Teaching Case

A 23-year-old woman, presented with a 6 month history of a painful hyperkeratotic swelling on the dorsal part of the left hallux. The lesion was tender to palpation and firm. The examination found a previous history of trauma.

The patient was referred to our department in order to undergo a CT. The CT showed a bony excrescence image, arising from the dorsal side of the last phalanx of the hallux (Figure1). There was no cortical disruption or periosteal apposition. The lesion outline were regular (Figure 2).
We do not find any invasion of the subcutaneous tissus.
Surgical excision of the lesion was performed and histopathological assessment of the specimen showed it to be consistent with a subungual exostosis, without any sign of malignancy.

Subungual exostosis is relatively uncommon with an incidence of 4.6% of all bone tumours, first described by Dupuytren in 1847.
It is an exophytic growth from the tuft of the terminal phalanx, most common in the big toe. However, there are reports of the subungual exostoses arising from the lesser toes, and from fingers as well [1]. It occurs at any age, but a slight predominance among females is noted.
The precise cause is not clear, but factors like trauma, chronic infection and local irritation are often implicated [2].
There is debate whether subungual osteochondroma is the same clinical entity as subungual exostosis.
The subungual exostosis is an osteo-cartilaginous benign tumour, the mature exostosis consists of normal appearing trabecular structure within a fibrocartilaginous cap broadly contiguous, whereas osteochondroma has a hyaline cartilage cap.
Clinically patients may present with swelling in the distal phalanx, with some degree of tenderness often after an episod of trauma, producing elevation and separation of the involved portion of its overlying toenail. The position of the lesion is almost always dorso-medial [3].
The radiographic and CT-scan can be helpful in detecting calcification and evaluating adjacent bone structures. The appearance is often diagnostic, consisting of a trabecular bony overgrowth, with or without a defined cortex, arising from the dorsal or dorsomedial aspect of the distal phalanx.
Usually, US demonstrates a heterogeneously hyperechoic lesion with well-defined margins, calcification, and a hypoechoic fibrocartilaginous cap that is usually less than 2cm in diameter with a relative hypovascularity or mild vascularity in the colour Doppler US. MR imaging is the best radiologic modality for depicting the effect of subungual exostosis on surrounding structures and for distinguishing between this lesion and osteochondroma. The fibrocartilaginous cap in subungual exostosis is hypointense with all MR imaging sequences, whereas the hyaline cartilage in osteochondroma has high signal intensity on T2-weighted images.

The differential diagnosis of subungual exostosis include: Osteochondroma, Nora lesion, Kirner deformity. Radiologically, subungual osteochondroma is located along the epiphyseal plate or more proximal to the plate and is often larger in size. Kirner's deformity is a palmo-radial curvature of the distal phalanx of the little finger. Nora lesion is clinical and radiological similarity to more common osteochondromatous tumours.

A number of treatment modalities have been reported, the excision with adequate removal of the osteochondral cap is the most reliable way to prevent the reoccurrence, and large amputation as a result. However, no case of malignant transformation has been reported [4].