The pyramid-shaped maxillary sinus (or antrum of Highmore) is the largest of the paranasal sinuses, located in the maxilla. It drains into the middle meatus of the nose through the semilunar hiatus. It is located to the side of the nasal cavity, and below the orbit.
Structure It is the largest air sinus in the body. It has a mean volume of about 10 ml. It is situated within the body of the maxilla, but may extend into its zygomatic and alveolar processes when large. It is pyramid-shaped, with the apex at the maxillary zygomatic process, and the base represented by the lateral nasal wall. It has three recesses: an alveolar recess pointed inferiorly, bounded by the alveolar process of the maxilla; a zygomatic recess pointed laterally, bounded by the zygomatic bone; and an infraorbital recess pointed superiorly, bounded by the inferior orbital surface of the maxilla. The medial wall is composed primarily of cartilage.
Walls The nasal wall of the maxillary sinus, or base, presents, in the disarticulated bone, a large, irregular aperture that communicates with the nasal cavity. In the articulated skull this aperture is much reduced in size by the following bones:
the uncinate process of the ethmoid above, the ethmoidal process of the inferior nasal concha below, the vertical part of the palatine behind, and a small part of the lacrimal above and in front. The sinus communicates through an opening into the semilunar hiatus on the lateral nasal wall. The medial wall is composed primarily of cartilage. On the posterior wall are the alveolar canals, transmitting the posterior superior alveolar vessels and nerves to the molar teeth. The floor is formed by the alveolar process, and, if the sinus is of an average size, is on a level with the floor of the nose; if the sinus is large it reaches below this level. Projecting into the floor of the antrum are several conical processes, corresponding to the roots of the first and second maxillary molar teeth; in some cases the floor can be perforated by the apices of the teeth. The roof is formed by the floor of the orbit. It is traversed by infraorbital nerves and vessels.
Features The infraorbital canal forms a ridge at the junction of the roof and anterior wall of the sinus; additional ridges are sometimes seen in the posterior wall of the cavity and are caused by the alveolar canals.
Innervation The mucous membranes receive their mucomotor postganglionic parasympathetic nerve fibres from the pterygopalatine ganglion. The superior alveolar (anterior, middle, and posterior) nerves, branches of the maxillary nerve provide sensory innervation.
Microanatomy The sinus is lined with mucoperiosteum, with cilia that beat toward the ostia. This membranous lining is also referred to as the Schneiderian membrane, which is histologically a bilaminar membrane with pseudostratified ciliated columnar epithelial cells on the internal (or cavernous) side and periosteum on the osseous side. The size of the sinuses varies in different skulls, and even on the two sides of the same skull.
Relations The roof of the sinus is also the floor of the orbit. Posterior to the sinus and its wall are the pterygopalatine fossa and the infratemporal fossa.
Variation After puberty, the maxillary sinus rapidly increases in size. Its size is variable in the adult; if large, it may extend into the zygomatic process and alveolar process of the maxilla. Extension into the maxillary alveolar process may cause the roots of the molars and even premolars to lie just beneath the floor of the sinus or even project through the floor and into the sinus; in such cases, the roots of the teeth are typically surrounded by a thin layer of bone, but may sometimes lie directly underneath the mucous membrane of the sinus. Projection of the roots into the maxillary sinus is more common in advanced age due to bone resorption. In such cases, tooth extraction can create a fistula between the oral cavity and the sinus that nevertheless usually resolves spontaneously. An accessory maxillary ostium (found in 0% to 42% of the population) is an additional natural opening in the lateral nasal wall that provides communication between the maxillary sinus and the nasal cavity, apart from the primary or natural maxillary ostium. It is usually located in the posterior fontanelle of the lateral nasal wall, part of the middle meatus. While this ostium is listed as an anatomical variation, it has a role in the development of maxillary sinusitis.
Development Maxillary sinus is the first paranasal sinuses to form. At birth, it is about 6 to 8 cm3 in volume, elongated, as is orientated in antero-posterior direction, located at the next to the medial orbital wall of the eye. The lateral wall of the maxillary sinus goes beneath the medial orbital wall during the first year of life, extends laterally pass the infraorbital groove by the age of four years, and reach the maxilla by the age of nine years. After the first permanent tooth erupted at the age of six to seven, aeration of maxillary sinus is the main growth feature. At the final phase of aeration, the floor of maxillary sinus is four to five millimetres below the floor of nasal cavity. However, timing of maxillary sinus growth is variable in different people.
Clinical significance
Maxillary sinusitis
Maxillary sinusitis is inflammation of the maxillary sinuses. The symptoms of sinusitis are headache, usually near the involved sinus, and foul-smelling nasal or pharyngeal discharge, possibly with some systemic signs of infection such as fever and weakness. The skin over the involved sinus can be tender, hot, and even reddened due to the inflammatory process in the area. On radiographs, there is opacification (or cloudiness) of the usually translucent sinus due to retained mucus. Maxillary sinusitis is common due to the close anatomic relation of the frontal sinus, anterior ethmoidal sinus and the maxillary teeth, allowing for easy spread of infection. Differential diagnosis of dental problems needs to be done due to the close proximity to the teeth since the pain from sinusitis can seem to be dentally related. Furthermore, the drainage orifice lies near the roof of the sinus, and so the maxillary sinus does not drain well, and infection develops more easily. The maxillary sinus may drain into the mouth via an abnormal opening, an oroantral fistula, a particular risk after tooth extraction.
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