Cutaneous metaplastic synovial cyst in the buttock: A comprehensive analysis of etiologic, clinical, ultrasonic, and pathological aspects

Cutaneous metaplastic synovial cyst (CMSC) is a rare cutaneous cystic lesion, which frequently occurs subsequent to trauma or surgery, forming through the reparative and healing process. It is often misdiagnosed as foreign body granuloma or recurrent epidermoid cyst. Histologically, it is characterized by a cystic structure with multiple villous projections pointing toward the cystic space, lined by cells resembling hyperplastic synovium. To date, only 32 cases have been reported in the English literature. In this article, we present two cases of CMSC with distinctive clinical features and ultrasonic findings. Furthermore, we found only three case reports of CMSC in the gluteal region. All of these cases were elderly patients and developed CMSC at the medial aspect of the buttock. Based on their unique clinical and ultrasonic characteristics, we propose that CMSC in the buttock may be considered a distinct subgroup within CMSC, demonstrating a spectrum of degenerative diseases along with ischiogluteal bursitis.