8 The Nose As an Aerodynamic Body
8.1 The Physics of the Nose and the Patient′s Subjective Complaints
The nose shares several aerodynamic properties in common with a turbine or jet engine. A comparison will aid in understanding the nose as an aerodynamic body and how its function can be improved by surgery ( Fig. 8.1 ).
In a jet engine, a gas or fluid stream is drawn into the front intake, is compressed, and then slowed in a diffusor before being accelerated and redirected. Air flowing through the nose is directed and modified in a similar way.
In the nasal vestibule, the constricted lumen accelerates and laminates the inspired air on its way to the internal orifice. The concave internal orifice acts like a concave lens in optics: the air stream is dispersed and directed into the expanding anterior nasal cavity, which is analogous to a diffusor. 1 Next it enters the functional cavity of the nose with the nasal turbinates. Aerodynamically, this cavity functions as a partitioned space that can perform its tasks of warming, filtering, and humidification (also olfaction) only when all portions of the cavity are reached by a slowed, turbulent stream of air. On entering the posterior nasal cavity, the now-conditioned air is accelerated and becomes less turbulent. 2 It then reaches the convex-shaped choana, which further converges the flow so that it can be delivered to the lower airways with the least possible resistance.
Aside from functional diagnostic tests, it is important in everyday practice to have an algorithm that can “interrogate” specific functional and morphologic structures and can actually select patients who require surgical modification of one or more functional problem zones. 3 In a study on the long-term results of septoplasties, Mlynski found that nasal breathing was improved in only 68% of the patients operated. 4 One approach to solving this problem may lie in a more complex consideration of the nose as an aerodynamic body.
8.2 Evaluating Dynamic Functional Elements
The dynamic functional elements discussed below play a role in the pathogenesis of impaired nasal breathing and should be evaluated before every operation. 4 , 5
8.2.1 Nasal Septum
All septal deviations are not the same. Small curves and pronounced ridges or spurs may have no functional significance. Sites of narrowing in the nasal airway have highly variable functional (aerodynamic) implications. From a functional standpoint, a centered septum is more important than a straight septum. 4 The height of the septum is also of functional importance ( Fig. 8.2 ).
8.2.2 Nasolabial Angle
The curvature of the nasal vestibule is a critical factor for optimum nasal airflow. It must be between 90° and 100°. A simple test is to rotate the nasal tip upward to see whether this maneuver improves nasal breathing. The correction of tip ptosis during septoplasty of the aging nose is appropriate for functional reasons if the nasolabial angle is less than 90°. Angles greater than 100° lead to decreased airflow through the upper functional space and impaired function.
8.2.3 Inferior Turbinates
The nasal turbinates form the air passages and functional space of the nasal cavity and provide the necessary morphology for normal airflow. Along with the septal cavernous tissue, they increase flow resistance and turbulence to promote greater contact between the air and mucosa. Overresection of the inferior turbinates, for example, cause flow to follow the path of least resistance and pass through the nose much too rapidly without adequate mucosal contact. The other portions of the nose are no longer ventilated, and important components of nasal respiratory function are lost. Consequently, any resection of the inferior turbinates should be done very sparingly. Overresections are irreparable. The goal of functional nasal surgery is not to maximize the cross-sectional area of the nasal airway, but to promote an optimum flow distribution ( Fig. 8.3 ).
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