Osteology of the orbit

Question 1

What separates the two orbits from each other?

Answer 1

the nasal cavity and the ethmoidal and sphenoidal air sinuses

Question 2

What are the 7 types of bones of which parts contribute to the walls of the orbit?

Answer 2

1. Frontal
2. Maxilla
3. Sphenoid
4. Zygomatic
5. Palatine
6. Ethmoid
7. Lacrimal

Question 3

What is the approximate shape of each orbit and what does it lead to?

Answer 3

quadrilateral pyramid, base is the orbital margin (yellow ring), apex narows at the optic canal

Question 4

Where is the orbit widest?

Answer 4

1.5cm behind the orbital margin

Question 5

How are the medial vs lateral walls of the orbit angled?

Answer 5

medial walls approx parallel to mid sagittal plane, whereas lateral walls angled approx 45 degrees to this plane

Question 6

In which direction is the orbital aperture directed?

Answer 6

forwards, laterally and slightly downwards

Question 7

In which direction do nerves and muscles passing from the apex into the orbit pass?

Answer 7

forward and laterally

Question 8

What are the approximate dimensions of the orbit - height, width and depth?

Answer 8

40mm height, 40mm width, 40mm depth

Question 9

What is the volume of the orbit and what proportion of this is occupied by the eye?

Answer 9

30ml total; one fifth occupied by eye (6ml)

Question 10

What are the 2 key bones which form the roof of the orbit?

Answer 10

1. Orbital plate of the frontal bone
2. Small area of lesser wing of the sphenoid

Question 11

What are the two key bones that form the lateral wall of the orbit?

Answer 11

1. Orbital surface of zygomatic bone anteriorly
2. Orbital surface of greater wing of sphenoid posteriorly

Question 12

What are the 4 key bones which form the medial wall of the orbit?

Answer 12

1. Frontal process of the maxilla
2. Lacrimal bone
3. Orbital plate of the ethmoid
4. Body of the sphenoid bone

Question 13

What are the 3 key bones which form the floor of the orbit?

Answer 13

1. Orbital plate of the maxilla
2. Orbital surface of the zygoma
3. Orbital process of the palatine bone

Question 14

What are 3 noteworthy features of the orbital roof?

Answer 14

1. Fossa for the lacrimal gland
2. Trochlear fossa (fovea)
3. Anterior and posterior ethmoidal canals

Question 15

Where in the orbital roof is the fossa for the lacrimal gland located?

Answer 15

in the anterolateral aspect of the roof, behind the zygomatic process of the frontal bone

Question 16

Where in the orbital roof is the trochlear fossa (fovea)?

Answer 16

in the anteromedial aspect of the roof, 4mm from the margin

Is the site at which the trochlea (small pulley) is attached; tendon of the superior oblique passes through the trochlea

Question 17

What is the trochlear fossa the site of?

Answer 17

the site at which the trochlea (ligamentous sling, acts as small pulley) is attached

Question 18

What passes through the trochlea?

Answer 18

the tendon of the superior oblique

Question 19

Where in the orbital roof are the anterior and posterior ethmoidal canals?

Answer 19

positioned at the junction of roof and medial wall above the frontoethmoidal suture

transmit the anterior and posterior ethmoidal nerves and vessels

Question 20

What is the function of the anterior and posterior ethmoidal canals?

Answer 20

transmit the anterior and posterior ethmoidal nerves and vessels

Question 21

What are the relations of the roof of the orbit (i.e. what lies above it)?

Answer 21

separates the orbit from the anterior cranial fossa and frontal lobes of the brain

anteriorly, the frontal sinus lies above the orbit

Question 22

What is the thickness like of the orbital roof?

Answer 22

it is thin and translucent except at the lesser wing of the sphenoid

Question 23

What are 2 key features of the medial orbital wall?

Answer 23

1. lacrimal fossa for the lacrimal sac
2. vertical sutures which separate the four bones that comprise this wall (ethmoid, lacrimal, maxilla, sphenoid)

Question 24

What is the shape of the medial orbital wall?

Answer 24

oblong and thin (0.2-0.4mm)

Question 25

Where within the medial orbital wall is the lacrimal fossa for the lacrimal sac?

Answer 25

bound by anterior and posterior lacrimal crests and is continuous below with the nasolacrimal canal

Question 26

Explain the locations of the fossa for the lacrimal gland, and the lacrimal fossa for the lacrimal sac?

Answer 26

the **_fossa for the lacrimal gland_** is in the **roof** of the orbit (in the anterolateral aspect of the roof behind the zygomatic process of the frontal bone), whereas the lacrimal fossa for the lacrimal sac is in the **medial** **wall** (bound by anterior and posterior lacrimal crests and continuous below with the nasolacrimal canal)

Question 27

How thin is the medial wall of the orbit?

Answer 27

it is the thinnest of the walls, largely transparent or semi-transparent the ethmoidal air sinuses can easily be seen through the wall in a dried skull

Question 28

What are the relations of the medial wall of the orbit i.e. what lies medial to it?

Answer 28

anterior to posterior: anterior, middle, posterior ethmoidal air cells and the sphenoidal sinus

Question 29

What can infection rapidly spreading through the thin walls of air cells in the ethmoidal/sphenoid sinuses cause?

Answer 29

orbital cellulitis

Question 30

Which is the thinnest of the walls of the orbit?

Answer 30

medial wall

Question 31

In what direction does the floor of the orbit slope?

Answer 31

slightly downwards from medial to lateral wall

Question 32

What are the 2 key features of the orbital floor?

Answer 32

1. inferior orbital fissure 2. infraorbital groove --\> becomes infraorbital canal

Question 33

What is the course of the infraorbital groove/canal?

Answer 33

the **infraorbital groove** runs forward from the **inferior orbital fissure** before reaching the orbital margin, the fissure becomes the **infraorbital canal** opens as the **infraorbital foramen** 4mm below the orbital margin on the anterior surface of the maxilla

Question 34

What are the relations of the orbital floor?

Answer 34

separates the orbit from the maxillary sinus

Question 35

What is the thickness of the orbital floor?

Answer 35

only 0.5-1mm thickness

Question 36

Which orbital wall is most likely to be damaged following trauma such as in orbital blowout fractures?

Answer 36

the orbital floor

Question 37

What are 4 key features of the lateral orbital wall?

Answer 37

1. Spina recti lateralis: small bony pine on the greater wing of the sphenoid 2. Zygomatic foramen 3. Lateral orbital tubercle 4. Foramina for small veins that communicate with middle cranial fossa

Question 38

What is the spina recti lateralis?

Answer 38

small bony spine on greater wing of sphenoid near apex of orbit gives origin to part of the lateral rectus muscle

Question 39

What is the zygomatic foramen?

Answer 39

foramen in lateral orbital wall that transmits the zygomatic nerve and vessels to the temporal fossa and cheek (zygomaticotemporal nerve and zygomaticofacial nerve)

Question 40

What is the lateral orbital tubercle?

Answer 40

tubercle within the lateral orbital wall forms the atachment of the check ligament of the lateral rectus, suspensory ligament (Lockwood's) of the eye, superior transverse ligament (Whitnall's) and aponeurosis of levator palpebrae superioris

Question 41

What are the 4 things that attach to the lateral orbital tubercle?

Answer 41

1. check ligament of lateral rectus (expansion of the sheaths of LR that attach to the zygomatic bone) 2. suspensory ligament (Lockwood's ligament) 3. superior tranverse ligament (Whitnall's ligament) 4. Aponeurosis of levator palpebrae superioris

Question 42

What are the 2 nerves that are transmitted in the zygomatic foramen (within the lateral orbital wall)?

Answer 42

**zygomaticotemporal** nerve and **zygomaticofacial** nerve

Question 43

What are the 3 lateral relations of the lateral orbital wall (anterior to posterior)?

Answer 43

1. skin 2. temporal fossa 3. middle cranial fossa

Question 44

What is the orbital margin?

Answer 44

thickened rim of bone that helps protect the orbital contents

Question 45

What are the 3 bones that comprise the orbital margin?

Answer 45

frontal bone zygomatic bone maxilla

Question 46

Of the lateral and medial margins of the orbital margin, which reaches further anteriorly?

Answer 46

medial

Question 47

Why is the medial margin of the orbital margin sharper in the lower half?

Answer 47

due to anterior lacrimal crest; it is indistinct superiorly

Question 48

What is the superior orbital fissure?

Answer 48

communication between the orbital and cranial cavities that lies between the roof and lateral wall of the orbit, bounded by the lesser and greater wings of the sphenoid bone

Question 49

What bones bound the superior orbital fissure?

Answer 49

bounded by the lesser and greater wings of the sphenoid bone

Question 50

How does the width of the superior orbital fissure change along its length?

Answer 50

wider at its medial end and narrowest at its lateral end

Question 51

How long is the superior orbital fissure?

Answer 51

22mm

Question 52

What separates the orbital fissure from the optic foramen above?

Answer 52

the posterior root of the lesser wing of the sphenoid bone

Question 53

What soft tissue structure spans between the narrow and wide parts of the superior orbital fissure?

Answer 53

the part of the **common tendinous ring** that gives origin to the **lateral rectus muscle**

Question 54

What are 5 structures that pass above or outside the tendinous ring / annulus spanning the superior orbital fissure?

Answer 54

1. lacrimal nerve 2. frontal nerve 3. trochlear nerve 4. superior ophthalmic vein 5. recurrent branch of the lacrimal artery

Question 55

Which bood vessel does the recurrent branch of the lacrimal artery anastamose with (and where may it more commonly travel rather than the superior orbital fissure)?

Answer 55

middle meningeal artery in a small cranio-orbital foramen lateral to the superior orbital fissure

Question 56

What are 5 structures that pass **within** the common tendinous ring that gives origin to the lateral rectus (and thus within the apex of the muscle cone) that spans the superior orbital fissure?

Answer 56

1. oculomotor nerve (superior and inferior divisions) 2. abducent nerve 3. nasociliary nerve 4. sympathetic root of the ciliary ganglion 5. inferior ophthalmic vein (variably)

Question 57

How many orbital cellulitis develop as a result of infection spread?

Answer 57

consequence of infection spreading from air sinuses to the orbit via the paper-thin medial wall (lamina papyracea)

Question 58

What is the anatomical location of the inferior orbital fissure?

Answer 58

lies between the lateral wall and floor of the orbit, below the superior orbital fissure

Question 59

What does the inferior orbital fissure form a communication between?

Answer 59

between the orbit and the infratemporal fossa and the pterygopalatine fossa

Question 60

What is the length of the inferior orbital fissure and where does it end?

Answer 60

runs forward and laterally for approximately 20mm, ends 20mm from the orbital margin

Question 61

Where is the inferior orbital fissure narrowest?

Answer 61

in the middle section

Question 62

What covers the inferior orbital fissure?

Answer 62

periorbita and a sheet of smooth muscle of unknown function, called the orbitalis or 'muscle of Müller'

Question 63

What are 4 things transmitted within the inferior orbital fissure?

Answer 63

1. infraorbital nerve 2. zygomatic nerve 3. branches from pterygopalatine ganglion 4. branches from inferior orbital vein

Question 64

What may branches of the inferior ophthalmic vein, tranmitted within the inferior orbital fissure, communicate with?

Answer 64

pterygoid venous plexus below

Question 65

What is the optic canal?

Answer 65

bony channel in the sphenoid that passes anteriorly, inferiorly and laterally (36 degrees) from the middle cranial fossa to the apex of the orbit

Question 66

Which bones form the optic canal?

Answer 66

the two roots of the lesser wing of the sphenoid

Question 67

How far apart are the optic canals anterior and posteriorly?

Answer 67

anteriorly: 30mm posteriorly: 25mm

Question 68

What is the shape of the optic canal and where is it narrowest?

Answer 68

funnel shaped, narrowest where its opening into the orbit is oval with sharp upper and lower borders and a prolonged roof (10-12mm in length)

Question 69

How long is the prolonged roof of the optic canal?

Answer 69

10-12mm

Question 70

What is the shape of the opening of the optic canal at the cranial aspect?

Answer 70

oval with a prolonged floor

Question 71

What are 2 important relations of the optic canal?

Answer 71

1. sphenoidal and posterior ethmoidal air sinuses (medial) 2. olfactory tracts (superior relations)

Question 72

What are 4 things transmitted within the optic canal?

Answer 72

1. **_optic nerve_**, with its meningeal coverings 2. **ophthalmic artery** 3. **sympathetic nerve fibres** originating from superior cervical ganglion that form plexus around internal carotid artery (accompany ophthalmic artery)

Question 73

What is the relation of the ophthalmic artery to the optic nerve within the optic canal?

Answer 73

ophthalmic artery lies below and lateral to the nerve within the dural sheath for part of its course

Question 74

Why is it thought that the orbital floor is more commonly involved in orbital blow-out fractures (even though the medial wall is thinner)?

Answer 74

because the floor lacks the buttress-like supports of the ethmoidal air cells and the protection of the nose

Question 75

Where may orbital contents herniate into in the event of orbital blowout fracture affecting the orbital floor?

Answer 75

maxillary sinus

Question 76

What are the 4 types of paranasal sinuses?

Answer 76

1. frontal 2. ethmoidal 3. sphenoidal 4. maxillary

Question 77

What are the paranasal sinuses?

Answer 77

air-filled cavities within the skull that are in communication with the nasal cavity via a series of apertures

Question 78

What are 3 functions of the paranasal sinuses?

Answer 78

1. warm and moisten air 2. add resonance to voice 3. lighten skull

Question 79

What is the way to remember the **cranial nerves** passing through the superior orbital fissure?

Answer 79

LFT SNIA: Loose foreskin trapping smegma never impressed anyone L: lacrimal nerve (V1) F: frontal nerve (V1) T: trochlear nerve (IV) S: superior division of oculomotor n (III) N: nasociliary nerve (V1) I: inferior division of oculomotor n (III) A: abducent nerve (VI)