A contribution to the functional morphology of articular surfaces

The variations of the joint shape are first analyzed in the acetabulum and the ulnar trochlear notch of man. Then an attempt is made to explain the basic shape of the lunate surface and the types of articular surface diverging from it with regard to causal histogenesis and the general theory of joint stress (Pauwels), as well as the theoretical distribution of stress in the perfect ball and socket joint (Kummer). Certain statements are made about the position, size, changes in direction and course of the resultant forces at the joint, statements which seem very likely to be accurate when compared to facts already known about the hip joint in man. Under these preconditions, the various forms of the observed articulating surfaces may be theoretically reconstructed in every case. In this connection, it is first shown that the preservation of the joint cartilage is also dependent on the period of time during which the resultant force is effective in the various areas of the articular surface. The term "relative constancy" is used to express this time factor. Two functional explanations are given for the presence of incisions and divisions in the lunate surface and in the trochlear notch: 1. It is assumed that the "relative constancy" of the resultant forces is of varying magnitude in the different areas of the articular surface. The incisions and divisions in the joint cartilage are thus seen as areas in which the resultant joint stress does not rise above the minimum value for preservation of the articular cartilage. 2. The radius of the convex articular surface can be slightly larger than the radius of the concave articular surface. Because of this incongruity no pressure will be transmitted in an area of the lunate surface or of the trochlear notch because of the defective contact. It follows that the cartilage-preserving stimulating pressure is also absent from this area. According to Pauwels, the magnitude and distribution of the stresses arising in the articulating surfaces may be determined from the varying density distribution of the subchondral bony tissue, as seen in x-rays. The density distribution of the subchondral bone at the ulnar socket shows clear basic parallels to the extent of the surface covered by joint cartilage. The relatively low density in the area of the notch is seen as being the result of lower stress, in accordance with the functional explanation of division of the cartilage surfaces. In photoelastic model experiments, the dependence of the degree of relative stress on the form of the articular surface is illustrated by means of the number of isochromes. Here, the distribution of density of the subchondral bony tissue corresponds to the local magnitude of stress in the corresponding model joint. Functional analysis of the tangential fiber layer of the joint cartilage at the femoral head and the elbow joint, shows basic agreement between the split line patterns and the trajectoral pattern, as described in articulating surfaces examined by Pauwels, Konermann and Molzberger...