Normal Anatomy

Question 1

Palpebral fissure length

Answer 1

25-30 mm

Question 2

Palpebral fissure height

Answer 2

8-12 mm

Question 3

Distance from upper lid margin to corneal light reflex (MRD 1)

Answer 3

3-5 mm

Question 4

Levator excursion

Answer 4

8-15 mm

Question 5

Distance from upper lid margin to upper lid crease

Answer 5

8-11 mm

Question 6

Average volume of adult orbit

Answer 6

30 cc

Question 7

Entrance height of adult orbit

Answer 7

35 mm

Question 8

Entrance width of adult orbit

Answer 8

40-45 mm

Question 9

Average medial wall length of adult orbit

Answer 9

40-45 mm

Question 10

Average distance from posterior globe to optic foremen in adult orbit

Answer 10

18 mm

Question 11

Length of orbital segment of optic nerve

Answer 11

25-30 mm

Question 12

Bones comprising roof of orbit

Answer 12

Frontal

Lesser wing of sphenoid

Question 13

Structures passing through supraorbital notch/foramen

Answer 13

Supraorbital vessels

Supraorbital branch of frontal nerve

Question 14

Bones comprising lateral wall of orbit

Answer 14

Greater wing of sphenoid
Zygomatic

NB: thickest and strongest of orbital walls

Question 15

Where is the lateral orbital tubercle of Whitnall located and what attaches to it?

Answer 15

Zygomatic bone

Lateral canthal tendon
Lateral horn of the levator aponeurosis
Check ligament of the lateral rectus
Lockwood ligament (the suspensory ligament of the globe)
Whitnall ligament

Question 16

Bones comprising the medial wall of the orbit

Answer 16

Ethmoid
Lacrimal
Maxillary bone
Lesser wing of sphenoid

NB: thinnest points of orbit are within medial wall (lamina papyracea between orbit and ethmoid sinuses, maxillary bone esp.posteromedial portion)

Question 17

Bones comprising inferior wall of orbit

Answer 17

Maxillary
Palatine
Zygomatic

Question 18

Within what bone is the superior orbital fissure located and what structures pass through it?

Answer 18

Separates lesser and greater wings of sphenoid

CNs III, IV, V1, VI
Superior ophthalmic vein (majority of venous drainage from orbit to cavernous sinus)
Sympathetic nerve fibers

Question 19

What are the boundaries of the inferior orbital fissure and what structures pass through it?

Answer 19

Sphenoid, maxillary, and palatine (lies between the lateral orbital wall and the orbital floor)

Question 20

Inferior orbital fissure

Answer 20

Bounded by the sphenoid, maxillary, and palatine bones

Transmits the maxillary division of CN V (V2) and branches of the inferior ophthalmic vein (leading to the pterygoid plexus)

Question 21

What is the orientation of nerves and blood vessels entering the orbit through the superior orbital fissure and how does this affect surgical approach to the optic nerve?

Answer 21

CN III (superior and inferior divisions), CN VI, and the nasociliary nerve (branch of V1) enter the orbit through the oculomotor foramen passing through the muscle cone

CN IV, the frontal and lacrimal nerves (branches of V1), and the superior ophthalmic vein do not pass through the muscle cone. They enter superotemorally (LFTS)

A superonasal approach to the optic nerve is used

Question 22

Optic canal

Answer 22

8-10 mm long

Located within lesser wing of sphenoid

Transmits optic nerve, ophthalmic artery, and sympathetic nerves

Question 23

Arterial supply of orbit

Answer 23

Arterial supply:
Ophthalmic artery is major (branch of internal carotid), internal maxillary and facial arteries are minor (branches of external carotid)

Ophthalmic artery travels underneath optic nerve through dura mater

Major branches of ophthalmic artery:

1. Branches to extraocular muscles
2. Central retinal artery (to optic nerve and retina)
3. Posterior ciliary arteries (long to ant. segment, short to choroid)

Question 24

Venous drainage of the orbit

Answer 24

Superior orbital vein is main vein

Originates in superonasal quadrant and extends posterior my through superior orbital fissure

Drains into cavernous sinus

May be seen as structure coursing diagonally through superior orbit on axial CT scans

Question 25

Maxillary division of CN V (V2)

Answer 25

Exits skull via foramen rotundum

Travels through ptergyopalatine fossa and gives off zygomatic branch

Becomes infraorbital nerve and travels anteriorly in floor of orbit through infraorbital canal - emerges on the face of the maxilla 1 cm below inf. orbital rim

Orbital blowout fractures commonly disrupt floor of orbit medial to infraorbital canal, leading to nerve compression and hypoesthesia in the V2 region

Question 26

Where are the EOMs innervated?

Answer 26

All recti muscles are innervated on the intraconal side 2/3 of the way posteriorly

CN IV crosses over the levator and innervates the superior oblique in its posterior third

The nerve to the inferior oblique travels anteriorly along the lateral aspect of the inferior rectus and innervates the inferior oblique on its posterior surface

Question 27

Where is the facial nerve in relation to the SMAS?

Answer 27

It is superficial to the SMAS in the upper face (so you want to keep your surgical plane deep to the SMAS)

It is deep to the SMAS in the lower face (so you want to keep your surgical plane superficial to the SMAS)