16

LASEK vs. LASIK: Comparison of Visual Outcomes

Neal J.Peterson, MD, Alice Z.Chuang, PhD, Rajy M.Rouweyha, MD,

and Richard W.Yee, MD

Hermann Eye Center, University of Texas Health Science Center at

Houston

Houston, TX

INTRODUCTION

Photorefractive keratectomy (PRK) has been demonstrated to be a safe and effective treatment of low to moderate myopia (1,2). Since the introduction and advancement of the excimer laser, increasingly excellent results are being achieved with PRK. Despite these excellent results, postoperative pain, a relatively long recovery period, and the development of stromal haze limit the role of PRK in the current refractive surgery arena

(3).

Laser in situ keratomileusis (LASIK) has largely replaced PRK. LASIK offers the advantages of minimal postoperative pain or discomfort, rapid visual recovery, good refractive stabilization, and absence of stromal haze development. These advantages, especially the rapid visual recovery and minimal discomfort, led to the rapid popularization of LASIK. This popularization continues to guide additional patients to select refractive surgery for the correction of their myopia.

Unfortunately, LASIK is not the perfect refractive surgery. Several studies comparing long-term outcomes of LASIK to PRK have failed to demonstrate any superiority of LASIK (4,5). Additionally, the creation of the lamellar flap in LASIK is associated with many well-described complications (6–11). These complications may potentially cause significant visual impairment and thus limit the usefulness of LASIK. The risks associated with flap formation have influenced some refractive surgeons to select PRK as their procedure of choice over LASIK. Recent advancements in wavefront-guided ablation technology have additionally increased the current interest in refractive techniques that do not require a lamellar flap (12,13).

The ideal refractive surgery would combine the safety of PRK with the comfort and rapid visual recovery of LASIK. In 1996, Azar (14) first performed a refractive surgery that had the potential to combine the advantages of these two refractive surgery techniques. The technique was called the “alcohol-assisted flap PRK.” Later, a slightly modified technique called laser epithelial keratomileusis (LASEK) was described and popularized by Massimo Camellin, MD in 1999 (15).

Camellin proposed that this hybrid of PRK and LASIK eliminates the shortcoming while still maintaining the advantages of each individual procedure. In LASEK, the

LASEK vs. LASIK 177

central corneal epithelium viability is maintained. Thus LASEK has the potential of providing a quick visual recovery with minimal discomfort while eliminating the need for a microkeratome and possible flap-related complications.

Since Camellin first coined the acronym LASEK, several alternate names have been used to describe this technique. Among these alternate names are laser-assisted subepithelial keratectomy (16), subepithelial PRK (17), laser subepithelial keratomileusis (18), and epi-LASEK (19).

In this chapter, we present the data from our own cohort of LASEK-treated and LASIK-treated eyes and review the current literature comparing the long-term outcomes of LASEK and LASIK performed in patients with low to high myopia.

PATIENTS AND METHODS

We retrospectively reviewed the records of 63 consecutive eyes (35 patients) treated with LASEK for myopia ranging between −1.50 to −14.75 diopters (D) (mean −7.40± 2.71 D) with and without astigmatism. Fourteen LASEK-treated eyes were corrected for monovision and were thus eliminated from refractive outcomes analysis. The remaining 49 emmetropia-targeted LASEK-treated eyes were evaluated for refractive outcomes analysis. A similar cohort of 84 consecutive eyes (43 patients) treated with LASIK for myopia ranging between −2.75 to −11.25 D (mean −6.10±2.17 D) with and without astigmatism was also analyzed. Twelve LASIK-treated eyes were corrected for monovision and were eliminated from refractive outcomes analysis. The remaining 72 emmetropia-targeted LASIK-treated eyes were evaluated for refractive outcomes analysis. All LASEK-treated eyes were assessed for corneal haze development. All LASEK-treated and LASIK-treated eyes were assessed for the number and type of enhancement procedures required over 12 months of follow-up.

A single surgeon (R.W.Y.) performed all refractive surgeries during a 14-month period using the Alcon Summit Autonomous LADAR Vision excimer laser (Orlando, FL). Ablation diameters ranged from 6 to 8 mm based on scotopic Colvard pupillometry measurements. Ablation depths varied from 18 to 174 µm based on preoperative myopia. Both spherical myopia and myopia with astigmatism were treated.

Uncorrected visual acuity, manifest refraction, best-spectacle Snellen visual acuity, stability of refraction, enhancement procedures performed, development of haze, and development of other complications were assessed before and up to 12 months after surgery. LASEK-treated and LASIK-treated eyes were examined on day 1, at 1 and 2 weeks, and at 1, 3, 6, and 12 months. Comparisons of the two techniques were made at these time points. Twelve-month data were available for 33 LASEK-treated and 36 LASIK-treated eyes.

LASEK Technique

A lid speculum is applied to ensure adequate exposure, and the cornea is marked inferiorly. This is followed by epithelial micro-rephination. Using a 60-to 80-µm deep calibrated blade, the trephine is centered on the pupil, pressed down, and rotated slightly. The 270-degree trephine is specially designed to leave a hinge of 60 to 80 degrees at the

LASEK, PRK, and excimer laser stromal surface ablation 178

12-o’clock position. We used two ring sizes for our epithelial micro-trephinations: 8.0 mm and 9.0 mm. The larger 9.0-mm trephine was used for larger treatment zones.

A cylindrical well with a diameter 0.5 mm wider than the incision is then centered on the pupil encircling the previous incision. Several drops of 20% ethanol solution (created by diluting nonpreserved absolute ethanol in balanced salt solution) are instilled into the well. This 20% ethanol solution is left in contact with the corneal epithelium for approximately 45 seconds. The ethanol solution is then removed using a dry weck cell sponge, followed by a thorough rinsing of the cornea epithelium with balanced salt solution.

The inferior margin of the epithelium is lifted using a blunt end of a number 69 Beaver blade and gently peeled toward the 12-o’clock position. As the epithelium is gathered toward the 12-o’clock position, the hinge left on micro-trephination maintains epithelial attachment. Once the epithelial flap is completely gathered at the 12-o’clock position, a standard small-spot PRK ablation is performed.

Once ablation is complete, the epithelial flap is repositioned using a fine canula or small spatula. Extra care is taken to smooth the epithelial surface without creating tears in the epithelium. Finally, a soft contact lens (B&L Purevision) is placed on the eye and left in position for 3 to 4 days to protect the epithelial flap. The lid speculum is removed, and epithelium and contact lens position are examined by slit-lamp microscopy.

Several specially designed surgical tools have been designed for use in LASEK. These tools include a micro-trephine with an 80-degree hinge (Janach J2900S) and the alcohol solution well (Janach J2905), which may be used in the initial steps of microtrephination and ethanol-assisted epithelial debridement. In addition, the epithelial microhoe (Janach J2915A) and repositioning spatula (Janach J2920A) may make flap removal and repositioning easier.

RESULTS

Baseline demographic and refractive characteristics of the LASEK and LASIK-treated groups are presented in Table 1. There was no difference in the age or sex ratio of the two groups, but a statistically significant difference in the degree of preoperative myopia was noted. LASEK-treated eyes were significantly more myopic than LASIK-treated eyes. The preoperative mean spherical equivalent (MSE) of the LASEK-treated eyes was −7.40±2.71 D, compared to −6.10±2.17 D in LASIK-treated eyes (p=0.002). This difference was caused by selection criteria used to determine good candidates for LASIK. Patients with insufficient corneal thickness for LASIK were given the option of LASEK for correction of their myopia.

After the initial healing period, LASEK-treated eyes obtained refractive outcomes comparable with that of LASIK-treated eyes. Figure 1 shows the cumulative percent of eyes with uncorrected visual acuity (UCVA) better than 20/40 for each treatment group at consecutive time intervals. After the first 2 weeks, there was no statistically significant

LASEK vs. LASIK 179

Table 1. Baseline Demographic and Refractive

Data.

*Mean spherical equivalent. †Statistically significant P < 0.05.

Figure 1 Cumulative percent of all LASEK and LASIK eyes with UCVA better than 20/40 at consecutive time intervals.

difference observed between the two groups and the percent of eyes that achieved UCVA better than 20/40 (p < 0.0001 at day 1, p < 0.005 at weeks 1 and 2, p > 0.05 at all other time points). The difference in refractive outcomes seen during the first 2 weeks of recovery was most likely caused by epithelial flap irregularity (20).

Figure 2 shows the cumulative percent of LASEK-treated eyes that obtained UCVA of 20/20, 20/25, and 20/40 at time points 1, 3, 6, and 12 months. The percent of eyes obtaining UCVA of 20/20 continued to increase from 52% (n=46) at 1 month to 67% (n=33) at 12 months. In contrast, the cumulative percent of eyes maintaining UCVA of at least 20/40 displayed a slight regressive trend from 93% (n=44) of eyes at 3 months to 88% (n=33) of eyes at 12 months.

LASEK, PRK, and excimer laser stromal surface ablation 180

Figure 2 Cumulative uncorrected visual acuity in LASEK-treated eyes.

Figure 3 Cumulative uncorrected visual acuity in LASIK-treated eyes.

Distribution of UCVA obtained in LASIK-treated eyes is portrayed in Figure 3. The percent of LASIK-treated eyes obtaining UCVA of 20/20 was 54% (n=70) at 1 month, and increased to 75% (n=36) by 12 months. Unlike the slight regressive pattern seen by 12 months in LASEK-treated eyes, the 87% (n=70) of LASIK-treated eyes with UCVA of at least 20/40 at 1 month increased to 97% (n=36) by 12 months.

LASEK, PRK, and excimer laser stromal surface ablation 182

equivalent, which may mask refractive errors through offsetting spherical and cylindrical components, the defocus equivalent takes into account only the absolute value of refractive errors. The defocus equivalent is calculated in the following equation: defocus equivalent = |sphere| + |½ cylinder|. Figures 6 and 7 depict the defocus equivalent of LASEK-treated and LASIK-treated eyes at 1 month and 12 months, respectively. In general, LASEK-treated eyes trended toward a slightly increased defocus equivalent from month 1 to month 12, whereas LASIK-treated eyes retained a stable defocus equivalent over the 12–month period.

Refractive stability was obtained within 1 month and was maintained for the entire year for LASIK-treated eyes. LASEK-treated eyes rapidly progressed to near emmetropia within the first month, but then proceeded to follow a gradual trend of regression over the next 12 months. The trend of refractive stability of both procedures is portrayed in Figure 8. Mean spherical equivalent for LASEK-treated eyes at 1 month was 0.10±0.73 D, and −0.24±0.67 D for LASIK-treated eyes (p=0.01). By 12 months, the mean

Figure 6 Percent eyes at consecutive defocus equivalents at 1 month.

LASEK vs. LASIK 183

Figure 7 Percent eyes at consecutive defocus equivalents at 12 months.

spherical equivalent was −0.68±1.08 D and −0.27±0.56 D for LASEK-treated and LASIK-treated eyes, respectively (p=0.06).

The safety of both procedures was exceptional. No eye treated with LASEK or LASIK had lost two or more lines of best-corrected visual acuity (BCVA) at 12 months. The distribution of eyes and lines of BCVA gained or lost at 12 months is depicted in Figure 9.

Complications

Overcorrections of greater than 0.5 D occurred in seven LASEK-treated eyes (n=45) and in four LASIK-treated eyes (n=70) at 1 month. No clinically significant difference was

LASEK, PRK, and excimer laser stromal surface ablation 184

Figure 8 Plot graph of mean spherical equivalent from pre-op to 12 months.

Figure 9 Change in BCVA at 12 months from baseline.

noted between the rates of overcorrection in the two groups (p=0.10). Overcorrection was more likely to occur in LASEK-treated eye with a higher degree of myopia. By 12 months, 1 LASEK (n=33) and 1 LASIK-treated eye (n=36) remained overcorrected.

Haze development is a concern in PRK-like procedures and is more common with deeper ablation depths. It has been proposed that the incidence of haze is decreased in

LASEK vs. LASIK 185

LASEK as compared to PRK (22–24). Our LASEK-treated eyes were followed-up and examined for signs of haze development during the 12-month follow-up period. The presence of haze was recorded as follows: 0=no haze, clear cornea; trace (grade 0.5)= haze barely discernable by slit-lamp microscopy; grade 1=haze easily seen with slit-lamp microscopy but does not affect vision; grade 2=dense haze that affects vision; grade 3=dense haze that obscures some iris details; and grade 4=dense haze that completely obscures iris details.

The incidence of haze developing at any point during the 12-month follow-up period in LASEK-treated eyes was 51% (n=63). Figure 10 shows the distribution of maximum

Figure 10 Maximum haze grade in

LASEK-treated eyes by 12 months.

haze development during the 12-month follow-up period. The mean grade of maximum haze development over 12 months was 0.50±0.63. Six eyes in five patients had clinically significant haze (grade 2+) develop. Clinically significant haze was more likely to develop in eyes with a higher degree of myopia, thus requiring deeper ablation depths. The mean degree of preoperative myopia in eyes with clinically significant haze developing was −9.75±2.44 D (range −7.88 to −14.50 D).

By 12 months, 11 eyes in eight LASEK patients had undergone regression for a rate of 17% (n=63). Four of the regressed eyes developed 2+ corneal haze during the regression process. At 12 months, all 11 eyes had BCVA of 20/25 or better. One additional patient had a posterior subcapsular cataract develop, along with late-onset 2+ corneal haze in one LASEK-treated eye at 12 months.

Eleven eyes in eight LASIK patients also underwent regression, for a rate of 13% (n=84). At 12 months, all 11 eyes had BCVA of 20/25 or better. One LASIK-treated eye developed epithelial in-growth that did not require retreatment. Additionally, one LASIKtreated eye incurred a vitreous detachment at 10 months. No intra-operative flap related complications were encountered.

Enhancement procedures were performed on LASEK-treated and LASIK-treated eyes that had undergone regression, had residual astigmatism, or that remained overcorrected. Enhancement procedures included second LASEK and LASIK procedures, along with

LASEK, PRK, and excimer laser stromal surface ablation 186

astigmatic keratotomy. Through the 12 months, 19 LASEK-treated eyes and 27 LASIKtreated eyes required enhancement procedures, for a rate of 30% (n=63) and 32% (n=84), respectively. Of the eyes that required enhancement, 11 (17%) of the LASEK-treated eyes, compared to 14 (17%) of the LASIK-treated eyes, had a second LASEK or LASIK procedure for enhancement. No statistical difference was noted between the rates of enhancement procedures, the type of enhancement procedures performed (AK vs. reLASIK/LASEK), or the number of enhancement procedures performed on a single eye.

DISCUSSION

We add our cohort of LASEK-treated eyes to the growing body of studies demonstrating LASEK to be a safe and effective treatment for a wide range of myopia with and without astigmatism. Our LASEK-treated eyes maintained uncorrected visual acuity comparable to their LASIK-treated counterparts in this study. Even though good visual acuity was maintained, LASEK-treated eyes followed a slight regressive pattern. Approximately −0.75 D of myopic regression was seen when mean spherical equivalents were followed from month 1 to month 12. This regressive trend may be due to the high degree of preoperative myopia treated with LASEK. This myopic regression in LASEK has not been previously reported.

Many studies report excellent long-term refractive outcomes for LASEK-treated eyes. Shahinian (16) followed a cohort of 146 LASEK-treated eyes for a 12-month period. He found that after 12 months, 56% of his LASEK-treated eyes had an UCVA of 20/20 whereas 96% maintained UCVA of 20/40 or better (n=55). In addition, the mean refraction of his cohort was stable and close to zero from month 1 to month 12, indicating no myopic regression. Claringbold (25) reports similar refractive outcomes in his cohort of 222 myopic eyes treated with LASEK. At 12 months, 82% of LASEK-treated eyes had an UCVA of 20/20, and 100% had UCVA 20/25 or better (n=84); 96.4% of eyes were within ±0.5 D of intended correction, and all eyes were within ±0.75 D.

Anderson et al. (19) conducted the largest study of LASEK-treated eyes to date. They treated and followed-up 343 eyes for up to 6 months. In this study, the authors found that 85% of LASEK-treated eyes were within ±0.5 D and 94% were within ±1.0 D of the intended correction at 6 months (n=115); 98% of eyes maintained UCVA of at least 20/40 through 6 months (n=122).

Autrata et al. (26) performed the LASEK study with the longest follow-up period. Their cohort consists of 92 patients with low to moderate myopia who underwent LASEK in one eye and PRK in the other. All eyes were followed-up for a 24-month period, without any patients lost to follow-up. At 24 months, 91% of LASEK-treated eyes had UCVA better than 20/40, with 62% of eyes within ±0.5 D and 92% within ±1.0 D of the intended refraction (n=92). Although not discussed in the study, their PRK and LASEK-treated eyes followed a myopic regression pattern similar to that seen in our cohort. Their mean spherical equivalent at 1 month was 0.81±0.95 D, which slowly regressed to −0.21±0.43 D by 12 months. From month 12 to 24, refraction stabilized. This 1.0 D of myopic regression is very similar to the 0.75 D of regression observed in our LASEK-treated eyes with low to high myopia.

LASEK vs. LASIK 187

Because of its enormous popularity, an increasing number of patients seek out refractive surgeons to perform LASIK for correction of myopia. As was seen in our cohort of patients, many potential refractive surgery candidates are prevented from having LASIK because of inadequate corneal thickness. In this situation, LASEK should be offered as the refractive surgery of choice. Several additional clinical situations are encountered when LASEK has been proposed as the primary refractive surgery modality. These situations include: patients with steep or flat corneas; patients involved in activities or occupations that predispose them to eye trauma and possible flap dehiscence; large pupils that require a wider and therefore deeper ablation; high myopia; keratoconus suspect; deep-set eyes; narrow palpebral fissure; glaucoma; filtering blebs; anterior scleral buckle; previous vitrectomy; optic nerve drusen; dry eye disease; and cases of patient apprehension caused by microkeratome use (14,16,19).

Elimination of the microkeratome is the major advantage of LASEK over LASIK. Avoiding use of the microkeratome prevents flap-related complications such as complete flap removal, flap slippage, diffuse lamellar keratitis (DLK), epithelial ingrowth, peripheral flap melt, flap dehiscence, buttonhole, incomplete flap, deep infectious keratitis, corneal perforation, and corneal ectasia (6–11,27,28).

The epithelial flap created in LASEK is not known to be associated with the same complications seen with the lamellar flap of LASIK. Even in the worst-case scenario, if the epithelial flap is completely removed during LASEK, the procedure simply becomes a standard PRK, which has been repeatedly shown to be a safe and effective treatment for myopia with excellent and predictable refractive outcomes.

Scerrati (29) describes two additional advantages of LASEK over LASIK. He found that his group of 30 LASEK-treated eyes had fewer aberrations as compared to 30 LASIK-treated eyes when comparing corneal topographic meridian values. This LASEK group also had better contrast sensitivity at 3 months and substantially increased contrast sensitivity at 6 months compared to the LASIK-treated eyes.

When Camellin first described LASEK, he proposed that it had the advantages of less postoperative pain and haze than is seen in PRK. Several studies have investigated these claims by conducting prospective, comparative, paired eye trials in which one eye is treated with PRK and the other with LASEK in the same patient (23,24,26,30).

All three studies designed to investigate corneal haze found that mean corneal haze was significantly less in LASEK-treated eyes as compared to their PRK-treated counterparts. Lee et al. (23) found that haze was significantly reduced at 1 month, but that there was no difference in the amount of haze at 3 months. Shah et al. (24) found LASEKtreated eyes had a statistically significant reduction in haze at 12 months as compared to their PRK paired eye. Autrata et al. (26) found a statistically significant reduction in mean corneal haze score at all time points investigated (1, 3, 6, 12, and 24 months).

Reported maximal mean corneal haze scores range from 0.46 (24) to 0.73 (26). Our maximal mean corneal haze score was 0.50±0.63 for the 12 months that LASEK-treated eyes were followed-up. This haze score is akin to previously reported scores, despite the high degree of preoperative myopia treated in our LASEK group.

Anderson et al. (19) reported clinically significant haze developing in 1.6% of LASEK-treated eyes (n=295). We observed clinically significant haze developing at a rate of 9.5% in our cohort of LASEK-treated eyes. The difference seen may be attributable to the high degree of preoperative myopia treated in our cohort. Shahinian

LASEK, PRK, and excimer laser stromal surface ablation 188

(16) reports that haze development was more common in eyes with higher preoperative myopia. In our cohort, eyes that developed clinically significant haze likewise had a higher degree of myopia than did eyes that remained haze-free.

Mild discomfort and foreign body sensation are seen in LASEK during the first few days postoperatively. Several studies have demonstrated less pain associated with LASEK than with PRK. Lee et al. (24) found that patients rated pain levels on a subjective pain scale significantly less in LASEK-treated than in PRK-treated eyes. Autrata et al. (26) also found that LASEK resulted in an overall decreased pain score on days 1 through 3, as reported by patients on a questionnaire of subjective pain levels. Anderson et al. (19) reported that 87% of LASE K-treated eyes experienced no postoperative pain (n=342).

One study contradicts these results and claims that LASEK results in more postoperative pain than PRK. Litwak et al. (30) found that 72% of their patients reported more discomfort on day 1 in LASEK-treated than PRK-treated eyes (n=25). This increased to 80% of patients reporting more discomfort in the LASEK eye by day 3.

It is postulated that the epithelial flap is responsible for the decreased incidence of haze and pain seen in LASEK by conferring mechanical and immunochemical protection. As such, much research has gone into determining the factors involved in this protection and the extent of cell viability maintained by the epithelial flap. Corneal epithelial cell viability has been demonstrated in cells exposed to 20% alcohol for up to 30 seconds (31–33). The viability of these cells fell off markedly when exposed to higher concentrations of alcohol, or duration exceeding 45 seconds. After exposing the corneal epithelial cell to alcohol for 60 seconds, the majority of cells had died (31).

Removal of the corneal epithelium causes damage to the underlying stromal keratocytes. Decreases in keratocyte density are known to lead to increased keratocyte density and increased collagen and extracellular matrix synthesis (24). This activation of stromal keratocytes is believed to play a role in the subepithelial haze seen in PRK.

Various cytokines are also known to function in stromal wound healing after excimer laser keratectomy. Among these cytokines are transforming growth factor β (TGF-β) and keratinocyte growth factor. TGF-β expression is increased in tears after PRK (34). Increased TGF-β expression and TGF-β receptor expression play a key role in the activation and proliferation of stromal keratocytes. Activation of stromal keratocytes leads to the deposition of extracellular matrix and the formation of corneal fibrosis, scarring, and haze (35). Lee et al. (36) demonstrated that levels of TGF-β1 and other cytokines released in the tears are decreased in eyes treated with LASEK than in eyes treated with PRK. This decreased cytokine production may be one of the reasons for less haze formation after LASEK. In addition, a viable epithelial flap further decreases stromal exposure to these cytokines by acting as a mechanical barrier blocking tears from reaching freshly ablated and healing stroma. Lastly, this epithelial flap acts as a biological therapeutic lens that protects the ablated stroma and may reduce pain levels.

In conclusion, LASEK offers a safe and effective treatment for low to high myopia. LASEK may be offered in many situations in which LASIK is not an option. Refractive outcomes for LASEK are excellent and comparable to refractive outcomes achieved with LASIK. By eliminating the use of a microkeratome, LASEK avoids any of the flaprelated complications seen in LASIK. The rate of haze formation in LASIK is low but may be more likely with deeper ablation depths. LASEK-treated eyes have less haze

LASEK vs. LASIK 189

formation and pain than do PRK-treated eyes. Finally, lower residual aberrations that are achieved in LASEK may maximize the advantages of wavefront-guided ablations.

REFERENCES

1.Gartry DS, Kerr Muir M, Marshall J. Excimer laser photorefractive keratectomy: 18-month follow-up. Ophthalmology; 1992; 99:1209–1219.

2.Salz JJ, Maguen E, Nesburn AB. A two-year experience with excimer laser photorefrative keratectomy for myopia. Ophthalmology; 1993; 100:873–882.

3.Alio JL, Artola A, Claramonte PJ. Complications of photorefractive keratectomy for myopia: two-year follow-up of 3000 cases. J Cataract Refract Surg; 1998; 24:619–626.

4.Hersh PS, Brint SF, Maloney RK. Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia. A randomized prospective study. Ophthalmology; 1998; 105:1513–1523.

5.Pop M, Payette Y. Photorefractive keratectomy versus laser in situ keratomileusis. A controlmatched study. Ophthalmology; 2000; 107:251–257.

6.Melki SA, Azar DT. LASIK complications: etiology, management, and prevention. Survey of Ophthalmology; 2001; 46:95–116.

7.Jacobs JM, Taravella MJ. Incidence of intraoperative flap complications in laser in situ keratomileusis. J Cataract Refract Surg; 2002; 28:23–28.

8.Lin RT, Maloney RK. Flap complications associated with lamellar refractive surgery. Am J Ophthalmol; 1999; 127:129–136.

9.Stulting RD, Carr JD, Thompson KP. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology; 1999; 106:13–20.

10.Knorz MC. Flap and interface complications in LASIK. Curr Opin Ophthalmol; 2002; 13: 242– 245.

11.Ambrosio R Jr, Wilson SE. Complications of laser in situ keratomileusis: etiology, prevention, and treatment. J Refract Surg; 2001; 17:350–379.

12.Panagopoolou SI, Pallikaris IG. Wavefront customized ablations with the WASCA Asclepion Workstation. J Refract Surg; 2001; 17:S608–S612.

13.Wilson SE, Mohan RR, Hong J-W. The wound healing response after laser in situ keratomileusis and photorefractive keratectomy: elusive control of biological variability and effect on custom laser vision correction. Arch Ophthalmol; 2001; 119:889–896.

14.Azar DT, Ang RT, Lee JB. Laser subepithelial keratomileusis: electron microscopy and visual outcomes of flap photorefractive keratectomy. Curr Opin Ophthalmol; 2001; 12:323–328.

15.Cimberle U, Camellin M. LASEK may offer advantages of both LASIK and PRK. Ocular Surg News Int; 1999; 10:14–15.

16.Shahinian L Jr. Laser-assisted subepithelial keratectomy for low to high myopia and astigmatism. J Refract Surg; 2002; 28:1334–1342.

17.Kornilovsky IM. Clinical results after subepithelial photorefractive keratectomy (LASEK). J Refract Surg; 2001; 17(supplement):S222–S223.

18.Dastjerdi MH, Soong HK. LASEK (laser subepithelial keratomileusis). Curr Opin Ophthalmol; 2002; 13:261–263.

19.Anderson NJ, Beran RF, Schneider TL. Epi-LASEK for the correction of myopia and myopic astigmatism. J Cataract Refract Surg; 2002; 28:1343–1347.

20.Rouweyha RM, Chuang AZ, Mitra S. Laser epithelial keratomileusis for myopia with the autonomous laser. J Refract Surg; 2002; 18:217–224.

21.Waring GO. Standard graphs for reporting refractive surgery. J Refract Surg; 2000; 16: 459– 466.

LASEK, PRK, and excimer laser stromal surface ablation 190

22.Lee JB, Choe C-M, Seong GJ. Laser subepithelial keratomileusis for low to moderate myopia: 6-month follow-up. Jpn J Ophthalmol; 2002; 46:299–304.

23.Lee JB, Seong GJ, Lee JH. Comparison of laser epithelial keratomileusis and photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg; 2001; 27:565–570.

24.Shah S, Sebai Sarhan AR, Dole SJ. The epithelial flap for photorefractive keratectomy. Br J Ophthalmol; 2001; 85:393–396.

25.Claringbold TV. Laser-assisted subepithelial keratectomy for the correction of myopia. J Cataract Refract Surg; 2002; 28:18–22.

26.Autrata R, Rehurek J. Laser-assisted subepithelial keratectomy for myopia: two-year follow-up. J Cataract Refract Surg; 2003; 29:661–668.

27.Mifflin M, Kim M. Laser in situ keratomileusis flap dehiscence 3 years postoperatively. J Cataract Refract Surg; 2002; 28:733–735.

28.Sridhar MS, Rapuano CJ, Cohen EJ. Accidental self-removal of a flap—a rare complication of laser in situ keratomileusis surgery. Am J Ophthalmoly; 2001; 132:780–782.

29.Scerrati E. Laser in situ keratomileusis vs. laser epithelial keratomileusis (LASIK vs. LASEK). J Refract Surg; 2001; 17(supplement):S219–S221.

30.Litwak S, Zadok D, Garcia-de Quevedo V. Laser-assisted subepithelial keratectomy versus photorefractive keratectomy for correction of myopia. A prospective comparative study. J Cataract Refract Surg; 2002; 28:1330–1333.

31.Gabler B, Winkler Von Mohrenfels C, Dreiss AK. Vitality of epithelial cells after alcohol exposure during laser-assisted subepithelial keratectomy flap preparation. J Cataract Refract Surg; 2002; 28:1841–1846.

32.Chen CC, Chang JH, Lee JB. Human corneal epithelial cell viability and morphology after dilute alcohol exposure. Invest Ophthalmol Vis Sci; 2002; 43:2593–2602.

33.Dreiss AK, Winkler Von Mohrenfels C, Gabler B. Laser epithelial keratomileusis (LASEK): histological investigation for vitality of corneal epithelial cells after alcohol. Klin Monatsbl Augenheilkd; 2002; 219:365–369.

34.Vesaluoma M, Teppo AM. Gronjagen-Riska C, Tervo T. Release of TGF-beta 1 and VEGF in tears following photorefractive keratectomy. Curr Eye Res; 1997; 16:19–25.

35.Kaji Y, Mita T, Obata H. Expression of transforming growth factor β superfamily and their receptors in the corneal stromal wound healing process after excimer laser keratectomy (letter). Br J Ophthalmol; 1998; 82:462–463.

36.Lee JB, Choe CM, Kim HS. Comparison of TGFβ1 in tears following laser subepithelial keratomileusis and photorefractive keratectomy. J Refract Surg; 2002; 18:130–134.

17 Topography-Based Aberration in LASEK

vs. PRK and LASIK

Michael K.Smolek, PhD, Stephen D.Klyce, PhD, and Loan Nguyen,

MD

LSU Eye Center

New Orleans, LA

Richard W.Yee, MD and John P.Stokes, MD

Hermann Eye Center, University of Texas Health Science Center at

Houston

Houston, TX

Marguerite B.McDonald, MD

Southern Vision Institute

New Orleans, LA

INTRODUCTION

The current state of the art of excimer laser vision correction encompasses three basic types of surgical procedures: photorefractive keratectomy (PRK), laser in-situ keratomileusis (LASIK), and laser epithelial keratomileusis (LASEK). Given the recently acquired ability to extract detailed optical aberration information from ocular and corneal topography wavefronts, interest has turned to how these three procedures compare in terms of their average optical performance. Does the LASEK procedure generate fewer aberrations than PRK or LASIK, or does it produce a similar outcome? Although all three procedures use excimer laser ablation to reshape the cornea, the methods differ in complexity and in the complications that may arise during and after surgery. Some of these complications directly affect optical aberrations and visual performance.

The PRK procedure was first described by Trokel in 1983 (1), and there soon followed a number of studies on the mechanisms and efficacy of the ablation process (2–4). PRK in blind-eye human studies was reported soon after Trokel’s description (5,6), and the first sighted-eye case report was presented by McDonald in 1989 (7). The PRK procedure requires the surgical removal of a circular patch of epithelium over the central cornea (typically by debridement), followed by precisely controlled reshaping of the stroma through tissue ablation with the excimer laser (8). After ablation, epithelial cells must proliferate and migrate over the ablated stromal surface from the intact peripheral epithelium, a process that usually takes 3 to 5 days. PRK complications can include corneal haze, halos, glare, postoperative pain, and a regression from the desired refractive correction (9,10).

Topography-based aberration in LASEK vs. PRK and LASIK 193

The LASIK procedure was patented by Peyman in the mid 1980s (11,12), later clinically introduced by Pallikaris in 1989 (13), and subsequently described by Burrato (14,15). The LASIK procedure requires the temporary lifting of a thinly cut flap of anterior stromal tissue under which the stromal bed is reshaped by the excimer laser. Creation of the flap by a microkeratome requires surgical expertise and sets it apart from ordinary PRK. LASIK has a number of clinically significant advantages over PRK, including reduced likelihood of haze and pain because of the retention of an intact central epithelium. In addition, LASIK shows less regression of the desired refractive effect. However, LASIK does have unique complications that include problems associated with the creation or replacement of the flap, epithelial ingrowth or other infiltrates occurring beneath the flap, and the possibility of corneal distension resulting from a stromal bed that is too thin to support the tensile forces arising from intraocular pressure (16,17).

LASEK was introduced by Camellin in 1999 (M. Camellin, MD. “LASEK May Offer the Advantages of Both LASIK and PRK.” Ocular Surgery News, International Edition, March 1999). Only a few reports on LASEK have been published to date (18–20). In a simplistic sense, LASEK is an attempt to use the best aspects of PRK and LASIK while avoiding the worst complications of either procedure. The typical LASEK procedure begins with the controlled application of diluted alcohol to initiate the release of epithelium from the stroma. The epithelial cells remain attached to one another in a sheet through retention of intercellular junctions, but the basement epithelial cells release their binding to the basement membrane. The intact epithelial sheet (typically circular) is carefully scraped from the central cornea, but an attachment to the cornea along a hinge of epithelium is retained. The bare stroma is then ablated by the excimer laser just as in PRK, and the sheet of epithelium is repositioned over the cornea. The LASEK procedure has been shown to significantly increase postoperative comfort for the patient as compared to PRK (19).

In the current study, we evaluated the corneal topography of LASEK, PRK, and LASIK for corneal wavefront aberrations preoperatively and at specific times in the early postoperative period. Data were compared to determine whether significant differences exist between LASEK and PRK or LASIK in terms of aberrations caused by corneal shape.

MATERIALS AND METHODS

This study was conducted in accordance with the Declaration of Helsinki for human research. All data were acquired retrospectively from historical examination records for two surgeons at two different clinics: Dr. McDonald for the Southern Vision Institute in New Orleans, Louisiana and Dr. Yee for the Hermann Eye Center at the University of Texas in Houston, Texas. All eyes in the study were treated for low to moderate amounts of myopic astigmatism. Although all of the corrections included astigmatism, in this report we use the generic acronyms of LASEK, PRK, and LASIK when referring to the three cohorts. All LASEK data came exclusively from the Hermann Eye Center, where the corneal topography was recorded with an EyeSys 2000 corneal topographer. All PRK and LASIK data were provided by the Southern Vision Institute and recorded with the Tomey TMS corneal topographer. Records were reviewed for quality of the topographic

LASEK, PRK, and excimer laser stromal surface ablation 194

examinations, and topographic maps suitable for analysis were collected for preoperative examinations and at follow-up examinations at 1 week, 1 month, 3 months, and 6 months, postoperatively. Different lasers were known to be used among the three cohorts, but no information was recorded to specify whether different laser software or nomograms were used within each cohort. Details about the LASEK, PRK, and LASIK patient cohorts are shown in Table 1.

All corneal topography was identically analyzed for corneal wavefront aberrations using CTView version 3.18 (Sarver and Associates, Merritt Island, FL). CTView is a

Table 1. Cohort Information.

software application that provides identical topographic and wavefront analysis to be performed with data from different commercial topography systems. CTView was set to its default settings using a 10-mm corneal diameter topographic fit to determine the focal point of the cornea and a 4-mm “entrance pupil” diameter for determination of the wavefront errors. Wavefront error was specified as the sum of the RMS (root mean square) error of Zernike polynomial coefficients describing wavefront shape, which avoided dealing with sign differences for terms in left and right eyes when raw Zernike coefficients are used (21). Only specific aberration types were collected for this study: total aberrations for the second to sixth order, low aberrations of the second order, highorder aberrations of the third to sixth order, second-order astigmatism, second-order defocus, third-order coma, and fourth-order spherical aberration.

RESULTS

Table 2 shows the mean manifest refraction data for the cohorts for the preoperative and 6-month periods. Note that the cohorts were not identical in that spherical error of the LASEK cohort was greater than that of the PRK and LASIK groups. Table 3 shows the mean spherical equivalent of the manifest refraction for preoperative and 6-month periods. The residual error indicates that the achieved LASIK correction was less than either LASEK or PRK. Table 4 indicates the mean attempted correction programmed into the laser algorithm for the corneal plane.

Corneal aberrations measured in microns of mean RMS error were extracted from topography maps and plotted as a function of time in months. Results indicated that LASEK topography had significantly less total corneal aberration (Fig.1) and significantly less second-order low-order corneal aberration than for the PRK or LASIK cohorts at 1 week and beyond. The low-order aberrations are virtually identical to total aberrations in terms of magnitude and trend and are not shown here. There was a large

Topography-based aberration in LASEK vs. PRK and LASIK 195

spike in total and low aberrations at 1 week for the PRK and LASIK cohorts, followed by a decline to aberration

Table 2. Mean Manifest Refraction.

*Values are in diopters at the spectacle plane.

Table 3. Mean Manifest Refraction Spherical

Equivalent.

*Values are in diopters at the spectacle plane.

error levels that were not significantly different from the preoperative state at 6 months. Meanwhile, LASEK exhibited a response that was completely reversed to that of PRK and LASIK. In LASEK, the total and low-order aberrations initially declined at 1 week and then returned to a level that was not significantly different from the preoperative level.

Both PRK and LASIK showed an immediate increase in defocus at 1 week and beyond (Fig. 2), whereas LASEK showed a rise only at 1 month and beyond. Curiously, the preoperative defocus aberrations were significantly greater for PRK and LASIK compared to LASEK. The cause for this result is not known.

The astigmatism aberration (Fig. 3) showed an immediate and significant decline at 1 week and beyond for LASEK, but the PRK and LASIK cohorts showed no significant decline. At 6 months, LASEK astigmatism was significantly less than LASIK. With slightly larger sample sizes to reduce variance, the 6-month difference between LASEK and PRK might become significant as well.

As expected, all three procedures exhibited an increase in their high-order corneal aberrations after surgery (Fig. 4). In particular, LASEK had significantly less high-order aberrations than PRK from 1 week to 3 months. This effect was caused primarily by the immediate and large postoperative increase in high-order aberrations seen with PRK, which gradually declined over time. In contrast, the LASEK cohort showed no immediate peak in postoperative high-order aberration, but the error increased at 1 month and plateaued thereafter. LASIK high-order aberration response showed an increase at 1 week, followed by a stabilization at 1 month and beyond. Postoperative LASEK and

LASEK, PRK, and excimer laser stromal surface ablation 196

LASIK results for high-order aberrations were therefore not significantly different except at 1 week. Curiously, the preoperative PRK group was significantly higher than both the LASEK and LASIK cohorts, which complicated the analysis.

With respect to individual high-order aberrations, fourth-order spherical aberration showed a large and significant increase for all three cohorts at the 6-month period compared

Table 4. Mean Attempted Correction.

*Values are in diopters at the corneal plane.

Figure 1 Total aberrations of the cornea as a function of time in months. Aberration is defined by the mean of the sum of RMS error in microns for Zernike decomposition from the second to sixth radial order. Error bars indicate ± standard error of the mean. Asterisks indicate time periods in when the LASEK cohort was significantly different from both the PRK and LASIK cohorts (p<0.05).

LASEK, PRK, and excimer laser stromal surface ablation 198

Figure 3 Astigmatism aberration of the cornea as a function of time in months. Aberration is defined by the mean of the sum of RMS error in microns for Zernike decomposition of mean. Asterisks indicate time periods when the LASEK cohort was significantly different the two second radial order astigmatism terms. Error bars indicate±standard error of the from both the PRK and LASIK cohorts (p<0.05). The single-barred cross indicates the time period when the LASEK cohort was significantly different from the LASIK cohort only.

to their preoperative values (Fig. 5). As seen with other aberrations, LASEK had a spherical aberration value at 1 week that was not different than the preoperative value. This lagging effect was not seen in the PRK or LASIK cohorts. Both the preoperative PRK and LASIK cohorts were significantly elevated in error compared to the LASEK group.

With respect to third-order coma (Fig. 6), both PRK and LASIK showed a spike at 1 week, whereas the LASEK value again exhibited a lagging effect and did not significantly change from the preoperative value. At 1 month and beyond, all three cohorts exhibited elevated coma. However, we found that the postoperative coma values

Topography-based aberration in LASEK vs. PRK and LASIK 199

for PRK did not significantly differ from the preoperative value, which was already significantly elevated when compared to the LASIK and LASEK cohorts.

Figure 4 High-order aberration of the cornea as a function of time in months. Aberration is defined by the mean of the sum of RMS error in microns for Zernike decomposition of the third through sixth radial order terms. Error bars indicate ± standard error of the mean. Asterisks indicate time periods when the LASEK cohort was significantly different from both the PRK and LASIK cohorts (p0.05). The double-barred cross indicates the time period when the LASEK cohort was significantly different from the PRK cohort only.

LASEK, PRK, and excimer laser stromal surface ablation 200

Figure 5 Spherical aberration of the cornea as a function of time in months. Aberration is defined by the mean of the sum of RMS error in microns for Zernike decomposition of the fourth radial order spherical aberration term. Error bars indicate±standard error of the mean. Asterisks indicate time periods when the LASEK cohort was significantly different from both the PRK and LASIK cohorts (p<0.05).

The mean best spectacle corrected visual acuity (BSCVA) results as a function of time are shown in Figure 7. Figure 8 shows the acuity data for preoperative (top) and 6–month data (bottom) expressed in terms of the frequency of eyes whose BSCVA is specified by the ability to read Snellen chart lines. Note that PRK and LASIK tended to have more cases of poor visual acuity for the preoperative period, which suggests that the preoperative corneal topographies for PRK and LASIK may have been more aberrated than that of LASEK. The graphical data for the high-order aberrations of coma for PRK (Fig. 6) and spherical aberration for both PRK and LASIK (Figure 5) tend to support the idea that the preoperative PRK and LASIK cohorts were more aberrated to begin with, and this seems to be reflected in the preoperative BSCVA data.

Topography-based aberration in LASEK vs. PRK and LASIK 205

the right; superior to the top). Medium green indicates zero aberration. Negative error is indicated by hot colors. Note the strong prevalence of astigmatism aberration in the averaged wavefront. The image simulation is of an 8.5-inch letter chart seen at 20 feet. The dark bar indicates the 20/20 Snellen acuity line and the bottom line indicates 20/10. Note that the 20/20 letters are barely legible because of the aberrations present in the preoperative state.

The map at the top of Figure 9 shows the average wavefront before LASEK surgery (n=20), and the map at the top of Figure 10 illustrates the aberration at 6 months after LASEK surgery (n=10). These wavefronts are presented using a right eye orientation, with a medium green contour indicating zero RMS error. The images at the bottoms of Figures 9 and 10 show the corresponding ray-traced simulations for distance eye charts (at 20 feet) imaged through the averaged corneal wavefronts at the top of the two figures. These simulations are in a sense estimations of the potential acuity with these averaged wavefront errors. In the preoperative condition shown in Figure 9, the 20/20 Snellen acuity line is barely legible (fifth line from top demarcated by horizontal bars), whereas the corresponding postoperative image in Figure 10 shows the relative lack of aberrations at 6 months and the potential ability to see 20/10 Snellen letters. Of course, the simulation does not take into account many other factors involved in seeing and is only indicative of the averaged wavefront data for many corneas and not a specific result for a single patient.

Topography-based aberration in LASEK vs. PRK and LASIK 207

individual maps and is presented in the orientation of the right eye (nasal to the right; superior to the top). Medium green indicates zero aberration. Negative error is indicated by hot colors. Note the strong prevalence of coma and spherical aberration in the averaged wavefront. The image simulation is of an 8.5-inch letter chart seen at 20 feet. The dark bar indicates the 20/20 Snellen acuity line and the bottom line indicates 20/10. Note that even the 20/10 letters are clearly legible, although all letters are surrounded by a fuzziness because of the high-order aberrations present at 6 months.

DISCUSSION

There are several interesting results from this study. First, it was shown that the LASEK procedure apparently produced fewer total and low-order aberrations despite the fact that the attempted refractive correction was larger. In particular, the LASEK procedure appeared to be more efficient in producing an astigmatic correction; however, this interpretation should be made with caution because both the PRK and LASIK procedures attempted smaller cylinder error corrections. The study did not look at the efficiency of correcting individual eyes; therefore, these average results do not take into account cylinder axis and vectorial changes in cylinder error.

A second finding is that all three procedures end up producing essentially similar levels of high-order aberration at 6 months. An increase in high-order aberration is expected with any refractive surgical procedure now in use, because of our limited understanding in controlling aberrations, such as spherical aberration and coma, even with customized corneal ablation procedures. This study suggests that in terms of all high-order aberrations combined, the LASEK method showed no particular advantage over PRK or LASIK by 6 months, but this result should be confirmed by a larger and better-controlled study.

A third finding is the distinctly different response among the three procedures at 1 week. The LASEK cohort tended to show no change at 1 week from the preoperative levels for most individual or grouped aberrations (except astigmatism). However, PRK and, to a lesser extent, LASIK tended to show a large increase in aberrations at 1 week. It is difficult to resolve the cause of this difference. It is known that apoptosis is present immediately after surgery and particularly documented in the case of PRK. Corneal

LASEK, PRK, and excimer laser stromal surface ablation 208

edema caused by apoptosis perhaps results in unusual topographic changes to the cornea, which elicit increased aberration. In LASEK, the epithelium is kept largely intact, and perhaps apoptosis may be reduced because of this. It is not known if bandage lens-fitting differences in the cohorts could cause corneal molding effects to reduce aberrations in the LASEK group at 1 week, which then is resolved by 1 month. Either theory is speculative until the 1-week LASEK data are replicated under more controlled conditions.

The early postoperative topographic and aberration increases that were measured are not demonstrated in the visual acuity of the patients. LASEK corneas tend to have slightly worse BSCVA at 1 month from the preoperative levels, whereas there is very little change in the PRK or LASIK acuities, and perhaps even a suggestion of an improvement. This tends to run contrary to the aberration findings that indicate that total and low-order aberrations are reduced in LASEK and high-order aberrations are approximately the same in all three cohorts. So, why should BSCVA be worse in LASEK at 1 month despite topographical results suggesting it should be improved in comparison to PRK or LASIK? The answer may lie in optical effects that cannot be measured by shape alone, namely in the transmission quality of light. It is possible that there is some form of forward-scattering or haze in the LASEK corneas at 1 month, and this effect has been noted in the literature. We did not look at the effects of haze in this study. It is clear, however, that whatever caused the diminished acuity in the LASEK group was transient, and by 6 months, LASEK acuity was significantly improved compared to 1 month. In fact, LASEK acuity at 6 months is better than that of the LASIK cohort, nearly significantly better than that of PRK, and appears to be on a trend toward continued improvement.

There are limitations in this study, which must be taken into account when assessing the clinical significance of the data. First, two different surgeons were used and, consequently, surgical techniques and environments may play a role in the reported differences in patient outcomes. It is also possible that the surgeons selected patients differently, and this might account for slightly worse preoperative BSCVA in the PRK and LASIK groups (Fig. 7) and with certain high-order aberrations, such as coma and spherical aberration, being elevated in preoperative PRK as compared to LASEK (Figs. 5 and 6). LASIK showed a preoperative elevation in error with spherical aberration but not coma.

The study also did not look specifically for differences in the laser systems, which may affect the efficiency or the smoothness of ablations, nor did we examine in detail the possibility of substantive and planned differences in the surgical goals for the three procedures. It is clear, for example, that the LASIK cohort was not designed to fully correct the cylinder error that was evident in the manifest refraction. This may have been planned as an adjustment to the specific laser system and tends to complicate a direct comparison of the cohorts. However, it should not be surprising to find that the correction of astigmatism with LASIK appeared to be less effective than that seen with LASEK because of this factor.

Finally, we do have concerns about comparing data from two different topographic instruments. Although CTView is intentionally designed to perform cross-platform data analysis using one or more topographic or ocular wavefront systems, there is no published data that indicate that the result is valid under all corneal conditions and for the aberration orders we measured. One might speculate that because the EyeSys topographer

Topography-based aberration in LASEK vs. PRK and LASIK 209

has a lower spatial resolution than the Tomey system for recording data points, this might result in smoother approximations of the corneal surface and, consequently, smoother wavefronts (hence reduced aberrations). However, most of the preoperative aberrations of the two systems were not significantly different from one another. In instances in which there was a difference, such as with third-order coma, two different corneal topographers were alike (LASIK and LASEK data) and data from the same topographer differed (PRK and LASIK). The same was not true with spherical aberration, a fourth-order aberration, in which both PRK and LASIK differed from LASEK at the preoperative period. We suspect that any difference between EyeSys and Tomey topographers may arise in highorder aberration terms but may be less of a concern in the low-order terms. So, the question of inter-topographer variability is moot, given that we do not yet know the radial order cutoff frequency at which the Zernike coefficient fit differs substantially from the actual wavefront surface for either the EyeSys or Tomey topographers.

CONCLUSIONS

There are some apparent topographical advantages with the LASEK procedure, particularly when compared to the PRK method. Although the attempted myopic astigmatism correction was higher in the LASEK cohort, the topographic result was generally better than that found with PRK or LASIK when measured with corneal wavefront aberrations. LASEK corneal aberrations tended to plateau at a constant value by 1 month, whereas PRK aberrations tended to drift for a longer period of time from an immediate postoperative extreme. LASIK aberrations tended to fall between the PRK and LASEK cohort values. Although some of the specific advantages of LASEK appear to be lost by 6 months, the LASEK method generally appeared to have less immediate postoperative variability, more stability over time, and less total and low-order aberrations in the early postoperative period. In particular, the correction of the astigmatism may be more effective with the LASEK approach. The initial worsening of BSCVA reported by LASEK patients at 1 month was unrelated to induced corneal aberrations and was temporary. Ideally, this comparative topographic aberration study should be repeated using a randomized, prospective approach with identical topographic measurement systems, identical laser systems, similarly selected cohorts, and the same refractive surgeon.

REFERENCES

1.Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol; 1983; 96:710–715.

2.Krueger RR, Trokel SL, Schubert HD. Interaction of ultraviolet laser light with the cornea. Invest Ophthalmol Vis Sci; 1985; 26:1455–1464.

3.Puliafito CA, Wong K, Steinert RF. Quantitative and ultrastructural studies of excimer laser ablation of the cornea at 193 and 248 nanometers. Lasers Surg Med; 1987; 7:155–159.

4.Hanna KD, Pouliquen Y, Waring GO, Savoldelli M, Cotter J, Morton K, Menasche M. Corneal stromal wound healing in rabbits after 193-nm excimer laser surface ablation. Arch Ophthalmol; 1989; 107:895–901.

LASEK, PRK, and excimer laser stromal surface ablation 210

5.Taylor DM, L’Esperance FA, Del Poro RA, Roberts AD, Gigstad JE, Klintworth G, Martin CA, Warner J. Human excimer laser lamellar keratectomy. A clinical study. Ophthalmology; 1984; 96:654–664.

6.L’Esperance FA, Taylor DM, Del Poro RA, Roberts A, Gigstad J, Stokes MT, Warner JW, Telfair WB, Martin CA, Yoder PR. Human excimer laser corneal surgery: preliminary report. Trans Am Ophthalmol Soc; 1988; 86:208–275.

7.McDonald M, Kaufman HE, Frantz JM, Shofner S, Salmeron B, Klyce SD. Excimer laser ablation in a human eye. Case report. Arch Ophthalmol; 1989; 107:641–642.

8.Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg; 1988; 14:46–52.

9.Loewenstein A, Lipshitz I, Varssano D, Lazar M. Complication of excimer laser photorefractive keratectomy for myopia. J Cataract Refract Surg; 1997; 23:1174–1176.

10.Brilakis HS, Deutsch TA. Topical tetracaine with bandage soft contact lens pain control after photorefractive keratectomy. J Refract Surg; 2000; 16:444–447.

11.Peyman GA, Katoh N. Effects of an erbium:YAG laser on ocular structures. Int Ophthalmol; 1987; 10:245–253.

12.Peyman GA. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. [Letter to editor]. Am J Ophthalmology; 1996; 122:284–285.

13.Pallikaris IG, Papatzanaki ME, Stathi EZ, Frenshock O, Georgiadis A. Laser in situ keratomileusis. Lasers Surg Med; 1990; 10:463–468.

14.Buratto L, Ferrari M. Excimer laser intrastromal keratomileusis: case reports. J Cataract Refract Surg; 1992; 18:37–41.

15.Buratto L, Ferrari M, Rama P. Excimer laser intrastromal keratomileusis. Am J Ophthalmol; 1992; 113:291–295.

16.Ambrosio R Jr, Wilson SE. Complications of laser in situ keratomileusis: etiology, prevention, and treatment. J Refract Surgery; 2001; 17:350–379.

17.Johnson JD, Azar DT. Surgically induced topographical abnormalities after LASIK: management of central islands, corneal ectasia, decentration, and irregular astigmatism. Curr Opinion Ophthalmol; 2001; 12:309–317.

18.Scerrato E. Laser in situ keratomileusis vs. laser epithelial keratomileusis (LASIK vs. LASEK). J Refract Surgery; 2001; 17:S219–S221.

19.Lee JB, Seong GJ, Lee JH, Seo KY, Lee YG, Kim EK. Comparison of laser epithelial keratomileusis and photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg; 2001; 27:565–570.

20.Azar DT, Ang RT, Lee JB, Kato T, Chen CC, Jain S, Gabison E, Abad JC. Laser subepithelial keratomileusis: electron microscopy and visual outcomes of flap photorefractive keratectomy. Curr Opinion Ophthalmol; 2001; 12:322–328.

21.Smolek MK, Klyce SD, Sarver EJ. Inattention to nonsuperimposable midline symmetry causes wavefront analysis error. Arch Ophthalmol; 2002; 120:439–447.