I am really pissed off for you! SI is part of your medical history - which by law is confidential information. No one can force you to reveal this, unless you would like to.
For this reason, I consider this one of those times when a little white lie is okay. If I am confronted by someone at work, or anyone I don't want to tell, I simply say, "car accident." A car accident can cause a wide variety of injuries, and is reasonable enough. Someone might suspect I am lying, but they have no cause to further confront me. If they push the issue, I have no problem saying, "I don't feel it's professional to discuss personal issues in the workplace. I'm sure it's not your intention to pry." If they have any sense at all, that should make them feel like an *****.