Preferential direction of the collagen fibrils in the subchondral bone of the hip and shoulder joint

We hypothesised that - due to bending and tension - there should exist a pr eferential direction of the collagen fibrils in the subchondral bone of the concave components of the hip and shoulder joint that results from bicentr ic or excentric loading, but there should be no preferential direction in t he convex joint partners. We therefore examined 25 human hip and 27 shoulde r joints, these being decalcified after maceration. To analyse the preferen tial direction of the collagen fibrils, we used the split line method. The subchondral plate was pierced at regular intervals with needles that had be en previously dipped in diluted ink. In the acetabulum, we found a predomin antly transverse direction of the split lines in the ventral and dorsal hor n of the lunate 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 preferen tial 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 speci mens. We interpret the split-line patterns in the acetabulum as an expressi on of the tensile stress that is encountured during a "spreading open" of t he socket upon bicentric (ventral-dorsal) loading in the physiologically in congruous joint. In the glenoid, the relatively weak bony support in the ve ntral and dorsal part of the articular surface map be responsible for bendi ng and tensile stress, particularly in view of excentric loading during dyn amic activity, and this could explain the observed anterio-posterior split line pattern. The results support the idea that the subchondral bone of con cave 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.