Cutaneous Metaplastic Synovial Cyst

Metaplastic synovial cyst of the skin is a recently recognized entity characterized by an intradermal nodule that usually occurs at the site of previous surgical trauma. Histologically, the lesion demonstrates a cystic structure with villous‐like projections and a lining resembling hyperplastic synovium. We have studied two patients with rheumatoid arthritis, aged 46 and 55 years, who presented with cystic nodules localized on the thumb and great toes, respectively, without any history of previous trauma or surgical procedures performed in the areas. The presence of vimentin and CD 68 positivity of the cells lining the cyst walls supports the similarities between normal and metaplastic synovium. We hypothesize that constant pressure on the great toe, repeated manipulation of the finger, and chronic inflammation around the affected joints may have played roles in the pathogenesis of the lesions in our patients.