Ординатура / Офтальмология / Английские материалы / Surgical Atlas of Orbital Diseases_Mallajosyula_2009

Fundamental to the understanding of orbital pathology and its surgical management is a sound working knowledge of the anatomy of the normal orbit in three dimensions. The goal of this chapter is to review the location of critical ocular adnexal, orbital and related craniofacial structures and the anatomic relationships between them.

OVERVIEW

The orbit is defined as the bony cavity containing the globe, extraocular muscles, fat, nerves and blood vessels. Although the orbit is often described as pyramidal in shape, the space is actually pear-shaped, with its largest horizontal and vertical diameters lying 1 cm past the orbital rim and adjacent to the equator of the globe. Average orbital volume is approximately 25-30 cc, of which the globe occupies approximately 7 cc of space. The lateral walls are oriented about 90° to one another and run 40 to 45 mm in length to the apex. The medial walls of each orbit run parallel to each other and measure 45 to 50 mm in length. The optical axes themselves are also parallel to the course of the medial walls, rather than the diverging central axes of the orbits. Therefore, each globe is tonically held in adduction by the extraocular muscles to maintain ocular alignment. These relationships and other important dimensions of the orbit are illustrated in Figure 1.1.

Orbital Osteology

which make up the four orbital walls: the frontal, sphenoid (greater and lesser wings), ethmoid, lacrimal, maxillary, palatine, and zygomatic bones (Figure 1.2).

The roof of the orbit is comprised of the frontal bone anteriorly, and the lesser wing of the sphenoid bone posteriorly (Figure 1.3). The overall thickness of the roof is significantly greater than that of either the medial wall or orbital floor and is therefore, relatively resistant to fracture. Within the lesser wing lies the optic foramen, through which the optic nerve exits the orbit via the optic canal. In about 30% of individuals, just above the frontosphenoid suture, lies the meningeal foramen through which the recurrent meningeal artery (a branch of the external carotid system) passes to anastomose with the lacrimal artery (a branch of the internal carotid system). This communication provides an important potential source of collateral blood flow to the orbit

The orbital rim is roughly in the shape of a spiral, with its starting and end points at the anterior and posterior lacrimal crests.1 There are seven bones

Figure 1.1: An axial view of the orbits demonstrating the dimensions and relationships between associated structures

should its primary supply via the internal carotid system become disrupted. When the meningeal foramen is absent, the middle meningeal artery courses directly via the superior orbital fissure.2 Other important bony landmarks include the lacrimal gland fossa in the temporal roof and the trochlear fossa anteromedially. Just superolateral to the trochlear fossa and at the medial one-third junction of the superior rim, lies the supraorbital notch which gives passage to supraorbital artery, vein, and nerve. In some individuals, this point of egress is completely enclosed and appears as the supraorbital foramen.3

The medial wall includes the ethmoid, maxillary, and lacrimal bones, as well as the lesser wing of the sphenoid (Figure 1.4). Within the bony suture line separating the frontal from the ethmoid bone, there are two important apertures, the anterior and posterior ethmoidal foramina. These foramina are the exit points for the anterior and posterior ethmoidal arteries and nerves, respectively. The anterior ethmoidal foramen is typically located approximately 24 mm posterior to the orbital rim and the posterior ethmoidal foramen lies approximately 36 mm posterior to the rim. The optic foramen, in turn, is located approximately 6 mm posterior to the posterior ethmoidal foramen. These foramina help the surgeon delineate the frontoethmoidal suture which is an important surgical landmark for the roof of the ethmoid sinus, or fovea ethmoidalis. The orbital roof slopes downward as it travels medially. Medial to the orbital space, just beyond the frontoethmoidal suture line, the fovea ethmoidalis continues in a downward plane and ends

Figure 1.4: Medial wall of the right orbit

Applied Anatomy of Orbit 5

sagittally just above the nasal cavity and below the anterior cranial fossa at the cribriform plate. Bony dissection of the medial wall above the suture line exposes the dura of the frontal lobe.

The ethmoid portion of the medial wall, the lamina papyracea, is extremely thin and is thus prone to fracture with trauma and to easily transmit infection from the ethmoid air cells into the orbit as subperiosteal abscesses. The medial wall thickens again in the area of the inferior suture between the ethmoid and maxillary bones. This maxilloethmoid strut4 provides support to the inferomedial orbital wall and often survives trauma which fractures the more superior aspects of the wall. At the anterior aspect of the medial wall is the lacrimal sac fossa, bounded by the anterior and posterior lacrimal crests. The anterior and posterior limbs of the medial canthal tendon insert on the anterior and posterior lacrimal crests, respectively.

The floor of the orbit consists of the maxillary, zygomatic and palatine bones. The maxillary bone forms the bulk of the floor while the zygomatic bone contributes anterolaterally and the palatine bone contributes to the posterior floor. A major landmark in this area is the infraorbital groove, which originates approximately 25-30 mm posterior to the orbital rim. The groove deepens and becomes an enclosed canal as it travels anteriorly within the floor to open again on the face of the maxillary bone at the infraorbital foramen on the maxillary face, 4-6 mm from the rim in adults.5 This pathway contains the infraorbital neurovascular bundle which is easily injured by floor fractures or inadvertent surgical dissection. Just medial to the infraorbital groove is the thinnest portion of the maxillary bone. Not only does this render the posteromedial part of the floor particularly susceptible to blowout fractures, but it also provides an area where bone can be removed with relative ease for inferior orbital decompression. The thicker, maxilloethmoid strut lies in the medial floor and provides support for the orbital soft tissues and the globe.4, 6 In the anteromedial floor, the bony nasolacrimal duct travels from the base of the lacrimal fossa in an inferior and usually slightly posterolateral direction through the maxillary bone in the lateral nasal wall to empty into the inferior meatus of the nose. The vector of the nasolacrimal duct shows considerable variability.

6 Surgical Atlas of Orbital Diseases

The lateral wall contains the zygomatic bone and the greater wing of the sphenoid which separates the posterolateral orbit from the middle cranial fossa. The posterior borders of the lateral wall are defined by the superior and inferior orbital fissures. The boundary between the lateral wall and roof is formed by the frontosphenoid suture, which transmits the recurrent meningeal artery. The anterior part of the lateral wall is comprised of the zygoma. Important landmarks in this region include the lateral orbital tubercle, or Whitnall’s tubercle, which is the insertion point of the posterior head of the lateral canthal tendon, the lateral horn of the levator aponeurosis, the check ligament of the lateral rectus muscle, and Lockwood’s ligament. The tubercle can be found just inside the orbital rim and approximately 11 mm below the frontozygomatic suture.7 The superoanterior zygoma also contains the zygomaticotemporal and zygomaticofacial canals through which branches of the lacrimal artery and the lacrimal and zygomatic nerves pass (Figure 1.5).

The Periorbita

The periorbita refers to the tough, fibromembranous lining of the bony orbit which acts as a physical barrier to infection and provides a scaffold to which other intraorbital connective tissues can attach. The regions of greatest adherence between this sheath and bone are at the orbital rim, suture lines, bony fissures, trochlear fossa and the lacrimal crests. At the orbital margins, at the arcus marginalis, the periorbita thickens and gives rise to the orbital septum. Deep in the orbit, the periorbita continue through the

superior orbital fissure and optic canal to become continuous with the dura. The potential space outside the periorbita is an important surgical plane. Access to the orbital walls in decompression surgery, for example, entails dissection between the bone and this overlying periosteal sheet.

The Orbital Apex

The orbital apex merits special attention, as the region in which many critical orbital structures converge and communicate with other important, periorbital spaces (Figure 1.6). Cranial nerves II through VI, major orbital vessels, and all of the extraocular muscles excluding the inferior oblique sit in tight proximity within the apex, and pathology in this region can produce profound deficits in vision and ocular motility.8, 9

The apex is defined by only three walls; the floor is absent in the far posterior orbit.1 Major bony landmarks include the superior orbital fissure, inferior orbital fissure, and the optic canal. The superior orbital fissure divides the sphenoid into the greater (lateral) and lesser (medial) wings and lies inferiorly and laterally to the optic foramen. This fissure measures approximately 20-22 mm in overall length and is separated into superolateral and inferomedial sections by the tendon of the lateral rectus muscle. The superotemporal part of the fissure lies above the annulus of Zinn, the fibrous ring formed by the common origin of the rectus muscles. The lacrimal, frontal and trochlear nerves, and the superior ophthalmic vein pass through this region as they traverse the apex. The inferomedial segment of

the superior orbital fissure, also called the oculomotor foramen, is located inside the annulus and transmits the superior and inferior divisions of the oculomotor nerve, the abducens nerve, sympathetic fibers, and the nasociliary nerve, a terminal sensory branch of the ophthalmic division of the trigeminal nerve10-12 (Figure 1.7).

The inferior orbital fissure bounds the greater wing of the sphenoid, separating it from the maxillary bone inferomedially. This fissure communicates primarily with the pterygopalatine fossa (Figure 1.8). The maxillary branch of the trigeminal nerve passes through the pterygopalatine fossa and subdivides into the infraorbital nerve which, in turn, travels anteriorly into the orbit via the infraorbital groove. The zygomatic nerve, another branch of the maxillary branch of cranial nerve V, enters the orbit through the inferior orbital fissure to provide sensory innervation to lateral orbit and cheek after passing through the zygomaticofacial foramen. Also within the pterygopalatine fossa is located the maxillary artery which gives rise to the infraorbital artery, part of the neurovascular bundle traveling through the infraorbital groove. Parasympathetic fibers originating from the pterygo-

palatine ganglion and terminating in the lacrimal gland are transmitted by the inferior orbital fissure as well.2 The inferior ophthalmic veins also pass from the orbit into the pterygoid plexus via the inferior orbital fissure.

The optic canal penetrates the superomedial orbital apex through the lesser wing of the sphenoid bone as the optic foramen. The canal is approximately 6 mm in diameter and 8-10 mm in length and houses the optic nerve and the ophthalmic artery. The canal runs along the upper, lateral wall of the anterior sphenoid sinus and in the floor of the anterior cranial fossa.

Figure 1.8: Lateral cross-section of the orbit

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The Cavernous Sinus

The cavernous sinus is a large venous sinus posterior to the orbital apex contained within a dural cleft which is situated lateral to the sphenoid sinus (Figure 1.9). Its tributaries consist of the ophthalmic, cerebral middle meningeal, and pterygoid veins. The left and right cavernous sinuses communicate with one another via small channels which run superior to the roof of the sphenoid sinus. Several critical structures pass through the cavernous sinus as they travel into the orbit. The carotid siphon and the sympathetics which ride along on its sheath traverse centrally. In the lateral wall of the cavernous sinus area embedded the oculomotor nerve, trochlear nerve, the ophthalmic and maxillary divisions of the trigeminal nerve, and the abducens nerve. The optic nerves course superomedially to the cavernous sinus. The optic chiasm is formed just above the anterior aspect of the cavernous sinus.

As in the orbital apex, pathological processes involving the cavernous sinus such as the formation of carotid-cavernous fistulas, inflammation or infection typically cause multiple cranial neuropathies affecting the eye.13 Further, because the right and left cavernous sinuses are interconnected, disease processes extending posteriorly from the orbit on one side can spread to the other via these spaces.

The Globe

The average-size globe measures approximately 23.5 mm in the horizontal meridian and 23 mm

vertically, with an anterior-posterior dimension of about 24 mm. Its overall volume is about 7cc. The globe is surrounded by a loose fascial sheath, or Tenon’s capsule, which is interconnected to the sclera by fine fibrous bands. The potential space between these layers is the episcleral space, and the areas of greatest adherence between them are approximately 1.5 mm from the limbus anteriorly, and posteriorly, at the optic nerve sheath. Tenon’s capsule is suspended inside the orbit by interconnections with fine connective tissue septae within the surrounding orbital fat. This sheath must be traversed by the nerves and blood vessels which supply the globe. Likewise, the extraocular muscles must penetrate Tenon’s layer to attach to the globe. As the muscles pass from outside to inside the episcleral space to fuse with the sclera, Tenon’s capsule makes attachments with the intermuscular septum, a fibrous network which encases and interconnects the extraocular muscles (Figure 1.10).14-18 Thus, following enucleation, orbital implants that are placed within Tenon’s capsule may demonstrate a fair amount of motility even if they are not sutured to the extraocular muscles themselves.18 The intermuscular septum and rectus muscles delineate the intraconal versus extraconal space.

Interconnections between the extraocular muscle sheaths and the periorbita comprise the check ligaments. Superiorly, the check ligaments consist of the fascial complex surrounding the upper lid retractors, the superior rectus and levator palpebrae muscles.19 Similarly, the inferior check ligament consists of the muscle sheaths surrounding the lower lid retractors, the inferior rectus and inferior oblique.

Medially, the muscle sheath of the medial rectus inserts just inside the posterior lacrimal crest, to the medial orbital septum and caruncle to collectively form the medial check ligament. The analogous check ligament of the lateral rectus inserts onto the lateral orbital tubercle of Whitnall.7

The Extraocular Muscles

The four rectus muscles originate at the annulus of Zinn, within the orbital apex. Specifically, the superior and medial recti originate adjacent to the lesser wing of the sphenoid, next to the optic canal. The inferior rectus originates from a portion of the annulus which extends from the body of the sphenoid bone to its great wing. The lateral rectus has a bifid origin from a tendinous segment of the annulus which extends across the superior orbital fissure from the greater to the lesser sphenoid wing and a more inferior portion which extends directly from the greater wing itself.

The superior rectus lies just underneath the levator palpebrae. Immediately beneath and medial to the superior rectus run the nasociliary nerve and ophthalmic artery. The superior edge of the medial rectus travels just under these structures. From its origin at the annulus, the inferior rectus closely follows the floor of the orbit until reaching the anterior orbit, where it becomes separated from the floor by the inferior oblique muscle as the latter crosses from the medial to the lateral wall, and by fat. Medial and superior to the lateral rectus, is found the ciliary ganglion which is usually adherent to the intraorbital segment of the optic nerve.

The superior oblique muscle also begins along the annulus of Zinn, superomedially and extends forward superiorly and along the junction between the orbital roof and the medial orbital wall. As it courses toward the anterior orbit, the muscle belly transitions to a tendinous segment as it reaches the trochlea, a pulley-like cartilaginous structure which lies approximately 6-10 mm posterior to the superomedial orbital rim.20, 21 From this point, the tendon passes posterolaterally, making a 54° angle, to attach to the globe. It is this course which gives rise to the main actions of the superior oblique, namely intortion and depression of the globe with contraction. During orbital surgery, caution to avoid injuring the trochlea must be taken to avoid

Applied Anatomy of Orbit 9

subsequent scarring and restriction of superior oblique muscle action, or Brown’s syndrome.22

The inferior oblique, unlike the other extraocular muscles, does not originate at the annulus, but from the periosteum of the anterior, inferomedial orbit on the maxillary bone. It courses posterolaterally, just beneath the inferior rectus to insert on the inferolateral globe, which gives rise to its main actions: extortion and elevation of the globe. The capsulopalpebral fascia and Lockwood’s ligament interdigitate with the muscular fascia of the inferior oblique.

The extraocular muscles approximate a spiral in the distance between their individual insertions and the corneal limbus, the so-called spiral of Tillaux (Figure 1.11). Beginning with the medial rectus, each successive muscle inserts farther from the limbus. Although there is individual variation, the average distances are: medial rectus, 5.5 mm; inferior rectus, 6.5 mm, lateral rectus, 6.9 mm, and superior rectus, 7.7 mm.23

Innervation to the extraocular muscles is largely carried by the oculomotor nerve (third cranial nerve), which supplies the medial, inferior, and superior rectus muscles, the inferior oblique, as well as the levator palpebrae superioris. The superior oblique and lateral rectus receive innervation from the

Figure 1.11: Spiral of Tillaux

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trochlear nerve (fourth cranial nerve) and the abducens nerve (sixth cranial nerve), respectively. The blood supply to the muscles is carried by muscular branches of the ophthalmic artery.

Lids

Understanding the general anatomy and dimensions of the major eyelid landmarks is important in the evaluation of structural disease of the lids and in planning surgical repair. The normal interpalpebral fissure height is 10-12 mm and the average length is 28-30 mm. The distance between the upper lid crease and lid margin measures approximately 8-11 mm at the pupillary axis. The highest point of the upper lid contour rests just nasal to the center of the pupil and the upper lid margin is typically located 1-2 mm below the superior limbus in adults. The lateral canthal angle sits approximately 2 mm higher than the medial canthus.

Both the upper and lower eyelids can be divided into the skin, orbicularis, orbital septum, orbital fat, lid retractors, tarsus, and conjunctiva. For descriptive purposes, the layers can be grouped into the anterior

and posterior lamellae. The anterior lamella contains lid structures which lie outside the orbit per se, and is comprised of the skin and the orbicularis oculi (Figure 1.12).

The skin of the lids is the thinnest of the body and unlike skin elsewhere, has no subcutaneous fat layer. The portion which lies anterior to tarsus is relatively firmly attached to deeper structures while the preseptal portions are loosely connected. Anterior to the margin, the skin contains the lash follicles with their associated sebaceous glands of Zeis and apocrine glands of Moll, as well diffusely distributed eccrine sweat glands.

The orbicularis muscle is a C-shaped complex of muscle fibers which functions to close the lids (Figure 1.13). It is divided into pretarsal, preseptal and orbital sections, all of which receive innervation from the facial nerve (seventh cranial nerve). The pretarsal and preseptal parts of orbicularis are primarily involved in involuntary closure of the lids, as elicited by the blink reflex. These fibers insert at the medial canthal tendon as deep and superficial heads. A subset of pretarsal orbicularis fibers, also called Horner’s muscle, inserts deep at the posterior lacrimal crest as the deep limb of the medial canthal

tendon, along the posterior aspect of the lacrimal sac and surround the canaliculi. These fibers are thought to provide a pumping action as they contract which facilitates tear drainage.24, 25 Horner’s muscle is also critical in maintaining close contact between the posterior aspect of the lid and the globe. The remaining pretarsal orbicularis inserts superficially within the anterior limb of the medial canthal tendon. Laterally, slips from the upper and lower lid pretarsal orbicularis insert onto the lateral canthal tendon which in turn, inserts on the lateral orbital tubercle. Medially, the deep head of the preseptal orbicularis inserts into the fascia surrounding the lacrimal sac while the superficial head inserts onto the anterior limb of the medial canthal tendon.

The orbital orbicularis is chiefly responsible for forced lid closure, such as winking or in blepharospasm. Medially, its insertions lie along the orbital rim and anterior medial canthal tendon. As they course laterally, these fibers overlie the zygoma and the elevators of the lateral mouth, the zygomaticus major and minor.

Underlying the pretarsal orbicularis and resting anterior to the tarsus is the muscle of Riolan which consists of small, horizontally oriented slips of muscle. These fibers appear grossly as the “gray line” of the lid margin which is posterior to the lash line and function to turn the lashes toward the eye during blinking. The “gray line” is a useful landmark in aligning the wound edges in marginal lid laceration repair.

The septum, orbital fat and posterior lamella, which consists of the retractors, tarsus and conjunctiva, are considered to be intraorbital structures. The septum is comprised of tough fibrous connective tissue arranged in sheets which originate from the periosteum of the orbital rims at the arcus marginalis. This structure acts as an relative barrier between the orbit and lid in limiting the deep spread of superficial hemorrhage and infection. In the upper lid, the septum fuses with the aponeurosis of the levator muscle, the primary upper lid retractor muscle, approximately 2-5 mm above the superior tarsal edge.26 In the lower lid, the septum condenses with the capsulopalpebral fascia as the two layers converge toward the inferior edge of the lower tarsal plate.27

Applied Anatomy of Orbit 11

The orbital septum lies anterior to the preaponeurotic orbital fat pads, which prolapse forward with any violation of the septum. As a natural consequence of aging, as the septum thins and stretches, these fat pads tend to gradually herniate anteriorly. In the upper lid, there are two distinct fat pads, the medial and central fat pads. The medial pad can be distinguished by its relatively white color. The medial palpebral artery typically runs within this pocket and care should be taken to avoid inadvertent laceration or cauterization during surgery. Laterally, there is usually little fat in the upper lid. Instead, this space is usually filled by orbital lobe of the lacrimal gland.

The lower lids contain medial, central and lateral fat pads. The medial pad lies just medial to the inferior oblique and as in the upper lid, has a characteristically white color and contains the lower palpebral artery. The central fat pad lies between the inferior oblique muscle and a fascial band that separates it from the lateral fat pad. The latter extends to the inferior edge of the lacrimal gland.28

The upper lid retractors consist of the levator muscle which is innervated by cranial nerve III, and the sympathetically innervated Müller’s muscle. The levator, which is the primary retractor, originates from a point just above the annulus of Zinn, from the lesser wing of the sphenoid bone. The levator complex includes a muscular component which is approximately 40 mm long extending from its origin on the lesser sphenoid wing just outside the annulus of Zinn, and the fibrous levator aponeurosis which is 14-20 mm in length. As the muscular portion courses forward in the orbit, it rides just above the superior rectus muscle and the two are interconnected by interdigitated fibrous bands. As they reach the equator of the globe, the levator broadens and transitions into its aponeurotic component. Medially and centrally, the aponeurosis inserts onto the anterior tarsal surface and passes through the orbicularis to insert onto pretarsal skin. These insertions create the upper lid crease. Medially, the aponeurosis separates into a single medial horn which inserts into the posterior lacrimal crest and becomes continuous with the medial canthal tendon complex. Similarly, the aponeurosis courses into a lateral horn which inserts into the lateral orbital tubercle, also