Concord Field Station

Evaluation of skeletal loading history in relation to mechanical effects on bone remodeling

A. Skeletal loading over a range of varying locomotor behaviors.
How variable is the loading pattern experienced by a bone element? Is the distribution of loading events through daily activity similar among different sites within a bone and different bones within an animal? How does this relate to bone remodeling patterns and the risk of bone fracture? These are questions that we seek to address in our studies on in vivoskeletal loading patterns. We have begun this work using the chicken tibiotarsus to evaluate the daily loading history of different sites within the bone. The goal of this work has been 1) to verify that a treadmill exercise pattern can impose a physiologically distinctive loading component relative to sedentary background loading, and 2) to determine how variable the loading distribution (in terms of sign and peak strain magnitude) is over a rull range of locomotor behaviors. Previous work has largely focused on steady speed, level treadmill gait. However, the skeletons of animals must withstand daily loading events over a much broader range of behaviors. Is the intrinsic organization of the musculoskeletal system and the distribution of loads that it transmits such that the loading patterns experienced by skeletal elements of the limb are maintained with low variability? Or are bone elements normally exposed to a much broader range of loading patterns than that suggested by steady state recordings of gait?

B. Effects of exercise and disuse on the development and maintenance of trabecular architecture in growing bone.
How does mechanical loading affect the development and maintenance of trabecular structure? How is the organization of trabeculae related to the pattern of stresses that must be transmitted? Classically, the formation and growth of trabeculae at the ends of bones and within bone processes have been thought to align to the principal stress trajectories that the bone experiences through functional use. This was first formlated by Wolff in the 1890s, becoming known as Wolff's Law, and has had a profound impact on modern studies of bone adaptation and remodeling. Previous studies of strains developed on the surface of the calcaneus of sheep and potoroos (a marsupial) indicates a fairly precise alignment of the principal trabecular arcades with the directions of principal stress. This likely results from the well-defined and uniform loading that the calcaneus experiences in transmitting force via the Achilles tendon. A similar pattern is suggestive in the femoral head of humans. We are interested in how exercise affects the development and maintenance of trabecular structure. When load bearing function is eliminated in adult animals, the trabeculae thin and elements are lost, but their basic alignment is retained, suggesting that the fundamental organization of trabecular structure is achieved during the initial growth of the bone. We hypothesize that if functional loading is eliminated earlier in growth, new trabeculae subsequently formed will be more randomly aligned due to the absence of a mechanical strain signal. We also wish to see how plastic trabecular architecture is when a shift in mechanical loading pattern is imposed. Can mature trabecular bone shift its basic alignment and organization to accomodate a change in principal stress direction?