Tracking adolescent bone remodeling through sex-specific femoral cross-sectional changes

This study investigated the presence and expression of medullary contractions-defined as reductions in the medullary area (MA)-during adolescence, with particular focus on sex-specific patterns and regional variability within the femur. We assessed changes in cortical area (CA) and MA both in absolute terms and relative to total cross-sectional area (TA) at two anatomical locations: the femoral midshaft and midneck. By integrating developmental and structural indicators, we aim to explore how the hormonal surge associated with pubertal maturation influences femoral cross-sectional geometry, providing novel insights into life-history processes as recorded in skeletal remains. The study sample included 75 femora (46 non-adults and 29 adults) from the medieval population of San Pablo Monastery (Burgos, Spain, 14th-18th centuries). Individuals were assigned to one of four maturity groups based on dental and skeletal indicators. Sex estimation was performed macroscopically for adults and late adolescents, and via enamel peptide analysis (amelogenin) for younger individuals. Cross-sectional properties (including TA, CA, and MA) were extracted from CT scans at the midshaft and midneck. Multiple linear regression models (log-transformed variables) were used to assess the relationship between MA/CA and TA, testing to evaluate the effects of sex and skeletal maturity. At the midneck, both CA and MA increased with TA, with medullary expansion being more pronounced in females with larger bones, likely reflecting the influence of pelvic morphology and femoral neck orientation. In contrast, at the midshaft, TA was not generally associated with MA; however, interactions revealed that females and adults exhibited steeper increases in MA relative to TA, despite having smaller medullary cavities. Although CA was generally higher in adult females, its proportional increase with TA was attenuated, especially at the midshaft. These patterns reflect a shared trend toward medullary contraction, differentially modulated by anatomical region and biomechanical context. Our findings provide evidence for medullary contractions during late adolescence, particularly in females; however, their expression varies by anatomical region. The midneck shows more morphological plasticity, likely reflecting regional loading asymmetries, whereas the midshaft displays canalization with modulated trajectories in larger bones. Proximity to peak bone mass appears critical for detecting this process, which may be transient and obscured in cross-sectional studies. Future studies should increase the sampling of late-adolescent females and integrate pelvic proxies and other bones to better understand the timing and biomechanics of medullary contractions.