Onycholemmal Horn: An Exceedingly Rare Subungual Tumor.

The onycholemmal horn is an exceedingly rare subungual tumor characterized by trichilemmal-like keratinization pattern. The only previous instance of onycholemmal horn dates back to 1983 when Haneke reported a keratotic subungual tumor clinically and histologically comparable to a trichilemmal horn. No other case of this condition has been reported so far. We present the case of a 72-year-old female patient, with a history of a slowly growing tumor originating from the nail bed epithelium of the left thumb. The lesion was surgically removed, and pathological examination was consistent with the diagnosis of an onycholemmal horn. Additionally, this study aims to elucidate the correlation between the onycholemmal and trichilemmal keratinization.