Purpose: To explore anatomic abnormalities causing congenital ulnar-angled thumb at the distal phalanx, so as to identify the differential diagnosis and guide for surgical treatment. Methods: Reviewing the clinical manifestations, x-ray films, and operative findings of 43 children with congenital ulnar-angled thumbs in the distal phalanges without radially inserting terminal tendons at the base of distal phalanx, and analyzing their anatomic abnormality as well as differential diagnosis. Results: There were 57 ulnar-angled thumbs in this study. Abnormal hypertrophic epiphyses were found in 35 thumbs. In x-ray, distal phalanx was in good apposition with its epiphysis and they got pretty close to the average of less than 1 mm. In operation, we measured the length of the radial side of the hypertrophic epiphyseal averaged 3.2 mm. Extra bones were found to exist between the proximal phalanges and distal phalanges in the remaining 22 triphalangeal thumbs. The preoperative radiography showed a larger distance between the distal phalanx and the extra bone, which averaged to more than 2 mm. The length of the radial side of the hypertrophic epiphyseal measured in operation averaged 3.7 mm. Conclusions: Abnormal hypertrophic epiphysis and triphalangeal thumb are two different causes for congenital ulnar-angled thumb at the distal phalanx. The distance between the hypertrophic epiphysis and the distal phalanx is much smaller in x-ray so we can distinguish it from a triphalangeal thumb before surgery.
