The conjunctiva is nearly transparent and well vascularized (Fig. 85.1). Adjacent to the limbus, it contributes to the rich anastamotic vascular network providing collateral flow to the anterior segment.¹ After topical 2.5% phenylephrine, normal conjunctival vessels constrict; the underlying anterior ciliary vessels do not. The anterior ciliary vessels, their scleral penetration sites and anastamotic branches to neighboring rectus muscles can be seen. The insertion sites of the four rectus muscles can be visualized. The anterior termination of the extraconal fat pad is seen through the conjunctiva roughly 10 mm posterior to the limbus. These features help determine if there has been prior surgery and guide placement of the surgical incision.

Fig. 85.1 Forceps grasp the left globe at the limbus in the inferior nasal quadrant (surgeon’s view). Small conjunctival vessels run anteriorly from the fornix, mostly meridional. Larger caliber anterior ciliary vessels are traveling on the scleral surface anterior to the insertion of the inferior rectus. The anterior edge of the extraconal fat is seen inferonasally and inferotemporally.

The conjunctiva permits full movement of the globe. With scarring the conjunctiva can limit globe movement.

Tenon’s capsule

Tenon’s capsule is a dense, translucent and nearly avascular fascial layer extending from the limbus to the optic nerve. Anteriorly, it is located just beneath the conjunctiva. The conjunctiva can easily be separated from the underlying Tenon’s capsule, beginning 1 mm posterior to the limbus. Three millimeters posterior to the limbus, Tenon’s capsule fuses with the underlying intermuscular septum, forming a single fascial layer.² Posterior to this zone, Tenon’s capsule envelops the globe, forming a potential cavity (sub-Tenon’s space).

In young people, Tenon’s capsule is thick, white and glistening, becoming thinner in adulthood. The bulbar surface of Tenon’s capsule is smooth, providing movement of one fascial plane over another.

Tenon’s capsule is penetrated by the four rectus muscles, just posterior to the equator of the globe. The penetration sites are approximately 10 mm posterior to the insertion of the rectus muscles. The inferior oblique muscle and superior oblique tendon penetrate Tenon’s capsule just anterior to the equator. The penetration sites of the extraocular muscles resemble a cuff on the sleeve of a sweater and represent a point of attachment between Tenon’s capsule and the capsule of the four rectus muscles, the inferior oblique muscle, and the superior oblique tendon. This point of attachment is an important landmark during strabismus surgery and in the recovery of lost muscles.

The penetration sites of the rectus muscles form a division between anterior and posterior Tenon’s capsule. Anterior Tenon’s capsule extends from the penetration sites to the limbus, while posterior Tenon’s capsule extends from the penetration sites to the optic nerve.

Tenon’s capsule separates the orbital fat from the globe. Anterior Tenon’s separates extraconal fat from the eye; posterior Tenon’s separates intraconal fat from the globe. Fibrous septae extend from the outer surface of Tenon’s capsule to the orbital periosteum.³ It is important to avoid violating Tenon’s capsule during surgery. Failure to do so permits fat to prolapse into sub-Tenon’s space. Scarring may result in fat adherence with restricted eye movement.

Muscle capsule and intermuscular septum

The rectus and oblique muscles and tendons are enveloped in connective tissue capsules. The capsules on the rectus muscles are well developed anterior to the site of the muscles’ penetration of Tenon’s capsule. With age, the capsules thin. Postoperative scarring is reduced if the muscle capsule is left intact in areas not involved with the procedure. Posterior to the penetration site in Tenon’s capsule, the muscle capsules become quite thin.⁴

The intermuscular septum is a thin, avascular layer of connective tissue that is continuous with the muscle capsules, extending from the border of each rectus muscle to the adjacent rectus muscles (Fig. 85.2). Anterior to the rectus muscles’ penetration of Tenon’s capsule, the intermuscular septum forms a thin, separate layer just beneath Tenon’s capsule, adjacent to the sclera. Posterior to the rectus muscles’ penetration, the intermuscular septum has been thought to separate intraconal from extraconal fat. However, the existence of this portion of the intermuscular septum has recently been questioned: it may be the anterior aspect of the connective tissue pulleys.⁵ The intermuscular septum is associated with the inferior oblique and forms a roughly 2 mm wide band, extending off the posterolateral margin of the muscle.

Fig. 85.2 Left medial rectus with Jameson hook behind insertion (surgeon’s view). The conjunctiva and Tenon’s capsule have been retracted over the tip of the hook, exposing the intermuscular septum, which remains intact on the superior pole of the medial rectus insertion. The anterior ciliary vessels are on the surface of the medial rectus tendon and the surface of the sclera.

Blood supply

The ophthalmic artery supplies the extraocular muscles. The lateral muscular branch of the ophthalmic artery supplies the lateral rectus, superior rectus, superior oblique, and levator palpebrae superioris. The lateral rectus is also supplied by the lacrimal artery. The medial muscular branch of the ophthalmic artery supplies the inferior rectus, medial rectus, and inferior oblique muscles. The inferior rectus and inferior oblique muscles are also supplied by the infraorbital artery.

Each of the four rectus muscles carries anterior ciliary vessels that contribute to the blood supply of the anterior segment; the two oblique muscles do not. In the classic description, each rectus muscle has two anterior ciliary arteries, except the lateral, which has one.⁶ This diagram of the ocular circulation is reproduced repeatedly in ophthalmic literature^6,⁷ and shows the anterior ciliary vessels traveling within the muscle and tendon. However, the vessels emerge from the substance of the muscle, posterior to the transition from muscle to tendon, and travel on the surface of the tendon⁸ (Figs 85.3 and 85.4). The number of anterior ciliary vessels is variable.^9,¹⁰

Fig. 85.3 Diagram of typical rectus muscle showing position of anterior ciliary vessels (ACVs). (A) Surgeon’s view of inferior rectus. The ACVs emerge from the substance of the muscle posterior to the transition from muscle to tendon. Anterior to this point, the ACVs travel on the surface of the muscle and tendon, following somewhat serpentine course in a plane that is continuous with the muscle capsule, continuing onto the surface of the sclera. (B) Transverse section of rectus muscle at position a-1, posterior to the point of emergence of the ACVs. The vessels are deep in the muscle, not visible to the surgeon. S = sclera; IM = intermuscular septum (continuous with the muscle capsule); T = Tenon’s capsule; C = conjunctiva. (C) Transverse section at position a-2 showing three ACV “groups” on the surface of the tendon, in a plane that is continuous with the capsule of the muscle and tendon. Each anterior ciliary vessel group can contain more than one anterior ciliary vessel. (D) Transverse section at position a-3: the tendon insertion site. The ACVs remain on the surface and continue onto the sclera, anterior to the tendon insertion.

Fig. 85.4 Transverse section of resected lateral rectus. A single ACV group is shown, which contains three vessels of significant size, surrounded by loose connective tissue, which is continuous with the capsule and separates the vessels from the underlying tendon. A potential surgical dissection plane exists between the vessels and the tendon (hematoxylin-eosin; original magnification, ×200).

The contribution of the anterior ciliary vessels to anterior segment blood flow has been studied by iris fluorescein angiography.¹¹ Tenotomy of either of the vertical rectus muscles produces a significant delay in vascular filling in the corresponding segment of the iris.¹² Surgery on the vertical rectus muscles impacts more on anterior segment blood flow than surgery on the horizontal rectus muscles and may influence the risk of anterior segment ischemia.

Innervation

The extraocular muscles receive innervation from the oculomotor (III), the trochlear (IV), and the abducens (VI) nerves. The abducens nerve innervates the lateral rectus. The trochlear nerve innervates the superior oblique. The oculomotor nerve divides into a superior and an inferior division. The superior division innervates the levator palpebrae muscle and the superior rectus muscle while the inferior division innervates the medial rectus, inferior rectus, and inferior oblique muscles. The nerves supplying the rectus muscles penetrate the bellies of the muscles at roughly the junction of the posterior and middle thirds of the muscles. The inferior oblique muscle receives its innervation lateral to the inferior rectus muscle. The superior oblique muscle receives its innervation from the inferior surface in the middle third of the muscle.

Sclera

The sclera is composed of highly organized collagen fibers. It ranges from 1 mm thick just posterior to the limbus to 0.33 mm thick just posterior to the rectus muscle insertions, where it is most at risk of perforation during strabismus surgery.

Pulleys of the orbit

Highly complex pulleys in the anterior orbit have been described. Each rectus muscle passes through an encircling ring of connective tissue near the equator of the globe, which attaches to the orbital wall. The pulleys are composed of collagen, bands of smooth muscle, and elastin.^13,¹⁴ They have an intimate and active relationship with the extraocular muscles, which can include acting as the functional origin and limiting side-slipping.^15,^16,¹⁷ Orbital malformations, disease, trauma, and surgical procedures that affect the integrity of the orbital pulleys may be significant in ocular motility disorders.

Rectus muscles

There are four striated rectus muscles arising from the annulus of Zinn in the apex of the orbit. Each is 40 mm in length and inserts on the sclera anterior to the equator of the globe. Following the “spiral of Tillaux,” the medial rectus inserts 5.5 mm from the limbus, the inferior rectus 6.5 mm, the lateral rectus 6.9 mm, and the superior rectus inserts 7.7 mm from the limbus (Table 85.1). The location of the insertions is actually somewhat variable.¹⁸ The insertions of the rectus muscles are curvilinear, particularly the vertical rectus muscles, in which the temporal border is further from the limbus than the nasal border.

Table 85.1 Rectus muscle dimensions

The medial rectus is responsible for adduction, the lateral rectus for abduction. The superior rectus supraducts and the inferior rectus infraducts; they have additional actions − adduction and torsion.

The medial rectus muscle courses anteriorly along the medial wall of the orbit inserting on the sclera with a 6 mm arc of contact with the globe. It is at risk for inadvertent damage during ethmoid sinus procedures and nasal pterygium excision. Without fascial attachments to an oblique muscle and its innate tension and relatively short arc of contact, the medial rectus is at the greatest risk of inadvertent loss during surgery.

The lateral rectus courses anteriorly along the lateral orbit inserting on the sclera with a 10 mm arc of contact with the globe. The insertion of the inferior oblique is in close proximity to the lower border of the lateral rectus, 8−10 mm posterior to the lower pole of the lateral rectus insertion. Due to fascial attachments between the lateral rectus and the inferior oblique, the surgeon can often locate the lateral rectus muscle if it becomes detached during surgery or trauma.

The inferior rectus muscle courses anteriorly, laterally, and inferiorly to insert on the sclera. The inferior rectus forms a 23° angle with the visual axis when the globe in the primary gaze position. It has fascial attachments to the inferior oblique muscle and the lower eyelid retractors (Fig. 85.5). If these attachments are not severed during recession or resection of the inferior rectus, eyelid fissure changes may occur. Connective tissue attachments between the superior rectus and the superior oblique may assist the surgeon in locating a ‘lost’ superior rectus muscle.

Fig. 85.5 Relationship of inferior rectus to lower lid: surgeon’s view from above. (A) Diagrammatic illustration. (B) Lower lid retractors on surface of inferior rectus muscle being held with forceps. (C) Thickened Tenon’s fascia beneath inferior rectus muscle seen as a glistening membrane with ridges between the muscle and the globe. (D) Inferior rectus muscle seen within Tenon’s capsule. The original muscle insertion is seen under the left muscle hook.

(Patient of Dr R. Morris.)

The superior rectus muscle courses anteriorly, laterally, and superiorly to insert on the sclera. It forms a 23° angle with the visual axis when the globe is in the primary gaze position. The superior rectus has fascial attachments to the superior oblique tendon and the levator palpebrae muscle. If the attachments to the levator palpebrae are not severed during recession or resection of the superior rectus, eyelid fissure changes may occur. Connective tissue attachments between the inferior rectus and the inferior oblique may assist the surgeon in locating a “lost” inferior rectus muscle.

Superior oblique muscle

The actions of the superior oblique are incyclotorsion, depression, and abduction. The superior oblique is an antagonist of the inferior oblique, with respect to torsion. The other actions are not strictly antagonistic with the inferior oblique.

The superior oblique muscle arises from periosteum in the orbital apex above the annulus of Zinn and travels anteriorly along the superomedial orbital wall. It becomes a cord-like tendon, passing through the trochlea to run in a posterolateral direction at an angle of approximately 51° to 54° with the sagittal plane. The tendon passes under the superior rectus and fans out, inserting on the sclera in the superotemporal quadrant (Fig. 85.6). The anterior pole of its insertion is located close to the lateral border of the superior rectus, beginning 4 to 6 mm posterior to the superior rectus insertion. Its tendon is thin and broad, extending posteriorly for about 11 mm, but with considerable variability. The posterior end of the insertion lies 6−7 mm from the optic nerve. The superior temporal vortex vein can exit the sclera temporal to the superior oblique insertion, under the insertion or sometimes split the fibers of the insertion and pass through them.

Fig. 85.6 Superior oblique tendon (held in lower hook) inserting beneath the superior rectus: surgeon’s view from above.

(Patient of Dr R. Morris.)

The superior oblique tendon can be abnormal.¹⁹ It may be redundant, lax, or misdirected. It can have an anomalous insertion nasal to the superior rectus or may insert into Tenon’s capsule and the trochlea, or be absent.²⁰ Intraoperative “exaggerated”^21,²² traction testing of the superior oblique can demonstrate laxity of the superior oblique tendon (Fig. 85.7).

Fig. 85.7 Strabismus hooks beneath superior oblique tendon demonstrating tendon laxity.

(Patient of Dr R. Morris.)

The nasal aspect of the superior oblique tendon has fascial relationships with the intermuscular septum, which envelops it to form a capsule. If the nasal intermuscular septum is thin, atrophic, or is damaged while performing a nasal tenotomy, the cut ends of the tendon may separate excessively, creating a superior oblique palsy. The ends of the tendon may be difficult to recover. There is a frenulum between the capsule on the underside of the superior rectus and superior surface of the superior oblique tendon. The frenulum must be lysed for significant procedures on either muscle (Fig. 85.8).

Fig. 85.8 Relationship between superior oblique and superior rectus: surgeon’s view from above. (A) Chavasse muscle spreading hook (left) beneath superior rectus and Stevens hook beneath the superior oblique tendon showing the fascial attachment between them. It passes obliquely between the crook of the Stevens hook to just by the insertion of the superior rectus. (B) Undersurface of disinserted superior rectus (below, held on sutures) showing fascial attachment between it and the superior oblique tendon.

(Patient of Dr R. Morris.)

Inferior oblique muscle

The actions of the inferior oblique are excyclotorsion, elevation, and abduction. The inferior oblique forms an antagonist pair with the superior oblique with respect to torsion. The additional actions of the inferior oblique are not strictly antagonistic with the superior oblique.

The inferior oblique muscle originates from the maxillary bone adjacent to the lacrimal fossa. It passes posteriorly and laterally, forming approximately a 51° angle with a sagittal plane passing through the origin of the muscle. After passing beneath the inferior rectus, the inferior oblique travels upward inserting on the sclera, close to the macula.²³ The tendon of the inferior oblique is 1−2 mm long, the shortest of the extraocular muscle tendons. The inferior oblique insertion is 8 to 12 mm posterior to the lateral rectus insertion, adjacent to its lower border. There is considerable variability in the location and configuration of the inferior oblique insertion.

The inferior temporal vortex vein lies close to the inferior oblique. It can be damaged when the surgeon isolates the inferior oblique muscle or when placing an inferior oblique muscle to be recessed at its new insertion site, which may be close where the vortex vein exits the sclera. The scleral exit of the inferior temporal vortex vein is variable, but is often close to the lateral border of the inferior rectus, 8−10 mm posterior to the lateral pole of the inferior rectus insertion.

The neurovascular bundle enters the inferior oblique muscle as it passes beneath the inferior rectus muscle. The neurovascular bundle may serve as the effective origin of the inferior oblique, particularly after anterior transposition of the muscle.²⁴

Overaction of the inferior oblique is common, particularly in congenital esotropia, intermittent exotropia, and superior oblique palsy. Overaction of the inferior oblique is clinically graded on a scale of 1−4.²⁵ Overaction of the inferior oblique must be distinguished from other causes of elevation of the globe in adduction. Overaction of the inferior oblique commonly coexists with dissociated vertical deviation.²⁶

General principles of surgery

Preoperative examination

The preoperative examination should include a comprehensive eye examination with history, sensory, and motor evaluation and refraction. Of particular importance are issues of prior surgery and which extraocular muscles were involved. It is not uncommon for adults to not recall childhood surgery. The examination should identify prior surgical scars as well as alterations in the course of the anterior ciliary vessels. Certain systemic and orbital disorders are associated with anatomic abnormalities affecting the extraocular muscles.

Preoperative counseling

The patient and/or the patient’s guardians must be informed regarding the indications, potential benefits, risks, and alternatives to the procedure. Realistic expectations are essential. The discussion includes the potential effects on binocular vision, the possibility of overcorrection, undercorrection, and future sequential deviations that may require additional treatment.

Anesthesia

For young strabismus patients, surgery is usually performed under general anesthesia. Because strabismus procedures are relatively quick, short acting inhalational anesthetic agents are typically utilized. Sevoflurane is the most commonly used inhalational agent and a laryngeal mask airway or endotracheal tube is employed for its administration. Using a right angle endotracheal tube or a flexible tube laryngeal mask airway avoids the airway interfering with the drapes and reducing exposure.

Postoperative nausea and vomiting in strabismus patients receiving general anesthesia is common. Short acting inhalational anesthetics reduce, but do not eliminate, this problem.^27,^28,²⁹ Dexamethasone and ondansetron decrease postoperative nausea and vomiting.^30,³¹

Patient preparation

Patient preparation is essential for efficient and safe surgery. Patient identification, diagnosis and laterality for surgery should be verified before operating. One drop of 2.5% phenylephrine is instilled in the operated eye(s) prior to the surgical preparation to provide vasoconstriction and dilate the pupil should postoperative fundoscopy be needed.

The patient should be positioned with the head level and the neck slightly extended (Fig. 85.9). The exposed skin is cleaned with aqueous povidone-iodine. One drop of 5% aqueous povidone-iodine is placed in the conjunctival cul-de-sac prior to surgery. The eyebrows are covered with drapes or an adhesive plastic strip (Fig. 85.10) and one is placed over the upper eyelid eyelashes. A separate speculum is used on each eye, when the procedure is bilateral.

Fig. 85.9 Patient correctly positioned for surgery with neck slightly extended.

Fig. 85.10 Surgical drape covering face while exposing both eyes for bilateral surgery.

Surgical instruments

A selection of strabismus hooks is essential (Fig. 85.11). Hooks with tips slightly longer than the width of the rectus muscle insertion allow the tendon to be spread evenly on the hook, which facilitates dissection and suture placement (Fig. 85.12).

Fig. 85.11 Strabismus hooks. From left to right: a Stevens tenotomy hook, a von Graefe hook, a Chavasse hook, a Helveston hook, and a Green muscle hook.

Fig. 85.12 Surgical use of muscle hooks. (A) Chavasse muscle hook under medial rectus. Note even spreading of the tendon on the hook and the anterior ciliary arteries. (B) Muscle spread evenly on Helveston hook.

(Patient of Dr R. Morris.)

Traction testing

Traction testing is performed at the beginning of every procedure. If abnormal, the test may be repeated several times intraoperatively as the muscles are released. The position of the surgeon should be consistent for each traction testing. Traction testing consists of two components: force generation and passive range of motion.

Force generation testing is performed with the patient awake and able to follow commands, with no anesthetic induced akinesia or hypokinesia of the extraocular muscles. It qualitatively assesses the strength of the extraocular muscles by stabilizing the globe in one position and asking the patient to voluntarily move the eye in specific directions.

Passive range of motion testing can be performed in the office or the operating room. It qualitatively assesses the resistance imparted by the extraocular muscles as well as orbital tissues during manual rotation of the globe. It can be performed with the patient awake or under general or local anesthesia. The “volume effects” of any anesthetic agent injected in the orbit may alter the passive range of the traction test.

Intraoperative traction testing can be performed using different techniques. Most commonly, it is performed with two toothed forceps placed 180° apart adjacent to the limbus. The rectus muscles are evaluated by rotating the globe medially, temporally, superiorly, and inferiorly, while placing gentle anterior traction on the globe, to place tension the rectus muscles.

To test the oblique muscles, the globe is rotated into extorsion and intorsion with gentle posterior pressure on the eye (retropulsion). This places the oblique muscles, with their anterior functional origins, under tension.

The exaggerated traction test of the superior oblique determines whether superior oblique laxity is present and may be helpful in Brown’s syndrome.³² It is performed by grasping the eye with two toothed forceps adjacent to the limbus. The eye is adducted while placing the eye in retropulsion, external rotation, and elevation. While maintaining this position, the eye is moved into abduction. A palpable “thump” is perceived with a normal superior oblique. An absent or reduced “thump” is evidence of superior oblique laxity. This test requires a considerable experience.

Incisions for strabismus surgery

Limbal incision

The limbal incision is a common approach for access to extraocular muscles (Fig. 85.13). The fusion of conjunctiva, Tenon’s capsule, and intermuscular septum a few millimeters from the limbus permits the three layers to be retracted together, providing direct access to the sclera. It provides an excellent view of the extraocular muscles.

Fig. 85.13 Limbal conjunctival incision: surgeon’s view from above. (A) Inferior conjunctival “relaxing” incision. (B) Superior conjunctival relaxing incision parallel to inferior conjunctival incision following completion of peritomy. (C) Accurate apposition of conjunctiva following rectus muscle surgery using four 7–0 Vicryl sutures.

(Patient of Dr R. Morris.)

This incision is ideal for the inexperienced surgeon. Compared to the fornix incision, the limbal incision does not require as much anatomic knowledge on the part of the surgical assistant.

Fornix or cul-de-sac incision

The fornix or cul-de-sac incision is also commonly employed. It reduces postoperative discomfort and places the surgical scar is beneath the eyelid (Fig. 85.14). It requires a skilled assistant, familiar with ocular anatomy.

Fig. 85.14 (A) Cul-de-sac conjunctival incision is made 8 mm from the limbus parallel to the fornix in the inferonasal quadrant. (B) Tenon’s capsule is held up with two Castroviejo forceps and incised at a right angle to the conjunctival incision. (C) A tenotomy hook engages the edge of the medial rectus behind the insertion. It is replaced with a Jameson or other long tipped hook to hold the entire width of the tendon just posterior to its insertion.

The fornix incision does not gain direct access to the sclera by penetrating the conjunctiva, Tenon’s capsule, and intermuscular septum together in a single incision. Proper placement is important, particularly to avoid bleeding, inadvertent entry into the orbital fat, and, potentially, surgery on the wrong muscle.

The conjunctiva is opened as an isolated layer. Depending on the location of the anterior termination of the extraconal fat, the incision is placed 1−2 mm anterior to the fornix extending for 8 mm parallel to the fornix. A second incision is made through Tenon’s capsule and the underlying intermuscular septum at a right angle to the conjunctival incision. It is important to avoid extending the incision in Tenon’s capsule more than 10 mm posterior to the limbus, due to the anterior border of the extraconal fat. When the sclera is exposed, a muscle hook is used to engage the extraocular muscle to be operated.

Fornix incisions may be performed in any of the four oblique quadrants. By employing only an inferior temporal and superior nasal incision, the surgeon can gain access to all six extraocular muscles. Avoid incising the plica semilunaris when performing an inferonasal quadrant fornix incision.

Swan incision

The Swan incision is made directly over the rectus muscle insertion.³³ It is less popular due to the interpalpebral location of the wound with scar formation and potential bleeding from the underlying anterior ciliary vessels. A modified Swan incision may be useful in reoperations.

Minimal incision

Minimally invasive incisions are performed via small openings away from the limbus placed over the location on the muscle in which sutures are required. This may reduce discomfort and scarring.³⁴^–³⁶

Extraocular muscle surgery

Principles

Attention to detail makes extraocular muscle surgery proceed smoothly. Confirmation of the proposed procedure should be re-verified immediately prior to starting surgery.

A history of prior muscle surgery warrants special consideration. Surgical scarring may alter the location of the extraocular muscles and other structures.

Careful dissection and hemostasis provide better exposure, reduce adhesions, and minimize muscle and vessel trauma. This diminishes the potential for inadvertent penetration of Tenon’s capsule and fat prolapse. Light bipolar cautery under saline irrigation reduces tissue damage, in comparison to thermal cautery: its judicious use prevents tissue shrinkage and excessive scarring.

A moistened flat cellulose sponge placed on the corneal surface throughout the procedure helps reduce drying of the cornea and postoperative discomfort.

Sutures

Spatulated, side cutting needles are a major advance in strabismus surgery. They provide for easier and safer scleral passes and reduce the risk of scleral perforation.

Most strabismus surgery is performed using absorbable sutures, such as 6-0 coated polyglactin 910. A number of needle choices are available. Double armed 6-0 coated polyglactin 910 needle choices for strabismus surgery include S-14, S-24, S-28, S-29, and TG-100 needles.

Non-absorbable suture is sometimes desirable: 5-0 and 6-0 polyester white braided suture is available with S-14 and S-24 spatulated needles.

Conjunctival closure can be accomplished with a wide variety of techniques including no suture, cautery, 6-0 gut, and 8-0 polyglactin. The choice of needle for conjunctival closure is less critical than for suture placement in the sclera.

Tissue adhesives

Tissue adhesives have been used in conjunctival closure during strabismus surgery ^37,³⁸ but have not gained popularity in securing the muscles to the sclera.³⁹^–⁴¹

Magnification

Some surgeons routinely use the operating microscope; most use magnifying loupes of 2× to 3.5× power. Wide field 3.5× loupes provide excellent magnification, a comfortable depth of field, and a field wide enough to include the entire orbit. However, higher magnification and a wide field create a larger ring scotoma than the more commonly used 2× non-wide field loupes.

Rectus muscle surgery

Proper exposure of the extraocular muscles is essential for successful strabismus surgery. Dissection of excess connective tissue at the suture line provides better exposure for precise suture placement and decreases the risk of slipped or lost muscles (Fig. 85.15). This requires excision of redundant muscle capsule and intermuscular septum. The muscle capsule and intermuscular septum can be lifted off the rectus muscle adjacent to the insertion and excised without disrupting the anterior ciliary vessels. Cauterizing anterior ciliary vessels prior to passing sutures or making incisions in the tendon or muscle reduces bleeding and decreases scarring.

Fig. 85.15 Muscle suture placement: double-armed suture through lateral rectus: surgeon’s view from above. (A) Central suture placement. (B) Superior locking bite. (C) Completed suture placement prior to muscle disinsertion.

(Patient of Dr R. Morris.)

Weakening procedures on the rectus muscles

Recessions

Recessions are the most common weakening procedure on the rectus muscles. The maximal effect is in the field of action of the recessed muscle. However, recessions also have a significant effect on primary gaze alignment. Recessions are generally more comfortable postoperatively than resections; causing less swelling, dellen formation, inflammation, and scarring.

Recession results in a small resection of tendon. The method and point of measurement for recessions varies. Because of this and other factors, a 5 mm recession performed by one surgeon is not identical to one performed by another. Suture placement 1 mm posterior to the insertion minimizes the concurrent resection and allows disinsertion to be performed without inadvertent suture transection. Techniques to place the suture securely in the tendon include placement of a central tie in the tendon and double locking sutures at each border of the tendon.

The preferred site for measuring a recession varies. Measurement from the limbus avoids the potential variability in the position of the muscle insertion.⁴² Those who measure from the insertion of the tendon feel the amount of recession is dependent upon the change in position relative to the original scleral attachment point, rather than the limbus. Measurement from the insertion should be determined prior to disinsertion; the elastic properties of the sclera allow the insertion to shift anteriorly following detachment of the muscle,⁴³ especially with tight extraocular muscles.

Recessions can be performed by attaching the muscle to the sclera at the desired position or by employing “hang-back,” “hemi hang-back,” or various modifications.

Direct fixation of the recessed muscle to the sclera is widely used (Fig. 85.16A,B). The scleral entry position of the needles is at the location of the desired recession and should be directly posterior to the original insertion site. Each pole should be spaced to maintain the width of the recessed muscle to avoid “central sag.” If sagging occurs, the center of the muscle can be sutured to the sclera (Fig. 85.17). Because the muscle is sutured directly to the sclera, it allows accurate supra- and infraplacement and “slanting” the recession, if desired, in “A” and “V” patterns.

Fig. 85.16 Rectus muscle recession. (A) Fixed recession (two single sutures). (B) Fixed recession double armed suture. (C) Hang-back recession (note central bowing). (D) Hemi hang-back recession. (E) Anchored hang-back suture. (F) Gobin’s loop recession.

Fig. 85.17 Inferior rectus fixed muscle recession, double-suture technique: surgeon’s view from above. (A) Needle placement through two scleral tunnels. (B) Muscle tied in recessed position. (C) Needle placed back through center of muscle. (D) Sutures tied down: note absence of central sag.

(Patient of Dr R. Morris.)

Hang-back and hemi hang-back techniques are commonly employed in recessions (Fig. 85.16C,D).The hang-back technique suspends the muscle on a suture attached at or near the terminal poles of the original insertion or can be located near the mid-portion of the original insertion (see Fig 85.20). Hang-back recessions may reduce scleral perforations by placing the sutures in thicker sclera at the original insertion. However, with large recessions, the muscle may side-slip or creep anteriorly during or after surgery; this is reduced by placing shallow scleral “belt-loops” close to the recessed muscle or using an anchored hang-back suture (Figs 85.16E and 85.18).

Fig. 85.18 Medial rectus. Anchored hang-back suture. (A) The needles of the double armed sutures (already attached to the muscle, as in Figure 88.13, and the muscle detached) are passed through the sclera. (B) Sutures passed through muscle insertion. (C) Sutures tied down: note minimal central sag.

(Patient of Dr R. Morris.)

In the hemi hang-back technique, the suture engages the sclera posterior to the original insertion site but anterior to the recessed muscle (Fig. 85.16D). Indications include: surgical exposure, thin sclera, scarring, the presence of an oblique muscle (particularly the superior oblique tendon), or a scleral buckle, glaucoma implant or other device on the sclera. Hemi hang-back sutures with large recessions may reduce the potential for side-slipping or muscle creep.

In a loop recession, a non-absorbable suture is placed through each pole of the muscle and sutured to the sclera behind the insertion (Fig. 85.16F).

Horizontal muscle recession

Dose-response tables for horizontal rectus recessions and resections are available. As a general rule, the medial rectus can be recessed a maximum of 7 mm without reducing adduction and the lateral rectus recessed up to 10 mm. Exceptions to these limits are common, particularly with restrictive and paretic strabismus, when much larger recessions are commonly employed. However, 7 mm bilateral medial rectus recessions may increase the risk of consecutive exotropia in children with congenital esotropia.⁴⁴^–⁴⁶ The dose-response with large recessions is non-linear and less predictable. The guideline values listed in the dose-response tables are most useful in uncomplicated strabismus (Tables 85.2 and 85.3).

Table 85.2 Esotropia^a

Table 85.3 Exotropia^a

The quantity of surgery required to correct horizontal deviations is dependent upon a number of factors, especially the magnitude of the preoperative deviation. Axial length and refractive error are not usually significant.⁴⁷ Recession of unoperated rectus muscles is more predictable than recession of previously operated muscles. Restrictive disorders, particularly thyroid orbitopathy, and recession of antagonist to a paretic muscle are associated with a reduced effect per millimeter of recession. Very large recessions combined with resections may result in limitation of movement.

Recessions can be measured using a curved ruler or calipers. Curved rulers follow the circumference of the sclera, measuring the length of the arc (Fig. 85.19). Calipers measure the length of the chord: this disadvantage can be reduced by measuring the total recession as two small chord lengths, whose sum is equal to a large recession.

Fig. 85.19 Curved ruler measuring 11 mm from medial limbus.

(Patient of Dr R. Morris.)

Inferior rectus muscle recession

Inferior rectus recessions are employed in complex non-concomitant vertical strabismus. Indications for inferior rectus recession include thyroid orbitopathy, blowout fracture, congenital fibrosis of the extraocular muscles, “double elevator” palsy, superior rectus palsy, superior oblique palsy in the contralateral eye, and anesthetic block induced vertical strabismus.

Recession of the superior rectus in the contralateral hypertropic eye is often preferred over recession of the inferior rectus in the hypotropic eye in non-restictive strabismus. In patients with over 15 prism diopters (PD) of deviation, recession of the ipsilateral inferior rectus and contralateral superior rectus can be considered.

Although inferior rectus recessions employ basic techniques similar to horizontal recessions, there are several differences. The inferior rectus is closely associated with the inferior oblique muscle and the retractors of the lower eyelid.

There are no dose-response tables for vertical rectus recessions or resections. On average, 1 mm of inferior rectus recession will result in correction of 3 PD of vertical deviation in primary gaze and 5 PD in downgaze.⁴⁸

Recessions of the inferior rectus for non-paretic, non-restrictive disorders should generally be limited to 5 mm or less; larger recessions commonly cause lower eyelid retraction. However, inferior rectus recessions of less than 5 mm may cause lower eyelid retraction in restrictive strabismus, particularly thyroid orbitopathy. Severe restrictive disorders often require large recessions resulting in significant retraction of the lower eyelid. The patient should be warned of this preoperatively.

Techniques to reduce lower eyelid retraction include:

1. Dissection and lysis of fascial attachments to the inferior rectus as far posteriorly as possible, including: intermuscular septum, check ligaments, and Lockwood’s ligament connections.

2. Suture the lower eyelid retractors to the inferior rectus.⁴⁹ An adjustable suture can be placed on the capsulopalpebral head and passed through the inferior rectus insertion. Adjustment of the capsulopalpebral head is performed after the inferior rectus adjustment.⁵⁰

3. Disinsertion of the lower eyelid retractors from the tarsal plate.⁵¹

Delayed overcorrection is a significant problem following inferior rectus recession. This is usually accompanied by limited depression in the operated eye with diplopia, particularly in downgaze. Delayed overcorrection is more likely to occur with thyroid orbitopathy treated with adjustable sutures. It occurs in 10−21% of cases.^52,^53,⁵⁴

A number of mechanisms may contribute to “late slippage” of the inferior rectus.^55,^56,⁵⁸ These include:

1. Thickened Tenon’s capsule under the inferior rectus muscle allowing the muscle to slip posteriorly after absorption of the sutures, forming a pseudo-tendon anterior to the insertion.

2. Inferior rectus attachments to Lockwood’s ligament and the lower eyelid retractors.

3. Hang-back adjustable sutures hold the recessed inferior rectus in place centrally. When the eye makes horizontal movements, the inferior rectus does not move fully with the eye; repeat side-slipping prevents firm reattachment of the muscle to the sclera.

4. Laterally directed traction of the inferior oblique may prevent adherence of the inferior rectus to the sclera.

5. Scarring around Lockwood’s ligament pulls the inferior rectus muscle anteriorly, creating slack in the anterior aspect of the muscle, weakening its depressor function.⁵⁷

6. Following a large recession, the inferior rectus is not in apposition to the globe when the muscle is contracting and the eye is in downgaze.⁵⁸ MRI studies demonstrate that an inferior rectus recession of 6.0 mm and a medial rectus recession of 7.5 mm in a normal eye leads to loss of inferior rectus apposition, with lower values in thyroid associated patients.⁵⁹

7. Ipsilateral contraction of the superior rectus in thyroid orbitopathy leads to late overcorrection.

Undercorrections after inferior rectus recessions are associated particularly with restrictive processes and reduced contractility of the opposing superior rectus. In severe thyroid orbitopathy, even recessions of 15 or more millimeters from the insertion may fail to correct the hypotropia. Significant restriction in elevation may persist, even while the inferior rectus is disconnected from the globe during surgery. Postoperative undercorrection in primary gaze with hypertropia in downgaze indicates a tight superior rectus. Restriction following orbital blowout fractures frequently is associated with alterations in the elastic properties of the surrounding orbital tissues.

Poor or absent superior rectus function is a cause for undercorrection. “Double elevator” palsy and superior rectus palsy are likely to be undercorrected with inferior rectus recessions alone. Failure of superior rectus development in congenital fibrosis of the extraocular muscles causes disappointing results following inferior rectus recession.^60,⁶¹ Retrobulbar and peribulbar block induced vertical strabismus can be complex and is often associated with paretic and restrictive components.⁶²^–⁶⁷

Bilateral recessions of the inferior rectus muscles can create exotropia in downgaze, forming an “A” or “λ” pattern, especially in thyroid orbitopathy where large inferior rectus recessions have been performed. Nasal displacement of the inferior rectus muscles reduces the tendency for exotropia in downgaze. However, this may be associated with intorsion.⁶³

Superior rectus muscle recession

There are also no dose response tables for superior rectus recessions or resections. Each millimeter of superior rectus recession generally corrects 3 to 4 PD of vertical deviation up to 6 mm of recession. Recessions of greater than 6 mm are avoided, except in dissociated vertical deviation (DVD), when hang-back recessions of up to 10 mm may be performed (Fig. 85.20).^64,⁶⁵ Superior rectus recessions beyond 4 mm require lysis of the fascial attachments between the underside of the superior rectus and the superior oblique tendon (see Fig. 85.8B). For very large recessions, it is important to dissect and cut fascial attachments and check ligaments on the superior rectus as far posteriorly as possible and identify the superior oblique tendon, both nasal and temporal to the superior rectus. The superior oblique insertion is in close proximity to the temporal border of the superior rectus. The thin fibers of the superior oblique near its insertion can be mistaken for intermuscular septum and inadvertently transected (Fig. 85.6).

Fig. 85.20 Superior rectus hang-back recession: surgeon’s view from above.

(Patient of Dr R. Morris.)

Superior rectus recessions do not commonly cause upper eyelid retraction.⁶⁶ Because of the potential problems associated with inferior rectus recessions, surgery for concomitant vertical deviations of up to 15 PD is often performed by superior rectus recession in the hypertropic eye.

Non-concomitant hypertropia that increases on downgaze is usually treated by recession of the ipsilateral superior rectus. In the absence of oblique dysfunction, hyperdeviations that increase in abduction are treated by superior rectus recession in the hypertropic eye. However, hyperdeviations that increase in adduction (without oblique dysfunction) are commonly treated by recession of the inferior rectus of the hypotropic eye. Deviations larger than 15 PD in primary gaze are often treated with recession of the superior rectus in the hypertropic eye and recession of the inferior rectus in the hypotropic eye.