Invaginated fibrokeratoma with matrix differentiation: a new histological variant of acquired fibrokeratoma.

We report two cases of acquired fibrokeratoma which present new clinical and histological features specific to the nail apparatus. There are three histological characteristics. (i) A deep epithelial invagination which develops from the proximal nail fold and is orientated in the same direction as, but is more proximal than, the normal matrix. (ii) The postero-inferior aspect of the cul-de-sac produced by the invagination acts as an accessory matrix, giving rise to a pseudo-nail plate. The granular layer is reduced or absent. (iii) This accessory nail apparatus lies on a conical dermal tumour, sharply demarcated from the surrounding dermis, with a wide base narrowing towards the tip. The finger-shaped structure is reminiscent of Kint's type I acquired fibrokeratoma.