The ethmoid bone is a light, spongy bone at the skull base that forms part of the nasal cavity and eye orbits.
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.
The ethmoid bone is a single, unpaired bone with a highly irregular and lightweight structure. It is composed of three main parts:
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.
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.
The ethmoid bone articulates with 13 bones:
The ethmoidal sinuses are air-filled cavities within the lateral masses of the ethmoid bone. They are divided into:
These sinuses vary in size and number and are important in sinus physiology and pathology.
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.
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).
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.
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.
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.
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.
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.
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.