Time Trends in Trajectories of Forearm Mineral Content and Bone Size during Childhood—Results from Cross-Sectional Measurements with the Same Apparatus Four Decades Apart

Evidence suggests that single photon absorptiometry (SPA)-measured forearm bone mineral density (BMD) is lower in contemporary children in Malmö than it was four decades ago, but the fracture incidence in the at-risk population (all Malmö children) has been stable during the same period. The aim of this study was to evaluate if improvements in skeletal structure over time may explain this observation. In 2017–2018 we measured distal forearm bone mineral content (BMC; mg/cm) and periosteal diameter (mm) in 238 boys and 204 girls aged 7–15 using SPA. Based on the SPA measurements, we calculated forearm BMD (mg/cm2), bone mineral apparent density (BMAD, mg/cm3), section modulus, and strength index (BMAD × section modulus). The results were compared with those derived from measurements of 55 boys and 61 girls of the same ages using the same scanner in 1979–1981. We used log-linear regression with age, sex, and cohort as predictors to investigate differences in trait trajectories (trait versus age slopes [mean percent difference in beta values (95% confidence interval)]). SPA-measured forearm BMC was lower at each age in 2017–2018 compared to 1979–1981 (a mean age and sex adjusted relative difference of 9.1%), the forearm BMC trajectory was similar in 2017–2018 to that in 1979–1981 (reference) [0.0%/year (−1.0%, 1.0%)], while the 2017–2018 forearm periosteal diameter trajectory was steeper [1.1%/year (0.3%, 2.0%)]. Since bone size influences both BMD (BMC divided by scanned area) and mechanical characteristics, the forearm BMD trajectory was flatter in 2017–2018 [−1.1%/year (−2.0%, −0.2%)] and the forearm section modulus trajectory steeper [3.9%/year (1.4%, 6.4%)]. Forearm strength index trajectory was similar [1.8%/year (−0.5%, 4.1%)]. The lower SPA-measured forearm BMD trajectory in contemporary children compared to four decades ago may be offset by changes in forearm bone structure, resulting in similar overall bone strength