Age and sex differences in estimated tibia strength: influence of measurement site.

Variability in peripheral quantitative computed tomography (pQCT) measurement sites and outcome variables limit direct comparisons of results between studies. Furthermore, it is unclear what estimates of bone strength are most indicative of changes due to aging, disease, or interventions. The purpose of this study was to examine age and sex differences in estimates of tibia strength. An additional purpose of this study was to determine which tibia site or sites are most sensitive for detecting age and sex differences in tibia strength. Self-identifying Caucasian men (n=55) and women (n=59) aged 20-59yr had their tibias measured with pQCT from 5% to 85% of limb length in 10% increments distal to proximal. Bone strength index, strength strain index (SSI), moments of inertia (Ip, Imax, and Imin), and strength-to-mass ratios (polar moment of inertia to total bone mineral content [BMC] ratio [Ip:Tot.BMC] and strength strain index to total BMC ratio [SSI:Tot.BMC]) were quantified. There were significant (p<0.01) site effects for all strength variables and strength-to-mass ratios. Site×sex interaction effects were significant (p<0.05) for all strength variables. Men had greater (p<0.01) values than women for all strength variables. Sex differences in Ip, Imax, Ip:Tot.BMC, SSI, and SSI:Tot.BMC ratios were the smallest at the 15% site and peaked at various sites, depending on variable. Site×age interactions existed for Imax, Ip:Tot.BMC, and SSI:Tot.BMC. There were significant age effects, Imax, Ip:Tot.BMC, and SSI:Tot.BMC, as values were the lowest in the 20-29 age group. Age and sex differences varied by measurement site and variable, and larger sex differences existed for moments of inertia than SSI. Strength-to-mass ratios may reflect efficiency of the whole bone architecture.