foot pain

i have had heel pain for 5 years now after my first child. I have worn custom orthodics for 4 years, have had several cortisone injections, and done pysical therapy with no improvement. the pain is worse and is now traveling to the front of my foot, and is extremely painfull in the bursa at both knees. medications have failed and xrays show no bone problems. what other tests can I have done to find out the cause of this burning constant pain. at night the pain is the worst, I sleep on my stomach and must elavate both feet with a pillow because of the stress put on my feet in that position. do you think a night splint would help at all? or could there be something else to look for that noone has thought of? any help would be greatly appreciated. thank you