Mechanical determinants of bone modeling.

In angulated long bone malunions in children, bone architectural changes arise which act consistently in kind, location, and direction, while the associated bone strain-stress patterns become definable as qualitative differences from the norm. Comparison of such information reveals that bone architectural adaptations correlate one-to-one with dynamic flexural strain orientation and magnitude but not with any single principal stress. The above plus other facts suggest six axioms which can explain many of the observed architectural adaptations of lamellar bone in response to specific mechanical bone-loading phenomena in normal as well as pathologic states. In essence those axioms state: In growing mammals and under repetitive, uniformly oriented, nontrivial, dynamic flexural strains, all lamellar bone surfaces drift in the concave-tending direction. As a result of these strains, a bone would adopt a size and configuration that minimize its flexural deformation during normal activities. The axioms provide reasonable explanations for inwaisting of vertebral bodies and long bone metaphyses, correction of malunions, increasing outside diameter and cortical thickness during growth, circular, elliptical, and triangular cross-sections, natural curvatures of whole bones, many architectural changes due to neuromotor abnormalities arising in early life, and the more rapid modeling seen in very young children or after recent fractures.