Subungual Exostosis & Subungual Tumours

What it is. A subungual exostosis, also called Dupuytren's exostosis, is a benign osteocartilaginous outgrowth: trabecular bone capped by fibrocartilage, arising from the dorsal or dorsomedial aspect of the distal phalanx. It is most common in the great toe, typically in adolescents and young adults, and a history of preceding trauma is often present.

Not an osteochondroma. The lesion is histologically and radiologically distinct from a true osteochondroma. Its cap is fibrocartilage rather than hyaline cartilage, its cortex and medulla show no continuity with the underlying bone, and it arises from the surface of the distal phalanx rather than the metaphysis of a long bone.

Presentation. A firm subungual or periungual nodule elevates and deforms the nail, with onycholysis and nail-plate elevation. It is often painful, especially under footwear pressure, and may ulcerate or bleed. The diagnosis is suggested clinically and confirmed on a plain radiograph, which shows the bony outgrowth projecting from the distal phalanx.

The reactive model. The prevailing model is reactive. Trauma or chronic irritation, which fits the post-trauma history and the pressure-exposed great toe, provokes proliferation of cartilage at the periosteal surface, and that cartilage undergoes endochondral ossification to lay down trabecular bone. The lesion therefore grows from a proliferating cartilage cap downward into mature bone.

A neoplastic element. A recurrent chromosomal translocation has also been reported in these lesions, which would suggest a neoplastic rather than purely reactive element. The two models are not mutually exclusive, and trauma may act on a lesion that is already clonal.

The three zones. From base to surface:

• A deep zone of mature trabecular bone arising from the distal-phalanx surface
• A middle zone of maturing cartilage undergoing endochondral ossification
• A superficial fibrocartilage or proliferating fibroblastic cap

This zonal growth is why the cap is fibrocartilage rather than the hyaline cap of an osteochondroma, why there is no cortico-medullary continuity, and why an early, cellular lesion can look alarming.

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The subungual exostosis is a common, benign lesion, but it is one of a wider group of subungual tumours, and the subungual space also harbours dangerous ones. The group spans:

• Benign solid lesions - glomus tumour, subungual exostosis, soft-tissue chondroma, keratoacanthoma, haemangioma and pyogenic granuloma
• Benign cysts - epidermal and mucoid
• Malignant tumours - squamous cell carcinoma and subungual melanoma

Separating them. A plain radiograph shows the bony lesions, and ultrasound or MRI help characterise the soft-tissue ones. Imaging is backed by a low threshold for biopsy.

The principle. A symptomatic subungual exostosis is treated by marginal excision, removing the exostosis with its fibrocartilage cap down to its base on the distal phalanx, often with partial nail-plate removal for access. Completeness, not width, is the goal: incomplete excision with the cartilage cap left behind is the main cause of recurrence.

The operation.

1. Under tourniquet/ring block
2. Avulse or partially remove the overlying nail plate for access; a dorsal or distal "fishmouth" incision exposes the lesion
3. Excise the exostosis at its base on the distal phalanx, down to the cortex with osteotome or rongeur
4. Remove the entire fibrocartilage cap
5. Always send the specimen for histology to confirm the benign diagnosis and exclude malignancy

The nail. Preserve and repair the nail bed and matrix where possible to limit nail deformity. The nail plate regrows over months.

Counselling. Complete excision is curative with low recurrence. Warn about possible transient nail-plate deformity and the small recurrence risk, and follow the patient up to confirm resolution.

Completeness of excision is the surgical principle, but it is not the only thing that decides whether a lesion comes back. A retrospective study of 74 patients who underwent dorsal-approach excision of a subungual exostosis or subungual osteochondroma, reviewed at a minimum of six months, tested the obvious candidates:

Reading the table. A skeletally immature patient carries the higher recurrence risk and should be followed and counselled accordingly, while size, duration and nail damage should not be used to stratify follow-up. The series pooled subungual exostosis with subungual osteochondroma, which are related but not identical lesions.

The burr. In the same series the technique using a burr appeared more effective than the alternatives. That sits with the principle that the retained cartilage cap is the source of recurrence: a burr takes the base flush to the phalangeal cortex in a way an osteotome and rongeur may not.

What the patient can expect. The series supplies the figures the operation otherwise lacks: 85% of patients were satisfied with the procedure and close to 80% rated the cosmetic result good or very good. Worse satisfaction was strongly associated with recurrence and, separately, with younger age and with the intensity of post-operative pain, so analgesia and expectation-setting are part of the result.