Understanding the Ethmoid Bone: Anatomy, Function, and Clinical Significance
Introduction
The ethmoid bone is a small, unpaired bone located in the midline of the anterior cranium. It is a lightweight, spongy structure. The numerous fibers of the olfactory nerve (CN I) pass through the cribiform plate of the ethmoid bone to innervate the nasal cavity with the sense of smell.
Anatomical Location
The ethmoid bone is one of the 8 bones of the cranium. It is situated in:
- The roof of the nasal cavity
- Between the two orbital cavities
The ethmoid bone also:
- Contributes to the medial wall of the orbit
- Forms part of the anterior cranial fossa, separating the nasal cavity (inferiorly) from the cranial cavity (superiorly)
- Forms a significant portion of the nasal septum and lateral nasal wall
Articulation
The ethmoid bone articulates with 13 other bones:
Paired:
- Nasal bones
- Maxillae
- Lacrimal bones
- Palatine bones
- Inferior conchae
Unpaired:
- Frontal
- Vomer
- Sphenoid bones
Structure
The ethmoid bone is made up of 3 parts:
1. The Cribriform Plate
- Forms the roof of the nasal cavity
- Pierced by numerous olfactory nerve fibers, giving it a sieve-like structure
- The crista galli is a superior projection providing an attachment point for the falx cerebri (a sheet of dura mater separating the two cerebral hemispheres)
2. The Perpendicular Plate
- A projection of bone descending from the cribiform plate
- Forms the superior 2/3 of the nasal septum
3. The Ethmoidal Labyrinths
- Two large masses located at either side of the perpendicular plate
- Contain the ethmoidal air cells (sinuses)
- Two sheets of bone form each labyrinth:
- Orbital plate: The lateral sheet of bone, also forming the medial wall of the orbit
- Medial sheet: Forms the upper lateral wall of the nasal cavity, from which the superior and middle conchae extend into the nasal cavity
Clinical Correlates
Ethmoid Fracture
The ethmoid bone can be fractured in cases of facial trauma, leading to:
- Fracture of cribiform plate: Branches of the olfactory bulb may be sheared, potentially causing anosmia (loss of sense of smell).
- Fracture of the labyrinth: May allow communication between the nasal cavity and the orbit, potentially causing air to enter the orbit (orbital emphysema).
CSF Rhinorrhea
A fracture to the cribiform plate may allow communication between the nasal cavity and the CNS. Consequently, cerebrospinal fluid can enter the nasal cavity and drain out from the nose. This manifests clinically as a clear, watery discharge from one side of the nose.
