Dr. Gauthier's website is clear when he says (in French) that tight pants "(jeans)" are responsible for the appearance of depigmentation spots in front and back of the knees.
In children, early depigmentation on knees disappears after discontinuation of use "jeans". Unfortunately, in a long term use, it can be hard to recover.
http://www.vitiligofriends.org/forum/topics/another-amazing-french-website
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