Several years ago I had this condition after two nasty bouts of flu. The pain in my chest was severe, so I called the university's Triage Nurse help line and was told it was most probably costochondritis. Apparently, the excessive coughing associated with my illness had inflamed the cartilage attaching the ribs to the breastbone, hence the pain. As the nurse predicted, it abated after about one week; I just took aspirin for it. In your case, it does seem that the staple could be causing constant irritation. BTW, triage nurses rock!