A 28-year-old woman with an unremarkable medical history presented with an enlarging nodule that had been growing under her left great toenail for 6 months. The patient monitored the nodule, hoping that it would resolve on its own, but found that it steadily increased in size and began to displace the nail, causing pain. At the time of presentation, the nodule measured approximately 10 mm in diameter, and there was significant (~80°) superior displacement of the nail (FIGURE 1).
An initial radiograph identified a 5.5-mm bony density arising from the dorsal surface of the left first distal phalanx with no significant degenerative changes (FIGURE 2). A subsequent magnetic resonance image confirmed the bony excrescence and noted marrow continuity. A thin amount of T2 bright signal was also observed, suggesting either a cartilaginous cap or soft tissue edema secondary to pressure on the nail bed (FIGURE 3).
Histologic examination demonstrated a thin (3 mm) cartilaginous cap overlying an area of mature fibrocartilage with no definite periosteum. The osseous component appeared to mature from the cartilage, and the marrow was focally fatty and fibrosed (FIGURES 4A and 4B). Expert consultation with the Joint Pathology Center confirmed a benign osteochondromatous lesion.
The histologic differential diagnosis of this patient’s lesion included subungual exostosis and osteochondroma. Based on the patient’s age, location of the lesion, and histologic findings, the final diagnosis was subungual exostosis.
Subungual exostoses are benign osteocartilaginous tumors that most commonly affect children and young adults. They predominantly manifest on the dorsomedial aspect of the tip of the great toe (~80%), but can occur on other digits of the foot or hand.1 They are caused by a proliferation of fibrous tissue under the nail bed. The fibrocartilage cap then undergoes endochondral ossification to woven bone and lamellar bone trabeculae. As these lesions mature, they establish continuity with the underlying bone in the phalanx.2 Subungual exostoses were once thought to represent a proliferative response to trauma, but further research has identified a recurrent t(X;6) (q22;q13-14) translocation, suggesting a neoplastic origin.3
Osteochondromas are also common benign tumors formed by endochondral ossification, although secondary transformation into low-grade chondrosarcomas is well-documented.1 Osteochondromas commonly affect younger patients. They occur at epiphyseal areas of developing bone and have a hyaline matrix and chondrocyte pattern similar to that of a normal epiphyseal area, with confluence to the underlying trabecular and cortical bone. They are not caused by previous trauma and generally only become symptomatic after they have grown large enough to cause mechanical problems.1
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