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The ethmoid bone is a small bone with a cuboidal structure that forms the lateral boundaries of the orbit, the anterior cranial fossa superiorly, and the nasal cavities inferiorly.[1] It is relatively light and has a spongy texture. The ethmoid bone consists of the cribriform plate, two ethmoid labyrinths, and the perpendicular plate.[2] The ethmoid bone encloses ethmoidal cells and is separated from the orbits by the lamina papyracea. The superior and middle turbinate arise from the ethmoidal lateral masses and become the superior and middle meatus. The ethmoid bone also hosts the infundibulum of the frontal sinus and the ostium of the maxillary sinus inside the osteomeatal complex. Located between the ethmoid plate and nasal conchae are the ethmoid sinuses.[2] The ethmoid articulates with 13 bones: the frontal, sphenoid, nasals, maxillae, lacrimals, palatines, inferior nasal conchae, and vomer.
Of all the paranasal sinuses, the ethmoid sinus is the most complex and most variable. The roof of the ethmoid bone labyrinth consists of the fovea ethmoidalis, and the olfactory fossae are grooves on the cribriform plate within the ethmoid bone.[9] Keros categorized the depth of the olfactory fossa into three types: Type I, which has a depth of 1 to 3 mm, Type II, which has a depth of 4 to 7 mm and Type III, which has a depth of 8 to 16 mm.[10] The greater the height of the lateral lamella, the greater the risk of its perforation into the anterior cranial fossa.[10] The ethmoid roof also varies in different populations. For instance, Alazzawi et al. found that, among 150 cases from Malaysia, China, and India, 80% of the specimens were Keros Type I and 20% of were Keros Type II. Keros Type II was also more common in males than in females.[11] However, Solares et al. discovered, among 50 CT scans in the United States, 83% of the specimens were Keros Type I, 15% were Type II, and 2% were Type III.[12]