Prediction of incident hip fracture risk by femur geometry variables measured by hip structural analysis in the study of osteoporotic fractures

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 = 0.72; 95% CI, 0.70, 0.74), but section modulus performed less well (C-index = 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 = 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).

FIG. 1

Age-adjusted univariate HRs (95% CI) for incident hip fracture associated with 1 SD change in each HSA variable measured in three regions of interest. The direction of change is indicated on the x-axis variable labels as increase (+) or decrease (−), and for convenience, the variables are ordered in ascending order of the hazard ratio per 1 SD change at the NN region. NKLENGTH, neck length; NSANGLE, neck-shaft angle; CSMI, cross-sectional moment of inertia; WIDTH, subperiosteal diameter; ENDDIA, endosteal diameter; LATDIS, lateral distance; CENTPO, centroid position; SECTM, section modulus; AVGBR, average buckling ratio; CSA, cross-sectional area; AVGCO, average cortical thickness.

FIG. 2

ROC curves depicting the practical discriminative ability of narrow neck region's cortical thickness, buckling ratio, CSA, and section modulus to identify women with incident hip fracture (n = 635) vs. those without hip fracture (n = 6839) in comparison with BMD.

FIG. 3

C-index statistics (AUCs) depicting discriminative ability of HSA variables in separately identifying cases of incident femoral neck fractures and trochanteric fractures vs. noncases. See Fig. 1 for abbreviations.