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This is the free full text of AppSurgeOn - 3D Skull Atlas.

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Ethmoid Bone

The ethmoid bone lies in the middle of the anterior skull base and forms part of the medial walls of the orbits, part of the nasal septum, and the roof and lateral walls of the nasal cavities. It is irregularly cuboidal shaped, composed of a horizontal perforated cribriform plate, a median perpendicular plate, and two lateral labyrinths containing the ethmoidal air cells. It articulates with the frontal, sphenoid, maxillary, palatine, inferior concha, vomer, nasal, and lacrimal bones.

  

 Parts

 Cribriform plate

The upper surface of the cribriform plate is divided by a median process, called the crista galli. Two crista galli alae arise from the anterior part of the crista galli, and the notch that contributes to the foramen caecum is situated between them, on the anterior surface of the crista galli. The anterior ethmoidal foramina are located lateral to the crista galli on both sides of the cribriform plate. Two grooves, housing the meningeal branch of the anterior ethmoidal arteries, extend on the two sides from the anterior ethmoidal foramina towards the foramen caecum.

 

 

Ethmoidal labyrinths

It contains the ethmoidal air cells, which, from a surgical point of view, are subdivided into anterior and posterior cells by the basal lamella of the middle concha.

Ethmoidal Air Cells

The anterior cells drain into the middle nasal meatus, while the posterior cells drain into the superior meatus. In vivo, many air cells’ walls are constituted by adjacent bones, such as the sphenoid conchae, the orbital process of the palatine bone, the lacrimal bone, and the frontal process of the maxillary bone. Each labyrinth is roofed by the fovea ethmoidalis of the frontal bone. Therefore, the air cells are superiorly open in the isolated ethmoid bone. The anterior and posterior ethmoidal grooves are likewise visible on the superior surface of the ethmoidal labyrinths.

The frontal sinus drainage pathway, which allows the content of the frontal sinus to drain into the nasal cavity, is also evident superiorly, as it connects the frontal sinus to the middle nasal meatus. The lamina papyracea forms the lateral wall of the labyrinth on each side.
The ethmoid bulla, the uncinate process, the middle nasal concha, and the superior nasal concha are visible on the medial side of the labyrinths. The ethmoid bulla and the uncinate process are separated by the narrow hiatus semilunaris, which gives access to the ethmoid infundibulum.

 

Perpendicular plate

It descends from the inferior surface of the cribriform plate approximately in the midsagittal plane, although it typically deviates slightly from the midline.

 

 

 

Articulations

The ethmoid bone articulates with the frontal, sphenoid, maxillary, palatine, inferior concha, vomer, nasal, and lacrimal bones. 

 

Anatomical Details

Anterior Ethmoidal Foramen

The foramen that transmits the anterior ethmoidal nerve and vessels to the nasal cavity. It is located anteroinferiorly on each side of the crista galli.

 

Anterior Ethmoidal Groove

Ethmoidal part of the anterior ethmoidal canal formed with the corresponding groove of the frontal bone. It carries the anterior ethmoidal artery.

 

Anterior Falcine Artery Groove

The groove for the meningeal branch of the anterior ethmoidal artery, which continues as anterior falcine artery. It runs from the anterior ethmoidal canal posteriorly to the foramen caecum anteriorly.

 

Cribriform Plate

The median, horizontal, perforated plate that constitutes part of the nasal roof, located in the ethmoidal notch of the frontal bone. The posterior portion of the cribriform plate is wider and less depressed than the anterior one. Its name reflects the presence of numerous foramina allowing branches of the olfactory nerves to reach the nasal cavities.

 

Crista Galli

Thick triangular process, located in the midsagittal plane, which arises from the anterior part of the cribriform plate. It extends upwards to give insertion to the falx cerebri, in particular with its curved posterior edge. Two small alae are visible on its anterior border. The crista galli may be pneumatized, showing bulges on its sides, which, instead, are typically smooth.

 

Crista Galli Ala

Small eminence on each side of the anterior border of the crista galli that articulates with the frontal bone constituting the foramen caecum.

 

Ethmoid Bulla

Bone swelling on the lateral wall of the middle meatus produced by middle ethmoidal air cells, which open into the middle meatus.

 

Ethmoid Infundibulum

Space bound by the uncinate process anteromedially, the ethmoid bulla posterolaterally, and the lamina papyracea anterolaterally. It extends anterosuperiorly from the middle meatus and communicates with the anterior ethmoidal sinuses. The terminal recess of the infundibulum is present if the upper end of the uncinate process reaches the lamina papyracea and not the skull base.

 

Ethmoidal Air Cells

The air cells of the Ethmoid bone are anatomically arranged into anterior, middle and posterior groups. From a surgical point of view, the air cells are subdivided in anterior and posterior cells by the basal lamella of the middle concha. In the disarticulated bone, many air cells are open, but in life, and in the articulated skull, they are closed by proximity to adjoining bones, except where they open into the nasal cavity.

 

Foramen Caecum Notch

Small notch completed by the corresponding notch of the frontal bone to form the Foramen Caecum.

 

Frontal Sinus Drainage Pathway

The narrow space in which the ostium of the frontal sinus opens. It is formed by the ethmoid infundibulum when the uncinate process reaches the skull base, while it corresponds to the middle meatus when the uncinate process inserts on the lamina papyracea.

 

Hiatus Semilunaris

Narrow passage between the uncinate process and the ethmoid bulla, through which the ethmoid infundibulum communicates with the middle meatus.

 

Lamina Papyracea

Lateral, vertical surface of the ethmoidal labyrinth. It is part of the medial orbital wall.

 

Middle Nasal Concha

Thin, convex lamella extending along the entire medial surface of the labyrinth, anteroinferior to the superior meatus. Its lower edge is thick, and its lateral surface is concave and forms part of the middle meatus.

 

Perpendicular plate

Thin, quadrilateral process that descends from the cribriform plate to form the upper part of the nasal septum. Although it is described to lay in the midsagittal plane, it usually deviates slightly from the midline. Its superior portion shows numerous grooves that carry medial filaments of the olfactory nerves.

 

Posterior Ethmoidal Groove

The ethmoidal part of the posterior ethmoidal canal formed with the corresponding groove of the frontal bone. It carries the posterior ethmoidal artery.

 

Superior Nasal Concha

Curved, thin lamella located posterosuperior to the basal lamella of the middle turbinate. It is visible in the posterosuperior portion of the medial surface of the ethmoidal labyrinth, and it bounds medially and superiorly the superior meatus.

 

Uncinate Process

A thin, curved process of a variable size, which projects posteroinferiorly from the labyrinth. Its upper edge constitutes the anterior boundary of the hiatus semilunaris in the middle meatus. The uncinate process  crosses the maxillary hiatus to articulate with the ethmoidal process of the inferior nasal concha.

 

 

References

Anatomy of the Human Body. Gray, H. Philadelphia: Lea & Febiger, 1918; Bartleby.com, 2000

Rhoton's Cranial Anatomy and Surgical Approaches. Rhoton AL. Lippincott Williams & Wil-kins; 2007

Paranasal sinuses: anatomic terminology and nomenclature. Stammberger HR, Kennedy DW, Anatomic Terminology G. Ann Otol Rhinol Laryngol Suppl 167:7-16, 1995 

Surgical anatomy of the anterior ethmoidal canal in ethmoid roof. Moon HJ, Kim HU, Lee JG, Chung IH, Yoon JH. The Laryngoscope. 2001;111(5):900-904. 

Anatomy of the ethmoid: CT, endoscopic, and macroscopic. Terrier F, Weber W, Ruefenacht D, Porcellini B. AJR American journal of roentgenology 1985;144(3):493-500. 

 

 

Authors

 

Francesco Belotti, MD

Neurosurgery Resident
University of Brescia (Italy) 
"Spedali Civili" Hospital Brescia (Italy) 
Scientific Team - UpSurgeOn

 

Federico Nicolosi, MD

Neurosurgeon
University of Milan
"Spedali Civili" Hospital Brescia (Italy)
Scientific Team - UpSurgeOn