I was told after my neck fusion that it's possible the disc above or below the fused level might start to break down from the added stress and strain. I've thought about that but I don't really get it. Doesn't having the bad segment fused stop motion and protect me from having this problem again?
You are referring to the concept of adjacent segment disease (ASD) -- a possible long-term complication of vertebral fusion. This is something that has been studied in relation to the lumber spine (low back) and well as cervical spine (neck) fusions.
It's clear from many studies that adjacent segment disease is a real phenomenon. It's not entirely clear why this happens. The idea that fusing one level will transfer load to the next higher or lower segment makes some sense.
The stress, force, or load through the spine during motion is the same with or without a fusion. In other words, having a fusion doesn't take away the load placed on the spine during compression or movement. And according to all we know about movement and physics, load applied to the vertebrae is shared and does transfer from one segment to another.
When a segment can't move because of a fusion, there is a certain amount of stress or load that is absorbed by the vertebral segment on either side. But studies don't show equal amounts of adjacent segment disease on both sides of the fusion so there may be other factors involved.
For example, there is a report from a recent study done at the University of Washington in Seattle that the spine tries to restore normal range-of-motion as much as possible, especially after a cervical (neck)fusion. That may mean increasing motion at other segments to the point of becoming hypermobile (too much motion).
It is thought that this hypermobility contributes significantly to adjacent disease (i.e., the segment next to the disc fusion starts to deteriorate). Hypermobility combined with increased load transferred to the adjacent segments affects more than just those levels.
Based on the results of this study, the authors suggest that there are more biomechanical effects on the entire cervical spine after a two-level fusion compared with either one-level fusion. Greater muscular effort in an attempt to maintain full motion after a two-level fusion may be the reason patients develop muscle fatigue and pain.