Discussion
Two accurate nasal cavity and paranasal sinus models were created, and
numerical simulation of nasal ventilation was performed. In our
research, in the simulation that was performed using the nasal cavity
model without the sinuses, the magnitude of the relationship was
correct, but the simulation value tended to be lower than the actual
measurement, although a similar tendency was
observed.15 By including the paranasal sinuses, the
simulation was expected to approach the measured value because it would
be closer to the actual shape; however, this was not the case. This
discrepancy can be attributed to the fact that the airflow in the model
shown in Figure 4 hardly entered the paranasal sinuses.
There are two possible explanations for the observation that Model 1 had
a higher nasal resistance value than did model 2. 1) In Model 1, the
cross-sectional area became smaller than that of Model 2 as it
approached the choana; thus, the flow velocity increased from the
continuity equation. As the flow velocity increased, the pressure
decreased based on Bernoulli’s principle.
2) The difference in the cross-sectional areas of the nasopharynx
compared with the choana was larger in Model 1 than in Model 2. Thus, an
energy loss occurred because of the reverse pressure gradient according
to the rapid expansion of the cross-sectional area; it became a pressure
loss, and the pressure drop became even larger (Fig. 6).
Ventilation into the sinuses remains uncertain, but it is unlikely that
there is no ventilation. In our study, Model 1 exhibited ventilation in
the maxillary and ethmoidal sinuses exclusively. Kumar et al. reported
the air flow in the periods pre- and post-endoscopic sinus surgery using
numerical simulation.16 However, the 3D model reported
by them in pre-surgery did not connect the nasal cavity with the frontal
sinus and maxillary sinus. Therefore, it was too difficult to create an
accurate 3D model. The natural ostium connecting the sinuses and the
nasal cavity may not be accurately created. To date, it has been
reported that the nasal sinus model is used to examine the correlation
between the nasal air permeability measurement and the numerical
simulation, and, although they are similar, they do not completely
match.14 Radulesco et al. reported the measured and
simulated nasal resistance before septoplasty using 22 nasal cavity
models. The perceptions of the patients and the measured and simulation
nasal resistance exhibited strong correlations. However, the measured
and simulation nasal resistance were poorly
correlated.17 Tretiakow et al. investigated the
workflow for creating a 3D model for accurate CFD.18We think that it is necessary to create an accurate nasal sinus model
for more accurate simulation.
Our results suggest that the length of the inferior turbinate and the
cross-sectional area of the nasopharynx and the choana affect the nasal
resistance value. Hariri et al. reported that inferior turbinate weight
loss reduced nasal resistance in 3 of 5 models, whereas it remained
unchanged in the remaining two models. In the two cases without change,
the nasal resistance was affected by factors other than the inferior
turbinate.19 The relationship between the length of
the inferior turbinate and the cross-sectional area encompassing the
nasopharynx, as in this example, may also affect the nasal resistance
value. However, the sample size used in our investigation was 2
volunteers, which was too small to conclude on the effect of nasal
resistance in this context. A larger sample size is needed for further
investigation.