Thumb duplication has been classified by Wassel(1). It is based upon the most proximal level of skeletal duplication:
Type 4 is the most common (50% of such duplications) followed by type 2 and then type 6. There is no equivalent in this classfication of a rudimentary digit attached by a short soft tissue bridge to the radial side of the hand (the equivalent of a Stelling A type duplicated digit on the the ulnar side of the hand). Classification may be confounded by the ossification patterns within the bones: delayed ossification may require serial films; two initially separate areas within a phalanx may fuse into one at a later date.
Ref: Wassel HD (1969). Clin Orthop 64: 175-195.
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