Strabismus surgery

Published on 08/03/2015 by admin

Filed under Opthalmology

Last modified 08/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 9042 times

CHAPTER 57 Strabismus surgery

Chapter outline


See Video image

Terminology, history, and epidemiologic considerations

The term strabismus is derived from a Greek word, strabismos, which means to squint. Strabismus has been recognized since antiquity. However, the first strabismus surgery was not reported until 1839 – a medial rectus myotomy by Johann Dieffenbach on a 7-year-old child with esotropia1. Within a few years, strabismus surgery was widely practiced throughout Europe. Strabismus is one of the most common ocular diseases in children. It is also present in many adults either because it was not corrected during childhood or secondary to a condition having an adult onset. The most common types of strabismus vary depending on the ethnicity of the population. In North America and Europe, esotropia is the most common type of strabismus. This probably reflects the high incidence of hyperopia in this population that predisposes patients to accommodative esotropia. However, in Asia, exotropia is the most common type of strabismus, perhaps because of the lower incidence of hyperopia in this population2.

Anatomical considerations

Gross anatomy of the extraocular muscles

There are six extraocular muscles – the medial, lateral, superior and inferior rectus muscles, and the superior and inferior oblique muscles. The rectus muscles all have insertions that are 10–11 mm in width. The four rectus muscles all arise from the apex of the orbit at the annulus of Zinn and then insert at varying distances from the limbus. The medial rectus muscle inserts closest to the limbus and the superior rectus muscle furthest from the limbus. The outward spiraling of the insertions of the rectus muscles, beginning with the medial rectus muscle and ending with the superior rectus muscle, is referred to as the spiral of Tillaux (Fig. 57.1). The inferior and superior rectus muscle insertions are slanted nasally whereas the medial and lateral rectus muscles are not usually slanted. The rectus muscles have varying arcs of contact with the globe – the longest being 10 mm for the lateral rectus muscle. Certain disorders can be associated with ectopia of the rectus muscles. For example, patients with craniosynostosis often have medial rectus muscles which are abnormally high and lateral rectus muscles which are abnormally low, which can result in a V-pattern mimicking overaction of the inferior oblique muscles.


Fig. 57.1 The spiral of Tillaux.

From Coats DK, Olitsky SE, editors. Strabismus Surgery and its Complications. Berlin: Springer, 2007.

The superior oblique muscle also arises in the apex of the orbit, but it has a long tendon that is redirected temporally after it traverses the trochlea, which is located just superior to the medial canthal tendon. Brown syndrome is associated with dysfunction of the trochlea–superior oblique tendon complex. The superior oblique tendon fans out and then inserts beneath the superior rectus muscle. It is attached to the superior rectus muscle by a frenulum, which is an important anatomical structure when performing large recessions of the superior rectus muscle or disinserting the superior oblique tendon. The insertion of the superior oblique tendon is more variable than that of the other extraocular muscles. The inferior oblique muscle arises near the lacrimal fossa and traverses the floor of the orbit to insert near the inferior border of the lateral rectus muscle overlying the macula.

There are two distinct layers to the rectus muscles – the global and orbital layers. The orbital layer inserts directly into the muscle pulleys while the global layer inserts into the globe3 (Fig. 57.2).

Physiology of the extraocular muscles

The extraocular muscles are some of the fastest and most fatigue resistant muscles. Unlike skeletal muscles, which are exclusively singly innervated, the extraocular muscles are both singly and multiply innervated4. Singly innervated muscle fibers are thicker and produce rapid all-or-none contractions, while multiply innervated fibers are thinner and produce slow, graded contractions. Thick fibers are primarily located in the global layer of rectus muscles while thin, tonic fibers are primarily located in the orbital layer of rectus muscles. In the oblique muscles, a core of thick fibers is surrounded by an outer layer of thin fibers. Thick fibers are believed to be primarily responsible for saccades while thin, tonic fibers are responsible for small, refixation movements of the eye.

The extraocular muscles have complicated actions based on the angle and location of their insertions. While the medial and lateral rectus muscles only have the primary actions of adducting and abducting the eyes respectively, the other four extraocular muscles also have secondary and tertiary actions in primary position (Table 57.1). All of these actions must be taken into account when planning strabismus surgery. The eyes are yoked together and the movement of one eye is closely linked to the movement of the fellow eye. Sherrington’s law of reciprocal innervations states that, when the agonist muscle receives a stimulatory impulse, the antagonist muscle in the same eye receives an inhibitory impulse or no impulse. Herring’s law of equal innervations states that the muscles responsible for rotating the eyes in a certain direction (yoke muscles) receive equal innervation thereby assuring that the eyes move in the same direction.

Tenon’s fascia and muscle pulleys

The globe is suspended in the orbit by Tenon’s capsule. Tenon’s capsule covers the entire globe except for the area immediately adjacent to the corneal limbus where it fuses with the conjunctiva. It is quite thick in children, but thins with age and is often diaphanous in elderly adults. The rectus muscles penetrate Tenon’s capsule close to the equator of the globe. The fascial sheath surrounding the rectus muscles is often referred to as the intermuscular membrane. The attachment of the inferior rectus muscle to the retractors of the lower lid is called the capsulopalpebral fascia. As a result, recessing the inferior rectus muscle can alter the position of the lower eyelid. The superior rectus also has fascial connections to the levator palpebrae superioris muscle.

The paths of the rectus muscles have been shown to be highly uniform in normal patients in primary position using high resolution magnetic resonance imaging5. In secondary positions of gaze, the paths of the rectus muscles exhibit small but consistent shifts opposite to the direction of gaze. Patients with strabismus generally have little if any change in their rectus muscle paths either in primary position or in other positions of gaze. Rectus muscles pass through connective tissue sleeves in Tenon’s fascia near the equator of the globe. Demer et al6. have labeled these fascial connections which suspend the rectus muscles in the orbit as ‘muscle pulleys’. Ectopic pulleys are a rare cause of strabismus and may be identified by high resolution magnetic resonance imaging7.

Fundamental principles

There are certain fundamental principles that should be followed when performing strabismus surgery: 1) whenever possible surgery should be performed on the eye with the worst vision. However, this is not always possible and is not an inviolate rule; 2) strabismus surgery should be performed only when reliable, stable motility measurements are available. This is particularly true in paralytic forms of strabismus, which may improve spontaneously over time; 3) reoperations should only be performed in the immediate postoperative period if it is suspected that a muscle has slipped or is lost. Generally, it takes several weeks for ocular alignment to stabilize after strabismus surgery and sometimes it can take longer. For this reason, what may appear to be an under or over-correction immediately after surgery may turn out to be excellent alignment after a longer follow-up; 4) only two rectus muscles in one eye should be operated on during any given surgical session to minimize the risk of anterior segment ischemia. However, it is usually fine to operate on a third and sometimes even a fourth rectus muscle once collaterals have been allowed to form to the anterior segment; 5) do not resect restricted muscles because of the risk of impairing ocular rotations; 6) the medial rectus should not be recessed beyond the equator (>6 mm) because of the high risk of inducing a consecutive exotropia associated with retroequatorial medial rectus recessions; 7) transposition procedures are usually necessary to correct the strabismus associated with a paretic muscle; 8) correcting a large vertical deviation with surgery will often cause spontaneous resolution of small associated horizontal deviations; 9) the conjunctiva should be recessed after large horizontal rectus muscle resections or advancements; 10) the adjacent rectus muscles should be identified prior to operating on oblique muscles to minimize the risk of operating on the wrong muscle; 11) when possible the superior rectus muscle should be recessed rather than the inferior rectus muscle to avoid changing the position of the lower eyelid; 12) caution should be exercised when operating on the superior oblique muscle in a patient with high grade stereopsis because of the risk of inducing vertical or torsional diplopia.

Goals of surgery

The primary goals for strabismus surgery are: 1) to restore binocular vision, 2) to improve ocular alignment, 3) to enlarge the field of single binocular vision, 4) to alleviate an abnormal head position, and 5) to improve the aesthetic appearance of a patient. However, the goals for strabismus surgery vary depending on the age of the patient and the type of strabismus. For a child with infantile esotropia, the primary goal is usually to make the child orthotropic. A secondary goal would be to restore the child’s binocularity. Ideally both of these objectives would be achieved with a single surgery. In the past a small residual angle of strabismus (±10 PD) following horizontal strabismus surgery was considered an excellent result. However, it is now increasingly recognized that this is an unsatisfactory criteria for ‘success’ since even a small angle consecutive exotropia often increases over time and may result in the need for multiple reoperations later in life. For a child with intermittent exotropia, the goal is usually to restore ocular alignment with a single surgery and to maintain high grade binocularity. While this goal may be achieved in a high percentage of patients after a short-term follow-up, the success rate continues to diminish over time as many of these patients develop recurrences of their exotropia and require reoperations. Therefore, a very long follow-up is necessary to determine the ultimate success of strabismus surgery. The goals for a patient with paralytic strabismus are often more modest. For a patient with a sixth nerve palsy, the goal is often only to restore ocular alignment in primary position. If there is complete paralysis of the abducens nerve, it is unrealistic to think that ocular alignment can be restored in lateral gaze. For a patient with a fourth nerve palsy who has large vertical fusional amplitudes, the goal may be to reduce the hypertropia enough that the patient can comfortably fuse their hypertropia. For a patient with complete third nerve palsy, the goal may be only to restore an area of binocular vision in primary position or improved cosmesis. For a patient who has had multiple reoperations, the goals may be to achieve stable ocular alignment with the eyes white, normal lid fissures, and full rotation of the eyes. If patients have unrealistic expectations for strabismus surgery, it is generally better for the surgeon to not proceed with strabismus surgery because of the risk of disappointing the patient.

Preoperative assessment


The preoperative evaluation of the strabismus patient begins with the history, which can be obtained directly from adult patients, and is usually obtained from the parents of children. Family history of ocular or relevant systemic diseases, especially malignant hyperthermia or other anesthetic complications and bleeding diatheses, should be discussed. In children, it is important to obtain information about the pregnancy, perinatal period, and the child’s overall growth and development to help to identify any underlying neurological or genetic diseases. The time of onset of strabismus should also be noted in all patients, and photographs should be reviewed to document the age of onset, characteristics of the strabismus, and any associated anomalous head positions. In older children and adults, it is important to determine what previous treatments have been utilized, and it is helpful to obtain operative reports for prior strabismus surgeries or orbital surgeries. Patients should always be asked about trauma that might have caused intracranial or orbital injuries. In cases of atypical strabismus, patients should be questioned for symptoms of underlying systemic or neurologic diseases such as myasthenia gravis, Graves’ disease, giant cell arteritis, neoplasm, and Chiari malformation. If warranted by the history, the patient should undergo a thorough neuro-ophthalmic evaluation prior to proceeding with strabismus surgery. Available neuroimaging studies should be reviewed and additional studies ordered if indicated.


External inspection is useful to observe visual behavior, obvious misalignment of the eyes, nystagmus, and anomalous head postures, especially in very small children who may become apprehensive and non-cooperative with a formal examination. External inspection should also be used to document facial asymmetry, ptosis, hyper- or hypotelorism, and prominent epicanthal folds, which may give the false appearance of strabismus.

Age-appropriate methods should be used to document visual acuity in all patients. Refractive errors should be corrected prior to proceeding with strabismus surgery. It is essential to perform cycloplegic refraction on all children and on adults with a history of childhood strabismus to identify an accommodative component, which might alter surgical management, since the accommodative component of the strabismus should initially be treated non-surgically. The presence of severe axial myopia may increase the risk of scleral perforation, and should be noted preoperatively.

A complete eye examination should be performed on all patients prior to strabismus surgery. Anterior segment examination including evaluation of the conjunctiva for scarring, which may indicate prior trauma or identify the location of prior extraocular muscle or scleral buckling surgery, and is especially helpful when the history is incomplete. In addition, the presence of a filtration bleb or glaucoma seton may alter surgical planning. Evaluation of the fundus may reveal macular pathology (e.g. epiretinal membranes, macular degeneration), which could impair the patient’s ability to fuse, thereby limiting the success of the strabismus surgery in restoring single binocular vision. In addition, chorioretinal scarring from previous strabismus surgery is important to document preoperatively.

The majority of the preoperative examination should focus on the sensory–motor evaluation. This evaluation should be tailored to the patient and his or her individual problem. It is important to identify whether an abnormal angle kappa or eccentric fixation is present, which can be done by having the patient fix on a light with each eye separately and noting the position of the corneal light reflex relative to the center of the pupil. A normal angle kappa is slightly positive (displaced slightly toward the nose). Ductions and versions are tested, and recorded. Overacting or underacting muscles should be identified and documented. In very young children, or in patients with very poor vision, the Hirschberg light reflex test or Krimsky test is used to estimate the ocular deviation, but when possible alternate prism and cover testing are preferred to document the magnitude of the deviation in the nine diagnostic positions of gaze and at near. In patients with vertical deviations, the alignment should also be tested with right and left head tilt. The AC/A ratio should be assessed in patients with a distance–near disparity with either the lens gradient or the heterophoria method. For patients with intermittent exotropia, the alignment should also be assessed at near with +3.00 D lenses and at distance with −2.00 D lenses to determine if a high AC/A ratio is present. Testing in the far distance and after monocular occlusion is also helpful since it may bring out a larger deviation16. Common patterns that can be elucidated with motility testing include ‘A’ patterns, which are often associated with superior oblique muscle overaction, and ‘V’ patterns, which are often associated with inferior oblique muscle overaction. Other common patterns include ‘T’ or ‘Y’ patterns, ‘X’ patterns, and lambda patterns. Orbital pulley and innervational anomalies can give rise to atypical strabismus patterns17.

Sensory testing can be helpful in predicting postoperative outcomes and in directing surgical management. For example, adult patients with anomalous retinal correspondence should be warned of an increased possibility of postoperative diplopia. Decreasing distance stereoacuity can be an indication that control of an intermittent exotropia is worsening and that surgical intervention is warranted18. Surgery on the superior oblique muscles should be approached cautiously in adult patients with high grade stereopsis because of the risk of inducing vertical or torsional diplopia. The sensory evaluation should be tailored to the individual patient.

Forced duction and force generation testing are especially useful in patients with incomitant strabismus to identify muscle restriction and paresis. Forced duction testing can be performed preoperatively in cooperative patients. The conjunctiva is anesthetized and grasped with toothed forceps, and the examiner attempts to rotate the globe into the field of gaze limitation and notes any restriction encountered. Force generation testing must be performed with the patient awake, and is used to identify whether muscle paresis is present. The examiner notes whether muscle force can be generated when the patient is asked to look into the direction of the paretic muscle.

Orbital imaging is not routinely performed in all strabismus cases, but can be helpful in identifying anomalous anatomy, identifying slipped or lost muscles, or assisting with surgical planning in complicated cases7.

Preoperative care of the strabismus patient

Once the surgeon and patient have agreed upon surgical intervention and the surgical plan has been established, the patient is scheduled for surgery. Although several office visits are sometimes needed to obtain reliable strabismus measurements and establish rapport with children and some adults, in other cases strabismus surgery can be scheduled after a single office visit. Written informed consent may be obtained on the day of surgery, but the communication process must take place before surgery is scheduled.

Patients should be questioned about pre-existing medical conditions and asked about any family history of anesthetic complications such as malignant hyperthermia, pseudocholinesterase deficiency or bleeding diathesis. Patients with complicated past medical histories should undergo a thorough preoperative evaluation by their physician to determine whether it is reasonable for them to proceed with elective surgery. We do not recommend routine blood work on healthy children preoperatively.

Patients or their parents should be provided with clear instructions about the time, date, and location of the surgery, and should be given explicit instructions about preoperative fasting requirements.

If strabismus surgery is planned for only one eye, a mark should be placed above this eye to ensure that surgery is performed on the correct eye. This is generally done at the same time informed consent is obtained. In addition, prior to initiating surgery in the operating room, there should be a call-to-order to confirm the site and procedure to be performed.


Strabismus surgery may be performed using topical, regional, or general anesthesia. In children it is almost always performed using general anesthesia, but it is often possible to perform strabismus surgery in adults using regional or local anesthesia. Pain is produced primarily by pulling on the muscles so, when using local anesthesia, the muscles should be stretched as little as possible. This is more easily performed when a muscle is being recessed than when being resected. Regional anesthesia can be achieved by the irrigation of a local anesthetic agent in the subTenon’s space. A subTenon’s irrigation obviates the many complications that can occur with a retrobulbar injection such as a retrobulbar hemorrhage and injecting anesthetic agent into the optic nerve sheath, which can track back to the brainstem and depress respiration.

Postoperative pain and nausea are common following strabismus surgery particularly when general anesthesia is used. Box 57.1 lists precautions that can be taken to minimize postoperative pain and nausea in children.

Operative techniques

Instruments and techniques of manipulation

A strabismus surgery tray should include a variety of muscle hooks, forceps, needle drivers, scissors, lid speculums, calipers, hemostats, and retractors. The instruments used will depend on the type of strabismus surgery, whether an assistant is available, and the preferences of the surgeon. The lids should be opened with a lid speculum. The most commonly used muscle hooks are the Graefe, Jameson, Green, Manson, and Stevens hooks. Grooved muscle hooks are also available, which can be particularly helpful when sewing sutures into a restricted muscle or a muscle which has previously undergone a large recession. Locking 0.5 forceps are particularly helpful when performing fornix-based surgery. Non-locking 0.3 forceps are preferable for limbal-based surgery. The position of the muscle can be measured with either a standard caliper which measures the tangent or a curved ruler which measures the arc of the tangent. While the curved ruler is more accurate for large recessions, the caliper works best when measuring a muscle to be resected. Both are suitable for small recessions. Blunt Westcott scissors are best used to dissect the intermuscular membrane, while Manson–Ablei scissors are ideal for disinserting a muscle. The long flat blade should be positioned behind the muscle when it is disinserted. Retractors are necessary when operating on the oblique muscles. A Manson hook is particularly helpful when performing limbal-based surgery to dissect the intermuscular membrane off of the muscle. Tucks of the superior oblique muscle require a tendon tucker.

Bridal sutures are helpful to position the eyes when performing limbal-based surgery. 4-0 silk bridal sutures are usually placed at 6 and 12 o’clock 1–2 mm posterior to the limbus when operating on the horizontal rectus muscles. Hemostats are then attached to the silk sutures and the eye is rotated to expose the muscle undergoing surgery. It is important that the silk sutures be passed into the sclera or episclera so the conjunctiva will not be distorted during surgery. When performing strabismus surgery on the superior or inferior rectus muscles using a limbal approach, a single bridal suture is placed directly in the sclera adjacent to the muscle undergoing surgery after creating a peritomy. The eye is then rotated away from the muscle undergoing surgery by attaching a hemostat to the silk suture. When operating on the inferior rectus muscle, a bridal suture is generally placed at the limbus at 6 o’clock and then the eye is rotated superonasally with a hemostat attached to the surgical drapes. When performing surgery on the superior rectus, a bridal suture is placed at 12 o’clock and the eye is rotated either inferotemporally or inferonasally depending on whether access is desired to the temporal or nasal aspect of the superior oblique tendon. It is often helpful to remove the lid speculum when operating on the superior oblique muscle and to use a Desmarres lid retractor to expose the superior oblique tendon.

Forced ductions and traction testing

Traction testing is helpful for diagnosing restricted rectus muscles. It can also be helpful in diagnosing Brown syndrome or a lax superior oblique muscle. When performed under general anesthesia, depolarizing muscle relaxants should be avoided since they can make it more difficult to interpret forced ductions. When testing the horizontal rectus muscles, the conjunctiva is grasped with fine forceps at 6 and 12 o’clock 1–2 mm posterior to the limbus. The globe is then abducted and adducted without retropulsion. It is often helpful to compare the forced ductions in both eyes since a comparison between the two eyes will often allow a subtle difference to be detected. When testing the vertical rectus muscles, the conjunctiva is grasped at 3 and 9 o’clock and the globe is elevated and depressed. The restriction can be confirmed when the muscle is secured on a muscle hook. When testing the superior oblique muscle, the globe is grasped at 4 and 10 o’clock for the right eye and 2 and 8 o’clock for the left eye (Fig. 57.3). The globe is then retropulsed and elevated in a superonasal direction. Once in this position, it is moved back and forth temporally and nasally to determine how taut the tendon is. In an eye with Brown syndrome, it is generally difficult to fully elevate the eye. In an eye with a lax superior oblique tendon the eye can not only be fully elevated, but there is little resistance to rocking the globe while it is retropulsed and elevated20,21. The degree of laxity can be graded and used to determine whether to tuck the superior oblique tendon. Traction testing can result in conjunctival hemorrhages and tears and corneal abrasions, and thus should be performed carefully.

Conjunctival incisions and methods of muscle reattachment

Choice of type and location of incision

Fornix conjunctival incisions are made through the bulbar conjunctiva between adjacent rectus muscles at a point 8–9 mm posterior to the limbus to avoid the extraconal fat pad that begins 10 mm posterior to the limbus22. Fornix incisions are almost always used for surgery on the oblique muscles, and can also be used for surgery on the rectus muscles. Advantages of the fornix incision include access to a greater number of extraocular muscles through a single incision, ease of construction and closure of the incision, less scarring, and greater patient comfort. Disadvantages of the fornix incision include poorer exposure, especially when exposure to the posterior orbit is needed, an increased risk of conjunctival tearing, inability to resect conjunctiva in cases of large rectus muscle resections, and greater reliance on a skilled assistant.

Limbal incision technique

Limbal incisions are commonly used when performing surgery on the rectus muscles (Video: Medial rectus muscle using a limbal incision.). The conjunctiva is grasped with forceps along one border of the muscle undergoing surgery, and blunt Westcott scissors are used to make a radial incision that begins at the limbus and extends about 3–4 mm posteriorly (Fig. 57.5A). It is important to not extend the conjunctival incision to the plica when operating on the medial rectus muscle and to the lateral canthus when operating on the lateral rectus muscle to avoid the formation of symblepharon. The scissors are then used to extend the incision along the limbus through an angle of approximately 3 clock-hours. Tenon’s capsule should be bluntly dissected from the conjunctiva prior to performing the conjunctival peritomy. A second radial incision is then created at the opposite end of the limbal incision (Fig. 57.5B).

Surgery on the rectus muscles

Hooking and dissecting rectus muscles using a limbal approach

Prior to hooking the rectus muscles, the blunt Westcott scissors should be used to separate the intermuscular membrane and Tenon’s capsule from the rectus muscles. A muscle hook with a blunt end (Graefe or Stevens hook) should then be advanced parallel to the margin of the rectus muscle and then rotated behind the muscle. It is important that the toe of the muscle hook be firmly pressed against the globe during this maneuver since the rectus muscle lies flat against the globe and the hook will otherwise only capture Tenon’s capsule. It is also important to consider the varying distances of the rectus muscles from the limbus. In addition, since the nasal edges of the superior and inferior rectus muscle insertions are generally closer to the limbus than the temporal edges, it is generally preferable to hook these muscles nasally rather than temporally. Once it has been determined that the muscle is securely hooked, the intermuscular membrane should be dissected off the insertion of the muscle. This can be facilitated by lifting the conjunctiva overlying the muscle with a Manson double ended strabismus hook (Fig. 57.7). The poles of the muscle should then be inspected to ensure that the entire muscle has been captured by the muscle hook. The Graefe or Stevens muscle hook should then be replaced with a Green or Jameson muscle hook, which have a toe at one end of the hook that will prevent the muscle from slipping off of the hook. When performing a recession, just enough of the intermuscular membrane should be removed to place the polyglactin suture in the muscle insertion. However, a more extensive dissection of the intermuscular membrane is required when performing a resection. In addition when performing a resection, a second Green or Jameson hook is placed to spread out the muscle.

Hooking and dissection of rectus muscles using a fornix approach

Hooking and dissecting the rectus muscles using a fornix incision is performed similarly to that described for a limbal incision. Blunt Westcott scissors are used to separate the intermuscular membrane and Tenon’s capsule in the quadrants between adjacent rectus muscles. A Stevens hook is used to identify the insertion of the rectus muscle, the muscle is placed on a Green or Jameson hook (Fig. 57.8A), and the conjunctiva is reflected over the hook (Fig. 57.8B). Blunt Westcott scissors are used to dissect Tenon’s fascia from the tip of the hook, and a ‘pole test’ is performed by sweeping a small hook around the pole of the muscle to assure that all rectus muscle fibers are contained on the Green hook (Fig. 57.8C). The small hook can then be used to pull Tenon’s fascia away from the rectus muscle so that is can be dissected off of the muscle with blunt Westcott scissors (Fig. 57.8D). For recessions of the horizontal muscles, dissection of the Tenon’s fascia is limited to the anterior portion of the muscles. For resections or for vertical muscles, the muscle is dissected posteriorly to allow for the desired amount of resection, and to separate the vertical muscles from the eyelid retractors.