A FUNCTIONAL MORPHOLOGICAL-STUDY OF THE TIDEMARK REGION OF ARTICULAR-CARTILAGE MAINTAINED IN A NON-VIABLE PHYSIOLOGICAL CONDITION

Abstract

Composite samples consisting of articular and calcified cartilage maintained in a non-viable physiological condition were subjected to static compression using a system of simultaneous micromechanical testing and interference light microscopy. Combined with transmission electron microscopy following glutaraldehyde fixation of the tissue under sustained load, this provided a unique observation of the response of the collagen framework in the tidemark region of articular cartilage to sustained compression loading. The tidemark in mature articular cartilage is seen to be highly variable in its morphological features, when viewed ultrastructurally. It incorporates variable amounts of internal stress which are relieved when the articular cartilage is separated from the calcified cartilage. Deformation of the articular cartilage can terminate abruptly at the tidemark. There is no evidence that the tidemark or calcified cartilage provided an intermediate layer between the compliant articular cartilage and the rigid subchondral bone. A smooth transfer of stress from the compliant to the rigid tissues might be achieved through changes in orientation and packing density of the collagen fibers in the articular cartilage adjacent to the tidemark. Various morphological responses of the collagen framework were observed in the tidemark region of articular cartilage following static compressive loading. In any given region, these responses were determined by the local form and orientation of the tidemark, the organization of the collagen fibers and the position of this region with respect to the compressive anvil. No evidence was obtained which suggested that the collagen fibers near the tidemark had a predominantly tensile role during direct compression. The observed process of compaction and collapse via a crimp formation is clearly non-tensile. Deformation involving lateral shear in regions nearer the extremities of joint contact would require that the collagen fibers provide an anchoring role and would thus be subjected to tensile loading.