The heterogeneity in femoral neck structure and strength

Abstract

Most measures of femoral neck strength derived using dual-energy X-ray absorptiometry or computed tomography (CT) assume the femoral neck is a cylinder with a single cortical thickness. We hypothesized that these simplifications introduce errors in estimating strength and that detailed analyses will identify new parameters that more accurately predict femoral neck strength. High-resolution CT data were used to evaluate 457 cross-sectional slices along the femoral neck of 12 postmortem specimens. Cortical morphology was measured in each cross-section. The distribution of cortical thicknesses was evaluated to determine whether the mean or median better estimated central tendency. Finite-element models were used to calculate the stresses in each cross-section resulting from the peak hip joint forces created during a sideways fall. The relationship between cortical morphology and peak bone stress along the femoral neck was analyzed using multivariate regression analysis. In all cross-sections, cortical thicknesses were non-normally distributed and skewed toward smaller thicknesses (p < 0.0001). The central tendency of cortical thickness was best estimated by the median, not the mean. Stress increased as the median cortical thickness decreased along the femoral neck. The median, not mean, cortical thickness combined with anterior-posterior diameter best predicted peak bone stress generated during a sideways fall (R 2 ¼ 0.66, p < 0.001). Heterogeneity in the structure of the femoral neck determines the diversity of its strength. The median cortical thickness best predicted peak femoral neck stress and is likely to be a relevant predictor of femoral neck fragility. ß

Difference in the trajectory of change in bone geometry as measured by hip structural analysis in the narrow neck, intertrochanteric region, and femoral shaft between men and women following hip fracture

“…The buckling ratio is, at best, a crude index of cortical instability based on an estimate of the average cortical thickness. It is well recognized that the femoral neck cortex is highly asymmetric and favors the physiologically loaded medial surface [51]. Use of the average cortex probably underestimates the contribution of local buckling to failure in a fall to the side, however, failure mechanisms cannot be reliably predicted from the rudimentary structural information present in a DXA scan.…”

Physical Activity for Strengthening Fracture Prone Regions of the Proximal Femur

“…Greatest compressive stresses and strains now occur in or about the region of the thin superolateral femoral neck while the thick inferomedial femoral neck is exposed to lower tensile stresses and strains (Fig. 1B) [20, 21, 23, 24, 41]. The net result is the exposure of the superolateral cortex and bone nearby to potentially injurious stress and strain, with high-speed video recordings revealing that fracture initiation during an in vitro simulated side-ways fall occurs within the superolateral cortex [42, 43].…”

“…These assumptions misrepresent the irregular cylinder structure of the femoral neck and the non-normal disruption of its cortical thickness. Cortical thickness within the femoral neck is heavily skewed toward smaller thicknesses such that mean cortical thickness overestimates thickness in 81% of regions along the femoral neck [41]. Also, the HSA approach does not allow any site-specific effects of physical activity to be established at the superolateral femoral neck.…”

Role of cortical bone in hip fracture

“…104 New imaging techniques now make it possible to investigate the detailed 3D structure of individual osteons in the femoral cortex and the anatomical relationships between them. 105,106 These offer new avenues for understanding how conversion of the cortex to trabecular bone, leading to cortical thinning, 104,107 develops and to how osteonal structures might help preserve the toughness of bone once a crack threatens to expand beyond the osteon's own territory. 108 Another cause of increased remodelling is the secondary hyperparathyroidism associated with reduced vitamin D levels, commonly seen in the elderly.…”