Abstract
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.
