Tension and bending, but not compression alone determine the functional adaptation of subchondral bone in incongruous joints

In the present study, we tested the hypothesis that tension and bending, ra ther than compression alone, determine the functional adaptation of subchon dral bone in incongruous joints. We investigated whether tensile stresses i n the subchondral bone of the humero-ulnar articulation are affected by the direction of muscle and joint forces, and whether the tensile stresses are large enough to cause microstructural adaptation, specifically a preferent ial alignment of the trabeculae and the subchondral collagen fibres. Using a previously validated finite element model of the human humero-ulnar joint , we calculated the contact pressure, the principal compressive and tensile stresses, and the strain energy density in the subchondral bone for variou s flexion angles. A bicentric (ventro-dorsal) pressure distribution was fou nd in the joint at 30 degrees to 120 degrees of flexion, with contact press ures of up to between 2.5 and 3 MPa in the ventral and dorsal aspects of th e ulnar joint surface, but less than 0.5 MPa in the centre. The principal t ensile stress in the subchondral bone of the trochlear notch quantitatively exceeded the principal compressive stress at low flexion angles (maximum 8 .2 MPa), and the distribution of subchondral strain energy density differed substantially from that of the contact stress (r=-0.72 at 30 degrees and r =+0.58 at 90 degrees of flexion). No important tensile stress was computed in the trochlea humeri. On contact radiography, we found sagittally orienta ted subarticular trabeculae in the notch, running tangential to the surface . Furthermore, we observed sagittally orientated split lines in the subchon dral bone of the notch of 20 cadaver joints, suggesting a ventro-dorsal ori entation of the collagen fibres. The trochlea humeri, on the other hand, di d not show a preferential direction of the subchondral split lines, these f indings confirming the predictions of tensile stresses in the model. We con clude that, due to the important contribution of tension to subchondral bon e stress, the distribution of subchondral density cannot be directly employ ed for assessing the long term distribution of joint pressure at the cartil age surface. The magnitude of the tensional stress varies considerably with the direction of the muscle and joint forces, and it appears large enough to cause functional adaptation of the subchondral bone on a microstructural level.