54 - Covariation Between Age-related Cortical Expansion and Bone Mineral Density Decline in Human Tibiae

Poster Board Number: 54
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Odd poster #s – first hour
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Abstract Body : As humans age, endosteal removal and periosteal deposition of bone results in limb long bones with decreased cortical thickness and expanded cross-sectional diameter. This change in bone geometry redistributes bone farther away from the neutral axis, which bolsters a bone’s resistance to bending. Given its effect on bending strength, this geometric shift in cortical bone has been characterized as being compensatory to bone loss since there is a temporal relationship with bone mineral density (BMD) decline. If this is the case, a local relationship between cortical expansion and BMD decline would also be expected, though few studies have examined variation of these processes within a cross-section. This study seeks to examine local variation in age-related cortical expansion and BMD decline both within cross-sections and along the length of the leg. We hypothesize that shape changes associated with cortical expansion and BMD declines will exhibit local variation and ask whether these processes covary locally within tibial cross-sections.

To address these questions, we scanned the legs of 126 living participants between ages 40 and 80 years to generate cross-sections of the tibia using a peripheral quantitative computed tomography scanner. Scans were taken at five locations along the bone’s length. Semi-landmarks were established around the endosteal and periosteal borders of each cross-section to assess cortical expansion across individuals, resulting in measures of cortical thickness and endosteal/periosteal distance from the centroid. These endosteal and periosteal semi-landmarks were then used to place volumes of interest at different locations around the cross-section to sample BMD.

Preliminary results indicate that age-related changes in cortical geometry and BMD are not correlated uniformly throughout tibial midshaft cross-sections. Specifically, these processes correlate strongly along the mediolateral axis and weakly along the anteroposterior axis. These results suggest that cortical expansion and BMD decline do not occur evenly throughout long bone cross-sections and are instead mediated by local factors.