Ethmoid bone anatomy: the cribriform plate, perpendicular plate and ethmoid sinus

Also called

Ethmoid bone

What it is

A delicate cube-shaped bone between the orbits.

Location

Lies behind the nose and in front of the sphenoid, forming part of the nasal septum and cavity.

Function

Supports the nasal cavity and olfactory nerve fibres; contains ethmoid air cells.

Body system

Skeletal · single, midline structure

FMA ID

FMA52740 (Foundational Model of Anatomy)

Key facts

• The cribriform plate allows olfactory nerve fibres to pass; the crista galli anchors the falx cerebri.
• Its orbital plate forms much of the thin medial wall of the orbit, and it carries the superior and middle nasal conchae.

Clinical relevance

Fracture of the cribriform plate can cause cerebrospinal fluid leakage from the nose.

The ethmoid is one of the eight cranial bones. It sits in the anterior cranium, at the roof of the nasal cavity and between the two orbital cavities. Its upper surface forms part of the floor of the anterior cranial fossa, its sides form part of the medial wall of each orbit, and below it forms a large part of the nasal septum and the lateral walls of the nasal cavity. (TeachMeAnatomy: The Ethmoid Bone.)

Kenhub describes it as a small, cuboidal, porous bone. It sits in the ethmoidal notch of the frontal bone, with the frontal bone in front, the sphenoid bone behind, and the vomer and inferior nasal conchae below. (Kenhub: Ethmoid bone.)

The ethmoid bone has four main parts: a horizontal cribriform plate, a vertical perpendicular plate that hangs down from it, and two ethmoidal labyrinths, one on each side of the perpendicular plate. The crista galli rises from the upper surface of the cribriform plate in the midline. (TeachMeAnatomy: The Ethmoid Bone.)

Each labyrinth has two thin bony sheets. The lateral sheet is the orbital plate, which forms the medial wall of the orbit. The medial sheet forms the upper part of the lateral wall of the nasal cavity, and the superior and middle nasal conchae project from it into the nasal cavity. (TeachMeAnatomy: The Ethmoid Bone.)

The orbital plate is also called the lamina papyracea, Latin for paper-thin. It is the main component of the medial orbital wall and the lateral surface of the ethmoid air cells. It meets the frontal bone above, the maxilla and palatine bone below, the lacrimal bone in front and the sphenoid behind. (Radiopaedia: Lamina papyracea.)

The cribriform plate (lamina cribrosa) is a sieve-like plate between the anterior cranial fossa and the nasal cavity. It is perforated by foramina for the olfactory nerves. The anterior ethmoidal nerves also pass down through it, through slits beside the crista galli, to reach the upper part of the nasal cavity. (Radiopaedia: Cribriform plate.)

The olfactory bulbs lie on the cribriform plate in two grooves, one on each side of the crista galli. The crista galli itself is a thick triangular crest that gives anterior attachment to the falx cerebri, the fold of dura mater between the cerebral hemispheres. It may contain marrow or air, and an air-filled crista galli can look like a mass on MRI. (Radiopaedia: Crista galli.)

The perpendicular plate (lamina perpendicularis) is a thin vertical sheet of bone that descends in the midline from the cribriform plate. Inferiorly it attaches to the septal cartilage and forms part of the nasal septum, the partition between the right and left nasal cavities. (Kenhub: Ethmoid bone.)

In a dry skull the bony nasal septum has two parts: the perpendicular plate of the ethmoid forms its upper part and the vomer forms its lower and posterior part. In life the septal cartilage fills the gap at the front, which is why the septum looks incomplete on a dry skull. (OpenStax Anatomy and Physiology 2e, 7.2: The Skull.)

The ethmoid sinus is not one cavity but a group of small air cells inside each labyrinth, 3 to 18 on each side, separated by thin bony walls. The basal lamella, the lateral attachment of the middle concha, divides them into anterior and posterior groups; older books also describe a middle group, which is now included in the anterior group. (Radiopaedia: Ethmoidal air cells.)

Anterior ethmoid cells drain into the middle meatus through the hiatus semilunaris, and posterior cells drain into the superior meatus. The cells are present at birth, grow quickly up to about 4 years and enlarge again from about 8 to 12 years. Their blood comes from the anterior and posterior ethmoidal arteries (from the ophthalmic artery) and the sphenopalatine artery, and their sensation from the anterior and posterior ethmoidal nerves, branches of the nasociliary nerve (CN V1). (Radiopaedia: Ethmoidal air cells.)

The ethmoid also shapes the ostiomeatal complex, the shared drainage pathway of the frontal sinus, anterior ethmoid cells and maxillary sinus. The uncinate process, a hook-like plate, forms the front edge of the hiatus semilunaris, and the ethmoid bulla, usually a single large air cell, lies above it. Narrowing of this area by normal variants can predispose to sinusitis. (Radiopaedia: Ostiomeatal complex.)

A common variant is concha bullosa, an air cell inside the middle concha, seen in about 35% of patients on imaging. It is usually harmless but, if large, can block the middle meatus and cause nasal obstruction. (Radiopaedia: Concha bullosa.)

The ethmoid articulates with 13 bones. Three are single: the frontal bone, the sphenoid bone and the vomer. Ten are paired: the nasal bones, maxillae, lacrimal bones, palatine bones and inferior nasal conchae. (TeachMeAnatomy: The Ethmoid Bone.)

The ethmoid ossifies by endochondral ossification, from a cartilage model. At birth the labyrinths are small and the perpendicular and cribriform plates are still mostly cartilage. The plates later join the labyrinths, and by about 2 years of age the parts have fused into a single bone. (Kenhub: Ethmoid bone.)

Radiopaedia notes that the anterior skull base is largely cartilaginous at birth and that ossification of the crista galli and perpendicular plate begins at around 2 months of age. (Radiopaedia: Crista galli.)

Ethmoid fractures usually follow facial trauma, such as hitting a dashboard or a fall from height. A fracture of the cribriform plate can shear the olfactory nerve fibres and cause anosmia (loss of smell). A fracture of the labyrinth can open a path between the nasal cavity and the orbit, so air enters the orbit (orbital emphysema). (TeachMeAnatomy: The Ethmoid Bone.)

A cribriform plate fracture can also connect the nasal cavity with the space around the brain, so cerebrospinal fluid drips from the nose (CSF rhinorrhoea). It typically appears as clear, watery discharge from one nostril. Many leaks stop on their own, but some need surgical repair. Spontaneous leaks can occur through congenital or acquired defects in the bone. (TeachMeAnatomy: The Ethmoid Bone.)

Both are single midline bones of the cranial base, and students often mix them up. The ethmoid lies in front, between the orbits and above the nasal cavity, and its cribriform plates are perforated by olfactory foramina. The sphenoid lies behind it in the middle of the skull base, has greater and lesser wings, and holds the pituitary gland in its sella turcica. The ethmoid contains many small ethmoid air cells, while the sphenoid body contains the sphenoid sinus, the most posterior of the paranasal sinuses. (OpenStax Anatomy and Physiology 2e, 7.2: The Skull.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Short ciliary nerve · nervous
• Nasociliary nerve · nervous
• Medial rectus · muscular
• Anterior ethmoidal nerve · nervous
• Lacrimal bone · skeletal
• Vomer · skeletal
• Inferior rectus · muscular
• Lacrimal sac · eye & ear
• Long ciliary nerve · nervous
• Ophthalmic artery · cardiovascular