Femoral neck shape and the spatial distribution of its mineral mass varies with its size: Clinical and biomechanical implications

Structural differences contributing to sex-specific associations between FN BMD and whole-bone strength for adult White women and men

Hip areal BMD (aBMD) is widely used to identify individuals with increased fracture risk. Low aBMD indicates low strength, but this association differs by sex with men showing greater strength for a given aBMD than women. To better understand the structural basis giving rise to this sex-specific discrepancy, cadaveric proximal femurs from White female and male donors were imaged using nano-CT and loaded in a sideways fall configuration to assess strength. FN pseudoDXA images were generated to identify associations among structure, aBMD, and strength that differ by sex. Strength correlated significantly with pseudoDXA aBMD for females (R2 = 0.468, P < .001) and males (R2 = 0.393, P < .001), but the elevations (y-intercepts) of the linear regressions differed between sexes (P < .001). Male proximal femurs were 1045 N stronger than females for a given pseudoDXA aBMD. However, strength correlated with pseudoDXA BMC for females (R2 = 0.433, P < .001) and males (R2 = 0.443, P < .001) but without significant slope (P = .431) or elevation (P = .058) differences. Dividing pseudoDXA BMC by FN-width, total cross-sectional area, or FN-volume led to significantly different associations between strength and the size-adjusted BMC measures for women and men. Three structural differences were identified that differentially affected aBMD and strength for women and men: First, men had more bone mass per unit volume than women; second, different cross-sectional shapes resulted in larger proportions of bone mass orthogonal to the DXA image for men than women; and third, men and women had different proportions of cortical and trabecular bone relative to BMC. Thus, the proximal femurs of women were not smaller versions of men but were constructed in fundamentally different manners. Dividing BMC by a bone size measure was responsible for the sex-specific associations between hip aBMD and strength. Thus, a new approach for adjusting measures of bone mass for bone size and stature is warranted.

Associations Among Hip Structure, Bone Mineral Density, and Strength Vary With External Bone Size in White Women

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.

Sex and External Size Specific Limitations in Assessing Bone Health From Adult Hand Radiographs

Morphological parameters measured for the second metacarpal from hand radiographs are used clinically for assessing bone health during growth and aging. Understanding how these morphological parameters relate to metacarpal strength and strength at other anatomical sites is critical for providing informed decision-making regarding treatment strategies and effectiveness. The goals of this study were to evaluate the extent to which 11 morphological parameters, nine of which were measured from hand radiographs, relate to experimentally measured whole-bone strength assessed at multiple anatomical sites and to test whether these associations differed between men and women. Bone morphology and strength were assessed for the second and third metacarpals, radial diaphysis, femoral diaphysis, and proximal femur for 28 white male donors (18-89 years old) and 35 white female donors (36-89+ years old). The only morphological parameter to show a significant correlation with strength without a sex-specific effect was cortical area. Dimensionless morphological parameters derived from hand radiographs correlated significantly with strength for females, but few did for males. Males and females showed a significant association between the circularity of the metacarpal cross-section and the outer width measured in the mediolateral direction. This cross-sectional shape variation contributed to systematic bias in estimating strength using cortical area and assuming a circular cross-section. This was confirmed by the observation that use of elliptical formulas reduced the systematic bias associated with using circular approximations for morphology. Thus, cortical area was the best predictor of strength without a sex-specific difference in the correlation but was not without limitations owing to out-of-plane shape variations. The dependence of cross-sectional shape on the outer bone width measured from a hand radiograph may provide a way to further improve bone health assessments and informed decision making for optimizing strength-building and fracture-prevention treatment strategies. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

High Cortico-Trabecular Transitional Zone Porosity and Reduced Trabecular Density in Men and Women with Stress Fractures

To determine whether stress fractures are associated with bone microstructural deterioration we quantified distal radial and the unfractured distal tibia using high resolution peripheral quantitative computed tomography in 26 cases with lower limb stress fractures (15 males, 11 females; mean age 37.1 ± 3.1 years) and 62 age-matched healthy controls (24 males, 38 females; mean age 35.0 ± 1.6 years). Relative to controls, in men, at the distal radius, cases had smaller cortical cross sectional area (CSA) (p = 0.012), higher porosity of the outer transitional zone (OTZ) (p = 0.006), inner transitional zone (ITZ) (p = 0.043) and the compact-appearing cortex (CC) (p = 0.023) while trabecular vBMD was lower (p = 0.002). At the distal tibia, cases also had a smaller cortical CSA (p = 0.008). Cortical porosity was not higher, but trabecular vBMD was lower (p = 0.001). Relative to controls, in women, cases had higher distal radial porosity of the OTZ (p = 0.028), ITZ (p = 0.030) not CC (p = 0.054). Trabecular vBMD was lower (p = 0.041). Distal tibial porosity was higher in the OTZ (p = 0.035), ITZ (p = 0.009), not CC. Stress fractures are associated with compromised cortical and trabecular microstructure.

The Oval-like Cross-section of Femoral Neck Isthmus in Three-dimensional Morphological Analysis

Objectives

To investigate the cross‐section shape of the femoral neck isthmus (FNI) in three‐dimensional reconstruction model of the femoral neck.

Methods

From December 2009 to December 2012, computed tomography (CT) data of bilateral hip joint from 200 consecutive patients (137 males and 63 females, 69.41 ± 9.21 years old, ranged from 50–85 years old) who underwent surgical treatments for proximal femoral fracture were retrospectively reviewed. The 3D model of the proximal femur was reconstructed, and the “inertia axis” method, which was applied to measure the long and short axes of the cross‐section of the FNI, was established. The cross‐sectional area and perimeter were calculated by a formula using the length of the long and short axes and then compared with the actual measured values by the software. Correlation between the descriptive parameters of the FNI cross‐section (area, perimeter, and eccentricity) and patients' demographics (age, height, and weight) was analyzed. Stepwise linear regression analysis was used to determine the main relevant factors.

Results

The ICC results showed excellent data reproducibility ranged from 0.989 to 0.996. There was no significant difference in the cross‐sectional area of the FNI between the actual measured values and the predicted values using the formula (732.83 ± 126.74 mm²vs 731.62 ± 128.15 mm², P = 0.322). The perimeter using the two methods showed narrow while significant difference (97.86 ± 8.60 mm vs 92.84 ± 8.65 mm, P < 0.001), the actual measured values were about 5 mm greater than the predicted values. The parameters (area, perimeter, and eccentricity) were significantly larger in male than female (P < 0.001). A positive correlation between the cross‐sectional area, perimeter, height, and weight was observed. The stepwise linear regression analysis showed that the regression equation of the FNI area was as follows: Y = −1083.75 + 1033.86 × HEIGHT + 1.92 × WEIGHT, R² = 0.489.

Conclusion

The cross‐section shape of the FNI appears to be oval‐like in the 3D model, which is separated according to the inertia axis, and the findings proposed an anatomical basis for the further study of the spatial configuration of cannulated screws in the treatment of femoral neck fractures.

Heterogeneity in microstructural deterioration following spinal cord injury

BACKGROUND

Modelling and remodelling adapt bone morphology to accommodate strains commonly encountered during loading. If strains exceed a threshold threatening fracture, modelling-based bone formation increases bone volume reducing these strains. If unloading reduces strains below a threshold that inhibits resorption, increased remodelling-based bone resorption reduces bone volume restoring strains, but at the price of compromised bone volume and microstructure. As weight-bearing regions are adapted to greater strains, we hypothesized that microstructural deterioration will be more severe than at regions commonly adapted to low strains following spinal cord injury.

METHODS

We quantified distal tibial, fibula and radius volumetric bone mineral density (vBMD) using high-resolution peripheral quantitative computed tomography in 31 men, mean age 43.5 years (range 23.5-75.0), 12 with tetraplegia and 19 with paraplegia of 0.7 to 18.6 years duration, and 102 healthy age- and sex-matched controls. Differences in morphology relative to controls were expressed as standardized deviation (SD) scores (mean ± SD). Standardized between-region differences in vBMD were expressed as SDs (95% confidence intervals, CI).

RESULTS

Relative to controls, men with tetraplegia had deficits in total vBMD of -1.72 ± 1.38 SD at the distal tibia (p < 0.001) and - 0.68 ± 0.69 SD at distal fibula (p = 0.041), but not at the distal radius, despite paralysis. Deficits in men with paraplegia were -2.14 ± 1.50 SD (p < 0.001) at the distal tibia and -0.83 ± 0.98 SD (p = 0.005) at the distal fibula while distal radial total vBMD was 0.23 ± 1.02 (p = 0.371), not significantly increased, despite upper limb mobility. Comparing regions, in men with tetraplegia, distal tibial total vBMD was 1.04 SD (95%CI 0.07, 2.01) lower than at the distal fibula (p = 0.037) and 1.51 SD (95%CI 0.45, 2.57) lower than at the distal radius (p = 0.007); the latter two sites did not differ from each other. Results were similar in men with paraplegia, but total vBMD at the distal fibula was 1.06 SD (95%CI 0.35, 1.77) lower than at the distal radius (p = 0.004).

CONCLUSION

Microarchitectural deterioration following spinal cord injury is heterogeneous, perhaps partly because strain thresholds regulating the cellular activity of mechano-transduction are region specific.

Globular structure of the hypermineralized tissue in human femoral neck

Bone becomes more fragile with ageing. Among many structural changes, a thin layer of highly mineralized and brittle tissue covers part of the external surface of the thin femoral neck cortex in older people and has been proposed to increase hip fragility. However, there have been very limited reports on this hypermineralized tissue in the femoral neck, especially on its ultrastructure. Such information is critical to understanding both the mineralization process and its contributions to hip fracture. Here, we use multiple advanced techniques to characterize the ultrastructure of the hypermineralized tissue in the neck across various length scales. Synchrotron radiation micro-CT found larger but less densely distributed cellular lacunae in hypermineralized tissue than in lamellar bone. When examined under FIB-SEM, the hypermineralized tissue was mainly composed of mineral globules with sizes varying from submicron to a few microns. Nano-sized channels were present within the mineral globules and oriented with the surrounding organic matrix. Transmission electron microscopy showed the apatite inside globules were poorly crystalline, while those at the boundaries between the globules had well-defined lattice structure with crystallinity similar to the apatite mineral in lamellar bone. No preferred mineral orientation was observed both inside each globule and at the boundaries. Collectively, we conclude based on these new observations that the hypermineralized tissue is non-lamellar and has less organized mineral, which may contribute to the high brittleness of the tissue.

Deterioration of Cortical and Trabecular Microstructure Identifies Women With Osteopenia or Normal Bone Mineral Density at Imminent and Long-Term Risk for Fragility Fracture: A Prospective Study

More than 70% of women sustaining fractures have osteopenia or "normal" bone mineral density (BMD). These women remain undetected using the BMD threshold of -2.5 SD for osteoporosis. As microstructural deterioration increases bone fragility disproportionate to the bone loss producing osteopenia/normal BMD, we hypothesized that the structural fragility score (SFS) of ≥70 units, a measure capturing severe cortical and trabecular deterioration, will identify these women. Distal radial images were acquired using high-resolution peripheral quantitative tomography in postmenopausal French women, mean age 67 years (range 42-96 years); 1539 women were followed for 4 years (QUALYOR) and 561 women followed for 8 years (OFELY). Women with osteopenia or normal BMD accounted for ~80% of fractures. Women ≥70 years, 29.2% of the cohort, accounted for 39.2% to 61.5% of fractures depending on follow-up duration. Women having fractures had a higher SFS, lower BMD, and a higher fracture risk assessment score (FRAX) than women remaining fracture-free. In each BMD category (osteoporosis, osteopenia, normal BMD), fracture incidence was two to three times higher in women with SFS ≥70 than <70. In multivariable analyses, associations with fractures remained for BMD and SFS, not FRAX. BMD was no longer, or weakly, associated with fractures after accounting for SFS, whereas SFS remained associated with fracture after accounting for BMD. SFS detected two-to threefold more women having fractures than BMD or FRAX. SFS in women with osteopenia/normal BMD conferred an odds ratio for fracture of 2.69 to 5.19 for women of any age and 4.98 to 12.2 for women ≥70 years. Receiver-operator curve (ROC) analyses showed a significant area under the curve (AUC) for SFS, but not BMD or FRAX for the women ≥70 years of age. Targeting women aged ≥70 years with osteopenia indicated that treating 25% using SFS to allocate treatment conferred a cost-effectiveness ratio < USD $21,000/QALY saved. Quantifying microstructural deterioration complements BMD by identifying women without osteoporosis at imminent and longer-term fracture risk. © 2019 American Society for Bone and Mineral Research.

Increased Cortical Porosity and Reduced Trabecular Density Are Not Necessarily Synonymous With Bone Loss and Microstructural Deterioration

Absolute values of cortical porosity and trabecular density are used to estimate fracture risk, but these values are the net result of their growth-related assembly and age-related deterioration. Because bone loss affects both cortical and trabecular bone, we hypothesized that a surrogate measure of bone fragility should capture the age-related deterioration of both traits, and should do so independently of their peak values. Accordingly, we developed a structural fragility score (SFS), which quantifies the increment in distal radial cortical porosity and decrement in trabecular density relative to their premenopausal mean values in 99 postmenopausal women with forearm fractures and 105 controls using HR-pQCT. We expressed the results as odds ratios (ORs; 95% CI). Cortical porosity was associated with fractures in the presence of deteriorated trabecular density (OR 2.30; 95% CI, 1.30 to 4.05; p = 0.004), but not if trabecular deterioration was absent (OR 0.96; 95% CI, 0.50 to 1.86; p = 0.91). Likewise, trabecular density was associated with fractures in the presence of high cortical porosity (OR 3.35; 95% CI, 1.85 to 6.07; p < 0.0001), but not in its absence (OR 1.60; 95% CI, 0.78 to 3.28; p = 0.20). The SFS, which captures coexisting cortical and trabecular deterioration, was associated with fractures (OR 4.52; 95% CI, 2.17 to 9.45; p < 0.0001). BMD was associated with fracture before accounting for the SFS (OR 5.79; 95% CI, 1.24 to 27.1; p = 0.026), not after (OR 4.38; 95% CI, 0.48 to 39.9; p = 0.19). The SFS was associated with fracture before (OR 4.67; 95% CI, 2.21 to 9.88) and after (OR 3.94; 95% CI, 1.80 to 8.6) accounting for BMD (both ps < 0.0001). The disease of bone fragility is captured by cortical and trabecular deterioration: A measurement of coexisting cortical and trabecular deterioration is likely to identify women at risk for fracture more robustly than absolute values of cortical porosity, trabecular density, or BMD. © 2018 The Authors. JBMR Plus Published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

The Influence of Cortical Porosity on the Strength of Bone During Growth and Advancing Age

PURPOSE OF REVIEW

Bone densitometry provides a two-dimensional projected areal apparent bone mineral density that fails to capture the heterogeneity of bone's material composition and macro-, micro-, and nano-structures critical to its material and structural strength. Assessment of the structural basis of bone fragility has focused largely on trabecular bone based on the common occurrence of fragility fractures at sites with substantial amounts of trabecular bone. This review focuses on the contribution of cortical bone to bone fragility throughout life.

RECENT FINDINGS

Accurately differentiating cortical and trabecular bone loss has important implications in quantifying bone fragility as these compartments have differing effects on bone strength. Recent advances in imaging methodology have improved distinction of these two compartments by (i) recognition of a cortico-trabecular transitional zone and (ii) quantifying bone microstructure in a region of interest that is a percentage of bone length rather than a fixed point. Additionally, non-invasive three-dimensional imaging methods allow more accurate quantification of changes in the cortical, trabecular, and cortico-trabecular compartments during growth, aging, disease, and treatment. Over 75% of the skeleton is assembled as cortical bone. Of all fragility fractures, ~ 80% are appendicular and involve regions rich in cortical bone and ~ 70% of all age-related appendicular bone loss is cortical and is mainly due to unbalanced intracortical remodeling which increases cortical porosity. The failure to achieve the optimal peak bone microstructure during growth due to disease and the deterioration in cortical and trabecular bone produced by bone loss compromise bone strength. The loss of strength produced by microstructural deterioration is disproportionate to the bone loss producing this deterioration. The reason for this is that the loss of strength increases as a 7th power function of the rise in cortical porosity and a 3rd power function of the fall in trabecular density (Schaffler and Burr in J Biomech. 21(1):13-6, 1988), hence the need to quantify bone microstructure.

Interpreting the three-dimensional orientation of vascular canals and cross-sectional geometry of cortical bone in birds and bats

Cortical bone porosity and specifically the orientation of vascular canals is an area of growing interest in biomedical research and comparative/paleontological anatomy. The potential to explain microstructural adaptation is of great interest. However, the determinants of the development of canal orientation remain unclear. Previous studies of birds have shown higher proportions of circumferential canals (called laminarity) in flight bones than in hindlimb bones, and interpreted this as a sign that circumferential canals are a feature for resistance to the torsional loading created by flight. We defined the laminarity index as the percentage of circumferential canal length out of the total canal length. In this study we examined the vascular canal network in the humerus and femur of a sample of 31 bird and 24 bat species using synchrotron micro-computed tomography (micro-CT) to look for a connection between canal orientation and functional loading. The use of micro-CT provides a full three-dimensional (3D) map of the vascular canal network and provides measurements of the 3D orientation of each canal in the whole cross-section of the bone cortex. We measured several cross-sectional geometric parameters and strength indices including principal and polar area moments of inertia, principal and polar section moduli, circularity, buckling ratio, and a weighted cortical thickness index. We found that bat cortices are relatively thicker and poorly vascularized, whereas those of birds are thinner and more highly vascularized, and that according to our cross-sectional geometric parameters, bird bones have a greater resistance to torsional stress than the bats; in particular, the humerus in birds is more adapted to resist torsional stresses than the femur. Our results show that birds have a significantly (P = 0.031) higher laminarity index than bats, with birds having a mean laminarity index of 0.183 in the humerus and 0.232 in the femur, and bats having a mean laminarity index of 0.118 in the humerus and 0.119 in the femur. Counter to our expectation, the birds had a significantly higher laminarity index in the femur than in the humerus (P = 0.035). To evaluate whether this discrepancy was a consequence of methodology we conducted a comparison between our 3D method and an analogue to two-dimensional (2D) histological measurements. This comparison revealed that 2D methods significantly underestimate (P < 0.001) the amount of longitudinal canals by an average of 20% and significantly overestimate (P < 0.001) the laminarity index by an average of 7.7%, systematically mis-estimating indices of vascular canal orientations. In comparison with our 3D results, our approximated 2D measurement had the same results for comparisons between the birds and bats but found significant differences only in the longitudinal index between the humerus and the femur for both groups. The differences between our 3D and pseudo-2D results indicate that differences between our findings and the literature may be partially based in methodology. Overall, our results do not support the hypothesis that the bones of flight are more laminar, suggesting a complex relation between functional loading and microstructural adaptation.

Quantifying sex, race, and age specific differences in bone microstructure requires measurement of anatomically equivalent regions

INTRODUCTION

Individuals differ in forearm length. As microstructure differs along the radius, we hypothesized that errors may occur when sexual and racial dimorphisms are quantified at a fixed distance from the radio-carpal joint.

METHODS

Microstructure was quantified ex vivo in 18 cadaveric radii using high resolution peripheral quantitative computed tomography and in vivo in 158 Asian and Caucasian women and men at a fixed region of interest (ROI), a corrected ROI positioned at 4.5-6% of forearm length and using the fixed ROI adjusted for cross sectional area (CSA), forearm length or height. Secular effects of age were assessed by comparing 38 younger and 33 older women.

RESULTS

Ex vivo, similar amounts of bone mass fashioned adjacent cross sections. Larger distal cross sections had thinner porous cortices of lower matrix mineral density (MMD), a larger medullary CSA and higher trabecular density. Smaller proximal cross-sections had thicker less porous cortices of higher MMD, a small medullary canal with little trabecular bone. Taller persons had more distally positioned fixed ROIs which moved proximally when corrected. Shorter persons had more proximally positioned fixed ROIs which moved distally when corrected, so dimorphisms lessened. In the corrected ROIs, in Caucasians, women had 0.6 SD higher porosity and 0.6 SD lower trabecular density than men (p<0.01). In Asians, women had 0.25 SD higher porosity (NS) and 0.5 SD lower trabecular density than men (p<0.05). In women, Asians had 0.8 SD lower porosity and 0.3 SD higher trabecular density than Caucasians (p<0.01). In men, Asians and Caucasians had similar porosity and trabecular density. Results were similar using an adjusted fixed ROI. Adjusting for secular effects of age on forearm length resulted in the age-related increment in porosity increasing from 2.08 SD to 2.48 SD (p<0.05).

CONCLUSION

Assessment of sex, race and age related differences in microstructure requires measurement of anatomically equivalent regions.

A method for the measurement of dispersion curves of circumferential guided waves radiating from curved shells: experimental validation and application to a femoral neck mimicking phantom

Our long-term goal is to develop an ultrasonic method to characterize the thickness, stiffness and porosity of the cortical shell of the femoral neck, which could enhance hip fracture risk prediction. To this purpose, we proposed to adapt a technique based on the measurement of guided waves. We previously evidenced the feasibility of measuring circumferential guided waves in a bone-mimicking phantom of a circular cross-section of even thickness. The goal of this study is to investigate the impact of the complex geometry of the femoral neck on the measurement of guided waves. Two phantoms of an elliptical cross-section and one phantom of a realistic cross-section were investigated. A 128-element array was used to record the inter-element response matrix of these waveguides. This experiment was simulated using a custom-made hybrid code. The response matrices were analyzed using a technique based on the physics of wave propagation. This method yields portions of dispersion curves of the waveguides which were compared to reference dispersion curves. For the elliptical phantoms, three portions of dispersion curves were determined with a good agreement between experiment, simulation and theory. The method was thus validated. The characteristic dimensions of the shell were found to influence the identification of the circumferential wave signals. The method was then applied to the signals backscattered by the superior half of constant thickness of the realistic phantom. A cut-off frequency and some portions of modes were measured, with a good agreement with the theoretical curves of a plate waveguide. We also observed that the method cannot be applied directly to the signals backscattered by the lower half of varying thicknesses of the phantom. The proposed approach could then be considered to evaluate the properties of the superior part of the femoral neck, which is known to be a clinically relevant site.

Unmet needs and current and future approaches for osteoporotic patients at high risk of hip fracture

This review provides a critical analysis of currently available approaches to increase bone mass, structure and strength through drug therapy and of possible direct intra-osseous interventions for the management of patients at imminent risk of hip fracture.

PURPOSE

Osteoporotic hip fractures represent a particularly high burden in morbidity-, mortality- and health care-related costs. There are challenges and unmet needs in the early prevention of hip fractures, opening the perspective of new developments for the management of osteoporotic patients at imminent and/or at very high risk of hip fracture. Amongst them, preventive surgical intervention needs to be considered.

METHODS

A European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO)/International Osteoporosis Foundation (IOF) working group reviewed the presently available intervention modalities including preventive surgical options for hip fragility. This paper represents a summary of the discussions.

RESULTS

Prevention of hip fracture is currently based on regular physical activity; prevention of falls; correction of nutritional deficiencies, including vitamin D repletion; and pharmacological intervention. However, efficacy of these various measures to reduce hip fractures is at most 50% and may need months or years before becoming effective. To face the challenges of early prevention of hip fractures for osteoporotic patients at imminent and/or at very high risk of hip fracture, preventive surgical intervention needs further investigation.

CONCLUSION

Preventive surgical intervention needs to be appraised for osteoporotic patients at imminent and/or at very high risk of hip fracture.

Increased incidence of femoral fractures in small femurs and women undergoing uncemented total hip arthroplasty - why?

The aim of this study was to investigate the relationship between the geometry of the proximal femur and the incidence of intra-operative fracture during uncemented total hip arthroplasty (THA). We studied the pre-operative CT scans of 100 patients undergoing THA with an uncemented femoral component. We measured the anteroposterior and mediolateral dimensions at the level of division of the femoral neck to calculate the aspect ratio of the femur. Wide variations in the shape of the femur were observed, from round, to very narrow elliptic. The femurs of women were narrower than those of men (p < 0.0001) and small femurs were also narrower than large ones. Patients with an intra-operative fracture of the calcar had smaller and narrower femurs than those without a fracture (p < 0.05) and the implanted Corail stems were smaller in those with a fracture (mean size 9 vs 12, p < 0.0001). The variability of the shape of the femoral neck at the level of division contributes to the understanding of the causation of intra-operative fractures in uncemented THA.

Evaluating accuracy of structural geometry by DXA methods with an anthropometric proximal femur phantom

DXA-derived bone structural geometry has been reported extensively but lacks an accuracy standard. In this study, we describe a novel anthropometric structural geometry phantom that simulates the proximal femur for use in assessing accuracy of geometry measurements by DXA or other X-ray methods. The phantom consists of seven different interchangeable neck modules with geometries that span the range of dimensions in an adult human proximal femur, including those representing osteoporosis. Ten repeated hip scans of each neck module using two current DXA scanner models were performed without repositioning. After scanner specific calibration, hip structure analysis was used to derive structural geometry. Scanner performance was similar for the two manufacturers. DXA-derived HSA geometric measurements were highly correlated with values derived directly from phantom geometry and position; R² between DXA and phantom measures were greater than 94% for all parameters, while precision error ranged between 0.3 and 3.9%. Despite high R² there were some systematic geometry errors for both scanners that were small for outer diameter, but increasing with complexity of geometrical parameter; e.g. buckling ratio. In summary, the anthropometric phantom and its fabrication concept were shown to be appropriate for evaluating proximal femoral structural geometry in two different DXA systems.

Gender differences in the heritability of musculoskeletal and body composition parameters in mother-daughter and mother-son pairs

Bone mass and body composition traits are genetically programmed, but the timing and gender and site specificities of their heritability are unclear. Mother-child correlations of bone mineral density (BMD) and bone mineral content, lean mass, and fat mass were studied in 169 premenopausal mothers and their 239 children. Heritability estimates of lean mass, fat mass, BMD, and area were derived for each gender and pubertal stage. There were significant correlations for most densitometry-derived variables at the spine, hip, femoral neck (FN), and total body (r=0.192-0.388) in mother-postmenarcheal daughter pairs, for bone areas at all sites in early puberty (r=0.229-0.508) and for volumetric-derived density at FN and spine (r=0.238-0.368) in mother-son pairs. Fat mass correlations were significant in both genders after puberty (r=0.299-0.324) and lean mass in postmenarcheal girls only (r = 0.299). Heritability estimates varied between 21% and 37% for mother-daughter and 18% and 35% for mother-son pairs for density-derived variables and between 26% and 40% for body composition variables. Maternal inheritance of bone traits is expressed in early-pubertal boys for several skeletal site traits but consistently involves most site traits in girls and boys by late puberty. Body composition inheritance is more variable.

Factors affecting the outcome of fractures of the femoral neck in children and adolescents: a systematic review

Fractures of the femoral neck in children are rare, high-energy injuries with high complication rates. Their treatment has become more interventional but evidence of the efficacy of such measures is limited. We performed a systematic review of studies examining different types of treatment and their outcomes, including avascular necrosis (AVN), nonunion, coxa vara, premature physeal closure (PPC), and Ratliff's clinical criteria. A total of 30 studies were included, comprising 935 patients. Operative treatment and open reduction were associated with higher rates of AVN. Delbet types I and II fractures were most likely to undergo open reduction and internal fixation. Coxa vara was reduced in the operative group, whereas nonunion and PPC were not related to surgical intervention. Nonunion and coxa vara were unaffected by the method of reduction. Capsular decompression had no effect on AVN. Although surgery allows a more anatomical union, it is uncertain whether operative treatment or the type of reduction affects the rate of AVN, nonunion or PPC, because more severe fractures were operated upon more frequently. A delay in treatment beyond 24 hours was associated with a higher incidence of AVN.

A three-dimensional axis for the study of femoral neck orientation

A common problem in the quantification of the orientation of the femoral neck is the difficulty to determine its true axis; however, this axis is typically estimated visually only. Moreover, the orientation of the femoral neck is commonly analysed using angles that are dependent on anatomical planes of reference and only quantify the orientation in two dimensions. The purpose of this study is to establish a method to determine the three-dimensional orientation of the femoral neck using a three-dimensional model. An accurate determination of the femoral neck axis requires a reconsideration of the complex architecture of the proximal femur. The morphology of the femoral neck results from both the medial and arcuate trabecular systems, and the asymmetry of the cortical bone. Given these considerations, two alternative models, in addition to the cylindrical one frequently assumed, were tested. The surface geometry of the femoral neck was subsequently used to fit one cylinder, two cylinders and successive cross-sectional ellipses. The model based on successive ellipses provided a significantly smaller average deviation than the two other models (P < 0.001) and reduced the observer-induced measurement error. Comparisons with traditional measurements and analyses on a sample of 91 femora were also performed to assess the validity of the model based on successive ellipses. This study provides a semi-automatic and accurate method for the determination of the functional three-dimensional femoral neck orientation avoiding the use of a reference plane. This innovative method has important implications for future studies that aim to document and understand the change in the orientation of the femoral neck associated with the acquisition of a bipedal gait in humans. Moreover, the precise determination of the three-dimensional orientation has implications in current research involved in developing clinical applications in diagnosis, hip surgery and rehabilitation.

Skeletal biology over the life span: a view from the surfaces

The biocultural interpretation of skeletal remains is based upon the foundation of skeletal biology. In this review we examine the current state of skeletal biology research outside of the mainstream anthropology literature. The focus is on the structural changes of bone development and growth, and modeling and repair in the four bone surfaces: periosteal, Haversian, endosteal, and trabecular. The pattern of skeletal changes is placed within the framework of the human life span. New perspectives and direction of research on the environmental, biological, and genetic influences on modeling and remodeling processes are discussed chronologically at each bone surface. Implications for biological anthropologists are considered. This approach emphasizes variation in skeletal biology as a dynamic record of development, maturity, and aging.

In peripubertal girls, artistic gymnastics improves areal bone mineral density and femoral bone geometry without affecting serum OPG/RANKL levels

Peripubertal artistic gymnasts display elevated areal bone mineral density at various bone sites, despite delayed menarche and a high frequency of menstrual disorders, factors that may compromise bone health. The concomitant improvement in femoral bone geometry and strength suggested that this type of physical activity might have favourable clinical impact.

INTRODUCTION

The purpose of this study is to evaluate the effect of artistic gymnastics (GYM) on areal bone mineral density (aBMD), femoral bone geometry and bone markers and its relationship with the osteoprotegerin (OPG)/rank-ligand (RANKL) system in peripubertal girls.

METHODS

Forty-six girls (age 10-17.2 years) were recruited for this study: 23 elite athletes in the GYM group (training 12-30 h/week, age at start of training 5.3 years) and 23 age-matched (± 6 months; leisure physical activity ≤ 3 h/week) controls (CON). The aBMD at whole body, total proximal femur, lumbar spine, mid-radius and skull was determined using dual-X-ray absorptiometry. Hip structural analysis (HSA software) was applied at the femur to evaluate cross-sectional area (CSA, cm(2)), cross-sectional moment of inertia (CSMI, cm(4)), and the section modulus (Z, cm(3)) and buckling ratio at neck, intertrochanteric region and shaft. Markers of bone turnover and OPG/RANKL levels were also analysed.

RESULTS

GYM had higher (5.5-16.4%) non-adjusted aBMD and adjusted aBMD for age, fat-free soft tissue and fat mass at all bone sites, skull excepted and the difference increased with age. In the three femoral regions adjusted for body weight and height, CSA (12.5-18%), CSMI (14-18%), Z (15.5-18.6%) and mean cortical thickness (13.6-21%) were higher in GYM than CON, while the buckling ratio (21-27.1%) was lower. Bone markers decreased with age in both groups and GYM presented higher values than CON only in the postmenarchal period. A similar increase in RANKL with age without OPG variation was observed for both groups.

CONCLUSION

GYM is associated not only with an increase in aBMD but also an improvement in bone geometry associated with an increase in bone remodelling. These adaptations seem to be independent of the OPG/RANKL system.

Development of a three-dimensional detection method of cam deformities in femoroacetabular impingement

OBJECTIVE

The description of femoral head sphericity and related risk for femoroacetabularimpingement is currently limited to an angular estimate-the alpha angle-whose relevance and accuracy have been challenged. We developed a three-dimensional approach for both automated digital measurement of the alpha angle and the detection of camdeformities. Accuracy and diagnostic relevance of the alpha angle estimated by means of the oblique axial and multiple radial plane protocol were compared with the computed results.

MATERIALS AND METHODS

Using subject-specific statistical information of the femur head and mid-neck region, a method was developed to accurately compute the maximum alpha angle and to define aspherical eccentric areas at the femoral head-neck junction. The method was evaluated on 102 dry cadaver femur specimens.

RESULTS

Average detection limit for bony prominences at the head-neck transition was 0.98 mm. Pixel size of the investigated CT data was 0.79 mm. Mean maximum computed alpha angle of the femurs with cam-type morphology as identified by the morphological method was 67.72° (range 53.04-88.02°). Mean maximum computed alpha angle of the femurs without cam deformity was 47.65° (range 38.67-59.81°). Alpha angle estimates obtained by means of the multiple radial plane protocol correlated better (R = 0.88) and showed higher diagnostic agreement (phi = 0.77) with the 3D computational analysis compared to the oblique axial protocol (R = 0.60; phi = 0.67).

CONCLUSIONS

The alpha angle seems to be a relevant screening tool when obtained by 3D computed analysis or when estimated according to the multiple radial plane protocol. Estimates obtained by means of the oblique axial protocol have insufficient diagnostic and measurement accuracy.

Is bone loss the reversal of bone accrual? Evidence from a cross-sectional study in daughter-mother-grandmother trios

Bone adapts to mechanical loads applied on it. During aging, loads decrease to a greater extent at those skeletal sites where loads increase most in earlier life. Thus, the loss of bone may occur preferentially at sites where most bone has been deposited previously; ie, bone loss could be the directional reversal of accrual. To test this hypothesis, we compared the bone mass distribution at weight-bearing (tibia) and non-weight-bearing (radius) bones among 18-year-old girls, their premenopausal mothers, and their postmenopausal maternal grandmothers. Bone and muscle properties were measured by pQCT, and polar distribution of bone mass was obtained in 55 girl-mother-maternal grandmother trios. Site-matched differences in bone mass were compared among three generations. The differences between girls and mothers and between mothers and grandmothers were used to represent the patterns of bone mass accrual from early adulthood to middle age and bone loss from middle to old age, respectively. Compared to the mothers, 18-year old girls had less bone mass in the anterior and medial-posterior regions of the tibial shaft, while the grandmothers had less bone in the anterior and posterior regions. In contrast, the bone mass differences in the radial shaft between girls and mothers and mothers and grandmothers were relatively uniform. We conclude that both bone accrual and loss are direction-specific in weight-bearing bones but relatively uniform in non-weight-bearing bones. Bone loss in old age is largely, but not completely, a reversal of the preferential deposition of bone in the most highly loaded regions during early life.

A method for three-dimensional evaluation and computer aided treatment of femoroacetabular impingement

Several theoretical models have shown that the range of motion of the hip joint is impaired in patients with femoroacetabular impingement, and that the acetabular cartilage is at risk of being damaged as a result of abnormal shear stresses, even during normal everyday activities. Computer aided technologies might add to the early diagnosis and adequate treatment of such lesions. This paper describes the technique, theories and algorithms we have developed for patient-specific detection, analysis and computer aided surgery of femoroacetabular impingement. Currently available models applicable to femoroacetabular impingement offer modeling based on collision analysis of a constrained hip joint. Such an approach implies that neither the femur nor the acetabulum can be analyzed completely separately for the presence of structural lesions responsible for the impingement problem. Moreover, a constrained model does not allow for comprehensive prediction of the possible locations and extent of secondary cartilage lesions (so-called contre-coup lesions) of the posterior acetabulum opposite the anterior impingement site. We report a new technique for the subject-specific morphological analysis of the proximal femur, acetabulum and hip joint. The technique offers a number of advantages compared to currently used techniques for the diagnosis and evaluation of hip impingement, and has direct orthopaedic applications as it allows computer aided planning and minimally invasive surgery for patients with femoroacetabular impingement.

Three-dimensional assessment of cam engagement in femoroacetabular impingement

PURPOSE

The purpose of our study was to 3-dimensionally assess cam engagement in male patients with symptomatic femoroacetabular impingement during motion.

METHODS

A total of 13 hips with cam-type impingement were investigated. Patient anatomy and clinical range of motion were determined. After 3-dimensional segmentation and reconstruction, the dynamic behavior of the cam lesion was investigated for the previously analyzed motions.

RESULTS

Important differences in the pattern of cam engagement were noticed. Abutment of the cam lesion and the acetabular cartilage was observed in flexion in 7 hips (54%) and in abduction in 11 hips (84%). Internal rotation with the hip in 90° of flexion caused intrusion of the cam lesion into the joint in 10 of the investigated cases (77%). Neutral rotation did not cause any conflict between the cam lesion and acetabulum for the measured range of motion. A similar area in the anterosuperior quadrant of the acetabulum appeared to be involved in the different motions that were analyzed.

CONCLUSIONS

Typically, during internal rotation in 90° of flexion, the centromedial portion of the cam lesion was found to abut against the anterosuperior quadrant of the acetabular cartilage. During abduction and flexion, this appeared to be the lateral and medial portions of the cam lesion, respectively. All motions causing cam intrusion appeared to focus on the same cartilage area of the acetabulum in its anterosuperior quadrant.

Familial resemblance and diversity in bone mass and strength in the population are established during the first year of postnatal life

Familial resemblance and diversity in bone structure and strength in adulthood are determined in part during growth. Whether these characteristics are established during gestation or shortly after birth is not known. Total-body, lumbar spine, and femoral neck size and mass and indices of tibial bending strength and distal radial compressive strength were measured using bone densitometry and quantitative computed tomography in 236 girls at 18.5 years of age. Among them, 219, 141, and 105 girls had crown-heel length (CHL) and weight recorded at birth and at 6 and 12 months of age, and then height and weight were recorded at 3, 5, 10, 13, and 15 years of age in 181, 176, 127, 111, and 228 girls, respectively. Of these girls, 101 and 93 girls also had bone structure assessed at 11 and 13 years of age, respectively. Similar bone measurements were made once in 78 mother-father pairs. CHL and weight at birth did not correlate or did so weakly with bone traits in girls at 18 years of age. By contrast, CHL at 6 months correlated with the height, bone traits, and strength at puberty and at 18 years of age (r = 0.24-0.56, p < .001) in girls and with their parents' height and bone traits (r = 0.15-0.37, p < .05). When the girls' CHL at 6 months was stratified into quartiles, the absolute and relative differences in bone traits observed at puberty (approximately 11.5 years) were maintained as these traits tracked during the ensuing 7 years. Similarly, weight at 6 months correlated with the girls' bone traits at puberty and 18 years of age (r = 0.22-0.55, p < .05). During puberty and at 18 years of age, the girls' bone traits correlated with the corresponding traits in their parents (r = 0.32-0.43, p < .01). It is concluded that familial resemblance in bone structural strength and the position of an individual's bone traits relative to others in adulthood are likely to be established during the first year of life. Thus susceptibility to bone fragility late in life has its antecedents established early in life.

Rapid growth produces transient cortical weakness: a risk factor for metaphyseal fractures during puberty

Fractures of the distal radius in children have a similar incidence to that found in postmenopausal women but occur more commonly in boys than in girls. Fractures of the distal tibia are uncommon in children and show no sex specificity. About 90% of lengthening of the radius but only 30% of lengthening of the tibia during puberty occur at the distal growth plate. We speculated that more rapid modeling at the distal radial metaphysis results in a greater dissociation between growth and mineral accrual than observed at the distal tibia. We measured the macro- and microarchitecture of the distal radial and tibial metaphysis using high-resolution peripheral quantitative computed tomography in a cross-sectional study of 69 healthy boys and 60 healthy girls aged from 5 to 18 years. Bone diameters were larger but total volumetric bone mineral density (vBMD) was lower at the distal radius (not at the distal tibia) by 20% in boys and by 15% in girls at Tanner stage III than in children of the same sex at Tanner stage I (both p < .05). In boys at Tanner stage III, total vBMD was lower because the larger radial total cross-sectional area (CSA) had a thinner cortex with lower vBMD than in boys at Tanner stage I. In girls at Tanner stage III, the larger total radial CSA was not associated with a difference in cortical thickness or cortical vBMD relative to girls in Tanner stage I. Cortical thickness and density at both sites in both sexes after Tanner stage III were greater than in younger children. Trabecular bone volume fraction (BV/TV) was higher in boys than in girls at both sites and more so after puberty because trabeculae were thicker in more mature boys but not in girls. There was no sex- or age-related differences in trabecular number at either site. We infer that longitudinal growth outpaces mineral accrual in both sexes at the distal radius, where bone grows rapidly. The dissociation produces transitory low cortical thickness and vBMD in boys but not in girls. These structural features in part may account for the site and sex specificity of metaphyseal fractures during growth.

Potential of first arriving signal to assess cortical bone geometry at the Hip with QUS: a model based study

There is evidence that quantitative ultrasound (QUS) at the proximal femur involves waves propagating circumferentially in the cortical shell. These waves, in theory, convey information on bone geometrical and material characteristics and thus have the potential to improve fracture risk prediction. In this work, the time-of-flight (TOF) of the first arriving signal (FAS) that corresponds to waves propagating circumferentially is considered. The relationships between the TOF of the FAS and geometrical features of the femoral neck are investigated. Five neck cross-sections from 11 human femurs from females (>65y) were extracted from 3-D X-ray quantitative computed tomography (XR-QCT) datasets. Geometrical parameters including cross-sectional area (CSA), minimum moment of inertia (I(min)) and section modulus (Z) were computed from femoral neck cross-section images. Two-dimensional numerical simulation of US propagation through femoral neck was performed and TOF of the FAS was estimated. TOF was best correlated with I(min): R(2)=0.82 (p=10(-4); RMSE=1390mm(4) [13%]) in the region between proximal and mid-femoral neck and with Z: R(2)=0.55 (p<10(-2); RMSE=480mm(3) [17%]) and CSA: R(2)=0.62 (p<5.10(-3); RMSE=22mm(2) [7%]) in the mid-femoral neck. The results suggest that QUS has the potential to assess proximal femur strength based on estimates of geometrical properties of the cortical shell.

Quantitative computed tomographic assessment of the effects of 24 months of teriparatide treatment on 3D femoral neck bone distribution, geometry, and bone strength: results from the EUROFORS study

We studied the changes in bone distribution, geometry, and bone strength based on 3D quantitative computed tomography (QCT) of the femoral neck (FN) in subjects receiving teriparatide (TPTD). Fifty-two postmenopausal women with severe osteoporosis were analyzed. Patients were divided into three subgroups based on their prior treatment with osteoporosis drugs: treatment-naive (Tx-naive; n = 8), pretreated (pre-Tx; n = 12), and pretreated showing an inadequate response to treatment (inad. pre-Tx; n = 32). QCT scans were performed at baseline and after 6, 12, and 24 months of treatment and were analyzed with Mindways QCT-PRO BIT software. Minimum and maximum section modulus, buckling ratio (BR), and cross-sectional area (CSA) were calculated as measurements of bending strength, risk of buckling, and bone apposition, respectively. After 24 months of TPTD treatment, areal and volumetric FN BMD increased significantly by 4.0% and 3.0%, respectively, compared with baseline. Decreases in cortical volumetric BMD occurred in locations not adversely affecting minimum bending strength indicators. Cortical CSA increased by 4.3%, whereas total CSA remained unchanged over the study duration, indicating that endosteal but no periosteal growth was observed. Strength parameters for buckling did not change at 6 and 12 months but improved significantly at 24 months. Measures of bending strength showed a trend toward improvement. Changes tended to be larger in individuals at higher risk of buckling failure. Prior antiresorptive treatment may delay response to TPTD, but based on the small magnitude of the mostly insignificant changes at 6 months, this does not appear to lead to an interim phase of reduced bone strength. In summary, FN QCT provides a tool for detailed longitudinal investigation of bone strength indices in vivo for different loading modes, yields insight into underlying structural changes, and provides relevant mechanostructural information beyond dual-energy X-ray absorptiometry. Continuous TPTD treatment for 24 months improves FN bone strength parameters.

Low volumetric BMD is linked to upper-limb fracture in pubertal girls and persists into adulthood: a seven-year cohort study

The aetiology of increased incidence of fracture during puberty is unclear. This study aimed to determine whether low volumetric bone mineral density (vBMD) in the distal radius is associated with upper-limb fractures in growing girls, and whether any such vBMD deficit persists into adulthood. Fracture history from birth to 20 years was obtained and verified by medical records in 1034 Finnish girls aged 10-13 years. Bone density and geometry at distal radius, biomarkers and lifestyle/behavioural factors were assessed in a subset of 396 girls with a 7.5-year follow-up. We found that fracture incidence peaked during puberty (relative risk 3.1 at age of 8-14 years compared to outside this age window), and 38% of fractures were in the upper-limb. Compared to the non-fracture cohort, girls who sustained upper-limb fracture at ages 8-14 years had lower distal radial vBMD at baseline (258.9+/-37.5 vs. 287.5+/-34.1 mg/cm(3), p=0.001), 1-year (252.0+/-29.3 vs. 282.6+/-33.5 mg/cm(3), p=0.001), 2-year (258.9+/-32.2 vs. 289.9+/-40.1 mg/cm(3), p=0.003), and 7-year follow-ups (early adulthood, 307.6+/-35.9 vs. 343.6+/-40.9 mg/cm(3), p=0.002). There was a consistent trend towards larger bone cross-sectional area in the fracture cohort compared to non-fracture. In a logistic regression model, lower vBMD (p=0.001) was the only significant predictor of upper-limb fracture during the period of 8-14 years. Our results indicate that low BMD is an important factor underlying elevated upper-limb fracture risk during puberty, and that low BMD in pubertal girls with fracture persists into adulthood. Hence low vBMD during childhood is not a transient deficit. Methods to monitor vBMD and to maximise bone mineral accrual and reduce risks of falling in childhood should be developed.

Women and men with hip fractures have a longer femoral neck moment arm and greater impact load in a sideways fall

INTRODUCTION

In a case control study, we report that women and men with hip fractures have a longer moment arm of the force applied on the proximal femur during a sideways fall, a structural feature that may contribute to fracture risk. The impact load and its direction during a sideways fall onto the greater trochanter are partly determined by the geometry of the proximal femur. We hypothesized that the hip geometry in elderly with hip fractures produces a greater impact on the hip during a sideways fall.

METHODS

We studied 41 female (77.2 +/- 9.9 years) and 22 male (76.2 +/- 12.1 years) patients with hip fractures and 40 female (85.7 +/- 6.0 years) and 17 male (84.3 +/- 10.1 years) controls. Hip geometry was analyzed on the nonfracture hip in patients and left hip in controls using dual-energy X-ray absorptiometry.

RESULTS

There was no difference in areal bone mineral density (aBMD), hip axis length, femoral neck axis length, or neck-shaft angle between cases and controls. However, the moment arm of the force on the hip during a sideways fall was 7.3% and 9.5% longer resulting in 5.6% and 9.1% greater moment in such a fall in female and male cases relative to their respective controls independent of height and weight (all p < 0.056). In multivariate logistic regression analysis, only the moment arm length in a sideways fall was associated with increased risk of hip fracture in females (odds ratio = 1.91, 95%CI: 1.14-3.20 for each SD increase in moment arm length of sideways fall, p = 0.02) and males (odds ratio = 2.69, 95% CI, 1.19-6.09, p = 0.01).

CONCLUSIONS

A longer moment arm in the sideways fall increases the resultant force applied to the hip predisposing to hip fracture.

Skeletal growth and peak bone strength

Bone size, shape and internal architecture, and not just bone mass, account for differences in bone strength between individuals, sexes and races. The differences in bone morphology in old age - whether an individual's bone size and mass occupy the 5th, 50th or 95th percentile - is determined early in life. Bone traits track from the position established early in life. Genetic and environmental factors establish the morphological features of bone through the cellular machinery of bone modelling and remodelling which adapts bone to its loading circumstance by modifying its size and shape and the distribution of its mass. The need for both strength for loading and lightness for mobility are achieved by deposition of bone where it is needed and removal of bone from where it is not. The machinery has enormous capacity during growth, as can be seen in the bone structure of the elite athlete, but not during advancing age because of changes in the cellular machinery itself and in systemic hormonal regulatory factors.

A novel DXA-based hip failure index captures hip fragility independent of BMD

Capability of a novel dual-energy X-ray absorptiometry (DXA)-based hip failure index (HiFI) to discriminate between hip fracture cases and controls was evaluated. Given the constraints of planar DXA, the femoral neck was assumed a foam-filled ( approximately trabecular bone), thin-walled ( approximately cortical bone) sandwich structure, while HiFI estimated the critical force sufficient to buckle the wall of such a structure. Proximal femur DXA data from 1379 women aged 65yr and older, 268 with prior hip fracture were used. Comparison between standard areal bone mineral density (BMD), femur strength index (FSI), and HiFI was based on areas under receiver operatoring characteristic curves (AUC). The mean femoral neck BMD (SD) was 0.689 (0.109) g/cm(2) among the cases and 0.768 (0.119) g/cm(2) among the controls; the mean FSI 1.33 (0.36) and 1.54 (0.41), and the mean HiFI -0.28 (0.14) and -0.18 (0.15), respectively; all intergroup differences were highly significant (p<0.001). The intergroup difference for HiFI remained significant (p<0.002) after adjusting for age and BMD or FSI. The AUCs were 0.696 (95% confidence interval [CI]: 0.661-0.730) for BMD, 0.665 (0.630-0.700) for FSI, and 0.701 (0.666-0.736) for HiFI. In conclusion, HiFI may capture structural traits that account for femoral neck fragility independently of BMD or FSI. Obviously, the use of actual geometric and structural information from three-dimensional imaging of the femoral neck would help diminish the crude assumptions of the present DXA approach and reveal the true potential of the HiFI approach to gauge hip fragility and identify at-risk individuals for hip fractures.

Shape, structural properties, and cortical stability along the femoral neck: a study using clinical QCT

This study used clinical quantitative computer tomography (QCT) to obtain detailed estimates of the structural properties and cortical dimensions of cross-sections (CSs) along the femoral neck (FN). The computer tomography scans of both proximal femora of 27 postmenopausal women (mean age 81, range 65-86yr) with osteoporosis were processed and analyzed. The cross-sectional shape, cortical and trabecular bone area, and section moduli under different fall directions were calculated. Furthermore, each CS was divided into 8 sectors and cortical thickness and buckling ratio were estimated for each octant. The cross-sectional shape was found to be increasingly elliptic and both tensile and compressive section moduli increased significantly (by a factor of up to 1.8) from the proximal to distal half of the FN. The section modulus was dependent on the fall direction; it was maximal when falling 20 degrees anterior and at its lowest (reduced by as much as 37%) when falling 50 degrees posterior on the greater trochanter. The cortex was significantly thinner (< or =1mm) in the anterior, superoanterior, superior, superoposterior, and posterior octants than the inferomedial aspect of the FN. In conclusion, multiple site measurements are required for a comprehensive assessment of FN structural properties, which can be studied based on clinical QCT.

Bone quality: the material and structural basis of bone strength

The material composition and structural design of bone determine its strength. Structure determines loads that can be tolerated but loads also determine structure. Bone modifies its material composition and structure to accommodate loads by adaptive modeling and remodeling. Adaptation is successful during growth but not aging because accumulating insults, including a reduction in the volume of bone formed in the basic multicellular unit (BMU), increased resorption in the BMU, increased remodeling rate in midlife in women and in some men because of sex hormone deficiency, and in both sexes in old age as a consequence of secondary hyperparathyroidism and reduced periosteal bone formation, all of which compromises the material composition of bone and its structure. An understanding of the mechanisms of adaptation and failed adaptation provides rational approaches to interventions that can prevent or restore bone fragility.

Femoral neck BMD is a strong predictor of hip fracture susceptibility in elderly men and women because it detects cortical bone instability: the Rotterdam Study

We studied HSA measurements in relation to hip fracture risk in 4,806 individuals (2,740 women). Hip fractures (n = 147) occurred at the same absolute levels of bone instability in both sexes. Cortical instability (propensity of thinner cortices in wide diameters to buckle) explains why hip fracture risk at different BMD levels is the same across sexes.

INTRODUCTION

Despite the sexual dimorphism of bone, hip fracture risk is very similar in men and women at the same absolute BMD. We aimed to elucidate the main structural properties of bone that underlie the measured BMD and that ultimately determines the risk of hip fracture in elderly men and women.

MATERIALS AND METHODS

This study is part of the Rotterdam Study (a large prospective population-based cohort) and included 147 incident hip fracture cases in 4,806 participants with DXA-derived hip structural analysis (mean follow-up, 8.6 yr). Indices compared in relation to fracture included neck width, cortical thickness, section modulus (an index of bending strength), and buckling ratio (an index of cortical bone instability). We used a mathematical model to calculate the hip fracture distribution by femoral neck BMD, BMC, bone area, and hip structure analysis (HSA) parameters (cortical thickness, section modulus narrow neck width, and buckling ratio) and compared it with prospective data from the Rotterdam Study.

RESULTS

In the prospective data, hip fracture cases in both sexes had lower BMD, thinner cortices, greater bone width, lower strength, and higher instability at baseline. In fractured individuals, men had an average BMD that was 0.09 g/cm(2) higher than women (p < 0.00001), whereas no significant difference in buckling ratios was seen. Modeled fracture distribution by BMD and buckling ratio levels were in concordance to the prospective data and showed that hip fractures seem to occur at the same absolute levels of bone instability (buckling ratio) in both men and women. No significant differences were observed between the areas under the ROC curves of BMD (0.8146 in women and 0.8048 in men) and the buckling ratio (0.8161 in women and 0.7759 in men).

CONCLUSIONS

The buckling ratio (an index of bone instability) portrays in both sexes the critical balance between cortical thickness and bone width. Our findings suggest that extreme thinning of cortices in expanded bones plays a key role on local susceptibility to fracture. Even though the buckling ratio does not offer additional predictive value, these findings improve our understanding of why low BMD is a good predictor of fragility fractures.

Construction of the femoral neck during growth determines its strength in old age

Study of the design of the FN in vivo in 697 women and in vitro in 200 cross-sections of different sizes and shapes along each of 13 FN specimens revealed that strength in old age was largely achieved during growth by differences in the distribution rather than the amount of bone material in a given FN cross-section from individual to individual.

INTRODUCTION

We studied the design of the femoral neck (FN) to gain insight into the structural basis of FN strength in adulthood and FN fragility in old age.

MATERIALS AND METHODS

Studies in vivo were performed using densitometry in 697 women and in vitro using high-resolution microCT and direct measurements in 13 pairs of postmortem specimens.

RESULTS

The contradictory needs of strength for loading yet lightness for mobility were met by varying FN size, shape, spatial distribution, and proportions of its trabecular and cortical bone in a cross-section, not its mass. Wider and narrower FNs were constructed with similar amounts of bone material. Wider FNs were relatively lighter: a 1 SD higher FN volume had a 0.67 (95% CI, 0.61-0.72) SD lower volumetric BMD (vBMD). A 1 SD increment in height was achieved by increasing FN volume by 0.32 (95% CI, 0.25-0.39) SD with only 0.15 (95% CI, 0.08-0.22) SD more bone, so taller individuals had a relatively lighter FN (vBMD was 0.13 [95% CI, 0.05-0.20 SD] SD lower). Greater periosteal apposition constructing a wider FN was offset by even greater endocortical resorption so that the same net amount of bone was distributed as a thinner cortex further from the neutral axis, increasing resistance to bending and lowering vBMD. This was recapitulated at each point along the FN; varying absolute and relative degrees of periosteal apposition and endocortical resorption focally used the same amount of material to fashion an elliptical FN of mainly cortical bone near the femoral shaft to offset bending but a more circular FN of proportionally more trabecular and less cortical bone to accommodate compressive loads adjacent to the pelvis. This structural heterogeneity was largely achieved by adaptive modeling and remodeling during growth-most of the variance in FN volume, BMC, and vBMD was growth related.

CONCLUSIONS

Altering structural design while minimizing mass achieves FN strength and lightness. Bone fragility may be the result of failure to adapt bone's architecture to loading, not just low bone mass.