Etmoid: What is it? Anatomy, Ossification, Function and Pathologies of A.Griguolo

Generality

The ethmoid, or ethmoid bone, is the uneven bone of the skull, located between the two eye sockets, under the frontal bone and anterior to the sphenoid bone.

The ethmoid is a cubic-shaped bone, which is characterized by 4 anatomical elements: the cribroso plate, the vertical plate and the two ethmoid labyrinths.

The ethmoid is an important bone, because: it contributes to the formation of the anterior cranial fossa, constitutes the medial wall of the eye sockets, forms the roof of the nasal cavities and the nasal septum, fixes the nasal cartilage, engages the cerebral sickle and, finally, through the holes of the cribroso plate, it allows the passage of the nerve fibers of the olfactory nerve (I cranial nerve).

What is the Etmoid?

The ethmoid, or ethmoid bone, is an unequal bone of the skull, known to form the medial wall (or inner wall) of the eye sockets, part of the nasal cavity and part of the nasal septum .

From the cubic form, the ethmoid is one of the 8 bony elements that make up the so-called neurocranium (or cranial box ); the neurocranium is the upper skeletal complex of the skull, which has the important task of enclosing and protecting the brain .

Short anatomical revision of the Skull: Neurocranium and Splancnocranium

Introduction: before proceeding with the description of the ethmoid, it is necessary to briefly review the subdivisions of the skull.

The skull of the human being can be divided into two main skeletal complexes: the aforementioned neurocranium and the splancnocranium .

Deputy to act as a container for the brain, the neurocranium is a structure composed of 8 bone elements: the frontal bone, the two temporal bones, the two parietal bones, the occipital bone, the sphenoid bone and already mentioned ethmoid bone.

Destined to give shape to the face, on the other hand, the splancnocranium is a structure consisting of as many as 14 bones, which are: the two zygomatic bones, the two tear bones, the two nasal bones, the two palatine bones, the two lower nasal horns, the two maxillary bones, the vomer and the mandible.

Origin of the term Ethmoid

The term "ethmoid" comes from ancient Greek, to be precise from the word " ethmos " (" ἠθμός "), which means " sponge ".

The use of the term "ethmoid" is due, as will be seen later, to the particular spongy aspect of the cranial bone described in this article.

Anatomy

The ethmoid is a cubic bone, characterized by a porous appearance that makes it look like a sponge .

Component of the so-called anterior cranial fossa, the ethmoid is anatomically divided into 4 parts:

The cribroso plate, also called cribriform plate or horizontal plate,
The vertical plate, also known as perpendicular plate, e
The 2 ethmoid labyrinths .

Position: Where is the Etmoid?

The ethmoid resides along the midline of the face, between the upper section of the nasal cavities and the anterior portion of the encephalon, in the middle of the two ocular orbits (of which it constitutes the inner wall), under the frontal bone and anterior to the sphenoid bone .

Parts of the Etmoid

CRIBROSOUS PLATE OR CRIBIFORM PLATE

The cribroso plate is the horizontal bone plate, which constitutes the central upper portion of the ethmoid and from which the ethmoid labyrinths branch off, laterally, and the vertical plate, below.

The cribroso plate helps to form the roof of the nasal cavities and represents the ethmoid portion forming part of the anterior cranial fossa.

The cribroso dish is important above all because:

It is the supporting surface of the two olfactory bulbs . Located above the cribroso plate, the olfactory bulbs are the centers for processing olfactory information perceived by the olfactory epithelium and destined to reach the brain, through the nerve fibers of the olfactory nerve (I cranial nerve);
It has a porous surface, that is characterized by small holes. These holes are the channels through which the nerve fibers of the olfactory nerve pass, connecting the olfactory epithelium of the nasal cavities to the aforementioned olfactory bulbs.
In other words, the cribiform dish is the ethmoid portion which, through its porosity, allows communication between the olfactory bulbs and the olfactory epithelium;
It is provided, in an upper position, with a vertical prominence, which prefers the name of crista galli (or cockscomb ). So called for the similarity to the crest of a rooster, the crista galli is the part of the ethmoid that rises to separate the two olfactory bulbs and on which it finds a very particular trait of the meningeal dura mater, called the cerebral sickle .

Curiosity

The human being perceives the smells when the aromatic molecules of the latter reach the olfactory epithelium that covers the nasal cavities; the olfactory epithelium is in fact equipped with receptors capable of transmuting the odor perceived in a nerve impulse and sending it, through the nerve fibers of the olfactory nerve, to the olfactory bulbs, first, and to the encephalon, then.

Within this framework, the olfactory bulbs are comparable to the centers of processing of the nervous signal, while the encephalon represents the final site of information processed along the previous lines of the olfactory pathway.

VERTICAL PLATE

The vertical plate of the ethmoid is the thin polygonal bone plate, which develops inferiorly to the cribroso plate, acting as a perpendicular of the latter.

Oriented in the opposite direction to the crista galli, the vertical plate is important because:

With its lower tract, it helps to form the nasal septum. The nasal septum is the bone lamina that separates the nasal cavities;
Its posterior border is the portion of ethmoid that communicates with the sphenoid and the vomer (two other bones of the skull);
Its lower edge is a fixation site for the nasal cartilage .

ETMOIDAL LABYRINTHS

The ethmoid labyrinths are two considerable bony masses, which arise at the lateral margins of the cryogenic plate and develop downwards, acting as elements of separation between the nasal cavities and the eye sockets.

From the structural point of view, each ethmoid labyrinth is characterized by: The ethmoid sinuses . Also known as ethmoid cells or ethmoid cells, the ethmoid sinuses are cavities inside the ethmoid labyrinths, filled with air and provided with an opening that puts them in communication with the nasal cavities.

For each ethmoid labyrinth, it is possible to recognize 3 groups of ethmoid cells: the anterior group, the median group and the posterior group (NB: the parameter of distinction of these 3 groups is, as it is easy to understand, the position occupied inside of the ethmoid labyrinth).
The ethmoid sinuses are examples of paranasal sinuses, therefore they serve to: improve the perception of odors, amplify the sounds and the voice emitted through the vocal cords, make the skull less heavy and humidify-heat-purify the inhaled air;

The orbital plate . The orbital plate is a thin bone lamina, which forms the orbital surface of each ethmoid labyrinth and closes the ethmoid sinuses laterally.
In fact, the orbital plate is the ethmoid portion that constitutes the medial wall of the eye sockets.
The upper nasal cornetto and the median nasal cornet . The superior nasal cornetto and the median nasal cornetto are the bony formations that constitute the nasal wall of the ethmoid (ie the surface of the ethmoid that "looks" towards the nasal cavities) and internally close the ethmoid sinuses.

What are the other paranasal sinuses?

The paranasal sinuses are cavities filled with air with a seat inside the cheeks and forehead, resulting from the particular anatomy and arrangement of the ethmoid, sphenoid, frontal and maxillary bones.

In addition to the aforementioned ethmoid sinuses, the paranasal sinuses include: the sphenoid sinuses, the frontal sinuses and the maxillary sinuses.

Borders and Reports

The ethmoid is in contact with as many as 13 bones of the skull ; to be precise, the ethmoid bone borders with 3 unequal cranial bones - which are the frontal, the sphenoid and the vomer - and 10 even cranial bones - which are the two lower nasal horns, the two palatine bones, the two tear bones, the two maxillary bones and the two nasal bones.

The ethmoid is bordered by 2 bones of the neurocranium (the frontal and the sphenoid) and 11 bones of the splancnocranium (the vomer, the lower nasal horns, the palatine bones, the tear bones, the maxillary bones and the nasal bones).

Joints

To link the ethmoid to the 13 bones of the neighboring skull are joints of a fibrous type (therefore not very mobile), which the experts identify with the generic name of cranial sutures .

More in detail, among these fibrous articulations, there are:

The spheno-ethmoid suture, which joins the sphenoid to the ethmoid (it is an unequal articulation);
The fronto-ethmoid suture, which joins the frontal bone to the ethmoid (is unequal);
The ethmoid-concal suture, which connects the ethmoid to the inferior nasal cornet (it is even);
The ethmoid-lacrimal suture, which joins the ethmoid to the lacrimal bone (it is even);
The ethmoid-maxillary suture, which joins the ethmoid to the maxillary bone (it is even);
The ethmoid-nasal suture, which connects the ethmoid to the nasal bone (it is even);
The ethmoid-vomerary suture, which connects the ethmoid to the vomer.

Ossification

The ethmoid is a bone whose definitive formation depends on the activity of 3 ossification centers : a center of ossification on each portion destined to become an ethmoid labyrinth and a center of ossification on the portion destined to become the vertical plate.

The ossification centers located where the ethmoid labyrinths take shape begin their activity between the fourth and fifth months of intrauterine life. From them depends the generation not only of ethmoid labyrinths, but also of a large part of the cribroso dish (the rooster crest is excluded);
The ossification center located where the vertical plate will form begins to activate during the first year of extrauterine life. Not only the vertical plate, but also the rooster crest derives from this ossification center;
As a rule, the fusion of the various components of the ethmoid in the ossification phase takes place around the second year of extrauterine life.

Function

Experts recognize the ethmoid at least 5 functions, which are:

Contribute to the formation of the anterior cranial fossa;

Constitute the medial wall of the eye sockets;
Form the roof of the nasal cavities;
Constitute the nasal septum and anchor (on the lower edge of the vertical plate) the nasal cartilage;
Hook (on the rooster crest ) the cerebral sickle;
Through the various holes on the cryogenic plate, allow the passage of the nerve fibers that make up the olfactory nerve and connect the olfactory epithelium to the olfactory bulbs.

diseases

As a result of violent facial trauma, the ethmoid may be fractured .

In particular, the fractures of the cryogenic plate of the ethmoid and the fractures of the ethmoid labyrinths deserve a mention.

In most cases, fractures of the ethmoid are the result of facial traumas following car accidents or falls from great heights.

Fracture of the Cribroso Plate: the possible consequences

Fractures of the cryogenic dish of the ethmoid cause pain in the face and can be associated with:

Anosmia (inability to smell), if they have also involved a lesion of the nerve fibers of the olfactory nerve, which pass through the pores of the ethmoid,

and / or

Cerebrospinal rhinorrhea (loss of cerebrospinal fluid from the nose), if they have created an abnormal pathway between the nasal cavities and one of the cerebral ventricles in which the cerebrospinal fluid is contained.

Fracture of the ethmoidal labyrinths: the possible consequences

In addition to facial pain, fractures of ethmoid labyrinths can be associated with orbital emphysema, a condition characterized by the passage and accumulation in the soft tissues surrounding the eye sockets of the air contained in the ethmoid sinuses.