Use of skeletal and soft tissue imaging to guide the surgical repair of misclassified Wassel type VI thumb duplication with convergent deformity.

Surgical treatment of Wassel type Ⅵ radial polydactyly with convergent deformity is technically challenging. We found that 17 of our cases exhibited cartilaginous connections between the bases of the radial and ulnar thumb's metacarpals. To better assess the anatomic features and guide surgical design, we used X-ray and MRI imaging to characterize the adduction of the ulnar metacarpal, radial subluxation of the metacarpophalangeal joint, narrowness of the first web, and thenar muscle distribution. Ulnar deviation of the metacarpal bases was 36.18±11.27˚, metacarpophalangeal joint radial deviation was 48.88±16.03˚, and intermetacarpal angle was -1.53±7.79˚. The first web measured approximately two-thirds the size of the contralateral side. Two different types of composition of the carpometacarpal joints and a transverse muscle was described. Treatment involved osteotomy of the first metacarpal's base, release of the transverse tissue, reconstruction of the thenar muscle attachment points, and reconstruction of the first-index web space with multiple interphalangeal skin flaps. Postoperative JSSH scores (evaluation sheet for polydactyly of the thumb presented by JSSH Congenital Hand Committee) were excellent (16/17). Web space size was similar to that on the opposite side and thumb alignment was good. No cases required additional surgical revisions. Furthermore, the risk of misclassifying Wassel type VI polydactyly is associated with convergent deformities, which may be caused by the limitations of X-ray imaging. Bifurcation of the first metacarpal is the initiating factor of the deformity. In contrast, dislocation of the metacarpophalangeal joint and narrowness of the first web are caused by bone and soft tissue factors. Therefore, performing osteotomy followed by soft tissue release and reconstruction can help achieve desirable results. LEVEL OF EVIDENCE: IV.