MRI's are not the be all and end all of imaging techniques. Different technicians can read them differently and they don't pick up everything anyway. They are excellent instruments but unfortunately you can still have an injury if nothing shows up on the MRI. Check out the rather disheartening article below for more information on the pro's and con's of MRI's.

If you have sciatica you almost by definition have a back problem. Your nerves are getting tweaked not in your legs where you feel them but way up in your back.

Good luck in dealing with it. I've had alot of success with my admittedly mild sciatica using Dr. Egoscue's stretching techniques. You can pick up his book on Amazon.com if you're interested. 

http://online.wsj.com/article/0,,SB1045520088439966623,00.html