Ethmoid Bone
Os Ethmoidale
location_on Between the orbits, forming part of the anterior cranial fossa floor and nasal cavity roof
The ethmoid is a delicate, sponge-like bone situated between the orbits that forms a significant portion of the nasal cavity walls and roof. It is the lightest bone of the skull and contains numerous ethmoidal air cells (sinuses). The cribriform plate, its superior surface, is perforated by tiny holes that transmit olfactory nerve fibers, making it essential for the sense of smell.
Key Anatomical Features
- Cribriform plate has tiny perforations for olfactory nerve filaments
- Crista galli is a vertical midline projection for attachment of the falx cerebri
- Perpendicular plate forms the upper portion of the nasal septum
- Superior and middle nasal conchae increase turbulent airflow in the nasal cavity
- Orbital plate (lamina papyracea) forms the paper-thin medial wall of the orbit
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| No direct muscular attachments | The ethmoid primarily provides structural support | N/A |
| Falx cerebri (dural attachment) | Crista galli | Anchors the dural fold between cerebral hemispheres |
| Medial palpebral ligament (indirect) | Near the lacrimal fossa on the orbital plate | Supports medial eyelid structures |
| Middle turbinate mucosa | Middle concha | Warms, humidifies, and filters inspired air |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Fronto-ethmoidal suture | Fibrous suture | Frontal bone |
| Ethmo-vomerine junction | Fibrous suture | Vomer |
| Ethmo-sphenoidal junction | Fibrous suture | Sphenoid bone |
Common Pathologies
CSF rhinorrhea
Fracture of the cribriform plate can cause cerebrospinal fluid to leak through the nose. Presents as clear watery nasal discharge, especially when leaning forward.
Ethmoidal sinusitis
Infection of the ethmoid air cells can spread to the orbit causing orbital cellulitis, one of the most common causes of proptosis in children.
Anosmia
Loss of smell from shearing of olfactory nerve fibers as they pass through the cribriform plate, commonly from head trauma.
Orbital blowout fracture
The paper-thin lamina papyracea can fracture from blunt orbital trauma, allowing orbital contents to herniate into the ethmoid sinuses.
Clinical Relevance
The cribriform plate is a weak point in the skull base; anterior cranial fossa fractures through this area present with CSF rhinorrhea and anosmia. Nasal packing should never be passed superiorly to avoid penetrating the cribriform plate. The lamina papyracea is so thin that ethmoid sinus infections readily spread to the orbit.
Development and Ossification
The ethmoid ossifies from three endochondral centers: one for the perpendicular plate and one for each labyrinth. Ossification begins during the 5th fetal month. The bone is largely cartilaginous at birth, with the cribriform plate completing ossification by age 2.
Did You Know?
- Ethmoid comes from the Greek ethmos meaning sieve, referring to the perforated cribriform plate
- The ethmoid contains more individual sinuses than any other bone, sometimes exceeding 18 air cells
- The lamina papyracea is named from the Latin for paper-like plate due to its extreme thinness
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