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Normal and five-fingered hand: comparative X-ray morphometry in the post-natal age.

M P Bondioni¹, L Casati², A G Salvi³

¹Department of Radiology, Department of Medical and Surgical Specialties, Radiological Sciences and Public Health, University of Brescia, Italy. mariapiabondioni@gmail.com.

Folia Morphologica

|July 10, 2020

Summary

This summary is machine-generated.

This study on the five-fingered hand (5-FH) suggests the missing segment in the normal human thumb is the metacarpal. Comparative X-ray analysis reveals bone segment homology, clarifying hand development.

Keywords:

five-fingered hand hand post-natal ossification pattern hand segments morphometry homology triphalangeal thumb

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Area of Science:

Human anatomy and evolutionary biology
Developmental biology and genetics
Radiology and medical imaging

Background:

The five-fingered hand (5-FH) is a rare human phenotype with three phalanges in the first ray.
A long-standing debate questions whether the missing element in the typical human thumb is the metacarpal or a phalanx.
This study investigates hand development using comparative X-ray morphometry.

Purpose of the Study:

To determine the identity of the missing bone segment in the human thumb.
To analyze bone segment homology in normal hands and five-fingered hands (5-FH).
To resolve the debate regarding the first ray's skeletal structure.

Main Methods:

Comparative X-ray morphometry of long bones in normal hands (n=20) and 5-FH (n=9).
Assessment of relative segment length and index of growth rate (IGR) for each ray.
Utilized the third ray as a reference for homology analysis in normal hands.

Main Results:

Homology of all five segments was confirmed in 5-FH transverse lines.
Relative length assessment in normal hands was adjusted using the third ray as a reference.
Analysis supported homology between the first metacarpal and proximal phalanges, and the first proximal phalanx and mid-phalanges.

Conclusions:

The study suggests the first metacarpal is homologous to the 2nd-5th proximal phalanges.
The first proximal phalanx is homologous to the 2nd-5th mid-phalanges.
These findings indicate the missing segment in the normal human thumb is the metacarpal.