Variation in Cortical Bone Distribution in the Aging Adult Appendicular Skeleton

This study considers the effects of age on the distribution of bone in the adult skeleton. Age effects on the skeleton have been studied for diagnosis of osteoporosis or as mechanical compensatory changes to bone shape with loss in density. However, adult skeletal morphology is the result of a lifetime of genetic, dietary, activity, and biochemical factors. With these influences, it unclear at what age(s) bone geometry shifts to adapt to the physiological and mechanical demands placed on it, or, how these adaptations vary within and between bones.

This research addresses these questions by examining skeletal data obtained from the William M. Bass Donated Skeletal Collection (UT). This collection includes modern individuals with documented age at death, and a digital collection of computed tomography (CT) scans to allow quantification of bone morphology through non-destructive means. The scans of 208 individuals are used to test the hypothesis that bone loss with age is sexually dimorphic, affected by body weight and activity, and varies within and between bones as a result of differential loading environments. A total of 30 cross-sectional images from six skeletal elements (humerus, radius, ulna, femur, tibia, and fibula) are examined from each individual. Fifteen geometric properties quantifying bone mass, shape, strength, and rigidity are compared from each cross-section.

Results indicate that geometry is sexually dimorphic and varies across the skeleton. The relationship of geometric properties to age is not linear; skeletal response to aging is more complex than previously understood. Loading environments in the limbs significantly affect bone geometry. In the upper limb, the radius and ulna appear to have complimentary loading-sharing capabilities. Distal limb bones (radius, ulna, and fibula) do not significantly differ in many geometric properties, which has clinical applications for assessment of fracture risk.

Physiological processes associated with senescence are known to decrease bone mass in adults, but the skeletal responses in shape, strength, and rigidity to cumulative effects are rarely studied concomitantly. This study applies conclusions from clinical and anthropological literature to a modern skeletal sample with known demographics, finding that sex, age, and mechanical loading influence the gross morphology of appendicular bone.