Onychomatricoma: A Rare Case of Unguioblastic Fibroma of the Fingernail Associated With Trauma

Histologically, the tumor is fibroepithelial or biphasic with stromal and epithelial components. It has a lobulated and papillary growth pattern with 2 distinct areas that correspond to 2 anatomic zones.³ The base of the tumor corresponds to the proximal anatomic zone, which begins at the root of the nail and extends to the cuticle. This area is composed of V-shaped keratinous zones similar to the normal matrix. If the nail is removed prior to excision, these areas can be avulsed, leaving clear clefts. The superficial aspect of the tumor corresponds to the distal anatomic zone, which is located in the region of the lunula. This area is composed of multiple digitate or fingerlike projections with a fibrous core and a thick matrical epithelial covering.³ These digitations extend into small cavities in the nail plate, which can be visualized as clear channels or woodwormlike holes in hematoxylin and eosin–stained specimens. A biphasic fibrous stroma also can be observed with the superficial dermis being cellular with fibrillary collagen and the deep dermis more hypocellular with thicker collagen bundles.^3,4

An analysis of keratins in the nail matrix, bed, and isthmus showed that OM has the capacity to recapitulate the entire nail unit with differentiation toward the nail bed and isthmus.⁶ It appears that the mesenchymal component has an inductive effect that can lead to complete epithelial onychogenic differentiation.⁶

Due to the histological differences among the described cases of OM in the literature, a new classification based on the spectrum of epithelial to stromal ratio of stromal cellularity and the extent of nuclear pleomorphism was proposed in 2004.⁷ The prominent feature of the unguioblastoma type of OM is epithelial, while the cellular stroma is the prominent feature in the unguioblastic fibroma type. Atypical unguioblastic fibroma refers to a tumor with increased mitotic activity and nuclear pleomorphism among the stroma.⁷

Most OM tumors follow a benign clinical course; however, complete excision is advised to include the normal nail matrix proximal to the lesion, which may prevent recurrence and serves as a primary treatment.

Conclusion
Onychomatricoma is a benign neoplasm of the nail matrix that may be triggered by trauma; however, due to the weak association, further observations and studies should be conducted to substantiate this possibility. Patients with the classic clinical presentation possibly may be spared a nail avulsion and biopsy. Onychomycosis occurs in the setting of OM, and culture and treatment are unlikely to change the appearance or course of this nail condition.