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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4416_Библиотеки_им_академика_М_И_Перельмана
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Fig. 2.24: Anterior ethmoidal artery canal seen as a notch in the upper medial wall of orbit.
31Chapter 2 Sinonasal Anatomy: Structure-wise
ANATOMY OF OLFACTORY FOSSA AND
MEDIAN ANTERIOR SKULL BASE ASB
■ Anterior skull base: Constituted by parts of frontal bone (orbital plates),
ethmoid bone (cribriform plate), and sphenoid bone (lesser wing).
■ Orbital plates of frontal bone form the posterior wall of frontal sinus and
fovea ethmoidalis (or ethmoid skull base/roof of ethmoid sinus); then
continue posteriorly as planum sphenoidale.
■ Cribriform plate forms the roof of nasal cavity. It is also referred to as
lamina cribrosa and the olfactory nerves traverse through its perforations.
e cribriform plate is joined laterally by the lateral lamella (LL) to the
fovea ethmoidalis. LL has a vertical orientation.
• Olfactory fossa—houses the olfactory bulb. It is a depression in the
cribriform plate and bound by crista galli medially, medial lamella
of cribriform plate inferiorly and lateral lamella of cribriform plate
laterally (Fig. 2.25). Olfactory bulb is best seen on T2W coronal MR
images (Fig. 2.26.)
• Asymmetry in level of olfactory fossa can be seen in up to one-third of
the population.
■ Keros classication6 of depth of olfactory fossa (Figs. 2.27A and B): Depth
of the olfactory fossa depends on the height of the LL
• Type 1 lateral lamella 1–3 mm
• Type 2 lateral lamella 4–7 mm
• Type 3 lateral lamella 8–16 mm.
Section 1 Normal Anatomy and Imaging32
Fig. 2.25: Olfactory fossa boundaries. Olfactory fossa (dot) bounded by crista galli medially,
lateral lamella of cribriform plate laterally and medial lamella of cribriform plate inferiorly.
Fig. 2.26: Olfactory bulb (arrows). As seen within olfactory fossa on MRI T2W coronal
image.
■ Type 1 is the shallowest olfactory fossa while type 3 is the deepest.
■ e anterior ethmoid artery (AEA) is a branch of the ophthalmic artery
(branch of internal carotid artery). It passes through the anterior
ethmoidal canal/foramen (seen as a notch on the superomedial orbital
wall) to the ethmoidal labyrinth and then to the anterior skull base.
■ It is identied on the lateral wall of the olfactory fossae as horizontal
lucencies (anterior ethmoid sulcus).
A B
Figs. 2.27A and B: Types of olfactory fossa.
■ Lateral lamella is thinnest bone in anterior skull base, hence predisposed
to injury during surgery
• Middle turbinate attaches to lateral lamella of cribriform plate
anteriorly, can be injured during turbinate handling
• Anterior ethmoidal artery also traverses the lateral lamella.
■ Fovea ethmoidalis—orbital plate of frontal bone that joins lateral lamella
of cribriform plate medially to lamina papyracea laterally.
■ Rudmik and Smith7 assessment of height of anterior ethmoid skull base
(ESB)—vertical distance between plane of ESB and mid orbital plane as
seen on coronal section showing anterior ethmoidal artery is measured
(Fig. 2.28)
• High ESB—vertical distance >7 mm
• Moderate ESB—vertical height 4–7 mm
• Low ESB—vertical height < 4 mm.
33Chapter 2 Sinonasal Anatomy: Structure-wise
ANATOMIC VARIANTS OF ASB
■ Type 2 olfactory fossa is the most common and least prone to injury while
type 3, the least common, is most prone to injury
• Angle between medial and lateral lamella can vary. Greater the angle,
more the risk for injury during surgery.
■ Lower ESB, greater the chance of injury during surgery
• Anterior ethmoidal artery can hang within a bony mesentery and is
then prone to injury
• Aerated crista galli can attenuate frontal ostium (Fig. 2.29).
Section 1 Normal Anatomy and Imaging34
Fig. 2.28: Anterior ethmoid skull base height. Measured as vertical distance between
fovea ethmoidalis and mid orbital plane (1.05 cm here). Keros type 3 and Rudmick & Smith
high type ASB.
Fig. 2.29: Aerated crista galli. Coronal image shows aerated crista galli.
ANATOMY OF POSTERIOR ETHMOID AIR CELLS
■ Ethmoid bone is a midline bone separating nasal cavity from the brain.
It has four parts:
• e cribriform plate (horizontal part): olfactory nerve bers pass
through it and olfactory bulb lies on it
• Crista galli: ick midline triangular structure on which falx cerebri
attaches
• Vertical plate (forms part of nasal septum)
• Lateral masses or labyrinths on each side.
e posterior ethmoidal air cells are located posterior to basal lamella
and anterior to sphenoid sinus (Figs. 2.30A and B)
A B
Figs. 2.30A and B: Posterior ethmoid cells. Sandwiched between bulla ethmoidalis and
sphenoid sinus.
35Chapter 2 Sinonasal Anatomy: Structure-wise
Fig. 2.31: Posterior ethmoid skull base height. Distance between roof of posterior
ethmoidal cells and roof of maxillary sinus.
■ Medial to lamina papyracea and lateral to superior turbinate
■ ese are fewer as compared to anterior ethmoid air cells
■ ese air cells drain via their separate ostia into the superior meatus
■ Height of posterior ethmoid sinus can be measured as the distance
between roofs of maxillary sinus and posterior ethmoid sinus as seen on
coronal images (Fig. 2.31).
Section 1 Normal Anatomy and Imaging36
ANATOMIC VARIANTS OF POSTERIOR ETHMOID CELLS
■ Onodi cell/sphenoethmoidal air cell
• ese are posterior ethmoid cells that extend superolateral to
sphenoid sinus
• An oblique or horizontal septum, if seen within sphenoid sinus is
suggestive of their presence (Fig. 2.32A)
• A cruciform septation within sphenoid air cell suggests bilateral onodi
cells (Fig. 2.32B)
• Onodi cells have a close relationship with the optic nerve and internal
carotid artery, and can predispose these structures to injury during FESS.
■ Low lying posterior ethmoid cells can predispose to skull base penetration
during FESS.
A B
Figs. 2.32A and B: Onodi cells. Posterior ethmoidal cells lying superolateral to sphenoid
sinus giving cribriform sign on coronal image.
ANATOMY OF SPHENOID SINUS AND
MEDIAN CENTRAL SKULL BASE CSB
■ Body of sphenoid bone houses the SS; laterally demarcated by petrooccipital suture.
■ Sphenoid sinus usually bilateral and asymmetric with vertical septation
within.
■ Anteriorly the roof of the SS is bound by planum sphenoidale, and
posteriorly is the sella turcica. Planum sphenoidale continues anteriorly
with ethmoid bone. Anterior clinoid process marks the transition
between anterior skull base (ASB) and central skull base (CSB).
■ Anterior wall is connected to perpendicular plate of ethmoid and vomer
in the midline, and can be displaced by an Onodi cell.
■ Lateral walls are thin and can even have dehiscent areas. It is related to
critical structures (optic nerve, cavernous sinus, ICA).
■ Sphenoid sinus drains via its ostium and sphenoethmoidal recess into
the superior meatus; it lies at the medial aspect of its anterior wall (Figs.
2.33A and B). e ostium is approximately 1.5 cm superior to the posterior
choanae. In the majority, it lies just medial to the superior turbinate
A B
Figs. 2.33A and B: Sphenoid sinuses drain via sphenoid ostia into sphenoethmoid recess.
37Chapter 2 Sinonasal Anatomy: Structure-wise
A B
Figs. 2.34A and B: Types of sphenoid sinus based on degree of pneumatization.
■ Sphenoethmoidal recess is a narrow corridor which is bound superiorly
by cribriform plate, medially by nasal septum and laterally by superior
turbinate. Inferiorly it leads to the superior meatus. e SS as well as
posterior ethmoid cells drain into it.
■ Types of sphenoid sinus based upon extent of pneumatization (Figs.
2.34A and B)
• Nonpneumatised sphenoid sinus
• Conchal sphenoid sinus—small air cell not reaching up to sella
• Presellar sphenoid sinus—reaching up to anterior wall of sella
• Sellar sphenoid sinus—reaching below sellar oor; most frequent.
■ Critical structures in close relationship with sphenoid sinus (Fig. 2.35)
• Optic nerve canal in relationship with superior wall of sphenoid sinus
• Internal carotid artery canal in relationship with posterolateral wall of
sphenoid sinus
• Pterygoid/vidian canal along the inferior wall
• Foramen rotundum—along the lateral wall
• Lateral craniopharyngeal wall: Defect along lateral wall of sphenoid
sinus due to fusion defect between basisphenoid and greater wing of
sphenoid.
Section 1 Normal Anatomy and Imaging38
Fig. 2.35: Central skull base critical structures in close relation to sphenoid sinus.
ANATOMIC VARIANTS OF CSB
■ Poor pneumatization
• Trans-sphenoidal skull base surgery is relatively contraindicated.
■ Presence of intrasinus septae
• Variable, vertical/horizontal/oblique
• Must be mentioned in report with attention to any attachment with
critical structures especially the carotid arteries
• Horizontal septae point to onodi cells.
■ Dehiscent walls of bony walls of any of these critical structures should be
carefully looked for
■ Pneumatized anterior clinoid process (Fig. 2.36)
• Results in formation of opticocarotid recess between optic nerve canal
above and carotid artery below
• Optic nerve canal may be dehiscent in up to four-fths of these cases.
Fig. 2.36: Aerated anterior clinoid process—seen to surround the optic nerve canal on
the left side.
RELEVANT ORBITAL ANATOMY FIGS. 2.37A AND B
e relevant orbital anatomy is given in Table 2.2.
A B
Figs. 2.37A and B: Boundaries of orbit. [Medial wall: Lamina papyracea (arrow); Roof:
Frontal bone (arrowhead); Floor: Roof of MS (block arrow)].
39Chapter 2 Sinonasal Anatomy: Structure-wise
Table 2.2: Relevant orbital anatomy.
Wall Bone Relationship to sinuses
Medial wall Frontal bone (orbital process)
Lamina papyracea (ethmoid bone)
Lacrimal bone
Palatine bone (orbital process)
Lateral wall Zygomatic bone
Greater wing of sphenoid
Floor (inferior wall) Maxillary bone
Zygomatic bone
Palatine bone
Roof Frontal bone and lesser wing of sphenoid Floor of frontal sinuses
Lateral wall of ethmoid
sinuses
Roof of MS
Infraorbital nerve passes
through it
REFERENCES
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(Eds). Diseases of the Sinuses: Diagnosis and Management. London, UK: B.C.
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3. Wormald PJ, et al. e International Frontal Sinus Anatomy Classication (IFAC)
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Section 1 Normal Anatomy and Imaging40
4. Folbe AJ,Svider PF,Eloy JA. Anatomic considerations in frontal sinus surgery.
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6. Keros P. On the practical importance of dierences in the level of the cribriform
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