Nasal cavities are located in the midface, separated by a median septum; they communicate posteriorly with the nasopharynx through the choanae. The floor of nasal fossae is the hard palate, the cribriform plate and the planum ethmoidalis compose its roof, that separate nasal cavities from the anterior cranial fossa. The nasal septum is made of a "hard" portion (perpendicular lamina of the ethmoid, vomer), and of a "soft" part (septal cartilage).
The lateral nasal wall has a more difficult anatomy, an understanding of it being critical for endonasal surgery planning.
The complex anatomical arrangement of the lateral nasal wall can be more easily understood if related to the embryologic development of turbinates and ethmoid (Wolf et al. 1993).
The ethmoid turbinates originate from ridges in the lateral nasal wall of the fetus. Each of the five ridges (ethmoidoturbinals) has an ascending (more vertical) and descending (more horizontal) portion, being - therefore - similar to the turbinate in the adult. Between the fetal ridges are six grooves. Some ridges and grooves will fuse or disappear - partially or totally - during fetal development to ultimately result in the nasal turbinates of the adult. Key points are:
The first ethmoidoturbinal regresses, i.e. will not develop into a turbinate. The agger nasi is considered a residual of the ascending portion, while the uncinate process is presumably a remnant of the descending portion.
The descending part of the first groove (located between the first and second ethmoidoturbinals) becomes the ethmoidal infundibulum and middle me-atus, whereas the ascending part becomes the frontal recess.
The second ethmoidoturbinal gives rise to the bulla lamella, a lamina attached to the lateral nasal wall, a structure that can be observed in few patients because it is usually pneumatized, therefore appearing as the bulla ethmoidalis.
The third and fourth ethmoidoturbinals develop, respectively, into the permanent middle and superior turbinates; and the fifth ethmoidoturbinals become the supreme (uppermost) turbinate.
The superior and uppermost meatus develop from third and fourth fetal grooves.
As a result, ridges fully develop into thin laminae crossing the entire ethmoid to project into the nasal cavity (turbinates) or give rise to incomplete laminae, like the uncinate process. Each bony lamina has a constant portion inserting into the lateral nasal wall (ground lamella) and additional attachments to the lamina cribrosa or to the fovea ethmoidalis (superiorly).
In most subjects, four ground lamellae are usually present, almost each one obliquely oriented, somewhat parallel to one another. The uncinate process - an incompletely developed lamella - is the first one, followed by the bulla lamella. If the latter extends vertically up to the ethmoid roof, the frontal recess becomes separated from the rest of the ethmoid. Pneumatization of this lamella results in the development of the ethmoidal bulla. The third, more constant and complete ground lamella consists of the lateral insertion of the middle turbinate on the lamina papyracea. This lamella separates anterior from posterior ethmoidal cells. A fourth lamella is made by the lateral attachment of the superior turbinate. Occasionally, a fifth lamella may be observed when the supreme turbinate does not regress (Kim et al. 2001).
The spaces among the ground lamellae (interturbinal meatus) are further subdivided by transverse bony septa, resulting in several cells that communicate with the interturbinal meatus only through a small ostium. Rarely these septa are absent. In this latter condition, the original framework of the labyrinth organized into single large cells - corresponding to the interturbinal meatus - can be observed.
Variations or anomalies in the development of ground lamellae and septa will result in a great variability of number, size and morphology of the single ethmoid cells and may additionally reflect on the ratio between the volume of anterior versus posterior cells.
The middle meatus plays a crucial functional role, as in this area the secretions of several sinuses are collected, namely:
• The ethmoid bulla (roof of the meatus)
• The anterior ethmoid cells and maxillary sinus, both through the hiatus semilunaris, a subtle fissure located in front and below the ethmoid bulla
• The frontal sinus, through the frontal recess
The inferior meatus, although the largest, has a less relevant functional role: in this space only the distal opening of the nasolacrimal duct is found.
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