Triphalangism in thumb polydactyly: an anatomic study on surgically resected thumbs.

Surgically resected thumbs in preaxial polydactyly were submitted to anatomic dissection to detect a triphalangeal thumb. Radiologically, two particular categories of thumbs with duplication at the metacarpophalangeal joint were seen. The surgically excised thumbs of either the radial or the ulnar member were preserved for dissection. Depending on the number of phalanges as well as on osteocartilaginous structures, the thumbs were classified into three groups. In the first group, the thumbs consisted of three phalanges but had absent joint formation between the phalanges and metacarpal. The second group consisted of three phalanges with two well-formed joints between them. The third group of thumbs also had three phalanges but had only one interphalangeal joint between them. In all three groups, morphologic features and clinical criteria are discussed.