Osseous Metaplasia and Mature Bone Formation With Extramedullary Hematopoiesis in Trichilemmal Cyst

This study reviewed 88 cases of trichilemmal cysts (TCs) from Bellaria Hospital to assess the incidence of osseous metaplasia. It found that 3.4% of TCs exhibited ossification, a phenomenon previously considered extremely rare. Ossification was associated with cyst wall rupture and involved membranous ossification without cartilaginous precursors. One unique case also showed extramedullary hematopoiesis (EMH), marking the first documented instance of EMH in ossifying TCs. The findings suggested that ossification in TCs might be more common than previously thought, with dystrophic calcification potentially contributing to bone formation.