Sixteen so-called rudimentary supernumerary fingers were compared with ten true sixth fingers (pedunculated postminimi) and five digital traumatic neuromas. The absence of neural proliferation in true sixth fingers and the histologic similarity of the acquired traumatic neuromas and the "rudimentary supernumerary digits" support the interpretation that the last-named represent neuromas rather than abortive attempts to form a sixth finger.
Shapiro L, Juhlin EA, Brownstein MH. Rudimentary PolydactylyAn Amputation Neuroma. Arch Dermatol. 1973;108(2):223-225. doi:10.1001/archderm.1973.01620230023009