Results
The Orbit from an Inferomedial Perspective: The Endoscopic Endonasal Approach
Medial Relationships: The Ethmoid Bone
The endoscopic approach through the lamina papyracea has demonstrated many advantages
as compared with the traditional medial orbitotomies for certain orbital lesions.[2]
[3] Endoscopic treatment of sinus diseases may be associated with serious orbital complications
when the extremely thin or even dehiscent lamina papyracea is mistaken for ethmoid
cells.
The medial orbital wall, orbital apex, and medial optic canal can be readily accessed
through an endonasal approach. The medial orbital wall is composed of the frontal
process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid bone,
also called lamina papyracea, and the body of the sphenoid. The quadrangular orbital
plate of the ethmoid separates the orbit from the nasal cavity and it articulates
superiorly with the medial edge of the orbital plate of the frontal bone. The anterior
and posterior ethmoidal canals are formed by the anterior and posterior ethmoidal
notches that exist in both of these plates ([Fig. 1]). These canals transmit the anterior and posterior ethmoidal branches of the ophthalmic
artery and the nasociliary nerve of the ophthalmic division of the trigeminal nerve
and open into the anterior cranial fossa along the lateral edge of the cribriform
plate.[4] The cranial openings of the ethmoidal canals are located at the anterior and posterior
limits of the ethmoidal cribriform plate and represent two important surgical landmarks
for endonasal approaches to the anterior fossa and orbit; they divide the anterior
fossa floor and nasal cavity roof into frontal (anterior to the anterior ethmoidal
canal), cribriform (between both canals), and planum areas (posterior to the posterior
ethmoidal canals), and similarly, the orbit into bulbar, retrobulbar, and apical parts.[1] The anterior ethmoidal artery runs along the cranial base after exiting the orbit
from posterior to anterior toward the cribriform plate. The anterior and posterior
ethmoidal arteries may be totally covered by bone or freely hanging into the ethmoidal
labyrinth, placing them at risk of inadvertent injury during surgery.[5] The posterior ethmoidal artery is usually smaller than the anterior and may be absent
in 30% of the cases. The anterior ethmoidal canal may be found on average 21-mm posterior
to the medial orbital rim. The posterior ethmoidal canal is located approximately
14-mm posterior to the anterior ethmoidal canal, and the optic canal can be found
on average 7-mm posterior to the posterior ethmoidal canal ([Fig. 1], part 1).[1]
Fig. 1 Medial and inferior anatomical relationships of the orbit. The orbit from an endoscopic
endonasal perspective, 0-degree endoscopic view. (A) Identification of the right ethmoid bulla and uncinate process in the middle nasal
meatus after careful medial displacement of the middle nasal turbinate. (B) Sagittal dissection of the left nasal cavity, the middle turbinate has been completely
removed. The bulla ethmoidalis and the uncinate process have been partially divided.
The space between the uncinate process and the bulla is the inferior semilunar hiatus.
(C) The anterior and posterior ethmoidal canals with the respective arteries are visualized
at the superior edge of the lamina papyracea and cranial base after removal of the
ethmoidal cells. These canals divide the anterior fossa floor and nasal cavity roof
into frontal (anterior to the anterior ethmoidal canal), cribriform (between both
canals), and planum areas (posterior to the posterior ethmoidal canals), which approximately
correspond to the to the bulbar, retrobulbar, and apical parts of the orbit. (D) Relationship of the posterior ethmoidal canal with the orbital apex posteriorly:
the optic canal prominence is located superior to the lateral opticocarotid recess,
a triangle-shaped depression superolateral to the carotid prominence. The lateral
opticocarotid recess corresponds to the optic strut base intracranially, and the clinoid
segment of the internal carotid artery is located medially. (E) Endonasal view of the posterior wall of the sphenoid sinus. (F) The inferior wall of the orbit corresponds to the superior wall of the maxillary
sinus. The infraorbital nerve crosses through the infraorbital canal until it exits
via the infraorbital foramen located in the inferior orbital rim. (G) Close view of the lamina papyracea. The medial wall of the maxillary sinus has been
removed but the underlying mucosa has been preserved. (H) The lamina papyracea has been resected exposing the periorbita and the mucosa of
the medial wall of the maxillary sinus has been removed. (I) The periorbita underneath the lamina papyracea has been incised and the orbit fat
has been removed, exposing the neurovascular orbital contents through the medial orbit
wall. (J) The superior wall of the maxillary sinus has been removed medially to the infraorbital
nerve and the periorbita has been incised, exposing the neurovascular orbital contents
through the medial and inferior orbit walls. Note how the infraorbital branch of the
internal maxillary artery follows the infraorbital nerve at the infraorbital groove
and receives an anastomosis from the ophthalmic artery. (K): Detailed view of the orbital apex and the optic canal. (L) Specimen divided in the sagittal plane, the ethmoid labyrinth and the medial wall
of the maxillary sinus have been removed exposing the infraorbital canal and the orbital
plate of the ethmoidal bone (lamina papyracea). A., Anterior; ICA, internal carotid
artery; M, muscle; N, nerve; Post, posterior.
Besides contributing to the medial orbital wall, the cubical-shaped ethmoid bone also
contributes to the nasal septum (perpendicular plate) and nasal cavity roof (cribriform
plate).
On the lateral nasal wall, the supreme (if present), superior, and middle turbinates
form part of the ethmoid bone, while the inferior turbinate is a separate bone, the
inferior concha. The turbinates' major axes follow an oblique direction, with the
posterior part lower than the anterior part.[6]
[7] The middle turbinate insertion constitutes an important surgical landmark and follows
three different planes: superiorly, the anterior third attaches to the lateral end
of the cribriform plate in a sagittal plane; the middle third turns laterally to lie
in a coronal plane (basal lamella) and attaches to the lamina papyracea; and the posterior
third is attached to palatine bone, after rotating inferiorly to become horizontal.[8] Therefore, the middle turbinate must be handled with great care as is attached to
the paper-thin cribriform plate above and excessive manipulation can cause a fracture
and consequent cerebrospinal fluid leak. Up to 35% of the patients may present a pneumatized
middle turbinate. This anatomical variant, known as concha bullosa, is important from
a surgical perspective as it may decrease the surgical working area.[9]
Beneath each of the nasal turbinates, the corresponding meatuses can be identified.
Two important ethmoidal structures are located in the middle nasal meatus, a bulging
elongated osseous structure anteriorly called the uncinate process and apneumatized
structure posteriorly that corresponds to the ethmoidal bulla.[8]
The uncinate process is a thin hook-shaped bone projection from the ethmoid labyrinth.
Superiorly, the uncinate process may attach to the lamina papyracea, anterior skull
base, middle nasal turbinate, or a combination of these. Anteriorly, the uncinate
process attaches to the posterior edge of the lacrimal bone and inferiorly, to the
ethmoidal process of the inferior nasal concha. Its posterior edge is free and is
covered by mucosa.[6]
Posterior and above the uncinate process is the ethmoidal bulla, the largest and most
constant of the ethmoid cells. The bulla is formed by the pneumatization of the bulla
lamella or second ethmoid basal lamella ([Fig. 1], part 1). The retrobullar recess is a space that may be present between the posterior
surface of the bulla ethmoidalis and the basal lamella of the middle turbinate, and
the suprabullar recess may be present if the bulla does not reach the skull base.[8]
Between the anterior face of the bulla and the posterior edge of the uncinate process
there is a two-dimensional crescent-shaped opening called the inferior semilunar hiatus.
This semicircular space continues laterally as a three-dimensional slit-like space
called the ethmoidal infundibulum.[6] The ethmoidal infundibulum is bordered medially by the uncinate process and inferior
semilunar hiatus, laterally by the lamina papyracea, inferiorly by the maxillary sinus
ostium, and posteriorly by the ethmoidal bulla. The anterior ethmoidal cells also
drain into the ethmoidal infundibulum, and in 50% of cases, the frontal sinus. On
the other hand, the posterior ethmoidal cells drain into the superior meatus.[7] The basal lamella of the middle turbinate is an important surgical landmark as it
serves as a clear separation between the anterior and posterior ethmoid air cells.[8]
Once the uncinate process is excised, the ethmoidal bulla and the basal lamella of
middle turbinate constitute the main surgical corridor to the posterior ethmoid and
eventually to the orbital plate laterally.
The ethmoidal labyrinths consist of numerous paper-thin ethmoidal air cells. The medial
surface of the labyrinth forms part of the superolateral nasal wall from the inferior
surface of the cribriform plate to the middle turbinate. The lateral surface of the
labyrinth forms part of the medial orbital wall (lamina papyracea; [Fig. 1], part 2).[6] The roof of the ethmoidal labyrinth is called the fovea ethmoidalis and joins the
cribriform plate through a thin piece of bone called the lateral lamella.[8] The relationship of the fovea ethmoidalis with the lateral lamella and cribriform
plate in the coronal plane can present many variations.[7]
The posterior ethmoidal air cells are often situated immediately in front of the optic
canal which is located on an average of 7-mm posterior to the posterior ethmoidal
canal.[1] Removing the posterior ethmoid sinus and the adjacent part of the sphenoid sinus
will expose the medial wall of the optic canal ([Fig. 1]).[10] When the most posterior ethmoid cell pneumatizes superiorly and posteriorly above
the sphenoid sinus, this posterior ethmoid cell is called a sphenoethmoidal cell or
Onodi's cell. This is important from a surgical perspective as the optic nerve may
be intimately related to its lateral wall and it places the optic nerve at increased
risk of injury during posterior ethmoidectomy.[8]
Inferior Relationships: The Maxillary Sinus and Related Structures
The maxillary sinus is the entrance to several important surgical corridors, and its
medial and superior wall provide access to the inferomedial wall of the orbit. The
transmaxillary approach was traditionally performed using a sublabial incision in
the gingivobuccal sulcus (Caldwell–Luc approach).[11] Over the last several decades the endoscopic endonasal route has been added to the
surgical armamentarium, either replacing previous transmaxillary approaches or utilizing
endoscopic assistance after a Caldwell–Luc approach.[12]
The pyramid-shaped maxillary sinus, the largest of the paranasal sinus, is located
in the body of maxilla. The floor is formed by the alveolar and palatine processes
of the maxilla.
The anterior wall is formed by the facial surface of the maxilla and contains the
canalis sinuosus, a groove for the anterior superior alveolar nerve and vessels. The
roof of the maxillary sinus forms the inferior wall of the orbit and is traversed
by the infraorbital nerve in its groove and canal.[8] The posterior wall is formed by the infratemporal surface of the maxilla and forms
the anterior border of the pterygopalatine fossa. This posterior wall contains the
alveolar canals which transmit the posterosuperior alveolar nerves to the molar teeth.
The lateral apex extends into the zygomatic process of the maxilla.[13] The medial wall separates the sinus from the nasal cavity and is the site of the
maxillary hiatus, the natural sinus opening that will provide surgical entrance to
access the orbit through this corridor. The maxillary hiatus is located just below
the roof of the sinus and opens into the inferior part of the of the ethmoidal infundibulum,
between the middle and posterior thirds of it, at the middle meatus.[8] This opening is partially closed by the uncinate process of the ethmoid bone, its
margins should be identified and removed as a first step in performing the antrostomy
to widen the access to the maxillary sinus ([Fig. 1]). The root of the middle nasal concha attaches to the lateral nasal wall near the
junction of the orbit and the maxillary sinus.[7]
[8]
The infraorbital nerve arises from the maxillary division of the trigeminal nerve
at the pterygopalatine fossa. After entering the orbit via the inferior orbital fissure
(IOF), it usually courses anteriorly through the infraorbital groove and canal. The
infraorbital canal is located anteriorly within the orbital floor or superior wall
of the maxilla. The infraorbital nerve then exits the canal through the infraorbital
foramen of the anterior maxilla ([Fig. 1]).[6]
[10] Sometimes, however, the infraorbital canal protrudes into the maxillary sinus separate
from the orbital floor. This variant may lead to inadvertent nerve injury during endoscopic
surgery. The infraorbital nerve traversing the lumen of the maxillary sinus can be
found in more than 10% of the patients ([Fig. 1F]) and is even more prevalent in the setting of another anatomical variant, an ipsilateral
infraorbital ethmoid cell (Haller's cell), especially if the nerve is contained within
the lamellae of the cell.[14] This ethmoidal cells that develop into the maxillary sinus may narrow the drainage
pathway of the maxillary ostium or the infundibulum ethmoidal and also obstruct distal
regions, narrowing the space for the passage of the endoscope and surgical instruments.[8] Preoperative image studies can help to identify these variants and avoid nerve damage.
During certain surgical procedures, inadvertent damage to the nasolacrimal apparatus
may occur as well.[15] The lacrimal drainage pathway includes the superior and inferior lacrimal canaliculi,
the lacrimal sac, and the nasolacrimal duct. The lacrimal sac lies in the nasolacrimal
groove at the inferomedial aspect of the orbit, beneath the medial canthal ligament.
Endonasally this region corresponds to the area just anterior to the origin of the
axilla of the middle turbinate.[10] The nasolacrimal groove is formed anteriorly by the frontal process of the maxilla
and posteriorly by the lacrimal bone.[13] The medial canthal ligament is firmly attached to the anterior and posterior margins
of the lacrimal groove. If a medial orbital approach is performed, the medial canthal
ligament should be sharply divided or elevated in such a way that it can be preserved
for appropriate reattachment during the closure.[10] The lacrimal sac drains inferiorly through the nasolacrimal duct which opens into
the inferior meatus, approximately 2 cm behind the nostril.[7] Care must be taken during the uncinectomy and maxillary antrostomy to avoid injury
to the nasolacrimal duct.[15]
The Orbit from a Posterior and Superolateral Intracranial Perspective
Superior Relationships: The Orbitofrontal Cortex and Surrounding Dura Overview
The orbitofrontal cortex, also known as ventromedial prefrontal cortex, lies immediately
above the orbits ([Fig. 2]). Although considerable individual variability has been found, the orbitofrontal
cortex is usually formed by the lateral, medial, anterior, and posterior orbital gyri,
and the orbital sulci.
Fig. 2 Lateral and superior anatomical relationships of the orbit. (A) Sagittal view of the medial left orbit. The orbital plate of the ethmoidal bone
(lamina papyracea), the medial wall of the maxillary sinus and the superior wall of
the maxillary sinus medially to the infraorbital nerve have been removed. The periorbita
have been excised and the orbital fat has been resected. The intracranial portion
of the optic nerve and the ophthalmic artery are observed entering the optic canal.
(B) A frontotemporal craniotomy has been performed on the right side and the lesser
wing of the sphenoid bone has been drilled. The meningoorbital band, a dural fold
located on the lateral side of the SOF, is exposed. Note how this dura bridge tethers
the frontotemporal basal dura to the periorbita, and therefore should be carefully
incised to allow further exposure of the SOF, paraclinoid, and cavernous sinus region.
(C) View of the orbit and cranial base from superior, the brain has been resected. On
the left side, the dura covering the anterior and middle fossa floor has being excised,
the superior wall of the frontal and ethmoid sinuses, the orbital roof, superior periorbita,
and orbital fat have being resected. (D) Right oblique intracranial view. The dura, brain, superior wall of the ethmoid sinus,
superior and lateral orbit walls, periorbita, and orbit fat have being excised. The
structures located in the SOF, IOF and the main contents of the cavernous sinus are
demonstrated. Note how the SOF superior is posteriorly related to the cavernous sinus
and the optic canal opens medially into the intradural space. (E) Right lateral intracranial view. The neurovascular structures accessing the orbit
through the SOF and IOF are seen. The orbitofrontal cortex, also known as ventromedial
prefrontal cortex, lies immediately above the orbits. The lateral compartment of the
SOF constitutes the corridor through which the superior ophthalmic vein, the trochlear,
frontal and lacrimal nerves enter the orbit. The central compartment, situated just
behind the annular tendon, transmits through this tendinous ring the superior, and
inferior divisions of the oculomotor nerve, the abducens and nasociliary nerves, and
the sensory and sympathetic roots of the ciliary ganglion. The IOF opens into the
temporal, infratemporal, and pterygopalatine fossae. (F) Right lateral intracranial view. The lateral rectus muscle has been divided and
retracted laterally. The infraorbital nerve arises from the maxillary division of
the trigeminal nerve at the pterygopalatine fossa and enters the IOF. A, artery; CN,
cranial nerve; ICA, internal carotid artery; IOF, inferior orbital fissure; M, muscle;
N, nerve; SOF superior orbital fissure, V vein.
The medial and lateral orbitofrontal arteries provide the blood supply to the orbitofrontal
cortex. The medial orbitofrontal artery is the first branch of the A2 segment of the
anterior cerebral artery. It branches distal to the anterior communicating artery
and passes anteriorly and inferiorly toward the frontobasal surface of the olfactory
sulcus to reach the level of the planum sphenoidale. It terminates at the orbitofrontal
cortex, supplying the orbital gyri and inferomedial portion of the frontal lobe. The
lateral orbitofrontal artery most frequently branches off the superior trunk of the
middle cerebral artery at the M2 segment. It passes between the frontal and temporal
lobes from posteroinferior to anterosuperior to arrive at the posterolateral orbitofrontal
cortex surface, where it primarily supplies the lateral orbitofrontal cortex. A remarkable
variability in the number of their branches and anastomoses has been described.[6]
When either a frontotemporal or a supraorbital craniotomy are the selected routes
to approach the orbit, the frontal dura that covers the orbitofrontal cortex and the
related orbitofrontal arteries has to be carefully dissected free from the orbital
roof and the cortex gently retracted. The orbitofrontal cortex is connected with sensory
regions, as well as limbic system structures, playing an important role in memory,
emotion, and the cognitive process of decision making; inadvertent surgical damage,
although uncommon during orbital surgery, may lead to a pattern of disinhibited behavior.[16]
The dural layers and their organization around this area are complex and relevant
in surgery. The intracranial dura is a thick collagenous sheath composed of a periosteal
layer that faces the bone and a meningeal layer that faces the brain. These layers
are distinguished as separate sheaths at the venous sinuses and optic canal. The meningeal
layer fuses with the epineurium of the cranial nerves and with the adventitia of the
vessel as they emerge from the cranium through the cranial foramina or fissures. An
exception is the optic nerve in which, after conforming the optic nerve sheath, the
meningeal layer blends into the outer scleral layers. The periosteal layer of dura
is continuous with the pericranium through the cranial sutures and foramina and with
the periorbita through the superior orbital fissure (SOF) and optic canal.[6] Hence, at the SOF, the dura covering the middle fossa and cavernous sinus (CS) blends
into the periorbita of the orbital apex, but also into the annular tendon from which
the rectus muscles arise.[17] The annular tendon, also called the annulus of Zinn, adheres to the dural sheath
of the optic nerve and it is attached to the periosteum along the upper, medial, and
lower margins of the optic canal; its lower portion extends horizontally from the
sphenoid body below the optic foramen and optic strut to attach to a bony prominence
at the midportion of the lateral edge of the SOF.[13] Consequently, when the frontotemporal basal dura is fully retracted, a dural fold
is formed on the lateral side of the SOF. This superficial dural bridge that tethers
the frontotemporal basal dura to the periorbita, named by some authors the meningoorbital
band, contains few a small dural veins and the orbitomeningeal artery ([Fig. 2]).[18] Coagulation and incision of this dura fold will allow sufficient retraction of the
temporal dura and therefore improved exposure of the SOF, which is potentially the
most posterior limit of the superior orbitotomy, and will also provide a wider exposure
for approaching the paraclinoid and anterior CS region. Once the dura fold is carefully
incised and detached from the periorbita, the dura of the temporal lobe can then be
dissected from the middle fossa floor to proceed medially peeling the outer layer
(periosteal dura) away from the inner layer (meningeal layer) of the CS, exposing
the cranial nerves that pass through the CS on their course to the orbit when the
surgery requires it.[18]
[19]
The meningoorbital foramen, also called cranioorbital foramen, is located in the greater
wing of the sphenoid, usually anterior to the lateral aspect of the SOF, but it can
be lateral or even confluent with its lateral end, and it can have multiple openings
in up to 15% of the cases.[20] The cranioorbital foramen connects the middle cranial fossa with the orbit and contains
an anastomosis between the orbital branch of the middle meningeal artery, known as
meningolacrimal artery, and a recurrent meningeal branch of the lacrimal artery that
exits the orbit through the SOF, courses laterally below the sphenoid ridge, and turns
forward through the cranioorbital foramen to supply the periorbita.[10] This foramen is present in approximately 55% of the cases, while in the remaining
cases the anastomotic ramus usually enters the orbit through the SOF.[20] This small foramen is important from a surgical perspective because it constitutes
a potential source of hemorrhage that surgeons must take into account during deep
orbital dissection and its intraoperative identification could avoid unexpected bleeding
that may complicate surgery at the orbital apex.[21]
Posterior and Posteroinferior Relationships
The three important osseous openings that connect the orbit with the intracranial
and extracranial spaces are the SOF and IOF and the optic canal. The SOF is posteriorly
related to the CS, the optic canal opens posteromedially into the intradural space
and the IOF opens into the temporal, infratemporal, and pterygopalatine fossae.
Superior Orbital Fissure and Cavernous Sinus
The SOF is a bony cleft located between the orbital apex anteriorly and the CS posteriorly,
constituting a potential route of extension of orbital lesions toward the CS and middle
cranial fossa and vice versa. Its lateral aspect is narrow and is formed by the lesser
and greater wing of the sphenoid bone and a small portion of the frontal bone at its
lateral apical margin, whereas its medial edge is wider and is formed by the sphenoid
body. The lower end of the SOF blends inferiorly with the posterior end of the IOF.
The superior margin of the SOF is delimited by the lower surfaces of the lesser wing,
the optic strut, and the anterior clinoid process. The lower margin, delimited by
the junction of the greater wing with the sphenoid body, is located at the level of
the lower edge of the CS, with some of its venous spaces extending forward along these
margins. The lower margin is separated from foramen rotundum by a bony bridge, the
maxillary strut ([Figs. 2], [3]).[10]
[17]
Fig. 3 Anterior anatomical relationships of the orbit. (A, B) A anterior and oblique overview of the osseous components of the right orbit, which
is formed by seven different bones: the frontal, zygomatic, sphenoid, lacrimal, ethmoid,
palatine bones, and maxilla. Note the paper-thin consistency of the lamina papyracea,
and its relationship with the anterior and posterior ethmoidal canals, within the
superior edge of the lamina papyracea and the medial edge of the orbital plate of
the frontal bone. The canals offer exit to the anterior and posterior ethmoidal arteries
(which are branches of the ophthalmic artery) and the nasociliary nerve (division
of the ophthalmic nerve). The nasolacrimal groove is formed anteriorly by the frontal
process of the maxilla and posteriorly by the lacrimal bone. The optic canal is observed
close to the posterior ethmoidal canal and contains the optic nerve, ophthalmic artery
and sympathetic nerve fibers. The SOF and IOF are identified. Note how the lower end
of the SOF blends inferiorly with the posterior end of the IOF. Note the infraorbital
foramen of the anterior maxilla and the supraorbital notch at the supraorbital rim.
(C) Oblique view of the right orbit. The skin surrounding the orbit, extraocular muscles
and orbital fat have been removed, and the maxillary sinus opened. The medial and
lateral canthus are demonstrated. The supra-orbital nerve emerges from the supra-orbital
foramen and the supratrochlear nerve is located medially between the supraorbital
foramen and the trochlea of the superior oblique muscle. The inferior orbital wall
has been removed medially up to the infraorbital canal. The infraorbital nerve leaves
the orbit through the infraorbital foramen and divides into the palpebral, nasal and
superior labial branches, supplying sensory innervation of the skin of the lower eyelid,
medial cheek, lateral nose and upper lip, and the mucosa of the anteroinferior nasal
septum and oral mucosa of upper lip. (D) Right anterior view of the orbit after skin removal. The orbital and palpebral portions
of the orbicularis oculi muscle are observed in the medial half of the orbit. The
lacrimal part of the orbicularis oculi muscle lies underneath these structures, extending
behind the lacrimal sac and attaching to the lacrimal bone. The orbicularis oculi
muscle surrounds the orbital rim circumferentially and extends into the lids, temple
and cheek, having its fibers interdigitated with the occipitofrontalis and the corrugator
muscles. On the lateral side of the orbit, the orbicularis oculi muscle has been resected
and the two dense plates of connective tissue know as tarsi are observed. (E) Right lateral view to illustrate the course and location of the temporal branches
of the facial nerve. The temporal division of the facial nerve emerges directly through
the parotid gland and divides in three rami: anterior, middle and posterior. The temporal
branches are located superficial to the temporoparietal fascia (also known as superficial
temporal fascia). The middle ramus (frontal ramus), which innervates the frontalis
muscle, is located anteroinferior to the frontal branch of the superficial temporal
artery. The frontalis muscle is continuous laterally with the temporoparietal fascia
and is medial to the superior temporal line. (F) Dissection of the right temporal area. The temporoparietal fascia and the frontalis
muscle have been reflected anteriorly and part of the temporoparietal fascia has been
resected to expose the frontal ramus of the facial nerve and the superficial temporal
artery that run on this layer. The temporal fascia is continuous with the pericranium
medially and divides 2 to 3 cm above the zygomatic arch into two layers, superficial
and deep. In between these layers the interfascial fat pad is encountered. To protect
the frontal ramus of the facial nerve the dissection of the skin flap is performed
between the deep and superficial layers of the deep temporal fascia (interfascial
dissection) or under the deep layer (subfascial dissection). (G): Right frontotemporal exposure within trifacial temporal dissection. The interfascial–subpericranial
flap has been folded anteriorly. A, artery; IOF, inferior orbital fissure; M, muscle;
N, nerve; Palp, palpebral; SOF, superior orbital fissure.
The annular tendon is located in the anterior upper half of the medial part of the
fissure and divides the fissure into three compartments: lateral, central, and inferior.
The lateral compartment is situated at the lateral aspect of the SOF outside the annular
tendon and constitutes the corridor through which the superior ophthalmic vein, the
trochlear, frontal, and lacrimal nerves enter the orbit. The central compartment,
situated just behind the annular tendon, transmits through this tendinous ring the
superior and inferior divisions of the oculomotor nerve, the abducens and nasociliary
nerves, and the sensory and sympathetic roots of the ciliary ganglion. The inferior
compartment is located below the annular tendon and transmits the inferior ophthalmic
vein. This compartment contains some of the orbital fat that extends backward into
the fissure and in its lower margin a posterior extension of the orbital smooth muscle.[10]
[17] The orbital smooth muscle constitutes a landmark to reach the SOF from an endoscopic
endonasal perspective, therefore also useful for CS and orbital apex regions approaches.[22]
When accessing the lateral segment of the SOF, the opening should be attempted through
its upper margin, as the superior ophthalmic vein and the large Sylvian veins that
empty into the CS are located at the lower margin of it. Special care must be taken
to not damage the trochlear nerve, which passes along its upper margin above the ophthalmic
nerve.[17]
The posterior SOF transmits and is related to the CS. The CS is a vascular structure
defined traditionally as a four-walled dural envelope that runs from the SOF and anterior
clinoid process (ACP) anteriorly to above the petrous apex and posterior clinoid process
posteriorly.[23] It lies lateral to the dorsum sellae and sphenoid body and medial to the junction
of the greater wing of the sphenoid bone. This venous plexus maintains connections
to the superior and inferior ophthalmic veins, superior and inferior petrosal sinuses,
basilar sinus, and middle and inferior cerebral veins.
The main arterial structure encased in the CS is the intracavernous portion of the
internal carotid artery (ICA), which is accompanied and surrounded by a sympathetic
nerve plexus. The ICA enters the CS after passing under the petrolingual ligament,
follows an ascending course and forms a posterior bend lateral to the posterior clinoid
process. The ICA then follows a horizontal trajectory and forms a bend anteriorly.
The ICA leaves the CS through its superior wall medial to the anterior clinoid process.
The intracavernous ICA gives rise to branches that supply the structures contained
in the CS, sellae, and tentorium. These branches display some degree of variability,
with the most consistent being the meningohypophyseal and inferolateral trunk. The
meningohypophyseal trunk, the largest intracavernous branch, usually arises from the
posterior bend of the intracavernous ICA and further divides into three branches:
the tentorial, dorsal meningeal and inferior hypophyseal arteries. The inferolateral
trunk usually arises from the lateral portion of the ICA as it courses anteriorly.[24]
The III, IV, and VI cranial nerves and the first division of the V nerve also pass
through the CS on their course to the SOF. The VI cranial nerve is located completely
inside the CS, medial to the ophthalmic division of the V nerve. The III, IV, and
V1 nerves lie within the dura of the lateral sinus wall ([Fig. 2]).[25]
[26]
Optic Canal
The optic canal is located in the sphenoid bone medial to the SOF extending in a superior
medial direction from the orbital apex to the optic foramen ([Figs. 2] and [3]). The optic canal is funnel-shaped, narrower anteriorly near the orbit, and is bounded
laterally by the lesser wing of the sphenoid and anterior clinoid process and medially
by the body of the sphenoid.[27] The anterior root of the lesser wing of the sphenoid forms the roof of the optic
canal and the posterior root or optic strut forms the floor.[10] The optic strut, or posterior root of the lesser wing of the sphenoid bone, is a
bony bridge running between the sphenoid bone and the lower medial margin of the base
of the anterior clinoid process.[25] It separates the SOF from the optic canal and foramen, conforming the junction of
the upper and medial walls of the SOF and the lateral edge and floor of the optic
canal.[13] The inferior surface of the optic strut also forms the anterior part of the roof
of the CS.[19]
The optic canal transmits the optic nerve, ophthalmic artery, and sympathetic nerve
fibers. The optic nerve follows a superomedial trajectory from the orbit, whereas
the ophthalmic artery is located inferomedially within the proximal portion of the
canal and inferolaterally within the distal portion.[1] The ophthalmic artery constitutes the major supply to the orbit and in the majority
of the cases is a branch of the supraclinoid portion of the ICA. However, instead
of always arising in the subarachnoid space, the ophthalmic artery can arise in the
CS, be a branch of the middle meningeal artery or even arise as duplicate arteries
of nearly equal size ([Fig. 2]).[1]
[24]
The walls of the canal can be thinned by the proximity of the sphenoid sinus, and
sometimes if the sphenoid sinus is well pneumatized, the medial bony layer may be
even absent, leaving the optic nerve separated from the sinus by only the dural nerve
sheath and mucosa.[28] This anatomic variation should be carefully addressed in preoperative images and
kept in mind especially during endoscopic endonasal surgery, but also during intracranial
approaches to prevent postoperative CSF leaks. If there is an Onodi's cell or sphenoethmoidal
cell, the optic canal and often the ICA can be seen inside the cell and care must
be taken not to injure them during an endonasal approach.
The portion of the optic nerve proximal to the optic foramen is covered by the falciform
process or ligament, a reflected leaf of dura mater that extends medially from the
anterior clinoid process across the top of the optic nerve to the sphenoid limbus.[25] Compression of the optic nerve against the sharp edge of the falciform process due
to an expanding lesion or surgical maneuvers may result in visual loss. The length
of nerve covered just by the falciform process without underlying bone can vary from
1 mm to 1 cm and special care must be taken not to coagulate the falciform ligament
and carefully divide it to release the optic nerve, as the lack of bone could lead
to nerve injury.[28] The trajectory of the ophthalmic artery in relation to the optic nerve at the optic
canal must be also remembered to prevent iatrogenic lesion to this artery during opening
of the falciform ligament.[1]
Once the optic nerves emerge from the optic foramina, they are directed posteriorly,
superiorly, and medially toward the optic chiasm. The optic chiasm lies above and
usually behind the chiasmatic sulcus, a groove connecting both optic foramina.[28] The chiasmatic sulcus is located on the superior surface of the sphenoid bone, bounded
anteriorly by the planum sphenoidale and posteriorly by the tuberculum sellae.[10] From the chiasm, the optic tracts follow a posterolateral direction around the cerebral
peduncles to enter the incisural spaces.[28]
Inferior Orbital Fissure
The IOF is located between the lateral wall and floor of the orbit posteriorly. The
IOF has long anterior and posterior edges and narrow medial and lateral ends. The
fissure is formed laterally by the zygomatic bone, posterolaterally by the greater
wing of the sphenoid bone, anteriorly by the orbital surface of the maxilla, and a
short segment of the orbital process of the palatine bone and medially by the sphenoid
body.[10]
The anterolateral portion of the IOF communicates with the temporal fossa inferiorly.
This portion contains only smooth muscle and adipose tissue, being a suitable point
to perform the bony cuts needed to remove part of the roof, and lateral walls when
performing frontotemporal or lateral approaches.[1] The medial portion of the fissure connects the orbit to the infratemporal fossa
that is located below the greater sphenoid wing.[13] This segment contains the entrance of the infraorbital branch of the maxillary nerve
and the infraorbital branch of the internal maxillary artery into the infraorbital
groove. The posteromedial part of the IOF communicates below with the pterygopalatine
fossa and, through this, with the nasal cavity[1] ([Fig. 2]).
Other neurovascular structures passing through the fissure are the zygomatic nerve
and branches of the inferior division of ophthalmic vein that communicates with the
pterygoid plexus. The zygomatic nerve branches off the maxillary nerve in the pterygopalatine
fossa to enter the orbit by the IOF and to course along the lateral orbital wall,
where it divides into zygomaticofacial and zygomaticotemporal branches, which exit
the orbit through identically named foramina.[10] The orbital smooth muscle (Müller's muscle) spans the upper entire length of the
fissure and blends into the periorbita, perineurium of the infraorbital nerve, and
periosteum of the maxillary bone, with some of its fibers reaching posteriorly the
inferior wall of the CS and the SOF.[22] The superior surface of the orbital smooth muscle is in intimate relationship with
the inferior rectus muscle, inferior branch of the oculomotor nerve, and inferior
ophthalmic vein.[29]
The Orbit from an Anterior Perspective
Direct access to the orbital contents with no need for bone removal can be achieved
using either transcutaneous or transconjunctival approaches. While a pure transconjunctival
approach gives access to the orbit through the conjunctiva of the inferior eyelid,
the transcutaneous approaches can be used in the periorbital area including superior
and inferior eyelids. The transconjuctival approach has the advantage of avoiding
skin incisions; however, the exposure achieved may be limited. If an increased surgical
corridor is required, a lateral canthotomy can be associated. The lateral canthotomy
increases the risk of lower lid malpositionpostoperatively.[30] In cases of transcutaneous approaches (superior palpebral/eyebrow incisions or inferior
palpebral incisions: subciliary, subtarsal, or infraorbital), an understanding of
the underlying muscular, neural, and tendinous anatomy is key.
The orbicularis oculi muscle lies just below the loosely attached layer of skin of
the eyelids. The orbicularis oculi muscle surrounds the orbital rim and extends into
the lids, temple, and cheek. There are three defined portions of the orbicularis muscle
as follows: the orbital, palpebral, and lacrimal parts. The orbital portion lies circularly
around the margins of the orbit and is the sphincter muscle of the eyelids. The palpebral
portion is situated in the margins of the upper and lower eyelids and closes the eyelids.
The lacrimal portion lies the most deep, extending behind the lacrimal sac, and attaching
to the lacrimal bone, and is important in tear transport.[6]
The orbital portion arises from the medial palpebral ligament, nasal process of the
frontal bone, and frontal process of the maxilla. Its lateral upper fibers blend with
the occipitofrontalis and the corrugator muscles. Many of its upper fibers are inserted
also into the skin and subcutaneous tissues of the eyebrow. The palpebral portion
arises from the bifurcation of the medial palpebral ligament and inserts on the lateral
palpebral raphe, which is a ligament located on the outer canthus of the eye ([Fig. 3]). The lacrimal portion is located behind the medial palpebral ligament and arises
from the posterior lacrimal crest and adjacent part of the orbital surface of the
lacrimal bone.[10]
The tarsi, two dense plates of connective tissue that serve as the structural support
of the eyelids, are located underneath the palpebral portion of the orbicularis oculi
muscle.
The aponeurosis of the levatorpalpebræ muscle is attached to the anterior surface
of the upper tarsus. The tarsi have rigid attachments to the periosteum through the
canthal tendons medially and laterally.[10] The medial ends of the tarsi are attached by the medial palpebral ligament to the
frontal process of the maxilla in front of the lacrimal crest, crossing the lacrimal
sac. As the ligament crosses the lacrimal sac, a strong aponeurotic lamina from its
posterior surface expands over the sac to attach the upper part of the posterior lacrimal
crest.[31] The lateral ends of the tarsi are attached by the lateral palpebral ligament, a
much weaker structure than the medial palpebral ligament, to a tubercle on the frontosphenoidal
process of the zygomatic bone immediately within the orbital margin.[21]
The orbital septum is an extension of the periorbita that runs into the eyelid, separating
the orbital contents from the lid contents. This membranous sheet extends to the tarsus
from the orbital rim, where it attaches to the bone and becomes the periorbita inside
the orbit and periosteum along the anterior edge of the orbit.[21] In the upper eyelid, the septum blends with the aponeurosis of the superior levator
muscle, and in the lower eyelid with the anterior surface of the tarsus.[10]
The medial canthus therefore consists of the attachment of the orbicularis oculi muscle
and tarsus through the medial palpebral ligament, lacrimal sac, and a fibrous extension
from the fascia, covering the medial rectus muscle attached to the lacrimal bone immediately
posterior to the posterior lacrimal crest, known as medial check ligament.[21] The lateral canthus consists of the attachment of the tarsus, the aponeurosis of
the superior levator muscle and the lateral check ligament (a fibrous extension from
the fascia covering the lateral rectus muscle) to the tubercle located on the zygomatic
bone, whereas the fusion of the upper and lower orbicularis oculi occurs superficially
at the lateral palpebral raphe ([Fig. 3]).[31]
Eventually, when accessing orbital lesions requires an anterolateral craniotomy with
supraorbital rim removal or lateral orbitotomy, knowledge and preservation of the
temporal branches of the facial nerve and frontal branches of the trigeminal nerve
is essential.
The frontal nerve arises from the ophthalmic division of the trigeminal nerve at the
lateral wall of the CS, passes through the lateral part of the SOF outside the annular
tendon and courses over the levator muscle to divide into the supratrochlear and supraorbital
nerves within the orbit. The supraorbital and supratrochlear nerves exit the orbit
anteriorly through a notch or foramina in the supraorbital rim with the supraorbital
and supratrochlear artery respectively, both branches of the ophthalmic artery. The
supratrochlear nerve passes between the supraorbital foramen and the trochlea of the
superior oblique muscle in a superior direction over the forehead ([Figs. 2] and [3]).[6]
[10] The supraorbital nerve may be released if necessary by removing bone from the lower
margin of its foramen. Inadvertent injury of the supraorbital or supratrochlear nerve
causes sensory disturbances of the forehead and scalp. The supraorbital notch constitutes
an important surgical landmark for craniotomies that include removal of the supraorbital
rim, like the orbitozygomatic and supraorbital approaches. The supraorbital notch
has been traditionally used as medial craniotomy limit to avoid entry into the frontal
sinus and supratrochlear nerve injury.[32] However, its relationship with the frontal sinus and landmark reliability have been
questioned.[33] Careful study of a preoperative CT is mandatory to define the limits of the frontal
sinus.
The temporal branch of the facial nerve arises inside the parotid gland, pierces the
parotidomasseteric fascia below the zygomatic arch and then above the zygomatic arch
divides into its three terminal rami: anterior, middle, and posterior. The middle
ramus, also called frontal ramus, innervates the frontalis muscle. The frontalis muscle
is medial to the superior temporal line, and is embedded within the galea ([Fig. 3]). This layer is continuous laterally with the temporoparietal fascia (also known
as superficial temporal fascia) over the area of the temporalis muscle. Deep to the
temporoparietal fascia lies the temporal fascia (also known by some authors as deep
temporal fascia), which divides into a deep and superficial layer 2 to 3 cm over the
zygomatic arch. In between the superficial and deep layers of the deep temporal fascia
lies the interfascial fat pad (also known as superficial fat pad). The frontal ramus
of the facial nerve usually runs in the plane of the temporoparietal fascia; however,
sometimes, the frontal ramus goes into the interfascial fat pad and then becomes again
superficial to enter the frontalis muscle.[34]
[35] When the skin flap is reflected anteriorly, and the temporalis muscle is reflected
inferiorly to perform an anterolateral craniotomy, the temporalis muscle dissection
can put these nerves in jeopardy. Although the majority of the cases have a good aesthetic
result, injury of the frontal ramus may carry undesirable cosmetic defects as permanent
or transient frontalis muscle palsy. To avoid frontal ramus damage, special care must
be taken and different techniques like interfascial dissection (between the superficial
and deep layers of the deep temporal fascia) or subfascial dissection (below the deep
layer) have been proposed ([Fig. 3]).[32] When performing a supraorbital approach, injury to the frontal ramus should be also
prevented. Dissection deep to the frontalis muscle close to the periosteum may prevent
nerve damage. Other technical considerations, like subperiosteal dissection on the
zygomatic arch, and avoidance of excessive inferior reflection of the scalp flap can
also protect the frontal ramus from inadvertent injury.[36]
