That's a loaded question - there are so many things that can affect the fusion area. It depends on how much pain we're talking about. If it's bad enough that you've been referred to a Pain Management Program, then it's possible the hardware might be installed wrong. If it's pain that just requires some Percocet or Vicodin on an as-needed basis, it's probably scar tissue buildup and spinal stenosis, which is common in patients who've had fusions.
I've had 3 so far - C5/6/7 in '96, L4/5/S1 in '99, and L3/4 in '06. I've been in a formal Pain program for 11 years, but have dealt with spinal pain for a lot longer than that. In my case, I was told a long time ago it was 'Failed Fusion Syndrome', or Post Laminectomy Syndrome, whatever you want to call it. Essentially if there's no visible cause for the pain, the chalk it up to residual pain from the procedure.