Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Striae

The striae should be treated just when it cross the optical zone, affecting the visual acuity. On a striking way, we will be able to notice striae on the optical zone with no complaints. On the other hand striae out of the optical zone can not be perceived.

Surgical Technique

The only way to remove the striae of the Bowman membrane, is to take out the epithelium of the disc (Figures 23-8, 23-9).

Therefore, it’s total desepithelization is the first

step to the surgery. After, we wash the hole surface

Figure 23-10: Hidrodissection of the disc.

in order to eliminate the loosen epithelial cells. During the second step, we do the hydrodissection described over (Figure 23-10).

The third step is similar to the folds technique. We revert and rehydrate abundantly the stroma of the disc. On the forth step, we relocate the disc and brush the surface of the Bowman membrane with movements from the center to he disc periphery, in all directions, till all the striae has disappeared (Figures 23-11, 23-12).

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Subjects Index

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Canrobert Oliveira, M.D.,

Director of the Hospital de Olhos de Brasilia – Brasilia DF Brazil

Av. L2 Sul Q 607 Modulo G Brasilia DFCEP 70.200-670

Phone 55 61 242 4000 Fax 55 61 244 49 10 www.hobr.com.br hob@hob.com.br

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Subjects Index

One of the complications of LASIK is the formation of folds or striae in the corneal flap, which cause irregular astigmatism and loss of visual acuity. Striae may form during surgery due to the incorrect repositioning of the flap over the stromal bed, or in the early postoperative period if the disc is slightly displaced by blinking.

In the early days of LASIK, flaps created with large hinges were easily correctly repositioned. Nowadays, however, to perform ablations with large optical zones and peripheral ablations to correct hyperopia, we try to create a flap of minimum hinge. In these cases, it is important to make epithelial reference marks and correctly align these when we reposition the flap over its bed. If the reference marks are misaligned, this means that the flap is wrinkled or slightly displaced leading to the formation of folds and striae. It is important to check for correct alignment using the slit-lamp, and if this complication is noted, to immediately lift the flap, hydrate it and reposition it by careful alignment.

It was thought in the past that the nasal flap was a cause of striae formation, since it favored displacement during blinking in the early postoperative course. Displacement is less common with a superior flap but striae still appear. We generally observe horizontal striae in nasal flaps and vertical striae in superior flaps.

Striae formation is also more common in the case of a thin corneal disc. Thus, good prophylactic practice is to plan for a corneal disc of 160 m in thickness.

The LASIK surgeon should be aware of the complication and try to avoid it. If detected, folds and striae need to be rapidly treated. We present a new technique for the treatment of flap striae.

the eye showing folds and striae of the flap. We subsequently decided to perform a new treatment protocol described below.

SURGICAL TREATMENT

Lifting of the Flap Using a Spatula (figure 24-2).

Using the slit-lamp, the flap edge is identified and lifted slightly using the tip of an insulin needle and the lifting procedure completed under the surgical microscope. In such cases, we avoid making epithelial reference marks, since the disc is incorrectly positioned and we would induce the same error when repositioning it over the stromal bed.

Figure 24-2: Treatment: lifting the flap with a spatula.

Checking the Stromal Bed (figure 24-3).

Once the flap is lifted, we check the stromal bed for any irregularity which could lead to the formation of striae. In the present case, the bed was perfectly uniform and even. The technique applied was aimed at smoothing out the wrinkled corneal disc. First, we scrape the edges of the corneal epithelium to avoid peripheral epithelialization, since at the end of treatment the disc will be stretched and will therefore be larger than the exposed bed.

Figure 24-3: Treatment: checking the stromal bed.

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Figure 24-4: Treatment: repositioning the flap.

Massaging the Flap Using a Spatula Over a Contact Lens (figure 24-5).

A soft contact lens is placed on the cornea and using a flat spatula the flap is massaged over the contact lens. The contact lens allows us to perform an intense massage without damaging the corneal

Figure 24-5: Treatment: massaging over a contact lens.

disc and evenly distributes tension preventing the formation of further striae. When folds are considerable, the epithelium covering a stria may rupture. Other times, the epithelium readapts to the underlying flattened disc without breaking. We commence the massage over the pupillary area (where it is most important to achieve an even flap) performing circular movements. When we observe a highly marked fold, we place the spatula parallel to the fold and massage in a perpendicular direction. While we perform these maneuvers it is important to keep irrigating the flap on its stromal side to prevent adhering to the bed. If the disc adheres, massage is ineffective. Similarly, we frequently hydrate the epithelial side of the disc to prevent it sticking to the contact lens and damaging the epithelium. The massaging procedure is the most important step of treatment and should be performed for as long as considered necessary. We generally massage for about an hour.

Direct Massaging of the Flap (figure 24-6).

After massaging, the contact lens is removed, the stromal bed is rehydrated, and with the spatula directly on the cornea, we undertake a smoothing out maneuver from the hinge towards the opposite edge of the flap. This direct massaging of the flap over the cornea favors the adhesion of the flap over the stromal bed. It is important to wet the epithelial side of the corneal disc to avoid damaging the epi-

Figure 24-6: Treatment: direct massaging of the cornea.

thelium with the spatula and to perform this direct massage until the flap has completely adhered to the bed.

Appearance of the Cornea After

Treatment (figure 24-7).

Despite the fact that folds appear to persist, the stromal interface is seen to be perfectly even on slit-lamp examination. What we are really seeing at first, are the imprint of past folds on the epithelium. After treatment, mydriatic and antibiotic drops are instilled. We also recommend the use of a non-op- pressive dressing for 12-24 hours to avoid displace-

Figure 24-7: Final image after treatment.

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Chapter 24

ment due to blinking and favor the adequate epitheliazation of the cornea.

Outcome

After 12 hours, we removed the dressing and confirmed the disappearance of the striae (figure 24- 8). The cornea was reepithelialized and uncorrected visual acuity was 20/25. A week later, corrected visual acuity was 20/20 and an even cornea was shown on topography.

Figure 24-8: Outcome one day after treatment.

Conclusion

Any stria or fold affecting the pupillary area should receive early treatment during the first 24-48 hours following LASIK. Unlike other methods of treatment, massaging the flap with a spatula over a contact lens is an effective method of removing flap striae as late as one month after surgery.

SUGGESTED READINGS

1.Carpel EF, Carlson KH, Shannon S. Folds and Stria in Laser in situ Keratomileusis Flaps. J Refract Surg 1999;15: 687-690.

2.Gimbel HV, Peters NT, Iskander NG, Penno EA. Laser in situ Keratomileusis Flap Complications and Management. J Refract Surg 2000;16: s223-225.

3.Gutierrez AM. Treatment of flap folds and striae following lasik. In: Buratto L, Brint SF, eds. LASIK: Surgical Techniques and Complications. Thorofare, NJ: SLACK Incorporated; 1999; 557-562.

Jairo E. Hoyos, M.D.

E-mail: los@lix.intercom.es

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Introduction

Laser in situ keratomileusis (LASIK) for the surgical correction of myopia is gaining acceptance as a versatile refractive surgical procedure. Quick visual rehabilitation, minimal postoperative discomfort, and the ability to correct high degrees of myopia with little postoperative corneal haze are some reasons for LASIK’s popularity over other surgical vision correction options.1-4 However, postoperatively detected corneal ectasia is a possible complication of the procedure, even with temporary improvement in vision.5-7 Because the maximal corneal stromal tissue will be photoablated from the central cornea, the thickness of that area becomes important when LASIK is performed for high refractive errors with large ablation depths.7 In viewing the risk of creating iatrogenic keratectasia by removing excessive stromal tissue during LASIK, we have found that the mechanism have not yet been completely clarified and some analysis has been attempted in other to provide a more accurate understanding of keratectasia after LASIK.

Corneal Stromal Changes

Induced by LASIK

All excimer laser refractive procedures modify the refracting power of the cornea by altering the anterior corneal curvature using photoablation. The procedure substantially weakens the mechanical strength of the cornea because the biomechanically effective thickness of the cornea is reduced by the thickness of the lamella plus the keratectomy

depth.7,8 Biomechanical weakening may only become manifest months after a surgical procedure and some surgeons have recommended that at least 250 to 300 mm of residual posterior stroma be left untouched to ensure adequate biomechanical corneal strength to minimize the risk of keratectasia.9-11 Performing LASIK in patients with high myopia and a thin cornea should result in posterior stromal bed thickness less than 250 mm.12 The keratectasia is probably due to the action of intraocular pressure (IOP) on that weakened cornea, producing a corneal steepening revealed on corneal topography. This steepening appears to be greater in older patients and provides further evidence of the ability of IOP to produce ectasia in a thinned cornea.10 The correction of myopia involves the relative flattening of the central rather than the peripheral cornea, which reduces the anterior corneal curvature and thus the refractive power of the treated area. This becomes a particularly contentious issue when, in the absence of classic clinical evidence of keratoconus, inferior steepening of the cornea seen on corneal topographic scan after LASIK suggests the possibility of subclinical keratoconus.11,12 Keratoconus usually starts during puberty and, in most cases, it takes more than 5 years to manifest clinically. At any stage, the cornea may return to its original tensile strength or at least increase its stiffness, leading to an abortive form of keratoconus, forme fruste keratoconus (FFK).7 The diagnosis of FFK is not always easy. In some cases clinical signs such as the Fleischer ring are present without any progression of myopia and astigmatism over years. Also, corneal topography may look keratoconus-like, with a temporal inferior steepened area. In other cases, however, no clinical diagnostic signs can be detected

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Chapter 25

at the slit lamp; an asymmetric bow-tie pattern on corneal topography may be the only diagnostic hint to explain the slightly tilted Javal mires described and classified by Amsler.8-12 Asymmetric bow-tie patterns, however, appear to be more frequent than symmetric patterns in a population of normal eyes. To substantiate the relationship between FFK and iatrogenic keratectasia after LASIK, the populations of large prospective studies currently underway should be investigated for FFK cases.13 Until a clinical relationship between FFK and complications after LASIK has been established or refuted, we should consider FFK a contraindication for LASIK.

Corneal Evaluation Using the

Orbscan Topography System

Computer-assisted videokeratoscopes are now used in clinical practice, and videokeratography has enhanced our ability to detect early keratoconus in a quantifiable and reproducible manner. Evaluation of the cornea in refractive surgery routinely includes some form of 2-dimensional corneal surface mapping. However, axial power maps commonly produced by most videokeratography systems do not correlate well with refractive changes. Most computerized videokeratoscopes used in clinical practice are Placido-disk-based systems. In this system, a Placidodisk pattern is projected onto the cornea, video-im- age captured, and analyzed by computer. Axial power calculated from Placido-based systems represents only local or refractive power of the cornea within the paraxial region (approximately the central 2.5 mm zone of the cornea). The limitations of these systems have been discussed and the Placido skew ray error, information reflected from only tear film, and sensitivity to focus and alignment. Furthermore, Placidobased systems cannot distinguish convex from concave shape changes on corneal surface. Therefore z- height calculations from Placido-based data have severe accuracy limitations.13-16

The Orbscan Topography System (Orbtek, Inc.) is a 3-dimensional (3-D) slit-scan topography system designed for analysis of the cornea and anterior chamber surfaces. It uses a calibrated video and scanning slit-beam system to independently measure the x, y, and z locations of several thousand points

on each surface of the cornea and anterior chamber. The instrument therefore measures elevation, from which shape and power calculations are derived. From the slit images, the anterior and posterior corneal surfaces can be reconstructed mathematically based on ray tracing and triangulation between the light emitter, the corneal surfaces, and the sensor video capture.17 We used the Orbscan Topography System to evaluate a series of postoperative patients submitted by LASIK (Figures 25-1 through 25-4).

Using the reconstructed anterior and posterior corneal surfaces, it is possible to apply ray-trac- ing models to calculate the cornea’s total effective lens power. Thus, the Orbscan produces total optical power maps that describe the Snellen refractive power of the cornea as a 3-dimensional structure, mapped locally onto the front surface using a false color scale. The power calculation is based on tracing parallel rays traveling through the anterior surface and, subsequently, posterior corneal surface.18 Therefore, the results should correlate more closely with the manifest change in refractive power than paraxially calculated maps of the anterior surface alone.

Analysis of the Orbscan color maps can be focused on quantitative topographic parameters at 3 points: the central point of the cornea; the apex, the point with maximum reading on the anterior elevation best-fit sphere map (anterior elevation BFS); and the thinnest point, the spot with minimum value on the pachymetry map. Analysis of these parameters include the following:

1.Location – radius (the distance from the central point of the cornea) and semimeridiam (designated from 0 to 360 degrees, proceeding counterclock-wise from 3 o’clock in both right and left eyes).

2.Elevation – compared to a best-fit sphere. The anterior BFS is calculated to best match the anterior surface. This match is determined using a least squares method. The displayed map data represent the sphere subtracted from the eye surface in millimeters. The difference between the sphere and the eye surface is expressed as the distance radially from the center of the sphere rather than perpendicular to a plane.

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