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A Novel Radiographic Subclassification of Postaxial Polydactyly Type A
Holly Cordray, BS1, Apurva S. Shah, MD MBA1,2, Eliza Buttrick, BA2 and Shaun D. Mendenhall, MD1,2, (1)Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, (2)The Children's Hospital of Philadelphia, Philadelphia, PA

Introduction: In Type A postaxial (ulnar) polydactyly, the small finger is duplicated as a well-developed accessory digit that articulates with the bony structure of the hand. The condition shows considerable morphologic diversity, yet no system for further classification has gained broad acceptance in clinical practice. We aimed to develop a novel subclassification for Type A postaxial polydactyly based on radiographic morphology.
Materials & Methods: All cases of Type A postaxial polydactyly at a major pediatric hospital from 2012-2022 were reviewed. Demographic and clinical characteristics along with radiographs from the most recent preoperative imaging session were extracted from the electronic medical record. In the first phase, radiographs were compared to determine morphologic themes and devise a subclassification system. In the second phase, 2 attending pediatric hand surgeons used the proposed system to classify all radiographs from the cohort. Interrater reliability was determined by Cohen’s κ.
Results: The cohort included 58 Type A hands in 37 patients (56.8% bilateral). Patients had concomitant preaxial (radial) polydactyly in 8.1% of cases, and 16.2% of cases were syndromic. A positive family history of polydactyly was reported in 43.2% of cases. Racial identities were 45.9% White, 29.7% Black, 2.7% Asian American Pacific Islander, 5.4% multiracial, and 16.2% other. The cohort was 86.5% non-Hispanic and 54.1% male. Radiographs were available and sufficient for 53 hands. Six subtypes emerged from radiographic analysis (Table 1; Figure 1). Our system is based on the level of skeletal duplication in a distal-to-proximal sequence, reflecting how proximally the polydactyly extends. In this cohort, 34.0% of cases were classified as Type A-I (Hypoplastic), 11.3% as Type A-II (Phalangeal Duplication with Divergence), 15.1% as Type A-III (Bifid/Broadened Metacarpal), 13.2% as Type A-IV (Hypoplastic Duplicated Metacarpal), 11.3% as Type A-V (Metacarpal-Phalangeal Fusion), and 1.9% as Type A-VI (Triplication); 13.2% did not achieve agreement on initial ratings. Interrater reliability was strong (κ = 0.83).
Conclusions: This 6-subtype system provides a more nuanced classification of Type A postaxial polydactyly. From preliminary interrater reliability, the system is feasible to implement in clinical practice. Our system may help guide more tailored care for children with this congenital hand difference.

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