Introduction
The ethmoid bone, a small, unpaired bone nestled in the anterior cranium’s midline, plays a crucial role in our sense of smell and facial structure. This lightweight, spongy structure houses the olfactory nerve fibers, which pass through its cribiform plate to innervate the nasal cavity.
Anatomical Location
The ethmoid bone is one of eight cranial bones, strategically positioned:
- Roof of the nasal cavity: Separating it from the cranial cavity above.
- Between the orbital cavities: Contributing to the medial wall of each orbit.
- Anterior cranial fossa: Forming part of this structure.
- Nasal septum and lateral nasal wall: Contributing significantly to their formation.
Articulation
The ethmoid bone articulates with a total of 13 bones:
Paired:
- Nasal bones
- Maxillae
- Lacrimal bones
- Palatine bones
- Inferior conchae
Unpaired:
- Frontal bone
- Vomer
- Sphenoid bone
Structure
The ethmoid bone comprises three main parts:
1. Cribriform Plate:
- Forms the roof of the nasal cavity.
- Characterized by numerous olfactory nerve fiber perforations, giving it a sieve-like appearance.
- Features the crista galli, a superior projection that serves as an attachment point for the falx cerebri (a dura mater sheet separating the cerebral hemispheres).
2. Perpendicular Plate:
- A bony projection descending from the cribiform plate.
- Forms the upper two-thirds of the nasal septum.
3. Ethmoidal Labyrinths:
- Two large masses flanking the perpendicular plate.
- Contain the ethmoidal air cells (sinuses).
- Composed of two bony sheets:
- Orbital plate: The lateral sheet 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.
Clinical Correlates
The ethmoid bone’s location and delicate structure make it susceptible to injury, potentially leading to:
Ethmoid Fracture:
- Common in facial trauma.
- Cribriform plate fracture: Can shear olfactory bulb branches, causing anosmia (loss of smell).
- Labyrinth fracture: May create communication between the nasal cavity and orbit, potentially leading to orbital emphysema (air trapped in the orbit).
CSF Rhinorrhea:
- Cribriform plate fracture can allow communication between the nasal cavity and the central nervous system.
- Cerebrospinal fluid may leak into the nasal cavity and drain through the nose.
- Presents as a clear, watery discharge from one nostril.
