Unfortunately, adding a fusion to a preexisting one is somewhat common. The same genetics that existed to allow the fuzed level to wear out initially and need a fusion still exists. When the next level wears out, it is called adjacent segment disease (ASD) This disorder is so common, it does have a name. Normally, the level that wears out is the very next level. It is more uncommon that the level 2 levels away wears out. L2-3 is separated from L4-5 by L3-4.
Dr. Corenman