Do I need a bone graft or HA paste for sliding genioplasty?

In my practice a 5mm gap is generally considered the maximum gap that can be expected to fill in with bone without grafting. There are other considerations as well. The patient's age and the amount of simultaneous forward projection. An 8 mm projection is near the maximum of horizontal advancement that can be achieved with a genioplasty. Together these 2 changes are significant for potential instability without grafting. My personal preference would be to add bone putty rather than HA to fill in the gap and improve the contour irregularities created by this large advancement. The bone putties are commercially available and do not add significant costs to the procedure. The bone putty acts as a scaffold for new bone growth. I feel there are less chances for "step" defects with grafting.

If you want to ensure intergap healing between the new inferior chin position and the superior fixed bone, use allogeneic (tissue bank) block or corticocancellous chips to do the fill-in. There is no need to make an actual bone graft or use a synthetic material like HA. This not only expedites the bone healing but also hedges against significant deepening of the labiomental sulcus.