Residency · Residency · Otolaryngology
Sinonasal Anatomy and Endoscopic Landmarks
Overview
A thorough understanding of sinonasal anatomy is the foundation for safe and effective endoscopic sinus surgery (ESS). The lateral nasal wall contains the key structures and landmarks encountered during functional endoscopic sinus surgery. Critical relationships between the sinuses and the orbit, skull base, and carotid artery must be appreciated to minimize surgical complications.
Nasal Septum
Composed of: perpendicular plate of ethmoid (superior), vomer (posteroinferior), quadrangular cartilage (anterior). Kiesselbach plexus (Little area): anterior septum; anastomosis of anterior ethmoidal artery, sphenopalatine artery, superior labial artery, greater palatine artery -- most common site of anterior epistaxis. Woodruff plexus: posterior nasal cavity near posterior end of inferior turbinate; posterior epistaxis source.
Lateral Nasal Wall
Turbinates
Inferior turbinate: independent bone; largest; covered by highly vascular, erectile mucosa; not part of the ethmoid bone. Middle turbinate: part of the ethmoid bone; attaches superiorly to the skull base (lateral lamella of cribriform plate), laterally to the lamina papyracea, posteriorly to the palatine bone. Basal lamella of the middle turbinate: divides anterior from posterior ethmoid cells. Concha bullosa: pneumatized middle turbinate; may contribute to ostiomeatal obstruction. Paradoxical middle turbinate: laterally curved, may obstruct. Superior turbinate: landmark for sphenoid sinus ostium (located medial to superior turbinate). Supreme turbinate: present in ~60%; most superior, rudimentary.
Uncinate Process
Thin, sickle-shaped bone arising from the lateral nasal wall. Anterior attachment: lacrimal bone and frontal process of maxilla. Inferior attachment: inferior turbinate and palatine bone. Superior attachment varies (determines frontal sinus drainage pathway): Attaches to lamina papyracea: frontal sinus drains medial to uncinate into middle meatus. Attaches to skull base or middle turbinate: frontal sinus drains lateral to uncinate into ethmoid infundibulum. First structure removed in uncinectomy (first step of FESS).
Ethmoid Infundibulum
Three-dimensional cleft between the uncinate process medially and the lamina papyracea laterally. Drains the maxillary sinus (via the natural maxillary ostium) and anterior ethmoid cells. Hiatus semilunaris: the slit-like opening between the uncinate and ethmoid bulla, connecting infundibulum to the middle meatus.
Ostiomeatal Complex (OMC)
Functional unit, not a discrete anatomic structure. Includes: maxillary ostium, ethmoid infundibulum, hiatus semilunaris, ethmoid bulla, middle meatus, frontal recess. Obstruction of the OMC is the key event in sinusitis pathophysiology.
Paranasal Sinuses
Maxillary Sinus
Largest sinus; pyramidal shape. Natural ostium: high on the medial wall, drains into the ethmoid infundibulum (not the most inferior point). Accessory ostium: present in 10-30%; located in the posterior fontanelle. Roof = orbital floor; contains infraorbital nerve. Floor: above the premolar and molar tooth roots. Posterior wall: relates to pterygopalatine fossa (internal maxillary artery, V2).
Ethmoid Sinuses
Anterior ethmoid cells: drain into the middle meatus; anterior to the basal lamella. Agger nasi cell: most anterior ethmoid cell; anterior and superior to the frontal recess; key landmark for frontal sinus surgery. Ethmoid bulla: most consistent and usually largest anterior ethmoid cell; located posterior and superior to the hiatus semilunaris. Haller cell (infraorbital ethmoid cell): ethmoid cell extending along the orbital floor over the maxillary ostium; may narrow the infundibulum. Suprabullar and frontal bullar cells: may extend into the frontal recess. Posterior ethmoid cells: drain into the superior meatus; posterior to the basal lamella. Onodi cell (sphenoethmoid cell): posterior ethmoid cell that pneumatizes lateral and superior to the sphenoid sinus; places the optic nerve and internal carotid artery at risk during posterior ethmoidectomy.
Frontal Sinus
Drains via the frontal recess (not a true duct) into the middle meatus. Frontal recess boundaries: agger nasi anteriorly, ethmoid bulla posteriorly, lamina papyracea laterally, middle turbinate medially. KUHN classification of frontal cells: Type 1: single frontal recess cell above agger nasi. Type 2: tier of cells above agger nasi. Type 3: cell extends into frontal sinus. Type 4: isolated cell within the frontal sinus. International Frontal Sinus Anatomy Classification (IFAC): newer, more comprehensive system.
Sphenoid Sinus
Ostium located in the sphenoethmoidal recess, medial to the superior turbinate. Typically located approximately 30 degrees from floor of nose, 7 cm from anterior nasal spine. Pneumatization types: conchal, presellar, sellar (most common). Critical relationships: Optic nerve: superolateral wall; dehiscent in 4-8%. Internal carotid artery: lateral wall; dehiscent in 8-25%. Sella turcica: superior wall (pituitary surgery landmark). Cavernous sinus: lateral. Vidian canal (nerve of pterygoid canal): floor -- landmark to ICA.
Critical Structures at Risk
Orbit (Lamina Papyracea)
Paper-thin bone separating the ethmoid sinuses from the orbit. Dehiscent in up to 10%. Breach leads to orbital fat prolapse, orbital hematoma, diplopia, or vision loss. Medial rectus muscle is immediately lateral.
Skull Base (Fovea Ethmoidalis)
Roof of ethmoid sinuses; thinnest at the lateral lamella of the cribriform plate (Keros classification).
| Keros Type | Olfactory Fossa Depth | Lateral Lamella Risk | Prevalence |
|---|---|---|---|
| Type I | 1-3 mm | Low | ~12% |
| Type II | 4-7 mm | Moderate | ~70% (most common) |
| Type III | 8-16 mm | High (long, thin lamella) | ~18% |
Keros classification: Type I: 1-3 mm depth of olfactory fossa (low risk). Type II: 4-7 mm depth (moderate risk) -- most common. Type III: 8-16 mm depth (high risk) -- long, thin lateral lamella vulnerable to penetration. Asymmetry between sides in up to 10%. Breach leads to CSF leak, intracranial injury, pneumocephalus.
Anterior Ethmoidal Artery
Branch of ophthalmic artery; crosses from the orbit through the anterior ethmoid foramen. May course through the skull base or hang in a mesentery below it (particularly dangerous configuration). Landmark: approximately at the level of the frontal recess, posterior to the frontal beak. Injury: retraction into orbit may cause orbital hematoma (sight-threatening emergency).
Posterior Ethmoidal Artery
Located 12-15 mm posterior to the anterior ethmoidal artery. Approximately 5-7 mm anterior to the optic nerve, Smaller than the anterior ethmoidal artery.
Nasolacrimal Duct
Courses in the lateral nasal wall within the lacrimal bone, anterior and inferior to the uncinate process. At risk during aggressive anterior uncinectomy or maxillary antrostomy extended too far anteriorly. Injury causes epiphora (excessive tearing).
<image>Endoscopic view of the right lateral nasal wall showing key anatomic landmarks for functional endoscopic sinus surgery. Labeled structures include: inferior turbinate, middle turbinate, uncinate process, ethmoid bulla, hiatus semilunaris, and the natural maxillary sinus ostium visible through the infundibulum. The middle meatus is shown with the endoscope positioned between the septum and middle turbinate. Anatomical orientation markers indicated.</image>
<image>Coronal CT scan of the paranasal sinuses with annotated surgical anatomy. Labels include: crista galli, cribriform plate, fovea ethmoidalis, lateral lamella (Keros classification), lamina papyracea, ethmoid bulla, uncinate process, maxillary sinus natural ostium, ethmoid infundibulum, middle turbinate, inferior turbinate, nasal septum, and orbital floor. Keros type II configuration shown on one side and type III on the other to demonstrate asymmetry risk.</image>
<image>Sagittal cross-section diagram of the frontal recess and surrounding cells. Structures labeled include: frontal sinus, frontal beak, agger nasi cell, ethmoid bulla, suprabullar cell, frontal bullar cell, anterior ethmoidal artery crossing point, skull base, and middle turbinate attachment. The drainage pathway of the frontal sinus through the frontal recess into the middle meatus is traced with an arrow. KUHN type 3 frontal cell shown as a variant.</image>
<image>Axial CT scan through the sphenoid sinus with critical neurovascular structures labeled. The optic nerve canals are shown in the superolateral walls, the internal carotid arteries in the lateral walls, the sella turcica posterosuperiorly, and the vidian canals in the floor. An Onodi cell is illustrated on one side, demonstrating the optic nerve coursing through the posterior ethmoid cell lateral and superior to the sphenoid sinus. Dehiscence of the bony covering over the ICA is highlighted.</image>
Clinical Pearls
The uncinate process is the first structure removed in FESS; its superior attachment determines the frontal sinus drainage pathway. Always identify the maxillary sinus natural ostium before creating or enlarging an antrostomy; connecting a surgical antrostomy to an accessory ostium creates a recirculating mucociliary pathway. The Onodi cell is the most dangerous variant in posterior ethmoidectomy -- always look for it on preoperative CT; the optic nerve may be dehiscent within it. Keros type III represents the highest-risk skull base configuration due to a long, thin lateral lamella that is easily penetrated. The anterior ethmoidal artery hanging in a mesentery below the skull base is a high-risk configuration; retraction into the orbit after injury can cause a retrobulbar hematoma requiring emergent lateral canthotomy. Always perform a systematic review of the CT before surgery: check for Onodi cells, Keros classification (both sides), carotid dehiscence, lamina papyracea dehiscence, and frontal recess anatomy. The 30-degree endoscope is the workhorse of sinus surgery; 45- and 70-degree scopes are used for frontal sinus work and inspecting the maxillary sinus.
References
- Stammberger H, Posawetz W. "Functional endoscopic sinus surgery: concept, indications and results of the Messerklinger technique." Eur Arch Otorhinolaryngol. 1990;247(2):63-76.
- Keros P. "On the practical significance of the level of the lamina cribrosa of the ethmoid." Z Laryngol Rhinol Otol. 1962;41:808-813.
- Wormald PJ. "The agger nasi cell and the frontal recess." In: Endoscopic Sinus Surgery: Anatomy, Three-Dimensional Reconstruction, and Surgical Technique. 4th ed. Thieme; 2018.
- Lund VJ, Stammberger H, Fokkens WJ, et al. "European position paper on the anatomical terminology of the internal nose and paranasal sinuses." Rhinology. 2014;Suppl 24:1-34.
