ANATOMY OF EXTRAOCULAR MUSCLE ( EOM)

In this article we discuss in detail about the anatomy of the extraocular muscles in the eye. and the angle of the eye. We will begin with classifying the various eye muscles and then delve deep into the anatomy of individual muscles and their supporting structures like muscle pulleys and fascias. If you are an audio visual learner you can visit my video on the anatomy of Extraocular muscles here. Also you may refer to the text below as a read along notes for the same.

Table of Contents

Classification of Ocular Muscles

The muscles of the eye are divided as follows :-

INTRAOCULAR MUSCLES ( IOM) : muscles located within the eyeball. These are namely the Ciliary muscle, dilator pupillae and sphincter pupillae.
EXTRAOCULAR MUSCLES ( EOM) : These muscles are situated around the eyeball. They can further be divided into Voluntary EOM and Involuntary EOM.

INVOLUNTARY EOM : These extraocular muscles are not within our voluntary control. The superior and inferior tarsal muscles basically form the involuntary EOM .
VOUNTARY EOM: these muscles are within our control. The Recti ,Obliques and the levator form the voluntary extraocular group of muscles

Voluntary Extraocular Muscle Group

There are four recti, 2 oblique muscles and levator palpebrae superioris forming the voluntary extraocular muscle group-

LEVATOR PALPEBRAE SUPERIORIS
SUPERIOR RECTUS
INFERIOR RECTUS
LATERAL RECTUS
MEDIAL RECTUS
SUPERIOR OBLIQUES
INFERIOR OBLIQUES

Tenon's Fascia and Muscle Pulleys

Apart form the intermuscular septum there are other fascial attachments as well in the eye.
A thin membrane arises from the optic nerve sheath and extends to the corneal limbus, enveloping the eyeball .
This fascial membrane is known as the tenons fascia or the fascia bulbi.
The tenons fascia seperates the eyeball from the fat in the orbit.
ATTACHMENTS OF TENONS FASCIA:–
POSTERIORLY : It isfused with optic nerve sheath
ANTERIORLY : It fuses with intermuscular septa 3 mm posterior to limbus.

Now, the extraocular muscles arise posteriorly in the orbit outside the tenons fascia. As they travel anteriorly they pierce the tenons capsule to reach their insertion. As they do that, they carry along with them a sleeve of tenons fascia along. This fascial sleeve is referred to as the muscle pulleys

The muscle pulleys are made up of collagen, elastin, smooth muscle
They suspend the muscle from adjacent orbital wall, connect to the tenons capsule.
Muscle pulleys act as functional origins of the rectus muscle.
Also they prevent the displacement of the muscle during the movement of the eyeball.
For detailed understanding refer to my video on extraocular muscle anatomy

TENONS FASCIA is a protective fascial covering of the eyeball, in which the extraocular muscles are suspended and supported.
IMPORTANCE: in Extraocular muscle surgeries access to muscles is not possible without opening up of the tenons fascia.
The muscles pierce the tenons capsule/ come in contact first with tenons capsule at a distance of 10mm from the limbus.

Anatomy of the Rectus Muscle

The term rectus means “straight“. Therefore recti have a straight course in the orbit . There are four recti muscles named according to their locations in the eye.
The superior rectus is situated superiorly to the eyeball. The inferior rectus is situated inferior to the eye. The medial rectus is situated medial to the eye and the lateral rectus is situated lateral to the eye.
All the recti are stabilized and interconnected with each other with fibrous connective tissue known as the intermuscular septum.
The Recti along with the intermuscular septum form a cone in the orbit. This cone is called the extraocular muscle cone.
The extraocular muscle cone divided the orbit into two parts:-

EXTRACONAL SPACE : The space in orbit outside the muscle cone.
INTRACONAL SPACE: the space within the muscular cone is known as intraconal space.

Understanding Common Origin Of the Recti (Annulus Of Zinn)

A fascial ring is attached at the apex of the orbit enclosing the optic canal and the superior orbital fissure. This fascial ring is known as the annulus of zinn/common tendinous ring
The four recti muscle have a common origin. They arise from this Common tendinous ring.
Note: the obliques DONOT arise from the annulus of zinn.

Clinical Nugget : Pain in Optic Neuritis

Superior rectus and Medial rectus have dense adhesions with Dural sheath of the optic nerve near their origins from the annulus of zinn.
Therefore in OPTIC NEURITIS , these adhesions causes painful eye movements in the up gaze and on adduction of eye.

Course Of the Recti in Orbit

Originating from the common tendinous ring the recti have a relatively straight course.
In order to understand the course and angles of the muscles, it is important to understand the anatomy of the orbit.
The orbital axis and the optical axis have an angle between them of about 23 degrees.
The superior and the Inferior rectus follow the orbital axis and travel at an angle of 23 degrees to the visual axis.
The medial rectus passes almost parallel to the medial wall of the orbit
The lateral rectus passes along the lateral wall of the orbit and forms an angle of 45 degrees to the visual axis.

Insertion of the Recti Muscles (SPIRAL OF TILLAUX)

The muscles originating from the annulus of Zinn, ultimately inserts into the anterior aspect of the sclera of eyeball.
However they are inserted at variable distance from the limbus.
Superior rectus = 7.7 mm from the limbus
Lateral rectus= 6.9 mm from the limbus
Inferior rectus muscle =6.5 mm from the limbus
Medial rectus = 5.5 mm from the limbus.
This can be remebered using the mnemonic – “SLIMS”

If we join all the insertions of the recti muscle we get a spiral line .

This imaginary line connecting the insertions of the recti muscles of the eye is called the spiral of tillaux

Clinical Nugget : Importance of Spiral Of Tillaux

SPIRAL of TILLAUX is an important landmark in strabismus surgery.
It helps a surgeon to identify the exact location of the extraocular muscle insertion by measuring the corresponding distance from the limbus.
Also Sclera is thinnest at the insertion of the rectus (0.3mm )
This is also a common site for perforation during severe blunt trauma to globe
Therefore the knowledge about the insertion of the muscles is extremely important

Important Dimensions of Each Rectus Muscle.

Now let us discuss some important anatomical points and dimensions about each rectus muscle

Medial Rectus Muscle Relevant Anatomy and Dimensions

Origin : medial part of the annulus of Zinn .
Insertion : 5.5 mm from the limbus.
Width: 10.3 mm
Length:40.8 mm
Tendon Length :3.7 mm
It has the SECOND SHORTEST tendon in eye.
Important Relations of Medial Rectus :Lateral to medial rectus is the optic nerve in the optic canal. It has strong adhesions with the dural sheath of the optic nerve.
ACTION : Adduction

Lateral Rectus Muscle Relevant Anatomy and Dimensions

Origin : Lateral part of the annulus of Zinn .
Insertion : 6.9 mm from the limbus.
Width: 9.2mm
Length:40.6 mm
Tendon Length :8 mm
Important Relations of Lateral Rectus :Lateral to lateral rectus are the structures present in the lateral compartment of the superior orbital fissure. These are the superior ophthalmic vein, lacrimal nerve, trochlear nerve and the frontal nerve
Medial to lateral rectus are the components of the medial compartment of the superior orbital fissure namely, the occulomotor nerve, nasociliary nerve and abducens nerve
ACTION : Abduction

Inferior Rectus Muscle Relevant Anatomy and Dimensions

Origin : Inferior part of the annulus of Zinn .
Insertion : 6.5 mm from the limbus.
Width: 9.8 mm
Length:40 mm
Tendon Length :5.5 mm
Important Relations of Inferior Rectus :
INFERIOR RELATIONS: Inferior oblique muscle is situated below the Inferior rectus muscle (Inferiro oblique is closer to the orbit.) It is also related to the infraorbital nerve and vessels and inferior ophthalmic veins.
SUPERIOR RELATION : Inferior Division of third nerve
ACTION : Depression ,Adduction and Extorsion ( refer video)

Superior Rectus Muscle Relevant Anatomy and Dimensions

Origin : Superior part of the annulus of Zinn .
Insertion : 7.7 mm from the limbus.
Width: 10.6 mm
Length:41.8 mm
Tendon Length :5.8 mm
Important Relations of Superior Rectus :
INFERIOR RELATIONS: Near the insertion the superior oblique insertion is inferior/below the superior rectus
SUPERIOR RELATION : Levator palpebrae superioris
ACTION : ELEVATION , Adduction and intortion ( refer video)

Anatomy of Obliques

We have two oblique muscles in each eye. They are the superior oblique muscle and the inferior oblique.
The obliques do not arise from the common tendinous ring
They have a more complex course than the recti.
Their insertions are not straight. Instead they have oblique insertions.
Let us discuss them individually :-

Superior Oblique Muscle Relevant Anatomy and Dimensions

Origin : From the periosteum of lesser wing of SPHENOID BONE ( medial and superior to Optic foramen)
Insertion : Complex Fan shaped insertion explained below.
Width: 10.8 mm
Length:40mm
Tendon Length :20 mm
LONGEST and THINNEST of all EOM
Important Relations of Superior Oblique:
SUPERIOR RELATION : Superior rectus
ACTION : Intortion, Depression and Abduction ( refer video)

Important points about Course Of Superior Oblique

After arising from the lesser wing of sphenoid bone, the superior oblique muscle travels parallel to the medial wall of the orbit.
At the superomedial aspect of the orbit the muscle meets a pulley called TROCHLEA.
the superior Oblique passes through the trochlea and changes its direction now.
It bends and makes an angle of 54 degrees with the trochlea.
Then it travels POSTERIORLY and LATERALLY and get inserted in behind the equator below the superior rectus insertion.
The part of superior oblique present before it enter the trochlea is known as the PRETROCHLEAR PART and the part present after it has passed through the trochlea is known as the POST–TROCHLEAR PART .
Pre-trochlear part length : 40mm
Post trochlear part length : 20mm

Important angles Related to Superior Oblique

Angle between the insertion of Superior Oblique and visual axis : 51 degrees
Angle with which the direction of Superior Oblique muscle changes at trochlea:- 54 degrees
Angle between Superior rectus and superior Oblique directions : 105 degrees

Important Points about The Insertion of the Superior Oblique

IMPORTANT SURGICAL ANATOMY OF INSERTION

The insertion of superior oblique is not as straight as recti.
The Superior Oblique has a fan shaped insertion because of which the anterior fibres are more laterally placed than the posterior fibres.
Anterior end of the superior oblique insertion is 12–14mm from limbus and 3-4mm from the superior rectus.
These anterior fibres of the superior Oblique are responsible for INTORTION
The posterior end of insertion is 17–19mm behind the limbus and 13.8mm from the superior rectus insertion.
Posterior fibres of the Superior Oblique are associated with DEPRESSION
The total length of insertion is 7–18mm. It is in close proximity to vortex veins.

Clinical Nugget : Trochlea

Trochlea is the pulley of Superior oblique
It lies at superior medial angle of orbit
It is Partly bony ( medial aspect closer to the medial wall of orbit)
Its lateral part is cartilaginous
It is attached to the frontal bone through spina trochlear.
The Superior Oblique muscle originating from the lesser wing of sphenoid becomes tendinous at a distance of 10 mm from the trochlea.
The superior Oblique bends by an angle of 54 degrees at the trochlea.
The Trochlea consists of a bursa known as the TROCHLEAR BURSA, which prevents the wear and tear of the superior oblique tenson as it passes and moves in it.
Although the true origin of the Superior Oblique is from the Lesser wing of sphenoid, The trochlea act as its FUNCTIONAL ORIGIN.
this is explained in our video on anatomy of EOM part 2

Inferior Oblique Muscle Relevant Anatomy and Dimensions

Origin :It is the only extraocular muscle originating from the anterior orbit. It originates from orbital surface of the maxilla (orbital floor) lateral to the lacrimal groove.
Insertion : Variable complex insertion( explained below)
Width: 9.6 mm
Length:37 mm
Tendon Length :1-2 mm
SHORTEST MUSCLE of all EOM
Important Relations of Inferior Oblique :
SUPERIOR RELATION : Inferior rectus
ACTION : EXTORTION ,ELEVATION and Abduction ( refer video)

Important points about Course Of Inferior Oblique

The orbital apex is formed by the optic canal and the superior orbital UPWARDS POSTERIO- LATERALLY .
It passes INFERIOR to the inferior rectus and finally inserts behind the Lateral rectus muscle behind the equator.

IMPORTANT SURGICAL ANATOMY OF INSERTION

Anterior end of insertion is 10 mm behind the lateral rectus and is responsible for EXTORSION of eye.
The posterior end of insertion is 1mm below the fovea . These fibres are responsible for ELEVATION OF EYE.
The muscle is related to inferior vortex vein

IMPORTANT NUGGET : The Recti are inserted anterior to the equator whereas the obliques are inserted poterior to the equator.

Anatomy Of Superior Orbital Fissure

The orbital apex is formed by the optic canal and the superior orbital fissure.
The Superior orbital fissure is a cleft between the greater and lesser wing of the sphenoid bone that serves as a vital communication between the middle cranial fossa and the orbit.
The superior orbital fissure
The tendinous ring of zinn ,which is the common origin of the four rectus muscles divides the superior orbital fissure into three compartments .

LATERAL / SUPERIOR LATERAL COMPARTMENT

Situated lateral to the annulus of Zinn
Trochlear(4rth nerve)
Lacrimal
Frontal nerves
The superior ophthalmic vein.

MIDDLE/CENTRAL COMPARTMENT

Superior and Inferior branches of the oculomotor nerve
Nasociliary nerve
Abducens nerve
Both the sensory and the sympathetic root of the ciliary ganglion.

MEDIAL / INFEROMEDIAL COMPARTMENT

Situated medial to the annulus consists of –
inferior ophthalmic vein.
For better understanding check out the video

Nerve Supply of Extraocular Muscles

Levator and superior rectus–>Superior division of the occulomotor nerve ( 3rd cranial nerve)
Medial, inferior rectus and inferior Oblique–>Inferior division of the occulomotor nerve.(3rd cranial nerve)
Superior Oblique >Trochlear nerve (4rth cranial nerve)
Lateral rectus: Abducens nerve ( 6th cranial nerve)

Blood Supply of Extraocular Muscles

The blood supply of the extraocular muscles is derived from the ophthalmic artery
The ophthalmic artery has two types of muscular arteries ; medial muscular arteries and lateral muscular arteries.
These muscular arteries further branch to form 7 anterior ciliary arteries.
The medial ciliary arteries supply the medial rectus, inferior rectus and the inferior oblique.
The lateral arteries supply the superior rectus, superior oblique and lateral rectus .

Clinical Nugget : Anterior ciliary arteries and the Anterior Segment Ischemia

There are a total of 7 anterior ciliary arteries in number.
There are 2 for each rectus except the LATERAL RECTUS which has only one anterior ciliary artery (Mnemonic: LR is a loner)
Additionally the anterior ciliary arteries along with the posterior ciliary arteries are responsible for formation of MAJOR ARTERIAL CIRCLE & EPISCLERAL ARTERIAL PLEXUS

CLINICAL SIGNIFICANCE

70% of the blood supply in the arterial circle is contributed from the anterior ciliary arteries and only 30% is contributed from the posterior ciliary arteries.
Therefore it is advised that we do not operate on more than 2 recti muscle in one eye in order to not damage the anterior ciliary arteries
This leads to patient developing a risk ANTERIOR SEGMENT ISCHEMIA.
Surgeries on the vertical muscles are more risky; as at those meridians there are no posterior ciliary arteries.

Conclusion

So that was the comprehensive anatomy of all the extraocular muscles for you. We covered the anatomy of the recti and the obliques. For anatomy of the levator palpebrae superioris you can visit the post on anatomy of eyelid. In the next post we shall be discussing about the eye movements in detail.