I had a ACDF (C5-C7) over two years ago. I was in more pain when I came out of surgery than I had ever had before, and it has never improved. The doctor says my fusion looks great, but because it took me a year and a half to get my surgery approved by workers comp, I now have permanent nerve damage. I have never been able to return to work, nor will I ever be able to work again. I have been in pain management for two years. I am on Fentynel patches 100mcg and Oxycontin. Trying to control the pain is the only hope that I have. I will also require more surgery eventually, the disks above and below my fusion are already herniated.
Finding a good pain clinic should be able to help with your pain. Good luck.
Having had a cervical fusion myself, the risks of additional deterioration of the adjacent discs are a potential complication of this type of surgery. I am not sure why the plate was removed from your C5-6 level at the time of the C7-8 surgery as the plate and screws provide stability while the bone knits together (cadaver bone with your vertebrae). I would have thought a larger plate covering both disc areas now fused would have been done.
As far as "slippage" goes, without a plate/screws stabilizing the area, you are reliant only on the cadaver bone to hold everything together until complete fusion occurs which could take 6-8 months. If the bone has not knit together, it is a sign of psuedoarthrosis or failed fusion. While this is a small percentage of complications post fusion surgery it does occur. A bone stimulator applied to the area sometimes helps with the fusion process.
As for the permanent spinal cord damage -- it would also mean you will not be completely pain free and will have residual symptoms that can not be alleviated. However, the level of pain you are currently experiencing seems too high to have any quality of life. If the fusion problems are addressed, perhaps these symptoms will decrease.
Have you consulted a Pain Management Specialist? Long-term management of pain following a fusion is usually needed. Unfortunately, once a neck patient you are always a neck patient!
Best wishes ----