Intradiploic epidermoid cyst of frontal bone with spontaneous fistulization to eyelid.

Dermoid and epidermoid cysts are choristomas that form due to the entrapment of epithelial remnants within the bony suture lines during embryologic development, around third and fifth week of gestation. Although these rare cysts, particularly the epidermoids, are most often seen in the cranial bones, they only constitute about 1% of all cranial tumors. Approximately, 25% of cranial epidermoids form in the diploic space of the membranaous bones of the skull, hence called intradiploic cysts.