Implications for treatment of variations in length of the first metacarpal in different types of triphalangeal thumbs.

Abnormal function in the triphalangeal thumb is partly due to its extra length, which is due not only to the extra phalanx, but also to differences in the length of the first metacarpal. This study investigated whether the additional length of the first metacarpal is influenced by the growth plate location alone, or also by the type of triphalangeal thumb. Fifty-nine hands in 37 patients with triphalangeal thumbs were examined for thumb type (delta 31, trapezoid nine and full type 19), growth plate location and relative length of the first metacarpal. The first metacarpals in all three types of triphalangeal thumbs were significantly longer than in the normal population. The length of the first metacarpal was related to the site of the growth plate. The type of triphalangeal thumb did not affect the length. These findings suggest that a corrective procedure on the first metacarpal should be considered in all types of triphalangeal thumbs.