Could be inflamation but I can't help but wonder if also could be from surgical positioning on the table. I just wonder of that sometimes because I had acdf and woke up with horrible right shoulder pain which was severe. I still get nasty flare ups in the shoulder. I remember years ago during my cancer surgery which was on my leg --I was awake during that surgery but numb from the spine down--- I remember being on the table with arms extended and tied down and legs were streched so far apart almost to the point of doing the splits. Im not sure if they use any kind of traction device during cervical fusion but u were flipped on both sides--I only had anterior but my brother had posterior and I know posterior is harder on recovery because they have to cut all that muscle on the back on the neck. The only problem is because of bleeding no antinflamtory meds are allowed for long time after fusion surgery.