Why do railroads use timber ties on bridges and concrete ones on the ground?

What is the procedure for bridges?

• I'm 29 and have a couple of congenitally missing teeth (upper incisors - 7 and 10). I'm on year 2 of Invisalign per my ortho. The teeth behind where the upper incisors should be grew partially into that spot and there was a gap. I also had a gap in between my two upper front teeth. Invisalign is moving the teeth behind the incisors back to where they should be (as well as moving my upper teeth outward in the gum towards the front of the mouth as far as we can, because I have a slight underbite), but we aren't sure if there is going to be enough space for implants where the missing incisors are. At year 1 of Invisalign, the roots for the teeth behind the upper incisors (the ones being moved back) were angled and apparently still where the teeth used to be before the teeth moved back. Year 2 is doing more movements, making more space, etc. But we still aren't sure if those roots are going to move or if there will be enough space. There's about 7mm of space on the right and about 6mm on the left so far, with over half of a year of Invisalign left. But I'm curious about option 2, just in case those roots are still there and we of course can't drill through them. A possible bridge was mentioned, if the roots haven't moved at the end of year 2. There would only be 2 teeth needed (the upper incisors) for a bridge. Which teeth would they grind down for this? I've heard that one on each side would ground down and would get a crown on it. Would having a bridge in that area cause me to have a lisp? I've had an awful lisp ever since I started Invisalign, but it's the slickness of the trays on the back of the upper front center teeth that are causing that lisp. But even thinking of a bridge wired together that would go behind my upper front center teeth makes me curious if that's also going to give me a lisp. I have already researched and found that bridges average 10-15 years in general for a lifespan. With two teeth ground down for that bridge to be glued to...how is it changed when a new bridge is needed? Are the teeth ground down further? How many times can a new bridge be gotten? At 29, that concerns me...the thought of getting 3-4 new bridges over the remainder of my life. How do they get the old bridge off when a new one is needed?

• Answer:

  http://travelandsmile.files.wordpress.com/2010/05/bridge1.jpg There is a pic of a bridge and what the dentist does to the teeth on the side. Since your 2 missing teeth aren't next to each other and it has 2 teeth inbetween the spaces, you'll need 2 bridges.