Sinus airflow after FESS using models and fluid dynamics
Reviewed by Suki Ahluwalia

This is a very interesting study from Australia and New Zealand looking at flow of air into the nose and sinuses after FESS surgery. There is plenty of data regarding computational fluid dynamics for preoperative cases but not much for post-surgery. The group studied six patients, who all had their post-FESS sinonasal cavities reconstructed as models. They were not done preoperatively since data exists to suggest that sinus ventilation is negligible preoperatively using the same modelling techniques. Two patients had revision procedures, with more comprehensive dissections after initial mini-FESS and thus eight models were made, making this the largest study. One patient additionally had a modified Lothrop procedure. High quality MRI or CT scans were used at least four months postoperatively to reconstruct the sinonasal cavity using a medical imaging software package and then meshed to produce the models. Two inhalation flow rates were studied at five and 15L/min to mimic resting and mild exercise in adults. A sinus ventilation rate was calculated by creating a surface layer over the neo ostium and looking at flow rates and velocity of air entering and leaving this area. A quantification of total air was also made. General trends in inhalation airflow were observed and split between streams: a superior flow towards the ethmoids, maxillary and sphenoid areas, and an inferior flow stream, along the nasal septum and into the nasopharynx. The superior flow stream was limited by the internal nasal valve and air tended to reach the ethmoid roof and then flow into the sphenoid. Air entering the maxillary sinus went to the posterior superior area of the sinus and then exited inferiorly. Minimal air entered the sinus if a limited FESS was performed and negligible air to the frontal sinus unless a modified Lothrop procedure had been done despite comprehensive FESS. Data examining the airflow through the sinus related to the size of the ostium suggested that a larger ostium promoted more airflow. A mini-MMA resulted in a near zero maxillary sinus ventilation rate but larger MMAs demonstrated a higher ventilation rate 11-40mls/s or 4-16% of total airflow, at a rate of 15L/min. For mega anotrostomies, the ventilation rate was even higher, except for the patient with the modified Lothrop since more air was diverted into the ethmoid via the septal window. A positive linear correlation exists between size of maxillary antrostomy and rate of maxillary inflow, which was better at 15L/min than 5L/min. A comprehensive FESS model showed the ethmoid flow rate to be 4-51ml/s or 2-20% total airflow, however in the Lothrop patient this was much higher due to the septal window created. In another comprehensive FESS the flow was very low, and this contributed towards a finding of a weakly positive correlation between ethmoid opening and ventilation rate. At 5L/min inhalation rate, frontal sinus flow was negligible in all models except the modified Lothrop model. This improved at 15L/min and also unilaterally in another two FESS cases. For models who had undergone wide sphenoidotomies, the ventilation rate of the sphenoid sinus had improved and there was a positive linear correlation. Total airflow entering the frontal and sphenoid sinuses was lowest compared to the higher flow maxillary and ethmoid post-surgery. Airflow for patients who had undergone a mini-FESS was also lower compared to comprehensive FESS.

Quantification of airflow in the sinuses following functional endoscopic sinus surgery.
Siu J, Dong J, Inthavong K, et al.
RHINOLOGY
2020;58(3)257-65.