PHYSIOLOGICAL INCONGRUITY OF THE HUMERO-ULNAR JOINT - A FUNCTIONAL PRINCIPLE OF OPTIMIZED STRESS-DISTRIBUTION ACTING UPON ARTICULATING SURFACES

Investigations into the distribution of subchondral bone density in th e human elbow have suggested that the geometry of the trochlear notch deviates from a perfect fit with the trochlea, and that the load is tr ansmitted ventrally and dorsally rather than through the centre of the humero-ulnar joint. We therefore decided to make a quantitative asses sment of the degree of incongruity between the two components in 15 hu man specimens (age distribution 60 to 93 years) with different types o f joint surface. Polyether casts of the joint cavity were prepared und er loads of 10, 40, 160 and 640 N. The thickness of the casts was then measured at 50 predetermined points, and an area distribution of the width of the joint space represented in a two-dimensional template of the trochlear notch. The reproducibility of this procedure was tested by image analysis. At a load of 10 N, only a narrow space was present ventrally and dorsally in the joint, but in the depths of the trochlea r notch a width of 0.5 to 1 mm was recorded in the centre, and up to 3 mm at its medial and lateral edges. Specimens with continuous articul ar cartilage showed a lower degree of incongruity than those with a di vided articular surface. As the load was increased to 640 N, however, the original incongruity between the articular surfaces disappeared al most completely. The joint surfaces became more congruous, probably be cause of the viscoelastic properties of the articular cartilage and th e subchondral bone, and the contact areas merged in the centre of the joint. It is suggested that this physiological incongruity brings abou t an optimal distribution of stress over the articular surface during the transmission of the load, and it may lead to better nourishment of the articular cartilage by providing intermittent mechanical stimulat ion and circulation of the synovial fluid.