Synovial metaplasia of the skin.

We present two female patients, aged 15 and 45 years, with synovial metaplasia of the skin. Both lesions, localized in the knee and hand respectively, arose after preceding local trauma. Case 1 had intradermal irregular cystic spaces in the adjacent myxoid stroma of which large polygonal eosinophilic cells were found. In contrast, case 2 was characterized by a longitudinal space within the lower dermis and subcutis which was lined by a membrane similar to hyperplastic synovium. The cells of the membrane showed an eosinophilic spindle shaped cytoplasm with processes towards the lumen. In both cases the eosinophilic cells, strongly suggestive of fibroblasts, showed staining for vimentin only, whereas no reactivity could be obtained with antibodies to actin, desmin, S-100 protein, Factor VIII related antigen, cytokeratin, epithelial membrane antigen, carcinoembryonic antigen and CD 68. The absence of CD 68 positivity differs from that seen in normal and hyperplastic synovium. Delayed wound healing around a nidus seems the most likely cause of the change.