I think the reason is that if you don't reconstruct the area by putting the bone flap back and putting some plastic over the hole, then the dura, the lining of the brain, is in immediate contact not with the skull but overlying muscle. As that scars down, whenever those muscles contract, like whenever you move your head or neck, and the muscle contracts, it pulls on the dura, a pain sensitive structure and that can cause a severe headache. When we do have medically intractable headaches after a retrosigmoid, I encourage patients to think about having a very minor procedure. We open that wound, remove the scar tissue and put a small plastic plate in the bony defect, so that the dura is no longer in contact with the muscle."
I had my AN removed at the London Clinic in 2003 by Gerald Brookes by retrosigmoid. They did not put any plate over the hole in the skull. I have severe problems if I do physical work e.g. in the garden, using e.g. a heavy hedge cutter held up to head height. I get severe headaches, and pains in my neck and shoulder. This post is very helpful and makes me think it may help to have this plate inserted, in case this is exacerbating my headaches.
I had an AN of less than 1cm but I suffered total hearing loss, which it would seem was unlucky, and like many others continue years after to suffer very bad tinnitus. Like other members, this is always worse in noisy environments such as supermarkets (with the freezers and air con fans) and restaurants, and in a noisier car. What is worse, is that although I have good hearing in my left ear, if I listen to music on e.g. an MP3 player, it causes a terrible aftermath of tinnitus in my deaf ear - so bad that I have more or less resigned myself to not listening to music except quietly in the background. For a musician that it pretty tiresome and depressing.
Also I have found that certain foods seem to also cause me major problems with dizziness and headaches - cheese, and spinach (the iron??), and any salty foods. I am finding I have to control what I eat far more than I expected, and it does seem salty foods are bad news for me.