Cutaneous Metaplastic Synovial Cyst: A Case Report.

Cutaneous metaplastic synovial cyst (CMSC), presents as a solitary, tender subcutaneous nodule that usually occurs at the site of previous surgery or trauma. Histologically, the lesion is characterized by a cystic structure with villous-like projections that lined by metaplastic synovial tissue. The main cause remains unclear, but trauma is presumed to be a precipitating factor, as most reported cases have a history of antecedent cutaneous injury. Here we present a case of CMSC in a 51 year old man, presented with a painless deep-seated dermal nodule in the medial aspect of left ankle without history of any trauma or surgery in this site. Immuno-histochemistry study reveals positive reaction for CD68 in the cystic wall and negative reactions for S-100. CMSC is a unique lesion and worthy to attention, and should be included in the differential diagnosis of deep dermal cutaneous cysts.