HIP DEXA SCANS FROM OSTEOPOROSIS STUDIES

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A number of large population studies have acquired dual energy x- ray absorptiometric (DEXA) scans of the hip to study rates of bone mass loss with age as well as sex and ethnic differences in those rates. There is a growing realization that in addition to bone mass, structural details are important indices of bone strength. Their measurement should permit a better understanding of biomechanical implications of reduced bone mass in the aging hip. Such details are obscured in the conventional bone mass measurement but DEXA scans acquired in these large population studies are archived as a quantitative projection image of the hip. Although not designed for the purpose, these images can be re- analyzed to extract geometric properties which should provide insight into biomedicanical adaptations and alterations of the aging hip, or alternatively, used in a limited (two dimensional) engineering analysis to estimate the strength loss associated with the reduced bone mass. The process involves the use of specialized interactive computer programs which yield the (cortical equivalent) cross-sectional areas, cross-sectional moments of inertia and bone widths of the entire proximal femoral as a continuum including the proximal shaft, intertrochanteric region and femoral neck. Dimensions such as the femoral neck length, hip axis length and neck-shaft angle are also computed as well as bending moments for specific loading conditions. The method has been used to extract structural information from approximately 15, 000 hip scans acquired in the third National Health and Nutrition Examination Survey (NHANES III) as well as a subsample of the Study of Osteoporotic Fracture (SOF). Application of the method to the entire SOF data set as well as data from the European Prospective Osteoporosis Study (EPOS) are planned. These analyses should permit improved understanding of specific patterns of structural change associated with aging as well as the differences in those patterns in ethnic groups with disparate hip fracture rates. It should also provide information permitting a biomechanical explanation of why certain dimensions such as hip axis length, are associated with increased hip fracture risk. Because structural geometry is, unlike bone mineral density, strongly body size dependent, proper size scaling methods immune to cohort effects, vertebral collapse and wasting in the very old will have to be devised so that age trends can be elicited from cross-sectional sample data. A structural analysis of bond mass data in studies with good anthropometric data should also yield more biomechnically meaningful ways of scaling conventional bone mass measurement data for body size and shape for improved clinical management.