Subungual exostosis of the little toe: MRI features

Clinical History

A 15 year old boy presented with generalised swelling of right little toe with no prior history of injury.

Imaging Findings

A 15 year old boy presented with a 3-month history of an enlarging, minimally painful swelling of the tip of right little toe. The swelling was generalised with no clear cut demarcations from the underlying bone and was firm in consistency with minimal tenderness on palpation. The overlying nail was thickened and spoon-shaped. No similar swellings were found elsewhere. Radiography showed bony erosion of the distal phalynx of little toe (figure 1). MRI showed a lesion enveloping the terminal phalanx of the little toe from below and laterally, with further extension of it between the dorsal surface of the bone and the nail. The lesion was low signal on the T1 sequence (figure 2a), high signal on the fat suppressed sequence (figs 2b - d) and there was no enhancement within it following gadolinium administration. No signal void to suggest calcification within the mass was demonstrated. The scan confirmed pressure erosion of the tuft of the distal phalanx. Surgical excision of the lesion was performed and histopathological assessment of the specimen showed it to be consistent with a subungual exostosis with predominant chondroid metaplasia.

Discussion

Subungual exostosis is a rare osteo-cartilaginous benign tumour arising usually from the distal phalynx of the toes, more commonly from the hallux. Very rarely they can be found in the hands. The term ‘exostosis’ is a misnomer as there is no continuity with the underlying bone. Females are affected twice as males and the lesions occur mostly during the second decade. The precise cause is not clear, but factors like trauma, chronic infection and local irritation are often implicated. The underlying pathology is the initial growth of fibro-cartilaginous tissue, which later ossifies to a varying degree. It differs from osteochondroma, which is a congenital lesion arising from the metaphysis of the underlying bone. Clinically patients may present with swelling in the distal phalynx, with some degree of tenderness often after an episode of trauma. The position of the lesion is almost always dorso-medial. Possible differential diagnoses for the lesion include osteochondroma, glomus tumour, squamous cell carcinoma and melanoma. Radiologically, the appearances can vary according to the amount of calcification. Generally the outgrowth has features of cancellous bone without a defined cortex. During the pre-ossifying stage the underlying bone can be eroded due to the compression by the tumour. Surgical excision and curettage of the base is the treatment of choice. Rarely recurrences can occur if the base is not excised completely.

Differential Diagnosis List

Final Diagnosis

Subungual exostosis of the liitle toe

URL:	https://www.eurorad.org/case/5467
DOI:	10.1594/EURORAD/CASE.5467
ISSN:	1563-4086