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

Surgical Importance of the Pterygopalatine Fossa

I. INTRODUCTION

1. Contains critical neurovascular structures
2. Communicates with 6 adjacent spaces via 7 foramina/fissures
3. Acts as a conduit for spread of disease (infection, tumour, perineural invasion)
4. Is a target for multiple surgical procedures

II. BOUNDARIES

III. CONTENTS

A. Maxillary Nerve (V2)

• Purely sensory; enters from the middle cranial fossa via foramen rotundum
• Passes anteriorly and exits as the infraorbital nerve through the inferior orbital fissure
• While in the PPF, gives rise to:
  • Zygomatic nerve (to orbit)
  • Posterior superior alveolar nerve (to upper molar teeth and adjacent gingiva)
  • Two ganglionic branches (to pterygopalatine ganglion)
• All upper teeth receive innervation from V2 branches passing through the PPF

B. Pterygopalatine (Sphenopalatine) Ganglion

• The largest parasympathetic ganglion in the head
• Located just in front of the pterygoid canal, hanging from V2 by its two ganglionic branches
• The nerve of the pterygoid canal (Vidian nerve) enters it, carrying:
  • Preganglionic parasympathetic fibers from the greater petrosal nerve (branch of CN VII) - synapse in the ganglion
  • Postganglionic sympathetic fibers from the deep petrosal nerve (from carotid plexus) - pass through without synapsing
• Branches of the PPG carry parasympathetic + sympathetic + sensory fibers to:
  • Orbital branches - orbit, sphenoidal and ethmoidal sinuses
  • Greater and lesser palatine nerves - hard and soft palate
  • Posterior superior nasal nerves - lateral nasal wall
  • Nasopalatine nerve - nasal septum and anterior hard palate
  • Pharyngeal nerve - roof of nasopharynx

C. Terminal Part of the Maxillary (Internal Maxillary) Artery

• The terminal third of the maxillary artery lies within the PPF
• Its branches supply: upper teeth (posterior superior alveolar), palate (greater palatine), nose (sphenopalatine - terminal branch via sphenopalatine foramen), orbit (infraorbital), and pharynx (pharyngeal artery)
• The sphenopalatine artery is the terminal branch - the dominant blood supply to the nasal cavity

IV. COMMUNICATIONS (THE SURGICAL HIGHWAYS)

V. SURGICAL IMPORTANCE

1. Access for Epistaxis Control

• The sphenopalatine artery exits the PPF medially through the sphenopalatine foramen (posterior to the crista ethmoidalis - the key surgical landmark)
• Endoscopic sphenopalatine artery ligation (ESPAL) achieves >90% initial control rate for posterior epistaxis
• The crista ethmoidalis (small crest of the perpendicular plate of the palatine bone, anterior to the sphenopalatine foramen) is the consistent and reliable landmark to identify the sphenopalatine artery
• Critically, 97% of specimens have two or more branches medial to the crista ethmoidalis - the surgeon must control all branches

• When more proximal vascular control is needed, the internal maxillary artery can be ligated within the PPF by removing the posterior wall of the maxillary sinus (endoscopic maxillary artery ligation)
• Technique: maxillary antrostomy → removal of posterior maxillary sinus wall mucosa → drilling of bone → preservation then incision of PPF periosteum → exposure of PPF fat → identification of pulsatile maxillary artery → clip placement

2. Vidian Neurectomy (Surgery for Vasomotor/Allergic Rhinitis)

• Greater superficial petrosal nerve - carries preganglionic parasympathetic secretomotor fibers to lacrimal, palatine, and nasal glands
• Deep petrosal nerve - carries postganglionic sympathetic fibers

• Endoscopic approaches have replaced transantral approaches
• Success rate reported up to 91% for rhinitis control
• Complications: dry eye (epiphora), palate numbness - due to collateral injury to parasympathetic fibers to lacrimal gland and palate
• Alternative: posterior nasal neurectomy (transection of the posterior nasal nerve at the sphenopalatine foramen - avoids the above complications as it targets postganglionic fibers distally)

3. Route of Tumour Spread and Perineural Invasion

• Sinonasal tumours (maxillary sinus, ethmoid, skull base): direct and perineural spread into PPF via the posterior maxillary wall
• Nasopharyngeal carcinoma: spreads superiorly and laterally through the PPF
• Juvenile Nasopharyngeal Angiofibroma (JNA): classically extends into the PPF, causing characteristic bowing/expansion of the posterior wall of the maxillary sinus on CT (Holman-Miller sign) - the expanded PPF filled with the tumour is a hallmark CT finding
• Adenoid cystic carcinoma and other malignancies: travel along V2 retrogradely through the foramen rotundum to reach the middle cranial fossa - perineural invasion
• Conversely, tumours from the middle cranial fossa (meningioma, pituitary, trigeminal schwannoma) can extend into the PPF through foramen rotundum

4. Access for Skull Base Surgery (Endoscopic and Open)

• Provides endoscopic access to the infratemporal fossa (ITF), Meckel's cave, petrous apex, cavernous sinus, and middle cranial fossa
• The PPF serves as the corridor to these deep spaces
• Full mobilization of the PPF contents (with vidian nerve sacrifice if necessary) allows retraction of the entire PPF to expose the deeper ITF
• Carotid artery identification is critical if dissection extends into the ITF

• Medial maxillectomy → trans-PPF corridor → sphenopalatine artery ligation → removal of posterior maxillary sinus wall → Kerrison rongeurs/drill to remove orbital process of palatine bone → full PPF mobilization
• Sacrifice of the descending palatine artery (if needed) allows access to deeper ITF

5. Sphenopalatine Ganglion Block

• The SPG is accessible transnasally (posterior to the middle turbinate) or via the greater palatine canal
• Indications: cluster headache, migraine, trigeminal neuralgia, atypical facial pain, postoperative pain following sinus surgery
• Block can be performed with topical anaesthetic via the nasal route or by injection via the greater palatine foramen
• As the SPG lies just deep to the nasal mucosa (covered by only 1-2 mm of connective tissue and mucosa), transnasal access is straightforward

6. Dental and Palatal Surgery

• The posterior superior alveolar nerve (from V2 in PPF) is the basis of posterior superior alveolar nerve block - anaesthesia of maxillary molars
• The greater and lesser palatine nerves (descending from PPF through the palatine canal) provide sensation to the hard and soft palate
• Greater and lesser palatine nerve blocks are used for palatal surgery, rhinoplasty, and cleft palate repair
• All upper teeth receive their nerve supply and blood supply through branches that pass through the PPF - injury here can cause global upper dental anaesthesia

7. Route of Spread in Sinonasal Infections

• The PPF communicates with the orbit via the inferior orbital fissure - a potential route for spread of sinonasal infection or abscess to the orbit
• Communication with the middle cranial fossa (via foramen rotundum and Vidian canal) means deep PPF infections can ascend intracranially

VI. SURGICAL ANATOMY SUMMARY TABLE

• Gray's Anatomy for Students - Pterygopalatine Fossa, Gateways, Contents (pp. 1132-1134)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1 & 2 - Vascular supply of lateral nasal wall; Skull base anatomy
• Cummings Otolaryngology Head and Neck Surgery - Endoscopic sphenopalatine artery ligation; Vidian neurectomy; Coronal plane resections into PTF/ITF; Juvenile Angiofibroma
• KJ Lee's Essential Otolaryngology - Pterygopalatine (Pterygomaxillary) Fossa: Boundaries, Contents, Communicating spaces