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Ethmoid bone
Ethmoid bone
Ethmoid bone
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Ethmoid bone
Ethmoid inside the orbit (brown)
Animation of the ethmoid bone
Details
Identifiers
Latinos ethmoidale
MeSHD005004
TA98A02.1.07.001
TA2721
FMA52740
Anatomical terms of bone

The ethmoid bone (/ˈɛθmɔɪd/;[1][2] from Ancient Greek: ἡθμός, romanized: hēthmós, lit. 'sieve') is an unpaired bone in the skull that separates the nasal cavity from the brain. It is located at the roof of the nose, between the two orbits. The cubical (cube-shaped) bone is lightweight due to a spongy construction. The ethmoid bone is one of the bones that make up the orbit of the eye.

Structure

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The ethmoid bone is an anterior cranial bone located between the eyes.[3] It contributes to the medial wall of the orbit, the nasal cavity, and the nasal septum.[3] The ethmoid has three parts: cribriform plate, ethmoidal labyrinth, and perpendicular plate. The cribriform plate forms the roof of the nasal cavity and also contributes to formation of the anterior cranial fossa,[4] the ethmoidal labyrinth consists of a large mass on either side of the perpendicular plate, and the perpendicular plate forms the superior two-thirds of the nasal septum.[3] Between the orbital plate and the nasal conchae are the ethmoidal sinuses or ethmoidal air cells, which are a variable number of small cavities in the lateral mass of the ethmoid.[5][6]

Articulations

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The ethmoid articulates with thirteen bones:

Development

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The ethmoid is ossified in the cartilage of the nasal capsule by three centers: one for the perpendicular plate, and one for each labyrinth.

The labyrinths are first developed, ossific granu[clarification needed]. At birth, the bone consists of the two labyrinths, which are small and ill-developed. During the first year after birth, the perpendicular plate and crista galli begin to ossify from a single center, and are joined to the labyrinths about the beginning of the second year.

The cribriform plate is ossified partly from the perpendicular plate and partly from the labyrinths.

The development of the ethmoidal cells begins during fetal life.

Function

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Role in magnetoception

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Main article: Magnetoception

Some birds and other migratory animals have deposits of biological magnetite in their ethmoid bones which allow them to sense the direction of the Earth's magnetic field. Humans have a similar magnetite deposit (ferric iron), but it is believed to be vestigial.[7]

Clinical significance

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Fracture of the lamina papyracea, the lateral plate of the ethmoid labyrinth bone, permits communication between the nasal cavity and the orbit on the same side of the body through the inferomedial orbital wall, resulting in orbital emphysema. Increased pressure within the nasal cavity, as seen during sneezing, for example, leads to temporary exophthalmos.

The porous fragile nature of the ethmoid bone makes it particularly susceptible to fractures. The ethmoid is usually fractured from an upward force to the nose. This could occur by hitting the dashboard in a car crash or landing on the ground after a fall. The ethmoid fracture can produce bone fragments that penetrate the cribriform plate. This trauma can lead to a leak of cerebrospinal fluid into the nasal cavity. These openings let opportunistic bacteria in the nasal cavity enter the sterile environment of the central nervous system (CNS). The CNS is usually protected by the blood–brain barrier, but holes in the cribriform plate let bacteria get through the barrier. The blood–brain barrier makes it extremely difficult to treat such infections, because only certain drugs can cross into the CNS.

An ethmoid fracture can also sever the olfactory nerve. This injury results in anosmia (loss of smell). A reduction in the ability to taste is also a side effect because it is based so heavily on smell. This injury is not fatal, but can be dangerous, as when a person fails to smell smoke, gas, or spoiled food.[3] In fact, older people with anosmia were more than four times as likely to die in five years compared to those with a healthy sense of smell.[8]

Additional images

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  • Ethmoid bone from above.
    Ethmoid bone from above.
  • Perpendicular plate of ethmoid.
    Perpendicular plate of ethmoid.
  • Ethmoid bone (view from behind).
    Ethmoid bone (view from behind).
  • Ethmoid bone from the right side.
    Ethmoid bone from the right side.
  • Side view of the skull.
    Side view of the skull.

See also

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This article uses anatomical terminology.

References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ethmoid bone

View on Grokipedia
from Grokipedia
The ethmoid bone is an unpaired, cuboidal cranial bone located at the roof of the , forming part of the and the medial walls of the orbits, and characterized by its light, spongy, pneumatized structure enclosing ethmoidal air cells. Positioned anterior to the and inferior to the , the ethmoid bone separates the from the nasal and orbital regions, contributing significantly to the and upper respiratory anatomy. It consists of three main components: the horizontal , which features foramina for the passage of filaments and supports the attachment of the ; the unpaired perpendicular plate, which projects downward to form the posterior ; and the paired ethmoidal labyrinths, which house the ethmoidal air cells and are separated from the orbits by the thin lamina papyracea. The bone articulates with up to 13 other cranial and facial bones, including the frontal, sphenoid, lacrimal, , maxillae, and inferior nasal conchae, providing structural stability to the midface and facilitating sinus drainage through the osteomeatal complex. Functionally, it supports olfaction by housing , aids in air humidification, warming, and within the nasal passages, and contributes to and ventilation via its air cells. Developmentally, the ethmoid bone ossifies through endochondral mechanisms beginning around 25 to 28 weeks of gestation, with its sinuses present at birth and pneumatizing further during childhood. Clinically, the ethmoid bone's delicate structure makes it vulnerable to trauma, where fractures can lead to complications such as leakage, orbital , or , and it is implicated in conditions like chronic rhinosinusitis, often addressed through .

Anatomy

Components

The ethmoid bone presents a distinctive butterfly-shaped morphology when viewed superiorly, characterized by a central body flanked by lateral expansions. This lightweight, spongy structure is located at the midline of the anterior base, contributing to the separation between the and the . The bone comprises three primary components: the , the perpendicular plate, and the paired ethmoidal labyrinths. The forms a horizontal, sieve-like sheet positioned at the roof of the and the floor of the . It is perforated by numerous cribriform foramina, which transmit filaments of the (cranial nerve I) from the to the olfactory bulbs in the . The plate also contains shallow depressions known as foveolae ethmoidales, which house branches of the anterior and posterior ethmoidal nerves. The perpendicular plate is a thin, central, unpaired vertical lamina that constitutes the superior portion of the . It descends from the midline of the and extends inferiorly to articulate with the bone, thereby dividing the into left and right halves. The ethmoidal labyrinths are paired cuboidal masses situated laterally to the perpendicular plate, each enclosing a network of ethmoidal air cells that form the ethmoidal sinuses. These labyrinths project medially as the curved superior and middle nasal conchae, which increase the surface area within the . The lateral walls of the labyrinths are thin and paper-like, known as the lamina papyracea, forming part of the medial orbital walls. Overall, the ethmoid bone relates anteriorly to the roof via the , laterally to the medial walls of the orbits through the lamina papyracea of the labyrinths, and superiorly to the base of the .

Articulations

The ethmoid bone forms numerous articulations with surrounding cranial and facial bones, primarily through fibrous sutures that ensure structural integrity of the , , and orbital walls. These connections, often serrated in nature, interlock the irregular surfaces of the bones to distribute mechanical stresses and maintain stability during mastication and head movements. Anteriorly, the ethmoid bone articulates with the via the frontoethmoidal suture, which runs along the posterior margin of the and the superior aspect of the orbital plates, forming a serrated junction that separates the from the . This suture contributes to the stability of the skull base by anchoring the ethmoid's roof-like structure. Posteriorly, the ethmoid connects to the through the sphenoethmoidal suture, located at the junction of the sphenoidal crest with the perpendicular and s, as well as along the body of the sphenoid; this articulation provides a firm posterior boundary for the ethmoid and enhances overall rigidity. Medially, the perpendicular plate of the ethmoid bone attaches to the vomer along its posterior border, forming the superior portion of the bony nasal septum that divides the nasal cavity; this connection, reinforced by a fibrous suture, supports midline stability and prevents septal deviation. Laterally, the ethmoidal labyrinths establish connections with the lacrimal, maxillary, and palatine bones through their medial and inferior surfaces, while the thin orbital plates (lamina papyracea) attach superiorly to the frontal bone, anteriorly to the lacrimal bone, and inferiorly to the maxillary and palatine bones, collectively forming the medial walls of the orbits. These serrated sutures around the labyrinths provide lateral reinforcement, protecting the orbital contents and linking the facial skeleton to the neurocranium. Inferiorly, the ethmoid bone articulates with the inferior nasal conchae along the lateral masses of the labyrinths, where the conchae groove into the ethmoidal surfaces, and with the nasal bones anteriorly via the superior margins of the perpendicular plate, supporting the nasal roof and enhancing ventilatory pathway stability.

Vascular and Neural Supply

The arterial supply to the ethmoid bone is primarily provided by the anterior and posterior ethmoidal arteries, both of which arise as branches of the within the . The anterior ethmoidal artery enters the ethmoid bone through the anterior ethmoidal , located in the frontoethmoidal suture along the medial orbital wall, and supplies the anterior ethmoidal air cells, , and anterosuperior . Similarly, the posterior ethmoidal artery passes through the posterior ethmoidal , situated posterior to the anterior in the same suture line, to vascularize the posterior ethmoidal air cells, posterior , and adjacent . Venous drainage of the ethmoid bone follows the arterial pathways via the anterior and posterior ethmoidal veins, which converge into the ophthalmic venous plexus and ultimately drain into the superior and inferior ophthalmic veins through the superior orbital fissure. These veins also communicate with the cavernous sinus, forming potential routes for intracranial spread of infection. Neural innervation of the ethmoid bone derives from the anterior and posterior ethmoidal nerves, branches of the nasociliary nerve, which itself is a division of the ophthalmic nerve (CN V1, trigeminal nerve). The anterior ethmoidal nerve traverses the anterior ethmoidal foramen to provide sensory innervation to the mucosa of the anterior ethmoidal sinuses, anterosuperior nasal cavity, and external nasal skin via its external nasal branch. The posterior ethmoidal nerve enters through the posterior ethmoidal foramen, supplying sensory fibers to the posterior ethmoidal sinuses and the dura overlying the ethmoid roof. Additionally, the cribriform plate of the ethmoid bone transmits filaments of the olfactory nerve (CN I) for the sense of smell. Lymphatic drainage from the ethmoid bone varies by region: anterior portions drain to the submandibular lymph nodes, while posterior aspects route to the retropharyngeal nodes. The anterior and posterior ethmoidal foramina, critical for these vascular and neural structures, lie along the frontoethmoidal suture, with the anterior foramen typically 24 mm posterior to the anterior lacrimal crest and the posterior foramen about 6-12 mm further back. Clinically, these arteries pose a risk of significant epistaxis if injured, as their branches contribute to the vascular network of Little's area (Kiesselbach's plexus) on the anterior nasal septum, a common site of nosebleeds.

Development

Embryology

The ethmoid bone originates primarily from neural crest-derived during early embryogenesis, with cells migrating from the dorsal around the fourth week of gestation to populate the developing craniofacial region. These cells, originating from the and levels, contribute to the formation of the anterior cranial base, including the ethmoid precursors, while paraxial mesoderm plays a limited role confined to more posterior cranial elements. The migration of these cells is guided by signaling pathways such as FGF and BMP, establishing the mesenchymal framework for the chondrocranium. By the sixth to seventh week of , the nasal placodes—ectodermal thickenings induced around the fifth week—stimulate the underlying to form the initial anlage of the ethmoid bone as part of the nasal capsule, a cartilaginous precursor to the chondrocranium. This capsule encases the developing nasal cavities and olfactory structures, with the ethmoid anlage appearing as condensations of mesenchymal tissue that begin chondrification, laying down the foundational cartilage model for the bone's labyrinths and plates. Concurrently, the optic placodes influence the positioning of ethmoidal precursors adjacent to the orbital regions, ensuring integration with the developing eyes. During the eighth week, the ethmoidal precursors undergo migration and fusion, where bilateral components of the nasal capsule converge midline to form the and lateral labyrinths, while the mesethmoid emerges to outline the plate. This process is influenced by interactions with the first derivatives, particularly through the frontonasal prominence, which contributes mesenchymal support to the plate's development along the . By the end of the eighth week, these cartilaginous elements establish the basic architecture of the ethmoid, setting the stage for later .

Ossification

The ethmoid bone primarily develops through , with substantial postnatal progression following its partial prenatal formation from precursors. At birth, the labyrinths are small and partially ossified, forming the lateral masses, while the , perpendicular plate, and remain predominantly cartilaginous. Ossification of the perpendicular plate and initiates from a single endochondral center in the mesethmoid during the first year of life, proceeding upward from the base. Multiple centers contribute to the bone's maturation, including those in the labyrinths that expand postnatally and integrate with the midline structures. The begins ossifying around 2 months of age, often by extension from the labyrinth centers or independent foci, achieving substantial by 2 years, though complete solidity may extend to . The labyrinth centers continue developing, with full and expansion occurring by 12-14 years as the ethmoidal air cells pneumatize. Fusion of the perpendicular plate with the labyrinths typically happens around the second year, while overall integration of centers, including the perpendicular plate, completes by . Full pneumatization of the ethmoidal cells, driven by expansion and sinus outgrowth, progresses through childhood and concludes by 12 years of age. Growth patterns of the ethmoid bone are influenced by the enlarging and paranasal sinus development, resulting in progressive expansion of the labyrinths and refinement of the plates. Histologically, the initial postnatal bone tissue consists of woven bone deposited by osteoblasts at the ossification fronts, which remodels over time into organized lamellar bone for enhanced strength and vascular integration. This transition supports the bone's adaptation to mechanical stresses from adjacent structures like the and orbits.

Functions

Structural Roles

The ethmoid bone plays a critical role in the mechanical of the , providing essential support and compartmentalization within the nasal and cranial regions. Its intricate contributes to and reinforcement of key facial cavities, while its pneumatized elements help distribute forces during mechanical stress. This bone's contributions are vital for maintaining the spatial integrity of the upper face and anterior cranium. The perpendicular plate of the ethmoid bone forms the superior portion of the nasal septum, effectively dividing the left and right nasal cavities and providing a midline structural barrier that enhances the overall stability of the nasal framework. This plate articulates superiorly with the cribriform plate and extends downward to integrate with the vomer and septal cartilage, ensuring a robust partition that supports the nasal vault's architecture. Through its , the ethmoid bone constitutes the floor of the , forming a thin, perforated shelf that separates the from the while offering foundational support to the frontal lobes. This plate's sieve-like perforations accommodate neural passages but maintain sufficient bony density to bear and contribute to the base's load-bearing capacity. The ethmoidal labyrinths, comprising the lateral masses of the bone, form significant portions of the lateral walls of the and the medial walls of the orbits, thereby providing bony enclosures that protect and position the eyeballs within their sockets. These labyrinths, separated from the orbital contents by the delicate lamina papyracea, offer structural reinforcement to the orbital rims and nasal conchae, facilitating the precise alignment of ocular and nasal structures. The ethmoid bone houses the ethmoidal air cells, which are integral to the paranasal sinus system and serve to lighten the overall weight of the by replacing dense with air-filled cavities, while also contributing to in the nasal passages. These cells, numbering variably from 3 to 18 per side and clustered into anterior, middle, and posterior groups, pneumatize the labyrinths and integrate with adjacent sinuses to optimize the craniofacial skeleton's mass distribution. Biomechanically, the ethmoid bone's extensive pneumatization renders it fragile, particularly in the thin lamina papyracea, which can under impact but aids in absorbing and distributing traumatic forces across the medial orbital wall to prevent more severe cranial injuries. This design allows the bone to act as a sacrificial structure in high-impact scenarios, mitigating propagation of to adjacent robust bones like the frontal or sphenoid.

Sensory and Respiratory Roles

The of the ethmoid bone serves as a critical pathway for the (cranial nerve I), allowing approximately 20 to 40 filaments from the olfactory receptor neurons in the nasal to pass through its numerous foramina and with mitral cells in the olfactory bulbs above the . These unmyelinated axons, bundled as fila olfactoria, transmit sensory signals for the detection and discrimination of odorants, enabling the essential for flavor perception, environmental navigation, and social behaviors in humans. The superior and middle ethmoidal conchae, projecting from the ethmoid bone into the nasal cavity, significantly increase the mucosal surface area, facilitating the humidification and warming of inhaled air to near body temperature and saturation levels before it reaches the lungs. By inducing turbulent airflow through their scroll-like structures, these conchae promote efficient particle filtration, pathogen trapping in mucus, and enhanced gas exchange preparation, thereby protecting the lower respiratory tract from desiccation and irritation. The ethmoidal sinuses, a network of air-filled cells within the ethmoid bone, contribute to voice resonance by acting as acoustic chambers that amplify and modulate sound waves during phonation, adding timbre to speech. Additionally, these sinuses aid in pressure equalization during respiration and activities like swallowing or yawning, as their ostia connect to the nasal cavity, allowing air exchange to balance ambient pressure changes and prevent discomfort. Autonomic innervation of the ethmoid bone's associated mucosa, primarily via the posterior ethmoidal nerve and branches of the vidian nerve, regulates glandular secretion through parasympathetic stimulation that increases mucus production for lubrication and defense, while sympathetic input promotes to control nasal patency and reduce excessive secretion. This neural interaction maintains in the nasal environment, responding to environmental stimuli like allergens or temperature shifts. In non-human vertebrates, such as birds, magnetite crystals in the ethmoidal region may support magnetoception for geomagnetic navigation, with iron-rich particles detected histologically in species; however, evidence for a similar mechanism in humans remains limited and inconclusive.

Clinical Significance

Pathologies

Ethmoid sinusitis refers to of the ethmoid sinuses, which can be acute or chronic. Acute ethmoid sinusitis typically presents with symptoms such as facial pain or pressure around the eyes and , nasal , purulent nasal discharge, and . Common bacterial pathogens in acute cases include Streptococcus pneumoniae and Haemophilus influenzae, often following viral upper respiratory infections. Chronic ethmoid sinusitis shares similar symptoms but persists beyond 12 weeks, with Staphylococcus aureus being a predominant pathogen in many cases. Fractures of the , a thin bony structure in the ethmoid bone, commonly arise from blunt . These injuries can disrupt the , resulting in cerebrospinal fluid (CSF) , where clear fluid leaks from the . Such fractures increase the of ascending bacterial infections, potentially leading to if the CSF leak persists beyond a week. Additionally, damage to the olfactory nerves passing through the plate often causes , or loss of smell, which may be partial or complete. Congenital anomalies affecting the ethmoid bone include , a blockage of the posterior nasal passages due to failed recanalization during fetal development. This condition, often a mix of bony and membranous tissue, leads to severe nasal obstruction, particularly in bilateral cases. Hypoplastic ethmoidal labyrinths represent underdevelopment or absence of the ethmoid air cells, resulting in reduced sinus volume and potential associated craniofacial malformations. Ethmoidal tumors, such as (also known as olfactory neuroblastoma), originate from the olfactory neuroepithelium lining the and upper . This rare accounts for about 3-6% of sinonasal tumors and typically presents with unilateral nasal obstruction, epistaxis, and . A significant complication of ethmoid sinusitis is , arising from direct spread of infection through the thin lamina papyracea separating the ethmoid sinuses from the . This is particularly prevalent in pediatric patients, where ethmoid sinusitis is the most common form of acute in children under 5 years and accounts for the majority of sinus-related orbital infections. Orbital complications occur in approximately 5-7% of acute sinusitis cases overall, with ethmoidal involvement implicated in up to 85% of pediatric orbital infections from .

Surgical Considerations

Surgical considerations for the ethmoid bone primarily involve procedures addressing chronic sinusitis, mucoceles, or neoplasms within its air cells, with a shift toward minimally invasive techniques to minimize morbidity. Historically, external ethmoidectomy approaches, such as the Lynch-Howarth procedure, were standard from the early 20th century through the 1970s, involving transcutaneous incisions for access to the ethmoid labyrinth; however, these carried higher risks of scarring and cosmetic defects. The advent of functional endoscopic sinus surgery (FESS) in the mid-1980s, pioneered by pioneers like Messerklinger and Stammberger, revolutionized treatment by enabling transnasal access to ethmoid air cells, reducing complications and improving outcomes for conditions like refractory sinusitis or tumor resection. In modern practice, FESS facilitates targeted ethmoidectomy, beginning with uncinectomy to expose the ethmoid infundibulum, followed by sequential removal of anterior and posterior ethmoid air cells using a 0- or 30-degree and microdebrider. The procedure clears obstructed air cells while preserving mucosa to maintain ciliary function, often extending to the frontal recess for complete drainage. Preoperative imaging is crucial: (CT) scans, with coronal and sagittal reconstructions in bone windows, delineate bony architecture, such as ethmoid labyrinth pneumatization and skull base slope (e.g., via classification for fovea ethmoidalis depth), guiding safe dissection. (MRI) complements CT by evaluating invasion in neoplastic cases, identifying dural involvement or perineural spread without . Key anatomical landmarks during FESS include the middle turbinate axilla for initial orientation, the ethmoid bulla as the first air cell encountered, and the basal lamella separating anterior and posterior ethmoids; proximity to the skull base (fovea ethmoidalis and ) and orbital lamina papyracea demands meticulous adherence to these to avoid breaches. Variations like cells, present in up to 95% of cases, can narrow the frontal recess and alter surgical trajectories, necessitating individualized planning. The anterior and posterior ethmoidal arteries, often dehiscent along the skull base, serve as critical vascular landmarks to prevent hemorrhage. Procedural risks remain significant due to the ethmoid's adjacency to vital structures. Iatrogenic (CSF) leak occurs in approximately 0.2-1% of FESS cases overall, rising to about 1-2% in procedures involving the , often repaired intraoperatively with fat grafts or multilayer closure to avert . Orbital injury, including penetration of the lamina papyracea leading to or vision loss, affects roughly 0.1-0.5% of cases, requiring immediate recognition (e.g., fat prolapse) and potential orbital decompression. Vascular damage to ethmoidal arteries can cause epistaxis necessitating packing or , with incidence under 1% in experienced hands. systems and intraoperative monitoring further mitigate these risks in revision or tumor surgeries.

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