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.
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.
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.
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.
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.
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.
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.
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 …
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.
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.
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.
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.
In the slice view If we compare before and after virtual implants, we also do not see any significant difference in airflow velocity.
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.
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.
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.
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.
Same thing for the left side.
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.
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.
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.
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.
I designed a simplified 3d model of a nasal cavity (just one side) to test different scenarios such as adding an implant. You can see that I grouped the superior turbinate and the middle turbinate in order to simplify the model. I need a simple model because it is better to make future geometry change.
The model is simpler but it respects the cross-sectional area of my real nasal cavity, you can see my post about the cross-sectional area measurements that I have written here. Also as you can see it simulates a pretty large inferior turbinectomy and a total turbinectomy in the middle section area. For the flow I took 7.5l/min, it is the respiratory flow at rest. Here are the first results I get...
The airflow in the area of the absent inferior turbinate swirls and is very slow (less than 0.6m/s) unlike the airflow in the area of the middle turbinate (about 3m/s). This is not very surprising, it can be seen in quite a few CFD studies on empty nose syndrome. The speed values also match which means that my 3d model and the flow rate value taken are pretty accurate.
In the coronal section view, we can see that the velocity is higher between the middle turbinate and the septum but also in the zone at the bottom of the middle turbinate.
Now what I can say about these results is that too low airflow velocity greatly reduces sensations in the area of the absent inferior turbinate (inferior meatus). This is one of the big problems of the ENS.
In future articles, I will simulate an implant in the inferior meatus zone to see the change in airflow.
First of all, I found this study that compares the cross-sectional area of a control group with an ENS group. I found this interesting so I do the same for myself, and here are the results.
We can see that there is no doubt that my nasal cavity (in blue) is more empty than the control group. The information about the control group is pulled from the study cited at the beginning of this article.
At a distance of 20 mm from the nostrils, my nasal cavity is more empty than the control group but not too much. But after 40 mm the value explodes and it reaches up to 2.5 times the control value at a distance of 50 mm to the nostrils. So clearly I need more volume from 40mm to 70mm.
I think it is better to have a stable cross-sectional area all along the nasal cavity even if a little higher than normal rather than reaching the normal value at the beginning of the nasal cavity and then leaving a big hole afterward. Of course, the ideal would be to be able to reach the value of the control group all along the nasal cavity.
I made a second graph that compares the cross-sectional area from the right and left nasal cavity in order to see the imbalance.
Here we can see that the area until 40 mm from the nostrils of the right nasal cavity is pretty stable thanks to my lateral wall implant. After this value, the area explodes and reaches more than 200 mm². Which is more than 2 fold the control group. I don't have the cross-sectional area data from the right and left nasal cavities of the control group, only the two together. But we can speculate that the nasal cavities of the control groups are balanced so about 100 mm² each. The left side is worse, it reaches almost 300 mm² at a distance of 50 mm, which is almost 3 times the control value! However, this is the side where I have almost 50% of the turbinate left, but it is also the side where I have never had an implant or injections.
However, although my left nasal cavity is emptier than the right, I don't feel that it is worse. First because on the left I still have 50% of the turbinate while on the right nothing. Thanks to the half of the turbinate still present on the left, the surface of the mucosa is not the same between the two nasal cavities. This is what we will see in this third graph.
We can therefore see that the perimeter of the mucosa is greater on the left than on the right. We are talking about perimeter here because these are slices in 2 dimensions but it reflects the value of the surface of the mucosa. So we can clearly see on the graph that the surface of the mucosa is higher on the left than on the right thanks to the remaining half of the turbinate. So although my left side is emptier than the right I don't feel it is worse.
Note that the larger the mucosa surface, the more blood vessels, TRPM8 sensors there are, and the larger the exchange surface between the air and the mucosa.
To conclude this article, I think that the cross-sectional area and the surface of the mucosa are interesting information that can help to know where to place an implant. And also it's I think interesting to quantify the lack of volume.
Now that we have seen how to 3d print the nasal cavity in slices we need parts to stack/de-stack easily the slices. So I have designed parts called "slices holders" which will be also been 3d printed in plastic. The slices will be glued to the "slices holders"
I I'll take pictures of the set when it's made ...
Go to https://www.prusa3d.com/, click on software and select your OS. Install the prusa slicer.
This software is a "slicer", it is used to prepare the 3d model to be 3d printed, to transform the STL file into a G-code readable for the 3d printer.
But in our case, we will just use this software to edit the 3d model. But of course, if you have a prusa 3d printer you will also use this software to 3d print the slices.
So open the .STL file that you have just saved with SLICER.
Click on file, then import STL/OBJ etc ...
Click on rotate and turn the model 90° to put it vertically.
Once it done click on cut and set the thickness you want, I set 10 mm.
Finally click on perform cut.
You will obtain that.
Now select the model and cut a slice again, organize the slices as you want but it is better to do it in the cut order.
Right click on each slices one by one and export as STL.
And that's it, now you have all the slices ready to be 3d printed where you want.
You can find a friend or a member of your familly which have a 3d printer or use an onlise service like shapeways or sculpteo for example.
An example of a 3d printed nasal cavity.
In this tutorial, I will explain how to turn your DICOM data pulled from your CT-SCAN into a 3d printable file.
Go to https://www.slicer.org/ download the software and install it.
Open it and click on import DICOM files, search the directory of your DICOM files and click on import.
Click on show DICOM database, click on your name and finally double click on the slices you are interested in, here sinus. The slices will load.
Now click on the scrolling menu and choose volume rendering.
Click on display ROI, white lines will appear.
You can click on the white dot next to Volume, your head will appear in 3d. But it is optional.
You also can select a preset, here I choose lung, like that you can see inside the nasal cavity.
Now crop the desired zone using the color dot.
When you are happy with your cropped zone click on the little magnifying glass, then write crop, click on crop volume and finally switch to module. After that click on the apply button. Wait a moment it can take a while.
Now return to the Volume rendering worshop and click on display ROI in order to hide white lines
Once it's done, click again on the little magnifying glass and write editor, then click on switch to module.
A little window will appear, click OK.
Click on the threshold effect button.
The slices will turn green, now you need to adjust the left cursor in order to paint all your soft tissues green.
I found that generally the good numbers are between -200 and -400 but it may depend on CT-SCAN.
Finally, click ok the apply button.
It's almost over, we've done the biggest part.
Now click on data in the menu.
Right-click on your "name files" cropped-label and click on Convert labelmap to segmentation mode.
A line tissue will appear, again right-click on it and Export visible segment to models.
Wait a moment, it can take a while, a line cropped-label-segmentation-models will appear and also your 3d model on the right window.
Click on file, then save or ctrl + s.
In the window, uncheck all boxes except tissue.
Click on Poly data (.vtk) and choose .STL.
Click on change directory for selected files and choose a directory you want to save the 3d model file.
Finally click on save and voila, well done !
This .STL file is a 3d file that can be used to 3d print the model directly, but as you can imagine it is not useful to 3d print the model in this state, we will see nothing. We need to see inside the nasal cavity.
That is why in the next tutorial I will explain how to cut the model into slices in order to see inside. The slices will be stackable.
The first one is hydrolized collagen, some studies tend to prove that it can improve skin health, hydration, thickness etc ... Exemple this study. Hydrolyzed collagen is a type of collagen with low molecular weight in order to be better absorbed by the body and reach the skin and other organs. So I ask myself if this product can help skin, maybe it can help the mucosa too because mucosa is made of collagen. So I took 5g/day for two months which is a dosage often seen in the studies. After one week I noticed some skin improvement, a little smoother, but no airflow sensation improvement. I cannot say if my mucosa was better after the two months or not but I can say that I feel no difference in sensation and hydration.
The second one is Hyaluronic Acid, yes the same thing that can be injected into our turbinates or lateral wall in order to regrow it temporarily. Oral HA supplementation seems to also improve skin health, hydration, thickness, reduce wrinkles etc ... Again, some studies tend to prove that, but there are small studies. The common dosage seems to be between 120 mg/day. I have not yet tried this product but maybe it is worth a try...
I try to know how to rebuild the nasal cavity after an inferior turbinectomy, where to add volume in priority ?
Some ENTs think that the head of the turbinates is the most important because it is where it's easiest to add resistance. And they often think that nasal resistance is the most important parameter in the ENS.
In this study, Dr Nayak and others researchers/doctors prove that it is possible to reduce significantly the symptoms of ENS without adding resistance. He implanted cartilage in the inferior meatus (the place of the inferior turbinate) and they have seen with CFD that the airflow is better redistributed between middle meatus and inferior meatus.
This other study shows that without inferior turbinate the most part of the airflow is redirected in the middle turbinate compared to healthy subjects. And they found a correlation between peak wall shear stress in the inferior meatus and airflow feeling sensation. Peak wall shear stress is the quantity of friction of air on the mucosa.
What we can say with all of this data? I think that it is useless and may be harmful to just add volume to the beginning of the nasal cavity ( head of turbinates) because that will increase wall shear stress just at the beginning of the nasal cavity and not all along. And that will not solve the redistribution airflow problem. Without adding volume all along the inferior meatus the airflow will stay in the middle meatus and the sensation will remain bad.
The solution is to rebuild the nasal cavity like before turbinectomy, with the same distribution of the volume. It can be with adding volume in the remaining inferior turbinate if it is possible or adding cartilage on the lateral wall. But you have understood it is very important to add volume not just at the beginning of the nasal cavity but all along like a healthy nose. In the graph below, we can see that a healthy nose has a stable cross-sectional area all along the nasal cavity, but of course, it is not the case with an inferior turbinectomy. The reconstruction of the nasal cavity must tend towards these values.