The ethmoid bone is an unpaired bone of the skull known for its complex and irregular shape, positioned deep within the face. Located centrally, it contributes significantly to the architecture of the nasal cavity and the orbits, or eye sockets. Its name derives from the Greek word “ethmos,” meaning sieve, reflecting its unique, porous structure. This delicate, spongy bone acts as a partition, separating the nasal cavity from the cranial cavity that houses the brain.
Location and Position in the Skull
The ethmoid bone occupies a recessed, midline position in the anterior skull, largely hidden by surrounding facial and cranial bones. Situated between the two orbital cavities, it forms a substantial portion of their medial walls and acts as a boundary between the braincase, eye sockets, and nasal airway.
Superiorly, the ethmoid bone completes the floor of the anterior cranial fossa, the lowest and most forward section of the braincase. This location places it immediately below the frontal lobe of the brain. Inferiorly, it forms the roof of the nasal cavity, the air passage directly behind the nose.
The bone articulates with numerous surrounding bones, stabilizing the central facial skeleton. It connects with the frontal bone above and the sphenoid bone posteriorly, which forms the back of the eye socket. Additional articulations include the nasal bones, lacrimal bones, and maxillae.
Detailed Anatomical Structure
The ethmoid bone is composed of three primary structural components: the cribriform plate, the perpendicular plate, and the two ethmoidal labyrinths. The most superior part is the cribriform plate, a thin, horizontal sheet of bone that forms the roof of the nasal cavity.
The cribriform plate is perforated by numerous tiny openings, or foramina, which give the bone its sieve-like appearance. These small holes allow the filaments of the olfactory nerves to pass from the nasal mucosa into the cranial cavity, connecting with the olfactory bulb and enabling the sense of smell. Projecting upward from the cribriform plate is the crista galli, a small, triangular crest of bone that serves as an attachment point for the dura mater.
The perpendicular plate extends vertically downward from the cribriform plate, contributing to the superior portion of the nasal septum. This vertical partition divides the nasal cavity into two halves. While thin, the plate provides structural support for the airway and helps ensure proper airflow.
On either side of the perpendicular plate are the paired ethmoidal labyrinths, also referred to as the lateral masses. These cube-shaped masses contain a honeycomb arrangement of air-filled spaces called the ethmoidal air cells, which form the ethmoid sinuses. The lateral surface of each labyrinth is a smooth, thin plate known as the lamina papyracea, which forms the majority of the medial orbital wall.
The medial surface of the labyrinths projects into the nasal cavity, creating the superior and middle nasal conchae, or turbinates. These curved, shelf-like structures condition inhaled air by increasing the surface area of the nasal lining, warming, humidifying, and filtering the air before it reaches the lungs.
Clinical Relevance and Vulnerability
The ethmoid bone’s delicate, thin-walled structure and central location make it susceptible to damage and infection. Due to its close proximity to the brain and the eye, trauma to the midface can result in severe complications. High-energy impacts, such as those sustained in motor vehicle accidents, can lead to naso-orbito-ethmoid (NOE) fractures, involving the nasal bones, orbits, and the ethmoid complex.
Fractures of the cribriform plate are a significant concern because they breach the barrier between the nasal cavity and the braincase. This damage can cause cerebrospinal fluid (CSF) to leak into the nasal cavity, a condition called CSF rhinorrhea. This leakage provides a direct pathway for bacteria from the nose to enter the cranial cavity, greatly increasing the risk of infections such as meningitis.
Damage to the olfactory nerve filaments passing through the cribriform plate is a common result of trauma, often leading to anosmia, the complete loss of the sense of smell. Fractures involving the ethmoidal labyrinths can also compromise the lamina papyracea, allowing air from the nasal cavity to enter the orbital tissues, resulting in orbital emphysema.
The ethmoidal air cells within the lateral masses are frequent sites of inflammation and infection, known as ethmoid sinusitis. The narrow drainage pathways of these sinuses can easily become blocked, trapping mucus and pathogens. This condition is managed with antibiotics or, in persistent cases, surgical intervention to widen the drainage channels.