| The role of mechanics in bone maintenance has been recognized for over a hundred years; although the relative importance compared to other factors is still unknown. This dissertation evaluates the calcaneus as a site for studying bone adaptation. A novel technique for modeling the calcaneus is developed and applied to examine the effects of altered loading on calcaneal density and morphology. The first study develops and validates a contact-coupled finite element model of the foot relating external daily loading to calcaneal stresses and strains. Analyses showed that peak calcaneal loading occurs at 70% of the stance phase of walking and 60% of the stance phase of running. The ligament, tendon, and joint contact forces scaled with the external moment. The second study applies the mathematical model of bone adaptation to the finite element model of the calcaneus to assess the effect of different lifetime activity levels including running, moderate and intense exercise intervention, and age-related decreases in activity levels. The simulations predicted that lifetime exercisers had substantially increased bone density over sedentary individuals. The simulations of moderate exercise intervention predicted density increases from 0.8 to 2.8% after one year, while intense exercise resulted in gains of 2.5 to 8.5%. Simulations of age-related bone loss suggested that over a 20% decrease in bone density between the ages of 30 to 65 could be explained by a decreasing activity level. In the third study, physiological adaptation rates were determined based on clinical data of calcaneal bone loss during bedrest. Simulations of bedrest using CT data demonstrated that regional density heterogeneity in the calcaneus resulted in different volumetric resorption rates and absolute quantities of bone lost. The percentage of bone lost varied by 33% when a 5 mm diameter region-of-interest was used. Utilizing larger regions-of-interest, the percent of bone lost varied by 10% and the absolute density change varied by over 200%. This dissertation establishes a numerical model of the calcaneus to quantitatively examine mathematical theories of bone remodeling by providing the critical link between external forces acting on the foot during daily activity, in vivo calcaneal stresses, and bone adaptive response. |
