An onychomatricoma is a subungual tumor of the fingers and toes, described in 1992. 
The terminology describing onychomatricomas has slowly been adjusted over time. In 1992, the original description termed this tumor an onychomatrixoma, based on the description of a filamentous, tufted tumor in the matrix. Subsequently, the term onychomatricoma was introduced into the English literature in 1995.  The term onychomatricoma is most descriptive and breaks down as onycho (nail) matric (matrix) oma (tumor).
Currently, histologic criteria have been used to establish subclassifications of onychomatricomas as 1 of 3 types. The clinical relevance of these subclassifications has not yet been well established, but they are described in Histologic Findings.
The etiology of onychomatricomas is not yet known. Patients do not usually report any history of trauma.
One literature review has noted that onychomycosis may be found more commonly in association with toe onychomatricomas; almost half the reported cases have had a fungal association. However, the significance of onychomycosis in these patients is not yet well established and may or may not play a role in the pathophysiology of onychomatricomas.
Another theory espoused is that onychomatricomas are fibroproliferative in origin and are epithelial and connective-tissue hamartomas simulating the nail matrix. This theory is further supported by an electron microscopy study that associated onychomatricoma to a nail matrix origin.  On immunohistochemistry, onychomatricomas express the same pattern of cytokeratins and integrins as seen in normal nail matrix. Keratin expression patterns have also suggested differentiation toward the nail bed and the nail isthmus. 
One report has noted an onychomatricoma arising from the ventral part of the proximal nail fold, thus taking on a pterygium aspect.  A dorsal pterygium bridging the proximal nail fold may also be noted. In such cases, tumor is also produced by the ventral surface of the proximal nail fold (as opposed to most cases, in which only the matrix produces tumor). This odd variant produces 2 layers of plate sandwiched together, which makes diagnosis more difficult. 
Onychomatricomas were originally believed to be benign nail lesions. However, the literature suggests that malignant potential cannot be completely excluded.
In ultrastructure studies, E-cadherin and beta-catenin have been noted at the cell membrane in the epithelial invaginations, while P-cadherin is restricted to basal cells. In contrast to other matrical tumors, nuclear beta-catenin was not present, suggesting that onychomatricoma may lack the transcriptional activating role of beta-catenin noted in follicular and odontogenic matrical tumors. 
Genome studies have suggested chromosome 11 plays a role in this tumor. 
The frequency of onychomatricomas in the United States has not been determined. One epidemiological study reported no more than 80 known cases in the literature.  The frequency of onychomatricomas internationally has not been determined.
The most current literature suggests the male-to-female ratio for onychomatricomas is approximately 1:1.
Few long-term studies are available on onychomatricomas. In one case, surgical removal yielded no recurrence at 2-year follow-up. Similar results were also noted in a 2-year follow-up. Nail regrowth occurs without incident.
A continuum with potentially malignant onychomatricoma may exist. Awareness of the possibility is prudent.