Ectopic nail is a condition involving the development of an additional and independent nail in an abnormal site. Few cases of congenital ectopic nail have been reported in the worldwide dermatological literature1. In Korea, a case of congenital ectopic nail was reported by Lew et al.2. In addition, a report on a Korean patient with posttraumatic ectopic nail was published in the Journal of Pediatric Dermatology in 20163.

Herein, we describe a 62-year-old female patient with a congenital ectopic nail on the left 5th toe (Fig. 1A) that developed at birth. The nail had been felt tender for the last 2 years. There were no abnormal laboratory findings. Moreover, there were no abnormal bony deformities on radiological examination (Fig. 1B). She underwent surgery for removal of the congenital ectopic nail (Fig. 1A). Histopathologically, the congenital ectopic nail showed prominent hyperkeratosis (Fig. 2A). The expression of β-catenin was shown in the epidermal keratinocytes under the congenital ectopic nail (Fig. 2B). The dermis under the congenital ectopic nail showed CD10 expression (Fig. 2C), and the dermis turned bluish with the Alcian blue stain (Fig. 2D).

Ectopic nail, also known as onychoheterotopia, is a very rare disorder. It has been classified into an acquired type disorder caused by trauma and a genetically predisposed type disorder when there is no history of trauma. The pathogenesis of ectopic nail remains to be determined. However, various hypotheses, including the ectopic existence of a germ cell, rudimentary nail bed, and traumatic inoculation of onychocytes, have been proposed to explain the pathogenesis of ectopic nail4. Moreover, onychodermis for the specialized nail mesenchyme containing onychofibroblasts may serve an important function in the origin of ectopic nail5. The congenitally predisposed type is more common and can be attributed to digital deformity or bony malformation of the distal phalanx. Congenital ectopic nail can be associated with congenital palmar nail syndrome and Pierre Robin syndrome. Congenital ectopic nail usually develops on the fingers, whereas posttraumatic, acquired ectopic nail can occur on the fingers and toes. On histopathological examination, ectopic nail is characterized by a fully developed nail unit with an abnormal nail matrix. β-catenin, which plays an important role in hair and nail formation, is expressed in the nail matrix cells. The dermis under nail matrix can show the expression of CD10 and a positive stain with alcian blue. CD10, a cell surface metalloprotease, can be expressed in a variety of normal and neoplastic tissues, including the mesenchyme under the nail matrix and nail bed5. Ectopic nail can be confused with acquired digital fibroma, cutaneous horn, clavus, subungual exostosis, warts, rudimentary polydactyly, foreign body, hamartoma, and split nail deformity4. A fully developed nail unit on histopathological examination is important to diagnose ectopic nail. Ectopic nail can be removed through total surgical excision of the nail plate and nail matrix.