- Surgical Anatomy of Orbit
- Evaluation of Orbital Disorders
- Orbital Imaging
- Congenital Orbital Anomalies
- Orbital Infection and Inflammation
- Thyroid-associated Orbitopathy
- Orbital Fractures: A Conceptual Approach
- Orbitotomy
- Orbital Decompression
- Enucleation, Evisceration, Orbital Exenteration and Orbital Implants
INTRODUCTION
A comprehensive knowledge of the orbit and periorbital area aids in diagnosis and management of disorders in this region.
The two orbital cavities are situated on either side of the sagittal plane of the skull between the cranium and the skeleton of the face.
RELATIONS OF ORBIT
- Superiorly—anterior cranial fossa and frontal sinus
- Laterally—temporal fossa (anteriorly); middle cranial fossa (posteriorly)
- Inferiorly—maxillary sinus
- Medially—ethmoid sinus and anterior part of sphenoid sinus.
Due to the close proximity of these structures, any infection can spread across these regions. For instance, an infection of the ethmoid sinuses can easily invade the orbit through the thin lamina papyracea.
EMBRYOLOGY
Orbital Walls
- The orbital walls are derived from the cranial neural crest cells which expand to form the frontonasal process and maxillary process
- Inferior, medial, and lateral walls develop from the lateral nasal process and maxillary process
- Orbital roof is formed from capsule of forebrain
- Ossification of the orbital bones can be either enchondral or membranous
- First bone to develop is maxillary bone, around 6 weeks of intrauterine life. It develops from elements in the region of the canine tooth. Secondary ossification centers are in the orbitonasal and premaxillary regions
- Other orbital bones develop at around 7 weeks of intrauterine development
- Frontal, zygomatic, maxillary, and palatine bones have intermembranous origin
- Sphenoid bone has both enchondral and membranous origin
- Lesser wing of sphenoid—develops at 7 weeks of intrauterine development. It has enchondral origin
- Greater wing of sphenoid—develops at 10 weeks, has intermembranous origin
- Both wings join at 16 weeks of age.
- Ossification completes at birth, except the orbital apex
- In the early stages of development, human eyes are directed in opposite direction. With facial development, angle between optic stalk decreases and is about 68° in adults.
Size, Shape, and Volume of Orbit
Orbit is shaped like a quadrilateral pyramid, with the base anteriorly and apex directed posteriorly. The orbits are aligned such that medial walls are parallel to each other and lateral walls are perpendicular to each other. The angle between medial and lateral wall is 45°. The axis between visual axis and orbital axis is 23°.
The average dimensions of the orbit are as follows (Figs. 1A and B):
- Height of orbital margin—40 mm
- Width of orbital margin—35 mm
Figs. 1A and B: (A) Dimensions of the orbit; (B) Depth of Safe limit for intraorbital dissection of orbital walls during surgery.
- Volume of orbit—30 cm3
- Depth of orbit—40–50 mm
- It is approximately 1 cm shorter from the lateral orbital rim to apex
- The widest circumference of the orbit is 1.5 cm inside the orbital rim at the level of lacrimal recess.
OSTEOLOGY
The orbit is composed of seven bones: frontal, lacrimal bone, zygoma, maxilla, ethmoid, sphenoid, and palatine bone (Fig. 2).
Superior Wall or Roof
- Formed by:
- Orbital plate of frontal bone
- Lesser wing of sphenoid
- Concave in shape
- Separates orbit from anterior cranial fossa
- Optic foramina in lesser wing of sphenoid:
- Transmits optic nerve and ophthalmic artery from middle cranial fossa.
- The medial aspect has fovea for trochlea, 4 mm behind the orbital margin
- The lateral most aspect accommodates fossa for lacrimal gland, behind the zygomatic process of frontal bone
- The superior orbital rim has a notch at the junction of medial one-third and lateral two-thirds:
- Transmits supraorbital nerve and vessels-supplies forehead.
Medial Orbital Wall
Composed of four bones, from anterior to posterior:
- Frontal process of maxilla
- Lacrimal bone
- Orbital plate of ethmoid
- Body of sphenoid.
Orbital plate of ethmoid is the largest part of medial orbital wall. It is very thin (papyraceous). It separates orbit from ethmoid sinuses.
The frontoethmoidal suture line marks the approximate level of ethmoidal sinus roof, hence any dissection above this line should be avoided as it will expose the cranial cavity.
Anterior and posterior ethmoid foramina are present behind the medial orbital rim at the junction of medial wall and roof. They transmit anterior and posterior ethmoidal arteries (branches of ophthalmic artery) and ethmoidal nerves (branches of nasociliary nerve). The anterior ethmoidal foramen is located at a distance of 24 mm from the anterior lacrimal crest, while the posterior ethmoidal foramen is located at a distance of 36 mm from the anterior lacrimal crest (Fig. 3).5
Lacrimal fossa is a depression in the inferomedial orbital rim. It is formed by maxillary (anterior part) and the lacrimal bone (posterior part). It is bounded by two projections, i.e. anterior lacrimal crest of maxillary bone and posterior lacrimal crest of lacrimal bone. Lacrimal bone is thin whereas the maxillary bone is quite thick. If maxillary bone is predominant in the lacrimal fossa, then osteotomy becomes quite difficult during dacryocystorhinostomy (DCR) surgery.
Nasolacrimal canal lies in the inferomedial part of orbit through which nasolacrimal duct traverses.
The nasolacrimal duct is 3–4 mm in diameter, it passes backward, downward, and laterally to open into the inferior meatus under the inferior turbinate.
Sutura longitudinalis imperfecta of Weber lies in the frontal process of maxilla just anterior to the lacrimal fossa. This suture runs parallel to the anterior lacrimal crest. Small branches of infraorbital artery pass through this groove to supply the nasal mucosa. The presence of these vessels should be anticipated in any lacrimal sac surgery to avoid intraoperative bleeding.
Disruption of medial wall leading to nasoorbitoethmoidal fracture (NOE fracture) or any lateral displacement of the walls leads to hypertelorism.
Any trauma to frontal process of maxilla where medial canthal ligament is attached, leads to telecanthus.
Lateral Wall
- Formed by:
- Greater wing of sphenoid (posteriorly)
- Zygoma (anteriorly)
- Thickest bone
- Greater wing of sphenoid separates orbit from middle cranial fossa. Congenital absence of this bone in cases like neurofibromatosis results in pulsatile proptosis due to orbital encephalocele
- Whitnall's tubercle:
- 4–5 mm behind the lateral orbital rim
- 1 cm inferior to frontozygomatic suture
- Attachments:
- Lateral canthal tendon
- Lateral horn of levator aponeurosis
- Lateral rectus check ligament
- Suspensory ligament of lower lid (Lockwood's ligament)
- Lacrimal gland fascia
- Orbital septum.
- The frontal process of zygomatic bone and zygomatic process of frontal bone are thick bones and thus protect globe during injury
- Posterior part of this lateral wall is thin (about 1 mm), composed of orbital plate of greater wing of sphenoid and posterior zygomatic bone
- The superior orbital fissure (between lateral and superior walls of orbit) and the inferior orbital fissure (between lateral and inferior walls of orbit) transmit important structures
- Zygomaticosphenoid suture is an important landmark for lateral orbitotomy. Superiorly the bony incision is usually made just above the frontozygomatic suture. The lateral wall removal is completed by fracturing the bone at the zygomaticosphenoid suture
- The recurrent meningeal branches of the ophthalmic artery (internal carotid supply) exit the orbit via the frontosphenoid suture to anastomose with the middle meningeal artery (external carotid supply)
- The zygomaticofacial and zygomaticotemporal neurovascular structures leave the orbit via their respective foramina.
Orbital Floor
- Triangular in shape and shortest of all the walls
- Formed by:
- Orbital plate of maxilla
- Zygoma
- Palatine.
- Separated from lateral wall by inferior orbital fissure
- Inferior orbital fissure weakens the floor. Blow out fractures usually occur medial to it
- Medially, it is bounded by maxilla ethmoidal strut. It is important to preserve this during orbital decompression surgery to avoid hypoglobus and postoperative diplopia
- Infraorbital groove becomes a canal anteriorly, through this groove passes the infraorbital nerve and artery (maxillary division of trigeminal nerve and the terminal 6branch of internal maxillary artery). They exit through the infraorbital foramen to supply the lower eye lid, cheek, upper lip, and upper anterior gingiva
- The infraorbital foramen is located about 6–10 mm below the infraorbital rim
- The inferior oblique muscle arises anteromedially, immediately lateral to the nasolacrimal canal.
FISSURES AND CANAL
Various important nerves and vessels are transmitted through fissures and canals in the orbit (Fig. 4).
Superior Orbital Fissure
- Location—between greater and lesser wing of sphenoid. It lies between roof and lateral walls of orbit (Fig. 5).
- Also known as sphenoidal fissure
- It is 22 mm long, largest communication between orbit and middle cranial fossa
- It usually narrows laterally and widens medially, below the optic foramen
- Its tip is about 30–40 mm from frontozygomatic suture
- Its medial part is separated from optic foramen by posterior part of lesser wing of sphenoid
- The annulus of Zinn, a tight fibrous ring, divides the superior orbital fissure into intraconal and extraconal spaces.
Structures passing through upper part:
- Lacrimal nerve
- Frontal nerve
- Trochlear nerve
- Superior ophthalmic vein.
Structures passing through annulus of Zinn:
- Superior division of 3rd nerve
- Nasociliary nerve
- Sympathetic root of cervical ganglion
- Inferior division of 3rd nerve
- 6th nerve
- Sympathetic fibers.
Structures passing through lower part, outside the annulus of Zinn:
Inferior ophthalmic vein.
Inferior Orbital Fissure
- Location—lies between lateral wall and floor of the orbit. It is about 1 cm posterior to the inferolateral orbital rim
- It is also known as sphenomaxillary fissure
- It is about 20 mm long
- The orbit communicates with the pterygopalatine and infratemporal fossa
- Structure passing:
- Maxillary division of the trigeminal nerve
- Zygomatic nerve
- Branches from the sphenopalatine ganglion
- Branches of the inferior ophthalmic vein leading to the pterygoid plexus
- The maxillary division of trigeminal nerve and the terminal branch of internal maxillary artery enter the infraorbital groove and canal to become the infraorbital nerve and artery.
Optic Foramen
- The foramen is present in the lesser wing of sphenoid lies medial to the superior orbital fissure and is separated from it by a bony optic strut
- Conveys the optic nerve and ophthalmic artery
- The optic foramen is about 6.5 mm in diameter
- In adults the optic canal is 8–10 mm long and 5–7 mm wide
- Any trauma to optic canal, can result in injury to optic nerve.
ORBITAL SOFT TISSUES
- Orbital septum
- Periorbita
- Orbital fat
- Extraocular muscles
- Lacrimal system.
Orbital Septum
- It is anterior soft tissue boundary. It is extension of periorbita (Fig. 6)
- It originates from arcus marginalis. It extends from tarsus to orbital rim
- It acts as a physical barrier, separates the orbital contents from eyelids
- It is covered anteriorly by preseptal orbicularis muscle and skin.
Periorbita
- It is periosteal lining of the orbital walls
- It is firmly attached at the suture lines, the foramina, the fissures, the arcus marginalis, and the posterior lacrimal crest
- Posteriorly, the periorbita is continuous with the optic nerve sheath where the dura is fused to the optic canal
- Periorbita thickens on the orbital surface of the optic canal and the medial aspect of the superior orbital fissure and gives rise to the tendinous attachments of the four rectus muscles, the levator superioris, and the superior oblique muscle. This tendinous ring is called the annulus of Zinn.
Tenon's Capsule
- Also known as fascia bulbi or bulbar sheath
- It is dense, elastic, and vascular connective tissue that surrounds the globe
- It begins from perilimbal area and extends posteriorly along the globe till optic nerve and fuses with dural sheath and sclera
- It is separated from sclera by periscleral lymph space which is in continuation with subdural and subarachnoid space.
Orbital Fat
- Acts as a cushion to orbital structures
- In upper eyelid it lies anterior to levator complex and posterior to orbital septum
- It is divided into compartments by connective tissue septa
- Infratrochlear nerve and medial palpebral artery branch of the ophthalmic artery course through the medial fat pad
- In lower eyelid, medial fat pad is separated from central pad of fat by inferior oblique muscle.
Extraocular Muscles
Each orbit contains six extraocular muscles that function together to move the eye:
- Rectus muscles (4)—superior, inferior, lateral, and medial recti muscle
- Oblique muscles (2)—superior and inferior
- Other muscles—the levator palpebrae and Müller's muscle (Fig. 7).
Recti Muscles
The rectus muscles originate at the annulus of Zinn, a fibrous tendon that encircles the optic foramen. The annulus of Zinn is divided into the superior Lockwood tendon and the inferior tendon of Zinn. The inferior tendon gives origin to parts of medial and lateral recti and entire inferior rectus muscle. The superior tendon gives origin to part of medial and lateral recti and all of the superior rectus muscle. The attachments of superior and medial recti muscles are close to the dural sheath of optic nerve. Thus causing pain during 8extreme eye movements in retrobulbar neuritis. The recti are inserted 6–8 mm posterior to the limbus into the sclera.
Actions:
- Medial rectus—Adduction
- Lateral rectus—Abduction
- Superior rectus—Elevation, adduction, intorsion
- Inferior rectus—Depression, adduction, extorsion.
Oblique Muscles
The superior and inferior oblique muscles originate separately from the posterior orbital wall.
The superior oblique muscle arises from the sphenoid bone superomedial to the optic canal. It courses in the forward direction lying above the medial rectus, and through a cartilaginous pulley (the trochlea) attached to the frontal bone. Thereafter, the tendon passes posterolaterally, running inferior to the tendon of the superior rectus to insert into the posterior sclera.
The inferior oblique muscle arises from the maxilla at the anteromedial floor of the orbit, passes in a posterolateral direction, immediately inferior to the inferior rectus to insert into the posterior sclera.
Actions:
- Inferior oblique—extorsion, elevation, abduction
- Superior oblique—intorsion, depression, abduction.
Nerve supply: The superior oblique muscle is supplied by the trochlear nerve, the lateral rectus by the abducent nerve, and all the other extraocular muscles are supplied by the oculomotor nerve.
Levator Palpebrae Superioris
It is a striated muscle. It helps in elevation of the eyelid.
Origin
Levator palpebrae superioris (LPS) arises from the under surface of lesser wing of sphenoid just above and in front of optic foramen, and usually it is blended with the origin of superior rectus muscle. From this attachment this ribbon-like muscle passes forward below the roof on top of the superior rectus muscle.
Insertion
Levator palpebrae superioris inserts into the skin of the upper eyelid and upper tarsal plate.
Nerve Supply
It is innervated by the superior division of 3rd cranial nerve.
Müller's Muscle
It is a smooth muscle which acts as an eyelid elevator.
Origin
It arises from the inferior aspect of LPS.
Insertion
Inserted into the upper edge of tarsal plate.
Nerve Supply
It is innervated by sympathetic fibers.
SURGICAL SPACES (FIG. 8)
- Subperiosteal space
- Peripheral orbital space
- Muscular cone.
Subperisoteal Space
- It lies between orbital bones and periorbita
- It is limited anteriorly by strong adhesions between periorbita and orbital margins.
Extraconal Space
- It is bounded peripherally by periorbita and centrally by four recti muscles and their intermuscular septa, anteriorly by orbital septum and posteriorly by:
- Peripheral orbital fat
- Muscles—superior and inferior oblique, LPS
- Nerves—lacrimal, frontal, trochlear, anterior and posterior ethmoidal
- Vessels—superior and inferior ophthalmic veins
- Lacrimal gland
Intraconal Space
- Bounded anteriorly by Tenon's capsule, peripherally by four recti and intermuscular septa and posteriorly continuous with peripheral space
- Contents:
- Central orbital fat
- Nerves—optic nerve, oculomotor, abducens, nasociliary, ciliary ganglion
- Vessels—ophthalmic artery, superior ophthalmic vein.
Sub-Tenon's Space
It lies between sclera and Tenon's capsule.
LACRIMAL APPARATUS
Lacrimal apparatus comprises of lacrimal gland and its excretory passage.
Lacrimal Gland
The lacrimal gland lies in the superotemporal orbit, in lacrimal fossa of the frontal bone. It measures about 20 mm by 12 mm. It is divided into larger orbital and smaller palpebral part by levator aponeurosis. The gland is composed of numerous secretory units known as acini which progressively drain in to small and larger ducts. About 2–6 ducts from the orbital lobe pass through the palpebral lobe joining with the ducts from the palpebral lobe to form 6–12 tubules to empty into the superolateral conjunctiva. Hence, damage to the palpebral lobe may block drainage from the entire gland. About 20–40 accessory lacrimal glands of Krause are located in the superior conjunctival fornix, about half this number is located over the lower fornix.
Nerve Supply
Nerve supply is innervated by branches from 5th and 7th cranial nerves, sympathetic supply to lacrimal gland is via the nerves from the superior cervical ganglion. The parasympathetic fibers are supplied via the 6th nerve. Sensory supply is via the branches of trigeminal nerve.
Vessels
- Arterial supply—lacrimal artery branch of ophthalmic artery
- Venous drainage—ophthalmic veins
- Lymphatic drainage—preauricular lymph nodes.
Lacrimal Excretory System
The lacrimal excretory system begins with punctum, about 0.3 mm in size. It lies at the medial end of each eyelids, at the junction of ciliated and nonciliated part. The punctal opening widens into ampulla, makes a sharp turn to drain into the canaliculi. The canaliculi measures 0.5–1 mm in diameter and courses parallel to the lid margins. It has two parts—vertical 2 mm in size and horizontal about 8 mm in length. The superior canaliculus is slightly shorter than the inferior canaliculus. In 90% of individuals the superior and inferior canaliculi merge into a common canaliculi before draining into lacrimal sac.10
There is a valve at the junction of common canaliculus and lacrimal sac known as the Rosenmuller valve. The lacrimal sac resides in the lacrimal fossa. It measures about 12–15 mm vertically and 4–8 mm anteroposteriorly. It opens into nasolacrimal duct, which is about 15 mm in length. It has interosseous and meatal part. It is directed downward, backward, and laterally and opens in inferior meatus. Valve of Hasner is found at the lower end of the nasolacrimal duct at the level of inferior meatus of nose. Imperforate Hasner's valve in new born infants results in congenital nasolacrimal obstruction (Fig. 9).
Vessels
- Arterial supply—palpebral branch of ophthalmic artery, angular and infraorbital arteries, and nasal branch of sphenopalatine
- Venous—angular and infraorbital vessels above and nasal veins below
- Lymphatic drainage—submandibular and deep cervical nodes.
Nerve Supply
Infratrochlear and anterior superior alveolar nerves.
ORBITAL NERVES
Orbit contains seven nerves. It comprises of:
- Component of central nervous system—Optic nerve
- Motor nerves—Oculomotor, trochlear, and abducens nerves
- Sensory nerve—Ophthalmic division of trigeminal nerve (V1) and some contribution from maxillary division (V2)
- Autonomic center—Ciliary ganglion.
Optic Nerve
Optic nerve is the second cranial nerve
- It is about 4 cm in length
- Parts:
- Intraocular—1 mm
- Intraorbital—30 mm
- Intracanalicular—5 mm
- Intracranial—10 mm
- Intraocular: The nerve fibers start from axons of ganglion cell layer of retina, converge on the optic disc, and pierce the layers of the eye. It is 1.5 mm in diameter and expands to 3 mm behind sclera as it receives myelin sheaths
- Intraorbital: Extends from back of eye to optic foramina
- Its course is tortuous
- The entire intraorbital optic nerve is surrounded by meningeal and arachnoidal sheaths in continuation with the respective intracranial layers
- The optic nerve lies within the muscular cone
- Relations:
- Long and short ciliary nerves and arteries surround the optic nerve before they enter the eyeball
- Ophthalmic artery, superior ophthalmic vein and nasociliary nerve crosses the nerve superiorly from lateral to medial side
- Between optic nerve and lateral rectus muscle lies ciliary ganglion, nasociliary nerve, divisions of oculomotor nerve, and abducent and sympathetic nerve.
- Nerve passes through the optic canal to enter the middle cranial fossa.
- Intracanalicular:
- Ophthalmic artery crosses inferiorly from medial to lateral side
- Sphenoid and posterior ethmoidal sinuses lie medially to it, thus results in retrobulbar neuritis following infection.
- Intracranial—Lies above cavernous sinus and combines with opposite nerve to form optic chiasma.
Motor Nerves
- Oculomotor nerve (III)
- Abducens nerve (VI)
- Trochlear nerve (IV).
Oculomotor Nerve (Fig. 10)
- The two branches enter the muscular cone and diverges
- The superior division moves up on the lateral side of optic nerve. It supplies the LPS and superior rectus muscles
- The inferior division is located inferiorly and outside the optic nerve and then splits to supply the medial rectus, inferior rectus, and the inferior oblique
- The branch to inferior oblique travels along the lateral border of inferior rectus and enters the inferior oblique
- From the branch to inferior oblique a small branch arises which goes to ciliary ganglion to form its parasympathetic root. After synapses the fibers of third nerve combine with sympathetic fibers to constitute short ciliary nerve which supplies ciliary muscle and iris sphincter.
Trochlear Nerve (Fig. 11)
- The trochlear nerve enters the orbit just outside the annulus of Zinn, having crossed superior to the oculomotor nerve in the lateral wall of the cavernous sinus
- It travels forward in the orbit crossing from lateral to medial above the origin of LPS to enter the lateral border of superior oblique at the junction of the posterior third and anterior two-thirds.
Abducens Nerve (Fig. 12)
- The abductor nerve starts in the medial part of the superior orbital fissure inside the annulus of Zinn and outside the branches of the oculomotor nerve
- It travels along the medial surface of lateral rectus piercing the muscle at the junction of the posterior third and anterior two-thirds.
Sensory Nerves (Fig. 13)
- The trigeminal nerve supplies sensory innervation to the orbit and surrounding structures. It originates at the lateral and ventral portion of the pons
- Most of the supply is from ophthalmic division (V1) of trigeminal nerve with some contribution from maxillary division (V2)
- The ophthalmic division extends from the trigeminal ganglion and passes through the cavernous sinus to the orbit via the superior orbital fissure.
- Before entering the orbit through superior orbital fissure it divides into lacrimal, frontal, and nasociliary branches
- The lacrimal and frontal nerves enter the fissure outside the annulus of Zinn and travel forward in the superior orbit
- The lacrimal nerve, the smallest branch of ophthalmic nerve, travels along the superior border of the lateral rectus and supplies the postganglionic secretomotor fibers to the lacrimal gland, and sensory fibers to surrounding conjunctiva and upper eyelid. The parasympathetic fibers travel from the lacrimal nucleus in the pons to the greater superficial petrosal nerve via the nervus intermedius, to the vidian nerve, to the sphenopalatine ganglion, to the zygomatic branch of the maxillary nerve, to the zygomaticotemporal nerve, and to the lacrimal nerve, to innervate the lacrimal gland
- The frontal nerve, largest division of ophthalmic nerve, divides into the supraorbital and supratrochlear nerves. The supraorbital nerve moves anteriorly above LPS, leaves the orbit through the supraorbital notch, and supplies the forehead, scalp, upper eyelid, and frontal sinus. The supratrochlear nerve, medial one, runs anteriorly above the trochlea and supplies medial part of the forehead and upper eyelid
- The nasociliary branch enters the orbit through the annulus of Zinn. It crosses the optic nerve and passes forward between the superior oblique and medial rectus muscles. Its branches into the anterior and posterior ethmoidal nerves, two or three long posterior ciliary nerves to the globe, short ciliary nerves which pass through the ciliary ganglion, and do not synapse. It terminates as the infratrochlear nerve which supplies the medial canthus and the tip of the nose. The ethmoidal nerves contribute branches to the nasal cavity and external nose. The long ciliary nerves carry sympathetics from the superior cervical ganglion responsible for dilatation of the pupil
- The maxillary division of the trigeminal nerve leaves the middle cranial fossa through the foramen rotundum and enters the pterygopalatine fossa
- Within fossa after giving off sphenopalatine, posterior superior alveolar, and zygomatic branches, the main part of nerve passes through the inferior orbital fissure to enter the infraorbital sulcus as the infraorbital nerve. It exits at the infraorbital foramen and supplies the lower lid skin, conjunctiva, cheek, and the upper lip
- Within the infraorbital canal, the infraorbital nerve gives off the anterior superior alveolar branch supplying the upper front teeth
- The zygomatic branch of V2 passes through the inferior orbital fissure and divides into zygomaticotemporal and zygomaticofacial branches that supply the skin overlying the lateral orbit and zygoma. The zygomaticotemporal branch also gives secretomotor fibers to the lacrimal nerve that supplies lacrimal gland.13
Ciliary Ganglion (Fig. 14)
- It is peripheral parasympathetic ganglion
- It is located near the orbital apex
- Lies between the optic nerve and lateral rectus
- It has three roots:
- Motor or parasympathetic root—comes from the inferior branch of the third cranial nerve (by the inferior oblique branch). Its fibers mainly innervate the ciliary muscle and, to a lesser extent, the sphincter of the iris.
- Sympathetic root—a branch of the carotid plexus which enters the orbit via the common tendinous ring
- Sensory root—a long and fine fiber which rejoins the nasociliary nerve where it enters the orbit. This supplies the eye and the cornea.
- Filamentous branches of the ganglion, 8–10 in number, are called the short ciliary nerves and they move forward around the optic nerve, together with ciliary arteries and the long ciliary nerves.
VASCULAR SUPPLY OF ORBIT
Arterial Supply (Fig. 15)
- Ophthalmic artery, first main branch of internal carotid artery, provides the main arterial supply to the orbit, with some contributions from maxillary and middle meningeal artery, branches of external carotid artery
- The ophthalmic artery originates from the internal carotid medial to the anterior clinoid process, as it exits the cavernous sinus. The ophthalmic artery courses on the inferior aspect of the optic nerve and enters the orbit through the optic canal
- At the entrance, it lies lateral to optic nerve and medial to ciliary ganglion. Then accompanied by the nasociliary nerve, it turns medially, crossing the optic nerve superiorly and below the superior rectus muscle. It then moves forward between superior oblique muscle and the medial rectus muscle. It terminates by splitting into two different arteries, the supratrochlear artery and the angular artery
- Branches are divided into three groups—ocular, orbital, and extraorbital
- Ocular branches:
- Central retinal artery
- Ciliary arteries
- Collateral branches to optic nerve
- Orbital branches:
- Lacrimal artery
- Muscular arteries
- Periosteal branches
- Extraorbital branches:
- Posterior and anterior ethmoid arteries
- Supraorbital artery
- Medial palpebral artery
- Dorsal nasal artery
- Supratrochlear artery.
Ocular Branches (Fig. 16)
Central Retinal Artery
- First and smallest branch of ophthalmic artery
- End artery
- It continues till papilla and then gives the terminal branches.
Ciliary Arteries
- Three types—long posterior ciliary artery, short posterior ciliary artery, and anterior posterior ciliary artery
- Long posterior ciliary arteries:
- Commonly two in number
- Arise in the ophthalmic artery at the point at which it crosses over the optic nerve. These enter the sclera not far from where the optic nerve entry point
- It runs in the epichoroidal space till the ciliary body, where it divides into upper and lower branches and encircle iris
- Anastomose with artery ciliary arteries to form major arterial circle of iris
- Short posterior ciliary arteries:
- Seven in number
- Moves in forward direction around the optic nerve and pierce sclera and supply choroid
- At equator it anastomoses with long posterior ciliary artery, anterior ciliary artery, and major arterial circle of iris.
- Anterior ciliary arteries:
- Seven in number
- Arise from muscular branches and pass in front, over the tendons of the rectus muscles.
Cilioretinal Artery
- Present in 15–20%
- Enters from lateral aspect of optic nerve
- Supplies retina between disk and macula.
Orbital Branches
Lacrimal Artery
- Arises near the optic canal above and lateral to optic nerve. It travels superolaterally above the lateral rectus muscle to supply lacrimal gland
- Branches:
- Lateral palpebral artery supplies upper and lower eyelid and anastomoses with medial palpebral artery
- Zygomatic artery which passes through zygomatic facial and zygomatic temporal foramina
- Muscular branch to lateral rectus
- Recurrent meningeal anastomoses with the middle meningeal artery.
Muscular Branches
- Supply the extraocular muscles
- Accompany oculomotor nerve along their course.
Extraorbital Branches
Supraorbital Artery
- Runs between levator muscle and periorbita
- Leaves orbit through superior orbital foramen along with the nerve
Supratrochlear Artery
- It is the terminal branch
- Pierces the orbital septum above superior oblique pulley.
Ethmoidal Arteries
Anterior Ethmoidal Artery
- Enters anterior ethmoidal canal
- Supplies anterior and middle ethmoidal cells, frontal sinus, meninges, anterior nasal mucosa, and skin of nose.
Posterior Ethmoidal Artery
- Runs between medial rectus and superior oblique muscle to enter posterior ethmoidal canal
- It supplies posterior ethmoidal sinuses, dura of anterior cranial fossa, and upper nasal mucosa.
Medial Palpebral Artery
- It arises below the superior oblique pulley
- Descends behind the lacrimal sac
- It pierces the orbital septum and forms peripheral and marginal arterial arches; runs between the orbicularis oculi and tarsal plate.
Lateral Palpebral Artery
- Branches from lacrimal artery
- Supplies eyelid
- Anastomoses with medial palpebral artery.
Dorsal Nasal Artery
- Terminal branch
- Pierces the orbital septum and passes above the medial palpebral ligament and descends to nose
- Supplies lacrimal sac and anastomose with facial artery.
Infraorbital Artery
- A terminal branch of the maxillary artery
- It courses through the inferior orbital fissure into the infraorbital sulcus where it gives off branches to the orbital fat and muscular branches to the inferior rectus and inferior oblique muscles prior to entering the infraorbital canal to exit at the infraorbital foramen
- It anastomoses with the angular artery and the inferior palpebral vessels.
Venous Drainage (Fig. 17)
The venous drainage of the orbit is through the valve less superior and inferior ophthalmic veins.
Superior Ophthalmic Vein
- Larger of the two
- It is formed superomedially near the trochlea by the union of the angular, supraorbital, and supratrochlear veins
- It communicates with central retinal vein, receive inferior ophthalmic vein and two vorticose veins from upper part of eyeball
- Leaves the orbit through superior orbital fissure and drains into the cavernous sinus (Fig. 18).
Inferior Ophthalmic Vein
- It is usually formed anteriorly as a plexus within the inferomedial orbital fat
- It also communicates with the pterygoid plexus via the inferior orbital fissure and facial vein
- Receives muscular branches and inferior vorticose veins
SUGGESTED READING
- Cornelius CP, Mayer P, Ehrenfeld M, et al. The orbits—Anatomical features in view of innovative surgical methods. Facial Plast Surg. 2014;30(5):487–508.
- Martins C, Costa e Silva IE, Campero A, et al. (2011). Microsurgical anatomy of the orbit: The rule of seven. Hindawi Publishing Corporation, Anatomy Research International. [online] Available from https://www.hindawi.com/journals/ari/2011/468727/abs/. [Accessed January, 2019].
- Patanaik VVG, Bala S, Sangla RK. Anatomy of the bony orbit-some applied aspects. J Anat Soc India. 2001;50(1):59–67.
- René C. Update on orbital anatomy. Eye (Lond). 2006;20(10):1119–29.
- Rootman J, Stewart B, Goldberg RA. Orbital anatomy. In: Orbital Surgery: A Conceptual Approach, 1st edition. Philadelphia, New York: Lippincott-Raven; 1995. pp. 75–150.
- Turvey TA, Golden BA. Orbital anatomy for the surgeon. Oral Maxillofac Surg Clin North Am. 2012;24(4):525–36.