Hi,
Please see the following section for information that will be useful:
http://www.lasik.md/learnaboutlasik/precautions.php
The basic issue is that laser vision correction thins the cornea and there is a limit to how much thinning can take place. However, there are few, if any, absolute rules as to when a cornea is too thin. Therefore, more conservative surgeons switch to PRK to conserve thickness earlier than more aggressive surgeons.