Characterising cortical density in the mid-tibia: intra-individual variation in adolescent girls and boys

Background: Inter-individual differences in cortical bone volumetric density (CoD), such as those related to sex, are a product of differences in remodelling rates. While cortical bone is often treated as a uniform tissue, remodelling rates also vary within individual bones. This level of adaptation has largely been overlooked in analyses of peripheral quantitative computed tomography (pQCT) images. Further, such variation in CoD has never been assessed in growing bones. We hypothesised that CoD varied significantly within the same cross-section of the mid-tibia of adolescents. We further hypothesised that due to the profound impact of oestrogen on remodelling, this variation would be different between sexes. Methods: Subjects were 183 adolescents (99 girls and 84 boys) in grade 6 and 7 with a mean age of 12.1 years. We used age at peak height velocity to adjust for maturational differences between sexes. Image data from a mid-tibia pQCT scan of each subject were assessed regionally within eight sectors distributed about the cortex and aligned by the anterior tibial crest. We used a repeated measures general linear model to assess intra-individual variation in CoD while controlling for differences in ethnicity, maturity, height, weight, physical activity level and total cross-sectional bone area (ToA). Results: Sector based variation in CoD was significant (p<0.001), with the anterior cortex having lower density than the posterior cortex. The largest percentage difference (anterior vs posteromedial sectors) was 12.2%. A significant sector*sex interaction (p = 0.018) was detected; however, its impact was relatively small with girls having 1.1-3.6% denser bones than boys depending on the sector (2.7% average difference). Conclusions: The magnitude of the variation in CoD across sectors within individuals of both sex was far greater than the mean differences between the sexes. This finding indicates that the microstructural variation within the mid-tibia is detectable by pQCT and its magnitude suggests an important level of adaptation to loading.