Development and Surgical Anatomy of the Ethmoid Bone; Variants of Ethmoid Air Cells: Surgical Significance and Relation to the Ethmoid Labyrinth

I. DEVELOPMENT OF THE ETHMOID BONE

Embryological Origin

"All permanent ethmoidal structures are present at birth and develop from these crests and the furrows between them."

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1

II. SURGICAL ANATOMY OF THE ETHMOID BONE

A. Gross Structure

• Roof of both nasal cavities (cribriform plate)
• Lateral wall of the nasal cavity (medial sheet)
• Medial wall of the orbit (orbital plate / lamina papyracea)
• Nasal septum (perpendicular plate)

1. Two rectangular ethmoidal labyrinths (one on each side)
2. Cribriform plate - connects the labyrinths across the midline
3. Perpendicular plate - descends from the cribriform plate to form the upper nasal septum
4. Crista galli - triangular projection on the superior surface of the cribriform plate that anchors the falx cerebri

B. Parts in Detail

• Fills the ethmoidal notch of the frontal bone
• Separates the nasal cavity below from the anterior cranial fossa above
• Perforated by multiple olfactory nerve fibers (CN I)
• Slopes downward as it passes posteriorly
• The crista galli projects superiorly from its midline

• Quadrangular in shape
• Descends in the midline from the cribriform plate
• Forms the upper part of the bony nasal septum

• Lateral sheet (Orbital plate / Lamina Papyracea): flat, forms part of the medial wall of the orbit - extremely thin and surgically vulnerable
• Medial sheet: forms the upper part of the lateral nasal wall; bears the superior and middle conchae (turbinates), and the ethmoidal bulla bulge inferiorly

C. Skull Base Anatomy (Fovea Ethmoidalis)

• Slopes downward approximately 15 degrees from anterior to posterior and from lateral to medial
• Attaches to the lateral lamella medially
• The posteromedial region is theoretically at greater risk during ESS due to its lower height

D. Keros Classification

• Increasing Keros type = increasing length of thin lateral lamella = increasing risk of CSF leak and intracranial penetration during ESS.

E. Key Relationships

• Anterior ethmoidal artery: enters the ethmoid anteriorly; visible on CT as a conical projection from the orbit; lies in the skull base just posterior to the frontal recess - at risk during anterior ethmoidectomy and frontal sinus surgery
• Posterior ethmoidal artery: located just anterior to the sphenoid sinus in larger posterior ethmoidal cells
• Lamina papyracea: separates the ethmoid cells from orbital contents - dehiscence risk during surgery
• Optic nerve: most closely related to the posterior ethmoid/sphenoid region, especially in the context of Onodi cells

III. THE ETHMOID LABYRINTH AND ITS FUNCTIONAL UNITS

1. Sagittal segment - attaches to skull base at the lateral lamella
2. Coronal segment - forms the basal lamella proper (anterior/posterior ethmoid boundary)
3. Axial segment - attaches to the lateral nasal wall; entry point for a terminal branch of the sphenopalatine artery

IV. VARIANTS OF ETHMOID AIR CELLS: SURGICAL SIGNIFICANCE

1. Agger Nasi Cell (ANC)

• Definition: Pneumatization of the bony mound (agger nasi) at the attachment of the middle turbinate to the lateral nasal wall - the most anterior of all ethmoid cells
• Incidence: 98.5% of CT scans - most constant ethmoid cell
• Relation to ethmoid labyrinth: Most anterior anterior ethmoid cell, located just posterior to the superior aspect of the nasolacrimal duct and lacrimal sac
• Surgical significance:
  • A large, well-pneumatized ANC creates a small frontal beak (large anteroposterior distance of frontal recess) - key in frontal sinus surgery
  • May pneumatize far superiorly into the frontal sinus - can be mistaken for the frontal sinus itself when viewed endoscopically from below
  • Most common mistake: removing the floor and posterior cell wall but leaving the cap/dome in the frontal recess, causing iatrogenic frontal sinus obstruction
  • Forms the anterior wall of the frontal recess

2. Ethmoid Bulla (Bulla Ethmoidalis)

• Definition: Largest and most consistent anterior ethmoid air cell; arises from the 2nd ethmoturbinal
• Attachments: Laterally to the lamina papyracea; variable attachments to skull base and basal lamella
• Relation to ethmoid labyrinth: The most identifiable landmark within the anterior ethmoid
• Surgical significance:
  • Superiorly, its anterior wall can extend to the skull base, forming the posterior limit of the frontal recess
  • Posteriorly, may blend with the basal lamella or leave a retrobullar recess between itself and the basal lamella
  • Creates clefts and spaces (sinus lateralis = suprabullar + retrobullar recesses)
  • Complete removal of the ethmoid bulla is critical to define the medial orbital wall as a landmark for surgery
  • A partially resected ethmoid bulla lamella can scar the frontal recess, leading to frontal sinus obstruction
  • Giant ethmoid bulla may narrow or obstruct the middle meatus and infundibulum

3. Infraorbital Ethmoidal Cell (IOC) - Previously "Haller Cell"

• Definition: Anterior ethmoid cell that pneumatizes into the orbital floor above the maxillary sinus ostium
• Relation to ethmoid labyrinth: Anterior ethmoid cell extending inferolaterally into the floor of the orbit and roof of the maxillary sinus
• Surgical significance:
  • May compromise patency of the maxillary sinus natural ostium - if its common wall with the maxillary sinus ostium is not adequately resected, edema may develop and obstruct the ostium
  • The lateral wall of the IOC may be attached to the infraorbital nerve canal - must be removed carefully to avoid infraorbital nerve injury
  • On CT: seen as an air cell lying along the floor of the orbit, medial to the infraorbital canal, superior and lateral to the maxillary sinus ostium (narrows the inferior ethmoidal infundibulum)
  • Note: The term "Haller cell" is now discouraged; current nomenclature favors "infraorbital ethmoidal cell" (IOC)

4. Onodi Cell (Sphenoethmoidal Cell / SEC)

• Definition: Posterior ethmoid cell that pneumatizes posteriorly and laterally over the superolateral aspect of the sphenoid sinus
• Incidence: Approximately 30% of patients
• Relation to ethmoid labyrinth: Most posterior ethmoid cell; extends beyond the anterior face of the sphenoid sinus
• Key anatomical relationship:
  • Onodi cell lies superolateral; sphenoid sinus lies inferomedial
  • The optic nerve courses along the superolateral wall of the Onodi cell (rather than the sphenoid sinus)
  • The internal carotid artery (ICA) may also project along this wall
• Surgical significance:
  • Optic nerve and ICA at markedly increased risk of iatrogenic injury in unrecognized Onodi cell
  • Can be mistaken for the sphenoid sinus, leading to incomplete sphenoid surgery
  • Identified on coronal CT as a horizontal septation within the sphenoid sinus, posterior to the bony choanal arch
  • Application of the maxillary sinus roof/orbital floor landmark helps identify the true sphenoid ostium
  • "Onodi cells pneumatize over the optic nerve placing the optic nerve at risk for injury during surgery." (Scott-Brown's, Box 87.16)

5. Suprabullar Cell

• Definition: Ethmoid cell located above the ethmoid bulla without pneumatizing into the frontal sinus
• The fovea ethmoidalis forms its roof
• Distinction from frontal bulla cell: suprabullar cell does NOT extend into the frontal sinus

6. Frontal Bulla Cell

• Suprabullar cell that pneumatizes into the frontal sinus along its posterior wall
• Can obstruct frontal recess drainage

7. Supraorbital Ethmoid Cell

• Ethmoid cell located posterolateral to the frontal sinus ostium, pneumatizing lateral to the lamina papyracea and superolateral to the orbital roof (orbital plate of frontal bone)
• The anterior ethmoidal artery typically lies within the posterior wall of this cell along or immediately beneath the skull base - at high risk during surgery

8. Concha Bullosa

• Definition: Pneumatization (aeration) of the middle turbinate - the most common turbinate variant
• Incidence: Found in ~28% with sinus disease and ~26% without (essentially a normal variant)
• Relation to ethmoid labyrinth: The air cell within the middle turbinate is effectively an anterior ethmoid air cell
• Surgical significance:
  • May obstruct the osteomeatal complex (OMC) when large
  • Pneumatization may involve only the vertical (sagittal) portion - difficult to resect
  • Rarely, the uncinate process, inferior turbinate, or superior turbinate may also pneumatize - recognition prevents surgical confusion
  • Incidental finding alone does not mandate surgery - most patients are asymptomatic

9. Aerated Crista Galli

• May communicate with the frontal recess
• Obstruction of its ostium can lead to chronic rhinosinusitis and mucocele formation
• Surgical significance: Must be differentiated from ethmoid air cells to avoid surgical penetration of the cranial vault

V. CLINICAL/SURGICAL IMPORTANCE SUMMARY

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1 - Ch. 87 (Development and Functional Anatomy), Ch. 101 (Complications)
• Cummings Otolaryngology Head and Neck Surgery - Ch. 38 (Anatomic Variations), Ch. 44 (Ethmoidal Complex, Anatomic Variations)
• KJ Lee's Essential Otolaryngology - Ch. 26 (Ethmoid Labyrinth, Air Cell Variants, Keros Classification, Skull Base Anatomy)
• Gray's Anatomy for Students - Ethmoid Bone