This is the biggest problem with spine fusions for disc damage or degeneration. Researchers around the world are studying the links between cause and effect. Does the fusion cause more damage than it repairs? Fusion increases stability at the painful level but often increases the load above and below it. Over time the disc at those levels starts to degenerate.
A recent study from Japan advises doctors to think twice before fusing the lumbar spine when disc changes are already present nearby. From all the studies done so far, it looks like you have about a 50-50 chance of getting good results.