Augmenting Functional Adaptation: Does Obesity have a Systemic Effect on Bone Strength Properties in Humans?

This study considers the mechanical and neuroendocrine-metabolic effects of obesity on cortical bone and joint morphology throughout the human skeleton. Obesity has primarily been associated with changes in lower limb bone morphology, attributed to local mechanical responses; however, it is known that systemic metabolic shifts concomitant with obesity also influence bone turnover and cell signaling. Thus, the interaction of these mechanical and metabolic effects should be considered, rather than either factor in isolation. The presented research addresses this interaction by examining skeletal data obtained the William M. Bass Donated Collection (University of Tennessee), a modern collection with documentation representing obese and non-obese individuals. Much of the collection has also undergone x-ray computer tomographic (CT) scanning, providing the means to assess bone morphologies beyond the external surface. The scans of 114 individuals are used here to test the hypothesis that obese individuals have increased cortical bone strength properties throughout the skeleton due to both mechanical and systemic effects, while the linear joint dimensions remain unaffected. A total of 22 cross-sections from six skeletal elements (cranial vault, humerus, radius, femur, tibia, fibula), representing three mechanically disparate regions (cranial vault, upper limb, lower limb), and linear dimensions from three articulations (shoulder, hip, and knee) are examined for each individual. Results indicate that obese individuals exhibit larger cross-sectional geometric properties for the humerus, femur, tibia, and fibula relative to normal mass individuals, and the load bearing bones display the greatest magnitudes of difference. Furthermore, whole-diaphyses data indicate that variability in bone robusticity decreases along a proximal-to-distal gradient. Equivocal cranial vault results require further investigation, although the present study suggests that there are minute, if any, macroscopic differences in cranial vault properties between obese and normal mass individuals. Articular dimensions are found to be constrained relative to the diaphyseal cross-sectional measures. Both biomechanical and systemic stimuli are known to affect bone and adipose tissues in known capacities but are rarely examined together. The study presented here applies conclusions from the experimental literature to a human skeletal sample with known demographics, finding that both biomechanical and neuroendocrine-metabolic factors influence macroscopic bone morphology throughout the skeleton