CONTACT SURFACES OF THE HUMAN HUMEROULNAR JOINT AS FUNCTION OF PRESSURE FORCES, THEIR CORRELATION WITH SUBCHONDRAL MINERALIZATION AND ARTICULAR SURFACE-MORPHOLOGY IN THE INCISURA TROCHLEARIS

Evaluation of the stress distribution in joints can be obtained direct ly from contact areas and pressure forces, and also indirectly from th e functional adaptation of the connective tissues. Therefore 8 human h umero-ulnar joints, fixed in formalin, were examined for size and posi tion of contact areas (polyether casting/Vidas image analyser) and the ir dependence upon the joint forces (Zwick material testing machine). The distribution of subchondral mineralisation was assessed, using CT osteoabsorptiometry. Depending on the joint force, the contact areas i ncrease from about 10% of the total surface (20 N) to approximately 60 % (1280 N). With weak forces they are localised ventrally and dorsally in the joint, with more powerful forces they run together centrally. With a divided articular surface they join at about 160 - 640 N, with a continuous surface, at about 40 - 80 N. Divided joint surfaces show a bicentric mineralisation pattern of the subchondral bone with ventra l and dorsal maxima. Continuous surfaces, on the other hand, usually s how central maxima. Both the mineralisation pattern and the position o f the contact areas suggest a physiological incongruity of the humero- ulnar joint surfaces, which vanishes with increasing pressure due to v iscoelastic deformation of articular cartilage and subchondral bone. M ore marked incongruity is postulated for the divided surfaces than for the others. The consequent peripheral transmission of pressure seems to involve a functional principle, which, present in several human joi nts, leads to both optimal distribution of the stress and better nutri tion of the articular cartilage.