Ethmoid Bone

The ethmoid bone is a small, delicate, and deeply situated cranial bone located between the two orbits and anterior to the sphenoid bone. Despite its size, it is structurally and functionally one of the most complex bones in the skull. It forms a critical part of the anterior cranial base, the medial walls of the orbits, and the roof and lateral walls of the nasal cavity. It also houses the ethmoidal air cells, contributing to the paranasal sinuses.

Structure

The ethmoid bone is a single, unpaired bone with a highly irregular and lightweight structure. It is composed of three main parts:

• Cribriform plate
• Perpendicular plate
• Labyrinths (lateral masses)

The bone contains multiple thin bony plates, air-filled spaces, and articulates with several facial and cranial bones, making it both fragile and important in multiple systems including respiratory, olfactory, and visual.

Location

The ethmoid bone is located centrally within the skull base, between the nasal cavity and the orbits. It lies anterior to the sphenoid bone and posterior to the nasal bones. The ethmoid forms part of the nasal septum, the roof of the nasal cavity, the medial walls of the orbits, and contributes to the anterior cranial fossa.

Articulations

The ethmoid bone articulates with 13 bones:

• Frontal bone
• Sphenoid bone
• Nasal bones (2)
• Maxillae (2)
• Lacrimal bones (2)
• Palatine bones (2)
• Inferior nasal conchae (2)
• Vomer

Parts and Key Features

1. Cribriform Plate

• Forms the horizontal roof of the nasal cavity and part of the floor of the anterior cranial fossa.
• Perforated by multiple foramina that transmit the olfactory nerve fibers (CN I) from the nasal mucosa to the olfactory bulbs.
• Delicate and thin; easily fractured in head trauma.

2. Crista Galli

• Vertical, midline bony ridge projecting superiorly from the cribriform plate into the anterior cranial fossa.
• Attachment point for the falx cerebri (a dural fold between cerebral hemispheres).

3. Perpendicular Plate

• Vertical plate projecting inferiorly from the cribriform plate.
• Forms the superior portion of the nasal septum, contributing to the midline division of the nasal cavity.

4. Ethmoidal Labyrinths (Lateral Masses)

• Composed of multiple thin-walled air cells (ethmoidal sinuses).
• Form the lateral walls of the nasal cavity and the medial walls of the orbits.
• Contain two scroll-shaped projections:
  • Superior nasal concha
  • Middle nasal concha
• These conchae increase surface area for warming, filtering, and humidifying inhaled air.

Functions

• Olfaction: Cribriform plate allows olfactory nerves to reach the olfactory bulbs.
• Respiration: Nasal conchae regulate airflow and trap particles during breathing.
• Support: Helps form part of the nasal cavity, orbital wall, and anterior cranial fossa.
• Sinus drainage: Ethmoidal air cells contribute to mucus production and drainage into the nasal cavity.

Ethmoidal Sinuses

The ethmoidal sinuses are air-filled cavities within the lateral masses of the ethmoid bone. They are divided into:

• Anterior ethmoidal cells: Drain into the middle nasal meatus via the infundibulum.
• Middle ethmoidal cells: Drain into the middle meatus through the ethmoidal bulla.
• Posterior ethmoidal cells: Drain into the superior nasal meatus.

These sinuses vary in size and number and are important in sinus physiology and pathology.

Development and Ossification

The ethmoid bone develops from cartilaginous precursors through endochondral ossification. Ossification centers appear during the fetal period, but the bone remains incompletely ossified at birth. The ethmoidal air cells begin to form in early childhood and continue developing into adolescence.

Clinical Relevance

1. Fracture and CSF Rhinorrhea

The cribriform plate is extremely thin and vulnerable to fracture in facial trauma or skull base injuries. If fractured, cerebrospinal fluid (CSF) can leak through the nose — a condition known as CSF rhinorrhea. It may also result in the loss of smell (anosmia).

2. Sinusitis

The ethmoidal air cells are prone to infection, especially in conjunction with maxillary or frontal sinusitis. Ethmoid sinusitis can cause pain between the eyes, headache, and sometimes orbital complications if the infection spreads.

3. Orbital Cellulitis

Due to its proximity to the orbit, infection of the ethmoid sinuses can spread to the orbital cavity, leading to orbital cellulitis — a serious condition that can impair vision or cause intracranial complications.

4. Anosmia

Damage to the olfactory nerve fibers passing through the cribriform plate can cause anosmia (loss of smell). This can result from trauma, tumors, or congenital defects.

5. Tumors

Rare tumors like esthesioneuroblastoma may originate from the olfactory epithelium lining the cribriform plate. Such tumors can invade both the nasal cavity and anterior cranial fossa.

6. Endoscopic Surgery

The ethmoid bone is a key structure during endoscopic sinus surgery. Surgeons navigate through the ethmoid labyrinth to reach deeper sinus cavities. Understanding its delicate anatomy is essential to avoid complications like orbital penetration or CSF leaks.

Radiological Appearance

On CT scans, the ethmoid bone appears as a lattice-like structure between the eyes. The cribriform plate is a thin horizontal line, and the ethmoidal sinuses are seen as small, air-filled spaces. The bone is best assessed in coronal views during sinus imaging.

Associated Foramina and Canals

• Anterior and posterior ethmoidal foramina: Transmit vessels and nerves from the orbit to the nasal cavity and anterior cranial fossa.
• Cribriform foramina: Transmit olfactory nerve fibers (CN I).