MORPHOMECHANICS OF THE HUMERO-ULNAR JOINT .2. CONCAVE INCONGRUITY DETERMINES THE DISTRIBUTION OF LOAD AND SUBCHONDRAL MINERALIZATION

Background: A deeper joint socket (concave incongruity) is found at mo st angles of flexion of the humero-ulnar joint and maintained over a w ide range of physiological loading. It is, however, unclear how far th is incongruity affects the distribution of load and subchondral minera lization of this joint as compared with a congruous configuration, Met hods: Two nonlinear, axisymmetrical finite element models with two car tilage layers were constructed, one congruous and one incongruous, wit h a joint space of realistic magnitude. The distribution of subchondra l mineralization was determined by computed tomography osteoabsorptiom etry in the same six specimens that were investigated in the first par t of the study, and compared with the biomechanical data obtained ther e and the predictions of the models. Results: In the congruous case, t he center of the socket is highly loaded, whereas the periphery does n ot experience mechanical stimulation. A central bone density maximum i s predicted. With concave incongruity the position of the contact area s shifts from the joint margin towards the center as the load increase s, and the peak stresses are considerably lower, A bicentric ventro-do rsal distribution pattern of subchondral mineralization is predicted, and this is actually found in the six specimens. Conclusions: Concave incongruity is shown to determine load transmission and subchondral mi neralization of the humero-ulnar joint. It is suggested that this shap e leads to a more even distribution of stress, provides intermittent s timulation of the cartilaginous tissue, and has beneficial effects on the metabolism, nutrition, and lubrication of the articular cartilage during cyclic loading. (C) 1995 Wiley-Liss, Inc.