Like Omers said, SI is your own personal issue. You shouldn't feel the need to bring people, just because they are your friends, into your personal issues if you don't want to. If they see your scar tell them exactly what you said and leave it at that. It is none of their business. Until you feel 110% comfortable with telling them you shouldn't. Until you know they are your friends, through anything, don't tell. I wish you the best of luck and my heart goes out to you.