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Hip scans of U.S. adults aged 20-99 years acquired in the Third National Health and Nutrition Examination Survey (NHANES III) using dual-energy X-ray absorptiometry (DXA) were analyzed with a structural analysis program. The program analyzes narrow (3 mm wide) regions at specific locations across the proximal femur to measure bone mineral density (BMD) as well as cross-sectional areas (CSAs), cross-sectional moments of inertia (CSMI), section moduli, subperiosteal widths, and estimated mean cortical thickness. Measurements are reported here on a non-Hispanic white subgroup of 2,719 men and 2,904 women for a cortical region across the proximal shaft 2 cm distal to the lesser trochanter and a mixed cortical/trabecular region across the narrowest point of the femoral neck. Apparent age trends in BMD and section modulus were studied for both regions by sex after correction for body weight. The BMD decline with age in the narrow neck was similar to that seen in the Hologic neck region; BMD in the shaft also declined, although at a slower rate. A different pattern was seen for section modulus; furthermore, this pattern depended on sex. Specifically, the section modulus at both the narrow neck and the shaft regions remains nearly constant until the fifth decade in females and then declined at a slower rate than BMD. In males, the narrow neck section modulus declined modestly until the fifth decade and then remained nearly constant whereas the shaft section modulus was static until the fifth decade and then increased steadily. The apparent mechanism for the discord between BMD and section modulus is a linear expansion in subperiosteal diameter in both sexes and in both regions, which tends to mechanically offset net loss of medullary bone mass. These results suggest that aging loss of bone mass in the hip does not necessarily mean reduced mechanical strength. Femoral neck section moduli in the elderly are on the average within 14% of young values in females and within 6% in males.

Graphene nanoribbons (GNRs) with widths down to 16 nm have been characterized for their currentcarrying capacity. It is found that GNRs exhibit an impressive breakdown current density, on the order of 10 8 A/cm 2 . The breakdown current density is found to have a reciprocal relationship to GNR resistivity and the data fit points to Joule heating as the likely mechanism of breakdown. The superior current-carrying capacity of GNRs will be valuable for their application in on-chip electrical interconnects. The thermal conductivity of sub-20 nm graphene ribbons is found to be more than 1000 W/m-K.Keywords: Graphene, Breakdown current density, Nano ribbons, Maximum current * raghu@gatech.edu, Ph: 404 385 6463 2 Graphene is a promising electronic material because of many interesting properties like ballistic transport 1 , high intrinsic mobility 2 , and width-dependent bandgap 3 . Graphene, in its 2D form, has been shown to have a high thermal conductivity 4 of around 5000 W/m-K pointing to its potential use as an on-chip heat spreader.Graphene nano ribbons (GNRs) have been predicted to be superior to Cu in terms of resistance per unit length 5 for use as on-chip interconnects. A high current-carrying capacity is critical for interconnect applications and reliability. There have been a number of studies on carbon nanotube (CNT) breakdown current density, and the current-carrying capacity of single-walled CNTs 6 is found to be on the order of 10 8 A/cm 2 ; in carbon nanofibers, the breakdown current density (J BR ) has been measured 7 to be around 5x10 6 A/cm 2 . Electrical breakdown has been used to burn away successive shells in a multi-wall CNT 8,9 . More recently, electrical breakdown has been used to obtain semiconducting CNTs from a mixture of CNTs since metallic ones burn away at a lower breakdown voltage 10 . Theoretical projections suggest that J BR of graphene should be on the same order as for CNTs. However, little experimental evidence exists on the electrical breakdown of either 2D graphene or 1D GNRs. In this work, it is experimentally shown that GNRs demonstrate an impressive J BR . A simple relation between J BR and nanowire resistivity is seen to emerge from the experimental data.Few-layer graphene (1-5 layers) is used as the starting material (see supporting material 11 ).Each device consists of parallel ribbons fabricated between sets of electrodes, Fig. 1. The ribbon width between a pair of electrodes is designed to be the same for all the parallel ribbons. The range of widths studied in this work is 16nm

We compared 7-month changes in bone structural properties in pre-and early-pubertal girls randomized to exercise intervention (10-minute, 3 times per week, jumping program) or control groups. Girls were classified as prepubertal (PRE; Tanner breast stage 1; n ‫؍‬ 43 for intervention [I] and n ‫؍‬ 25 for control [C]) or early-pubertal (EARLY; Tanner stages 2 and 3; n ‫؍‬ 43 for I and n ‫؍‬ 63 for C). Mean ؎ SD age was 10.0 ؎ 0.6 and 10.5 ؎ 0.6 for the PRE and EARLY groups, respectively. Proximal femur scans were analyzed using a hip structural analysis (HSA) program to assess bone mineral density (BMD), subperiosteal width, and cross-sectional area and to estimate cortical thickness, endosteal diameter, and section modulus at the femoral neck (FN), intertrochanter (IT), and femoral shaft (FS) regions. There were no differences between intervention and control groups for baseline height, weight, calcium intake, or physical activity or for change over 7 months (p > 0.05). We used analysis of covariance (ANCOVA) to examine group differences in changes of bone structure, adjusting for baseline weight, height change, Tanner breast stage, and physical activity. There were no differences in change for bone structure in the PRE girls. The more mature girls (EARLY) in the intervention group showed significantly greater gains in FN (؉2.6%, p ‫؍‬ 0.03) and IT (؉1.7%, p ‫؍‬ 0.02) BMD. Underpinning these changes were increased bone cross-sectional area and reduced endosteal expansion. Changes in subperiosteal dimensions did not differ. Structural changes improved section modulus (bending strength) at the FN (؉4.0%, p ‫؍‬ 0.04), but not at the IT region. There were no differences at the primarily cortical FS. These data provide insight into geometric changes that underpin exercise-associated gain in bone strength in early-pubertal girls. (J Bone Miner Res 2002;17:363-372)

ABSTRACT:The role of bone tissue's geometric distribution in hip fracture risk requires full evaluation in large population-based datasets. We tested whether section modulus, a geometric index of bending strength, predicted hip fracture better than BMD. Among 7474 women from the Study of Osteoporotic Fractures (SOF) with hip DXA scans at baseline, there were 635 incident hip fractures recorded over 13 yr. Hip structural analysis software was used to derive variables from the DXA scans at the narrow neck (NN), intertrochanter (IT), and shaft (S) regions. Associations of derived structural variables with hip fracture were assessed using Cox proportional hazard modeling. Hip fracture prediction was assessed using the C-index concordance statistic. Incident hip fracture cases had larger neck-shaft angles, larger subperiosteal and estimated endosteal diameters, greater distances from lateral cortical margin to center of mass (lateral distance), and higher estimated buckling ratios (p < 0.0001 for each). Areal BMD, cross-sectional area, cross-sectional moment of inertia, section modulus, estimated cortical thickness, and centroid position were all lower in hip fracture cases (p < 0.044). In hip fracture prediction using NN region parameters, estimated cortical thickness, areal BMD, and estimated buckling ratio were equivalent (C-index ‫ס‬ 0.72; 95% CI, 0.70, 0.74), but section modulus performed less well (C-index ‫ס‬ 0.61; 95% CI, 0.58, 0.63; p < 0.0001 for difference). In multivariable models combining hip structural analysis variables and age, effects of bone dimensions (i.e., lateral distance, subperiosteal diameter, and estimated endosteal width) were interchangeable, whereas age and neck-shaft angle were independent predictors. Several parsimonious multivariable models that were prognostically equivalent for the NN region were obtained combining a measure of width, a measure of mass, age, and neck-shaft angle (BMD is a ratio of mass to width in the NN region; C-index ‫ס‬ 0.77; 95% CI, 0.75, 0.79). Trochanteric fractures were best predicted by analysis of the IT region. Because section modulus failed to predict hip fracture risk as well as areal BMD, the thinner cortices and wider bones among those who fractured may imply that simple failure in bending is not the usual event in fracture. Fracture might require initiation (e.g., by localized crushing or buckling of the lateral cortex).

Heavier individuals have higher hip BMD and more robust femur geometry, but it is unclear whether values vary in proportion with body weight in obesity. We studied the variation of hip BMD and geometry across categories of body mass index (BMI) in a subset of postmenopausal non-Hispanic whites (NHWs) from the Women's Health Initiative Observational Cohort (WHI-OS). The implications on fracture incidence were studied among NHWs in the entire WHI-OS. Baseline DXA scans of hip and total body from 4642 NHW women were divided into BMI (kg/m 2 ) categories: underweight (<18.5), healthy weight (18.5-24.9), overweight (25-29.9), and mild (30-34.9), moderate (35-39.9), and extreme obesity (>40). Femur BMD and indices of bone axial (cross-sectional area [CSA]) and bending strength (section modulus [SM]) were extracted from DXA scans using the hip structure analysis (HSA) method and compared among BMI categories after adjustment for height, age, hormone use, diabetes, activity level, femur neck-shaft angle, and neck length. The association between BMI and incident fracture was studied in 78,013 NHWs from the entire WHI-OS over 8.5 ± 2.6 (SD) yr of follow-up. Fracture incidence (cases/1000 person-years) was compared among BMI categories for hip alone, central body (hip, pelvis, spine, ribs, and shoulder girdle), upper extremity (humerus and distal), and lower extremity (femur shaft and distal but not hip). Femur BMD, CSA, and SM were larger in women with higher BMI, but values scaled in proportion to lean and not to fat or total body mass. Women with highest BMI reported more falls in the 12 mo before enrollment, more prevalent fractures, and had lower measures of physical activity and function. Incidence of hip fractures and all central body fractures declined with BMI. Lower extremity fractures distal to the hip trended upward, and upper extremity incidence was independent of BMI. BMD, CSA, and SM vary in proportion to total body lean mass, supporting the view that bones adapt to prevalent muscle loads. Because lean mass is a progressively smaller fraction of total mass in obesity, femur BMD, CSA, and SM decline relative to body weight in higher BMI categories. Traumatic forces increase with body weight, but fracture rates at the hip and central body were less frequent with increasing BMI, possibly because of greater soft tissue padding. There was no evident protective effect in fracture rates at less padded distal extremity sites. Upper extremity fractures showed no variation with BMI, and lower extremity fracture rates were higher only in the overweight (BMI = 25-29.9 kg/m 2 ).

Longitudinal, dual-energy X-ray absorptiometry (DXA) hip data from 4187 mostly white, elderly women from the Study of Osteoporotic Fractures were studied with a structural analysis program. Cross-sectional geometry and bone mineral density (BMD) were measured in narrow regions across the femoral neck and proximal shaft. We hypothesized that altered skeletal load should stimulate adaptive increases or decreases in the section modulus (bending strength index) and that dimensional details would provide insight into hip fragility. Weight change in the ϳ3.5 years between scan time points was used as the primary indicator of altered skeletal load. "Static" weight was defined as within 5% of baseline weight, whereas "gain" and "loss" were those who gained or lost >5%, respectively. In addition, we used a frailty index to better identify those subjects undergoing changing in skeletal loading. Subjects were classified as frail if unable to rise from a chair five times without using arm support. Subjects who were both frail and lost weight (reduced loading) were compared with those who were not frail and either maintained weight (unchanged loading) or gained weight (increased loading). Sixty percent of subjects (n ‫؍‬ 2559) with unchanged loads lost BMD at the neck but not at the shaft, while section moduli increased slightly at both regions. Subjects with increasing load (n ‫؍‬ 580) lost neck BMD but gained shaft BMD; section moduli increased markedly at both locations. Those with declining skeletal loads (n ‫؍‬ 105) showed the greatest loss of BMD at both neck and shaft; loss at the neck was caused by both increased loss of bone mass and greater subperiosteal expansion; loss in shaft BMD decline was only caused by greater loss of bone mass. This group also showed significant declines in section modulus at both sites. These results support the contention that mechanical homeostasis in the hip is evident in section moduli but not in bone mass or density. The adaptive response to declining skeletal loads, with greater rates of subperiosteal expansion and cortical thinning, may increase fragility beyond that expected from the reduction in section modulus or bone mass alone. (J Bone Miner Res 2001;16:1108-1119)