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Introduction

First of all, what is Flowgy? Flowgy is a software dedicated for CFD of the nasal cavity with tools for virtual surgery. In this article, I will compare the results that I got with my industrial software (Onshape and Simscale) and the results with Flowgy. I would like to thank Pr Manuel Burgos for loaning me the software and answering all my questions.

Simscale - streamlines - left side
Flowgy® www.flowgy.com - streamlines - left side

I set the max speed at 3,25 m/s in both software. We can see the same thing, most of the airflow passes through the middle meatus. Note that on Flowgy I just showed the airflow in the left nasal cavity to see more clearly what is going on.

Cuts - Velocity

Simscale - cuts view - max velocity 3,5 m/s (red)
Flowgy® www.flowgy.com - cuts view - max velocity 2,5m/s (red)

The main difference in the two views is the max speed, it is set at 3,5 m/s in Simscale and 2,5 m/s in Flowgy because I was not able to set 3,5m/s in Flowgy. But we can roughly see the same thing, the speed of the airflow is very lower in the inferior meatus. It's reassuring to see the same thing because it means that the 3d model I drew of my nasal cavity was quite accurate.

Wall Shear Stress

Now the really interesting thing in Flowgy is that there are other data available, one of them that is particularly interesting in the ENS context is the Wall Shear Stress (WSS). I already mentioned WSS in other articles. I will explain a bit more about what WSS is and why it is particularly interesting data in the ENS context. WSS is the pressure exerted by the airflow to the walls, the unit is the Pascal (Pa). The higher the gradient of velocity near the wall the higher the WSS. It is an important value because it is the WSS that gives the airflow sensation.

Flowgy® www.flowgy.com - left side - Wall Shear Stress - max pressure 1 Pa (red)
Flowgy® www.flowgy.com - cuts view - Wall Shear Stress - max pressure 1 Pa (red)

The pressure higher than 1 Pa is represented in red. We can see that the max WSS is located mostly in the anterior part and also in the middle meatus which corresponds to the velocity of the airflow. So we want to have a better distribution of the WSS between the middle and inferior meats. This is what I will try to do with virtual implants in the next article.

Airflow humidity

Flowgy® www.flowgy.com - cuts view - mini humidity 70% (blue)

In this view is represented the humidity of the airflow, we can of course see that it increases as it passes through the nasal cavity. We can see that part of the airflow is at 92% humidity in the last cut. The values are certainly not very precise. By default in the simulation parameters, the mucosa is at 100% humidity which is a little optimistic given that we often have the mucosa dry because of the ENS. At the same time, the value of the inspired air is set to 20% humidity which is pessimistic because often the outside air is more humid than this. What will be interesting is the comparison before/after the virtual implants so the input parameters are not so important.

Airflow temperature

Flowgy® www.flowgy.com - cuts view - mini temperature 300°K (blue)

Same thing here with the temperature of the airflow, in the simulation parameters the mucosa is set at 309,65°K which is 36,65°C. The inhaled air is set at 21,15°C. The temperature of the mucosa is also maybe a little optimistic in the context of ENS. I think the entrance to the nose is colder than that due to the lack of mucosa. But I can't quantify it and it will depend on the aggressiveness of the turbinectomy. In this illustration, the minimum temperature is set at 300°K (27°C) in blue.

Conclusion, what’s next?

Flowgy gives us really interesting data which will be valuable to compare before and after the addition of virtual implants. I'll talk about it in the next article, but I think it's no longer possible to do nose surgery in 2022 without first having done a virtual surgery to see what it will give in terms of airflow, WSS, etc …
Now I will try to design virtual implants in order to improve airflow distribution, WSS distribution, etc …

Introduction

As planned I designed virtual implants in my nasal cavity in order to improve airflow distribution and decrease cross-sectional area to tend toward a healthy nose.

Virtual implants placement

Virtual implants

In red are the virtual implants that I designed, you can see that I added a lateral wall implant behind the existing one on the right side. It measures approximately 30 mm long, 10mm high, and 5mm thick.
On the other side, we can say that it is two implants, a floor, and a lateral wall implant.
The floor implant measures approximately 35 mm long, 6mm wide, and 3 mm thick.
And the lateral wall implant is 35 mm long, 10 mm high, and 2 mm thick.
So I don't know if it is even possible to add an implant behind the existing one but I think that it is a good idea to keep the cross-sectional area constant all along the right side.
On the left side, I added two implants which aim to reduce the space between the existing inferior turbinates and the walls.

Cross-sectional area change

We can see that the change starts at 40 mm from the nostrils until 70 mm. And now we reach a maximum of around 400 mm² instead of 500 mm², but it is still far from a healthy nose. So we can already say that the implants are not big enough. But my first goal was to try to have a better flow distribution and a constant cross-sectional area.

Results

Left side
Right side

We can quickly see that there is almost no change in the airflow distribution and velocity. There is maybe a bit more airflow in the inferior meatus but it is not a crazy change.

Before virtual implant

After virtual implant

Slices

In the slice view If we compare before and after virtual implants, we also do not see any significant difference in airflow velocity.

Before virtual implants
After virtual implant

I made a slice with the same distance from the nostrils of the two 3d models (before and after virtual implants). And I placed a dot to measure the velocity at the same location. After virtual surgery, the velocity is around twice higher 0,26 m/s instead of 0,14 m/s in the post-virtual surgery 3d model. But it is just a little area, we can see the color change and it is twice higher yes but 0,26 m/s is still a very low velocity to the 2 m/s in green for instance. So I speculate that this change is almost not perceptible in terms of airflow feeling.

Conclusion and next step

So what I can say with those data is these virtual implants have almost changed nothing in the velocity and the airflow distribution. It's possible that if these implants were done in reality I would feel almost no improvements. But one thing, ENS symptoms are not just due to airflow problems, there is also the health of the mucosa, the nasal cycle which is almost dead with cutted turbinates and maybe others things to take into account.

What is the next step?

Find better implants placement, also maybe bigger implants to decrease cross-sectional area … I don't know.
Recreate true turbinates… Ok it is not possible in reality.

Introduction

Finally, I was able to make a 3d model of my nasal cavity usable with a CFD software.
For those who don't know what is CFD, it just mean Computational Fluid Dynamics which is the study of fluids, in our case the air.
I took 15 l/min for the flow rate and I did not apply temperature conditions on the walls.
Just for information each nostril has a surface of around 68 mm² and the beguining of my pharynx 420 mm². And you can see my cross-sectional area here, in my precedent article.

Results

Right side view

In this view, the airflow is represented by tiny tubes. We can see that almost all the airflow passes through the middle meatus at a velocity between 1,5 and 2 m/s. On the opposite in the inferior meatus zone, there is almost no airflow and the velocity is very low, less than 0,5 m/s. Near the nostrils, in some areas, the velocity is pretty high around 3 m/s.

Left side view

Same thing for the left side.

Slices view

In this view, the nasal cavity is sliced, at the left of the illustration it is near the nostril, and at the right near the pharynx. We just see the velocity not the quantity of airflow, and we can see the same thing that in the other views, very low velocity in the inferior meatus despite my right nasal wall implant.

Results interpretation

The results are not very surprising, some studies come to the same conclusion about the airflow distribution. In this study for instance they showed the same thing, an important reduction of the airflow in the inferior meatus (25.8% ± 17.6%) . They also showed a reduction in wall shear stress (WSS) in the inferior meatus area, which is the force of friction on the walls of the mucosa. Less shear stress means less airflow sensation. And I'm not sure but less airflow quantity and less velocity mean less wall shear stress. In my simulation, I was not able to measure the wall shear stress due to software limitations but since the velocity and the airflow are very low in the inferior meatus then we can say that the WSS too. And according to this same study, the symptoms "of "suffocation" and "nose feels too open" were also found to be significantly correlated with peak WSS around the inferior turbinate. So in others words less WSS in the inferior meatus zone mean more symptoms of suffocation and the feeling of the nose too open.

Investigation of the abnormal nasal aerodynamics and trigeminal functions among empty nose syndrome patients

Conclusion and next step

Now that we clearly see that one of the problem in ENS is the airflow distribution, I will try to fix that with some virtual implants placed in differents area. The goal of course is to tend towards the same airflow distribution of a heathy nose.

Limitations

Just to be clear, I'm not a reasearcher nor a CFD specialist, so the results can be wrong due to many things. If you see an error in the conditions or the reasoning, please can contact me.

This website is created by Aurélien RUMIANO
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