Bone mass in young adults - determinants and fracture prediction

University dissertation from Orthopaedics

Abstract: Bone mass in adolescents and young adults was studied with regard to its age-related change and its determinants. Methodological aspects of bone mass measurements and their ability to predict future fractures were investigated. Bone mineral content (BMC) and bone mineral density (BMD) were assessed by the Single Photon Absorptiometry (SPA) and Dual Energy X-ray Absorptiometry (DXA) techniques. We demonstrate that results, when normative bone mass data are being collected, may depend on whether the study is population-based with randomly selected probands or selected by other means. Participation rate may also influence the outcome. In the same study population, higher BMD levels at several skeletal sites were found by increasing the participation-rate from 61.9 to 83.6%. A limited but significant reduction of forearm cortical BMD in women (<70 years) over the past two decades was found by comparing two samples of healthy women (n=271 and n=155). We conlude that this may be due to life-style changes. In a cross-sectional study of 332 subjects (175 men), age 15-42 years, peak bone mass occurred for both sexes at approximately age 20 in the hip and age 30 in the forearm, lumbar spine and total body. The drop in BMD following peak bone mass was most pronounced in the hip. In this study, 112 subjects (57 men) were studied longitudinally (mean 3.4 years). A high degree of conformity between cross-sectional and longitudinal bone mass data in the description of the age-related change in BMD was found. In a population-based study of 39 girls and 48 boys, childhood weight was found to be predictive of adolescent total body BMC but not total body BMD. This suggests that growth determines the size of the skeleton, whereas the density within that bone envelope is to a greater extent governed by other factors such as physical activity. Familial resemblance in BMD was studied in 40 biological daughter-mother-grandmother triads and 20 biological daughter-mother pairs. A significant correlation was found between age- and maturity-adjusted BMD Z-scores at all sites in the D-M relationship (r=0.25 - 0.39), only in the head for the M-G relationship (r=0.40) and not at all in the D-G relationship. This implies that environmental factors may modify the genetic impact on familial resemblance. In a study of active (n=96) and former (n=25) female fotball players we found higher BMD values in active players when compared with matched controls at several skeletal sites, most pronounced in the hip. The BMD advantage over controls was preserved in former players that had ended their active careers on an average 9.7 years prior to the study. A single bone density measurement of the forearm in a study including 410 women, was found to be predictive of a future fracture on a 25 year perspective. The relative risk (RR) of a fracture with a 1 SD decrease in BMD was 1.33 for all fragility fractures and 1.66 for a hip fracture.

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