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Laser in situ keratomileusis versus surface ablation: Visual outcomes and complications

Ghadhfan, Faisal MD, FRCS; Al-Rajhi, Ali MD, FRCS, FRCOphth; Wagoner, Michael D. MD

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Journal of Cataract & Refractive Surgery: December 2007 - Volume 33 - Issue 12 - p 2041-2048
doi: 10.1016/j.jcrs.2007.07.026
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Abstract

During the past 2 decades, corneal refractive surgery has undergone a significant evolution—from unpredictable, technically difficult procedures performed by a few experts to millions of technically easier, safer, and reasonably predictable surgeries performed by thousands of ophthalmologists. The modern era of refractive surgery began in 1983 with the experimental introduction of the 193 nm argon fluoride excimer laser by Trokel et al.1 and continued in 1987 with its first use in human corneas to perform photorefractive keratectomy (PRK) by McDonald et al.2 Lamellar refractive surgery, pioneered by Barraquer3 during the 1960s, was also simplified by the introduction of the automated microkeratome in 1988 by Ruiz and Rowsey (IOVS 1988; 29:ARVO Abstract 55). In the early 1990s, the automated microkeratome was modified and incorporated into laser in situ keratomileusis (LASIK).4,5 Since then, LASIK has become the most popular refractive procedure in the world and is considered by many, but not all, experts to be the refractive procedure of choice.

Although LASIK is the most commonly performed laser refractive corneal surgery in the world today,4 many ophthalmologists believe that surface ablation with PRK may be a safer option. Surface ablation can be performed by mechanical (M-PRK) or laser (T-PRK) removal of the epithelium or after epithelial flap formation using alcohol (laser-assisted subepithelial keratectomy [LASEK]) or a microkeratome (epi-LASIK). Few studies, however, have addressed the issue of stromal versus surface ablation in terms of visual outcomes (efficacy) or complication rates (safety). These studies report contradictory results with respect to the superiority of LASIK over surface ablation techniques or even the differences between methods of performing surface ablation6–37 (U. Cimberle, MD, M. Camellin, MD, “LASEK May Offer the Advantages of Both LASIK and PRK,” Ocular Surgery News International, March 1999, pages 14–15).

The present study retrospectively compared LASIK with the surface ablation techniques of M-PRK, T-PRK, and LASEK in a consecutive series of pa-tients treated with a single excimer laser and a single microkeratome.

PATIENTS AND METHODS

Approval from the Institutional Review Board of King Khaled Eye Specialist Hospital was obtained for the study. The charts of patients who had corneal excimer refractive procedures by staff and fellows between July 1, 2004, and June 30, 2005, were reviewed. The inclusion criteria were myopia from –1.50 to –10.00 diopters (D) with astigmatism less than 4.00 D. Exclusion criteria were fewer than 3 months follow-up or previous corneal surgery.

The decision to perform LASIK or surface treatment was at the discretion of the operating surgeon. Patients with evidence of forme fruste keratoconus or significant ocular surface disease were not offered refractive surgical intervention. Of the patients found suitable for refractive surgery, LASIK was generally selected unless there were contraindications or concerns due to corneal thickness. Surface treatment was selected in most eyes with central corneal pachymetry less than 500 μm. It was also used exclusively if the calculated residual stromal bed was less than 250 μm, although some surgeons used a cutoff between 250 μm and 300 μm as indications for surface treatment.

All cases were performed using previously described techniques.19,38,39 Briefly, excimer ablations were performed with the EC-5000 excimer laser (Nidek Co., Ltd.). All flaps were created with the LSK2 Carriazo-Barraquer manual microkeratome (Moria SA). Epithelial removal was performed for LASEK with a 20% concentration of alcohol for 45 seconds or a 25% concentration for 30 seconds, depending on surgeon preference. Mechanical epithelial removal was performed with a combination of a Weck-Cel sponge (Medtronic) and a scalpel blade. Before ametropic laser ablation, transepithelial removal was performed using the phototherapeutic keratectomy (PTK) mode of the excimer laser with a diameter of 7.0 to 9.0 mm and a depth of 45 to 55 μm (180 to 220 pulses), depending on surgeon preference.

Mitomycin-C (MMC) 0.02% was used for 15 to 60 seconds in some cases at the discretion of the operating surgeon. At the end of each procedure, a bandage contact lens was applied to the surface-treated eye and was kept in place until reepithelialization was complete. Topical prednisolone acetate 1.0% and ofloxacin 0.3% were applied to all LASIK and surface-treated eyes at the conclusion of each procedure. After LASIK and surface treatment, prednisolone acetate 1.0% and ofloxacin 0.3% were used 4 times daily for approximately 1 week. Some surgeons prescribed nonsteroidal antiinflammatory drops for pain relief in surface-treated eyes. After 1 week, most surgeons discontinued all topical medications except unpreserved topical lubricants in LASIK eyes; however, some used a slow steroid taper over 1 to 2 weeks. For surface ablations, most surgeons used fluorometholone 0.1% twice daily for 1 month and once daily for 1 month along with unpreserved topical lubricants. Most patients were examined 1 day; 1 week; and 1, 3, and 6 months after the procedure unless a clinical course warranted more frequent follow-up examinations.

Preoperative data collected from patients' charts included age, sex, best spectacle-corrected visual acuity (BSCVA), manifest and cycloplegic refractions, spherical equivalent (SE) refractive error, and corneal topography and pachymetry by ultrasound and Orbscan (Bausch & Lomb). Intraoperative data included surgical procedure, use of MMC, intended correction, and complications and their management. Postoperative data included the final uncorrected visual acuity (UCVA), BSCVA, refraction, enhancement, and complications and treatment. Outcome measures for visual outcomes were percentage of eyes achieving a final UCVA of 20/30 or better, percentage of eyes achieving a postoperative UCVA within ±2 lines of the preoperative BSCVA, refractive accuracy within ±0.50 D, and difference between the final postoperative UCVA and the preoperative BSCVA measured by Snellen and logMAR acuity. Outcome measures for safety were percentage of eyes losing more than 2 lines of BSCVA and complication rates.

The data were entered into a Microsoft Excel spreadsheet and analyzed for each study group. The data were further analyzed after stratification according to mean SE (MSE) into low to moderate myopia group (MSE <–6.00 D) and a high myopia group (MSE ≥−6.00 to 11.25 D). The logMAR scale was used to compare the differences between preoperative BSCVA and final postoperative UCVA. Categorical variables were compared using the Fischer exact test. Contingency tables were constructed for categorical variables. For continuous variables, mean values were compared across groups. A P value less than 0.05 was considered statistically significant.

RESULTS

Of the 696 eyes that met the inclusion criteria, 464 had LASIK, 104 had LASEK, 69 had M-PRK, and 59 had T-PRK (Table 1). During the first 6 months of the study period, surface techniques accounted for 12.6% of the procedures. This increased to 20.4% during the last 6 months of the study.

Table 1
Table 1:
Demographics and preoperative evaluation of LASIK and surface ablation groups.

All cases were performed by or under the supervision of 14 experienced staff surgeons. All 14 surgeons performed LASIK, including 4 who performed this procedure exclusively. Of the 10 surgeons who also performed surface procedures, 6 performed LASEK, 5 T-PRK, and 4 M-PRK. One surgeon performed all 4 techniques during the study period, 3 performed 3 techniques, and 6 performed 2 techniques.

Mitomycin-C was used at the conclusion of 24 cases (3.4%). These included 18 eyes (10 T-PRK, 6 M-PRK, 2 LASEK) with an SE refractive error of 6.00 D or greater and 6 eyes (5 M-PRK, 1 T-PRK) with an SE refractive error less than 6.00 D.

There was no significant difference between the treatment groups in mean age, duration of follow-up, MSE, percentage of eyes with low to moderate versus high myopia, and mean keratometry. Whereas the ratio of men to women was similar in the LASIK and LASEK treatment groups, there was a statistically significant bias toward women in the M-PRK and T-PRK treatment groups (P<.001). Mean corneal thickness, measured by ultrasound pachymetry and Orbscan, was significantly less in eyes treated with surface ablation than in those treated with LASIK (P<.001). Eyes selected for LASIK were significantly more likely to have a preoperative BSCVA of 20/20 or better than those receiving surface treatment (P = .006) (Table 2).

Table 2
Table 2:
Results of LASIK and surface ablation in the low to moderate myopia group.

Visual Outcomes

Low to Moderate Myopia

Results in eyes with low to moderate myopia that had LASIK or a surface ablation procedure are shown in Table 2. The percentage of eyes achieving UCVA of 20/30 or better was similar between all study groups. There were no statistically significant differences between study groups in the percentage of eyes that obtained a final UCVA within ±2 lines or better of preoperative BSCVA or refractive accuracy within ±0.50 D of emmetropia.

The mean difference between logMAR final postoperative UCVA and preoperative BSCVA was significantly lower after T-PRK than after LASIK (P = .03) or LASEK (P = .04). Based on this parameter, M-PRK also produced statistically significantly better results than LASIK (P = .01) and LASEK (P = .01). Differences between enhancement rates were insufficient to account for the differences in better visual outcomes after T-PRK and M-PRK.

High Myopia

Results in eyes with high myopia that had LASIK or a surface ablation procedure are shown in Table 3. Eyes treated with T-PRK were significantly more likely to obtain a final UCVA of 20/30 or better than eyes treated with LASIK (P = .02), LASEK (P = .002), or M-PRK (P =.0003). A higher percentage of eyes obtained a final UCVA within ±2 lines of the preoperative BSCVA with T-PRK than with LASIK, LASEK, and M-PRK, but the differences were not statistically significant (P =.13, P = .13, and P = .08, respectively).

Table 3
Table 3:
Results of LASIK and surface ablation in the high myopia group.

There was a smaller mean difference between logMAR final postoperative UCVA and preoperative BSCVA after T-PRK than after LASIK, LASEK, and M-PRK; however, the differences were not statistically significant (P = .11, P = .33, and P = .20, respectively). Transepithelial PRK was significantly more likely to achieve a final refractive error within ±0.50 D of emmetropia than LASIK (P = .02) and M-PRK (P = .04), but not LASEK (P = .07).

There was a significantly increased tendency to use MMC at the conclusion of T-PRK (P<.001) and M-PRK (P = .01) than after LASEK. Mitomycin-C was used in 10 eyes (45.5%) after T-PRK for high myopia. Its use was associated with a slightly smaller, statistically insignificant difference between the logMAR final postoperative UCVA and the preoperative BSCVA compared with eyes in which it was not used (−0.056 versus −0.089) and with a higher percentage of the final UCVA within ±2 lines of the preoperative BSCVA (100% versus 92%) or percentage of eyes within ±0.50 D of target refraction (100% versus 92%). These small differences were insufficient to account for the differences in the outcome measures between the T-PRK group and the other 3 groups.

Safety

Major complications occurring during or after LASIK or surface treatment are shown in Table 4. By definition, flap complications only occurred in eyes treated with LASIK. Intraoperative flap complications occurred in 14 eyes (3.0%) (7 incomplete, 6 buttonhole, 1 complete). Major postoperative flap complications occurred in 4 eyes. No trends could be identified to suggest that surgeon experience contributed to the rate of intraoperative or postoperative flap complications.

Table 4
Table 4:
Major complications of LASIK and surface ablation.

Major complications not related to the flap were more common after LASEK than after LASIK, M-PRK, or T-PRK (10.6% versus 2.2%, 2.9%, and 1.7%, respectively); however, the differences did not reach statistical significance (P = .12, P = .07, and P = .06, respectively). The occurrence of persistent epithelial defects and/or recurrent epithelial erosions in 5 eyes (4.8%) and 2 eyes (1.9%), respectively, accounted for most of the increased complications after LASEK compared with the other procedures.

Haze resulting in the loss of 2 or more lines of BSCVA occurred in 6 eyes (3 LASEK, 2 LASIK, 1 T-PRK). There was no significant correlation between the amount of attempted correction and the development of vision-compromising haze. Of the 3 eyes with significant post-LASEK haze, the spherical refractive errors were −2.75 D, −3.00 D, and −6.37 D. The spherical refractive error was −4.50 D in the T-PRK eye that developed significant haze. One post-LASIK case was in an eye with a spherical refractive error of −4.50 D and an incomplete flap that had laser ablation at the time of the initial cut. The other post-LASIK case was an eye with a spherical refractive error of −9.00 D in which no other complications occurred.

Of eyes treated for low to moderate myopia, loss of more than 2 lines of BSCVA occurred in 1 eye (0.3%) after LASIK due to haze. After treatment of high myopia, loss of more than 2 lines of BSCVA occurred in 1 eye (2.7%) after LASEK with no clear cause and in 1 eye (0.7%) after LASIK secondary to haze, but not in any eye having M-PRK or T-PRK.

DISCUSSION

The present study assessed the visual outcomes and safety of LASIK versus surface ablation procedures for low to moderate and high myopia. Because the primary patient motivation for having the procedure is to “trade” spectacle-dependent vision for spectacle-independent vision, assessment of efficacy must measure how effectively this goal has been met by a given procedure. This study assessed 4 criteria for visual outcome: (1) percentage of eyes achieving a final UCVA of 20/30 or better, (2) percentage of eyes achieving a final postoperative UCVA within ±2 lines of the preoperative BSCVA, (3) refractive accuracy within ±0.50 D, and (4) actual change between the preoperative BSCVA and final postoperative UCVA in each patient, measured by Snellen and logMAR acuity. It is our impression that the actual change between the preoperative BSCVA and final postoperative UCVA is the main factor by which patients judge their subjective satisfaction with refractive surgery.

Visual outcomes for low to moderate myopia was similar in all 4 groups with respect to the percentage of eyes achieving a final UCVA of 20/30 or better, percentage of eyes with a final postoperative UCVA within ±2 lines of the preoperative BSCVA, and refractive accuracy. However, when the mean change in the final postoperative UCVA was compared with the preoperative BSCVA, a significantly smaller difference was found after T-PRK and M-PRK than after LASIK or LASEK. These results suggest that for low myopia, T-PRK and M-PRK may give slightly better overall visual outcomes than LASIK or LASEK.

The disparities in visual outcomes for high myopia were greater when all criteria were analyzed. Transepithelial PRK was significantly more likely to provide a final UCVA of 20/30 or better than LASIK, LASEK, and M-PRK. In addition, T-PRK was more likely than the other 3 procedures to yield a final UCVA within ±2 lines of the BSCVA, to have a smaller mean change in Snellen and logMAR acuity between the final postoperative UCVA and preoperative BSCVA, and to have refractive accuracy within ±0.50 D; however, there were not enough cases to establish statistical significance in terms of refractive accuracy except when comparing T-PRK with LASEK and M-PRK. These differences between T-PRK and the other treatment methods remained consistent, even when a separate analysis was performed in eyes in which adjunctive MMC had or had not been used.

By definition, only LASIK had intraoperative or postoperative flap complications, although this was not a factor in the differences in visual outcomes. Intraoperative flap complications were associated with a loss of more than 2 lines of the BSCVA in only 1 eye. Postoperative flap complications did not result in significant loss of the BSCVA. There were no cases of keratectasia during the follow-up, probably because of the use of surface ablation procedures mainly in eyes with thinner central corneal pachymetry.

Differences in non-flap–related postoperative complications were also insufficient to account for differences in the visual outcomes between the study groups. The prevalence of non-flap–related postoperative complications was acceptably low and comparable with that with T-PRK (1.7%), LASIK (2.2%), and M-PRK (2.9%).

The overall postoperative complication rate was 10.6% after LASEK. The occurrence of persistent epithelial defects (5 eyes) or subsequent recurrent epithelial defects (2 eyes) after LASEK accounted for most of the increased prevalence of postoperative complications in this group, compared with the complications after LASIK and the other surface treatment modalities. The only case of bacterial keratitis occurred after LASEK. Fortunately, loss of more than 2 lines of BSCVA did not occur in any of the 10 eyes with epithelial-related complications.

Every effort was made to exclude patients with ocular surface disease from refractive surgical intervention; thus, selection of patients on the basis of this factor cannot account for the increased rate of ocular surface-related complications after LASEK compared with the rate after the other surface ablation techniques. Because the only variable between these groups was the method of epithelial removal, it is reasonable to hypothesize that alcohol-related toxicity contributed to poor reepithelialization and adhesion. Unfortunately, no patient was treated with epi-LASIK during the study period, eliminating the possibility that mere replacement of the epithelial flap, rather than alcohol toxicity, accounted for the epithelial abnormalities in these eyes.

In conclusion, treatment of low to moderate myopia with T-PRK and M-PRK generally yielded better results than treatment with LASIK or LASEK. In addition, treatment of high myopia with T-PRK generally produced better results than treatment with M-PRK, LASIK, and LASEK. The accurate and smooth removal of the epithelium using the PTK mode of the excimer laser before the ametropic treatment may have contributed to the favorable outcomes of T-PRK.

Our study was limited because it was retrospective and nonrandomized, involved multiple surgeons, and was performed without wavefront technology. Variations in epithelial removal techniques, use of MMC, and postoperative regimens, particularly with respect to threshold for performing enhancements, may have influenced some observations. Nonetheless, it was a consecutive series in which a single excimer laser and microkeratome were used and preoperative and postoperative evaluations and examinations were done under standardized conditions. A randomized prospective series performed in this or other settings with a limited number of experienced surgeons using wavefront technology should be performed to confirm the possible superior visual outcomes and safety of T-PRK suggested in the present study.

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