Maxillary hiatus

The maxillary hiatus (Hiatus of maxillary sinus; Opening of maxillary sinus) is a large, irregular opening located on the nasal surface of the maxilla that provides communication between the maxillary sinus and the middle meatus of the nasal cavity.

Anatomical Description

The maxillary hiatus is situated on the medial wall of the maxillary sinus, corresponding to the nasal surface of the maxillary bone. In the dried skull, it appears as a large, roughly oval or kidney-shaped deficiency in the bone. However, in life, its effective size is substantially reduced by the overlapping of several adjacent structures:

• Superiorly: The uncinate process of the ethmoid bone and the ethmoidal bulla partially cover the hiatus.

• Inferiorly: The inferior nasal concha (inferior turbinate) and its maxillary process overlap the lower portion.

• Posteriorly: The perpendicular plate of the palatine bone covers part of the posterior aspect.

• Anteriorly: A portion of the lacrimal bone may contribute to narrowing the opening.

Functional and Clinical Significance

After reduction by these bony and mucosal structures, the remaining patent opening is called the ostium of the maxillary sinus (or natural ostium), which typically measures only 2-5 mm in diameter. This ostium opens into the ethmoidal infundibulum within the middle meatus, beneath the hiatus semilunaris.

The maxillary hiatus is clinically important for several reasons:

• Sinus drainage: The natural ostium derived from the hiatus is the primary drainage pathway for the maxillary sinus. Its relatively superior position on the medial sinus wall means that gravity-dependent drainage is limited, predisposing to mucus retention.

• Sinusitis: Obstruction of the ostium by mucosal edema, polyps, or anatomical variants is a key factor in the pathogenesis of maxillary sinusitis.

• Endoscopic sinus surgery: The hiatus and its surrounding landmarks (uncinate process, ethmoidal infundibulum, hiatus semilunaris) are critical reference points during functional endoscopic sinus surgery (FESS). Uncinectomy and widening of the natural ostium (middle meatal antrostomy) are performed in this region to restore sinus ventilation and drainage.

• Accessory ostia: In some individuals, additional openings (accessory ostia) may be present in the membranous portions of the hiatus, posterior or inferior to the natural ostium, which can be relevant during surgical planning.

Embryology

The maxillary hiatus forms during fetal development as the maxillary sinus begins to pneumatize from the ethmoid infundibulum into the body of the maxilla. The hiatus represents the original communication between the developing sinus and the nasal cavity, which becomes progressively narrowed as surrounding bones grow and overlap.