Case Report
A 41-year-old man with no dermatologic history presented for a skin examination. During a full-body skin examination, a lesion was identified on the right third toe that was partially visible underneath the nail plate. The patient stated that the lesion had been present for many years and did not appear to be growing but did cause occasional pain. On examination a 1-cm verrucous, hyperkeratotic, tan papule was noted at the distal end of the nail bed causing partial onycholysis (Figure 1). It was not tender to palpation.
Figure 1. A 1-cm firm, hyperkeratotic subungual papule on the right third toe.
A shave biopsy was obtained of the visible portion of the lesion, which revealed hyperkeratosis, acanthosis, and a population of dermal spindle cells in a myxoid stroma that could not be definitively identified. Special stains were nondiagnostic. The patient was referred to dermatologic surgery for rebiopsy of the lesion after removal of the nail plate. Mature bone was seen embedded in the dermis (Figure 2), and a diagnosis of subungual exostosis was made. Radiography of the digit confirmed a bony excrescence from the tuft of the toe, and the patient was referred to orthopedic surgery for definitive excision. There was no evidence of recurrence at 1-year follow-up.
Figure 2. Underneath a hyperkeratotic and acanthotic epidermis, a focus of mature bone was seen embedded in the dermis (H&E, original magnification ×20).
Comment
Subungual exostosis is an uncommon benign bone tumor located beneath or adjacent to the nail bed on the dorsal aspect of the distal phalanx.1 Although it can occur on any digit, 70% to 80% of cases have arisen on the distal phalanx of the hallux.2 Both sexes are equally susceptible. The majority of lesions occur during the second or third decades of life and usually are asymptomatic unless there is trauma or infection. Growth of the lesion over time can cause lifting or deformity of the nail plate and can cause slight discomfort while walking if located on the great toe.3 Common differential diagnoses include osteochondroma, wart, fibroma, paronychia, myositis ossificans, and pyogenic granuloma.3,4 Diagnosis can be confirmed with radiography, which should be performed prior to any biopsy or invasive procedure. In our patient, initial radiography could have obviated the need for 2 biopsies prior to definitive excision. Histopathologic evaluation typically reveals mature trabecular bone (Figure 2) surrounded by a fibrocartilage cap.
Subungual exostosis begins as an area of proliferating fibrous tissue with cartilaginous metaplasia located beneath or adjacent to the nail bed on the dorsal aspect of the distal phalanx.1 This cartilage undergoes enchondral ossification and is converted to trabecular bone. As the lesion grows and matures, the cartilaginous cap blends imperceptibly with the nail bed and comes into continuity with the underlying distal phalanx.1,3 This process continues until the lesion fuses completely with the distal phalanx.1 Although the cause of subungual exostosis has not been clearly established, chronic irritation, trauma, and chronic infections are considered causative factors of fibrocartilaginous metaplasia.4