Application of in vivo bone strain measurement techniques to problems of skeletal adaptations

The shape of a bone and its microscopic structure have long been viewed as influenced by genetic and functional factors. Two functional factors of particular significance for the skeleton, weight-bearing and muscle action, affect bone by deforming it slightly. Until recently, it was possible only to hypothesize about normal patterns of deformation (or strain) on bone and how changes in functional factors might influence these strain patterns. With the advent of in vivo bone strain measurement techniques, it is possible to directly observe both normal and abnormal patterns of bone strain. Accumulating evidence from observations of a variety of bone strain patterns has contributed greatly to our understanding of relevant parameters for skeletal adaptations. Among strain parameters currently under investigation are peak bone strains, strain rates, number of strain cycles, and bone strain distributions. Results of in vivo bone strain studies have shown that all of the above parameters most likely exert some influence on bone. For example, strain distribution and/or peak strain may be a critical osteogenic signal, while number of strain cycles may provide a critical remodeling stimulus. In addition, some popular concepts of skeletal adaptations to mechanical stresses are currently being challenged. For example, the idea that bone represents a minimum amount of material arranged with maximum efficiency may no longer be tenable since, in some cases, bones seem to be arranged in a fashion that tends to increase rather than decrease bone strain levels.