Structural trends in the aging femoral neck and proximal shaft: Analysis of the Third National Health and Nutrition Examination Survey dual-energy X-ray absorptiometry data

Hip scans of U.S. adults aged 20-99 years acquired in the Third National He alth and Nutrition Examination Survey (NHANES III) using dual-energy X-ray absorptiometry (DXA) were analyzed with a structural analysis program. The program analyzes narrow (3 mm wide) regions at specific locations across th e proximal femur to measure bone mineral density (BMD) as well as cross-sec tional areas (CSAs), cross-sectional moments of inertia (CSMI), section mod uli, subperiosteal widths, and estimated mean cortical thickness, Measureme nts are reported here on a non-Hispanic white subgroup of 2719 men and 2904 women for a cortical region across the proximal shaft 2 cm distal to the l esser trochanter and a mixed cortical/trabecular region across the narrowes t point of the femoral neck. Apparent age trends in BMD and section modulus were studied for both regions by sex after correction for body weight. The BMD decline with age in the narrow neck was similar to that seen in the Ho logic neck region; BMD in the shaft also declined, although at a slower rat e. A different pattern was seen for section modulus; furthermore, this patt ern depended on sex. Specifically, the section modulus at both the narrow n eck and the shaft regions remains nearly constant until the fifth decade in females and then declined at a slower rate than BMD, In males, the narrow neck section modulus declined modestly until the fifth decade and then rema ined nearly constant whereas the shaft section modulus was static until the fifth decade and then increased steadily. The apparent mechanism for the d iscord between BMD and section modulus is a linear expansion in subperioste al diameter in both sexes and in both regions, which tends to mechanically offset net loss of medullary bone mass. These results suggest that aging lo ss of bone mass in the hip does not necessarily mean reduced mechanical str ength. Femoral neck section moduli in the elderly are on the average within 14% of young values in females and within 6% in males.