Thank you again, Kevan, for your interesting insights.
Before I go over them one by one: you say that my MPA is not steep. What is this based on? I’ve noticed that under the Ceph Measurement “Upper Occlusal Plane - Lat. Incl.” both surgeons use different pre-op values: the first says it measures 9.41°, while the second states 7.09°. I thought I understood that an angle between 8 and 10 is considered ideal. So at 9.41, I’m still on the higher end of that range, no? And would a slight CCW rotation of 1 degree perhaps be useful?
Unscientifically speaking, I do have the feeling that when I hold my head as ‘correctly’ upright as possible (Frankfort Horizontal Plane), my occlusal plane seems to point quite strongly downward toward the ground.
From what I understood CCW - if a possibility - is a way to get some advancement, better lip support, etc. without much of the negatives (e.g. longer philtrum, nasal flaring, etc.). If CCW really isn't an option for me then I will opt for a smaller maxille advancement (e.g. 3mm) to find the middle ground between aesthetic advantages and possible disadvantages. Although 6mm still doesn't look like a lot on paper..
The basis of an angle measurement in maxfax relates to basic geometrical relationships and measures thereof and not 'feelings'.
The first part of your question about the MPA (or any angle) is based on elementary geometry, for example a line can be drawn from 2 points and an angle can be formed from 2 lines. If one line is a diagonal one and the other line is a horizontal one, the angle of inclination that the diagonal line has with the horizont can be measured and the angle of inclination the diagonal line has with the horizont is the same thing as how many degrees it's rotated away from the horizont. Although a diagonal line is rotated away via the angle of inclination it has with (for example) the horizon, movement along the line is not a 'rotation'. Movement along it is a 'translation'; a combination of horizontal and vertical displacements like walking up a hill is which isn't a rotation.
The second part of your question as it applies to an angle called the MPA depends on it being intuitively obvious that the border of the mandible is oriented along a diagonal line AND a diagonal line is formed by connecting 2 points along the border of the mandible. From there, the angle of inclination with the horizontal can be measured. It's just a matter of knowing which points to use along the line of the mandible.
The 3rd part of your question as it applies to the spectrum or range of MPAs; whether or not the MPA is closer to the range of being 'steep' (high angle) or closer to the range of being 'flat' (low angle) is cross referenced with normative values of MPAs. An MPA can be within the range of the spectrum as far as normative ranges go but closer to either low or high angle. When an MPA is closer to the low angle spectrum, CCW-r (via posterior downgraft) would lower the angle.
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Maxfax relationships have quite a number of angle measures. As to the OP, doing a search for ranges of OPs calls up the following entry: [The normal angle of the occlusal plane, in relation to the Frankfort horizontal plane, is generally considered to be 8° ± 4°. This means that a normal occlusal plane typically forms an angle of between 4° and 12° with the Frankfort horizontal.] What can be concluded from that is that an OP within the range of 7.09 deg and 9.41 is also within the range of 4 deg and 12 deg. So, not steep enough to justify altering via CCW-r.
Although CCW-r via posterior downgraft is a way to get extra lower jaw advancement than one would otherwise get without it, it is also one aimed at MINIMIZING maxillary advancement so a request for extra maxillary advancement runs counter to that. As to the MPA and OP, it is the (extent of) DEVIATIONS from normative values for both the OP and the MPA that justify it.
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