Ординатура / Офтальмология / Английские материалы / Master Techniques in Cataract and Refractive Surgery_Hampton Roy, Arzabe_2004

SURGICAL PROCEDURE

The Intacs surgical technique is as outlined in Figure 11-5.

Preoperation

30 minutes before surgery

10 mg of Valium (Roche, Nutley, NJ) orally

In both eyes

Drop of proparacaine

Drop of alphagan

Drop of acular

Minutes before surgery

Drop of proparacaine

3 minutes before surgery

Drop of proparacaine

1 minute before surgery

Drop of proparacaine

Prep insert lid speculum

Drop of proparacaine

Preparation

The patient is prepped by washing upper and lower lid and lashes with 5% povidone solution and then carefully blotting dry any residual moisture. The drying is extremely important as we use steri strips to isolate the lashes. Any moisture on the lid or lashes can prohibit the strips from sticking. Improper lash covering exposes the risk of the lashes coming into contact with an instrument or the Intacs themselves. Additionally, uncovered lashes can stroke across the wound and operative field and disseminate bacteria attached to the lashes or oily debris. Because we are inserting an inert object into a open channel in nonvascularized tissue, attention to a “no touch” technique is mandatory. Also, draping the lids in a manner as to isolate the lashes from the field is necessary.

The Intacs procedure has 6 primary steps: the marking of the geometric center, the incision, the creating of the pocket, the dissection of the channels, the insertion of the Intacs, and the closure of the wound.

Ambience

The surgery is performed in a clean minor surgical office room. The surgical team wears surgical scrubs, as well as masks and gloves. The patient dons a bonnet and lays on a small gurney with a special positioning pillow so as to position the iris plane parallel with the floor. A low power binocular microscope with moderate illumination is adequate.

Prior to the preparation, drape, and gloving, the eyelids are retracted with the index finger and thumb of the left hand and a central pachymetry reading is taken for both eyes. Ninety µm are subtracted from the central thickness reading. The resultant number will be used for the diamond knife setting for the singular incision. Alternatively, 68% of the pachymetry reading over the incision site can be used.

The Marking of the Geometric Center

The crossed hair marker is firmly placed centered over the anatomical limbus with the cross hairs lined with the 90and 180-degree meridian (Figure 11-6). Taking care to line the horizontal wire intersecting the inner and outer canthus as well as squaring the vertical wire so that it appears to vertically bisect the upper and lower lid, prevents one from ending up with an incision that tilts nasally or temporally, or that rotates the incision nasally or temporally. Early on, I found the tendency to have my incisions more temporal than nasal. We now have a cross hair reference point for the incision marker, which is now applied having been coated with gentian violet. Keep in mind the orientation of the wire cross hair imprint on the cornea as you line up the “superior T” of the marking apparatus. It is relatively easy to get too much gentian violet on the marker and one can end up with

Figure 11-6. Marking the cornea.

Figure 11-7. The incision.

Figure 11-8. The incision (ctd.).

a wide smeared incision mark that is confusing when trying to decide on and make the incision.

The Incision

The incision is made with a double-sided diamond corneal incisional blade.

I use a “dual track” knife to make the 1.8-mm radial incision. The same advantages gained by employing this knife in any corneal incision technique are appreciated. Namely, the ability to cut a square edged even depth incision while avoiding central encroachment (Figure 11-7) is expedited as shown in Figure 11-8. The making of the incision is one of the critical surgical steps that needs undivided attention to the following details (Figure 11-9).

The knife is held perpendicular and pressed firmly straight down until one sees and feels the foot plate indenting the cornea surface.

The knife is moved posteriorly 1.8 mm. Direct visualization of this end point is difficult if one keeps the knife per-

Figure 11-9. The incision (ctd.).

pendicular. For this reason, practice making this length of incision on some practicing media (a boiled egg works satisfactorily) and noting the relationship of the front of the diamond foot plates to an easily visualized landmark on the cornea marks. This way, you’ll know that, for example, when the front edge of the footplate is halfway down the incision mark, one has made a 1.8-mm incision. When the front of the footplates reaches this mark, one had made a 1.8-mm length incision (Figure 11-9).

When the posterior destination with the diamond knife has been reached, the diamond is then pushed centroid until the dull part of the leading edges is stopped (Figure 11-10). One then knows that the endpoint of the incision traverse has been reached. This makes sure that one has a squared incision front and back and the incision is long enough to accept the glide guide as well as the glide dissectors.

The incision should never come closer than within 1 mm of the limbus.

Figure 11-12. Pocketing hook.

INCISIONAL ISSUES

If the incision is too short, one will struggle with the proper depth and size of the pocket. One then gets tight strangulated guide passage with a subsequent torquing of the blades. This can be compounded by weak suction, and this can result in a decrease diameter of the optical zone. Also, one can get an elliptical appearing placement of the Intacs that will cause some undercorrection and/or induced astigmatism.

If the incision is too long, one can have induced astigmatism against the rule due to relaxation of the “annulus tension.” Also, one can cut into the perilimbal blood vessels and end up with a pathway for vascular ingrowth.

If the incision is too shallow:

•Mildly too shallow—One can get under correction as the Intac does not support enough tissues at the periphery to get adequate peripheral lift and cord shortening to induce adequate flattening.

•Significantly too shallow—One gets photophobia, induced thinning over the segments and decreased effect. These Intacs need to be removed.

The Creation of the Pocket

Once an incision of proper depth and length is made, the creation of the pocket becomes the next focus. Most Intacs surgeons have moved to using the “prolate” system of intacs surgery. In doing so the awkward pocket dissector has been eliminated and we use the pocket hook (often referred to “a Sinskey on steroids”) exclusively to create the pockets.

If the incision is of proper length, one should be able to place the pocket dissector directly into and to the base of the incision without using any torque or tilt. I slightly elevate the toe 10 degrees up so that I go directly down and can feel a real end point when the heal reaches the bottom of the incision. This confident end point reinforces the feeling that an adequate depth has been achieved. I now tilt the toe down from its elevated angle so that the pocket dissector is fully in contact with the bottom of the incision along its entire length (Figures 11-11 and 11-12).

One is now ready to create the semicircular “pocket” that will serve as the gateway for the channel dissectors to create the intac channel. I like to visualize the pocket and its subsequent right and left channels as the sleeve of a jacket or sport coat. If the entry way at the shoulder is adequate and easy access to the sleeve, finding the gateway with one’s hand becomes easy, thus facilitating the sliding of one’s arm down the sleeve with undue impediment. We have all experienced trying to put our arm in a coat whose sleeves are unduly awkward to get to or off-centered. The frustration of not being able to slide one’s arm down the sleeve transforms the somewhat simple task of putting on a coat from a unconscious automatic task to a frustrating wrestling match often entailing stripping mechanical change and injury to the coat sleeve. This same aggravation can come to past in intacs sur-

Figure 11-13. Placing Intacs inserts.

Figure 11-14. Placing Intacs inserts (ctd.).

Figure 11-15. Suturing.

gery if one has an inadequate pocket down in which to slide the guide, dissector, and finally the intacs themselves.

Once the pocket dissector is parallel to the endothelium a twisting windshield wiper force is employed. This is effected by twirling the handle of the dissector between your fingers and your thumb. If some form of countertraction is not employed, the eye will simply move with you and your dissection will be inadequate.

Some surgeons use countertraction with a forceps to facilitate the dissection. However, I have found that having the patient look opposite of the direction in which I am attempting to make the dissection to be a most effective means establishing the necessary force. In addition, one does not stimulate a painful response from the patient that frequently occurs when one grasps the conjunctiva with toothed forceps for counter traction. I also come slightly posterior on each end of the arc so as to effect a pocket of 245 degrees.

Intacs Insertion

The intacs are grasped with the special insertion forceps and slid into their respective channel. The final length is pushed away from the incision. It is critical that either segment end does not lie under the incision (Figures 11-13 and 11-14).

Incision Closure

A single 10-0 nylon suture is placed to close the incision. The tightening knot should be buried. Avoid overtightening. The suture is removed from 1 to 4 weeks (Figure 11-15).

Postoperative Care

Antibiotic steroid drops are used 4 times daily for 1 week. Additionally we use a non-steroidal anti-inflammatory drop 4 times daily for the first 72 hours. Oral analgesics are used the first evening of surgery as needed. Patients are encouraged to use light ice packs for 15 minutes out of each hour for the first 4 hours for additional discomfort.

Outcomes

A comparison of the original FDA study group as well as over 900 eyes after approval is seen. Almost 80% of the patients achieved 20/25 or better visual acuity in both groups (Figure 11-16).

Keratoconus and Ectasia

Joseph Colin, MD, reported success in using intacs to treat contact lens intolerant patients with keratoconus.

When the intac is inserted into the ectatic kerataconic cornea and a new limbus at the 8-mm corneal diameter is created. The arc length is shortened thus flattening the cornea as well as symmetrically uplifting the sagging corneal apex back toward its centroid position. When we next insert

superiorly, the second intacs, the “fabric of the cornea” is “pulled” superiorly further rounding the cornea and moving the center of the apex of the cornea toward the optical axis. A new contour is induced, providing a more stable platform for contact lens fitting.

Intacs has also been used successfully to stabilize ectasia following LASIK and PRK.

Intacs Removal

In the event the intacs must be removed, the patient is prepped and draped similar to the initial placement procedure, and inserts can be removed in just a few minutes. The corneal topography and refractive status returns to the original status stabilizing within 3 months.2

THE INCISION

For inserts in place less than 6 months, re-open the original incision with a Sinskey hook by simple blunt dissection through the epithelial layer.

For inserts in place for longer than 6 months, or in individuals demonstrating aggressive incision healing, re-open the original incision to a depth of approximately 100 µm using a guarded diamond knife. A Sinskey hook can then be used to achieve blunt separation of the stromal layers to the full depth of the incision by gently stroking back and forth the full length of the incision until containing the original base.

THE CHANNEL

Locate appropriate channel depth by gently depressing the floor of the incision with the Sinskey hook.

With the Sinskey hook at the incision floor, exert minimal pressure to one side of the incision by rotating the tip of the Sinskey hook to separate intrastromal layers previously delaminated. Perform this maneuver on both sides of the incision.

Bluntly separate the channel with the Sinskey hook while advancing along the channel toward the insert. (The Sinskey hook is the same position used to advance the inserts after initial placement.)

LOCATING THE INTACS INSERTS

“Clean” the presenting face of each insert to remove any re-apposed stromal tissue by carefully rotating the tip of the Sinskey hook 2 clock hours above and below each insert-positioning hole

Engage the positioning hole with the tip of the Sinskey hook by placing the Sinskey hook underneath the insert and rotating the tip “up” into the positioning hold

Gently begin to pull and rotate each insert out of its respective channel

Figure 11-16. Month 1 comparison.

If necessary, have patient move eye in opposite direction of intacs inserts rotation

Repeat “cleaning” maneuver as necessary

INCISION REPAIR

Hydrate incision size and channel with BSS using an irrigating cannula

Suture wound with single 10-0 nylon suture

Remove suture after 2 weeks

This concept is under high power has the tectonic property of:

Increase thickness generates increased corneal flattening

Unique flattening as it shortens the corneal cord length and produce flattening across entire cornea

In contrast excimer causes preferential flattening in the center of the cornea. Intacs maintain the positive asphericity of the cornea. The central 3 to 4 mm of the cornea is still steeper than the midperipheral zone. Typically one sees a blue zone inside the edge of the segments with a zone that is slightly less blue just central to that.

REFERENCES

1.Colin J, Cochener B, Savary G, et al. Intacs inserts for treating keratoconus. Ophthalmology. 2001;108:1049-14.

2.Clinch TE, Lemp MA, Foulks GN, Schanzlin DJ. Removal of intacs for myopia. Ophthalmology. 2002;109:1441-1446.

The exiting field of refractive surgery has advanced geometrically over the past 25 years. This growth may be due to a combination of factors, while increased safety and precision are probably the most influential. Other contributing factors have increased consumer awareness; the increased number of refractive surgeons and laser centers, as well as the expanded approval of new technology and treatment modalities.

Because the surface of the cornea provides approximately two-thirds of the refractive power of the eye, changing its curvature provides the most common means of changing the eye’s refractive error.

Refractive surgery for myopia can be divided into several categories. Incisional surgery includes those procedures that involve partial thickness incisions in the cornea such as RK and AK. Lamellar surgery involves removal of tissue parallel to the surface of the cornea to change its curvature. Intraocular surgery generally involves the introduction of a high refractive index lens into the anterior or posterior chamber with or without removal of the crystalline lens. This chapter will limit the discussion to lamellar surgery, namely LASIK, LASEK, and photorefractive keratectomy (PRK).

Myopia is a condition with an overall incidence of 25% in the general population.1 In myopia, the parallel rays of light entering the eye are brought into focus at a location anterior to the retina. The total refractive power of the eye is greater than that required for emmetropia; this is due to either a steep cornea or a long eye. Correction of myopia with spectacles, contact lenses or refractive surgery allows the incoming rays to come into clear focus in the retina.

The normal cornea has a prolate shape (greater curvature centrally than peripherally) (Figure 12-1). Laser vision correction procedures reverse this natural prolate shape of the cornea and decrease the central corneal curvature (to create an oblate shape) (Figure 12-2).

PATIENT SELECTION

Many patients arrive to the surgeon’s office well informed about refractive surgery having read professional or lay literature. It is important for the surgeon to be able to provide the patient with current information for relevant procedures so that the appropriate procedure can be selected during the surgeon-patient interaction.

The first step in the evaluation should be to determine the goals that a patient has in seeking refractive surgery. Also

Figure 12-1. Normal corneal imaging with Orbscan (Bausch & Lomb, Rochester, NY). Note symmetry of the corneal anterior surface elevation, anterior surface curvature, posterior surface, and pachymetry.

important is a review of ocular and systemic conditions. Visual acuity is measured using manifest and cycloplegic refraction. Pupil size, ocular dominance testing, and distance and near vision with and without correction should also be documented. Anterior and posterior segment examinations are performed to rule out other conditions that may adversely affect the surgical result. Glaucoma is more common among myopic patients than in the general population.2 A careful assessment of the optic nerve and measurement of the intraocular pressure are also necessary.

Pachymetry measurements are performed to make sure that the cornea is of normal thickness. Computerized corneal topography is now used routinely in the assessment of preoperative and postoperative refractive surgery patients. This can help to screen for subclinical keratoconus or other corneal diseases (Figure 12-3). Extreme keratrometric values (flatter than 41.00 or steeper than 47.00) or abnormal corneal thickness should be identified. Rigid contact lens wearers should be out of their contact lenses for 3 to 4 weeks, and soft contact lens wearers need 2 weeks without lenses. Wavefront analysis is increasing in usage, as the ability to treat higher order aberrations improves.

The possibility of monovision should be discussed with patients near the presbyopic age. A discussion on glare and halos, the possibility of under and overcorrection, as well as any special considerations should take place with the patient. Appropriate reading materials are given to the patient for education.

This initial examination is a good occasion to counsel and assess the patient’s goals to make certain they are realistic. Informed consent should include a discussion of the most frequent side effects and potential risks involved with the surgery.

Figure 12-2. Post-LASIK corneal imaging with Orbscan. Note the symmetry of the flattening of the anterior curvature.

Figure 12-3. Keratoconus suspect imaging with Orbscan. Note the inferior steepening associated with inferior thinning and inferior elevation.

PRK

PRK was developed in the late 1980s as the first laser vision correction procedure. In October of 1995, PRK became the first FDA-approved laser treatment for the correction of myopia and eventually myopic astigmatism.

The excimer laser is used to reshape the surface of the cornea by removing anterior stromal tissue. The process by which the excimer laser removes corneal tissue is nonthermal ablative photodecomposition. Photons at extremely high energy are emitted towards the corneal tissue molecules and cause ejection of the fragments without thermal damage.3

Laser delivery patterns include broad beam, scanning slit, and flying spot. Broad beam lasers deliver a particular diameter beam of laser through a diaphragm that can expand or contract to modulate the beam size. Typically, the beam starts small and expands as the laser is delivered. The main advantage of broad beam lasers is a shortened operative time,

which results in less time for stromal hydration to change throughout the procedure. The main disadvantage is that broad beam lasers resulted in central islands because the emitted laser plume masked the cornea from successive laser pulses. New laser software addresses this by applying more treatment to the central cornea. Scanning excimer lasers including scanning slit and flying spot provide a smoother ablation than the older broad beam lasers. Additionally, the profile can produce aspheric ablations and larger diameter ablations. Scanning lasers can achieve any ablation profile, which is an advantage for irregular or asymmetric corneas.4 Some lasers such as the VISX (Santa Clara, Calif) system have a combination of mechanisms that allow for large and small treatment areas through a system termed variable spot scanning. This combines the advantage of a shorter treatment time by treating large areas all at once, as well as the flexibility of treating smaller areas asymmetrically when needed with a small diameter beam.

An important terminology in refractive surgery is the optical zone size and entrance pupil. The pupil that is seen when looking at an eye is termed the entrance pupil, which is approximately 0.5 mm anterior to and 14% larger than the real pupil.5 It is a virtual image of the real pupil formed by the cornea. There is an ongoing debate within surgeons regarding the best point to use for centration during the refractive procedure. Some surgeons use the corneal intercept of the visual axis which is the point where the cornea meets the line joining the fixation point to the fovea while other use the entrance pupil or the corneal light reflex.5

Advancement in technology continues to improve refractive outcomes. Eye tracking devices rely on infrared lasers or cameras to follow small eye movements and move the laser ablation beam accordingly. Preliminary studies have shown better UCVA, BCVA, and centration with eye-tracking devices.6,7 Corneal ablation patterns that may be different based on the specific optics of the eye in a manner more specific than sphere and cylinder are now becoming a reality.8,9 Time will tell whether these formats will provide better visual results.

The best results with the lowest incidence of complications occur in the lower ranges of myopia and astigmatism.10 In Phase III trials for PRK with the Summit ExciMed UV200LA laser (Summit Technology, Waltham, Mass), approximately two-thirds of the patients had 20/20 or better uncorrected acuity, >90% were 20/40 or better uncorrected, and 77.8% had postoperative refractions within 1.00 D of the target outcome.11

The higher the refractive error, the greater the chance of regression and corneal haze.12 The depth of the ablation that is required to achieve a given refraction result for myopia is defined by the Munnerlyn equation:13

•Equation 1: depth (mm) = [diameter (mm)] 2 x 1/3 power (D)

Smaller OZs are associated with greater degrees of regression, as well as haze.14 The optical zone size and depth must be optimized to avoid excessive wound healing that occurs in deep ablations and the excessive haloes, edge glare, and irregular astigmatism found with small optical zones.15

PRK has been supplanted by LASIK as the predominant refractive procedure. However, there are situations where PRK may be preferred to LASIK. These situations include patients with anterior basement membrane dystrophy (ABMD), corneal thinning, small and deep-set orbits, superficial corneal scars, very steep or flat keratometry values, anterior scleral buckles, glaucoma patients after trabeculectomy, optic nerve disease, risky occupation or activity, and corneal ectasia.

Indications

The principles applicable to patient selection for excimer laser PRK are no different than those for any other refractive procedure. Two fundamental criteria are:

•Realistic expectations, motivation, and awareness of potential complications and side effects

•Stability of preoperative refractive error

Excimer laser photorefractive keratectomy results appear to be more reproducible for patients who have lower amounts of myopia.16,17

Contraindications

Laser vision correction has a higher risk in patients with collagen vascular, autoimmune, or immunodeficiency diseases; women who are pregnant or nursing; patients with signs of keratoconus and patients taking isotretinoin (Accutane, Roche, Nutley, NJ) or amiodarone (Cordarone, Wyeth, Madison, NJ).

Other conditions with potential adverse outcome include: ophthalmic herpes simplex or herpes zoster, or other systemic diseases likely to affect healing such as diabetes and atopic disease.18 In patients with progressive myopia or astigmatism, the results will not be stable.

Surgical Treatment

PRK is performed under topical anesthesia with the patient under the microscope. The patient should be relaxed but not oversedated. Communicating to the patient what will occur during the procedure will alleviate much of the patient’s anxiety. Preoperatively, the patient may receive antibiotic, corticosteroid, nonsteroidal, and anesthetic drops. The contralateral eye is taped shut so that the patient does not crossfixate. A speculum is used to keep the eyelids open.

Figure 12-4. Cellulose sponge disc with alcohol is placed over the cornea for 20 to 60 seconds to loosen the epithelium before removal for PRK or LASEK.

EPITHELIAL REMOVAL TECHNIQUE

Quick and precise epithelial removal is critical for a good PRK result. The central corneal epithelium can be removed by a variety of available techniques, including manual scraping, mechanical rotating brushes, laser ablation combined with manual scraping (laser-scrape), or laser ablation alone (transepithelial approach). Debridement should take as little time as possible to avoid corneal hydration changes that may affect the outcome.

1.Alcohol removal is our current preferred technique. Absolute alcohol is mixed with balanced saline solution to dilute to 20% and is applied to the corneal epithelium on a 7 to 10 mm cellulose sponge disc for 20 to 120 seconds (Figure 12-4). The epithelium can then be removed without resistance. A metal 7 to 10 mm optical zone marker can alternatively be used to hold the alcohol within the marker followed by irrigation to help lift the epithelium (Figure 12-5).

2.Mechanical debridement consists of removing the epithelium bluntly with a blade, or disposable excimer spatula (Figure 12-6). A relatively blunt instrument helps prevent cutting Bowman’s membrane.

3.Laser-scrape technique uses the laser to remove 40 mm of the epithelium followed by the use of a blade or excimer spatula to remove the remaining debris.

4.Transepithelial laser removal uses the excimer laser set to a depth of approximately 50 mm (200 pulses) with the beam set to its widest aperture. There is evidence that the incidence of haze is less with this method.6 Many lasers do not allow this option for large treatment diameters.

Figure 12-5. Optical zone marker can also be used to hold alcohol before epithelial removal with PRK or LASEK.

Figure 12-6. Mechanical debridement of corneal epithelium using PRK spatula.

5.Rotary brushes are 6.5 and 9 mm in diameter and are used to remove epithelium.

The surgeon should always verify the entered computer data before starting the ablation (Figure 12-7). For PRK, the spectacle correction is adjusted to the corneal plane to take into account the vertex distance. The microscope should be focused on the corneal surface. The patient should be instructed to fixate on the target light and adequate centration over the pupil should be maintained at all times. This centration is essential in order to achieve the expected visual results (Figure 12-8). Tracking systems are now incorporated in most excimer lasers, which aid in the maintenance of centration.