ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Fig. 4 Streamlines through the ostia at peak loads of inspiration and expiration.
5. DISCUSSION
The most striking effect of having one or more maxillary sinus AOs is a very large
increase in ventilation through the sinus, compared with sinuses having only the NO.
The absence of any AO almost completely prevented air from passing through the
maxillary sinus. There are two possible reasons for the significant increase of air
ventilation through sinus with AO compared to sinus without AO. Firstly, the distance
between AO and middle meatus is much shorter than the distance between NO and
middle meatus, which makes the gas exchange between sinus and nasal airway easier.
Otherwise, in the absence of AO, the uncinate process prevents the airflow in the nasal
cavity from approaching the sinus. Secondly, connected to the middle portion of middle
meatus, the velocity magnitude around the AO is much higher than around NO
connecting to the superior of the middle meatus.
The mechanism of air ventilation through sinus with AO is complex. Our results
showed that it could be the shear force of the airflow in the middle meatus which drives
the air to flow through ostia. One of the evidences is that in sinuses I and II drastic
vortexes appeared near AOs as a sign of shear driven flow 7 . In addition, the streamlines
were parallel to the cross-sections of AOs, while perpendicular to the cross-sections of
NOs. Therefore it might be the shear force of the airflow in the middle meatus that
induced vortexes near AOs and promoted sinus ventilation. This effect could be more
clearly observed in an idealized model of sinus created by Hood et al. 8 .
An increase beyond two ostia is not necessarily associated with further increased flow,
since the flow through sinus I (with two AOs) was comparable to flow through sinus II
(with one AO). This increased ventilation of sinuses with AOs is complex. Under high
flow conditions mimicking nose blowing, in sinuses II, the sinus flow rate increased,
compared to the same sinus at 15 L/min. Sinus flow rates remained the same in sinus I,
with three ostia, under both high and low nasal flow conditions. Our simulations also
show small transient airflow reversals at both the beginning and end of respiratory
phases, during high flow conditions in sinus I, and at both high and low flow conditions
in sinus II. This finding could be caused by the inertial of air.
The sinuses differed in the direction of airflow with respiratory phase. Sinus I (two
AOs) exhibited flow reversal, with the flow through the NO moving in the same
direction as the gross nasal flow for both inspiration and expiration while sinus II (one
AO) showed the opposite, with flow through the AO always moving into the sinus,
despite the direction of nasal flow. Similar patterns of mucus flow were observed in
recurrent sinusitis patients, where mucus entered through AO and exited through NO 3 .
Also, mucus transport speed is reported to be proportional to sinus airflow rate 9 . With
only two examples, it is not possible to extrapolate to all multi-ostial sinuses, but the
present study has indicated that the presence of more than one ostium can radically
change sinus ventilation, and this could affect sinus function, mucus drainage, and
susceptibility to disease. Considering the reported significant prevalence of AO, there
will need to be systematic investigation of the influences of AO on sinus pathologies.