ABSTRACT
The nail apparatus can give rise to a huge variety of benign tumors, most of which may also be found in variable frequency in the skin but often then look different; however, some of them are specific for the nail. The first described nail-specific tumor is onycholemmal horn, which bears a striking resemblance to trichilemmal horn. Onychomatricoma, onychocytic matricoma, and onychopapilloma derive from the matrix whereas the various onycholemmal cysts and tumors have their origin in the nailbed. Other epithelial tumors are nevoid or derive from sweat glands. Subungual keratoacanthoma—by some considered to be a variant of squamous cell carcinoma—is a rare lesion with a characteristic pain symptomatology. Mesenchyme-derived tumors are common with ungual fibrokeratoma being the most frequent one. Of the vascular tumors, subungual glomus tumors are the best known and stand out by their excessive pain elicited upon pressure, shock, and cold. Subungual lipomas are rare and may grow to a size to distort the nail. Osteochondroma and subungual exostosis are distinguished by their particular cartilage caps: hyaline in osteochondroma and fibrous in the latter. Neurogenic tumors are rare, and subungual neurofibroma is only observed as an isolated tumor. Benign melanocytic lesions originating in the matrix usually cause longitudinal melanonychia whereas those of the nailbed remain visible as a dark spot under the nail. Melanonychia may be due to melanocyte activation, lentigo, nevus, or melanoma and it is the responsibility of the clinician to make the correct diagnosis or provide a good biopsy for histopathology.
