[The biphalangy of the thumb. General review of the literature]

Abstract

The thumb has only two phalanges whereas the other fingers present three. For two millennia, many anatomists tried to solve this riddle. Four different theories can be found in the review of literature, to explain what element misses in the radial column. In the first theory (Galen), the first metacarpal should be the proximal phalanx of the thumb. The particular ossification of the first metacarpal advocates for this theory. In the second theory (Sappey), the first metacarpal should result from the fusion between the true first metacarpal and the proximal phalanx of the thumb, and the proximal epiphysis should be constituted from both metacarpal and phalangeal parts. In the third theory (Paturet), the middle phalanx is thought to be missing. Apart morphological considerations, the main argument refers to symbrachydactyly, which consists in decreasing digital ray length (by reducing the middle phalanx), and usually affects digits excluding the thumb. In case the thumb is affected (in complex symbrachydactylies), the reduction concerns thus the first metacarpal, suggesting that the thumb middle phalanx has already disappeared. In the fourth theory (Pfitzner), the distal phalanx of the thumb should result from the fusion between the second and third phalanges of the thumb. This mechanism, named symphalangy, is well known in the foot, especially in the fifth toe, but also for the other toes, and even in the two ulnar fingers.