Advancement limit for genioplasty

A colleague asked in the FB Forum of Clinical Orthodontics group about the limit of advancement of the chin symphysis during genioplasty.

Here’s my answer:

1-Limit of AP advancement
The maximum advancement of the distal fragment is acheived when the lingual cortex of the symphysis is aligned with the buccal cortex of the symphysis. You can not go beyond that unless you do a tenon-mortese osteotomy of the chin.
The assessment of the “ideal amount” is determined by the position of the lower incisor tip to A-Pg line. You should aim for ideal /1-APg = 1 to 2 mm.
2- Soft tissue change
Most study report the ratio of the average soft tissue change to the average hard tissue change .
However, the report mention a ration Pg-Pg’ of 1: 0.6 (Bell& Dan, 1973) to 1: 0.95 (Polido et al, 1991)
I just finish a study with the help of a graduate student of U. MTL on the assessment of soft tissue change versus hard tissue change following a functional genioplasty. Among aother things, we did a regression analysis to find a predictive model of soft tissue change vertically and horizontally. I cannot say more than that. But you will know if it survive the reviewing process.
There is one important benefit of a functional genioplasty that is either ignored or unknown. It is the potential of bone regeneration/apposition above the distal fragment.
I am posting 2 exemples of maximum advancement and significant bone apposition.