Figure 1.

Theoretical framework of bone functional adaptation at the diaphysis. Mechanical loading [1] on a bone of a given stiffness [2] will produce a strain stimulus [3], which, if greater than customary, may elicit bone formation modeling [A] and targeted remodeling [B]. Formation modeling deposits new bone, primarily on the endocortical and periosteal surfaces within the diaphysis, whereas targeted remodeling aims to remove bone damaged by fatigue loading. In the case of disuse and a lower than customary strain stimulus, disuse-mediated remodeling [C] and resorption modeling [D] may occur. Both processes begin with bone resorption centered on or near the endocortical surface of the diaphysis. In the case of disuse-mediated remodeling, resorption is coupled with formation. In the case of modeling, resorption is independent of formation. Thus, bone modeling and remodeling occur on either end of the strain stimulus spectrum. Remodeling [4] primarily alters tissue-level mechanical properties [5], whereas modeling [6] alters the bone morphology [7]. Bone morphology and tissue-level properties collectively determine whole-bone stiffness [2], which then influences the strain response to subsequent bouts of mechanical loading.