M01.02.012 The Ethmoid Bone

The ethmoid bone is a small, unpaired bone located in the midline of the anterior cranium, which is the superior aspect of the skull that encloses and protects the brain. The term ‘ethmoid’ originates from the Greek ‘ethmos,’ meaning sieve, reflecting its lightweight, spongy structure.

In this article, we will explore the anatomy of the ethmoid bone, including its location, relations, and structure.

Anatomical Structure

The ethmoid bone is one of the eight bones of the cranium. It is situated at the roof of the nasal cavity and between the two orbital cavities. It contributes to the medial wall of the orbit and forms part of the anterior cranial fossa, where it separates the nasal cavity (inferiorly) from the cranial cavity (superiorly). It also forms a significant portion of the nasal septum and the lateral nasal wall.

The olfactory nerve (CN I) has a close anatomical relationship with the ethmoid bone. Its numerous nerve fibers pass through the cribriform plate of the ethmoid bone to innervate the nasal cavity with the sense of smell.

Parts of the Ethmoid Bone

The ethmoid bone is made up of three parts:

1. Cribriform Plate
2. Perpendicular Plate
3. Ethmoidal Labyrinth

Cribriform Plate

The cribriform plate forms the roof of the nasal cavity. It is pierced by numerous olfactory nerve fibers, giving it a sieve-like structure. Projecting superiorly from the cribriform plate is the crista galli, which provides an attachment point for the falx cerebri, a sheet of dura mater that separates the two cerebral hemispheres.

Perpendicular Plate

The perpendicular plate descends from the cribriform plate, forming the superior two-thirds of the nasal septum.

Ethmoidal Labyrinths

The ethmoid bone contains two ethmoidal labyrinths. These are large masses located on either side of the perpendicular plate, which contain the ethmoidal air cells (sinuses). Two sheets of bone form each labyrinth:

• Orbital Plate: The lateral sheet of bone, which also forms 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.

Articulations

The ethmoid bone articulates with 13 other bones:

• Paired Bones: Nasal bones, maxillae, lacrimal bones, palatine bones, inferior conchae.
• Unpaired Bones: Frontal, vomer, and sphenoid bones.

Clinical Relevance

Ethmoid Fracture

The ethmoid bone can be fractured in cases of facial trauma, such as hitting the dashboard in a collision or a fall from height. Signs and symptoms of fracture are related to the anatomy of the ethmoid bone:

• Fracture of Cribriform Plate: Branches of the olfactory bulb may be sheared, causing anosmia (loss of sense of smell).
• Fracture of the Labyrinth: This may allow communication between the nasal cavity and the orbit, potentially causing orbital emphysema as air enters the orbit.

CSF Rhinorrhoea

A fracture to the cribriform plate may allow communication between the nasal cavity and the central nervous system, leading to cerebrospinal fluid (CSF) entering the nasal cavity and draining from the nose. This manifests as a clear, watery discharge from one side of the nose, known as CSF rhinorrhoea.

These leaks normally stop spontaneously and can be managed conservatively; however, surgery is sometimes required. Spontaneous CSF rhinorrhoea can also occur due to congenital or acquired defects in the ethmoid bone.