The paranasal sinuses assist with several mechanical and physiological functions. These include humidifying inspired air, lightening the skull, regulating intranasal pressure, enhancing olfaction and adding resonance to the voice. Additionally, it has been postulated that the sinuses produce mucous that is free of environmental contaminants. This mucus is then secreted into the superior and middle meatus in order to mitigate the contamination present in the mucous that is in contact with the air and its contaminants.
These functions would be compromised in instances where the paranasal sinuses are filled with fluid and are unable to be drained. The anterior paranasal sinuses (anterior ethmoid air cells, maxillary, and frontal) drain their contents by way of the ostiomeatal complex (unit). This cluster of ostia and their corresponding drainage channels are located on the lateral wall of the nasal cavity, deep to the middle concha. Contents of the anterior paranasal sinuses that enter the ostiomeatal complex will subsequently drain into the middle meatus before travelling into the nasopharynx. This article will evaluate the boundaries, composition and anatomical variations of the ostiomeatal complex.
When the middle concha is reflected superiorly, the borders and components can be more readily appreciated. Anteriorly, the unit is in open communication with the anterior nasal cavity. The agger nasi and atrium of the middle meatus also have an anterosuperior, and anterior (respectively) relations to the complex. The superior aspect of the ostiomeatal complex is limited by the basal lamella. The basal lamella is a thin piece of lamina papyracea that attaches the middle turbinate to the lateral wall of the nasal cavity. It continues posteriorly along the length of the middle turbinate and consequently forms the posterior border of the space. Inferiorly, the ostiomeatal unit has an open communication with the middle meatus. The drained contents of the unit will travel in the meatus along the superior border of the inferior concha, which forms the floor of the middle meatus. Medially, the complex is limited by the body of the middle concha. Laterally, the lamina papyracea of the ethmoid sinuses borders the complex.
The most striking feature of the ostiomeatal complex is the laterally protruding structure spanning the middle third of the middle concha, called the bulla ethmoidalis (ethmoid bulla). The convexed bulge is created by the presence of the middle ethmoidal air cells and as a result, it contains an ostium that drains this space.
Located anteroinferiorly to the ethmoid bulla, and posterior to the uncinate process, is a curved furrow called the hiatus semilunaris. It contains the maxillary ostium and an accessory maxillary ostium towards the inferior aspect of the groove and the ostia of the anterior ethmoidal air cells anteriorly. This groove is continuous with the ethmoidal infundibulum, which courses superiorly and deep to the anterior aspect of the middle concha and lateral to the orbital plate of the ethmoid bone. The frontal sinus is therefore able to communicate with, and drain its contents into the ostiomeatal complex because of the connection between its frontonasal duct (recess) and the ethmoidal infundibulum. The uncinate process, which forms the anterior and medial limitation of the hiatus semilunaris and the ethmoidal infundibulum (respectively), is a thin, crescent shaped, bony leaflet that emerges from the ethmoidal labyrinth posteroinferiorly.
Several types of anatomical variations, of either pre-developmental or post-developmental aetiologies, have been observed in the ostiomeatal complex. The structures most frequently affected include the uncinate process, nasal septum and middle turbinate.
The middle turbinate may be congenitally filled with air (pneumatized). Clinicians refer to this variation as a concha bullosa. Also, there are also instances where the middle turbinate develops in a sigmoid shape – with a superior convexity, and an inferior concavity. This condition is referred to as a paradoxical middle turbinate.
The uncinate process may deviate from its typical posteroinferior course and arise in a medial fashion that obstructs the middle meatus and the ethmoid infundibulum. In cases where hypoplastic maxillary sinuses are present, lateral elevation of the uncinate process until it fuses with the medial orbital wall can impede the integrity of the orbit.
Congenital or traumatic deviation of the nasal septum can result in compression of the middle meatus and subsequent obstruction of the drainage pathway. Complications of these variations arise when they obstruct the drainage pathway of the respective paranasal sinuses. Pneumatization of respective areas has been associated with reduced ventilation to the sinuses. As a result, stasis of the mucous within the sinus can result in sinopathies such as sinusitis and rhinosinusitis.