Onychomatricoma with dorsal pterygium: pathogenic mechanisms in 3 cases.

Onychomatricoma is a histologic diagnosis with a range of clinical and intraoperative presentations. This article focuses on the unusual, misleading clinical variants that nonetheless present with all the classic histologic criteria of the tumor. The histologic study enables one to connect the 3 cases reported in the article that apparently differ in their clinical appearance by linking them to a common pathology, ie, matrix pterygium. In all these cases pterygium results from matrix metaplasia of the ventral aspect of the proximal nailfold. When onychomatricoma develops from the matrix and covers the whole ventral aspect of the proximal nailfold, including the angle where it meets the dorsal epidermis, the clinical appearance resembles the pterygium observed in lichen planus. Histologically, however, the two epidermal layers are not joined by matrix granular metaplasia but by two nail plates placed next to one another. When onychomatricoma is confined to the matrix, sparing the ventral aspect of the proximal nailfold, the distal portion of the ventral side of the proximal nailfold may react to the pressure exerted by the matrix onychomatricoma on a differentiated matrix. The appearance then clinically mimics onychogenic Bowen's disease.