The cancellous structure in the calcaneus and its relation to mechanical stressing.

The structure of the cancellous bone in the calcaneus is comparable to a three-dimensional trajectorial system, best adapted to resisting bending stresses. This can be seen from the curve trend of the tensile fascicles. Since stressing takes place mainly in the sagittal plane, the adaptation of the structure must be examined in this plane. The photoelastic experiments prove that similar trajectorial patterns result from different types of loading (e.g., standing with and without muscular contraction, the transmission of forces before and after the fusion of the tuber apophysis); and therefore the stresses remain the same. The trajectorial patterns, already established from the experiments, correspond with those of the cancellous structure, which indicates that a state of adaptation exists. Using the definition given in Roux's "maximum-minimum" law, this means that the bone achieves maximum resistance with a minimum of material. The photoelastic experiments can also be successfully applied to a short, skeletal element, which has been stressed in a complex manner. Furthermore, the theory of causal histogenesis in the supporting tissues can be proved, since the position of the epiphyseal center coincides with that of a singular poing (hydrostatic pressure) in the trajectorial pattern. A division of the trajectorial pattern, into lateral and medial fascicles (as described by T. von Lanz and W. Wachsmuth) cannot be seen in the bone sections and consequently cannot be regarded as being factual.