Topographical variation in the calcified zone of upper femoral articular cartilage.

A series of 27 adult human femoral heads has been examined for topographical variation in 'remodelling' and other histological features of the calcified zone at the base of the articular cartilage. The specimens were obtained from necropsies; hip joints with osteoarthritic bone exposure were excluded. A tissue sample from the inferomedial aspect was compared with one from the femoral zenith, using a standard length along the articular contour at each site. Histological sections were cut in a plane vertical to that of the articular surface. A study was made of the various patterns seen within cartilage tidemarks when these were examined at high magnification in paraffin sections stained with Ehrlich's haematoxylin and eosin. Special attention was paid to the identification of tidemark segments which stained faintly and were not readily apparent. The tidemarks were mapped on a photomicrographic montage from each of the tissue samples. When a sample showed evidence of one or more extra phases of cartilage calcification, as indicated by the presence of more than one tidemark, the spatial extent of the extra calcification was quantified by linear measurement and by point counting on the photomicrographic montage. The mean of the results for the spatial extent of extra-phase calcification of the cartilage was greater for the inferomedial than for the zenith samples. However, it was also greater for samples where the articular surface showed minimal fibrillation than for samples where the surface was still intact, and it was noted that surface fibrillation was much more common in the inferomedial than in the zenith samples. Where there was more than one tidemark, the lowest sometimes showed gaps where it had been breached by an advance of ossification into the calcified cartilage. The mean value of the tangential extent of such gaps was similar at the two sites sampled. Focal contacts, where the uncalcified articular cartilage was in contiguity with calcified zone 'defects' containing tissue other than calcified hyaline cartilage, were more numerous at the femoral zenith than inferomedial to the fovea. Counts were also made of tangential shearing splits at the interface of the calcified and uncalcified cartilage. Subject to the reservation that genuine splitting may be difficult to distinguish from technical artifact, the mean number was closely similar at the two sites sampled. The interpretation of the findings is discussed in relation to remodelling changes in the cartilage base and to degenerative changes in the overlying articular cartilage.