Ethmoid Bone

Introduction

The ethmoid bone is an unpaired cranial bone located in the central region of the face, between the two orbits. It forms the roof of the nasal cavity and the floor of the anterior cranial fossa in the skull. The ethmoid bone plays a vital role in providing structural support and housing the olfactory nerve endings responsible for the sense of smell.

It is one of the most complex bones in the human skull, featuring several thin bony plates and cellular cavities. The highly porous nature of the ethmoid bone makes it very light and fragile. In this article, we will explore the anatomy and physiology of this multi-functional cranial bone.

Anatomy

Ethmoid Bone Parts

The ethmoid bone has several parts that enable it to carry out its complex functions:

Perpendicular Plate

Thin, quadrilaterals-shaped bony plate projecting downward from the cribriform plate
Forms the superior part of the nasal septum which divides the left and right air passages
Articulates with vomer bone to complete the nasal septum
Has a small opening for passage of the nasociliary nerve

Labyrinth (Lateral Masses)

Consist of extremely thin-walled bony plates bordering upper parts of nasal cavities
Contain numerous small air-filled cavities called ethmoidal cells or sinuses
Cells are arranged in anterior, middle, and posterior groups based on location
Anterior cells open into the middle meatus of the nasal cavity
Posterior cells open into the superior meatus
Sinuses make bone lightweight, resonate sound, humidify air, produce mucus

Orbital Plates

Irregularly shaped bony plates projecting upward to form part of the medial walls of orbits
Separate ethmoidal cells from the orbital cavities
Have small openings for passage of nerve branches and blood vessels

Cribriform Plate

Horizontal bony plate forming the roof of the nasal cavity
Contain multiple small foramina for passage of olfactory nerve fibers
Forms part of the base of the anterior cranial fossa
Has lateral grooves that become ethmoidal canals carrying nerves and vessels

The complex internal structure of the ethmoid bone facilitates its vital neurosensory and respiratory functions. The delicate arrangements of plates and cells make it vulnerable to fractures and infections.

Borders and Articulations

The ethmoid bone articulates with several other facial and cranial bones:

Frontal Bone

The ethmoid articulates anteriorly with the vertical portion of the frontal bone
This occurs at the chorionic junction between the nasal bones and frontal bone
The cribriform plates meet to form the crista galli structure

Sphenoid Bone

Posteriorly, the sphenoid bone forms the primary articulation
Specifically where the sphenoid sinus meets the posterior ethmoid sinuses
It forms a key part of the central skull base

Lacrimal Bone

The medial walls of the orbits contain the lacrimal bone
This is the main lateral articulation of the ethmoid bone
Separates the ethmoid from the eye sockets

Nasal Bone

Inferior nasal septum articulates with the nasal crest of the maxillary bone
Septal cartilage also attaches to the nasal bones here
Allows ethmoid to take part in nose infrastructure

Foramina and Openings

Cribriform Plate

Contains multiple openings transmitting olfactory nerves
Over 20 foramina in honeycomb pattern pierced by filaments
Allows passage of olfactory nerves through bone to the nasal mucosa

Additional Openings

Grooves for ethmoidal nerves, arteries, and veins
Small canals transmit vessels and nerves to the nasal cavity
Connect anatomy between the cranial vault and nasal region

Embryology

The ethmoid bone begins as cartilage during fetal development and undergoes endochondral ossification, unlike other cranial bones.

Formed from the anterior cartilaginous part of the base of the embryonic cranium
Ossification centers appear in the perpendicular plate during 4th fetal month
Additional centers occur in labyrinths, orbital, and cribriform plates by 5th month
Sinuses are visible by 5th month and expand considerably after birth
Bone ossifies fully before puberty but enlarges outward as sinus cells grow

Nerves

The ethmoid bone transmits olfactory nerves and contains branches of the ophthalmic and maxillary nerves:

Olfactory nerves – I cranial nerve fibers pass through holes in the cribriform plate to reach nasal mucosa
Nasociliary nerve – a branch of the ophthalmic nerve that runs in a groove on labyrinths
Anterior and posterior ethmoidal nerves – branches supply mucosa of sinuses

Blood Supply

Arterial Supply:

The ocular artery branches into the anterior and posterior ethmoidal arteries.
Sphenopalatine artery
- a branch of the maxillary artery
Septal branches of the superior labial artery

Venous Drainage

Through facial veins ultimately to the internal jugular vein

Lymphatic Drainage

Lymph vessels present near the ethmoid bone drain to deep cervical lymph nodes. Specific nodes include:

Superficial parotid nodes
Upper deep cervical nodes
Retropharyngeal nodes
Submandibular nodes

The rich nerve supply, blood vessels, and lymphatic channels allow the metabolic needs of the ethmoid bone to be met and linked to the rest of the head.

Location & Functions

The ethmoid bone sits in an anteromedial position within the central skull base. It articulates laterally with the orbital plate of the frontal and sphenoid bones. The ethmoid bone interacts with several other adjacent facial bones:

Frontal bone – anterior articulation
Sphenoid bone – posterior articulation
Lacrimal bone – lateral articulation
Nasal bone – inferior articulation

This central position allows the ethmoid bone to connect the nasal cavity to the orbits and cranial cavity. The integral functions of the ethmoid bone include:

Forming part of the medial walls of orbits – The orbital plates contribute to the inner orbital walls, providing muscular attachments around the eyes.
Forming the roof of the nasal cavity – The horizontally oriented cribriform plate creates the ceiling of the nasal cavity, just beneath the cranial base.
Housing the paranasal sinuses – The ethmoid bone contains numerous small air-filled spaces known as ethmoidal sinuses, divided into anterior, middle, and posterior groups.

The complex anatomy of the ethmoid bone enables it to provide both structural and functional support in the delicate central areas of the skull base and nasal cavity. The bone plays a vital role in olfaction, respiration, orbital integrity, and anterior cranial fossa stability.

Common Disorders of the Ethmoid Bone

Ethmoid Sinusitis

Causes and Symptoms

Caused by nasal inflammation, allergies, infections
Swelling blocks sinus drainage leading to mucus buildup
Symptoms include nasal congestion, facial pain, headaches
May also cause fever, sinus pressure, loss of smell

Diagnostic Procedures

Physical exam of nose, sinuses, throat, ears, etc.
Nasal endoscopy to visualize sinus openings
CT scan of sinuses to see blockages
Allergy testing and microbial cultures

Treatment Options

Nasal corticosteroid sprays and antihistamines
Saline irrigation for clearing mucus
Antibiotics if bacterial infection is present
Surgery if medications fail to improve drainage

Ethmoid Bone Fractures

Types of Fractures

Linear fractures of thin orbital or cribriform plates
Comminuted fractures with shattered bony pieces
Common in trauma from vehicle accidents

Signs and Symptoms

Periorbital bruising, pain, and swelling
CSF rhinorrhea or bleeding from the nose
Altered vision, ocular misalignment
May see CNS issues from trauma

Management and Healing

Conservative treatment for non-displaced fractures
Surgery for displaced bones or cerebral fluid leaks
Monitoring for meningitis risk and vision loss
Healing aided by bone grafting if needed

Nasal Polyps and Ethmoidal Tumors

Understanding Nasal Polyps

Abnormal mucous membrane growths in the nose/sinuses
Arise commonly from ethmoidal mucosa
Allergic rhinitis and chronic inflammation involved

Ethmoidal Tumors

Inverted papilloma – benign tumor but locally aggressive
Squamous cell carcinoma – malignant tumor
Esthesioneuroblastoma – a rare cancer of olfactory nerves

Screening, Diagnosis, and Treatment

Nasal endoscopy and CT scans detect lesions
Biopsy analyses tumor type
Polypectomy or surgical resection
Chemoradiation therapy for cancers

Summary

The ethmoid bone is an unpaired cranial bone located centrally in the skull between the orbital cavities and the nasal cavity. It has several vital functions including forming the roof of the nose, the medial orbital walls, and housing the paranasal sinuses.

The complex internal structure of the ethmoid bone features thin perforated plates and an intricate labyrinth of air cells. The main parts include the perpendicular plate, forming the nasal septum; the labyrinth holding the anterior, middle, and posterior ethmoidal sinuses; the orbital plates constituting the medial orbital walls; and the horizontal cribriform plate with multiple openings for olfactory nerves.

The central position of the ethmoid bone allows it to articulate with the adjacent frontal, sphenoid, lacrimal, and nasal bones. It also contains various openings and grooves that allow the passage of important nerves and blood vessels. The rich nerve supply provides innervation to the sinuses and nasal cavity.

Due to the fragile construction and vital location, the ethmoid bone is susceptible to fractures and common disorders like sinusitis. Chronic inflammation can impair sinus drainage leading to fluid buildup, pain, and infections. Trauma may cause linear cracks or fragmentation of the thin ethmoid walls. Tumors and polyps may also arise within the sinus mucosa.

Treatment often relies on endoscopic surgery to access the internal anatomy for drainage, bone repair, or lesion removal. A comprehensive understanding of ethmoid bone anatomy is critical for managing associated disorders.

FAQs

Where is the ethmoid bone located?

The ethmoid bone is located at the midline of the central skull base, between the nasal cavity and orbital sockets, behind the bridge of the nose.

What structures articulate with the ethmoid bone?

The ethmoid bone articulates with the frontal, sphenoid, lacrimal, and nasal bones.

What is the function of the cribriform plate?

The cribriform plate forms the roof of the nasal cavity and has multiple small openings that allow passage of olfactory nerve fibers carrying the sense of smell.

How many ethmoidal sinuses are there?

There are several ethmoidal air cell sinuses surrounding the nasal cavity, usually divided into anterior, middle, and posterior groupings.

What are some common disorders of the ethmoid bone?

Chronic inflammatory conditions like ethmoid sinusitis, nasal polyps, tumors, and fractural cracks or breaks are frequently associated with disorders.

Why is the ethmoid bone important?

The ethmoid bone is important because it supports the medial walls of the eye sockets, houses the sense of smell nerves, allows sinus resonance, and anchors central skull base stability.

References

Ethmoid bone. (2023, October 30). Kenhub. https://www.kenhub.com/en/library/anatomy/the-ethmoid-bone
Yu, M. (2023, July 24). Anatomy, Head and Neck, Ethmoid Bone. StatPearls – NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK544328/
Hayes, K. (2023, June 6). The Anatomy of the Ethmoid Bone. Verywell Health. https://www.verywellhealth.com/ethmoid-bone-anatomy-4689011
Ethmoid Bone | Complete Anatomy. (n.d.). www.elsevier.com. https://www.elsevier.com/resources/anatomy/skeletal-system/axial-skeleton/ethmoid-bone/23626#section-list-of-clinical-correlates-4
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Introduction