Cranial Base Flexure

[Alyssa Domico]

I agree that cranial base flexure is a point I am struggling with too. I feel like I do understand it way more than I did in residency – I knew cranial base flexure was an important consideration that could affect reference points and interpretation, but I don’t think I ever really knew how to apply it until this course. So thank you to all of those involved in making this course, I’ve really enjoyed it so far and it’s been a wonderful learning experience!

To continue with Samantha’s discussion and points, I’ve noticed as I’m practicing that I’m doing the calculations and adjustments correctly, but just not applying it 100% correctly. I saw a previous post from Feb 2023 of an example with a “flat cranial base,” and Carter had mentioned that we needed to take that one step further and use AN Perp instead of relying on our SNA/SNB adjustments. I am now finding that I’m doing all of the adjustments and calculations correctly, but I’m getting tunnel vision in using those adjustments and values. There’s been multiple practice cases in which I “do the math” right, but I put the wrong interpretation and have trusted the numbers more than just my visual interpretation. I understand we can’t always rely on cephalometrics and numbers for sure, though.

To clarify, from what I understand, cranial base flexure/slope is the difference between SN and FH (which should be around 7 to 8 degrees as Samantha mentioned). You can find this difference by subtracting the SN-MP value and the FMA value. Anything over 7-8 degrees is “steep” cranial base, and anything under 7-8 degrees is “flat” cranial base. If the value deems it is steep, you add the difference (between the SN-FH and the norm of 7-8 degrees) to SNA/SNB. If it is flat/low, you subtract the difference from SNA/SNB. Am I interpreting this correctly?

Thank you again for all of your help!!

[Alyssa Domico]

I thought two ABO sample cases were good examples of when “the math was right” but my interpretation was wrong. They are from the Sample Case Part II PPT.

Sample Case Part II – Sample Case #6 and Sample Case #10.

Sample Case 6, Question 1 : SNA 90, SNB 80, SN-MP 42, MPA 34 “Identify the skeletal findings”

They listed protrusive maxilla and retrusive mandible. I put protrusive maxilla only and did not include retrusive mandible. SN-FH was 8, so that’s right around normal so I didn’t make any adjustments.

Sample Case 10, Question 1: SNA 87, SNB 80, SN-MP 14, MP 8 “Identify the skeletal abnormalities”

They listed retrusive mandible. I had put protrusive maxilla, which wasn’t an answer on this case. I figured since SN-FH was 6, you could maybe subtract a degree or two from SNA and SNB, but then that would give SNA 85 and SNB 78. Visually, the mandible looked retrusive to me, but I got a little caught up in the numbers I think.

Are these interpretations affected by the fact that the patient in Sample Case 6 has a steep MPA, and the patient in Sample Case 10 has a low MPA? Thanks again!

[Chad Carter (Course Director)]

Your math is correct. And absolutely trust your eyes!

[Chad Carter (Course Director)]

Samantha,

To your original question we are not looking at the FMA angle or the SN-MP angle but using those to find the SN-FH so that we have a value for the cranial base. Does that help?

So your example of a normal SN-MP but a high FMA = would lead to a low SN-FH angle and therefore a flat cranial base. A high SN-MP and a low FMA would show a steep cranial base.

[Samantha Karloff]

I’m sorry, but this doesn’t quite answer my above question. I am curious if there are EVER times where you do not adjust the SNA/SNB/ANB even with a steep or flat cranial base when evaluating a lateral ceph. My specific question was if you would NOT adjust the SNA/SNB/ANB when their reference plane (SN-MP) was within normal range, while the FMA is NOT (i.e. you would instead adjust the other ceph numbers that base their maxillary or mandibular position off of FMA instead of SN-MP). I’m asking this, because as Alyssa mentioned above, there have been example cases where the cranial base is either flat or steep and the SN-MP has been within normal range but FMA has not, and the answer provided has shown that the SNA/SNB/ANB was NOT adjusted and I’m just trying to figure out why.

I’m just trying to make sure I don’t automatically jump to adjusting numbers or measurements if I shouldn’t be based on a measured cranial base angle.

Thanks so much in advance, and let me know if this doesn’t make any sense.

[Chad Carter (Course Director)]

Samantha,

First, trust looking at the ceph and your initial impression. Taking time to do a cranial base calculation may not always be worth the squeeze – for example you can easily identify they are Class II or Class III.

Do you have a sample ceph of what you are seeing? I’d like to look at it to make sure I can help clarify my thoughts.

Now we are all clear on how cranial base is calculated.

The three planes SN (the cranial base), Franfort Horizontal, and Mandibular plane contribute to make the three angles. So if FMA is giving you a “normal” number and SN-MP is “off” then that’s a clue for a cranial base calculation. Likewise if SN-MP is “off” but FMA is “normal” that would also be a clue to check the cranial base. So if either SN-MP or FMA are out of norms that helps us identify a steep or shallow cranial base. If its just a degree or two off then I don’t find it’s worth my time doing calculations but if it’s a cranial base of 15 degrees then the calculations could “help” me identify which jaw is at fault but I’ll also still be trusting my eye’s initial impression.