The Anatomy of the Extraocular Muscles

Tukezban Huseynova, MDSpecialist in Strabismus and Refractive Cornea,

Briz-L Eye Clinic, Baku, AzerbaijanTukezban@gmail.com

Embryology of Extraocular Muscles

• The fibers of the striated EOMs originate from condensations of cranial mesoderm, whereas their associated connective tissues and the orbital smooth muscles (SMs) originate from the neural crest.

Embryology of Extraocular Muscles

MyogenesisPrimary 11 weeks

Secondary

the trochlea

the soft pulley

The periocular connective

tissues

Embryology of Extraocular Muscles

Adult skeletal muscles retain a quiescent stem cell population that allows regeneration. After muscle damage has occurred, the regenerative process recapitulates development as the precursor cells in the adult, now known as satellite cells, proliferate to produce myoblasts, which fuse to form new muscles fibers.

Embryology of Extraocular Muscles

1. The types of contractile proteins expressed in a given muscle fiber

2. The apparatus that links activation of the muscle fiber from a motor nerve to the production of a muscle contraction, excitation-contraction coupling.

Contraction speed, in turn, is a function of two factors

Embryology of Extraocular Muscles• The central contractile unit of a skeletal muscle fiber is the sarcomere.• Each sarcomere is approximately 2 to 3 mm long.• End-to-End arrangement of sarcomeres produces the characteristic longitudinal banding pattern of striated muscle.

İnternal Structure of the EOM The oculorotary EOMs, although not the levator palpebrae superioris (LPS), consist of two distinct layers:

Global layer Orbital layer ( SIMFs and MIMFs)

Origins and paths of the EOM The rectus EOMs originate in the orbital apex from a common fibrous ring surrounding the optic nerve called the annulus of Zinn

The SO muscle originates from the periorbita of the superonasal orbital wall

The IO muscle originates much more anteriorly from the periorbita of the inferonasal orbital rim adjacent to the anterior lacrimal crest

Pulleys of the EOM Pulleys consist of discrete rings of dense collagen encircling the EOM and are about 2 mm in length coaxial Has less substantial collagenous sleeves around the EOM. Anteriorly, these sleeves thin to form slings convex to the orbital wall, and More posteriorly the sleeves thin to form slings convex toward the center of the orbit. The anterior pulley slings are also known as the intermuscular septa. The collagenous pulley ring forms the fulcrum of the pulley that inflects the EOM path.

Pulleys of the EOM

Pulleys of the EOM

The pulleys function as the mechanical origins of the EOMs and play a vital role in ocular kinematics, the rotational properties of the eye.

Normal pulleys form a natural mechanical substrate for Listing's law and other aspects of ocular kinematics, and they render ocular rotations effectively commutative.

Pulleys of the EOM Donder's law, stating that there is only one torsional

eye position for each com-bination of horizontal and vertical eye positions.

Listing's law, a specific case of the more general Don-der's law, states that any physiologic eye orientation can be reached from any other by rotation about a single axis, and that all such possible axes lie in a single velocity plane, Listing's plane.

Pulleys of the EOM

Listing's law is satisfied if the axis of ocular rotation shifts by exactly one half of the shift in ocular orientation for any eye movement.This is the so-called Listing's half-angle rule.

Pulleys of the EOM Shifts in horizontal rectus pulley position required to maintain the Listing's half-angle relation in tertiary positions of adducted elevation and adducted depression. Pulleys are depicted as rings. Paths of global layers of rectus extraocular muscles are shown in black; orbital layers inserting in pulleys are shown in gray. Pulley suspensions are also shown in gray.

Pulleys of the EOMShift of pulley location can produce an "A" & "V"

pattern of strabismus

Pulleys cause "V" Pulleys cause "A"

Pulleys of the EOM

• The ‘pulley zone’ is roughly at the junction of the middle and posterior third of the globe, similar to Listing’s plane

Pulleys of the EOM

• The pulleys which ‘inflect the paths of the muscle.’

A. Medial rectus pulleyB. Lateral rectus pulley

• The functional origin of the rectus muscles is at the pulleys

Pulleys of the EOM

• The orbital fibers insert into the pulleys of the horizontal recti and the global fibers insert into sclera.

Pulleys of the EOM

The muscle - tendon anterior to the pulleyA. Passes straight in primary positionB. Courses upward in upgazeC. Courses downward in downgazeD. The direction of the muscle posterior to the pulley does not change during up and downgaze

Pulleys of the EOM

Some combination of:

Upward displacement of the lateral recti Downward displacement leads to ‘A’ pattern

Pulleys of the EOM

Some combination of:

Downward displacement of the lateral recti Upward displacement of the medial recti leads to ‘V’ pattern

Pulleys of the EOMThe reasons that the strabismus surgeon is not likely to see the pulleys are several.

First, surgery of the extraocular muscles is carried out beneath anterior Tenon's capsule and in the plane of posterior Tenon's capsule.

Second, dissection carried posterior to the origin of anterior Tenon's capsule will expose extraconal fat, which both complicates surgery and obscures the surrounding anatomy, including the pulleys.

Third, although the pulleys are located in the orbital fat just behind the insertion of anterior Tenon's capsule, they are virtually impossible to identify for what they are.

Scleral İnsertion of EOM

Anatomic relations of rectus extraocular muscle insertions to the corneal limbus.

Smooth muscles of the orbit

SMs in four areas of the human orbit:

The inferior palpebral muscle,

The superior palpebral muscle,

The “orbital muscle” spanning the inferior orbital fissure, and

The “peribulbar muscle” surrounding the anterior aspect of the globe.

Physiology of EOMKey traits of muscle function

1. Contraction speed

a. The types of contractile proteins expressed in a given muscle fibers. b. The apparatus that links activation of the muscle fiber from a motor nerve to the productionof a muscle contraction, excitation – contraction coupling.

2. Fatique resistance is a direct consequence of cellular metabolism.

Physiology of EOM

• The contractile unit of a skeletal muscle – sarcomere• End – to – end sarcomere forms striated muscle

Thick agregates of myosin (Light and heavy chains)

Thin filaments(polymers and α- actin)

Physiology of EOM• 2 proteins modulate the contractile process

Tropomyosin Troponin

Attach to the α – actin backbone.

Physiology of EOMSarcomere

DarkAnisotropic band

(A band)contain actin and myosin

LightIsotropic band

(I band)contain actin

H zone (center)presence only myozin

M zonecontaining protein,bisect by H zone

Z line longitudinary boundary ofeach sarcomere.

Physiology of EOM

Physiology of EOM

Physiology of EOM

• Each end – to – end sarcomere is known as myofibril, myofibril are separated from each other by a membranous calcium storage system, the sarcoplasmic reticulum.

Physiology of EOMFiber Types:

1. Slow twitch, fatigue resistant (red or Type I)

2. Fast twitch, fatigue resistant (intermediate or Type II A)

3. Fast twitch, fatigable (white or type II B)

4. Fast twitch, intermediate (Type II C or II X/D)

Physiology of EOMSix distinct fiber types

1. Orbital singly innervated fiber (predominant)

2. Orbital multiply innervated fiber

3. Global red singly innervated fiber (1/3 of global fiber)

4. Global intermediate singly innervated fiber (1/4 of global fiber)

5. Global white singly innervated fiber (1/3 of global fiber)

6. Global multiply innervated fiber

Physiology of EOM

• The energy requirement for muscle contraction is by adenosine triphosphate (ATP) cleavage via a myofibrillar ATPase. Both anaerobic (glycolytic) or aerobic (mitochondrial oxidative) mechanism.

Surgical anatomy

Palpebral fissure size

• The average adult palpebral opening is 28 mm long and 10 mm high.

• An average 18-month-old child has a palpebral opening 20 mm long and 8.5 mm high.

Palpebral fissure size

• A newborn has a palpebral opening measuring 18 mm long and 8 mm high.

The size and shape of the palpebral opening should be considered at the outset of extraocular muscle surgery.

Extraocular muscles size• İn newborns, the posterior part of the globe is relatively

smaller than the anterior part, meaning that a recession of 3 mm could place the medial rectus at the equator. This is important information but not for strabismus surgery, which is not indicated anyway in the newborn because of immaturity of the binocular system.

• The insertion of the medial rectus in an infant can be closer than 5.5 mm from the limbus.

Extraocular muscles size

Palpebral fissue shape

• A patient with myelomeningocele and a straight lower lid margin simulating a mongoloid slant. This is a common but unexplained finding in such patients.

Palpebral fissue shape

• ‘V’ esotropia in a patient with antimongoloid palpebral fissures.

Palpebral fissue shape

• ‘A’ esotropia in a patient with mongoloid palpebral fissures.

Palpebral fissue shape• The palpebral fissure may be level, mongoloid, or

antimongoloid, depending on the relative positions of the medial and lateral canthi.

• If the outer canthus is higher than the inner canthus, a mongoloid palpebral slant exists (Figure 15). If the outer canthus is lower than the inner canthus, an antimongoloid palpebral slant exists.

Epicanthal folds

A.This patient demonstrates telecanthus with an interorbital dimension clearly more than one-half the interpupillary distance and also an exotropia.

B. This patient with telecanthus also has prominent epicanthal folds.

Epicanthal folds

• A skin fold originating below and sweeping upward is called epicanthus inversus. This deformity is frequently associated with blepharophimosis and ptosis. These three deformities, which may be combined with telecanthus, cause significant disfigurement and present a formidable therapeutic challenge.

Epicanthal foldsA. Centered pupillary light reflex

B. The ‘straightening’ effect of exposing more ‘white’ nasally. (This is shown in an older patient because it is difficult to photograph the younger child where the test is more effective.)

Epicanthal folds• Epicanthal folds obscure the

nasal conjunctiva in both patients, giving the appearance of esotropia. However, the light reflex is centered in the pupil in each case. This reflex indicates the presence of parallel pupillary axes and, therefore, straight eyes or absence of manifest strabismus. Cover testing must be performed eventually to confirm the presence of parallel visual axes because a large angle kappa* could hide a small manifest esodeviation.

Epicanthal folds• Epicanthal folds are present to some degree in

most infants and children during the first few years of life.

• First, the examiner demonstrates the centered pupillary reflexes with a muscle light. Second, the examiner carefully pulls the skin forward over the bridge of the nose to demonstrate the ‘straightening’ effect of exposing the medial conjunctiva or ‘white of the eye’ .

Epicanthal folds

*Angle kappa is the angle formed by the pupillary axis and the visual axis. A positive angle kappa is present when the visual axis is nasal to the pupillary axis. This simulates exotropia and is common. A negative angle kappa is present when the visual axis is temporal to the pupillary axis. This simulates esotropia and is much less common than positive angle kappa.

Conjunctiva

• The plica semilunaris is a fold in the conjunctiva located far medially in the palpebral fissure and is mostly below the midline. The caruncle, located just medial to the plica, is about 3 mm in diameter, covered with squamous epithelium, and often contains small hairs.

Conjunctiva

• The topographic landmarks of the conjunctiva important to the strabismus surgeon are the following:

A The fusion of the conjunctiva and anterior Tenon’s capsule with the sclera at the limbus

Conjunctiva

A. The limbusB. The plica semilunarisC. The caruncle

Tennon’s capsule• Tenon's capsule is a structure with definite body

and substance in childhood which gradually atrophies in old age but not to the same degree as conjunctiva.

• Tenon's capsule has an anterior and posterior part.

• Anterior Tenon's capsule is the vestigial capsulopalpebral head of the rectus muscles.

• This covers the anterior half to two-thirds of the rectus muscles in their sheaths as well as the intermuscular membrane.

Tennon’s capsule

Tennon’s capsule• The muscle hook is placed in a

‘hole’ created in intermuscular membrane adjacent to the muscle insertion and glides along bare sclera behind the rectus muscle insertion and is exposed at the opposite muscle border with a snip incision.With a limbal incision, the multiple layers and surfaces associated with the rectus muscles can be readily seen. Conjunctiva and anterior Tenon’s capsule shown here separated are actually fused and separated only with difficulty.

Tennon’s capsule

• Anterior Tenon's capsule is fused with the undersurface of conjunctiva and attaches to sclera at the limbus.

• Posterior Tenon's capsule is made up of the fibrous sheath of the rectus muscles together with the intermuscular membrane.

Surgical anatomy of the rectus muscles

• The spiral of Tillaux and the relationship of the rectus muscle insertions.

• Width of the rectus muscle insertions.

Characteristics of EOM• The extraocular muscles are similar to skeletal muscles though there are

differences undoubtedly related to the very specialized function of the extraocular muscles.

• Both skeletal and extraocular muscles have several types of twitch fibers, but the extraocular muscles are unique, having tonically contracting fibers not found in skeletal muscle.

• The twitch fibers of extraocular muscles are called Fibrillenstruktur, and the unique slow tonic fibers are called Felderstruktur.

• There are two muscle fiber layers in the medial and lateral recti. The shorter orbital layer inserts in the muscle pulley, and the longer global fibers insert into sclera at the muscle’s insertion.

Characteristics of EOM• The shorter orbital layer inserts in the muscle pulley, and

the longer global fibers insert into sclera at the muscle’s insertion.

• The muscle fibers are long, traversing the entire length of the muscle, or in some cases, nearly so.

• The blood supply of the extraocular muscles is rich, coming from the muscular branches of the ophthalmic artery.

• The extraocular muscles have the lowest innervation ratio of any of the muscles of the body; that is, they have the most nerve fibers per muscle fiber.

Surgical anatomy of Inferior oblique

• The inferior oblique A. from in front and B. from behind.

Surgical anatomy of Inferior oblique

• The inferior oblique behaves as if it had two potential origins and two potential insertions because of its union with Lockwood's ligament as it passes beneath the inferior rectus. In addition, at the mid-section of the inferior oblique is a stout neurovascular bundle, described in detail by Stager and associates, which acts both as a restraining anchor and a source of innervation.

Lockwood ligament

A. The ligament of Lockwood could be compared to a hammock supporting the globe.

B. The inferior oblique passes beneath the inferior rectus, through Lockwood’s ligament and orbital fat approximately 12 - 14 mm from the limbus.

Lockwood ligament

• The inferior fat pad is prominent and should not be disturbed during surgery of the inferior rectus

Superior oblique

• The superior oblique tendon is redirected to 54 from the frontal plane and passes posteriorly and temporally beneath the superior rectus.

Superior oblique

• The superior oblique remains attached to the superior rectus when the rectus is detached and pulled up.

Superior obliqueA. When the eye is rotated downward, the superior rectus is the intended distance in a very large ‘hang loose’ recession even if the superior oblique tendon - superior rectus union is intact.

B. When the eye returns to the primary position, the superior rectus could be pulled forward, reducing theamount of recession.

Superior oblique

S.O. tendon excursion 8 mm either side of primary

Superior oblique

Whitnall’s ligament

A. The relationship of Whitnall’s ligament and the superior oblique tendon. ‘Blind hooking’ the superior oblique tendon can damage Whitnall’s, producing ptosis.B. Whitnall’s ligament acts like a clothesline with orbital structures suspended.C. Nasal ptosis right eye from disruption of Whitnall’s ligament after hooking of the superior oblique tendon in a ‘blind sweep’ nasal to the superior rectus.

Trochlea

A. The trochlea attached to the medial orbital wall with the tendon entering and exiting.

B. With fascial tissues removed the superior oblique tendon seen exiting the trochlea through a cuff attached to the trochlea.

Trochlea

Dimensions of the trochlea

A. SaggitalB. Coronal

Trochlea

• Composite

Trochlea

A. CT scan showing trochlea on the left and no trochlea on the right.

B. Same patient demonstrating the superior oblique muscle on the right and no muscle on the left.

Trochlea

TrochleaA. Gaze positions showing ‘overaction’ of the right inferior oblique and underaction of the right superior oblique.

B. At surgery, absence of the right superior oblique tendon was confirmed.

Anterior segment blood supply

Schematic of the blood supply of the anterior segment from Saunders, et. al. ACA = anterior ciliary artery IMC = intramuscular circle LPCA = long posterior ciliary artery RCA = recurrent choroidal arteryFrom Saunders RA, et al. Anterior segment ischemia after strabismus surgery. Survey of Ophthalmology, 1994, 38(5):456-466. Used with permission.

Anterior segment blood supply

• The anterior ciliary arteries are dissected from the superficial capsular muscle tissue allowing repositioning of the muscle while leaving blood flow undisturbed.

Vortex vein

The four vortex veins are viewed from the posterior aspect of the globe.

A. Lateral B. Medial

Vortex vein

A.The superior temporal vortex vein is seen at the posterior insertion of the superior oblique. Vortex veins are not seen routinely during surgery on the superior rectus.

B. A vortex vein may be seen but rarely at either (or both) borders of the medial rectus.

Vortex vein

C. A vortex vein is seen routinely under the mid-belly of the distal inferior oblique.

D. Vortex veins are seen at one or both borders of the inferior rectus.

Growth of the eye from the birth through childhood

Sclera• At the limbus, the sclera is 0.8 mm

thick.• Anterior to the rectus muscle

insertions, it is0.6 mm thick.• Posterior to the rectus muscle

insertions, it is0.3 mm thick.• At the equator, it is 0.5 to 0.8 mm thick.• At the posterior pole, it is greater than

1 mm thick. The area of greatest surgical activity for the extraocular muscle surgeon coincides with the thinnest area of the sclera.

Sclera

• The sclera is thinnest, 0.3 mm, posterior to the rectus muscle insertion

Sclera

A. Keystone spatula, cutting tip downB. Keystone spatula, cutting tip upC. Hexagonal spatula, neutral cutting tipD. Reverse cutting - tends to cut in - can be placed sidewaysE. Curved cutting - tends to be cut out

Blood supply of the EOMs

Ophthalmic artery

Infraorbital artery supplies the rest of the inferior oblique muscle.

Lacrimal artery supplies the rest of the lateral rectus muscle.

Muscular artery

LateralSuperior rectus & a portion of the lateral rectus muscle.

Medial Inferior and medial rectus muscle and a portion of the inferior obliqueMuscle.

Superior Supplies superior oblique muscle.

Blood supply of the EOMs

The blood supply to the orbital layer is much more extensive than that of the global layer.

Left Ocular Muscles and Innervation

Right Ocular Muscles and Innervation

Thank you