PROCEDURE
1.
PRE-WORK
(before lab) review
the major bones and
openings related to the orbit from your atlas or on an articulated skeleton in
the lab.
(Netter011)
a) Observe that the orbit is similar in shape to a cone, with a wide front that narrows to the posterior end of the orbit.
b) Review the bones contributing to the orbit. These include the frontal bone making up the superior wall of the orbit, the zygomatic and sphenoid making up the lateral wall, the maxilla forming most of the floor, and the ethmoid and lacrimal bones making up the medial wall of the orbit.
c) The optic canal is formed in the lesser wing of the sphenoid at the posteromedial end of the orbit.
d) The superior orbital fissure lies between the lesser and greater wings of the sphenoid bone.
e) The inferior orbital fissure lies between the greater wing and maxilla. This fissure leads from the pterygopalatine fossa into the orbit.
f) In the frontal bone at or near the superior rim of the orbit is the supraorbital notch (foramen).
g) In the maxilla, at or near the inferior rim of the orbit is the infraorbital foramen.
1a. REVIEW (before lab) Using a mirror at home or the eye of
a friend examine external features of the eyeball.
(Netter094)
Note that in donors, dehydration may have caused a collapse of part of the eyeball. All of the features listed can best be seen in the eye of the living.
Pupil - The pupil is the black center of the eye. However, rather than actually being 'black' it is transparent and appears dark because light entering through the pupil is almost completely absorbed by tissues of the retina.
Iris - The iris is a thin circular structure responsible for changing the diameter of the pupil and exhibits different pigmentation based on the genetics of the individual (most commonly brown, green, blue, grey, and hazel).
Sclera - This is the white region surrounding the iris, although in donors the tissue may have darkened in color.
Perform the dissection steps below on both the left and right sides unless otherwise indicated in one of the steps.
Note that the dissection of the orbital contents proceeds systematically from superior orbital structures to deeper orbital structures. Carefully identify structures at each step before transecting muscles and altering relationships.
2. With the donor prone (face down) place two stacked wood blocks under the chin so as to lift the skull base into an approximately horizontal position.
3. Remove any remaining dura from the anterior cranial
fossa and observe the thin orbital plate of the frontal bone in the anterior
fossa.
(Figure713)
(Netter115)
(Photo7046)
4. Chisel around the perimeter of the bulge that is formed
in the plate by the orbital contents (i.e. the orbital plate of the frontal
bone).
(Figure713)
Take care not to strike the chisel too hard as the bone is usually quite thin.
5. Pick away bony pieces with forceps or bone cutters in
individuals with heavier bone density.
(Figure714)
(Photo7047)
If you observe a series of spaces or voids in the bone at the medial side, you have inadvertently broken into the ethmoidal air cells and should avoid going further medial.
6. The first structure you will encounter is the periosteum
of the bone of the orbit.
(Figure714)
(Photo7048)
In the orbit, this periosteum layer is known as the periorbita since it encloses most of the orbital contents.
The periorbita at the posterior end of the orbit becomes continuous with the dura covering the optic nerve.
7. Push a probe from the middle cranial fossa into the
optic canal above the optic nerve.
(Photo7049)
8. Following the path of the probe as a guide, cut
through the roof of the optic canal using bone cutters.
(Figure715)
(Photo7050)
9. Lift the lateral side of the periorbita and make a small incision in the tissue with scissors.
10. Insert a blunt probe below the periorbita through the
incision and by lifting the periorbita using the probe as a guide, cut
through the periorbita all the way around the rim of the opening in the orbital plate
to remove the periorbita.
(Figure716)
(Photo7051)
Observe that below the periorbita the orbital contents are typically surrounded by abundant periorbital fat.
As you dissect to follow structures in the next steps, you will need to carefully use forceps and a probe to clean out periorbital fat surrounding the orbital contents, being careful not to destroy the structures you are following.
11. Push
a probe into the superior orbital fissure above the ophthalmic (V1) nerve
branch.
(Figure717)
(Netter097)
(Photo7051)
12. Using the probe as a guide, with bone cutters cut
through the roof of the superior orbital fissure exposing the path of the
ophthalmic (V1) nerve.
(Figure717)
(Netter097)
(Photo7051)
13. Starting at the trigeminal ganglion (on the side it is
exposed), trace the
ophthalmic (V1) nerve anteriorly where it divides into several branches.
(Figure717) (Netter097)
(Photo7051)
Frontal nerve - This is the largest of the ophthalmic (V1) nerve branches, coursing directly anteriorly immediately inferior to the periorbita (and along the superior side of the levator palpebrae muscle). Follow the frontal nerve anteriorly to where it begins to branch (or reaches the bone of the superior orbital rim).
Lacrimal nerve - The lacrimal nerve splits from the ophthalmic nerve within the vicinity of the superior orbital fissure. The lacrimal nerve courses to the lateral side of the orbit along the superior side of the lateral rectus muscle to innervate the lacrimal gland. We will not follow this specifically
14. Using a Stryker saw cut through the superomedial edge
of the orbital rim just lateral to where the orbital bone meets the nasal
structures on both the left and right sides of the head (cut#1).
(Figure783)
(Photo7099a)
In many individuals this cut will pass through part of the frontal sinus such that you may observe a space/void in the frontal bone.
15. Using a Stryker saw cut through the superolateral edge
orbital rim approximately in line with the lateral corner of the eyelid on both
the left and right sides of the head (cut #2).
(Figure783)
(Photo7099b)
16. Grasp the excised segment of the orbital rim and
tilt/reflect the bone anteriorly separating soft structures from the bone so
that the bone segment can be removed.
(Photo7099c)
The supraorbital nerve (a branch of the frontal nerve) most commonly passes through a superior orbital notch and you can free the nerve easily as you reflect the bone segment.
In some individuals the bone closes over the nerve to form a foramen through which the nerve passes. If this is the case transect the nerve near the bone of the orbital rim to simplify removal of the bone segment.
17. Dissect in the superolateral aspect of the orbit for
the lacrimal gland.
(Figure784)
(Netter097)
(Photo7051)
If you have difficulty distinguishing the lacrimal gland from connective tissue, any mass present in the superolateral aspect of the orbit will be the lacrimal gland.
18. Returning to the frontal nerve, continue following the
nerve anteriorly where it branches into the supraorbital (and a smaller supratrochlear
nerve).
(Figure784)
(Netter097)
(Netter099)
(Photo7051)
Supraorbital nerve - The supraorbital nerve continues anteriorly within the orbit to where it will pass superficially via the supraorbital notch/foramen.
Supratrochlear nerve - This smaller nerve splits from the frontal nerve and courses to the medial side of the orbit passing superior to the trochlear muscle to innervate the glabella region of the face. We will not study this specifically.
19. Gently retract the frontal nerve medially, and observe
the levator palpebrae superioris muscle below the nerve.
(Figure784) (Netter097)
(Photo7053)
This is the most superior muscle in the orbit and the only orbital muscle that does not attach to the eyeball.
The levator palpebrae superioris muscle attaches via a flat aponeurosis into the upper eyelid and is responsible for elevating the upper eyelid.
20. Work a probe inferior to the levator palpebrae
superioris muscle along its lateral edge and separate the levator palpebrae
superioris muscle from the deeper superior rectus muscle.
(Figure784) (Netter097)
(Photo7054)
The levator palpebrae superioris muscle is innervated by a branch of the oculomotor (III) nerve.
21. Leaving the frontal nerve intact if possible, transect
the levator palpebrae superioris muscle close to the anterior end of the muscle and
reflect the levator palpebrae superioris muscle to expose the underlying
superior rectus.
(Figure718) (Netter096)
(Photo7054)
Superior rectus muscle - The superior rectus muscle attaches to the eyeball via a flat tendon anterior to the equator of the eyeball. The primary action of this muscle is to elevate the eyeball (move the pupil superiorly).
22. Transect the superior rectus muscle close to the eyeball
(leaving approximately a 1cm attached to the eyeball) and
reflect it posteriorly.
(Figure719) (Netter097)
(Photo7055)
The inferior (deep) side of the superior rectus muscle is innervated by a branch of the oculomotor (III)nerve.
23. On the medial side of the orbit, find the superior
oblique muscle.
(Figure784) (Netter097)
(Photo7052)
The muscle is typically more cylindrical (or worm) shaped than the flatter rectus muscles.
If the trochlear (IV) nerve innervates the superior oblique muscle.
24. Trace the superior oblique muscle anteriorly to where
it forms a tendon that passes
through the trochlea (pulley) and attaches to the eyeball at a location deep to
the superior rectus muscle.
(Netter096)
(Photo7052)
Superior oblique muscle - The superior oblique muscle runs along the superomedial side of the orbit. The tendon turns sharply posterolaterally through the trochlea to insert into the eyeball posterior to its equator. The primary action of the muscle is to intort the eye (rotate the upper eyeball inwards towards the midline).
25. Pick away fat in the region between the superior rectus muscle and the optic nerve.
26. Remove fat lateral to the optic nerve sufficient
to expose the lateral rectus muscle.
(Figure719) (Netter097)
(Photo7055)
(Photo7056)
Lateral rectus muscle - The lateral rectus muscle is innervated by the abducens (VI) nerve. Contraction abducts the eye (moves the pupil away from the midline).
27. Remove fat medial to the optic nerve sufficient to
expose the the medial rectus muscle.
(Figure719) (Netter097)
(Photo7055)
(Photo7056)
Medial rectus muscle - The medial rectus muscle is innervated by the oculomotor (III) nerve. Contraction of the medial rectus muscle adducts the eye (moves the pupil towards the midline).
CLINICAL EXERCISE - Lateral Orbital Canthotomy
- Injury to the bones around the orbit can result in bleeding into the space behind the eyeball. Since this is an enclosed space, it is critical to release that pressure before it can occlude blood flow into the eyeball that can lead to permanent vision loss.
- In this clinical exercise, you have the chance to perform a surgical intervention designed to allow the eyeball anterior movement releasing pressure buildup in the space behind the eye.
- Perform this procedure on your donor (or if the lateral
region is damaged use your hemi-head). For the steps
to take:
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click here ------>
28. After performing the lateral orbital canthotomy, use forceps to firmly retract the upper eyelid of only one eye superiorly.
The next steps enucleating (removing) the eyeball are to occur on only one side (preserve the side with the best orbital dissection intact).
29. Starting at the lateral orbital rim lift and separate
the superior eyelid from the eyeball using a scalpel to
cut the superior eyelid conjunctiva.
(Figure781) (Netter094)
This cut should allow the superior eyelid to be completely retracted medially away from the eyeball.
Conjunctiva - This is a membranous epithelium covering the inside of the eyelid which reflects back on itself to cover the sclera of the eye. The conjunctiva forms a complete barrier preventing any external object from passing around the eyeball to the back of the eye.
30. At the medial corner of the eye, cut the upper eyelid
and orbicularis oculi muscle through to the bone of the nose such that you can
completely remove the superior eyelid.
(Figure781) (Netter094)
31. Use forceps to firmly retract the lower eyelid
inferiorly and starting at the lateral orbital rim, use a scalpel to
cut the inferior eyelid conjunctiva following the
inferior orbital rim.
(Figure781) (Netter094)
This cut should be against the bone of the inferior orbital rim allowing the inferior eyelid to be completely retracted.
32. At the medial corner of the eye, cut the lower eyelid
and orbicularis oculi muscle through to the bone of the bone of the nose such
that you can remove the inferior eyelid.
(Figure781) (Netter094)
This should now have the superficial regions of tissue surrounding the eyeball excised down to the orbital rim and regions between the eyeball and orbit accessible.
33. With scissors (or a scalpel as needed) cut through any of the attachments of muscles or connective tissue between the eyeball and the orbit.
The goal is to cut through all the tissue holding the eyeball in place, which includes the extraocular muscles and the optic nerve.
You may need to 'pull' the eyeball to one side or the other, or superiorly, to get sufficient access to cut the connections to the eyeball. As you extract the eyeball you may see the inferior oblique, although this is not readily recognizable and we won't look for it specifically.
Inferior oblique muscle - The inferior oblique muscle runs along the inferior side of the orbit curving to attach to the lateral side of the eyeball. The primary action of the muscle is to extort the eye (rotate the upper eyeball surface away the midline).
34. Extract the eyeball anteriorly out from the orbit.
35. With a fresh scalpel blade, make a sagittal transection
of the removed eyeball and identify the major parts of the eye.
(Figure722) (Netter100)
(NetterBP026)
(Photo7058)
Cornea - This is the outermost curved sheet of transparent tissue which begins the process of bending light into the eye (accounting for about 2/3rd of the total optical power of the eye). This typically becomes opaque post-mortem.
Anterior chamber - Directly behind the cornea is an anterior chamber containing fluid.
Pupil - The pupil is the opening allowing light to reach the lens.
Iris - The iris is a thin circular structure containing a peripherally located smooth muscle sphincter (sphincter pupillae), which can contract to alter the diameter of the iris.
Lens - The lens is a dense biconcave structure that refracts light entering through the pupil onto the retina (accounting for the remaining 1/3rd of the eyes total optical power and deformable so ciliary muscles can adjust the shape to control focus).
In a donor, post-mortem changes result in lens opacity and the embalming has made the lens hard and non-deformable.
Lens replacement surgery is common, if the lens in your donor is thin and transparent then it is a plastic or glass insert following cateract surgery.
Ciliary muscle - This is a ring of smooth muscle that has small ligaments attached to the periphery of the lens allowing contraction to change the lens shape for focus control.
Vitreous chamber - The large chamber forming the bulk of the eyeball. This is filled with a transparent gel, vitreous humor, that maintains shape while providing optical transparency.
Retina - The neural sheet containing photoreceptive cells and other interneurons processing optical information. The final output of the retina is a series of axons that traverse the innermost side of the retina to exit the eyeball as the optic nerve.
Fovea centralis - If your retina is well preserved you may be able to observe a ~1-2mm wide depression in the retina almost directly opposite the lens. This region of the retina is the fovea and is responsible for sharp central vision. Projections from here constitute ~50% of all optic nerve axons.
Post-Lab Daily Clinical Review Cases
Each lab has several multiple choice practice cases to reinforce understanding of the material within the lab and associated lectures. These are optional to review at home for consolidating and testing understanding.
CHECKLIST
Skeletal Structures
Orbit
Orbital plate of the frontal
bone
Optic canal
Superior
Inferior orbital fissure
Supraorbital notch/foramen
Infraorbital foramen
Soft Structures
Periorbita
P
Levator palpebrae superioris muscle
Superior rectus muscle
Medial rectus muscle
Lateral rectus muscle
Superior oblique muscle
Frontal nerve
Optic nerve
Pupil
Iris
Sclera
Cornea
Anterior chamber
Lens
Ciliary body
Vitreous chamber
Retina