Classification

Thumb duplication has been classified by Wassel(1). It is based upon the most proximal level of skeletal duplication:

• type 1: bifid distal phalanx
• type 2: duplicated distal phalanx
• type 3: bifid proximal phalanx
• type 4: duplicated proximal phalanx
• type 5: bifid metacarpal phalanx
• type 6: duplicated metacarpal phalanx
• type 7: triphalangeal thumb

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.