Learning Objective: Describe the anatomy of the ethmoid bone, including its parts, articulations, relations, and clinical significance, such as ethmoid fractures and CSF rhinorrhoea.
The ethmoid bone is a small, unpaired, lightweight bone located in the midline of the anterior cranial fossa, between the orbits. Its name derives from the Greek ethmos (“sieve”) due to the cribriform plate, which is perforated by the olfactory nerve fibers (CN I). It contributes to:
- Medial orbital walls
- Roof of the nasal cavity
- Nasal septum
- Lateral nasal wall
Anatomical Structure
Cribriform Plate
- Forms the roof of the nasal cavity
- Perforated by olfactory nerve fibers (CN I)
- Superior projection: Crista galli → attachment for falx cerebri
Perpendicular Plate
- Descends from the cribriform plate
- Forms the superior two-thirds of the nasal septum
Ethmoidal Labyrinth (paired)
- Lateral to perpendicular plate
- Contains ethmoidal air cells (sinuses)
- Two bony sheets:
- Orbital plate: lateral wall of labyrinth; forms the medial orbital wall
- Medial sheet: forms the upper lateral wall of the nasal cavity; gives rise to the superior and middle conchae
Articulations
The ethmoid articulates with 13 bones:
- Paired: nasal, maxilla, lacrimal, palatine, inferior conchae
- Unpaired: frontal, vomer, sphenoid
Clinical Significance
Ethmoid Fracture
- Causes: facial trauma, dashboard injury, falls
- Cribriform plate fracture: may shear olfactory nerve fibers → anosmia
- Labyrinth fracture: can create communication between the nasal cavity and the orbit → orbital emphysema
CSF Rhinorrhoea
- Fracture of the cribriform plate can allow cerebrospinal fluid to leak into the nasal cavity
- Presents as a clear, watery discharge from one nostril
- Management: usually conservative; surgical repair may be required in persistent or spontaneous cases
