Is it common for the rod in a spinal fusion to break after 3 months?

It is not common for a rod to break at all, especially 3 months after surgery.  The rod is just a solid metal rod.  It is unlikely for the rod to be defective.  Factors that may lead to breakage of the rod include multi-level fusion (greater stress on the rod), very weak spine (often broken back places large forces across the rod), deformed spine requiring surgical correction (again places large forces across the rod), bent rod (may weaken the rod but is usually necessary to align the spine), smaller diameter rod (less strong but necessary at certain areas of the spine), large patient, too active patient or someone who falls or involved in an accident (increased forces across the rod).  Good Luck!