MAXILLARY SINUS

Development

The maxillary sinus (antrum of Highmore) is the first to develop. These structures are usually fluid-filled at birth. The growth of these sinuses is biphasic with growth during years 0-3 and 7-12. During the later phase pneumatization spreads more inferiorly as the permanent teeth take their place. Pneumatization can be so extensive as to expose tooth roots with only a thin layer of soft tissue covering them.

Structure

The adult maxillary sinus is a pyramid which has a volume of approximately 15 ml (34x33x23mm). The base of the pyramid is the nasal wall with the peak pointing toward the zygomatic process. The anterior wall has the infraorbital foramen located at the midsuperior portion with the infraorbital nerve running over the roof of the sinus and exiting through the foramen. This nerve can be dehiscent (14%). The thinnest portion of the anterior wall is just above the canine tooth--the canine fossa. The roof is formed by the orbital floor and transected by the course of the infraorbital nerve. The posterior wall is unremarkable. Behind this wall is the pterygomaxillary fossa with the internal maxillary artery, sphenopalatine ganglion and the Vidian canal, the greater palatine nerve and the foramen rotundum. The floor, as discussed above, varies in it's level. From birth to age nine the floor of the sinus is above that of the nasal cavity. At age nine the floor is generally at the level of the nasal floor. The floor continues to sink as the maxillary sinus pneumatizes. Because of the close relationship with the dentition dental disease can cause maxillary infection, and tooth extraction can result in oral-antral fistulae.

Vascular supply

Branches of the internal maxillary artery supply this sinus. These include the infraorbital (as it runs with the infraorbital nerve), lateral branches of the sphenopalatine, greater palatine, and the alveolar arteries. Venous drainage runs anteriorly into the facial vein and posteriorly into the maxillary vein and jugular vs. dural sinus systems.

Innervation

The maxillary sinus is innervated by branches of V2. Specifically, the greater palatine nerve and the branches of the infraorbital nerve.

Related structures

Nasolacrimal duct

The nasolacrimal duct drains the lacrimal sac and runs from the lacrimal fossa in the orbit down the posterior aspect of the maxillary vertical buttress and empties in the anterior aspect of the inferior meatus. The duct lies very close to the maxillary ostium. On average it lies 4mm-9mm anterior to the ostium.
Natural ostium

The natural maxillary ostium is located at the superior aspect of the medial wall of the sinus. Intranasally, it is usually in the posterior half of the ethmoid infundibulum, or behind the lower 1/3 of the uncinate process. The posterior edge of the ostia is continuous with the lamina papyracea, thus a reliable landmark for the lateral limit of surgical dissection. The ostium size averages 2.4 mm but can vary from 1 to 17mm. The ostium is much smaller than that actual bony defect, as mucosa fills this area and defines the extent of the opening. 88% of maxillary ostium are hidden behind the uncinate process and therefore cannot be visualized endoscopically.

Anterior/Posterior Fontanelles/Accessory Ostium

Two bony dehiscences of the lateral nasal wall/maxillary sinus medial wall exist (sometimes there is one large bone dehiscence. These are usually covered by mucosa. In some individuals the anterior or posterior fontanelles may be patent which results in an accessory ostium. They are nonfunctional ostia and serve to drain the sinus only if the natural ostium is blocked and intrasinus pressure/gravity moves material out of the ostium. Accessory ostium are usually found in the posterior fontanel.

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