Maintaining femoral neck cortical thickness may help prevent hip fracture. Fracture initiation probably starts superiorly at flaws, ie where the cortex is thinnest. Whole body computed tomography (QCT) is now being used to study cortical thickness but limited resolution (> 300 micrometers) makes in vivo estimates imprecise, whereas microscopy s resolution approaches 1 micrometer. We have therefore extended our microscopic studies on femoral neck biopsies to include men (14 cases, 26 controls) and women (50 cases, 23 controls), and here provide data on true cortical thickness in subjects with and without hip fracture.

Whole femoral neck cross-sections obtained at hemiarthroplasty (or at post-mortem in controls) were embedded in methacrylate, cut, stained and imaged at medium power. Image-J was used to define cortical boundaries and to measure cortical thicknesses at 5 degree intervals of arc from the cross-sections centre of area.

We confirmed that the mid-femoral neck (or narrow neck) site, defined as where the ratio of maximum to minimum neck diameter (max:min) is 1.4, shows great asymmetry, with the thick inferior cortical octant averaging over 3mm thickness (mean age 79 years inter-quartile range 74-85). In the superior 3 octants cortical thickness averaged 26% of that seen inferiorly. To assess statistical determinants of cortical thickness, the data were modelled with linear regression in octants after adjusting for subjects age, sex, max:min, and hip fracture status. To achieve normality of residuals the cortical thickness data were log-transformed. 95% of measured cortical thicknesses fell between 45% and 220% of the mean for octant. In the thinner, superior three octants, minimum thicknesses were just under 0.3 mm in the fracture cases ie close to 35% of the subjects mean for octant. Cases had about 17% thinner cortical thicknesses in all octants than controls, while female controls had cortical thicknesses that uniformly averaged 90% of male. In conclusion, compared to gender and age-matched controls, intra-capsular hip facture cases had generalized cortical thinning in all mid-neck octants. This disease effect contrasts markedly with the effect of normal ageing, which thins preferentially the mechanically under-loaded superior cortex and spares the infero-anterior cortex.

Introduction: Previously, 3D finite element models of the midfemoral neck (MFN) predicted maximal compressive stress in the inferior cortex and trabeculae during the heel-strike of normal gait. We hypothesised that older women (compared with younger women) would have relative preservation of inferior trabecular bone, reflecting continued walking activity into old age. Recently, we identified preservation of the infero-anterior femoral neck cortex in old age despite women at 85 having 55–80% thinner superior cortices than women aged 25. Here, we examined regional trabecular bone mineral density in an extended region of interest (ROI) of the MFN, to establish age-associated differences and their likely contribution to hip fracture in a cross-sectional study of 100 women aged 20–90 years using 64-slice computed tomography (CT).

Methods: 100 healthy women were recruited by decade from age 20 to 90 (inter-quartile range 38–72). Participants consented to an extension of a routine clinical pelvic CT scan (Siemens 64-slice CT) using a fixed threshold of 450mg/cm3 and Mindways Software (BIT-2) to analyse both hips (1mm slice thickness, 0.59 mm voxel size). The starting position for cross sections was a 1mm thick MFN slice where the femoral neck height to width ratio was 1.4, since this location along the neck axis has been shown to be highly reproducible and unaffected by age (mean 51%, SE 0.016%). 5 parallel 1mm thick slices were evaluated towards the midline. Age effects on trabecular BMD (tbBMD mg/cm3.) were evaluated using linear regression (by anatomical quadrant).

Results: Trabecular BMD was significantly lower at 85 than at 25 years old amounting to over 50% difference in 3 out of 4 quadrants: Supero-posterior tbBMD difference between 25–85 years by modelling −146mg.cm3., (absolute %) −53%, r 2 (age) 0.61, p < 0.0001; Infero-anterior −25mg.cm3., −24%, r 2 0.094, p < 0.002. In the infero-anterior quadrants (that receive maximal stress at the heel strike in normal gait) there was relative preservation (r squared only 0.094) and only a 24% difference between 85 and 25. Body weight had no association with tbBMD in any quadrant (unlike cortical thickness).

Conclusion: Trabecular bone was better preserved in the infero-anterior quadrant in elderly women than in the other 3 quadrants. Since tbBMD also makes a small but significant contribution to the reduction of the risk of failure of femoral necks ex-vivo, increasing trabecular bone in the remaining quadrants should be included in strategies to prevent hip fracture, alongside efforts to strengthen the cortex.

Introduction: Femoral neck (FN) fragility has been attributed to age-related bone loss, with increased loss in women. It has been shown that the mechanical properties of a supporting structure will also change with any alteration to the structure’s dimensions. The purpose of this study was to identify the age-related changes that take place in the morphology of the mid cross-section of the FN, and the implications for its mechanical properties in the different regions around the mid FN cross-section.

Materials and Methods: Measurements were taken from peripheral quantitative computed tomogram (pQCT) images of 81 cadaveric femurs (36 F, 45 M). The mid FN cross-section was segmented radially into eight regions and the cortical bone thickness (CT) and change of the centroid position (CP) of the FN cross-section were measured. The age-related effects of the corresponding changes in the proportion of cortical bone and the “resistance to bending” (section modulus, (Z)) were also measured.

Results: Four femurs were excluded because there were clear signs of OA being present. The maximum difference in regional CT between men and women, was less than 7% (Female: 3.07 ± 0.108mm; Male: 3.28 ± 0.123 mm (mean ± SEM) p =0.21). However, there were regional differences in CT between the young under fifty, (Un50, n=26) and the old, (Abv50), (ANOVAs for young vs old: CT p = 0.001 t 0.01). These effects were attributable to differences in the inferior region, where there was an increase in thickness of the cortical bone of 27% with senior status (Abv50: 3.44 ± 0.09mm; Und50: 2.70 ± 0.12mm. p = 0001) counter balanced by anterior and posterior loss. There was a corresponding change in CP, the distance of the superior, posterior, and superoposterior regions to the FN cross-section’s centroid, 7.6% (Abv50: 20.88 ± 0.28mm; Und50: 19.40 ± 0.47mm; p = 0.005); 6.7% (Abv50: 14.67 ± 0.2mm; Und50: 13.74 ± 0.32mm; p = 0.01); and 8%(Abv50: 17.95 ± 0.24; Und50: 16.61 ± 0.37), respectively. When these two measurements were combined (CP divided by CT) to provide the Local Buckling Ratio (BLR), where the higher the ratio the more unstable the structure, there were significant differences in superoanterior, 30%(Abv50: 15.8 ± 0.52; Und50: 12.1 ± 0.59;p=0.0001); anterior, 20%(Abv50: 10.1 ± 0.32; Und50: 8.3 ± 0.4; p=0.001); inferior, 35%(Abv50: 4.37 ± 0.14; Und50: 5.8 ± 0.34; p=0.0001); inferoposterior 18%(Abv50 8.6 ± 0.27: Und50: 7.36 ± 0.41; p=0.008); posterior, 29%(Abv50: 14.0 ± 0.33; Und50: 10.8 ± 0.5; p=0.0001) and superoposterior, 14%(Abv50: 14.6 ± 0.3; Und50: 12.8 ± 0.4; p=0.001), regions. There was no significant difference in bending resistance nor in the proportion of cortical bone.

Conclusions: A more uniform cortical thickness, seen in the young, would optimise fracture resistance to overloading from unusually loaded directions. Ageing was associated with a thickening of the inferior cortex and thinning of the cortex elsewhere. This effects the location of the area that is least susceptible to the loading forces experienced in stance – that is of the FN mid cross-section’s neutral bending axis – as it will be nearer to the inferior region. Such a change in the morphology will produce deterioration in the FN’s capacity to take a load as shown by the detrimental change in the LBR. This change may indicate that the potential for femoral neck fracture increases with age when load is applied in a direction different to normal stance eg through the greater trochanter.

Areal BMD (aBMD) is relatively poor at discriminating those patients at risk of hip fracture. This study tested the hypothesis that a measure of bending resistance, cross section moment of inertia (CSMI) and section modulus, derived from 3D peripheral quantitative computed tomography (pQCT) images made ex-vivo, would discriminate cases of hip fracture from controls better than areal bone mineral density.

The biopsies were from (n = 20, F) subjects that had suffered an intracapsular hip fracture. The control material (n = 23, F) was from post-mortem subjects. Serial pQCT 1mm thick cross-sectional images using the Densiscan 1000 pQCT clinical forearm densitometer were obtained, and matched for location along the neck. The image voxels were converted to units of bone mass, which were then used to derive the mass weighted CSMI (MWCSMI), section modulus and areal bone mineral density, (see Table).

The aBMD results showed that the difference between the means of the fracture cases compared to the controls was 9.9% (−0.061g/cm²; +0.0055g/cm², −0.127g/cm²; 95% confidence interval). However, the MWCSMI was 29.5% (−5966mm⁴; −8868mm⁴,−3066mm⁴; 95% confidence interval) lower in the fracture cases compared to the controls, while section modulus was 32.5% (−242mm³; −133mm³, −352mm³ 95% confidence interval) lower. When presented as Z scores the fracture cases had considerably lower section modulus Z scores (mean −1.27 SD, p=0.0001) than aBMD – Z scores (mean −0.5 SD, p=0.07). To simulate the forces experienced during a sideways fall, the model’s neutral axis was rotated by 210°. The results were similar for section modulus to those at 0°.

This study suggests that biomechanical analysis of the distribution of bone within the femoral neck may offer a marked improvement in the ability to discriminate patients with an increased risk of intracapsular fracture. Progress towards implementing this form of analysis in clinical densitometry should improve its diagnostic value.