Wiki Skin graft substitute

[vhersey2003]

My podiatrist did surgery on a patient's left foot for which he is in the global period. On about his third followup visit she started applying Puraply to the left foot. However, he now also has a right foot ulcer which she is applying the skin graft substitute to as , well. The codes I am using are 15275, 15004 and Q4172 for the material. My question is, since the graft application code is for total square cm, but the left foot is in postop and needs a mod 58 and the right foot is new so would be unrelated and need a 79 modifier, how would I bill that?