Hi,
Modifier 58 - It was explained to me that this modifier was for "staged" procedures. Those procedures for which you know will not be completed in one surgical session and for which the patient will have to return to OR for completion of the total procedure. An example I was given was "hemorrhoidectomies"; I was told that these are removed in stages so as to reduce patient discomfort and to produce the best results. In the case of this type of procedure, the "stages" of it are often times pre-scheduled. Going into the first "stage", you know there will be more to come.
Modifier 78 - Just the opposite....this is an "unplanned" return to the OR. The patient must return to the OR for problems that were unforseen and that have arisen. These returns to the OR are unplanned and therefore "unstaged" - i.e. complications.
Hope this helps!