I haven't had knee surgery, but I've had ACL problems, and I've studied a whole lot about knee surgery. There are a few different styles and degrees of knee surgery.
This stuff gets kinda...well, nasty, especially if you think about it happening to yourself. If you don't wanna get a bit queasy, skip it.
Blazin, if you don't really want to think about the surgery, I'd suggest you skip it. Otherwise, it's a decent rough sketch of a few knee operations.
Easiest kind is orthroscopic (still dont know how to spell it), and I kinda doubt this is what you're looking at - you'll probably need something more. You probably have had it before if you've had knee problems in the past. They basically poke three holes in around your kneecap and go through them to clean up cartilidge, etc.
Then there are the ligament replacement surgeries, where they'll go ahead and take donor bits and stick them in where you've got broken bits. Once again, this isn't complete knee surgery.
Total knee surgery is something along these lines... The top bit of your shin bone (tibia) is replaced by a metal plate. This metal plate has a plastic plate on top of it. The bottom bit of your femur is replaced/covered up by a metal cap, which rotates and slides against the plastic bit on the top of the tibia. The back of the kneecap is replaced with another metal and plastic bit to slide nicely against all the other metal bits.
All the metal bits will probably be attached by inserting a stem in the shaft of the bone, which is secured using bone cement. Most common is basically hammer and chisel to get it snug into the bone, but I've seen a robotic computer-guided drill do the reaming out bit before the implant goes in. Bone cement is basically a polymer broken down into its monomers which forms up by means of a chemical reaction after it is injected. The other way of attaching metal bits is to have a rough surface coated with stuff to encourage bone growth into the porous surface (hydroxyapetite coating).
Yes, it is a highly invasive surgery, but it is a very common one as well. It is still very true that you should find the most experienced doctor that you can, but in this case, you shouldn't have any problems with anyone that's worth their salt.
Effects of total knee replacement surgery-
The metal bits are strong as all heck. They won't break. Period. However, they can come loose, especially in a repetitive stress area like the knee.
The plastic bits are the main problem here. As the metal slides against it, particles will rub off. These bits of plastic build up in tissues, and generally are no good - they can cause inflammation, tenderness, and are usually too big to be removed by the body's natural waste disposal systems.
General lifetime of a knee replacement is (don't quote me on this) about 15-ish years. Because of this, it isn't usually recommended for anyone young enough to outlive the lifespan of the implant or to be active enough to destroy it.
Finally, the bone around the implant tends to weaken. Since the implant takes most of the stress off the bone, the bone will weaken.
So, ya, that's it in a nutshell, pretty much. Google searches pull up lots of info. Search "total knee replacement" for all the info you want. "Bone cement" is another good thing to look into, just so you know what they're squirting into you. "Computer assisted surgery" brings up nifty cutting edge stuff on how doctors use computers in the operating room. Look stuff up, be informed before stuff is done.
Best wishes, and I hope all goes well for you in the near future!
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You asked what to expect, and I didn't even say anything along those lines...
They'll put you under for the most of a day, the operation takes a few hours. You'll be sitting doing nothing for a few weeks for the implants to get nicely implanted. Then there'll be the regular physical therapy and whatnot to get you back on your feet. After all's said and done, you should be almost good as new. Take it a bit easy on the new knee. It's stronger than the natural one, but it'll wear out relatively easily, unlike the one you were born with.
Only 16!
and i hope you were wearing a helmet whilst doing laps of the hospital 
