Disproportionate, age-related bone loss in long bone ends: A structural analysis based on dual-energy X-ray absorptiometry

The width of long bone diaphyses apparently increase with age, a phenomenon that is suggested to have some positive impact on bone strength. On the ot her hand, these changes in size that are site-specific may cause a deterior ation in the local mechanical integrity of the whole bone. Physical activit y and calcium intake are known to be able to modify bone mass and size. It is, however, not known whether these lifestyle habits can modify the postul ated disproportionate changes in bone size. To address this question, bone mineral content (BMC)-derived estimates of cross-sectional areas (CSA) of f emur and radius in 158 premenopausal (mean age 43, standard deviation 2 yea rs) and 134 postmenopausal (63 (2) years), clinically healthy women with co ntrasting long-term histories in physical activity and calcium intake were determined from dual-energy X-ray absorptiometry (DXA) data. The DXA-obtain ed BMC correlated strongly with the actual CSA (r = 0.94) determined with p eripheral quantative computed tomography. The ratios between functionally i nterrelated CSA data (i.e., (radial shaft CSA/distal radius CSA), (trochant er CSA/femoral neck CSA), (femoral shaft CSA/trochanter CSA) and (femoral s haft CSA/femoral neck CSA)) were considered primary outcome variables. Neit her physical activity nor calcium intake separately or interactively were a ssociated with any CSA ratio. Age showed no interaction with physical activ ity or calcium intake but was independently associated with all CSA ratios, except the ratio of femoral shaft CSA to trochanteric CSA. This study indi cated clearly that a preferential reduction in the cross-sectional area occ upied by bone mineral occurs disproportionately at the long bone ends as co mpared with diaphyseal sites, and this apparently inherent, age-associated relative loss seems not to be prevented by physical activity or calcium int ake. In particular, given the utmost clinical relevance of the proximal fem ur region, an observed loss in femoral neck CSA of about 10% in contrast to about a 5% loss in trochanteric CSA warrants further investigation regardi ng its potential role as a predictor for hip fracture. Not only the local d ifferences in bone composition but also the biomechanical aspects are impor tant factors underlying these apparent changes in CSA at the studied skelet al sites.