Modified Radiographic Classification System for Congenital Thumb Duplication: An Analysis of 2,300 Thumbs in 2,108 Children

The objectives of this study were to (1) evaluate the radiographic characteristics of children with congenital thumb duplication (CTD) seen in our institution between August 2015 and April 2021; (2) introduce a modified radiographic classification system (MCS) capable of including all cases of CTD based on their radiographic pathoanatomy; and (3) evaluate the inter- and intrarater reliability of the new classification system.We retrospectively reviewed 2,108 patients with 2,300 CTDs. The MCS is based on the Wassel-Flatt and Chung et al classification systems and includes specific subtypes from the Rotterdam and modified Wassel-Flatt classifications. The MCS is characterized by 4 groups according to the anatomical morphology of the duplication: A (joint), B (epiphysis), C (bone), and D (soft tissues). Each group includes 4 subtypes according to the location of the CTD, with subtypes 1-3 extending from the distal phalanx to the metacarpal or interphalangeal joints, then to the carpometacarpal joint, and with subtype 4 only including the triphalangia of the main thumb.Among the 2,300 fingers, 360 (15.7%), 2 (0.1%), and 3 (0.1%) CTDs could not be classified according to the Wassel-Flatt, Chung et al, and Rotterdam classifications, respectively. According to the MCS, the 2 most common forms of CTD were A2 (680/2,300; 29.6%) and D2 (308/2,300; 13.4%). All cases could be classified according to this classification system. The MCS showed excellent intrarater (0.875) and interrater (0.851) reliability relative to the Wassel-Flatt (0.863 and 0.820, respectively), Chung et al (0.793 and 0.822, respectively), and Rotterdam (0.873 and 0.836, respectively) systems.The MCS is a potential radiographic classification for CTD that enables the classification of all patients and has excellent inter- and intrarater reliability.Existing classification systems do not allow classification of the full spectrum of CTD and are not always related to surgery, and some existing systems are complex, with many categories that are rarely encountered, or are difficult to use widely in clinical practice.