Cutaneous metaplastic synovial cyst.

Metaplastic synovial cyst, a recently described entity, is histologically characterized by a cystic cavity lined by metaplastic synovial tissue that is often hyperplastic and protrudes into its lumen as villous structures. Clinically, these lesions resemble suture granuloma and develop at the site of previous surgical trauma. We have studied 4 cases of this unusual entity and have investigated the immunohistochemical profile in an effort to determine the nature and derivation of their lining tissue. One of our cases is unique since the lesion was associated with a basal cell carcinoma. A history of previous surgery was noted in one, and history of trauma in 2 of the 4 cases. Our immunohistochemical observations confirmed a mesenchymal derivation for the "synovial like villous structures" but non-availability of a specific "synovial" marker does not permit us to conclude with certainty the synovial nature of the metaplastic tissue. Cutaneous metaplastic synovial cysts are unique lesions which deserve attention and should be included in the differential diagnosis of cutaneous cysts.