We studied the pathological anatomical structures of atypical Wassel Type VI thumb duplication to provide a theoretical basis for surgical treatment. Forty-seven patients with atypical Wassel Type VI thumb duplication were treated. We found that some of cases had an ulnar thumb without an extensor tendon, or without a flexor tendon, or without both. All the ulnar metacarpal bones were abnormal to variable degrees. On-top-plasty is currently the best procedure for the treatment of this type of anomaly. Flexor tendon reconstruction is an important factor in functional reconstruction of the thumb, and keeping the pulley and sheath intact is a key step in flexor tendon reconstruction. Placing the on-top-plasty level at the distal end of the metacarpal or at the proximal phalanx depends on the presence of the flexor tendon. According to the condition of the flexor tendon, the atypical Wassel Type IV duplication can be divided into two types: the absent type and the intact type.Level of evidence: VI.
Keywords: Atypical Wassel Type VI; classification; flexor tendon; thumb duplication.