Primary synovial chondrometaplasia. Histologic variations in the structure of metaplastic nodules.

Eight cases of primary synovial chondrometaplasia have been evaluated with respect to 1) number of metaplastic nodules, 2) their location in relation to the surface of the synovium, 3) the histochemical composition of the matrix of the nodules; number of nodules with 4) cystic degeneration, 5) osteoclastic/chondroclastic activity at the margin, 6) invasion of connective tissue and 7) calcification and/or ossification. Furthermore, the pleomorphism and hyperchromatism of the chondroid cells and their nuclei and number of mitoses were evaluated. The number of metaplastic nodules varied greatly, from a few to more than a hundred. Most nodules were located at the level of the original synovial surface or in the pedunculated prominences. Histochemically, two types of nodules were found and in about equal numbers. Connective tissue invasion, calcification and ossification were the most frequent changes; whereas cystic degeneration, which has not been described before, was the rarest. Only one of the cases showed all the different changes, and these within a considerable number of the nodules. Variation of the chondroid cells and their nuclei was considered as an expression of degenerative changes, and mitoses were not found. Two of the cases had a tumor-like accumulation of metaplastic nodules.