What a b-o-o-o-o-o-ring perennial topic!
This is the favorite topic of high heel fetish people.
Achilles tendons are not made of steel. They are
pliable like all other tendons.
If adult wears very high heel shoes for more than a year,
her Achilles tendons will shorten, but this condition
is not permanent. If she later wears lower heel shoes
for a few months, she will be able to wear flat shoes
eventually.
A young teenager should not wear very high heel shoes
all the time because the shoes will deform her growing
bones. This kind of deformation looks ugly (sharp edges
of bones are visible under the skin) and is permanent
because the bones are rigid, and foot bone surgery is
very difficult.
The only permanent body modification associated with
adults wearing high heel shoes is injury caused by poor
quality of the shoes. If you want to wear very high heel
shoes, do not even think about buying ready-made shoes.
The shoes must be custom made so that they fit perfectly
and there is enough room in the toebox for a thick
insert under the ball of the foot. More info:
http://www.medianet.pl/~andrew/shoes/comfy_heels.htm