EMBRYOLOGY AND ANATOMY
The maxillary sinuses are air-containing spaces that occupy the maxillary bone bilaterally. They are the first of the paranasal sinuses (e.g., maxillary, ethmoid, frontal, sphe – noid) to develop embryonically and begin as a mucosal invagination that grows laterally from the middle meatus of the nasal cavity at approximately the seventieth day of gestation. At birth the sinus cavity is still somewhat less than a centimeter in any dimension. After birth the maxillary sinus expands by pneumatization into the developing alveolar process and extends anteriorly and inferiorly from the base of the skull, dosely matching the growth rate of the maxilla and the development of the dentition. As the dentition develops, portions of the alveolar process of the maxilla, vacated by the eruption of teeth, become pneumatized. By the time a child reaches age 12 or 13, the sinus will have expanded to the point at which its floor will be on the same hortzontallevel as-the floor of the nasal cavity. Expansion of the sinus normally ceases after the eruption of the permanent teeth but will, on occasion, pneurnanze further, after the removal of one Of more posterior maxillary teeth, to occupy the residual alveolar process. The,sinus may then extend virtually to the crest of the edentulous ridge. In adults the apices of the teeth may extend into ‘ the sinus cavity and may be identffied readily in the dry skull lying in the sinus floor. The sinuses are lined by respiratory eplthellum-a mucous-secreting, pseudostratlfied, Ciliated, columnar epithelium-and periosteum. The cilia and mucus are necessary for the drainage of the sinus, because the sinus opening, or ostium” Is not In a dependent position but lies two thirds the distance up from the inferior part of the medial wall and drains Into the nasal cavity. The maxillary sinus opens into the posterior, or inferior, end of the semilunar hiatus, which lles In the middle meatus of the nasal cavity,’between the inferior and middle nasal conchae, The ostium remains at the level of the original lateral extension from the nasal cavity from which the sinus began formation in the embryo and the location of which is close to the roof of the sinus (Fig. 19-1). Beating of the cilia moves the mucus produced by the lining epithelium and any foreign material contained within the “Sinus toward the ostium, from which it drains Intothe nasal cavlty.
The maxillary sinus is the largest of the paranasal sinuses. it may be described as a four-sided pyramid, with the base lying vertically on the medial surface and forming the lateral nasal wall. The apex extends laterally into the zygomatic process of the maxilla. The upper wall, or roof, of .the sinus is also the floor of the orbit. The posterior wall extends the length of the maxilla and dips into the maxillary tuberosity. Anteriorly and laterally the’ sinus extends to the region of the first bicuspid or cuspid teeth. The floor of the sinus forms the base of the alveolar process. The adult maxillary sinus averages 34 mm in anteroposterior direction: 33 mm in height, and 23 mm in .width. Its volume is approximately 15 cc (Fig. 19-2).