Does the chin in genioplasty need to be in contact with mandible to grow back together?

There needs to be some bone contact following the genioplasty. For a lengthening genioplasty, I sometimes place a bone graft.

Larger advancements tend to have some bone "paste" if you will, added to the gap to help the bone heal faster and add some stability. Although it would eventually fill in it on its own, this provides some of the base work and accelerates the healing process.