LASIK has an extremely high success rate, with more than 98% of patients diagnosed as having myopia and undergoing wavefront-guided treatment achieving an uncorrected distance visual acuity (UDVA) of 20/40 or better.1,2 Outcomes have steadily improved with improvement in surgeon experience and technology.3
Depending on the patient’s initial refractive error, equipment used, and the patient’s satisfaction, occasional re-treatment of patients having previously undergone LASIK is inevitable, with studies reporting rates up to 25%.4–9 Early reasons for enhancement include overtreatment or undertreatment. The most common reason is regression (recurrence of the original refractive error), which can range from 5% to 28% (depending on the definition used).7,10,11 A recent study with an inclusive definition of regression (either refractive error < −0.50 diopters [D] or myopic shift > 0.50 D in the first year after LASIK) reported a 67% risk of regression for a microkeratome and 44% for a femtosecond flap.12
Traditionally, LASIK enhancements are performed by re-lifting the flap and then applying additional treatment to the stromal bed (“re-treatment”). However, the reported risk of epithelial ingrowth after re-treatment is as high as 20%.13–16 Recent case series indicated that the risk of epithelial ingrowth was lower when the femtosecond laser was used to cut the original flap.17,18 Re-treatment operative techniques may also affect the rate of ingrowth.19
Potential serious complications of ingrowth include decreased UDVA and corrected distance visual acuity (CDVA), glare, astigmatism, foreign body sensation, or melt.17 The risk of ingrowth appears to increase with time after the initial surgery.13 We rarely re-lift a flap for an enhancement after 1 to 2 years. Recutting flaps was largely abandoned after evidence of loss of CDVA and subjective dissatisfaction.20
Given the high visual demands of refractive surgery patients and concerns about the risk of re-treatment, interest in performing photorefractive keratectomy (PRK) enhancements has increased. Relatively few studies have described outcomes for surface ablation after LASIK.21–27
There has also been little discussion of any potential differences between PRK enhancement for hyperopic and myopic residual refractive errors. Furthermore, the fact that a PRK enhancement is being performed on and sometimes through a LASIK flap could require different planning and result in a different predictability of effect compared to primary laser vision correction or re-treatment.
We present the outcomes from a single practice of the largest known series of patients undergoing PRK enhancement after LASIK, including the largest known group of hyperopes.
Patients and Methods
This retrospective case review included all cases of PRK enhancement after LASIK performed in a single private practice from 2004 to 2012. Using the practice’s scheduling system, potential cases were identified by searching all cases of PRK during the study period. These charts were examined to identify those having PRK in an eye with previous LASIK. All data collection occurred after approval by an institutional review board.
Each case was performed according to the surgeon’s preference, including any adjustments to the treatment performed, nomogram adjustments, and mitomycin C use. Four cases used transepithelial PRK for epithelial removal, with the rest having alcohol and then manual removal of the epithelium. Mitomycin C use and time varied between surgeons. All surgeons used an antibiotic drop four times per day until the bottle was empty and a nonsteroidal anti-inflammatory drug for several days after surgery. The steroid regimen varied from fluorometholone four times per day for 3 weeks and then tapering; prednisolone acetate 1% four times per day for 2 weeks and then tapering; prednisolone four times per day for 1 month and then tapering; or loteprednol 0.5% four times per day for 1 month and then two times per day for 1 month.
The primary outcome measure was UDVA, with secondary outcomes including postoperative manifest refraction, CDVA, change in lines of vision, actual versus targeted change in spherical equivalent, and haze or other complications (eg, diffuse lamellar keratitis).
For analysis, patients were divided in two groups (hyperopic and myopic) based on their manifest refraction spherical equivalent before enhancement. Postoperative visits included day 1, bandage contact lens removal (days 4 to 14), month 1 (days 15 to 45), month 3 (days 46 to 135), month 6 (days 136 to 225), months 9 to 12 (days 226 to 547), year 2 (days 548 to 912), and year 3 (days ≥ 913).
Patients who did not have at least 226 days of postoperative follow-up were excluded, as were those with a non-plano target. Patients with visually significant cataract were also excluded from the analysis. The result of the first enhancement was used for those with multiple PRK enhancements (n = 3).
Student’s two-tailed t tests were used to compare the two groups and preoperative and postoperative enhancement data. Pearson’s chi-square test was used to compare percentages between the two groups, with reference to the overall population mean. Linear regression analysis was used to compare the change in spherical equivalent achieved for both the planned treatment spherical equivalent and spherical equivalent before enhancement. Multivariate linear regression analysis was performed for the most recent UDVA in both groups using eleven variables: LASIK age, PRK age, spherical equivalent before LASIK, cylinder before LASIK, time to PRK, corneal thickness before PRK, spherical equivalent before PRK, cylinder before PRK, spherical aberration, coma, and root mean square of higher-order aberrations.
Of the 75 patients who underwent PRK after LASIK, 43 met the study criteria. Four had visually significant cataract and were excluded. Thirteen had cataract deemed non-visually significant and were eligible if they met the other criteria. Two excluded patients had non-plano targets and the remaining 26 had in-sufficient follow-up information. Mean follow-up was 652 ± 422 days (range: 233 to 1,921 days) for included patients, 793 ± 576 days (range: 237 to 1,921 days) for those with hyperopia before enhancement, and 584 ± 293 days (range: 233 to 1,226 days) for those with residual myopia.
Of the study patients, 14 had a hyperopic subjective manifest refraction at the time of PRK enhancement. Table 1 shows the preoperative information for the entire study group and for hyperopia and myopia. The two groups were comparable, with a significant difference only in age at the time of LASIK.
Nineteen percent (1 hyperope [for residual hyperopia] and 7 myopes [2 for overtreatment and 5 for residual myopia]) underwent LASIK re-treatment before PRK enhancement.
On average, 85.6 months (range: 6 to 192 months) passed between LASIK and PRK enhancement. Only 1 patient (hyperopic group) had a manifest refraction that was hyperopic before LASIK.
All patients underwent a Wavescan-guided treatment using the VISX laser (Abbott Medical Optics, Abbott Park, IL). The mean physician adjustment in the hyperopic group was −0.21 ± 0.27 D (range: −0.75 to +0.2 D), with a mean nomogram boost of +6.8% (4/14 with no adjustment, 1/14 with +5%, and 9/14 with +10%). In the myopic group, the mean physician adjustment was +0.17 ± 0.31 D (range: −0.71 to +0.75 D), with a mean nomogram adjustment of +4.0% (16/29 with no adjustment, 1/29 with −3%, and 12/29 with +10%). Mitomycin C was used in all of the hyperopes (mean: 102 seconds; range: 12 to 120 seconds) and in 25 of 29 myopes (mean: 67 seconds; range: 0 to 120 seconds).
The mean UDVA was 20/39 before enhancement and 20/24 after enhancement (P < .002) in the hyperopic group and 20/45 before enhancement and 20/22 after enhancement (P < .0000001) for the myopic group.
Figure 1 shows the UDVA for the two groups. All of the hyperopes were 20/40 or better at their most recent postoperative visit and 50% were 20/20 or better. All of the myopes were 20/40 or better at their most recent postoperative visit and 65.5% were 20/20 or better. Figure 2 shows the change in lines of CDVA, with 85.7% of hyperopes and 66.7% of myopes showing stability or improvement.
(A) Uncorrected visual acuity in patients with hyperopic manifest refraction significantly improved after enhancement (n = 14, P < .002). (B) Uncorrected visual acuity in patients with myopic manifest refraction before and after enhancement (n = 29, P < .001).
Change in lines of corrected distance visual acuity after photorefractive keratectomy enhancement for patients with (A) hyperopic and (B) myopic manifest refraction.
The mean refractive error for the hyperopic group declined from +1.10 ± 0.71 (range: +0.13 to +2.25 D) to +0.38 ± 0.66 D (range: −0.75 to +1.38 D) at the final postoperative visit. For the myopic group, the refractive error improved from −1.21 ± 0.61 (range: −3.25 to −0.38 D) to +0.34 ± 0.45 D (range: −0.25 to +1.75 D). Figure 3 shows the improvement in manifest refraction cylinder after enhancement. Those undergoing hyperopic enhancement had a decrease in cylinder from 0.84 to 0.46 D (P = .02) and 71.4% had 0.5 D or less of cylinder. The myopic group did even better, with a decrease from 0.64 to 0.26 D (P < .002) for those with preoperative and postoperative enhancement refractions (n = 27). After enhancement, 66.7% had 0.25 D or less of cylinder and 92.6% had 0.5 D or less.
(A) The hyperopic group showed a significant decrease in cylinder on manifest refraction (0.84 to 0.46 D; P = .02). (B) The myopic group also showed a significant decrease in cylinder (0.64 to 0.26 D; P < .002), with most having 0 or 0.25 D. D = diopters
The actual change in spherical equivalent at the most recent follow-up visit was compared to the manifest refraction change needed to achieve the plano refractive target and to the treatment performed after physician adjustments for the hyperopes (Figure 4). With a perfect result and perfect predictability, both regressions would have a slope of 1 with a y-intercept of 0. The linear regression for the plano target showed good fit (R2 = 0.51), with a slope of 1.03 and a y-intercept of 0.4 D. The linear regression for the treatment performed was similar, with an R2 of 0.45, a slope of 1.03, and a y-intercept of 0.17 D.
(A) Comparison of the treatment effect of photorefractive keratectomy (PRK) enhancement for hyperopia to the effect needed to achieve a plano refraction. (B) Comparison of the treatment effect of PRK enhancement for hyperopia to the treatment performed by the surgeon. The blue line is a 1:1 effect and the red lines show the +1 and −1 D error lines. D = diopters
The same analysis was performed for the myopic group. In this case, the linear regression for the plano target had an R2 of 0.67, with a slope of 1.04 and a y-intercept of 0.29 D (Figure 5). The regression for the programmed treatment showed a lower R2 of 0.40, with a slope of 0.84 and a y-intercept of 0.63 D (Figure 5). The closer fit and more ideal slope of the plano target regression indicated that the effect of myopic PRK enhancement was closer to the surgeons’ expectations than the straightforward treatment applied would suggest.
(A) Comparison of the treatment effect of photorefractive keratectomy (PRK) enhancement for myopia to the effect needed to achieve a plano refraction. (B) Comparison of the treatment effect of PRK enhancement for myopia to the treatment performed by the surgeon. The blue line is a 1:1 effect and the red lines show the +1 and −1 D error lines. D = diopters
Figure 6 shows the stability of refraction in both groups for a period of time. In the hyperopic group, there was an initial overtreatment effect and then mild regression. In the myopic group, there was a longer overtreatment effect with no regression seen during the study.
(A) Stability of the manifest refraction for the (A) hyperopic and (B) myopic group. Initially, there was a small overtreatment effect with mild hyperopic regression for a period of time. The overtreatment effect lasted for a longer time period in the myopic group. The bars show the standard deviation. D = diopters
In the hyperopic group, the percentage of treatment achieved (−1 × achieved change in refractive error spherical equivalent / spherical equivalent of PRK treatment after physician adjustment) ranged from −260% to 642%, with a mean of 80%. Eliminating patients with a spherical equivalent of +0.5 D or less before enhancement resulted in a mean treatment achieved percentage of 93%, but the spread was still from −13% to 189%. On average, hyperopes underresponded to enhancement, whereas myopes tended to overrespond. One patient underwent a treatment aiming for a +0.25 D change in spherical equivalent, but experienced an actual change of +2.75 D. Besides this outlier, the treatment achieved percentage ranged from 51% to 240%, with a mean of 131% and 25th to 75th percentiles of 101% and 175%, respectively. Only 2 patients had a response of less than 90% of the spherical equivalent treatment.
Six hyperopes and 2 myopes were considered outliers (defined as < 50% or > 200% of the targeted spherical equivalent change achieved). Age, refraction before LASIK, corneal thickness, comorbidity, time since LASIK, and presence or absence of mitomycin C had no value in predicting an outlier. None had cataract, haze, or any complications after PRK.
However, 5 of the 6 hyperopes had minimal hyperopia to correct (≤ +0.5 D), exaggerating the effect of any deviation from a plano result. The remaining hyperope had a −0.4-D physician adjustment to decrease the amount of hyperopia corrected and became undercorrected. All hyperopic outliers had a UDVA between 20/15 and 20/30.
The two myopes had overcorrections despite physician adjustments to decrease the amount of myopia treated. The spherical equivalents being treated were −0.75 and −1.0 D, and the final UDVA was 20/40 and 20/25, respectively. Both patients underwent previous LASIK re-treatments and one had significant dry eye.
In the multivariate linear regression, none of the variables had a P value less than .05 in the hyperopic group. For the myopic group, LASIK age, PRK age, months to PRK, cylinder before LASIK, and coma all had P values less than .05. However, the LASIK and PRK age coefficients were small and of opposite sign, essentially canceling out age as a significant factor. The only coefficient of clinically significant magnitude was coma, with higher coma resulting in worse UDVA after PRK enhancement.
Safety and Complications
The mean CDVA remained the same before and after enhancement for the hyperopic group (20/18); only one patient declined from 20/15 to 20/30 and had a correction of −0.5 + 1.25 × 10 and +1.0 +0.5 × 80 before and after enhancement, respectively. UDVA remained 20/30 before and after enhancement. In the myopic group, the mean CDVA was 20/18 before enhancement and 20/19 after enhancement. Although one-third lost a line of CDVA at their final follow-up visit, 8 of 9 worsened from 20/15 to 20/20; in some cases, the technician may not have tried to refract to 20/15 on the final visit. Considering the best CDVA enhancement after PRK (rather than the final one), 4 patients (14%) with myopia lost one line of CDVA, one from 20/20 to 20/25.
Of the 4 patients who did not receive mitomycin C, 1 developed non-clinically significant haze (25%). Two patients who received mitomycin C had haze (5.1%), with a nonstatistically significant trend toward greater haze in those without mitomycin C (P = .14). Although all three cases of haze were graded as mild, 1 patient in the hyperopic group (with mitomycin C) had clinically significant haze that led to −0.75 D of myopia 1 year after enhancement. Still, the UDVA improved from 20/60 to 20/40 and the CDVA from 20/25 to 20/20 in this patient. There were no issues with re-epithelialization in those receiving mitomycin C.
Five patients (4 with Stage 1 and 1 with Stage 2) had diffuse lamellar keratitis, but none required a flap lift. One patient had a steroid-induced pressure spike.
In this series, PRK enhancement after LASIK was safe and effective in improving UDVA for both hyperopes and myopes. The entire study population had a UDVA of 20/40 or better and most had 20/20 or better.
A small number of patients experienced overtreatment or undertreatment based on the change in subjective manifest refraction. Although no predictive clinical characteristics were identified, these patients generally had good UDVA. One possible explanation is that multiple refractive surgeries created a multifocal cornea, resulting in a refractive power range that still provided good vision. A second possible explanation is the challenge of treating a previously flattened (oblate) cornea; the excimer laser may ablate differently or unpredictably compared to a primary case. Third, the corneal epithelium undergoes significant changes during healing after LASIK.28 A report on phototherapeutic keratectomy after LASIK suggests that merely removing the epithelium and performing mild smoothing without a refractive treatment may have significant refractive effects.29 Therefore, epithelial removal and subsequent healing during PRK may introduce an additional variable, affecting predictability for some patients.
Our data showed an undertreatment effect of approximately 0.2 D in hyperopes compared to what the surgeon planned; on average, the treatment plan contained an undercorrection of 0.2 D, suggesting that surgeons should not make any adjustment for hyperopic enhancements. The 3 patients with the smallest hyperopic treatments had no change in UDVA; as such, enhancements for a refractive error less than +1 D may not be effective.
The outcomes of myopic enhancement were closer to the preoperative target than the hyperopic results. However, the slope of the linear regression (comparing planned Wavescan treatment to actual treatment effect) was not as close to 1, suggesting surgeon adjustments had a critical role in producing good outcomes. There were only 7.4% undercorrections compared to the planned treatment, which reflects that a myopic enhancement was re-treating an already-flattened cornea because essentially all patients were myopic before LASIK. From our linear regression, a surgeon should use the following correction factor to adjust treatment of myopia: +0.75 D + (0.19 × spherical equivalent of patient’s refractive error).
Although only 4 patients did not receive mitomycin C, we believe that this case series somewhat supports the routine use of mitomycin C for PRK enhancement.21,26 Carones et al. strongly advised against PRK enhancement of regressed LASIK in 2001 (during the era before mitomycin C) because of dense haze and recurrent regression.22 Other series that did not use mitomycin C have reported higher rates of haze than we observed.24,27
Overall, our results of PRK enhancement after LASIK were excellent and largely consistent with prior studies that generally showed a UDVA of approximately 20/25 after enhancement.21,24–27 However, we believe that this case series has a few significant methodological advantages. First, we included only patients with at least 226 days of follow-up, which we believe is an important factor to consider given the delayed healing time after PRK. The mean follow-up of 652 days was significantly longer than that of any other case series found in the literature.
Second, we had the largest known samples of patients and hyperopes. We believe that separating enhancements based on refractive error prior to enhancement is important for multiple reasons. First, 98% of our patients were myopic before LASIK, and an eye that has regressed to myopia has undergone a different healing process from one that is hyperopic. Furthermore, the excimer laser spot pattern is different: hyperopic treatment is more peripheral and is treating a thinner part of the flap because most of these patients have a microkeratome flap. By contrast, mild myopic enhancements were presumably performed within the flap stroma. None of our patients had a maximum ablation depth greater than 55 μm, with a mean of 26 ± 9.7 μm.
The percentage of patients with visually significant cataract or cataract surgery was surprisingly high. Given the average interval of 7 years between LASIK and PRK enhancement in this series and the fact that many of these patients had high myopia originally, it is wise to assess lens opacity carefully in patients being considered for enhancement.
With careful preoperative planning, PRK enhancement is a highly successfully procedure when too much time has passed for safe re-lifting of a LASIK flap or when the original flap was abnormal. However, a few patients appear to have a less predictable response to further laser correction, and mentioning this in the informed consent process appears prudent.
- Bailey MD, Zadnik K. Outcomes of LASIK for myopia with FDA-approved lasers. Cornea. 2007;26:246–254. doi:10.1097/ICO.0b013e318033dbf0 [CrossRef]
- Schallhorn SC, Farjo AA, Huang D, et al. Wavefront-guided LASIK for the correction of primary myopia and astigmatism. Ophthalmology. 2008;115:1249–1261. doi:10.1016/j.ophtha.2008.04.010 [CrossRef]
- Yuen LH, Chan WK, Koh J, Mehta JS, Tan DT. A 10-year prospective audit of LASIK outcomes for myopia in 37,932 eyes at a single institution in Asia. Ophthalmology. 2010;117:1236–1244. doi:10.1016/j.ophtha.2009.10.042 [CrossRef]
- Agarwal A, Agarwal A, Agarwal T, Bagmar A, Agarwal S. Laser in situ keratomileusis for residual myopia after primary LASIK. J Cataract Refract Surg. 2001;27:1013–1017. doi:10.1016/S0886-3350(01)00868-9 [CrossRef]
- Alio JL, Montes-Mico R. Wavefront-guided versus standard LASIK enhancement for residual refractive errors. Ophthalmology. 2006;113:191–197. doi:10.1016/j.ophtha.2005.10.004 [CrossRef]
- Bababeygy SR, Zoumalan CI, Chien FY, Manche EE. Wavefront-guided laser in situ keratomileusis retreatment for consecutive hyperopia and compound hyperopic astigmatism. J Cataract Refract Surg. 2008;34:1260–1266. doi:10.1016/j.jcrs.2008.04.026 [CrossRef]
- Bragheeth MA, Fares U, Dua HS. Re-treatment after laser in situ keratomileusis for correction of myopia and myopic astigmatism. Br J Ophthalmol. 2008;92:1506–1510. doi:10.1136/bjo.2008.143636 [CrossRef]
- Randleman JB, White AJ Jr, Lynn MJ, Hu MH, Stulting RD. Incidence, outcomes, and risk factors for retreatment after wavefront-optimized ablations with PRK and LASIK. J Refract Surg. 2009;25:273–276.
- Hersh PS, Fry KL, Bishop DS. Incidence and associations of retreatment after LASIK. Ophthalmology. 2003;110:748–754. doi:10.1016/S0161-6420(02)01981-4 [CrossRef]
- Chen YI, Chien KL, Wang IJ, et al. An interval-censored model for predicting myopic regression after laser in situ keratomileusis. Invest Ophthalmol Vis Sci. 2007;48:3516–3523. doi:10.1167/iovs.06-1044 [CrossRef]
- Lyle WA, Jin JG. Retreatment after initial laser in situ keratomileusis. J Cataract Refract Surg. 2000;26:650–659. doi:10.1016/S0886-3350(00)00319-9 [CrossRef]
- Lin MY, Chang DCK, Hsu WM, Wang IJ. Cox proportional hazards model of myopic regression for laser in situ keratomileusis flap creation with a femtosecond laser and with a mechanical microkeratome. J Cataract Refract Surg. 2012;38:992–999. doi:10.1016/j.jcrs.2012.01.025 [CrossRef]
- Caster AI, Friess DW, Schwendeman FJ. Incidence of epithelial ingrowth in primary and retreatment laser in situ keratomileusis. J Cataract Refract Surg. 2010;36:97–101. doi:10.1016/j.jcrs.2009.07.039 [CrossRef]
- Vaddavalli PK, Yoo SH. Femtosecond laser in-situ keratomileusis flap configurations. Curr Opin Ophthalmol. 2011;22:245–250. doi:10.1097/ICU.0b013e3283479ebd [CrossRef]
- Perez-Sontonja JJ, Ayala MJ, Sakla HF, Ruíz-Moreno JM, Alió JL. Retreatment after laser in situ keratomileusis. Ophthalmology. 1999;106:21–28.
- Mulhern MG, Condon PI, O’Keefe M. Myopic and hyperopic laser in situ keratomileusis retreatments: indications, techniques, limitations, and results. J Cataract Refract Surg. 2001;27:1278–1287. doi:10.1016/S0886-3350(01)00981-6 [CrossRef]
- Kamburoglu G, Ertan A. Epithelial ingrowth after femtosecond laser-assisted in situ keratomileusis. Cornea. 2008;27:1122–1125. doi:10.1097/ICO.0b013e3181731439 [CrossRef]
- Letko E, Price MO, Price FW Jr, . Influence of original flap creation method on incidence of epithelial ingrowth after LASIK retreatment. J Refract Surg. 2009;25:1039–1041. doi:10.3928/1081597X-20090617-13 [CrossRef]
- Chan CC, Boxer Wachler BS. Comparison of the effects of LASIK retreatment techniques on epithelial ingrowth rates. Ophthalmology. 2007;114:640–642. doi:10.1016/j.ophtha.2006.06.062 [CrossRef]
- Rubinfeld RS, Hardten DR, Donnenfeld ED, et al. To lift or recut: changing trends in LASIK enhancement. J Cataract Refract Surg. 2003;29:2306–2317. doi:10.1016/j.jcrs.2003.08.013 [CrossRef]
- Alio JL, Pinero DP, Puche ABP. Corneal wavefront-guided photorefractive keratectomy in patients with irregular corneas after cornea refractive surgery. J Cataract Refract Surg. 2008;34:1727–1735. doi:10.1016/j.jcrs.2008.06.025 [CrossRef]
- Carones F, Vigo L, Carones AV, Brancato R. Evaluation of photorefractive keratectomy retreatments after regressed myopic laser in situ keratomileusis. Ophthalmology. 2001;108:1737. doi:10.1016/S0161-6420(01)00715-1 [CrossRef]
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- Saeed A, O’Doherty M, O’Doherty J, O’Keefe M. Laser-assisted subepithelial keratectomy retreatment after laser in situ keratomileusis. J Cataract Refract Surg. 2008;34:1736–1741. doi:10.1016/j.jcrs.2008.06.020 [CrossRef]
- Srinivasan S, Drake A, Herzig S. Photorefractive keratectomy with 0.02% mitomycin C for treatment of residual refractive errors after LASIK. J Refract Surg. 2008;24:S64–S67.
- Shaikh NM, Wee CE, Kaufman SC. The safety and efficacy of photorefractive keratectomy after laser in situ keratomileusis. J Refract Surg. 2005;21:353–358.
- Spadea L, Fasciani R, Necozione S, Balestrazzi E. Role of the corneal epithelium in refractive changes following laser in situ keratomileusis for high myopia. J Refract Surg. 2000;16:133–139.
- Faktorovich EG, Nosova E. Epithelial removal and phototherapeutic keratectomy for residual refractive error following LASIK in eyes with corneal epithelial basement membrane degeneration. J Refract Surg. 2009;25:723–729. doi:10.3928/1081597X-20090707-07 [CrossRef]
|Variable||Total (n = 43)||Hyperopes (n = 14)||Myopes (n = 29)|
|Age at LASIK (y)||39.20 (8.70)||43.10b (7.20)||37.20b (8.80)|
|Time between first LASIK and PRK (months)||85.60 (40.0)||72.70 (35.30)||91.90 (41.30)|
| Sphere||−6.24 (2.91)||−5.64 (2.89)||−6.56 (2.93)|
| Cylinder||0.97 (0.97)||0.89 (1.10)||1.01 (0.92)|
| Spherical equivalent||−5.76 (2.85)||−5.20 (2.93)||−6.09 (2.82)|
|LASIK postoperative month 1|
| Uncorrected visual acuity||20/26||20/28||20/25|
| Corrected visual acuity||20/20||20/21||20/20|
| Subjective MRSE||0.07 (1.14)||0.23 (0.55)||−0.01 (1.34)|
|% with microkeratome LASIK flap||94.70||91.70||96.20|
|% with LASIK re-treatment||20.90||28.60||17.20|