Cartilage of the patella. Topographical variation of glycosaminoglycan content in normal and fibrillated tissue.

The variation in the glycosaminoglycan content was studied at different sites of the patella, both where the cartilage was intact and where it showed varying degrees of fibrillation. It was found that when the cartilage surface was intact the glycosaminoglycan content was the same at the different sites of the patella. Local fibrillation always gave rise to a local lowering of fixed charge density, the magnitude of the latter correlating well with the visual assessment of the severity of the lesion. Even in the presence of severe lesions, if there was a site left on the patella where the cartilage was visually normal, its fixed charge density was also at a normal level. Thus, the depletion of glycosaminoglycans is a local phenomenon, limited to the area of fibrillation. The glycosaminoglycan content of normal cartilage is lower in the knee joint than in the hip. This fact, together with the existence of high pressure during load bearing, may be responsible for the greater frequency of destructive lesions affecting the cartilage of the patella compared with that of the hip.