LAUSR

BACKGROUND Five-fingered hand (5-FH) with complete developed phalanges is a rare phenotype observed so far only in humans and characterized by three phalanges of the 1st ray. A long-lasting, debated question is if the missing element of the normal hand 1st ray is the metacarpal or the phalanx. In this study, comparative X-rays morphometry of long bones in normal and 5-FH is carried out with the aim to face this question through homology analysis of long bone segments in the transverse and longitudinal line of normal hand and 5-FH. MATERIALS AND METHODS In the normal hand X-rays (n =20) and in a 5-FHX-rays series (n=9) the relative length of each segment on the ray total length and the index of growth rate (IGR) were assessed. The calculation of the first parameter in normal hand bi-phalangeal thumb was carried out on the 3rd ray total length in the same hand. RESULTS The parameters relative length and the proximal/distal growth rate asymmetry in the post-natal period (assessed through the IGR) confirmed in 5-FH the homology of all the five segment on the transverse line. In the normal control hand, the relative length assessment methodology was biased by the missing segment of the thumb, therefore, the reference to the 3rd ray total length in the same hand (instead of the 1st), allowed the homology analysis of the thumb metacarpal and 1st phalanx with the lateral segments (2nd- 5th ray) of the same hand. The 5-FH analysis was used to choose the more appropriate reference ray for the normal hand group. CONCLUSIONS The relative length and IGR comparative analysis in the two groups suggested homology of the (anatomical) 1st metacarpal with the 2nd - 5th proximal phalanges in the same hand and that of the (anatomical) 1st proximal phalanx with the 2nd-5th mid phalanges. These data suggest that the missing segment of the thumb normal hand is the metacarpal.