Finite element analysis of the proximal femur and hip fracture risk in older men

Abstract

Low areal BMD (aBMD) is associated with increased risk of hip fracture, but many hip fractures occur in persons without low aBMD. Finite element (FE) analysis of QCT scans provides a measure of hip strength. We studied the association of FE measures with risk of hip fracture in older men. A prospective case-cohort study of all first hip fractures (n = 40) and a random sample (n = 210) of nonfracture cases from 3549 community-dwelling men > or =65 yr of age used baseline QCT scans of the hip (mean follow-up, 5.6 yr). Analyses included FE measures of strength and load-to-strength ratio and BMD by DXA. Hazard ratios (HRs) for hip fracture were estimated with proportional hazards regression. Both femoral strength (HR per SD change = 13.1; 95% CI: 3.9-43.5) and the load-to-strength ratio (HR = 4.0; 95% CI: 2.7-6.0) were strongly associated with hip fracture risk, as was aBMD as measured by DXA (HR = 5.1; 95% CI: 2.8-9.2). After adjusting for age, BMI, and study site, the associations remained significant (femoral strength HR = 6.5, 95% CI: 2.3-18.3; load-to-strength ratio HR = 4.3, 95% CI: 2.5-7.4; aBMD HR = 4.4, 95% CI: 2.1-9.1). When adjusted additionally for aBMD, the load-to-strength ratio remained significantly associated with fracture (HR = 3.1, 95% CI: 1.6-6.1). These results provide insight into hip fracture etiology and demonstrate the ability of FE-based biomechanical analysis of QCT scans to prospectively predict hip fractures in men.

FIG. 1

Typical FE model of the proximal femur, showing 3D (A) and 2D sectional (B) views. The color-coding shows the spatial variation of material strength assigned to the individual finite elements. A vertical force was applied through the center of the femoral head using a PMMA mold (shown in white). Bending moments and torque but no shear forces could be transmitted at the distal end, and shear forces could not be transmitted through the trochanter PMMA mold.

FIG. 2

Correlations between baseline measures of the hip. (A) Femoral strength vs. total hip aBMD. (B) Load-to-strength ratio vs. aBMD. (C) Load-to-strength ratio vs. femoral strength.

FIG. 3

The distributions of the hip measurements at baseline using Kernel density estimate curves in men with subsequent hip fractures and nonfractured men. Kernel estimators can be regarded as nonparametric histogram smoothers and are used here to compare the distributions of baseline hip measures in the two groups. (A) Femoral strength. (B) Load-to-strength ratio. (C) Total hip aBMD by DXA.

FIG. 4

ROC curves for hip fracture prediction using the three baseline hip measures. Conventional cut-offs of hip fracture risk, a total hip DXA T-score of T = − 2.5 (male reference), and the theoretical cut point of φ = 1.0 for the load-to-strength ratio, are indicated.