• Comprised of two nasal bones fused in the midline to form a pyramidal shape

• Thicker superiorly than inferiorly

• Attachments of nasal bones:

1. Superiorly: nasal process of frontal bone

2. Laterally: frontal process of maxilla

• Pyriform aperture = Bony opening into the nasal cavity bounded as described below:

1. Superiorly: caudal margin of nasal bones

2. Inferiorly: alveolar process of maxilla

3. Laterally: frontal process of maxilla

4. Medially: nasal septum

Nasal Cartilages

• Upper lateral cartilage (ULC): fuses superiorly with the nasal bones; articulates inferiorly with the cephalic margin of the LLC, most often forming an interlocking scroll; thickens medially where it becomes continuous with the quadrangular cartilage of the septum, forming the cartilaginous portion of the nasal dorsum

• Lower lateral cartilage (LLC): provides the shape of the nasal tip; composed of medial and lateral crura

• Sesamoid cartilages: small cartilages located lateral to the lateral crus

Nasal Septum

• Comprised of both bony and cartilaginous components; lined by either a mucoperiosteal or mucoperichondrial layer.

1. Cartilage components = Quadrangular cartilage

2. Bony components = Perpendicular plate of ethmoid, vomer, crest of the maxillary bone, crest of the palatine bone; minor contributions from crest of sphenoid bone and nasal spine of frontal bone

• Membranous (mobile) septum: cartilage deficient membrane adjoining the columella to the caudal septum; site of hemitransfixion/transfixion incision

• Keystone area: corresponds to convergence of caudal margin of nasal bone, perpendicular plate of ethmoid, and cephalic margin of ULC and cartilaginous septum; failure to preserve this region can result in nasal collapse

• Vomeronasal organ (VNO or Jacobsen’s organ): auxiliary olfactory organ involved in the perception of pheromones in mammals; in humans, function is controversial as it largely regresses in utero; identified as a groove in the anterior-inferior nasal septum

Nasal Vestibule

• Serves as the entrance to the nasal cavity

• Lined by hair bearing skin, sebaceous and sweat glands

• Boundaries include nasal septum medially, LLC superiorly and laterally, and alveolar process of maxilla inferiorly

• Posteriorly bounded by the limen nasi (limen vestibule), formed by the caudal margin of the ULC; this coincides with the transition from the skin of the nasal vestibule to the mucosal surface (pseudostratified ciliated columnar epithelium) of the nasal cavity

Nasal Valves

• Important contributors to nasal airway resistance

• Regions at greatest potential for collapse resulting in nasal airway obstruction

1. Internal nasal valve: bounded by nasal septum, caudal edge of ULC, anterior face of inferior turbinate; normally forms 10°–15° nasal valve angle; narrowest cross-sectional area of the nasal cavity and site of greatest nasal airflow resistance; normally does not undergo any change in dimension during inspiration

2. External nasal valve: bounded by nasal ala laterally, nasal septum and columella medially; situated caudal to internal nasal valve; normally dilates during inspiration

Lateral Nasal Wall

• Bony contributions to the lateral nasal wall from anterior to posterior include:

1. Frontal process of maxilla

2. Lacrimal bone

3. Medial maxillary wall inferiorly, lamina papyracea (LP) superiorly

4. Perpendicular plate of the palatine bone

5. Medial pterygoid plate of the sphenoid bone – Latter two structures make up the sphenopalatine foramen.

• Lamella of the lateral nasal wall: five lamella from anterior to posterior

1. First lamella = uncinate process

2. Second lamella = ethmoid bulla (EB)

3. Third lamella = basal lamella of middle turbinate

4. Fourth lamella = basal lamella of superior turbinate

5. Fifth lamella = basal lamella of supreme turbinate (when present)

• Middle turbinate (MT): anterior end attaches to frontal process of maxilla forming the axilla of the MT; has the three sites of attachment oriented in three planes

1. Anterior portion of MT: oriented in sagittal plane; attached to junction of cribriform plate and lateral lamella; delimits fovea ethmoidalis (roof of ethmoid sinus) from cribriform plate

2. Middle portion of MT: oriented in coronal plane (vertical); also known as the basal (ground) lamella of MT; attached laterally to LP; delimits anterior from posterior ethmoid sinus

3. Posterior portion of MT: oriented in axial plane (horizontal); attached to the crista ethmoidalis of the perpendicular process of palatine bone just anterior to sphenopalatine foramen (potential site of postoperative epistaxis when MT is resected); preservation of this attachment point reduces risk of postoperative MT lateralization

• Possible variations of the MT include:

1. Concha bullosa: pneumatized MT; present in one-third of the population

2. Paradoxical MT: the lateral surface of the MT is convex rather than concave

• Meatus: air space located beneath and lateral to the turbinate of the same name

1. Inferior meatus: contains slit-like opening (known as Hasner’s valve) corresponding to site of drainage of the nasolacrimal duct, typically located ~1 cm posterior to anterior face of inferior turbinate

2. Middle meatus: site of drainage of frontal, anterior ethmoid, and maxillary sinuses

3. Superior meatus: site of drainage of posterior ethmoid and sphenoid sinuses; sphenoethmoidal recess located within posterior aspect of superior meatus

• Lacrimal (maxillary) line: endoscopic landmark corresponding to suture line along the medial orbital wall at the junction of the frontal process of the maxilla and lacrimal bone

• Site of attachment of the UP to the lateral nasal wall

1. Attachment to LP: frontal sinus drainage occurs medial to the UP, directly into the middle meatus; results in blind pouch of the superior portion of the ethmoid infundibulum (termed recessus terminalis); most common configuration (80%)

2. Attachment to fovea ethmoidalis or MT: frontal sinus drainage occurs lateral to the UP and directly into the ethmoid infundibulum; less common configuration (20%)

• Ethmoid infundibulum: three-dimensional trough bounded by the UP medially, the LP laterally and the EB posterosuperiorly; opens inferiorly at maxillary sinus ostium

• Hiatus semilunaris: two-dimensional, crescenteric window into the ethmoid infundibulum: bounded by UP anteriorly and EB posteriorly

• Nasal fontanelles: areas of bony dehiscence of the lateral nasal wall typically covered by mucosa of the middle meatus, connective tissue, and mucosa of the maxillary sinus; site of accessory ostia when mucosa absent; most commonly located in posterior fontanelle (up to 30% of people); posterior fontanelle located posterior to horizontal process of UP; anterior fontanelle located anterior to UP articulation with the inferior turbinate; posterior fontanelle located posterior to natural maxillary os and oriented in sagittal plane (compared to oblique orientation of natural maxillary os)

• Osteomeatal complex (OMC): refers to the region within the middle meatus bounded by the basal lamella of the MT posteriorly, MT medially, LP laterally, and fovea ethmoidalis superiorly; bears particular significance in the pathophysiology of acute bacterial rhinosinusitis as it represents a regional confluence of drainage pathways of the maxillary, anterior ethmoid, and frontal sinuses

• Nasolacrimal system: intimately related with the lateral nasal wall; tear drainage pathway as follows:

1. Upper and lower lacrimal punctum → Upper and lower canaliculus → Common canaliculus (present in 90%) → Valve of Rosenmüller (one-way valve preventing reflux into canaliculi) → Lacrimal sac → Nasolacrimal ductàValve of Hasner → Inferior meatus

• Lacrimal sac: a significant portion of the sac is located above the axilla of the MT (~0.9 cm) and above the common canaliculus (~0.5 cm); relevant during dacryocystorhinostomy to ensure adequate lacrimal sac exposure

Vascular Anatomy

External Nose Vascular Supply

• Facial artery branches (external carotid artery)

1. Angular artery → lateral nasal artery; supplies lateral surface of caudal nose

2. Superior labial artery → columellar artery; supplies columella and nasal tip

• Ophthalmic artery branches (internal carotid artery)

1. Dorsal nasal artery; supplies nasal dorsum

2. External nasal branch of anterior ethmoid artery; supplies nasal dorsum and tip

Nasal Cavity Vascular Supply

• External carotid artery branches

1. Internal maxillary artery (IMA) branches

– Descending palatine artery → greater palatine artery; courses through the greater palatine canal located along the posterior aspect of the perpendicular plate of the palatine bone; potential for injury with dissection of the posterior medial maxillary wall at its junction with the posterior maxillary wall; supplies anteroinferior nasal cavity, including nasal septum, via incisive foramen

– Pharyngeal branch of IMA: supplies posterosuperior nasal cavity and nasopharynx

– Sphenopalatine artery (SPA): enters nasal cavity via sphenopalatine foramen (SPF); multiple SPA branches and/or accessory foramina may exist; SPF located within superior meatus, between basal lamella of MT and superior turbinate

a. Posterior septal branch: courses along sphenoid rostrum inferior to natural sphenoid os; risk of injury when extending sphenoidotomy inferiorly; supplies nasal septum (vascular supply of pedicled nasoseptal flap)

b. Posterior lateral nasal branch: runs along the middle and inferior turbinates, providing their vascular supply

– Superior labial artery → alar branch; supplies nasal vestibule and anterior nasal cavity

• Internal carotid artery branches (both branches of ophthalmic artery)

1. Anterior ethmoid artery (AEA): larger than PEA; exits orbit between superior oblique and medial rectus muscle via anterior ethmoid foramen (~24 mm posterior to anterior rim of lacrimal fossa) within the frontoethmoid suture; courses anteromedially along the skull base within a bony canal (dehiscent in 40%) or below skull base within a mesentery; delineates posterior limit of frontal recess; enters anterior cranial fossa medially by piercing the lateral lamella, rendering it the weakest portion of the skull base (one-tenth as strong as the fovea ethmoidalis); supplies anterior one-third of the nasal cavity

2. Posterior ethmoid artery (PEA): exits orbit via posterior ethmoid foramen (~12 mm posterior to anterior ethmoid foramen, 6 mm anterior to optic ring); supplies posterior nasal cavity

• Nasal cavity vascular plexuses

1. Woodruff’s plexus: aggregation of vessels (posterior lateral nasal + pharyngeal branch of IMA) within the posterior inferior meatus; previously thought to be arterial, now shown to be venous in nature

2. Kiesselbach’s plexus (Little’s area): Located in the anterior nasal septum, most common site of anterior epistaxis; formed by posterior septal branch of SPA, AEA, greater palatine artery, and alar branch of superior labial artery

• Venous drainage of nasal cavity

1. Anterior and posterior ethmoid veins → superior ophthalmic vein → cavernous sinus

2. Sphenopalatine vein → pterygoid plexus

– As this is a valveless venous system, there is a potential for retrograde spread of infection intracranially (particularly the cavernous sinus).

3. Diploic veins of Breschet: venous drainage of the frontal sinus occurs via diploic veins, which pass through multiple anterior and posterior table foramina (Breschet’s canals); these develop into the frontal diploic veins of Breschet, which communicate with the dural sinuses (cavernous sinus and superior sagittal sinus); contribute to intracranial spread of infection

External Nose

• Branches of ophthalmic division of cranial nerve V (V1)

1. Infratrochlear nerve: supplies lateral nose above medial canthus

2. External anterior ethmoidal nerve: supplies nasal tip, dorsum, and ala

• Branches of maxillary division of cranial nerve V (V2)

1. Infraorbital nerve: supplies lateral nose and ala

2. Anterior superior alveolar nerve (branch of infraorbital nerve): minor contribution to nasal tip and columella

Nasal Cavity

• Branches of ophthalmic division of cranial nerve V (V1)

1. Internal nasal branch of anterior ethmoidal nerve: supplies anterior and superior regions of the nasal cavity

2. Posterior ethmoidal nerve: supplies posterior nasal cavity

• Branches of maxillary division of cranial nerve V (V2)

3. Sphenopalatine (nasopalatine) nerve (anastomoses with greater palatine nerve): supplies posterior nasal cavity

4. Anterior superior alveolar nerve: supplies anterior nasal cavity and nasal floor

Autonomic Innervation

Sympathetic Innervation

• Function: vasoconstriction

• Preganglionic: sympathetic chain

• Ganglion: superior cervical ganglion

• Postganglionic: deep petrosal nerve joining with greater superficial petrosal nerve (GSPN) → vidian nerve (passes through sphenopalatine ganglion without synapsing) → distributed to nasal mucosa with sensory nerves

Parasympathetic Innervation

• Function: vasodilation and mediation of nasal secretions

• Preganglionic: superior salivatory nucleus (CN VII) → nervus intermedius (leaves CN VII at geniculate ganglion with GSPN to form vidian nerve)

• Ganglion: sphenopalatine ganglion

• Postganglioinic: distributed to nasal mucosa with deep branches of V2

• Laterally: zygoma

• Medially: lateral nasal wall

• Posteriorly: pterygopalatine (PPF) and infratemporal fossa (ITF)

• Anteriorly: facial surface of maxilla

Foramen of Maxillary Bone

• Infraorbital foramen: infraorbital nerve, artery and vein; run along roof of maxillary sinus within infraorbital canal with potential for injury during surgical instrumentation if canal is dehiscent (14% of people)

• Incisive foramen: greater palatine artery and nerve

• Maxillary os: located within posterior one-third of ethmoid infundibulum

• Superior alveolar foramen: posterior, middle and anterior superior nerve, artery and vein

Endoscopic Maxillary Sinus Landmarks

• Roof of maxillary sinus: corresponds to floor of sphenoid sinus

• Posterior wall of maxillary sinus: corresponds to anterior face of sphenoid sinus

• Medial maxillary wall of maxillary sinus: with normal pneumatization, located in line with a vertical line drawn tangential to LP; can be lateral to LP if hypoplastic or atelectatic maxillary sinus

Ethmoid Sinus

Anterior Ethmoid Sinus (anterior to basal lamella of MT)

• Anterior ethmoid cells are generally more numerous but smaller.

1. Agger nasi: most anterior ethmoidal cell, projects anterior to the axilla of the MT creating a bulge in the lateral nasal wall; posterior limit forms the anterior border of the frontal recess; anteroposterior distance of the frontal recess largely determined by degree of pneumatization of the agger nasi cell; a large, well pneumatized agger nasi cell confers a small frontal beak, which gives rise to a large anteroposterior distance

2. Ethmoid bulla: largest anterior ethmoidal cell; attached laterally to LP; most constant and identifiable ethmoidal cell

4. Suprabullar cell: ethmoidal cell located above the EB which does not pneumatize into the frontal sinus; roof of this cell formed by fovea ethmoidalis (ethmoid roof/skull base)

5. Frontal bulla cell: suprabullar cell which pneumatizes into frontal sinus along posterior wall of frontal sinus

6. Supraorbital ethmoid cell: ethmoid cell located posterolateral to the frontal sinus ostium with pneumatization lateral to the LP and superolateral to the orbital roof (orbital plate of the frontal bone); anterior ethmoid artery typically located within the posterior wall of the cell along or immediately beneath the skull base

7. Haller cell (infraorbital ethmoid cell): most common anatomic variation within the maxillary sinus; ethmoid cell, most commonly originating from the anterior ethmoid sinus, which pneumatizes along the inferomedial orbit, thereby contributing to obstruction of the natural drainage pathway of the maxillary sinus

Posterior Ethmoid Sinus (posterior to basal lamella of MT)

• Posterior ethmoid cells are generally fewer but larger.

1. Onodi cell (sphenoethmoidal cell): posterior ethmoid cell which pneumatizes posteriorly over the superolateral aspect of the sphenoid sinus; when present, the optic nerve and occasionally the internal carotid artery can project along the superolateral wall of the Onodi cell rather than the sphenoid sinus, placing them at increased risk of injury intraoperatively; reported incidence of ~30%; identified on coronal CT scan as a horizontal septation within the sphenoid sinus, posterior to the bony choanal arch (Onodi cell located superolaterally, sphenoid sinus located inferomedially)

Keros Classification

• Classification system for assessing the depth of the olfactory fossa (corresponds to the length of lateral lamella); lateral lamella of cribriform plate is the thinnest bone in the skull base; increased risk of cerebrospinal fluid (CSF) leak during endoscopic sinus surgery (ESS) with increasing Keros type due to low cribriform fossa and increased length of thin lateral lamella (more subject to fracture)

1. Type 1: 1–3 mm (second most common configuration)

2. Type 2: 4–7 mm (majority of cases)

3. Type 3: 8–16 mm (rare)

• Fovea ethmoidalis (ethmoid roof): formed by orbital plate of frontal bone; slopes downward (~15°) from anterior to posteriorly and from lateral to medial; posteromedial region of fovea ethmoidalis theoretically at greater risk of injury during ESS given lower height; attaches to lateral lamella medially

• Lateral lamella: formed by ethmoid bone; forms lateral surface of cribriform fossa; thinnest and weakest bone in the skull base

• Cribriform plate: forms floor of cribriform fossa; perforated by multiple olfactory nerve fibers; also slopes downward as it passes posteriorly

Sphenoid Sinus

Walls and Recesses of the Sphenoid Sinus

• Planum sphenoidale: forms sphenoid roof; contiguous anteriorly with fovea ethmoidalis

• Sphenoid rostrum: forms sphenoid face and anterior floor; articulates anteriorly with the vomer and perpendicular plate of ethmoid forming a keel

• Sella turcica: Latin for turkish saddle; rounded projection along posterosuperior wall in a well pneumatized sphenoid sinus; forms floor of hypophyseal fossa (containing pituitary gland, middle cranial fossa); may be attenuated or anteriorly displaced in the presence of a pituitary macroadenoma; bounded anterosuperiorly by tuberculum sella and posteriorly by dorsum sella

• Lateral pterygoid recess (lateral recess): inferolateral pneumatization of the sphenoid sinus; common location for spontaneous CSF leak giving rise to encephalocele formation

• Lateral wall of sphenoid: forms medial wall of cavernous sinus; when well pneumatized, bony impressions of the internal carotid artery (partially dehiscent in 25%) and optic nerve (dehiscent in 6%) can be visualized

• Opticocarotid recess: bony depression between the optic nerve superiorly and the cavernous internal carotid artery segment inferiorly; located posterosuperiorly along the lateral wall of a well pneumatized sphenoid sinus

• Sphenoid intersinus septum: often asymmetrically divides the sphenoid sinus; must be removed with caution in transnasal endoscopic approaches to the skull base when inserts onto or in the vicinity of the carotid and/or optic canal

• Clival recess: forms posteroinferior wall of the sphenoid sinus if well pneumatized; separates sphenoid sinus from posterior cranial fossa

Classification of Sphenoid Pneumatization

• Conchal: absent pneumatization, sinus is filled by cancellous bone; rare

• Presellar: pneumatization anterior to a vertical line drawn through the tuberculum sella; second most common pattern

• Sellar: pneumatization extends beyond a vertical line drawn through the turberculum sella; majority of cases

• Postsellar: pneumatization extends beyond the dorsum sella

1. Arrested pneumatization (arrested development): developmental variant resulting in persistence of atypical fatty marrow within a non-pneumatized region of the sphenoid sinus; incidental finding upon imaging as patients are typically asymptomatic; distinct from sphenoid hypoplasia wherein a normal bony trabecular pattern is present within the sphenoid sinus; may be mistaken for skull base lesion; most commonly occurs in sphenoid sinus

Landmarks for Sphenoid Ostium

• 1.5 cm superior to the bony choanal arch

• One-third up from choana to skull base

• 7 cm at a 30-degree angle from the anterior nasal spine

• Between the nasal septum and posterior insertion of the superior turbinate (most reliable landmark)

Frontal Sinus

• Made up of two frontal sinuses, frequently asymmetric, separated by an intersinus septum located approximately in the midline

• Thick anterior wall, thin posterior wall (anterior wall of anterior cranial fossa)

• Frontal beak: thick bone of the frontal process of maxilla, anterior to the agger nasi, that projects posteriorly into the frontal recess, thereby limiting its anteroposterior distance

Frontal Recess

Frontal Sinus Cells

• Frontal ethmoidal cells: anterior ethmoidal cells in contact with the anterior wall of the frontal recess (frontal process of maxilla) Two classification schemes exist:

1. Kuhn Classification:

– Type 1: single cell above agger nasi cell

– Type 2: tier of cells above agger nasi cell

– Type 3: single cell pneumatizing into the frontal sinus

– Type 4: isolated cell within the frontal sinus

2. Modified Kuhn Classification (Wormald)

– Type 1 and 2: no change from previous classification

– Type 3: cell pneumatizing into the frontal sinus but less than 50% of the vertical height of the sinus

– Type 4: cell pneumatizing into the frontal sinus greater than 50% of the vertical height of the sinus

Change in classification was prompted by rarity of initial definition of Type 4 cell.

• Intersinus septal cell: pneumatization of the interfrontal sinus septum; originates medially, thereby displacing frontal sinus drainage pathway laterally

• Other relevant frontal recess cells include agger nasi cell, suprabullar cells, frontal bulla cells, and supraorbital ethmoid cells (see description above).