Bone and space flight: an overview

Even four centuries ago, Galileo Galile realized that the rigidity of the skeleton of terrestrial animals is related to its load bearing function, and is associated with the size and mass of the animal (Galilei 1974). This phenomenon of differences in mechanical properties also applies to the relatively small variations in the skeleton which occur between individuals of the same species, as well as to variations in bone properties within the same subject. Ward (1836) observed that the trabecular arrangement within the femoral head, the area now known as Ward’s triangle, showed patterns comparable to those found in the crossbeam structures of nineteenth century street lights. This very clearly illustrated natural mechanical engineering of the weight bearing properties of bone. Some 50 years later, Wolff postulated his law; ‘Das Gezetz der Transformation der Knochen’ (Wolff 1892). In this essay he discussed in more detail how the structure of bone reflects its mechanical usage history. The process of bone formation and bone remodeling according to its mechanical history is now generally known as functional adaptation, a term proposed by Roux (1895).