Pilomatricoma with Osseous Metaplasia in the Shoulder Joint: A Common Lesion with Rare Differentiation

Abstract

Pilomatricoma, also known as calcifying epithelioma of Malherbe, is a tumour that shows differentiation towards hair cortex cells. It was first described by Malherbe and Chenantais in 1880. It most commonly occurs in the first two decades of life, with a female preponderance, and accounts for 1% of all benign skin lesions. Pilomatricoma undergoes calcification and ossification, but ossification is rare and poorly documented. The head and neck region, as well as the upper extremities, are the most common sites. Here, the authors present a rare case of pilomatricoma with osseous metaplasia in a 15-year-old male who presented with shoulder swelling for one year. The patient had a firm to hard, mobile swelling over the right shoulder, which was not fixed to the overlying skin or underlying structures. Clinically, calcified epidermal cysts were considered. Excision was performed in a minor operation theatre, and the specimen was sent for histopathological examination. On gross examination, the specimen was a firm to hard, globular 1×0.8 cm tissue that was difficult to cut. Microscopic examination revealed a lesion composed of two types of epithelium: inner shadow cells and outer basophilic cells. Bony trabeculae with osteocyte rimming were present around the shadow cells, and giant cells were also observed. Based on the histopathological examination, the diagnosis of pilomatricoma with osseous metaplasia was made. The patient was discharged after one week and followed-up for three months, during which satisfactory wound healing and no local recurrence were observed. Pilomatricoma with osseous metaplasia should be considered as a differential diagnosis when a patient presents with a firm to hard nodule.