a jsf bone sticking out
Enclosed is a diagram that should help you visualize why there would be an increased ' antegonial notch' after (hefty) CCW rotation along with (a hefty) BSSO advancment. Technically speaking, the true gonial notch does not change as can be observed in the YELLOW part of the lower jaw. Hence, my term for what usually happens and what you are probably observing is: 'BONE STICKING OUT'. If a better technical term is needed, we could describe it as; 'step off anterior to antegonial notch' or 'pseudo antegonial notch'.
It is a direct function of the extent of the posterior CCW-r at the maxilla which is what allows the lower jaw to be advanced over a DECREASED angle of inclination. So, with the BSSO cut, advancement of the lower jaw ALSO comes with a rotation along the bsso CUT which is why you see part of the lower jaw bone sticking out BELOW the native antegonial notch.
Since it would be the space in white bordered by red in the diagram that is 'empty', the most straight forward solution to 'filling' that space would be with FILLER as to stream line the type of disconnect along the lower border of mandible. Custom implant (doesn't matter what material it is) would be over-kill for that sort of thing.