Use of the duplication range concept for understanding morphology and predicting prognosis in thumb polydactyly.

This study systematically and comprehensively analysed 129 thumb polydactylies in 122 patients using a duplicating range concept based on the level of skin and skeletal bifurcation. Numerical levels were defined along the longitudinal axis of the ulnar thumb duplicate from distal to proximal: level 0 (thumb tip) to level 6 (carpometacarpal joint). The relationships between duplication range and morphological parameters were evaluated. Nail asymmetry was associated with skin bifurcation levels 0 to 2. Proximal phalangeal asymmetry and interphalangeal joint deviation, associated with skin bifurcation levels were assigned scores of 1 to 2.5 and 1 to 2, respectively. Metacarpophalangeal joint deviation had a bimodal distribution, at levels 1.5 and 4 of the longitudinal axes. Morphological similarity was found in patients with the same duplication range. The duplication range concept could potentially improve our understanding of morphology variation and predicting prognoses in thumb polydactyly.Level of evidence: III.