LIEBERMAN, DE; PEARSON, OM; POLK, JD: Growth versus repair responses to loading in mammalian limb bones

Bones respond to mechanical loading either through growth (modeling) or repair (remodeling), but how and why bones modulate these two processes is poorly understood. Most models propose that modeling and remodeling are coordinated to maintain mechanical equilibrium, adapting functionally comparable regions to similar peak strains between 2,000-3,000 units of microstrain. We test here the alternative hypothesis that bones are not adapted to similar mechanical strengths. Instead, we propose that limb bones initially trade-off growth versus repair responses to mechanical loading at different locations in proportion to variations in the energetic costs of adding mass at those locations. Our model is tested through comparisons of exercised versus sedentary domestic sheep from three age categories: juvenile, subadult, and young adult. Histological analyses of midshaft growth and repair rates show that similar loads induce higher rates of growth in proximal midshafts and higher rates of repair in distal midshafts. Studies of cross-sectional, midshaft strains normal to the long axis of the bone indicate that distal elements are adapted to higher strains and maintain lower safety factors than more proximal elements. However, growth rates decline and repair rates increase as animals age. In addition, moderate levels of mechanical loading stimulate both processes significantly less in subadults and especially in adults.