The classification of duplicated thumb is based on the level of duplication, which ranges from type I, which is partial split of the distal phalanx, to type VII, which is complete split of the metacarpal (Table 37-1). The type corresponds to the number of abnormal skeletal elements present. For example, type III has three abnormal bones (two distal phalanges and a partially split proximal phalanx). Wassel type IV is the most common type (Fig. 37-1).

Genetic counseling is usually not necessary except for type VII, which is associated with other congenital anomalies and is inherited in an autosomal dominant pattern.

The duplicated elements are abnormal in size and shape, and the radial duplicate is typically smaller in size. The surgeon should examine the thumb for the level of duplication, the degree of hypoplasia of each component, stability of the involved joints, and position of the thumb with respect to the bony axis and first web space.

Preoperative radiographs are indicated to identify the size and number of duplicated skeletal elements and to determine whether the elements are attached (bifid) or separated (duplicate) (Fig. 37-2).

The osseous anatomy is abnormal, with varying degrees of bony hypoplasia as well as widening and angulation of the articular surfaces.