6

LASEK Techniques

Chun Chen Chen, MD

Taipei Municipal Jen-Ai Hospital,

National Yang-Ming University

Taipei, Taiwan

Joel Javier, MD, and Dimitri T.Azar, MD

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

Harvard Medical School

Boston, MA

In this chapter, we will review the LASEK techniques that are commonly used at the time of writing. We also compare the Azar technique, illustrated in Figure 1, with other LASEK techniques, described in chapters 7 through 11. We also review the preoperative medications, patient preparation, effect of alcohol on corneal epithelial sheet preparation, and repositioning of the epithelium on the stromal bed after excimer laser surface ablation.

PREOPERATIVE MEDICATION

Topical Anesthetics

There are several kinds of topical anesthetics that can be used, including tetracaine 0.5%, proparacaine 0.5%, and xylocaine 2% to 4%. Tetracaine is more epitheliotoxic, which may aid epithelial debridement. Xylocaine has slower onset but may have prolonged action. Proparacaine is less painful than teracaine on instillation.

Topical NSAIDS

Topical NSAIDs could be used both preoperatively and postoperatively for relief of pain. Diclofenac sodium 0.1% (Volteran, CIBA) and ketorolac tromethamine 0.5% (Acular, Allergan) are two common topical NSAIDs. However, there are some adverse effects of excessive use of topical NSAIDs, including toxicity and delayed epithelial healing.

Topical Antibiotics

The broad-spectrum antibiotics are used for prophylaxis of infection. A combination of antibiotic-steroid preparation such as TobraDex (Alcon) is commonly used. The disadvan-

LASEK, PRK, and excimer laser stromal surface ablation 54

Figure 1 Azar LASEK technique. (From Taneri S, Zieske JD, Azar DT. Evolution, Techniques, Clinical Outcomes, and Pathophysiology of LASEK: Review of the Literature. Surv Ophthalmol. NovemberDecember 2004 In press)

tage of tobramycin is the reduced effectiveness against streptococcal bacteria, although there is excellent staphylococcal and pseudomonal coverage. The introduction of fluoroquinolones offers the increased broad-spectrum coverage and low toxicity. Fluoroquinolones could be used in combination with aminoglycosides or steroids.

Application of Topical Medication

Preoperatively, drops are instilled at 5-to 10-minutes intervals starting proximately 15 to 30 minutes before the procedure. Topical anesthetic drops will be applied to both upper and lower fornix. It is helpful to achieve the even distribution of the anesthetic medication by asking the patient to look to the left and right directions. Topical anesthesia can also be applied by means of an anesthetic-soaked surgical spear to the upper fornix or conjunctival limbus to avoid excessive medication over epithelium. It is

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also useful to instill a few drops of topical anesthetic into the fellow eye to reduce blepharospasm and further improve the cooperation and fixation of patients.

PREPARATION OF THE EYELIDS

The periocular skin is cleaned with a solution of betadine, being careful not to allow it into the eye, because it is toxic to the epithelium. The skin is then dried with sterile gauze. Trimming the lashes is almost never necessary because of the use of adhesive plastic drapes.

PATIENT POSITIONING AND PREPARATION

Once the laser is programmed, the patient can be brought into the laser suite. Minimizing the actual time the patient is underneath the laser helps alleviate anxiety; thus, all preparation of the laser should be performed before the arrival of each patient. The name and operative eye should be confirmed. In presbyopic patients, the question of targeting monovision should be verified.

The patient should be positioned on the operating chair/table in a comfortable position that will ensure a stable, immobile, and appropriate position for the entire procedure. The patient should be advised to assume a comfortable and quiet position and not to move the arms or legs during the operation. The patient’s head should be positioned so that the invisible line that connects the forehead and chin through the center of the nose is perpendicular to the operating microscope and laser beam.

The fellow eye should be covered, ideally with a solid eye shield instead of a pressure patch. This allows the patient to keep both eyes open during the procedure to avoid a bell’s phenomenon in the operative eye.

Application of the Drape

To cover the eyelashes and skin, a drape with a central fenestration is preferred. The patient is asked to open the eyes wide. The drape is applied to the lower eyelid, including the eyelashes. The same procedure is performed on the upper eyelids.

Placement of the Speculum

The ideal speculum should provide maximal access to the globe, allow for temporal and superior surgical approaches, and provide maximal patient comfort when fully opened. The Liebermann adjustable wire speculum is a popular design that provides constant exposure and is quite resistant to squeezing of the patient. The unique 45-degree angle contours to the facial angle on the temporal side, permitting accessibility for temporal and superior techniques. The Machat adjustable wire speculum is a variation of the Liebermann design with increased angle of the arms to maximize the exposure of the globe.

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The surgeon frees and opens the two arms of the speculum using the adjustment screw to adapt it to the eyelid opening of the patients. The arms of the speculum are introduced individually under the edges of the eyelids, assisted by lid retraction, and avoiding contact with the corneal surface. The instrument is centered within the inter-palpebral space, and then the screw is turned until the arms sufficiently open the eyelids. The speculum must provide perfect tension to maintain centration and balance.

FLAP MARKING

The marking of the cornea allows the surgeon to have the precise reference points to realign the flap in the corneal beds. The cornea could be marked peripherally with pararadial lines or circles.

The marking instruments that are frequently used are Ruiz paracentral, Buratto, Machat, and Slade markers. They are wiped with gentian violet marking pad, such as Preinked Marking Pad (Visitec), and then placed on the peripheral cornea. Excessive use of gentian violet should be avoided, because this has been associated with corneal toxicity.

During the LASEK procedure, the flap adhesion to the corneal stromal bed is crucial for rapid epithelial healing and visual recovery. Meticulously marking of the cornea is important to facilitate perfect realignments of LASEK flap. A floral pattern by overlapping 3-mm circles around the corneal periphery is demonstrated by Azar et al. (Fig. 2A and Fig. 2B) (1).

CHEMICAL AGENTS FOR EPITHELIAL REMOVAL

Chemical agents have long been used to remove the corneal epithelium (2). Citron et al. found n-heptanol to be superior to scraping in terms of preservation and smoothness of the basement membrane after removal of the corneal epithelium in rabbit (3). Hirst et al. evaluated chemical versus mechanical removal of the epithelium in the monkey and rabbit and found iodine-cocaine and n-heptanol to be equivalent to mechanical scraping

(4).

For centuries, the alcohols have been appreciated for their antimicrobial properties (5). They are fast-acting, barely toxic with topical application, nonstaining, nonallergenic, and readily evaporate. For infection control purposes, ethyl alcohol (ethanol) and isopropyl alcohol (isopropanol) are alcoholic solutions most often used. Their antimicrobial efficacies are enhanced in the presence of water, with optimal concentrations being from 60% to 90% (7). The exact mechanism by which alcohols destroy microorganisms is not fully understood. The most plausible explanations for the antimicrobial action are coagulation (de-naturation) of proteins, e.g., of enzymatic proteins, and lipid dissolution, leading to the loss of specific cellular functions. Because the outer cellular structures are quite different between the eukaryotic and prokaryotic cells (cell membrane and cell wall, respectively), human cells are more resistant to alcohol as compared with microbial ones

(8).

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using a sponge soaked in 100% ethanol for 2 minutes and warned against its use (10). Agrawal et al. found increased anterior keratocyte loss and inflammation when using 70% isopropyl alcohol applied for 2 minutes (11). Helena et al. described increased keratocyte loss but decreased inflammation using 50% ethanol for 60 seconds (12).

EPITHELIAL SURVIVAL AFTER ALCOHOL APPLICATION

Concentrations of ethanol ranging from 10% to 30% are widely used to remove the corneal epithelium before PRK (9). Stein et al. reported that using dilute alcohol (25%) in 91 cases of PRK was a safe, effective, and predictive method of removing the epithelium (13). Abad et al. found that chemical de-epithelialization with dilute ethanol (18%) appears to be safe and effective and might promote faster rehabilitation (14). Managing the corneal epithelium as a hinge flap with 20% ethanol is a safe technique with faster visual rehabilitation and reduced haze compared with debridement of epithelium with alcohol (15). Gaber et al. used 0.1% tryphan blue to test the viability of the epithelial flap of human cadaver eyes after alcohol treatment. They observed the epithelial cells were vital up to 45 seconds of 20% ethanol exposure (16).

We have detected a doseand time-dependent effect of dilute alcohol on cultured corneal epithelial cells (5). The 25% concentration of dilute alcohol was the inflection point of epithelial survival (Fig. 3). Significant increase in cellular death occurred after 35 seconds of 20% alcohol exposure (Fig. 4). Forty seconds of exposure further increased apoptosis after 8 hours of incubation (Fig. 5). These findings are consistent with the clinical observations of varied epithelial attachment to the stromal bed after LASEK surgery.

Preparation of Ethanol

The ethanol could be diluted in distilled water (1,5,9,14,15,17–19) and balanced salt solution (BSS) (13,20,22). Theoretically, the ethanol would be more effective in water solution. However, the epithelial protection may be greater in BSS. At this time, it is not clear whether modification of the preparation of dilute alcohol would allow for better cell survival and adhesion in vivo (5).

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Figure 3 Fluorescein viability stain with calcein acetoxymethyl ester (AM)/ethidium homodimer of the cells after (A) 10%, (B) 20%, (C) 24%, (D) 25%, (E) 26%, and (F) 40% EtOH- H2O treatment for 20 seconds. Metabolically active cells convert nonfluorescent calcein-AM into green fluorescent polyanionic calcein and exclude ethidium homodimer (A). Damaged cell membranes allow permeation of ethidium homodimer

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and its binding to nucleic acids resulting in red fluorescence (F). Bar=50 µm. (G) Cellular survival after different concentrations of alcohol treatment for 20 seconds. The percentage of viable cells (with exclusive green fluorescence) was calculated by counting cells per 10 fields at 400× magnification. (From Chen CC, Chang JH, Lee JB, Javier J, Azar DT. Human corneal epithelial cell viability and morphology after dilute alcohol exposure. Published courtesy of Invest Ophthalmol Vis Sci 2002; 43(8):2593–2602.)

The method of preparing 18% ethanol is drawing 2 mL of dehydrated alcohol (American Reagent Laboratories, Shirley, NY) from ampules into a 12-mL syringe. Add the sterile water for injection to 11 mL and mix well (1).

Although it has been advocated that plastic syringes should be avoided for storage of alcohol because of the risk of contaminations by toxic monomers (17), we have used them successfully for the installation of alcohol through an irrigation and aspiration (I & A) trephine instead of the hollow metal handle of Carones LASEK pump (Fig. 6).

TREPHINATION

Purpose

The purposes of trephination are:

1.Delineation of the epithelium

2.Pre-incision of corneal epithelium

3.Reservoir of alcohol

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Figure 4 Fluorescein viability stain with calcein-AM/ethidium homodimer of cells exposed to 20% EtOH-H2O for

(A) 20, (B) 25, (C) 30, (D) 35, (E) 40, or (F) 45 seconds. Calcein-positive- green fluorescence indicates metabolically active cell, and ethidium homodimer positive-red fluorescence indicates damage to the cell membrane and binding to nucleic acids. Bar=50 µm. (G) Cellular survival with different exposure times. The percentage of viable cells was

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calculated from the number of green, red, and bicolored cells counter per 10 fields at 400× magnification. Control group was treated with 100% KSFM (0% ethanol). (From Chen CC, Chang JH, Lee JB, Javier J, Azar DT. Human corneal epithelial cell viability and morphology after dilute alcohol exposure. Published courtesy of Invest Ophthalmol Vis Sci 2002; 43(8):2593– 2602.)

Sizing

The size of the epithelial trephines should correspond to the optic zone of laser ablation. The various sizes of the trephines are presented in Table 1

Success

Carones LASEK Pump OZ Chambers (Fig. 6)

Optic zone (OZ) chambers have semi-sharp edges to delineate the epithelium. By centering the OZ chamber on the cornea, the epithelium is delineated on the edges by moving the chamber clockwise and counterclockwise. The pump dispenses premeasured amount of fluid into an optic zone chamber. After the push of a button, the pump dispenses a premeasured amount of dilute alcohol into the OZ chamber.

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H2O exposure was detected at 24 hours of incubation (C) (58.05±33.10) and after 40 seconds of EtOH-H2O exposure at 8 hours of incubation (D) (94.12% ±1.21 %). Substantially lower TUNEL positivity was seen after 8,12, and 24 hours of incubation with EtOHKSFM for 40-seconds (G: 0.65±0.02%, H: 7.11±1.49%, I: 4.52±1.05%). (J) TUNEL positivity after 8, 12, and 24 hours of incubation of 20% EtOH-H2O for 20 and 40 seconds and 2% EtOH-KSFM for 20 and 40 seconds compared to controls. Control groups were treated with 100% KSFM for 20 seconds. (From Chen CC, Chang JH, Lee JB, Javier J, Azar DT. Human corneal epithelial cell viability and morphology after dilute alcohol exposure. Published courtesy of Invest Ophthalmol Vis Sci 2002; 43(8):2593–2602.)

Table 1. Sizing Profile of the Trephines.

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Figure 10 Our current LASEK technique. (A) Multiple marks are applied around the corneal periphery, simulating a floral pattern. (B) An alcohol dispenser consisting of a customized 7-or 9-mm semi-sharp marker attached to a hollow metal handle serves as a reservoir for 18% alcohol. Firm pressure is exerted on the cornea and alcohol is released into the well of the marker. (C) After 25 to 30 seconds, the ethanol is absorbed using a dry cellulose sponge. (From Taneri S, Zieske JD, Azar DT. Evolution, Techniques, Clinical Outcomes, and Pathophysiology of LASEK: Review of the Literature. Surv Ophthalmol. NovemberDecember 2004 In press.)

ALCOHOL CIRCULATION

Alcohol will stay in the barrel of the trephine for 20 to 40 seconds (Fig. 10A and Fig. 11). Young patients or contact lens wearer may require longer time for the dilute ethanol to detach the epithelium. Firm pressure will be applied firmly to avoid alcohol leakage. After certain exposure periods (for example, 25 seconds), the ethanol is

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Figure 11 Alcohol circulation.

Figure 12 Alcohol absorption.

absorbed using a dry cellulose sponge (Weck cell or Merocel; Xomed, Jacksonville, FL) on the alcohol reservoir (Fig. 10B and 12). The corneal and conjunctival surface could then be rinsed thoroughly with balanced salt solution to prevent alcohol spillage onto the epithelium outside the barrel. Camellin et al. introduced a method of irrigation with

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The Vinciguerra-Carones LASEK spatula is specially designed to engage the edge of epithelial flap for easy lifting of the epithelial flap (Fig. 14B). The vaulted curve of the spatula matches the curve of the cornea. This curved spatula allows for easy placement of the flap after laser ablation and removes any wrinkle formation as the flap is placed.

The epithelium is detached with the hockey spatula or epithelial microhoe (Fig. 14C, D). The Janach epithelial micro-hoe is used to complete the incision. The epithelium is detached by making tiny movement almost perpendicular to the margin.

Personal Experience (Azar’s Technique) (1,5)

After anesthesia and application of a lid speculum, the cornea is marked with overlapping 3.0- mm circles around the corneal periphery. An alcohol dispenser consisting of a customized 9.5- mm semi-sharp marker, attached to a hollow metal handle, with a reservoir for the 18% alcohol, allows irrigation/aspiration of the alcohol after applying firm pressure on the central cornea. After 25 to 30 seconds, the solution is absorbed using the suction port and a dry cellulose sponge. One arm of a jeweler’s forceps (Fig. 15) or the Azar LASEK scissors (ASICO, right and left, Fig. 15), is inserted under the epithelium and traced around the delineated margin of the epithelium, leaving 2 to 3 clock hours of intact margin (Fig. 1 and Fig. 10A–C). The loosened epithelium can also be peeled as a single sheet using a Merocel sponge (Fig. 17), leaving a flap with the hinge still attached (Fig. 18). A hydrodissection cannula can be used to assist with lifting the epithelium (Fig. 16C).

Alternative Techniques

In Camellin’s technique, the precut margin is lifted with a hockey spatula to detach the epithelium, and the epithelial flap is gently detached, gathered, and folded up at the 12- o’clock position. Because the preincision is not always perfect, an epithelial microhoe is used to complete it. The hoe is pressed firmly downward and pulled approximately 1 millimeter toward the pupil center. Alternatively, the epithelium is detached with the short side of a hockey spatula, making tiny movements almost perpendicular to the margin. The flap is generated and completed along the entire arc of trephination up to the hinge (17–19).

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Figure 14 (A) Carones LASEK spatula (ASICO AE-2920). (B) Vinciguerra Carones LASEK spatula (ASICO AE-2922). (C) Janach epithelial detaching spatula (JANACH 2910A). (D) Janach epithelial microhoe (JANACH 2915A).

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Figure 16 Our current LASEK technique. (A) One arm of a modified Vannas scissors (note the knob at the tip of the lower arm) is then inserted under the epithelium and traced around the delineated margin of the epithelium, leaving a hinge of 2 to 3 clock hours of intact margin, preferably at the 12 o’clock position.

(B)The loosened epithelium is peeled as a single sheet using a Merocel sponge or the edge of a jeweler’s forceps, leaving it attached at its hinge.

(C)After laser ablation is performed, an anterior chamber cannula is used to hydrate the stroma and epithelial flap with balanced salt solution. (D) The epithelial flap is replaced on the stroma using the cannula under intermittent irrigation. (E) Care is taken to realign the epithelial flap using the previous marks and to avoid epithelial defects. The flap is allowed to dry for 2 to 5 minutes. Topical steroids and

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antibiotic medications are applied. (F) A bandage contact lens is placed. (From Taneri S, Zieske JD, Azar DT. Evolution, Techniques, Clinical Outcomes, and Pathophysiology of LASEK: Review of the Literature. Surv Ophthalmol. NovemberDecember 2004 In press.)

To maintain the viability of epithelial cells, Dr. Vinciquerra proposed a modification of the LASEK technique that preserves the connection between the corneal flap and limbus

Figure 17 Former variant of our technique using a Merocel sponge to fashion the epithelial flap. (From Taneri S, Zieske JD, Azar DT. Evolution, Techniques, Clinical Outcomes, and Pathophysiology of LASEK: Review of the Literature. Surv Ophthalmol. NovemberDecember 2004 In press.)

(22). The butterfly technique requires the use of the Vinciguerra PRK/LASEK spatula (Fig. 20) to impart a thin abrasion to the paracentral corneal epithelium, from 8 to 11 o’clock, to spare the optic zone. After positioning the LASEK OZ chamber that is connected to the LASEK pump, apply 20% alcohol for approximately several seconds. The length of time depends on the firmness of the epithelial adhesion noted during the initial abrasion, with a firmer adhesion requiring a slightly longer time. With the Vinciguerra-Carones LASEK spatula, cautiously dissect the epithelium from Bowman’s membrane up to the limbus. It is mandatory to keep the cornea well hydrated in order to preserve the

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extending from original sheet border (arrowheads) to the 1-day outer border (arrows). (B) The cell attachment and epithelial outgrowth were persistent until day 15. Bar=50 µm. (From Chen CC, Chang JH, Lee JB, Javier J, Azar DT. Human corneal epithelial cell viability and morphology after dilute alcohol exposure. Published courtesy of Invest Ophthalmol Vis Sci 2002; 43(8):2593–2602.)

Flap Edges and Electron Microscopy

The basal epithelial cell layer maintained a compact and regular arrangement. The basement membrane layer showed discontinuous and irregular extracellular matrix fragments. These fragments, however, were still attached to the basal epithelial cell layer, indicating that the point of separation was likely to be within the basement membrane or between the basement membrane and Bowman’s layer. The adherence of the basement membrane to the basal layer of the epithelium is significant because it is believed that the basement membrane provides the stability and support that keeps the epithelium intact even with manipulation, thereby preserving the integrity and viability of the entire epithelium. The presence of desmosomes provides anchoring mechanisms for the epithelium to adhere to the ablated stroma.

Electron Microscopy

The epithelial sheet specimens were obtained from patients undergoing PRK. The epithelial sheets were fixed in half-strength Karnovsky fixative (2% paraformaldehyde and 2.5% glutaraldehyde) in 0.2 M sodium cacodylate buffer (pH 7.4) overnight and postfixed in 1% osmium tetroxide in 0.2 M sodium cacodylate for 1.5 hour. After dehydration in graded alcohol, the eyes were embedded in epoxy resin (Epon-Araldite). Thick sections (1 µm) were stained with toluidine blue, and a suitable area containing basal layers was chosen. The blocks were trimmed accordingly, thinsectioned (80–90 Ǻ), stained with 2% uranyl acetate-Reynold’s lead nitrate, and examined with a transmission electron microscope (model 410; Philips, Eindhoven, The Netherlands).

Electron Microscopic Analysis of Epithelial Sheets Removed Using 20 %

Alcohol

Normal corneal epithelia are nonkeratinizing, stratified, squamous epithelia five to seven layers thick. Desmosomes are present along all cell membranes abutting other cell membranes. The cells of the basal layer are columnar, and hemidesmosomes are present along their basal plasma membrane adjacent to the basement membrane. Beneath the

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epithelium is a unilamellar basement membrane that overlies a thick collagen stroma through which anchoring fibers extend from the lamina densa (25,26).

Azar et al. studied the electron micrograph of four freed epithelial sheets, which were obtained from 20% alcohol exposure (Fig. 19). The freed epithelial sheet displayed normal stratification. The basal epithelial surface of isolated epithelial sheets showed blebbing of the basal cell membrane and autophagic vacuoles within the cytoplasm of the epithelial basal cells of the freed sheet in two of the four specimens. They also observed variable basement membrane complex configurations beneath the epithelial basal cells: unilamellar basement membrane with focal disruptions, irregular and discontinuous basement membrane with intact hemidesmosome, disruptions of basal cell membranes with absent basement membrane, and duplicated basement membrane containing dense bundles of anchoring fibrils. Gabeler et al. also demonstrated that the plane of separation after ethanol exposure in human cadaver eyes was between the lamina densa and the Bowman’s layer (16).

The viability of the ethanol-treated epithelial sheet was further studied in tissue culture for cell migration and attachment (5). One of the three specimens showed outgrowth and attachment of epithelial cells from the epithelial sheet at days 1 to 15 (Fig. 20). These findings were reinforced by the electron microscopic evaluations of the epithelial tissue specimen in vivo. The basal epithelial cell layer maintained a compact and regular arrangement. The basement membrane layer showed discontinuous and irregular extracellualr matrix fragments. These fragments, however, were still attached to the basal epithelial cell layer, indicating that the point of separation was likely to be within the basement membrane or between the basement membrane and Bowman layer. The adherence of the basement membrane to the basal layer of the epithelium is significant because it is believed that the basement membrane provides the stability and support that keeps the epithelium intact even with manipulation, thus preserving the integrity and viability of the entire corneal epithelium. The presence of desmosomes provides a possible anchoring mechanism for the epithelium to adhere to the ablated stroma (1).

Irrigation and Reapproximation

After excimer laser ablation, a 30-gauge Rycroft anterior chamber cannula (Becton Dickinson, Franklin Lakes, NJ) is used to wash the ablation site and epithelial flap with balanced salt solution to remove debris or remaining loose epithelial cells (Fig. 21). Washing the ablated surface and epithelial flap also facilitates the ease of repositioning of the flap by lubricating the stromal and epithelial surfaces. The folded epithelial sheet is then gently manipulated and repositioned onto the ablated stromal bed using the straight part of the Rycroft cannula and gentle intermittent irrigation. Exposure of the flap to unnecessarily large amounts of fluid may cause the epithelium to swell resulting in poor coaptation of the wound edges. The flap is allowed to float into position and is then meticulously realigned using the previously placed corneal markings to minimize epithelial defects.

After LASEK, there is usually a clear visual axis, slight flap edema and minimal to moderate conjunctival injections similar to LASIK. Flap-related complications of LASIK procedures such as striae are reported to occur in 1% to 3% of cases (Stark 1999). The incidence of striae causing a decrease in best corrected visual acuity (BCVA) is difficult

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to ascertain using current literature although Gimbel et al. reported a 0.6% decrease in BCVA in 1,000 cases caused by microwrinkling. Flap desiccation and contraction during laser ablation, wrinkling caused by stretching or drying, tenting, misalignments, irregular flap thickness, freecaps, epithelial defects, and even movements during eye drape and speculum removal may cause striae in LASIK. Biomicroscopy after the procedures is a good technique used by surgeons to detect striae and other complications which may not be properly visible under the operating microscope. Epithelial flap and contact lens displacement in LASEK may also be detected early with biomicroscopy. Prevention of these flap complications in LASEK can easily be avoided by careful manipulation during the reflection and repositioning of the hinged flap and prevention of flap dehydration.

Figure 21 Laser treatment shown (A) is followed by gentle irrigation with BSS (B). The epithelial flap is then gently reflected back onto the stromal bed (C) and the margins realigned (D).

In LASEK, epithelial flaps are of a more resilient nature. Epithelial flaps may be folded towards the hinge. Hydration of the epithelial tissue and careful realignment of the tissue after laser ablation produces minimal striae detectable by biomicroscopy, which do not seem to affect BCVA. Epithelial flaps in this respect are easier to manage as compared to LASIK flaps. Minimizing flap striae in both LASIK and LASEK procedures is necessary in achieving good postoperative visual acuity. There have been no published advantages or drawbacks associated with either meticulous realignment and coaptation of wound edges against poor flap repositioning in LASEK.

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Drying Time and Instrumentation

After the flap is properly reapproximated, it is then allowed to air dry for 5 minutes to allow adhesion of the epithelial flap to the underlying stroma. Previous electron microscopic studies of central epithelial tissue in vivo showed that the bonds between the basement membrane is responsible for providing support and stability to the epithelium that allows it to withstand manipulation. Desmosomes present are believed to facilitate the adherence of the epithelium to the treated stromal bed (A1). The time allotted for proper adhesion of the flap to the ablated stroma varies fro different authors; Lee et al. have suggested that a minute is adequate drying time for proper adhesion of epithelium and stroma.

Excessive drying may cause dehydration, resulting in flap shrinkage or retraction. Drying times may be titrated according to the surgeon’s technique. The flap may be kept moist by careful placement of a drop of balanced salt solution on the central part of the cornea.

There are different techniques used for the creation and repositioning of the LASEK flap. Janach describes the repositioning of the flap using a spatula (Janach, J2920A, Como, Italy). Lee also describes the use of the same repositioning spatula to flatten and smooth the central portion of the epithelial flap during reapproximation. The epithelial flap may also be repositioned with ease using an irrigating cannula. Repositioning of the flap may be performed without specialized instruments depending on the surgeons’ preference.

Contact Lens Application

Bandage contact lenses are placed on the epithelial flap after it has sufficiently adhered to the stroma to prevent external factors such as the blink reflex to dislodge the flap (Fig. 22).

Figure 22 Contact lens application after flap repositioning (A). Moving the contact lens around (B) will test if the epithelium is adherent to the stromal bed (note the change in contact lens position made evident by the change in contact lens markings).

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Although contact lenses prevent the displacement of the epithelial flap by eyelidmediated sheering forces, the epithelial cell adhesion may occur within 1 to 3 days after surgery. Theoretically, contact lenses with a flat base curve (8.6 to 8.7 mm) will be advantageous to flap adhesion because it will facilitate a slight pressure effect to prevent flap displacement. Steep base curve contact lenses may have an advantage in preventing flap movements and improving patient comfort. These lenses, however, must be fitted well because tight lenses may produce stromal edema, ciliary flush, and chemosis. The contact lens will also aid in the healing of the epithelium and minimize postoperative pain.

The epithelial flap is observed immediately after surgery with the contact lens in place. The adhesion of epithelium to the stromal bed is deemed sufficient if rotating the contact lens does not produce movement of the flap (negative contact lens spinning test). In the event that the epithelium is not adequately adhered to the underlying stroma, the contact lens should be removed, the epithelial flap positioned properly, and replacement of the contact lens performed with application of pressure in appropriate locations.

The process of migration, mitosis, and differentiation govern regeneration of the disrupted corneal epithelium. After corneal epithelial injury, plasma membranes of the cuboidal basal epithelial cells surrounding the site of injury start to flatten and extend pseudopodia into the injured area. During the migration of these epithelial cells, there is constant simultaneous creation and dissolution of attachments between proteins in the epithelial cells’ plasma membrane and the extracellular matrix components. The dynamic changes in cellular interactions extend to the adjacent cells allowing the mass of epithelial cells to migrate towards the injury site without losing contact with its anchors. Polymorphonuclear cells from the tear film remove remnants of damaged cells from the injury site and a monolayer of migrating basal cells begins to migrate onto the wound area at a rate of 0.75 µm/minute. (Schultz 1997). Production of migratory cells occurs in a mitotic zone 3 to 5 millimeters peripheral to the injury site. The cells overlying the cuboidal basal cells also move into the injury site after the migration of the basal cells. The migration of cells into the defect halts once contact inhibition is established. The complete covering of the injury site induces epithelial cell mass regeneration to its original thickness. Basal epithelial cells reform adhesion complexes and differentiate into intermediate wing and superficial squamous cells which differentiate by secreting keratin proteins that found the barrier proteins of the epithelium.

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