Anatomical landmarks are important in the context of functional endoscopic sinus surgery (FESS).  There are variants to the normal anatomy of the paranasal sinuses which may not be present in the same position in each individual.  Awareness of these structures limits the rate of complications associated with FESS and other sinus procedures.

Each paragraph contains a diagram followed by a short definition to aid in the visualisation of these structures.

UNCINATE PROCESS

The uncinate process is a bony process of the lateral nasal wall that covers the ethmoid infundibulum.  There is considerable variation in the size, shape and attachment of the uncinate process.  Direct correlation between the attachments of the uncinate process and maxillary hypoplasia.  Also important is the degree of uncinate rotation in relation to the lateral nasal wall.  Rotation to the lateral side brings the uncinate against the medial wall of the orbit.  This places the eye at risk during surgical procedures such as FESS.

THE FRONTAL RECESS
The frontal recess connects the frontal sinus with the nasal cavity.  Whether the frontal recess opens into the infundibulum is determined by the superior insertion of the uncinate process.  This determines whether the frontal recess and frontal sinus are at risk of inflammation spreading from the anterior ethmoid cells or the maxillary sinus.

APLASTIC FRONTAL SINUS
The frontal sinus begins to pneumatize from within the frontal recesss during the first few years of life.  17% of people have aplastic frontal sinuses meaning that the frontal sinus fails to pneumatize to become a paranasal sinus.

BULLA ETHMOIDALIS

The bulla ethmoidalis (EB) is the most constant of the ethmoidal cells and is the most constant landmark for surgery.  It is the largest of the anterior ethmoid cells.  The anterior ethmoidal artery runs in the roof of this cell.  The ethmoid bulla lies above the infundibulum.  It is suspended laterally from the medial orbital wall and can be seen following removal of the uncinate process.  The crescent-shaped space formed between the bulla ethmoidalis and the uncinate process is known as the hiatus semilunaris inferioris.

CONCHA BULLOSA

Concha bullosa is the term used to describe a pneumatized middle turbinate.  When large, a concha bullosa may compromise the middle meatus amd ostiomeatal complex.  Occlusion of the ostium of a concha bullosa may result in the development of a mucocele, the clinical features of which are nasal obstruction and headache.

BASAL LAMELLA OF THE MIDDLE TURBINATE

A sagittal section through the nose and ethmoid sinuses.  The middle meatus has been cut away to expose the basal lamella.  The basal lamella separates the anterior ethmoidal cells from the posterior ethmoidal cells.  At the posterolateral end of the middle turbinate, the sphenopalatine neurovascular bundle emerges from the sphenopalatine foramen.  Injection at this site provides excellent anasthesia and haemostasis.

LATERAL SINUS
The lateral sinus is a variably present space.  When present it is located posterior to the ethmoid bulla and anterior to the ground lamella.  It may extend above the ethmoid bulla as the suprabullar space, and communicate anteriorly with the frontal recess.  The lateral sinus normally drains into the posterosuperior aspect of the semilunar hiatus in the middle turbinate.  It can be obstructed by an enlarged ethmoid bulla, a concha bullosa or large uncinate process.

ONODI CELLS

The posterior ethmoid cells are larger and fewer in number than the anterior group.  Pneumatization of the sphenoid bone from the posterior ethmoidal cells is known as an Onodi cell.  Vital structures such as the carotis artery and optic nerve may run through this cell.  Careful disssection and imaging studies are essential to avoid complications during surgery to this area.

NASOLACRIMAL DUCT
The nasolacrimal duct drains the lacrimal sac.  It runs from the lacrimal fossa in the orbit down to the back of the medial wall of the maxillary sinus to empty into the inferior meatus.  The duct lies very close to the maxillary ostium (about 4mm) and is at risk during surgery.

AGGER NASI

The agger nasi cell derives its name from the Latin that means "nasal knoll".  It is the first cell to pneumatize in the new-born.  Radiologically, the cell of the Agger Nasi can be identified by coronal CT scan, and can be differentiated from the ethmoid sinus by the degree of pneumatization (the ethmoid is more pneumatized).

The surgical importance of the cell is in its anatomical relation with the adjacent structures. The ceiling of agger nasi forms the floor of the frontal sinus. The frontal recess is behind the posteromedial wall of the cell of the agger nasi.  Agger nasi generally grows laterally towards the lacrimal groove, when this does not occur, it can cause a narrowing of the frontal recess and obstruct the drainage of the frontal sinus.  Similarly,  if the agger nasi cell is excessively pneumatized it can cause obstruction of the frontal sinus ostium and can produce frontal sinusitis.

The lacrimal bones are located close to the lateral surface of agger nasi. Dehiscence of the lacrimal bone can cause ocular symptoms like the epiphoria.
 

A Coronal CT scan showing agger nasi cells (AN) and the nasolacrimal duct (NL).

ANTERIOR AND POSTERIOR FONTANELLES

Two bony dehiscences of the lateral nasal wall and the medial wall of the maxillary sinus exist.  These are usually covered with mucosa.  in the diagram opposite the anterior fontanelle (ANF) and posterior fontanelle (PNF) can be seen alongside the hiatus semilunaris (HS).  In some individuals the anterior and posterior fontanelles may be patent which results in an accessory ostium.  They are non-functional ostia and drain the sinus only if the natural ostium is blocked.  In this instance intrasinus pressure/gravity moves material out of the ostium.  Accessory ostia are usually found in the posterior fontanelle.

HALLER'S CELLS

Haller's cells are thought to arise from the anterior ethmoid sinus and project in a variable manner into the inferomedial aspect of the orbital floor, opposite the natural ostium for the maxillary sinus.  They can be an incidental finding, or when enlarged they can predispose to chronic inflammatory disease of the maxillary and frontal sinuses by narrowing the infundibulum.

SAGITTAL CT OF THE HEAD
Look at the following CT scan.  It shows some of the anatomical landmarks described above.  Think about the FESS procedure.  Can you see how difficult it might be to perform without injuring an anatomical landmark? Click on the CT scan to find out more information on imaging in the paranasal sinuses.

P=posterior ethmoid sinus B=ethmoid bulla AN=agger nasi BL=basal lamella of the middle turbinate  FS=frontal sinus U=uncinate process IT=inferior turbinate

SERIAL CORONAL CT OF THE HEAD
This diagram shows how close these accessory structures are to the orbital and cranial cavities.

AN=agger nasi cell B=ethmoid bulla U=uncinate process