[Three-phalangeal thumbs].

Many teratogenic mechanisms converge to create a three-phalangeal thumb: duplication, fusion and deletion. Fine observation of the position of growth centres and of associated hand malformations suggests that the intermediate delta phalanx of the three-phalangeal thumb results of the fusion of a three-phalanx ray with a two-phalanx thumb. The ulnar or radial side of the deviation is determined by the radial or ulnar position of this three-phalangeal ray fusing with the thumb. An opposable three-phalangeal thumb ulnarly deviated with a radial intermediate delta phalanx is the result of the fusion of a duplicated Wassel VII type thumb composed by a radial three-phalangeal ray and an ulnar two-phalangeal ray. In split hands, the three-phalangeal thumb is most often radially deviated because it presumably results from the fusion of a radial two phalanx thumb with the index, deleted in this fusion. The aligned opposable three-phalangeal thumb is quite uncommon and is probably the result of a fusion of two Wassel VII-type duplicated three-phalangeal rays. Conversely, the five-fingered hand or non-opposable three-phalangeal thumb, usually has no deviation and could result of a thumb deletion associated to an index duplication. This understanding of three-phalangeal thumbs makes usual classifications obsolete, but explains the various pathologic associations found in literature and in our own series. Treatment varies with age and takes in account all fundamental aspects of this congenital hand anomaly: the extraphalanx, the clinodactyly due to a delta phalanx, the thumb duplication or the absence of opposable thumb.