Arachnodactyly can be recognized from the appearance of the hands. There are three simple methods to definitively determine if arachnodactyly is present.
The thumb sign, or Steinberg sign, is protrusion of the thumb past the hypothenar border when the hand is clenched in a fist
The wrist sign, or Walker–Murdoch sign, is overlap of the fifth finger and thumb when they encircle the wrist of the opposite hand
The metacarpal index is a radiographic measure of arachnodactyly. It is the average of the length divided by the midpoint width for the second through fourth metacarpals (a value of 5.4 to 7.9 is normal, whereas >8.4 occurs in Marfan syndrome).
Arachnodactly is present in approximately 90% of cases of Marfan syndrome but is not diagnostic and may be seen in other diseases.