Contribution of In Vivo Structural Measurements and Load/Strength Ratios to the Determination of Forearm Fracture Risk in Postmenopausal Women

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Abstract
Bone structure, strength and load-strength ratios contribute to forearm fracture risk independently of areal BMD. Technological and conceptual advances provide new opportunities for evaluating the contribution of bone density, structure, and strength to the pathogenesis of distal forearm fractures. From an age-sratified random sample of Rochester, MN, women, we compared 18 with a distal forearm fracture (cases) to 18 age-matched women with no osteoporotic fracture (controls). High-resolution pQCT was used to assess volumetric BMD (vBMD), geometry, and microstructure at the ultradistal radius, the site of Colles' fractures. Failure loads in the radius were estimated from microfinite element (microFE) models derived from pQCT. Differences between case and control women were assessed, and the risk of fracture associated with each variable was estimated by logistic regression analysis. Given similar heights, estimated loading in a fall on the outstretched arm was the same in cases and control. However, women with forearm fractures had inferior vBMD, geometry, microstructure, and estimated bone strength. Relative risks for the strongest determinant of fracture in each of the five main variable categories were as follows: BMD (total vBMD: OR per SD change, 4.2; 95% CI, 1.4-12), geometry (cortical thickness: OR, 4.0; 95% CI, 1.4-11), microstructure (trabecular number: OR, 2.3; 95% CI, 1.02-5.1), and strength (axial rigidity: OR, 3.8; 95% CI, 1.4-10); the factor-of-risk (fall load/microFE failure load) was 24 % greater (worse) in cases (OR, 3.0; 95% CI, 1.2-7.5). Areas under ROC curves ranged from 0.72 to 0.82 for these parameters. Bone geometry, microstructure, and strength contribute to forearm fractures, as does BMD, and these additional determinants of risk promise greater insights into fracture pathogenesis.