 They are most often found deep to the anterior aspect of the ear drum, and a vestigial structure, the epidermoid formation, from which congenital cholesteatoma may originate, has been identified in this area.Not all middle ear epidermal cysts are congenital, as they can be acquired either by metaplasia of the middle ear mucosa or by traumatic implantation of ear canal or tympanic membrane skin.In addition, cholesteatoma inadvertently left by a surgeon usually regrows as an epidermal cyst.