Opening of sphenoidal sinus

The opening of the sphenoidal sinus (Sphenoid sinus ostium; Natural ostium) is a small, round or oval aperture located on the anterior wall (face) of the sphenoid sinus that provides the natural communication between the sphenoid sinus cavity and the nasal cavity. It opens into the sphenoethmoidal recess, a narrow space situated posterosuperior to the superior turbinate, thereby draining into the region of the superior meatus of the nasal cavity.

Morphometric characteristics

Each sphenoid sinus typically possesses a single natural ostium. CT-based studies report mean horizontal and vertical diameters of approximately 1.25 mm × 1.82 mm, though considerable individual variation exists. The ostium is present bilaterally in approximately 71.5% of individuals, while in about 10% it may not be identifiable on imaging. The distance between the two ostia averages 67 mm. The ostium is situated at roughly the vertical midpoint of the anterior sphenoid wall (approximately 45.5% of total sphenoid height measured from the planum sphenoidale), though this varies with pneumatization pattern in less pneumatized (presellar or conchal) sinuses, the ostium tends to be closer to the skull base.

Topographic relationships and surgical landmarks

The ostium is consistently located medial to the superior turbinate in approximately 83% of specimens, about 1 cm above the posteroinferior end of the superior turbinate, and within 1 cm of the nasal septum (midline). Key distances from anterior nasal landmarks include:

• Approximately 55-57 mm from the limen nasi (at an angle of ~3036°)

• Approximately 62-63 mm from the nasal sill

• Approximately 21 mm from the superolateral angle of the posterior choana

The posteroinferior end of the superior turbinate is considered the most reliable intraoperative landmark for locating the ostium. The posterior wall of the maxillary sinus also serves as an adjunctive depth landmark, as the sphenoid ostium lies on average only ~1.5 mm posterior to this level.

Factors influencing ostium position

Several anatomical variables significantly affect ostium localization:

• Pneumatization pattern: Postsellar pneumatization is associated with a more medial and inferiorly displaced ostium, while presellar/conchal types tend toward more medial and superiorly positioned ostia.

• Sphenoid rostrum pneumatization: The strongest predictor of non-medial (lateralized) ostium placement (OR = 15.86); also promotes superior displacement.

• Onodi cells (sphenoethmoidal cells): Present in ~3639% of sinuses, associated with inferior displacement of the ostium and a greater ostium-to-choana distance.

• Gender: Female sex is associated with a higher likelihood of medial ostium localization.

Functional significance

The sphenoid ostium serves as the sole drainage pathway for the sphenoid sinus. The mucous membranes lining the sinus are continuous with those of the nasal cavity through this ostium, and mucociliary clearance depends on its patency. Obstruction of the ostium whether from mucosal edema, polyps, or other pathology impairs drainage and predisposes to secondary bacterial sphenoid sinusitis. In endoscopic sinus surgery and transsphenoidal approaches, the natural ostium is the preferred entry point, as it preserves physiological mucociliary clearance pathways and avoids the creation of artificial openings that may lead to mucus recirculation.

Critical adjacent structures

Given the proximity of the sphenoid sinus to vital neurovascular structures including the optic nerve, internal carotid artery, cavernous sinus, and skull base precise preoperative CT evaluation of ostium position and its relationship to these structures is essential for safe surgical access.