I was ACDF 4-6, NOW 4-7 due to same thing. C5-6 was a malunion, or what they deem as pseudoarthrosis. We couldn't figure out if it had fused and broken or never fused at all. It had spent some time in there knocking around and was extremely painfull at times when it it would get into certain positions. There were A LOT of Doctors that looked at it, saw it, and then ignored it. I finally found one that took care of it and went back in through the front redid the first surgery, then performed his own at C6-7 and then locked it all down with a 3 level titanium plate. I was told I had EXCELLENT Bone quality, however I am battling arthritis with a lot of spurring. I had to wear a bone growth stimulator post second surgery and NO J COLLAR!

From dealing with this and having to be my own advocate I came across the FDA recommended guideline for pseudoarthrosis. It suggests that if fusion isn't achieved within 8 months then surgical intervention should be obtained. Now some Doc's I talked with said that the graft was contained and it really couldn't move or slip or be of any danger. I totally disagree! I could feel mine moving around, and it caused a lot of damage that I am dealing with now. It also was quite painful!
Hope that helps with some of it, but I couldn't speak to the posterior effects at all.