Stress distribution in the trochlear notch. A model of bicentric load transmission through joints.

In 16 cadaver humeroulnar joints, the distribution of subchondral mineralisation was assessed by CT osteoabsorptiometry and the position and size of the contact areas by polyether casting under loads of 10 N to 1280 N. Ulnas with separate olecranon and coronoid cartilaginous surfaces showed matching bicentric patterns of mineralisation. Under small loads there were separate contact areas on the olecranon and coronoid surfaces; these areas merged centrally as the load increased. They occupied as little as 9% of the total articular surface at 10 N and up to 73% at 1280 N. Ulnas with continuous cartilaginous surfaces also had density patterns with two maxima but those were less prominent, and in these specimens the separate contact areas merged at lower loads. The findings indicate a physiological incongruity of the articular surfaces which may serve to optimise the distribution of stress.