Here's how I look at it. But I could be wrong. So, all the more reason to cross reference with a surgeon as to potential trade-offs.
Assume an anterior downgraft of 'X' mm will rotate the maxilla-mandible complex CW-r by Y degrees.
Assume a symmetrical but opposite posterior impaction of 'X' will rotate the maxilla-mandible complex CW-r by Y degrees.
Since each rotation is is CW-r, they are additive. The total rotation would be 2Y degrees CW-r. It isn't a 'net' rotation of 2 opposite directions when one is subtracted from the other.
It could result in an UNWANTED total CW-r where the anterior front face could look all the more longer because the posterior impaction rotates upward (shortens) (in CW-r) but the anterior downgraft CW-r rotates the chin point downward and backwards.
Now, Y degree rotation can vary from person to person for every X mm of anterior downgraft or posterior impaction. But to keep the concept SIMPLE, let's say for every Xmm addition to one part and subtraction to the other part = Y degrees. So,
4mm addition to anterior max will effect a CW-r of 4 degrees and a 4mm subtraction to posterior max (impaction) will also effect a CW-r of 4 degrees. So, it doesn't matter what Y is for every X. What matters is that for X to be added to the front and X subtracted from the back, both are CW-rs and both added together can double the total CW-r which could affect a visually longer appearance to the frontal visage.
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For example, let's take each one individually and then add them.
1: Consider if you got only an anterior downgraft of 'X'mm but no posterior impaction. In EXCHANGE for the extra X mm frontal tooth show (which again is a CW-r), the CW-r of Y degrees, rotates the chin point down and backwards which in turn, will make the anterior face cast longer. The rotational position of the chin point affects how the face looks in frontal view.
2: Consider if you got only a posterior impaction (CW-r) with no anterior downgraft. The back of the maxilla rotates upward and gets shorter. So, now you have some extra shortness in back which in turn will give LESS posterior tooth show because you are SUBTRACTING posterior tooth show from the smile. In effect, you would be 'adding' more 'shortness' to the overall tooth show
3: If you add both of those individual rotations together, you are adding X more frontal tooth show to the overall smile but also subtracting X tooth show from the back. Posterior impaction is CW-r but also an upward rotation at the back of jaw. Hence SHORTENING takes place there. Anterior downgraft is also CW-r but it's also rotating the chin point down and backwards which will cast the front face longer in frontal perspective. So, when the posterior shortening is equal to the anterior lengthening (of X mm) and Xmm rotates the MMcomplex Y degrees, when you add 2 CW-rs together, you can get a total rotation of 2 Y degrees. It isn't a 'net' rotation where one is subtracted from the other. It's a TOTAL rotation; a total CW-r. The impaction in back shortens in back and the downgraft in front elongates in front. So, the trade off of doing both could be the frontal view of face casting all the more longer.
Again, I could be wrong on this and your plan might work or I could be right on this and your plan might not work out so well. So, all the more reason you need to cross reference this with the surgeon as to the potential trade-offs.
