Last revised by Patrick J Rock on 11 May 2021

Arachnodactyly refers to the physical finding of elongated, thin "spider-like" fingers, which are a classic feature of Marfan disease, but by no means pathognomonic 1.

The metacarpal index was historically-used as a radiographic criterion for arachnodactyly. To obtain it, the length of the second, third, fourth and fifth metacarpals is divided by their width at the mid shaft. An index of greater than 8.4 is considered the upper limit of normal 2. It is no longer used due to its poor specificity and sensitivity.

From the Ancient Greek "arachno" refers to spiders and "dactyly" means fingers.

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