(Anatomical Museum, University of Edinburgh.)]
_Morbid Anatomy._--The commonest type is that in which the articular
surfaces undergo degenerative changes. The primary change involves the
articular cartilage, which becomes softened and fibrillated and is worn
away until the subjacent bone is exposed. If the bone is rarefied, the
enlarged cancellous spaces are opened into and an eroded and worm-eaten
appearance is brought about; with further use of the joint, the bone is
worn away, so that in a ball-and-socket joint like the hip, the head of
the femur and the acetabulum are markedly altered in size and shape.
More commonly, the bone exposed as a result of disappearance of the
cartilage is denser than normal, and under the influence of the
movements of the joint, becomes smooth and polished--a change described
as _eburnation_ of the articular surfaces (Fig. 158). In hinge-joints
such as the knee and elbow, the influence of movement is shown by a
series of parallel grooves corresponding to the lines of friction
(Fig. 158).
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