2 Surgical Anatomy for Strabismus Surgery



Sylvia H. Yoo


Summary


Understanding the anatomy of the rectus and oblique muscles and their relationships to adjacent structures are essential to approaching strabismus surgery for successful outcomes, particularly in reoperations or congenital anomalies in which the normal anatomy has been altered or is abnormal.



rectus muscles – oblique muscles – spiral of Tillaux – Tenon’s capsule – anterior ciliary arteries – vortex veins

2 Surgical Anatomy for Strabismus Surgery



2.1 Extraocular Rectus and Oblique Muscles


Six extraocular muscles control the motility of each eye: the medial and lateral rectus muscles, the superior and inferior rectus muscles, and the superior and inferior oblique muscles (Fig. 2.1). The four rectus muscles arise from the annulus of Zinn, a tendinous ring which encloses the optic foramen and a section of the medial superior orbital fissure. All six extraocular muscles insert on the eye, where the associated anterior ciliary arteries may be more easily identified subconjunctivally than the muscles themselves. The insertions of the rectus muscles form the spiral of Tillaux (Fig. 2.2) according to their distances from the limbus. They are approximately 7 mm apart, and are slightly curved, so that the central insertion is closest to the limbus. 1 Foot plates, which are small attachments of the rectus muscle to the sclera, may be present just posterior to the insertions. While the functions of the horizontal rectus muscles are straightforward as adductors and abductors, the functions of the cyclovertical muscles are more complex due to their paths of action, which are diagonal to the visual axis when the eye is in primary gaze. Understanding the relationships of the functions of the cyclovertical muscles is useful for surgical planning in patients with vertical and torsional strabismus. See Table 2.1 and Table 2.2 for detailed anatomic and functional features of each muscle.

Fig. 2.1 The six extraocular muscles, from their origins to their insertions on the eye—(a) superior view and (b) lateral sagittal view. (Reproduced with permission from Schünke M, Schulte E, Schumacher U. Thieme Atlas of Anatomy: Head, Neck and Neuroanatomy, 2nd ed. Stuttgart: Thieme; 2016.)
Fig. 2.2 The rectus muscle insertions form the spiral of Tillaux, with the medial rectus closest to the limbus, and the superior rectus furthest from the limbus (right eye pictured, measurements in millimeters). (Reproduced with permission from Schünke M, Schulte E, Schumacher U. Thieme Atlas of Anatomy: Head, Neck and Neuroanatomy, 2nd ed. Stuttgart: Thieme; 2016.)























































Table 2.1 Extraocular muscle anatomy: origin, insertion, size, arc of contact

Muscle

Origin

Insertion

Size

Arc of contact (in mm)

Medial rectus

Annulus of Zinn, medial and inferior to the optic foramen

5.5 mm from the limbus

Muscle is 40.8 mm long


Tendon is 3.7–4.5 mm long, 10.3 mm wide

6–7


Inferior rectus

Annulus of Zinn, inferior to the optic foramen

6.5 mm from the limbus

Muscle is 40 mm long


Tendon is 5.5–7 mm long, 9.8 mm wide

6.5–7


Lateral rectus

Annulus of Zinn, spanning the superior orbital fissure

6.9 mm from the limbus

Muscle is 40.6 mm long


Tendon is 7–8 mm long, 9.2 mm wide

10–12


Superior rectus

Annulus of Zinn, superior and lateral to the optic foramen, inferior to the origin of the levator palpebrae superioris

7.7 mm from the limbus

Muscle is 41.8 mm long


Tendon is 5.8 mm long, 10.6 mm wide

6.5


Superior oblique

Superomedial to the optic foramen, near the origin of the levator palpebrae superioris


Functional origin is the trochlea located on the orbital rim at the anterior superomedial orbit on the frontal bone

Muscle becomes tendon just before the trochlea, then continues laterally and posteriorly under the superior rectus, passing through the Tenon’s capsule 2 mm nasal and 5 mm posterior to the nasal insertion of the superior rectus muscle and inserting on the posterior superotemporal quadrant of the eye behind the equator, 12–13 mm posterior to the limbus with a fan-like insertion which is parallel to the lateral margin of the superior rectus muscle and with anterior fibers oriented circumferentially

Muscle is 32–40 mm long


Tendon is 20–26 mm long, 10–18 mm wide with a cord-like segment as it passes from the trochlea under the nasal aspect of the superior rectus and then fans out into a wide insertion

8–12


Inferior oblique

Maxillary bone, inferolateral to the lacrimal sac fossa, just posterior to the orbital rim, may partially arise from the lacrimal sac fascia

Posterior inferotemporal quadrant in the area of the macula


10% have two muscle bellies

Muscle is 37 mm long and 9.6 mm wide at its insertion


Tendon is minimally present

15


Sources: Data from Glasgow BJ. Anatomy of the Human Eye. Mission for Vision. http://www.images.missionforvisionusa.org/anatomy/2005/10/eye-anatomy-human.html (published 2005; accessed November 1, 2019) and Lueder GT, Archer SM, Hered RW, et al. Basic and Clinical Science Course: Pediatric Ophthalmology and Strabismus. San Francisco, CA: American Academy of Ophthalmology; 2014.
























































Table 2.2 Extraocular muscle anatomy: blood supply, innervation, relationship to adjacent structures, actions

Muscle

Blood supply

Innervation

Relationship to adjacent structures

Actions

Medial rectus

Inferior/medial muscular branch of ophthalmic artery, giving rise to two anterior ciliary arteries

Inferior division of third cranial nerve (oculomotor nerve) on its lateral surface at the junction of its medial and posterior thirds

No fascial attachments to oblique muscles


Passes through the Tenon’s capsule 12 mm posterior to its insertion

Adduction


Inferior rectus

Inferior/medial muscular branch of ophthalmic artery and infraorbital artery, giving rise to two anterior ciliary vessels

Inferior division of third cranial nerve (oculomotor nerve) on its upper surface at the junction of the middle and posterior thirds

Attached to lower eyelid by fascial expansion of its sheath, 15–18 mm posterior to its insertion


Muscle sheaths of the inferior oblique and inferior rectus muscles combine to form Lockwood’s ligament from which the capsulopalpebral fascia, the major retractor of the lower eyelid, extends and travels parallel to the inferior rectus before inserting at the inferior tarsus


Inferonasal and inferotemporal vortex veins are near the nasal and temporal edges of the inferior rectus posteriorly

Primary depression (sole depressor in abduction)


Secondary extorsion


Tertiary adduction


Also depresses the lower eyelid


Lateral rectus

Lacrimal artery and/or superior/lateral muscular branch of the ophthalmic artery, giving rise to one anterior ciliary artery (may vary)

Sixth cranial nerve (abducens nerve) on its medial surface just posterior to the middle of the muscle

Connection with inferior oblique at the inferior oblique insertion

Abduction


Superior rectus

Superior/lateral muscular branch of the ophthalmic artery, giving rise to two anterior ciliary arteries

Superior division of the third cranial nerve (oculomotor nerve) under the muscle at the junction of the middle and posterior thirds

Fascial connections with the levator palpebrae superioris


Connection to underlying superior oblique tendon


Superonasal and superotemporal vortex veins are near the nasal and temporal edges of the superior rectus posteriorly

Primary elevation (sole elevator in abduction)


Secondary intorsion


Tertiary adduction


Also helps elevate the upper eyelid


Superior oblique

Superior/lateral muscular branch of the ophthalmic artery

Fourth cranial nerve (trochlear nerve) at its superolateral aspect of the posterior third of the muscle

Trochlea-tendon complex: Trochlea is U-shaped and fibro-cartilaginous, tendon and its fibrovascular sheath move through the trochlea like a telescope


While variable, the anterior aspect of the insertion is approximately 3–5 mm posterior to the temporal pole of the superior rectus muscle insertion, while the posterior aspect of the insertion is 13–14 mm posterior to the superior rectus muscle insertion, so that the superior oblique insertion is approximately 11 mm wide.


Superotemporal vortex vein is near the posterior aspect of the superior oblique tendon insertion

Primary intorsion (primarily anterior one-third fibers)


Secondary depression (posterior two-thirds fibers, sole depressor in adduction)


Tertiary abduction (posterior two-thirds fibers)


Inferior oblique

Inferior/medial muscular branch of the ophthalmic artery and infraorbital artery

Inferior division of the third cranial nerve (oculomotor nerve) at the posterior and upper aspect, approximately 15 mm nasal to the insertion, within a neurovascular bundle

In contact with the periosteum of the orbital floor near its origin, then separated from the floor laterally by orbital fat and covered by the lateral rectus and Tenon’s capsule


Muscle sheaths of the inferior oblique and inferior rectus muscles combine to form Lockwood’s ligament, which may act as the effective insertion of the muscle in inferior oblique weakening procedures. With inferior oblique anteriorization, the neurovascular bundle may act as the effective origin


Inferotemporal vortex vein loops along the posterior border of the inferior oblique

Primary extorsion


Secondary elevation (sole elevator in adduction)


Tertiary abduction

Sources: Data from Glasgow BJ. Anatomy of the Human Eye. Mission for Vision. http://www.images.missionforvisionusa.org/anatomy/2005/10/eye-anatomy-human.html (published 2005; accessed November 1, 2019) and Lueder GT, Archer SM, Hered RW, et al. Basic and Clinical Science Course: Pediatric Ophthalmology and Strabismus. San Francisco, CA: American Academy of Ophthalmology; 2014.

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Feb 6, 2021 | Posted by in OPHTHALMOLOGY | Comments Off on 2 Surgical Anatomy for Strabismus Surgery

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