Hi Greg.
I'm sorry you are still in so much pain.
How long ago was your fusion?
Have you continued to see the same spinal surgeon?
Have you gotten another opinion from another spinal surgeon that is not part of your spine surgeons practice or hospital?
CT scans with contrast dye is one that can be done to see what is going on.
MRI are not much help after a fusion, if you had hardware put in, because the hardware makes the pics blurry.
I suggest that you seek out other opinions from other spine surgeons and have further tests to see what may or may not be going on.
Did you have BMP used with your fusion?
Fran