MECHANICAL IMPLICATIONS OF HUMERO-ULNAR INCONGRUITY - FINITE-ELEMENT ANALYSIS AND EXPERIMENT

Previous studies show that the humero-ulnar joint is physiologically i ncongruous [Eckstein er al. (1995a) Anat. Rec. 243, 318-326] and exhib its a bicentric (ventro-dorsal) distribution of subchondral mineraliza tion [Eckstein er ni. (1995b) J. Orthop. Res. 13, 286-278]. We therefo re asked: (1) Does humero-ulnar incongruity bring about a bicentric di stribution of contact pressure? (2) Do tensile stresses occur in the s ubchondral bone of the trochlear notch that are in the same order of m agnitude as the compressive stresses? (3) Do ventral and dorsal maxima of subchondral bone density correlate with a bicentric distribution o f strain energy density? To that end, a two-dimensional finite element model was designed. The shape and material properties of the bones we re based on CT and the boundary conditions selected to agree with resi sted elbow extension at 90 degrees of flexion. The incongruity and con tact areas were determined experimentally from casts, and the pressure distribution with Fuji Prescale film. In the model and the experiment contact stresses above 2 MPa were recorded in the ventral and dorsal parts of the joint, and values below 0.5 MPa in the depth of the notch . In the model, tensile stresses of 2.9 MPa were observed in the subch ondral bone of the ulna, but not in the humerus. The subchondral strai n energy density yielded a bicentric pattern in a model with homogeneo us subchondral bone properties. It is shown that humero-ulnar incongru ity brings about a bicentric distribution of contact pressure, a tensi le stress in the notch that is in the same order of magnitude as the c ompressive stress, and a distribution of strain energy density that co rrelates with subchondral density patterns. (C) 1997 Elsevier Science Ltd.