Midterm Surgical Outcomes of Duplicated Thumbs According to New Classification

Surgical outcomes of duplicated thumbs differ depending on the branching type. The authors developed a new classification system and report surgical outcomes with an average 10.2-year follow-up. METHODS: A total of 529 patients with 562 duplicated thumbs were reviewed. Surgical anatomies were compared with radiographs, and then a new classification system was developed based on branching level and bone shape observed on the radiograph: distal type (D-type), including Wassel types I and II; proximal type (P-type), including Wassel types Ⅲ and Ⅳ, and four subdivisions (Po, Pa, Pb, and Pc) according to bone structure; and metacarpal type (MC-type), including Wassel types V and VI. All hands were assessed using the Japanese Society for Surgery of the Hand evaluation form, and factors causing poor outcomes were analyzed. RESULTS: There were 25% D-type, 59% P-type, and 14% MC-type hands; 2% of hands were not classified. Overall, 351 hands (63%) were directly assessed when patients reached 5 years of age. Seventeen percent of hands had fair results. Good results were achieved in 90% of D- and Po-type hands. Pa-, Pb-, Pc-, and MC-type hands had lower Japanese Society for Surgery of the Hand scores than did D- or Po-type hands. Pa- and Pb-type hands tended to develop interphalangeal joint malalignment and instability, whereas Pc- and MC-type hands developed disorders in the metacarpophalangeal joint with growth. CONCLUSIONS: The authors’ new classification system clarifies the potential pitfalls for each type of duplicated thumb. More than 90% of D- and Po-type hands obtained good results. Care should be taken with interphalangeal joint reconstruction for Pa- and Pb-type hands. Meticulous reconstruction of the metacarpophalangeal joint is essential for Pc- and MC-type hands. This analysis provides important information for surgeons and patients. CLINICAL QUESTION/LEVEL OF EVIDENCE: Risk, IV.