Anatomy overview and definition
The ethmoid bone is an unpaired cranial bone that forms part of the neurocranium and contributes to the skull base, the nasal cavity, and the orbit. It lies at the junction of the nasal cavity and the anterior cranial fossa, separating the nasal cavity from the brain above and helping define the medial wall of each orbit. Because of its central position between the two orbits and the nasal cavity, the ethmoid contributes to the roof of the nose, the inner corner of each eye socket, and part of the nasal septum. Anatomically, it is best described by its three main components: the cribriform plate, the perpendicular plate, and the ethmoidal labyrinth.
Core location and boundaries
Cribriform plate and anterior cranial fossa
The cribriform plate is the horizontal, sieve-like roof of the nasal cavity and the floor of the anterior cranial fossa. It transmits olfactory nerves as they pass from the nasal mucosa to the olfactory bulb. The cribriform plate forms the anterior floor of the brain case and sits directly above the nasal cavity, making it a key landmark at the base of the skull.
Perpendicular plate and nasal septum
The perpendicular plate is a thin, vertical sheet of bone extending downward from the cribriform plate. It forms the superior portion of the nasal septum, which divides the left and right nasal cavities. The top of the perpendicular plate connects with the nasal bones and frontal bone anteriorly, while its posterior margin often articulates with the sphenoid bone and, variably, with the vomer inferiorly.
Ethmoidal labyrinth and orbital relations
The ethmoidal labyrinth consists of a maze of thin-walled chambers called ethmoidal air cells, located between the orbit and the nasal cavity. Each labyrinth has a medial wall (the nasal wall) facing the nasal cavity and a lateral wall (the orbital wall) forming part of the orbit. As a result, the ethmoid bone separates the orbit from the nasal cavity and contributes to the medial orbital wall anteriorly, while the orbital plate forms part of the floor of the anterior cranial fossa.
Relations to surrounding structures
Positionally, the ethmoid is at the anteromedial skull base. Superiorly, it is separated from the brain by the dura and the thin plate of the anterior cranial fossa. Anteriorly, it articulates with the nasal bones and the frontal process of the maxilla. Laterally, it forms the medial orbital wall and participates in the orbital margin near the medial canthus. Inferiorly, it connects with the maxilla, palatine bones, and the vomer, completing the nasal septum posteriorly. Posteromedially, it relates to the sphenoid bone at the sphenoethmoidal recess. Understanding these relations is important for interpreting imaging and for surgical approaches through the anterior skull base.
Regional anatomy and surgical relevance
The ethmoid bone’s location at the roof of the nasal cavity and the medial orbit makes it central to sinus surgery, transskull-base approaches, and endoscopic procedures. The thin lamina papyracea of the ethmoid lateral wall is a well-known landmark in endoscopic endonasal surgery, helping surgeons identify the orbital contents and avoid injury. Because the cribriform plate forms the floor of the anterior fossa, it is a potential route for cerebrospinal fluid leaks after trauma or surgery in this region. The ethmoid air cells are also commonly involved in rhinosinusitis, and their proximity to the orbit means orbital complications can arise from sinonasal infection or surgery.
Summary of key landmarks
| Anatomical attribute | Verified detail | Source type |
|---|---|---|
| Primary location | Roof of nasal cavity and medial orbit; between the orbits at the skull base | Standard anatomy |
| Key parts | Cribriform plate (roof of nose/floor of anterior fossa), perpendicular plate (superior nasal septum), ethmoidal labyrinth (air cells, medial and orbital walls) | Standard anatomy |
| Relation to anterior cranial fossa | Forms part of the anterior cranial fossa floor; transmits olfactory nerve branches | Standard anatomy |
| Relation to orbits | Medial orbital wall contributed by the ethmoid; orbital plate helps form the anterior cranial fossa floor | Standard anatomy |
| Clinical relevance | Site for anterior skull base approaches; thin lamina papyracea in endoscopic surgery; cribriform plate can be a route for CSF leakage | Surgical/clinical references |
Practical notes for imaging and identification
On CT scans, the ethmoid is easily recognized as the bone containing multiple air cells between the orbit and the nasal cavity; the cribriform plate appears as the superior wall of the nasal cavity with small foramina for olfactory nerves. On MRI, the thin lamina papyracea and the relationship of the ethmoid to the intracranial structures are well visualized. In endoscopic skull-base surgery, landmarks such as the fovea ethmoidalis (anterior to the cribriform plate) and the skull base junction help define safe corridors. Recognizing the position of the ethmoid is essential to avoid injury to the orbit, the anterior cranial fossa, and the olfactory structures.
Clinical contexts and considerations
Because of its central location, ethmoid fractures can affect both the nasal cavity and the orbit, potentially causing diplopia, epistaxis, or cerebrospinal fluid rhinorrhea. Sinus surgery in the ethmoid region requires careful navigation to preserve orbital soft tissues and the skull base. The proximity of the ethmoid to the optic nerve and internal carotid artery means that disease or surgical injury in this area can have significant visual and vascular consequences. Knowledge of the precise location of the ethmoid bone supports accurate interpretation of imaging, procedural planning, and avoidance of complications.