Paranasal Sinus
The paranasal sinuses, arranged in pairs and in relation to each nasal cavity, comprise two groups, anterior and posterior. The former includes the maxillary sinus, the frontal sinus and the anterior ethmoidal cells, all of which communicate with the middle meatus. The posterior group consists of the posterior ethmoidal cells and the sphenoidal sinus communicating with the superior meatus.
The maxillary sinus is also known as the maxillary antrum. It exists at birth as a small but definite cavity adjacent to the middle meatus, and it enlarges gradually to reach its maximum dimensions about the twenty-first year with the eruption of the upper wisdom tooth. The sinus expands in the maxilla during the eruption of the primary dentition until it reaches the level of the floor of the nasal cavity about the seventh year. In adult life it is somewhat pyramidal in shape, its roof being formed by the floor of the orbit, its floor being in close proximity to the roots of the second dentition; its posterior wall lying in relation to the pterygopalatine fossa; its medial wall adjoining the lateral wall of the nasal cavity; and its anterolateral walls being superficial. The opening into the middle meatus, the maxillary ostium, is near the upper part of the cavity of the sinus, and is thus unfavourably placed for drainage. There may be one or more accessory ostia posterior to the main one.
The frontal sinus is rudimentary at birth, being represented by a small upward prolongation from the anterior end of the middle meatus, the nasofrontal duct. During childhood this duct enlarges upwards to reach the level of the orbital roof about the ninth year. Thereafter the sinus extends for a variable distance as a result of absorption of cancellous bone between the outer and inner tables of the frontal bone. The anterior wall is formed by the outer table; the posterior wall is related to the inner table which separates it from the frontal lobe of the brain; its floor forms part of the orbital roof; and the medial wall is a septum separating the two frontal sinuses. The opening of the frontal sinus is in its floor, and communicates with the middle meatus through the nasofrontal duct.
The ethmoid sinuses constitute a cell labyrinth and are present at birth as prolongations of the nasal mucosa into the lateral mass of the ethmoid bone. In adult life they vary in number, size and shape, and for clinical purposes they are classified as anterior and posterior, depending upon whether they communicate with the middle or the superior meatus. Medially the cell labyrinth lying in relation to the upper half of the nasal cavity, laterally-to the orbit, from which the cells are separated laterally by the lamina papyracea and to the maxillary sinus. The cells abut anteriorly on the frontal process of the maxilla, posteriorly they are related to the sphenoid sinus. Superiorly the ethmoid cells related to the frontal sinus and anterior cranial fossa.
The sphenoid sinus occupies the body of the sphenoid bone, and may be present at birth as a small indentation of nasal mucosa. The sinus varies greatly in size in the adult. The lateral wall is contiguous with the internal carotid artery, the cavernous blood sinus, first branch of the V cranial nerve, III, IV and VI cranial nerves (Fig 24); the roof is related to the frontal lobe, the olfactory tract, the optic chiasma and the pituitary gland lying in the hypophyseal fossa; the floor adjoins the pterygoid canal, roof of the nasal cavity, nasopharynx; while the medial wall is a septum separating it from its neighbour. Anterior wall is contiguous with ethmoidal cells. Behind the posterior wall lies the posterior cranial fossa. The ostium is placed high up in the cavity of the sinus.