tissue-type (bone, muscle, tendon, cartilage, vessels and adipose tissue). We present the application and early results of nanoCT imaging in various tissue types and how this ultra-high resolution imaging modality is capable of characterizing microstructures at levels of details previously not possible. Contrast-enhanced imaging techniques to enable soft-tissue visualization and characterization are also outlined.

Traits of the skeletal system are coordinately adjusted to establish mechanical homeostasis in response to genetic and environmental factors. Prior work demonstrated that this 'complex adaptive' process is not perfect, revealing a two-fold difference in whole bone stiffness of the tibia across a population. Robustness (specifically, total cross-sectional area relative to length) varies widely across skeletal sites and between sexes. However, it is unknown whether the natural variation in whole bone stiffness and strength also varies across skeletal sites and between men and women. We tested the hypotheses that: 1) all major long bones of the appendicular skeleton demonstrate inherent, systemic constraints in the degree to which morphological and compositional traits can be adjusted for a given robustness; and 2) these traits covary in a predictable manner independent of body size and robustness. We assessed the functional relationships among robustness, cortical area (Ct.Ar), cortical tissue mineral density (Ct.TMD), and bone strength index (BSI) across the long bones of the upper and lower limbs of 115 adult men and women. All bones showed a significant (p<0.001) positive regression between BSI and robustness after adjusting for body size, with slender bones being 1.7-2.3 times less stiff and strong in men and 1.3-2.8 times less stiff and strong in women compared to robust bones. Our findings are the first to document the natural inter-individual variation in whole bone stiffness and strength that exist within populations and that is predictable based on skeletal robustness for all major long bones. Documenting and further understanding this natural variation in strength may be critical for differentially diagnosing and treating skeletal fragility., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Functional adaptation may complicate the choice of phenotype used in genetic studies that seek to identify genes contributing to fracture susceptibility. Often, genetic variants affecting one trait are compensated by coordinated changes in other traits. Bone fracture is a prototypic example because mechanical function of long bones (stiffness and strength) depends on how the system coordinately adjusts the amount (cortical area) and quality (tissue-mineral density, TMD) of bone tissue to mechanically offset the natural variation in bone robustness (total area/length). We propose that efforts aimed at identifying genes regulating fracture resistance will benefit from better understanding how functional adaptation contributes to the genotype-phenotype relationship. We analyzed the femurs of C57BL/6J-Chr(A/J)/NaJ Chromosome Substitution Strains (CSSs) to systemically interrogate the mouse genome for chromosomes harboring genes that regulate mechanical function. These CSSs (CSS-i, i=the substituted chromosome) showed changes in mechanical function on the order of -26.6 to +11.5% relative to the B6 reference strain after adjusting for body size. Seven substitutions showed altered robustness, cortical area, or TMD, but no effect on mechanical function (CSS-4, 5, 8, 9, 17, 18, 19); six substitutions showed altered robustness, cortical area, or TMD, and reduced mechanical function (CSS-1, 2, 6, 10, 12, 15); and one substitution also showed reduced mechanical function but exhibited no significant changes in the three physical traits analyzed in this study (CSS-3). A key feature that distinguished CSSs that maintained function from those with reduced function was whether the system adjusted cortical area and TMD to the levels needed to compensate for the natural variation in bone robustness. These results provide a novel biomechanical mechanism linking genotype with phenotype, indicating that genes control function not only by regulating individual traits, but also by regulating how the system coordinately adjusts multiple traits to establish function., (Copyright © 2014 Elsevier Inc. All rights reserved.)

To determine whether peak vertical and horizontal impact accelerations were different while running on a track or on a treadmill, 12 healthy subjects (average age 32.8 ± 9.8 y), were fitted with a novel, wireless accelerometer capable of recording triaxial acceleration over time. The accelerometer was attached to a custom-made acrylic plate and secured at the level of the L5 vertebra via a tight fitting triathlon belt. Each subject ran 4 miles on a synthetic, indoor track at a self-selected pace and accelerations were recorded on three perpendicular axes. Seven days later, the subjects ran 4 miles on a treadmill set at the individual runner's average pace on the track and the peak vertical and horizontal impact magnitudes between the track and treadmill were compared. There was no difference (P = .52) in the average peak vertical impact accelerations between the track and treadmill over the 4 mile run. However, peak horizontal impact accelerations were greater (P = .0012) on the track when compared with the treadmill. This study demonstrated the feasibility for long-term impact accelerations monitoring using a novel wireless accelerometer.

Many genetic and environmental perturbations lead to measurable changes in bone morphology, matrix composition, and matrix organization. The skeletal system is highly adaptive, such that genetic and environmental perturbations affecting one trait are often accompanied by compensatory changes in other traits. This adaptation should be considered when attempting to draw meaningful conclusions about the role of a gene, for example. The challenge is to figure out which alterations can be attributed to the perturbation and which result from adaptive changes associated with establishing mechanical function. The goal of this chapter is to describe straight-forward biomechanical methods that can be used to determine whether a genetic or environmental perturbation affected bone strength. We introduce a systematic method for evaluating how bone strength was altered in the context of morphology and tissue-level mechanical properties, which are determined in large part from matrix composition, matrix organization, and porosity. We present this work as a first step toward screening mice for a phenotypic effect and for establishing the associated biomechanical mechanism by which function was altered. The outcome of these analyses generally provides insight into the next set of experiments required to further connect the cellular perturbation with the functional changes. The protocols were written to enable researchers without a background in engineering to conduct the assays or to enable researchers to better understand the outcomes of similar assays conducted by colleagues knowledgeable in engineering.