19

Management of LASEK Complications

Massimo Camellin, MD

Sekal Rovigo MicroSurgery

Rovigo, Italy

INTRODUCTION

A complication usually involves making intraoperative decisions to manage the procedure well and avoid unwanted consequences in uncorrected visual acuity (UCVA) and of best spectacle corrected visual acuity (BCVA). It is important to keep in mind that some complications can happen in the postoperative period. Short-term and long-term complications must be distinguished because they may manifest differently.

MANAGEMENT OF INTRAOPERATIVE COMPLICATIONS

An unavoidable complication that can occur, even with an experienced surgeon, is solution leakage on the conjunctiva. This is often caused by a sudden movement of the eye and immediately creates the burning sensation referred to by patients. Despite the lid edema and a hyperemia in the conjunctiva for the first few days, no other permanent lesions develop. If this occurs, we abundantly rinse the eye with diclofenac to reduce irritation. The use of a well for the alcohol (E.Janach S.r.L., Como, Italy) with a double edge reduces the risk of leakage (Fig. 1). After thoroughly drying the interior of the well with a cotton sponge, we always fill the cone with diclofenac to dilute the alcohol residue.

The epithelium and stroma may be strongly adherent because of a previous inflammatory event. These adherences must be managed with care. However, sometimes they are so stuck that some tearing is unavoidable. Tears may not leave a trace either in the short-term or in the long-term. However, if the tears are too wide, it is possible that the patient will feel more pain postoperatively. The risk of losing the flap is increased and, in most cases, the more adherent portion is close to the hinge. Because in this area there is no epithelium precut that allows the solution to flow under the epithelium, it is possible to re-apply the alcohol solution for 5 to 10 seconds to increase the ease in detaching this portion. Only after a certain amount of training and experience will a surgeon be able to manage this hard portion with small but strong movements of the spatula. If the hinge is completely broken, the flap becomes free and needs two spatulas to be re-positioned well. It may be difficult to recognize which side should be in contact with the stroma. We do not worry too much about this because the protection realized from the basal layer is efficient. When the flap gets lost or completely broken, we apply autologous serum (1) to encourage more rapid epithelium regrowth.

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Figure 1 Well for the alcohol solution.

Double edge.

MANAGEMENT OF SHORT-TERM POSTOPERATIVE

COMPLICATIONS

Pain

Pain is present in approximately 10% of cases. When we exclude intraoperative alcohol leakage as a cause, the pain must be controlled with general anti-inflammatory drugs (not very efficient) and with pupil dilation. The discomfort does not begin until 3 or 4 hours after surgery. This timeframe coincides with the reduced effect of the midriatic instilled at the end of the procedure. Henceforward, pain can be derived from the iris. These patients sometimes find comfort with ice application over the lids. A foreign body sensation is common after 3 days and is related to the debris present over the lens. Patients often feel relieved after lens removal. However, it is dangerous to take away the lens, too, because of the risk of flap damage. If flap healing is uncertain, our experience suggests exchanging the lens for a new one that is slightly wider.

Lens Loss

Loss of the contact lens may be caused by to the lens being too wide, excessive watering, or extreme eye movements while instilling drops. When this occurs during the first few hours, rarely does the flap stay attached. More commonly, lens loss results in flap loss. A new lens must be fitted and auto serum instilled. Postoperative care will likely be primary to photorefractive keratectomy (PRK).

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Flap Loss

Unfortunately, lens presence is not always proof of flap presence. When the patient experiences sudden pain and the lens is still in place, we must look for flap signs. An excellent way to check flap presence is to dye the eye with macromolecular fluorescein. This coloring agent, flowing under the lens, dyes only the stroma and allows us to be certain the flap is still present. This simple procedure can also be used to check epithelium renewal, thereby helping to decide whether take to away the lens.

Figure 2 Re-epithelialization delay after an enhancement followed by a flap tearing.

Flap Breaks

A flap break can be small or large and is often related to the lens being removed too early. In our experience, it occurs rarely and does not create problems, except in one enhancement case that involved two points of re-epithelialization delay, both in the midperiphery (Fig. 2). We believe it is better to leave the lens 1 additional day if flap integrity is in question.

Infection

We have had two cases of infection. In one case, influenza developed the day after surgery, as did bilateral conjunctivitis with a lot of secretion. Infiltrates were seen at the periphery of the flap (Fig. 3). We immediately disinfected and rinsed the area with betadine. The postoperative care was uneventful, with no residual opacity in the

Management of LASEK complications 227

Figure 3 Infiltration after influenza conjunctivitis.

stroma. The second patient had an epithelial-stromal infiltrate at the 12 o’clock position (Fig. 4A). This case was also treated with betadine rinsing and showed evidence of a faint opacity 1 month later (Fig. 4B).

Diffuse Opacities

Diffuse opacities, presumably sterile, were observed in three cases. In two patients who had bilateral surgery, only one eye of each patient developed this complication. There was no conjunctival hyperemia, but pain was more intense compared to the other eye (Fig. 5). In our practice, the incidence of diffuse opacity is three out of 1,000 cases. It is a serious complication until its refractive effects are reduced. Hyperopic astigmatism may be noted during the first few months in these patients. The cause is related to an epithelial thinning that, by flattening the surface, results in a hyperopic shift. We tried to cure these cases with topical steroids, thinking that it was a form of diffuse lamellar keratitis. But after having noticed that the epithelium was thinner, we suspected the cause to be slower epithelium renewal (Fig. 6). We discontinued all therapy, except for artificial tears, and the curvature progressively improved. Only a light haze remained after several months.

Management of LASEK complications 229

Figure 5 Diffuse opacity after

LASEK.

MANAGEMENT OF LONG-TERM POSTOPERATIVE

COMPLICATIONS

Undercorrection/Overcorrection

Undercorrection is not common in LASEK, but if it appears, it is usually related to laser problems. It can easily be managed with an enhancement procedure by removing the epithelium without alcohol and adding the undercorrected value to the laser. This maneuver has to be performed within 1 month after the surgery.

Late regression is very rare and can be related to an optical zone being too small, or to a small transition zone. The initial regression, however, is caused by the epithelium (Fig. 7) and can be partially controlled by steroids.

Aside from regression, the epithelial basal layer seems to also be responsible for haze production (2). We believe it is useful to try steroid therapy for 1 month, and only if the regression is persistent do we suggest re-treating the patient.

Overcorrection is more common after LASEK. Using the Nidek EC 5000 laser, we had to reduce the planned treatments by 10% for myopia up to −10 diopters (D) and progressively up to 20% for myopia of –10 D up to −20 D. The Camellin formula for Nidek EC 5000 is:

Laser setting=Myopia×(0.9+[myopia/200])

(1)

Management of LASEK complications 231

Figure 7 Early regression in high myopia (top right and left) and reduction in regression after 1 month of steroid therapy (bottom right and left).

We have not had the same effect with the Schwind Esiris laser; however, with this laser, we reduce 5% for treatment up to −10 and 15% progressively for treatments up to −20 D. However, our experience with this laser is relatively sparse and may not be enough to propose the correct regression formula. The cause of overcorrection in laser subepithelial keratomileusis (LASEK) may be the same as that observed in laser in situ keratomileusis (LASIK), which is a decreased inflammatory reaction. Less apoptosis means less collagen production from the keratocytes, therefore resulting in higher effective correction.

Haze

At present, haze is an uncommon complication in PRK (3) and in LASEK for low moderate myopia (Fig. 8). The good quality of lasers has also contributed to this low incidence. However, a delay in epithelialization can occur and cause a high degree of haze (4). Epithelialization delay was rarely observed in LASEK. In our statistics, 83.8% were re-epithelialized within 4 days and 100% within 6 days.

We realize that in some cases of LASEK, the flap can be lost during the first few days and the procedure converted to PRK. It is difficult to check flap presence underneath the contact lens, but based on the low percentage of haze observed, we may conclude that the flap was present and protected the stroma from apoptosis in the majority of cases. We had

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25% of haze in re-operations after cornea transplants. This value is strictly related to the presence of an incomplete flap. After PTK treatment only 10% developed haze again.

Figure 8 Incidence of haze after LASEK, including cases of LASEK in which the flap was lost or did not adhere (converted to PRK), showing +1 or more haze in 3.8% of patients.

Foreign Body Sensation

Dry eye and the related foreign body sensation are not common in LASEK. Only 25% experienced slight dry eye symptoms, mostly disappearing by 6 months. Only 3% had problems at 12 months (Fig. 9). This symptom is common only in the morning during the first lid opening and can be easily managed with artificial tears. We did not observe any late-onset epithelial keratitis or erosion.

CONCLUSIONS

The low percentage and low degree of serious side effects related to LASEK confirm its feasibility as a useful refractive procedure, especially because these side effects can be managed effectively.

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Figure 9 Dry eye symptoms in LASEK eyes lasting 6 months or less (25%) and 12 months or less (3%).

REFERENCES

1.Tsubota K, Goto E, Shimurra S, Shimazaki J. Treatment of persistent corneal epithelial defect by autologus serum application. Ophthalmology; 1999; 10:1984–1989.

2.Lee YC, Wang IJ, Hu FR, Y-Kao WW. Immunohistochemical Study of Subepithelial Haze after Phototherapeutic Keratectomy. J Refract Surg; 2001; 17:334–341.

3.Zoltan Zsolt Nagy Z, Fekete O, Suveges I. Photorefractive Keratectomy for Myopia with the Meditec MEL 70 G-Scan Flying Spot Laser. J Refract Surg; 2001; 17:319–326.

4.Seiler T, Holschbach A, Derse M. Complications of myopic photorefractive keratectomy with the excimer laser. Ophthalmology; 1994; 101:153–160.

20

Wavefront Analysis, Principles, and

LASEK Application

Ronald R.Krueger, MD

Cole Eye Institute, Cleveland Clinic Foundation

Cleveland, OH

Patrick C.Yeh, MD and Dimitri T.Azar, MD

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

Harvard Medical School

Boston, MA

INTRODUCTION

One of the major attractions of laser subepithelial keratectomy (LASEK) surgery is the potential application of customized wavefront-guided treatments. At the time of this writing, custom LASEK in the United States is primarily an off-label application. The preliminary results have been encouraging. This chapter focuses on the history, principles, and methods of wavefront analysis and their applications for customized LASEK surgery to correct high-order aberrations. The benefits of custom LASEK depend not only on accurate wavefront measurements but also on the state of accommodation and pupil size. The benefits are reduced when the pupil is small. The benefit would be greatest in patients in whom the pupil is larger and in scotopic situations, such as night driving. It is not clear whether eyes of keratoconus patients or those with debilitating induced aberrations secondary to previous refractive surgery will benefit from high-order corrections using custom LASEK surgery.

OCULAR ABERRATIONS AND WAVEFRONT ANALYSIS

In physical optics terms, light is considered as a wave that spreads in all directions. A wavefront in turn describes the shape of light rays emanating from a source that are in phase (1). Rather than limited to any given refractive surface, the wavefront describes the total effects of the optical system of the whole eye as the light passes through the pupil. In an ideal eye that is free of any aberrations, the wavefront forms a perfect plane perpendicular to the visual axis (Fig. 1A). However, when optical aberrations are present in actual eyes, the wavefront forms an imperfect surface rather than a plane (Fig. 1B). Wavefront aberrations are defined as the difference between the actual, aberrated wavefront surface, and the ideal wavefront plane (Fig. 2).

Wavefront errors are quantified in terms of root mean square (RMS) or square root of the sum of the squares of the deviation of the actual wavefront from the ideal wavefront

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of a given pupil size. The larger the deviations and the pupils, the higher the RMS error. The influence of aberration on retinal image quality can be simulated by computing the point-spread function (PSF) (Fig. 3). However, the resulting retinal image blur produced by ocular aberration is not always predictable by the RMS; some aberrations can act to cancel each other out, producing an overall better image (right), even though this image has a greater RMS wavefront error (Fig. 4).

Refractive error exists when light does not focus perfectly onto the retina. Traditionally, spectacles, contact lenses, intraocular lenses, and refractive surgeries have been the only method available to correct the spherical and cylindrical components of refractive error. These are classified as low-order aberrations, which account for approximately 85% of wavefront error. It was not until recently that we have the means of measuring and treating high-order ocular aberration in a clinical setting. These are other components of refractive error that have been referred to as “irregular astigmatism,” which cannot be corrected with traditional spherocylinder lenses. They are believed to represent approximately 15% of the wavefront error. When the image is optically perfect, physiologic visual acuity is still limited to between 20/8 and 20/10 largely by the virtue of photoreceptor diameter and receptor packing (2) (Fig. 5). High-order ocular aberrations are also known as refractive distortions, which limit the vision of healthy eyes to less than the retinal limits. Laser refractive surgery, both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), is known to increase the high-order ocular aberrations, especially spherical aberration and coma (Fig. 6) (3–7).

Wavefront aberrations are expressed mathematically using Zernicke polynomial expansions (8). There are three components to the low-order aberration: zero order (a constant), first order (tilt or prism), and second order (defocus and astigmatism). Loworder aberrations can be corrected with glasses, contact lenses, and conventional laser surgery (Fig. 7). High-order aberrations are fit to a more complex wavefront shape. Some of the more common higher-order aberrations include third order (coma and trefoil) (Fig. 8A) and fourth order (spherical aberration and secondary astigmatism) (Fig. 8B). Secondary coma and more complex situations are represented by aberrations of the fifth order and beyond (Fig. 9). Polynomials can be expanded up to any arbitrary order if sufficient numbers of measurements for calculations are made.

Figure 1 (A) A perfect plane wave of light perpendicular to the visual axis in an ideal eye represented by the Hartmann-Shack wavefront aberrometer. (B) An imperfect wavefront surface in an eye with optical aberrations represented by the Hartmann-Shack wavefront aberrometer.

Figure 3 The retinal image can be simulated by computing the point spread function (PSF) from the wavefront profile.

Figure 4 The resulting retinal image blur produced by the aberration is not always predicted by the RMS, as illustrated in this example. When defocus (left) and spherical aberration (middle) are combined, they tend to cancel each other out, producing an overall better image (right), even though this image has the greatest RMS wavefront error.

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Figure 9 Three-dimensional pictorial directory of Zernike aberration components.

HISTORY AND TYPES OF WAVEFRONT SENSING

Wavefront aberration detection and analysis have been based on either interferometry or ray tracing. The interferometric method has not been widely used because of difficulties in stabilizing the eye to construct proper reference surfaces for comparison. As a result, most of the current methods used for wavefront detection and reconstruction are based on ray-tracing principles (9). The concept was first introduced by Tscherning in the late 1800s and then later expanded by Hartmann in 1900. In 1971, Shack and Platt made modifications to the Hartmann wavefront sensor, which became known as the HartmannShack aberrometer (10). The Hartmann-Shack aberrometer was used to improve the images of satellites viewed from earth. In 1994, clinical application of this technology was first adapted successfully to ophthalmology by Liang et al in the measurement of the eye’s wave aberration (11,12). In 1997, this technology was furthered in its application, and high-resolution and noninvasive retinal imaging of microscopic structures the size of single photoreceptors in a living human retina was made possible for the first time (13,14).

In general, the wavefront sensors can be classified into three types: (1) outgoing wavefront aberrometry, as is used in the Hartmann-Shack aberrometer; (2) ingoing adjustable aberrometry, as in slit skioloscopy and spatially resolved refractometer; and

(3) retinal imaging aberrometry, as in Tscherning and Tracey retinal ray-tracing method.

Principles of Outgoing Wavefront Aberrometry

The Hartmann-Shack aberrometer is based on the principles of outgoing wavefront aberrometry. Incorporated in the Hartmann-Shack aberrometer is a sensor that consists of a matrix of small lenslets. The aberrometer projects a laser light into the eye to illuminate a small spot on the retina. The laser light reflects off the retina and emerges as a wavefront, which passes through the lenslet array and focuses into spots on a detector array captured onto a charge-coupled device (CCD) camera (Fig. 10A). For an ideal eye, the reflected plane wave would be focused into a perfect array of point images, with each image falling exactly on the optical axis of the corresponding lenslet (Fig. 10B). In an aberrated eye, however, the distorted wavefront would be focused into a displaced array of spots. The deviation of each spot from its corresponding lenslet axis is used to calculate the aberrations or the slope of the aberrated wavefront (Fig. 10C). Mathematical integration of the deviation yields the shape of the aberrated wavefront, expressed in terms of Zernike polynomials (15). Wavefront devices that use the Hartmann-Shack technology include (Alcon) (Fig. 11A), WaveScan WaveFront® (VISX) (Fig. 11B), ZyWave (Bausch & Lomb) (Fig. 11C), and WASCA Wavefron Aberrometer (Asclepion-Meditec) (Fig. 11D).

Each Hartmann-Shack wavefront device is linked to its own specific excimer delivery system for customized laser treatment (Table 1). At the time of this writing, the LADARWave®, (CustomCornea®, Alcon Inc., Fort Worth, TX) and WaveScan

system (Custom VISX, Inc., Santa Clara, CA) are currently the two Food and Drug Administration (FDA)-approved systems in the United States for wavefront-guided customized myopic LASIK treatment. ZyWave system (Bausch & Lomb, Rochester, NY) is expected to be the third wavefront device approved by the FDA for custom LASIK in the United States in the autumn of 2003.

perfect array of point images, with each falling exactly on the optical axis of the corresponding lenslet. (C) In an actual eye with ocular aberrations, the distorted wavefront is focused into a displaced array of spots. The shape of the aberrated wavefront is the mathematical integration of the deviations.

Figure 11 (A) (Alcon) wavefront system. (B) WaveScan WaveFront® (VISX) system. (C) ZyWave (Bausch & Lomb) wavefront system. (D) WASCA (AsclepionMeditec) aberrometer.

LADARWave® system captures approximately 240 wavefront data points in a dilated 7- mm pupil. Most peripheral spots are filtered out, giving approximately 188 to 195 usable data points (Fig. 12). The limbus is used as a reference point for centration. The positioning of the pupil and limbal rings are then transferred to the laser system. Once the eye is dilated, the conjunctiva is manually marked using a gentian violet dye marking pen

LASEK, PRK, and excimer laser stromal surface ablation 246

at the 3 and 9 o’clock positions 1 to 2 mm outside of the limbus while the patient is sitting upright behind the slit lamp. These reference marks are used to register the position of the wavefront measurement to ensure the customized ablation pattern is applied in the same orientation as the wavefront measured by the wavefront measurement device to account for cyclotorsion during ablation. The LADARVision® laser system uses a flying small-spot beam (0.8 mm) and a closed-loop tracking system to ensure continual proper orientation and alignment of the laser beam during custom treatment.

WaveScan system also captures approximately 240 wavefront data points within a 7-mm pupil aperture (Fig. 13). VISX Star S4 laser uses variable-shaped beams ranging in size from 0.65 to 6.5 millimeters. It uses an active 60-Hz video eye tracking system to maintain pupil alignment during customized ablation. Registration of the wavefront profile is assumed based on the orientation of the undilated pupil. Using information from the WaveScan, the unique wavefront pattern can be placed on a lens (PreVue® Lens). The lens is then fitted in a trial frame, allowing refractive surgery candidates to “preview” the potential visual results before the actual procedure (Fig. 14).

Zywave system captures approximately 70 to 75 wavefront data within a 7-mm pupil

aperture. Its associated laser delivery system, Technolas® is a scanning/ flying spot laser with beam diameter of 2.0 mm (Fig. 15). An active 120-Hz video eye tracker is used to maintain pupil alignment. Similar to VISX Star S4, registration of the wavefront information is also based on the orientation of the undilated pupil.

Table 1. Clinical Hartmann-Shack Wavefront

Sensors and Their Associated Excimer Laser

Delivery Systems.

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Figure 13 WaveScan device showing two-dimensional preoperative and postoperative aberration maps.

Principles of Ingoing Adjustable Aberrometry

Spatially resolved refractometry (SRR) and slit skioloscopy use the principles of ingoing aberrometry. In SRR, a reference light is fixated through the center of the pupil while a second light is passed through a 1-mm hole and directed toward the retina. The ingoing rays of light are then manually steered by the patient to overlap with the reference light to define the wavefront needed to cancel ocular aberration (Fig. 16A). Slit skioloscopy is an objective variant of SRR. Using the principle of retinoscopy, skiascopy optical path detection projects a moving slit into the eye. The projecting system consists of an infrared light emitting diode (LED) that emits light going through a chopper wheel with slit apertures. The wheel rotates constantly at high speed to scan the retina for each meridian. The slit light rays are reflected back through a receiving lens and are detected by sensors. The relative motion of the slit’s image is used to calculate the refractive error along each

segment (16) (Fig. 16B). The Nidek OPD (Optical Path Difference) scanning system is an example of slit skioloscopy (Fig. 16C).

Figure 14 PreVue lens in trial frame.

Figure 15 Schematic diagram illustrating PlanoScan laser ablation pattern in progression.

Principles of Retinal Image Aberrometry

Both Tscherning aberrometry and Tracey sequential ray tracing are based on the principles of retinal image aberrometry. Tscherning aberroscope consists of a collimated laser beam that illuminates a mask with regular matrix pin holes, creating 168 single light rays. These form a bundle of thin parallel rays that project a retinal spot grid pattern on the retina. The spot pattern formed on the retina is distorted according to the eye’s aberrations. This retinal pattern is then imaged through a small aperture onto a CCD camera by indirect ophthalmoscopy. The deviations of all spots from their ideal positions are measured, and the optical aberrations are calculated based on these values (Fig. 17A and 17B)(17). Alegretto Wavefront Analyzer (Wavelight) (Fig. 17C) and ORK Wavefront Aberrometer (Schwind) are based on Tscherning aberrometry. Tracey sequential ray-tracing aberrometry also measures the position of a thin laser beam projected onto the retina. An individual laser ray, directed into the eye parallel to the visual axis, is very rapidly (within 10 to 20 msec) scanned through a multitude of entry points onto the retina. The scanned spots are captured and connected in a retinal spot diagram (Fig. 18). The measured location of each ray as it exits the eye is calculated against the known position, and the wave aberration function is described by Zernike polynomials (18).

WAVEFRONT PROFILES

Each ocular aberration has a well-defined three-dimensional wavefront pattern. For instance, myopia assumes the shape of a bowl (Fig. 19A), whereas cylinder resembles a saddle (Fig. 19B). Coma has a comet-shaped pattern with restricted light passage directly adjacent to an area of accelerated light passage in the same meridian, resulting in a “bump and dip” configuration (Fig. 19C). Spherical aberration has a central focus on restricted light (area of hyperopia) surrounded by an accelerated ring of light (annulus of myopia), resembling a “sombrero hat” (Fig. 19D). Other high-order aberrations, such as trefoil (“Napoleon’s hat”) and quadrafoil (“plant stand”) (Fig. 19E), generally have lower values (Fig. 19F) (9).

A normal cornea with its normal prolate pattern also contains some high-order aberrations, but in low magnitude. The asphericity of the cornea increased nonlinearly in a positive direction (oblate) with the amount of myopic excimer laser treatment. The greater the amounts of correction, the progressively more oblate the corneal surfaces become. As a result, the spherical aberration increases in numerical value and wavefront size (19,20).

CLINICAL EXPERIENCE WITH WAVEFRONT-GUIDED

ABLATION AND LASEK APPLICATION

The clinical results are promising to date, with a reduction in induced aberrations compared with conventional myopic treatment, as well as improvement in reduction of preoperative spherical aberration (Fig. 20).

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Standard LASIK are known to induce an increase in optical aberrations, especially spherical aberration and coma (3–7,21,22). These surgically induced aberrations are believed to contribute to the deterioration of vision under scotopic lighting conditions after excimer laser refractive surgery. These aberrations are further increased in eyes with a larger pupil size and with larger attempted correction (21,22). For a large pupil, LASIK has been shown to induce more spherical aberrations than PRK (6). Moreover, the formation of a LASIK flap alone is known to induce optical aberration (5). These changes are believed to be caused by biomechanical and biological responses of the corneal tissue after laser ablation as a result of structural and shape change (23–25). Because of the growing concern with LASIK-induced aberrations and their potential compromise on the outcomes of custom wavefront ablation, surface ablation such as PRK and LASEK regained its popularity among many refractive surgeons. Nonetheless, just as the biomechanical effects of the cornea limit the predictability with custom surface ablation, PRK procedures are also shown to increase high-order optical aberrations in human eyes (3,4,6,26,27). Moreover, there are yet controlled studies, to date, confirming the superiority of custom LASEK over custom LASIK surgery.

Wavefront technology provides a sophisticated diagnostic tool that, combined with a flying spot excimer laser system and eye tracking system, allows measurement and correction of ocular aberrations that interfere with the quality of vision but cannot be improved with traditional spherocylindrical lenses. Short-term clinical results are promising, suggesting that customized treatments result in improved quality of vision than achieved with the conventional treatments in a great proportion of eyes. Nonetheless, practical challenges, such as the inability to predict and compensate for the biomechanical effects of the cornea after laser surgery, the effects of the flap, and corneal wound healing, need to be further evaluated and addressed before customized wavefrontguided treatments can be performed with greater precision and high fidelity.

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Figure 20 Spherical aberrations before and after LASIK treatment using the CustomCornea® (Alcon Inc., Fort Worth, TX) and conventional technique. Eyes treated using the CustomCornea® system have 50% less spherical aberration postoperatively than eyes treated using the conventional technique.

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20.Holladay JT, James JA. Topographic changes in corneal asphericity and effective optical zone after laser in situ keratomileusis. J Cataract Refract Surg;() 2002; 28(6): 942–947.

21.Lichter H, Staver PR, Thompson K. Frontiers in refractive surgery. Interwave aberrometry for custom ablation. Int Ophthalmol Clin; 2002; 42:41–54.

22.Oshika T, Miyata K, Tokunaga T. Higher order wavefront aberrations of cornea and magnitude of refractive correction in laser in situ keratomileusis. Ophthalmology; 2002; 109:1154–1158.

23.Roberts C, Mahmoud A, Herderick EE, Chan G. Characterization of corneal curvature changes inside and outside the ablation zone in LASIK. Invest Ophthalmol Vis Sci;() 2000; 41(suppl): S679.

24.Roberts C. The cornea is not a piece of plastic. J Refract Surg; 2000; 16:407–413.

25.Roberts C. Biomechanics of the cornea and wavefront-guided laser refractive surgery. J Refract Surg; 2002; 18:S589–S592.

26.Nagy ZZ, Palagyl-Deak I, Kovacs A. Wavefront-guided photorefractive keratectomy for myopia and myopic astigmatism. J Refract Surg; 2002; 18:S615–S619.

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