Subungual exostosis of the thumb
1. Department of Dermatology and STD, and Department of Pathology, JIPMER, Pondicherry - 605 006, India. firstname.lastname@example.org; email@example.com
A 19-year-old girl presented with painful raised skin lesions over the right thumb for the past one year that had ulcerated over the last one month. On examination, a lobulated nodule of bony-hard consistency, measuring around 2 × 2 cm was present over the lateral aspect of the tip of the right thumb. The lesion was tender and the overlying skin showed superficial erosions. Roentgenogram of the hand showed mature bone projecting from the distal end of the terminal phalanx of the right thumb forming a "Y"-like bifurcation. Histology from a bit of excised tissue from the lesion (thumb) showed evidence of mature trabecular bone with a fibrocartilaginous cap in the deep dermis. There was no evidence of malignant change on histology. Local excision of the entire lesion was done and there has been no recurrence till date.
Subungual exostosis is a benign bone tumor found on the distal phalanx of a digit, beneath or adjacent to the nail. Dupuytren gave the first description of the lesion in 1847 when he reviewed his experience with 30 patients suffering from subungual exostoses of the great toe . Nearly 80 percent of reported cases involve the great toe and the remaining cases involve other toes. Involvement of the finger by this lesion occurs very rarely [2, 3]. There are only scant reports of this entity in the literature. Furthermore, standard textbooks of bone tumors scarcely make mention of this entity and emphasize that the usual location is on the foot . Very often these lesions are misdiagnosed, leading to protracted morbidity from inadequate therapy or extreme treatments such as digital amputation or radiation therapy. We report a rare case of subungual exostosis on the thumb presenting as stony hard painful swelling with superficial erosions.
A nineteen-year-old girl presented with painful raised skin lesions over the right thumb for the past one year that had ulcerated over the last one month. There was no history of preceding trauma or history of bleeding from the lesion though pain on pressure was a prominent symptom. On examination a lobulated nodule of bony hard consistency, measuring around 2 × 2 cm was present over the lateral aspect of the tip of the right thumb (Figs. 1-3). The lesions were tender and the overlying skin showed superficial erosions. The nail appeared normal (Fig. 4). The patient had been earlier treated at another center with paring followed by topical salicylic acid application.
Roentgenogram of the hand showed mature bone projecting from the distal end of the terminal phalanx of the right thumb forming a "Y" like bifurcation (Figs. 5 and 6). Histology from a bit of excised tissue from the lesion (thumb) showed evidence of mature trabecular bone with a fibrocartilaginous cap deep in the dermis (Figs. 7-10). There was no evidence of malignant change on histology. Local excision of the entire lesion was done and there has been no recurrence till date.
Subungual exostosis is an uncommon benign bone tumor arising in the distal phalanx of a digit, beneath or adjacent to the nail bed. It is considered to be a rare variant of osteochondroma . Almost invariably, it is a solitary lesion, usually occurring in patients who are in the second or third decade of life and it has clinical manifestations that include pain, swelling and ulcerations of the nail bed or surrounding tissue secondary infection . The great toe is involved in most of the cases, followed by lesions on the other toes. A solitary exostosis occurring on the finger is a rarely reported entity and persistent changes in the skin of a finger without obvious cause should arouse ones suspicion that an underlying bony lesion is present. The skin overlying an exostosis may become thickened like a callus and ulcerate with repeated trauma, pain being a prominent symptom .
The pathogenesis of such exostosis is not clearly understood, however, the following have been suggested: trauma, teratologic abnormality, forme fruste of multiple hereditary exostoses, and cartilaginous rest . Trauma is often a precipitating factor and subungual exostosis may represent cartilaginous metaplasia occurring in response to acute or chronic irritation [1, 4]. Others postulate that there is a defect in the perichondrial node of Ranvier that allows an excrescence from the normal physeal growth process . Chronic infection seems to be the result rather than the cause of the underlying lesion . In the early stages of development, the lesion consists of proliferating fibroblasts in direct continuity with the nail bed where cartilaginous metaplasia can be seen. The cartilage gradually undergoes calcification and ossifies. These areas of enchondral ossification eventually progress to woven bone and then lamellar bone. Initially, proliferation of cartilage contributes to the exophytic growth of the lesion while later mature bone dominates the outgrowth . Histologically, the mature lesion consists of a base of trabecular bone with a proliferating fibrocartilaginous cap. In immature lesions, the cartilaginous cap is thick while the mature exostosis shows a thin cap of cartilage that has been largely replaced by trabecular bone. In the cartilage cap, hypercellularity, mitotic figures, and dark staining nuclei may be apparent, and, although these findings may be misinterpreted as malignant, the lesion is uniformly benign as there is lack of true anaplasia . Distinction from a subungual osteochondroma may be possible histologically because fibrous cartilage caps the bony outgrowth in exostosis and hyaline cartilage in osteochondroma .
Radiographically, the lesion is approximately 1 cm in diameter and projects from the dorsal or dorsomedial aspect of the distal portion of a terminal phalanx. It is composed of a mature trabeculated bone with attachment to the phalanx; the free end is flat, cupped and smooth, or irregular. The radiological findings may seem modest in comparison to the clinical complaints; there is a large radiolucent cartilaginous cap [5, 7]. There is no cortical disruption or other abnormality of the distal phalanx .
The differential diagnosis of subungual exostosis includes verruca, pyogenic granuloma, glomus tumor, carcinoma of the nail bed, subungual epidermal inclusion cyst, melanotic whitlow, and enchondroma. Like an exostosis, an enchondroma may involve the distal phalanx. However enchondromas are cartilaginous tumors arising in the medullary cavity of tubular bones and appear on x-ray as a radiolucent defects with expansion of the bone .
This case has been presented to highlight the rare occurrence of subungual exostosis on a finger. The term subungual exostosis could be misleading as in some instances, as in our case; the lesion may be periungual with no disturbance of the nail itself. Also, these exostoses are not related to the conventional osteochondromas which usually occur in long bones preformed in cartilage and nor are they found in patients with multiple hereditary exostoses . These lesions are benign and one must be wary of diagnosing them as sarcoma and resorting to radical treatment modalities. Local excision with complete removal of the cartilaginous cap yields good results with low recurrence rates.
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