Abstract
In an attempt to investigate the pathogenesis of the ankle region from a mechanical viewpoint we made a model of the skeleton of the lower extremity of D.A.P. resin, with with we conducted a two-dimensional photoelastic experiment to stress analysis developed under variously applied loads. Determinations were made further of the strength of parts of the bones (e.g., thickness of the cortex, hardness of the bony substance) to find out mechanical weak points. The results thus obtained were compared with x-ray pictures. The results were summarized as follows.
1) Under perpendicularly applied loads there were noted uniformly distributed compressive stresses but without any pronounced stress concentration on the ankle.
2) With loads applied to the inverted foot, tensile, bending stresses act on the fibula (the maximum value being found at the joint space level, while large compressive stresses act on the inner aspect of subtibial articular surface corresponding to the talus.
3) When forces are acted on the everted foot, tensile stresses develop along the medial margein of the tibia with the strain augmenting abruptly at its malleolus. The fibula is acted on by compressive, bending stresses, their maximum value being seen at a level a little higher than the articular space and shifted upward further with tibiofibular diastasis.
4) Lateral projection experiments: Under loads applied to the dorsi-or plantalflexed foot compressive, bending stresses act on the leg and compressive stresses, on the anterior and posterior part of the subtibial articular surface.
5) Principal stress trajectories are similar to the stracture of trabecules.
6) Whereas the diaphysis of leg bones has a great hardness of the bony substance and a thick cortex, lower portions of the same bones have a lesser hardness and a thinner cortex. This is corresponding with the observed peripheral stress distribution of the leg under perpendiculary applied load.
7) Simultaneous inspection of a x-ray picture of fracture of the malleolus and the stress distrubution shows the fracture line to proceed from the peak of tangential tensile stress or the point of convergence of maximum shearing stress and to run along the lines of maximum shear stress. The fracture line thus is quite in keeping with a maximum shearing stress trajectories diagram, suggesting that a bone, because of its structure and its component material, is relatively vulnerable to shearing stress.
