Elizabetho

molars provide forward force on all teeth to the front of them (as long as there is contact with the teeth in question).. Imagine marbles lined up being pushed by the last marble like a train engine. Wisdom molars (being the last teeth to errupt, and errupting around your age) can cause damage even as they errupt.

This is why, when a baby tooth is lost prematurely and not relaced by either a false tooth or a band and loop "space-holder", the adult teeth will invariably come in crooked because the baby and developing molars push so hard and so fast that the space will be lost and the incoming teeth will come in however best they can... the baby teeth act as "placeholders" for the adult teeth erruping behind them. the last molar in the mouth drives contact forward. You didn't ask about baby teeth, i know, but the point is to see how molars "bring up the rear" so to speak, and that is actually part of their physiological job in the whole mouth development, jaw growth, and tooth erruption sequence of events.

as the wisdom's crown develops under the bone (years before erruption), it can be pressing on the roots of the molars in front (the 2nd molar roots). this is most pronounced when the growing molar crown is tilted forward. It is easily confirmed by xray. and so, while it is errupting into the mouth, it can be damaging the roots of the 2nd molar.

there's another way for 3rds to cause problems while looking "normal" in the mouth. if the 2nd molar is tipped slightly toward the back, or has a slightly bulbous crown, a wisdom tooth errupting perfectly "straight" can still damage the second molar and apply undue force to all teeth that in contact in front.

when a second molar is extracted and not replaced, say from severe cavities, the third molar will drift forward 99.9% of the time -- sometimes even touching the first molar. again, an xray reveals the root angulations involved and you can tell immediately what's going on. this is extremely common with middle aged patients or actually, anyone who loses a second molar.

there are other more rare complications, but you can see just how powerful the effect of the 3rd molar is.

third molars are least traumatically removed when they have nearly errupted as the bone is spongier earlier in life. by middle age bone is more brittle, and complications during extraction are much more likely. the other thing is, by removing the thirds, you make the back surface of the seconds easier to keep clean. this is important, because at a certain point in life, say in your thirties or so, the 3rds become a liability -- they are very hard to keep clean, and they prevent you from being able to do a great job keeping the seconds clean. there's an increased risk of decay between the two molars that inevitably ends up down the path of large fillings and crowns. so hanging onto 3rds too long can be sort of a double whammy, by then, unless one is meticulous abt hygiene and flossing.

oh, and if the thirds are removed, the seconds will keep pressure applied forward, and ditto for the first molar as well; each molar is less forceful as you move forward. without molars at all, the teeth won't float around too much because they are mostly single rooted and they are angled differently from molars.

now you asked why it happens. well, if you get a chance to look at an xray of molars in the jaw, you will see they are tilted forward. the crown leans forward. so imagine this: when you put your jaws together (to chew or clench) it is the crowns that come into contact. the roots pretty much stay planted into the bone, but the center of rotation when vertical chewing force is applied is somewhere in the upper root stucture. but you can see how the result is that the crowns push forward as vertical force is applied because they are tilted to begin with.

there is yet another physiologic reason why molars tend to drive the teeth forward and the basic idea has to do with the way bone is remodelled by the body in response to use. (that is a normal feature of bone biology and fairly well understood. they take advantage of the bone's growth patterns when they do orthodontics). well you probably a little tired of ortho by this point, so i'll stop here, but congrats on finishing ortho. hope this helps clear up things!