[Preferential direction of collagen fibrils in the sub-chondral bone bone and the hip and shoulder joint].

We hypothesised that--due to bending and tension--there should exist a preferential direction of the collagen fibrils in the subchondral bone of the concave components of the hip and shoulder joint that results from bicentric or eccentric loading, but there should be no preferential direction in the convex joint partners. We therefore examined 25 human hip and 27 shoulder joints, these being decalcified after maceration. To analyse the preferential direction of the collagen fibrils, we used the split line method. The subchondral plate was pierced at regular intervals with needles that had been previously dipped in diluted ink. In the acetabulum, we found a predominantly transverse direction of the split lines in the ventral and dorsal horn of the lunatic surface, and these usually continued through the acetabular fossa. In the ventral part of the acetabular roof, arch-like orientations of the split lines were observed. In the glenoid cavity, a clearly preferential orientation was found in anterior-posterior direction, usually in the middle third of the articular surface. In the femoral and humeral heads, no preferential direction of the split lines was observed in any of the specimens. We interpret the split-line patterns in the acetabulum as an expression of the tensile stress that is encountered during a "spreading open" of the socket upon bicentric (ventral-dorsal) loading in the physiologically incongruous joint. In the glenoid, the relatively weak bony support in the ventral and dorsal part of the articular surface may be responsible for bending and tensile stress, particularly in view of eccentric loading during dynamic activity, and this could explain the observed anterio-posterior split line pattern. The results support the idea that the subchondral bone of concave joint partners encounters tension, leading to a preferential direction of the collagen fibrils. This can be considered as a functional adaptation of the subchondral bone on a microstructural level.