And guess what? I just realized this: Opal parts her hair in the back of her head when she shows Holly and Artemis the scar. Your pituitary gland is located behind your eyes. If she got the surgery there, they would have had to go through a large part of her brain to put it in the right place, that is if her 'fragile pixie skull' even held up to the surgery. It would've been easier to go through her nose.
Ech-em. *Holds up sign in fron tof chest that says, "WARNING: Science Geek about to make speech, if you are scientiasesentive"* Hello, fellow Artemis Fowl fans, you are about to recieve a lecture on the brain. Well, I will try to explain the best I can without an image of the brain. First, going through the face would have left a scar on Opal's face, so of course she wouldn't have it. So, there is a way to pratically do it (well, sort of pratically) if Opal is laying on her right side go (assuming north is up) north west through the part of the back of your skull where your hair begins. go straight up through the cerebelium and around the brain stem, leading you straight to the pit. gland. Not pratical and human surgery would never think of it, but these are fairies. Sorry, I just had to do that!