Association Between Thumb Hypoplasia, Radial Longitudinal Deficiency, and Radial Polydactyly

PURPOSE: The expression of the sonic hedgehog protein has been associated with induction of both radial longitudinal deficiency (RLD) and radial polydactyly in animal studies. We sought to investigate the co-occurrence of thumb hypoplasia/RLD and radial polydactyly in a large registry patient cohort and to investigate the clinical presentations of such patients. METHODS: The Congenital Upper Limb Differences (CoULD) registry was queried for all patients with contralateral diagnoses of thumb hypoplasia/RLD and radial polydactyly, as defined by the Oberg-Manske-Tonkin classification system, between June 2014 and January 2026. The demographic information, medical and musculoskeletal comorbidities, radiographs, and clinical images were collected. RESULTS: Ten patients were identified with contralateral diagnoses of thumb hypoplasia/RLD and radial polydactyly. The median age and interquartile range at time of enrollment was 1.67 (0.2-3.0), and 4 of the 10 were women. There was a wide range of diagnoses of thumb hypoplasia including type II, IIIA, IIIB, IV, V, and VII (unclassifiable), and two patients additionally had RLD (type 1 and type 3). There was a similarly a wide range of radial polydactyly classification, including type II, IV, V, VI, VII, and VIII (unclassifiable). Four had a syndrome diagnosis, four had medical comorbidities, and three patients had a history of other orthopedic conditions. CONCLUSIONS: This study demonstrated that with co-occurrence, there was no clear association in severity between thumb hypoplasia/RLD and radial polydactyly. There was minimal forearm involvement which has implications in timing of hand development. Additionally, there was a high degree of variability including syndromic conditions, as well as medical or orthopedic comorbidities. We recommend that clinicians are aware of this possible association and perform a thorough examination, as the identification of a mildly hypoplastic thumb would guide additional medical testing not typically performed in thumb polydactyly. TYPE OF STUDY/LEVEL OF EVIDENCE: Differential Diagnosis/Symptom Prevalence Study IV.