Nonunion means that the bones do not fuse properly.
For example, in my case, one rib did not fuse back together. The two ends grew right up next to each other, but did not finish fusing into one rib again.
Although nonunions are unpredictable, smoking or diabetes will increase a patient's risk.
Nonunion in a rib is no big deal, but nonunion between vertebrae is a problem. In that case, only the rods would be holding the vertebrae steady, and the curve may continue to grow.
The technical term for nonunion is pseudoarthrosis.