Triplication and tetraplication of the thumb: a modification of the Rotterdam classification.

We proposed a modification of the Rotterdam classification for thumb triplication and tetraplication. Twenty-one patients were included (24 cases of thumb triplication and four cases of tetraplication). These were analysed and classified according to a modification of the Rotterdam classification involving three steps; from the radial to ulnar side, we first identified each thumb on radiographs and gross appearance to divide into triplication or tetraplication. Second, we define the levels of duplication and established the nomenclature. Third, the aberrant features and their location were assigned for each thumb, again from radial to ulnar side. A surgical algorithm was also proposed. This modified classification may be helpful for characterizing the rare conditions of thumb triplication and tetraplication for use in patient and management and communication between surgeons.Level of evidence: III.