The osteons in all human long bones are grouped in two helical antirotary systems of opposite oblique directions, situated in the contralateral walls of the diaphysis. Several arguments support the hypothesis that this special architecture arises from functional adaptations and that it depends on the orientation of the first principal stress. A new macroscopic method based on the filling of vascular canals of the undecalcified, polished bone with India-ink enabled us to study the spatial architecture of the haversian bone in the entire diaphysis. The osteon directions in normal and atypical femurs was compared with the direction of the first principal stresses, determined analytically in a cylindrical tube model of the diaphysis subjected to a combination of bending, torsion and compression. Under combined loading with the bending moment in the frontal plane to the medial side and with the torque moment in the sense of external rotation, the direction of the first principal stresses corresponded with the direction of osteons in the diaphysis of the femur. In both cases, the first principal stresses, as well as the osteons, were oriented in the opposite oblique direction in the medial and lateral walls of the model and of the bones. Between the two oblique fields a sharp boundary with an atypical organization of the principal stresses and osteons existed. In atypical femurs the osteonal orientation was longitudinal (likely unloaded femurs) or rotated 90° (markedly anteriorly convex femurs).

These observations support the hypothesis of a causal relation between the loading mode and the dominant osteonal direction. The organization of the haversian bone seems a typical example of functional adaptation.