Intraosseous Epidermal Inclusion Cyst! Although several diseases affect the distal phalanges of the hand, there are not too many true intraosseous lesions to do so. The differential diagnosis for a finger distal phalanx intraosseous lesion usually includes metastasis, enchondroma, glomus tumor and epidermal inclusion cyst. Subungual exostosis and keratoacanthoma are less common. Epidermal inclusion cyst is thought to be caused by extension of an adjacent subungual soft tissue epidermal cyst into the bone. Because this is a benign and chronic process, the lesion has well defined sclerotic borders and a very narrow zone of transition in radiographs. On MRI, the diagnostic hallmark is intralesional high T1 signal intensity due to proteinaceous content. It also shows peripheral contrast enhancement and high T2 signal intensity. Sometimes we can see surrounding edema and inflammation related to cyst rupture. Glomus tumors, a benign hamartoma, have low intralesional T1 signal intensity and diffuse and avid enhancement. Enchondromas are common in the bones of the hand and usually show radiographic nonaggressive features, endosteal scalloping and may have chondroid matrix. Metastasis most often looks aggressive with ill-defined borders and associated periosteal reaction. Lung endobronchial tumors represent up to 40% of these metastasis (in case you see this on a test). In the past, the differential diagnosis of a distal phalanx lesion would include tuberculous dactylitis, also known as Spina Ventosa.
