BSSO and Gonial Angles

[eastcoastian1]

Interesting article I found that suggests standard BSSO surgery increases/widens the gonial angles a pretty significant amount. This is in contrast to the belief you can only widen the angles (or create the illusion you are) through CCW. Wondering what everyone’s thoughts are:

https://www.semanticscholar.org/paper/Posterior-mandibular-widening-secondary-to-sagittal-Sigaux-Lahon/7ba8b84eeef2c292ef09a21455a049333d5559b7

[Post bimax]

Interesting article I found that suggests standard BSSO surgery increases/widens the gonial angles a pretty significant amount. This is in contrast to the belief you can only widen the angles (or create the illusion you are) through CCW. Wondering what everyone’s thoughts are:

https://www.semanticscholar.org/paper/Posterior-mandibular-widening-secondary-to-sagittal-Sigaux-Lahon/7ba8b84eeef2c292ef09a21455a049333d5559b7

That's very interesting.  It seems to be a property of the BSSO itself since there was no correlation with degree of advancement. I can't understand how this actually happens.  Maybe something to do with the way the mandible is split?

[TheDancingQueen]

Is this a good thing? Shouldn't the angle be more narrow than wide?

[eastcoastian1]

Is this a good thing? Shouldn't the angle be more narrow than wide?

Haha, not if you’re a guy. Many guys get surgery with a huge priority on getting more width

[eastcoastian1]

That's very interesting.  It seems to be a property of the BSSO itself since there was no correlation with degree of advancement. I can't understand how this actually happens.  Maybe something to do with the way the mandible is split?

I’m confused a bit as well because unless you’re doing CCW, the back of the mandible/ramus isn’t being touched at all, no? Don’t understand how you can get an increase of width here.

Futhermore, there are plenty of people where BSSO alone actually makes their angle MORE narrow and face longer. Why did they get an increase in width if no CCW or impaction?

[Post bimax]

I’m confused a bit as well because unless you’re doing CCW, the back of the mandible/ramus isn’t being touched at all, no? Don’t understand how you can get an increase of width here.

Actually I can see how this might happen. Look at the video here (not for faint hearted), specifically the model at 5:57.

https://www.youtube.com/watch?v=Td9Hx6N1YiY&has_verified=1

You can see that the BSSO in this cut is split down the body of the mandible.  This means that after the cut, the 'outer' half of the mandible including the ramus is totally separated from the rest of the jaw.  Then, the inner body of the jaw is advanced.  But you'll notice that since the mandible is naturally flared at the back, advancing the 'inner' portion of the mandible will cause the 'outer' portion to flare out as well.  This might be what is pushing the ramus outward.

It looks like this would also put some torque on the TMJ which might be part of why some people have TMJ pain after a BSSO, although this usually resolves.

[eastcoastian1]

Actually I can see how this might happen. Look at the video here (not for faint hearted), specifically the model at 5:57.

https://www.youtube.com/watch?v=Td9Hx6N1YiY&has_verified=1

You can see that the BSSO in this cut is split down the body of the mandible.  This means that after the cut, the 'outer' half of the mandible including the ramus is totally separated from the rest of the jaw.  Then, the inner body of the jaw is advanced.  But you'll notice that since the mandible is naturally flared at the back, advancing the 'inner' portion of the mandible will cause the 'outer' portion to flare out as well.  This might be what is pushing the ramus outward.

It looks like this would also put some torque on the TMJ which might be part of why some people have TMJ pain after a BSSO, although this usually resolves.

If thats the case, wouldn't everyone getting BSSO have wider, squarer jaws? There are plenty who's angles don't change at all or actually become more narrow.

I do get what you're saying though. Theres actually an image in the paper that reflects this as well, showing the space space between the separated bone and the advancement itself causing the angles to "push out" more. 

[Post bimax]

If thats the case, wouldn't everyone getting BSSO have wider, squarer jaws? There are plenty who's angles don't change at all or actually become more narrow.

I do get what you're saying though. Theres actually an image in the paper that reflects this as well, showing the space space between the separated bone and the advancement itself causing the angles to "push out" more.

Presumably a BSSO setback to correct a class 3 skeletal relationship would have the opposite effect.  Also, it would depend on how flared your jaw is naturally.  I imagine different jaw shapes would yield different results.

What I don't understand is why there is no relationship between the degree of advancement and the amount of flaring.

[TheDancingQueen]

It looks like this would also put some torque on the TMJ which might be part of why some people have TMJ pain after a BSSO, although this usually resolves.

Are there any cases of people having permanent TMJ pain after a BSSO? Everyday I learn something new that stresses me out about getting a BSSO lol.

[eastcoastian1]

Presumably a BSSO setback to correct a class 3 skeletal relationship would have the opposite effect.  Also, it would depend on how flared your jaw is naturally.  I imagine different jaw shapes would yield different results.

What I don't understand is why there is no relationship between the degree of advancement and the amount of flaring.

Funny you say that. I feel that Class 3’s with long faces that get set back usually have the best outcomes when it comes to making the jaws more square.

[Post bimax]

Are there any cases of people having permanent TMJ pain after a BSSO? Everyday I learn something new that stresses me out about getting a BSSO lol.

There's always a risk of something like this.  For most people, it's just tender for a few months as the bones and muscles adjust.  There are no guarantees in JS and plenty of pitfalls.

[Post bimax]

Funny you say that. I feel that Class 3’s with long faces that get set back usually have the best outcomes when it comes to making the jaws more square.

There's a difference between perception and actual measurements.  It could be that shortening the face with the BSSO simply improves the facial width-height ratio, thereby increasing perceived squareness/width of the gonials, even if they are closer in absolute terms.  The 'pushing out' theory is just a bit of speculation by me so I don't actually know for sure what is going on here.

[kavan]

I took a look at the link Post Bimax gave at time frame 5:57 where they show the bone model.

It's because with the BSSO cut a GAP is left and with healing, bone growth fills in this gap. Looks like a 2mm gap on each side can be there which would explain the statistical average of 4mm bigonial increase in width. The observed much wider bigonial distance (BGD) they cite in the EARLY stages of healing (like a day after or so) would be swelling of bone while it's initiating the inflammatory/healing response.

Keep in mind the observation was made via direct X-ray focus on the posterior mandible. Looking at someones (outer) face post surgery or self assessments of posterior jaw looking more narrow most likely could kick up optical illusion based on RELATIVE distances. That's because when the anterior part of the mandible is moved forward, that area will look WIDER because it's being brought MORE into the foreground. Because that is so, the width increase at posterior mandible might not look visually obvious or could look relatively more narrow depending on how much forward the anterior mandible is advanced.

[kavan]

Presumably a BSSO setback to correct a class 3 skeletal relationship would have the opposite effect.  Also, it would depend on how flared your jaw is naturally.  I imagine different jaw shapes would yield different results.

What I don't understand is why there is no relationship between the degree of advancement and the amount of flaring.

From the bone model you cited and also from the study citing that increase of BGD is NOT a function of advancement, I would posit that the GAP from the BSSO cut is constant. Keep in mind it's the anterior mandible being displaced.

[Post bimax]

From the bone model you cited and also from the study citing that increase of BGD is NOT a function of advancement, I would posit that the GAP from the BSSO cut is constant. Keep in mind it's the anterior mandible being displaced.

Yeah your explanation makes sense, I see the gap now.  I'm curious as to why/how the gap is created, and if it is maintained along the entire cut.  It looks to me like the 'inner' and 'outer' pieces of the mandible are in contact along the anterior vertical portion of the cut.

Basically I'm wondering how the bigonial distance increases unless the 'inner' mandible piece is pushing on the 'outer' piece along the vertical portion of the cut.  Otherwise it seems like the surgeon would need to manually torque the 'outer' piece and fix it with screws.