Am I likely to get permanent numbness on my chin if I had this with a previous procedure?

I had a breast reduction of 2 cup sizes four years ago and permanently lost sensation in my nipples. I am planning on getting a sliding genioplasty (around 5 mm forward and 5 mm down--does that look realistic?) and am worried about the chance of having permanent numbness in my lips, which I would hate. Is there a percentage?