Purpose
To evaluate the visual outcome and change in higher-order aberrations (HOAs) 1 year post simultaneous wavefront-guided and aspheric photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).
Design
Prospective interventional case series.
Methods
Consecutive myopic patients undergoing PRK (38 eyes of 23 patients) and LASIK (42 eyes of 25 patients) using the Technolas 217z100 excimer laser (Technolas Perfect Vision) in a private laser clinic were included. Main outcome measures were uncorrected distance visual acuity (UDVA), manifest refraction (MRSE), HOAs, and spherical aberration (Z 4 0 ).
Results
At 1 year, 87% (32/37) of PRK eyes and 81% (30/37) of LASIK eyes had UDVA of 20/20 or better ( P = .75). Mean ± SD MRSE was −0.26 ± 0.31 diopters (D) in the PRK and −0.16 ± 0.34 D in the LASIK group ( P = .222). There was no significant increase in total HOA root mean square (RMS) in the PRK group. Mean ± SD total HOA RMS increased from 0.402 ± 0.14 μm to 0.496 ± 0.17 μm ( P = .013) in the LASIK group at 1 year. Z 4 0 increased from mean ± SD −0.045 ± 0.12 μm to −0.109 ± 0.15 μm ( P = .006, factor 2.42) in the PRK group and did not significantly increase in the LASIK group (mean ± SD −0.16 ± 0.17 μm to −0.17 ± 0.15 μm ( P = .469, factor 1.08) at 6 mm pupil.
Conclusions
Visual outcome was excellent in both treatment groups. HOAs were still increased following LASIK by a factor of 1.23 but not PRK. Z 4 0 was not induced by LASIK with the treatment algorithm but was negatively induced in PRK eyes.
Higher-order aberrations (HOAs) of the eye reduce the visual performance and are often increased or induced by refractive laser procedures. In particular, spherical aberration (Z 4 0 ) is induced following photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). A current challenge of laser refractive surgery is the reduction of pre-existing HOAs without inducing new aberrations to improve the visual outcome.
The advent of wavefront-guided technology and its evolution over the last 10 years has led to significant improvements in reducing the postoperative induction of HOAs and improving visual outcomes vs conventional treatment. Challenges still remain, however. Preoperative HOAs are rarely reduced and Z 4 0 is frequently still induced. Aspheric treatments reduce postoperative induction of Z 4 0 but do not treat pre-existing HOAs.
The Zyoptix Personalized Treatment Advanced (PTA) algorithm (Technolas Perfect Vision GmbH, Munich, Germany) is a wavefront-guided and aspheric treatment developed to both address pre-existing HOAs and minimize the postoperative induction of Z 4 0 and thereby, theoretically, provide the “ultimate” algorithm.
The aim of this study was to evaluate the outcome of treatment of myopia and myopic astigmatism with this algorithm 1 year post surgery in terms of visual acuity, refraction, HOAs, Z 4 0 , and subjective satisfaction with vision for patients treated by PRK and LASIK.
Methods
Patients undergoing primary PRK or LASIK for low, moderate, or high myopia with or without myopic astigmatism were consecutively included in this prospective study from September 1, 2009 to October 31, 2009 in the Mater Private Hospital, Dublin, Ireland.
In all cases, patients were 20 years or older with stable refraction for at least 1 year, corrected distance visual acuity (CDVA) of 20/32 or better, no signs of keratoconus or ocular surface disease, and sufficient central corneal thickness (CCT) to allow an estimated residual stromal thickness of 350 μm following the necessary ablation required by the myopic treatment.
Patients were excluded for treatment by LASIK if any 1 of the following criteria were met: 1) age 20−23 years, 2) preoperative CCT of 500 μm or less, 3) reduced tear production on unstimulated Schirmer testing (<15 mm wetting over 5 minutes) in the absence of symptoms or other clinical signs of dry eye disease, or 4) involvement in contact sports or the armed forces. These patients were treated by PRK. If none of the exclusion criteria were met, patients could choose to have treatment by PRK or LASIK. Each group was followed and analyzed separately for 1 year post surgery.
All patients underwent a complete ophthalmic examination preoperatively including measurement of uncorrected distance visual acuity (UDVA) and CDVA with an ETDRS chart, manifest subjective refraction, anterior and posterior segment biomicroscopy, applanation tonometry, and Schirmer testing. Corneal topography was performed with the Orbscan IIz (Technolas Perfect Vision). Aberrations were measured with the Zywave II aberrometer (Technolas Perfect Vision). This is a Hartmann-Shack aberrometer, which records aberrations up to and including the 5th-order Zernike coefficients. Soft contact lenses were discontinued for at least 1 week and rigid contact lenses for 4 weeks prior to measurement.
Wavefront examinations were performed as 3 consecutive measurements under mesopic conditions (undilated pupil) prior to surgery. A minimum pupillary diameter of 6 mm was required. The undilated measurement with the best alignment and highest concordance to the subjective refraction data was chosen for treatment. Fogging of the accommodative target was used during the measurement. Treatment was calculated using the Zyoptix Treatment Calculator software and the Zyoptix Advanced Nomogram. The Advanced Nomogram uses both the manifest and the aberrometer refraction and considers the effect of treating all HOAs in calculating the treatment sphere. The maximum optical zone (OZ), with minimum permissible 6 mm, allowing a minimum residual stromal thickness of 350 μm was selected. At 1 year, the Zywave measurements were repeated under the same mesopic conditions with an undilated pupil. If the minimum pupillary diameter of 6 mm was not reached, a drop of phenylephrine hydrochloride 2.5% (Minims; Bausch & Lomb, UK Limited, Surrey, United Kingdom) was used to dilate the pupil in accordance with the guidelines of the manufacturer. This was to enable all wavefront measurements for the preoperative and postoperative analysis to be standardized for a 6-mm pupil under noncycloplegic conditions.
All procedures were performed by 1 surgeon (M.O’K.) using the Technolas 217z100 excimer laser (Technolas Perfect Vision). In bilateral cases, surgery was performed a week apart. Corneas were anesthetized preoperatively with proxymetacaine hydrochloride 0.5% drops (Minims; Bausch & Lomb, UK Limited) and an eyelid speculum was placed. For PRK, the corneal epithelium in an 8.5 mm diameter was loosened with a 20% alcohol solution and peeled with a blunt spatula and discarded. The ocular surface was irrigated with balanced salt solution and dried with a surgical sponge (Merocel; Medtronic Xomed Inc, Jacksonville, Florida, USA). For LASIK, the Zyoptix XP microkeratome was used to create 8.5- to 9.5-mm superior hinged flaps of 120 μm thickness. Tissue ablation was performed using the Zyoptix PTA algorithm with a postoperative target of emmetropia in all cases. Iris recognition and multidimensional rotational eye tracking were used during the ablation. Mitomycin C 0.02% was used following the laser ablation in PRK cases for 30 seconds when the calculated ablation depth was ≥100 μm. A soft contact lens (Purevision; Bausch & Lomb Inc, Rochester, New York, USA) was applied at the end of the PRK procedure.
Postoperative PRK patients received topical chloramphenicol 1% (Chloromycetin Redidrops; Goldshield Group Ltd, Surrey, United Kingdom) 4 times daily for 1 week followed by topical dexamethasone 0.1% (Maxidex; Alcon Laboratories (UK) Ltd, Herts, United Kingdom) 4 times daily for 3 weeks, then twice daily for 3 weeks. The soft contact lens was removed when reepithelialization was complete at 3 to 5 days postoperatively. LASIK patients received fluorometholone 0.1% (FML Liquifilm; Allergan Inc, Irvine, California, USA) and chloramphenicol 1% (Chloromycetin; Goldshield Group Ltd) 4 times daily for 1 week. Follow-up was at 1 day, 3 days (for PRK), 1 week, 1 month, 3 months, 6 months, and 1 year postoperatively.
Main outcome measures were efficacy, predictability, stability, safety, HOAs, and subjective quality of vision. Efficacy was evaluated by the UDVA at 1 year. Predictability was evaluated by the mean manifest refraction spherical equivalent (MRSE) at 1 year, by comparison of the attempted spherical equivalent (SE) refraction to the achieved SE refraction, by the percentage of eyes with postoperative SE refraction within 0.50 diopters (D) and within 1.0 D of emmetropia, and by the percentage of eyes with postoperative refractive astigmatism of ≤0.50 D and ≤1.0 D. Stability was evaluated by the percentage of eyes with change in postoperative SE refraction of >0.50 D from 3 to 12 months. Safety was evaluated by the change in CDVA at 1 year in terms of the number of eyes that lost or gained 1 or more lines of acuity.
At 1 year, patients were asked to rate their overall level of satisfaction with their vision on a scale of 1 to 4 (1 = very dissatisfied, 4 = very satisfied). Patients were asked whether they would choose to have the same surgery again based on their experience and whether their postoperative uncorrected vision was worse than, the same as, or better than the preoperative spectacle-corrected and/or contact lens–corrected vision.
Statistical analysis was performed using SPSS for Windows (version 16.0, SPSS, Inc). A t test for related samples was used for comparison of preoperative to postoperative data within each group. An independent samples t test was used for comparisons between the PRK and LASIK groups. Wavefront aberrometry data were analyzed using nonparametric tests for non–normally distributed data. Total HOA root mean square (RMS) and RMS of total third-, fourth-, and fifth-order aberrations were compared preoperatively and postoperatively for each group using a Wilcoxon signed rank test for related samples. Z 4 0 was analyzed separately, retaining the positive or negative sign of the coefficient. For comparisons between the PRK and LASIK groups, an independent samples Mann Whitney U test was used. All wavefront measurements were analyzed at the 6-mm pupil size. To compensate for mirror symmetry of right and left eyes, the signs of the coefficients for horizontal terms in the left eye were reversed when combining data for right and left eyes for analysis.
Multiple linear regression analysis was performed to determine which variables independently predicted change (positive induction) in HOA and in Z 4 0 . For the dependent variable change in HOA, the independent variables included in the model were age, sex, preoperative HOA level, magnitude of the attempted SE correction, OZ diameter, and LASIK vs PRK surgery. For the dependent variable change in Z 4 0 ,the independent variables included in the model were age, sex, preoperative Z 4 0 level, preoperative total HOA without Z 4 0 , magnitude of the attempted SE correction, OZ diameter, and LASIK vs PRK surgery. Standardized regression coefficients (beta) and P values for each variable in the model were calculated. Variables with P value >.05 were eliminated from the model in a stepwise manner to determine the variables with a statistically significant impact on positive induction of HOA or Z 4 0 . The beta value for each significant variable is reported and represents the numerical influence of that variable (slope of the regression line) on the change in HOA or Z 4 0 . The R square value (R 2 ) for each model is reported and represents how much of the variance in the dependent variable is explained by the model. For all tests, a P value of <.05 was considered statistically significant.
Results
Eighty eyes of 48 patients were treated during the study period. Thirty-eight eyes of 23 patients were treated by PRK and 42 eyes of 25 patients were treated by LASIK. The baseline characteristics are shown in Table 1 . There were no statistically significant differences between the groups in terms of preoperative sphere, cylinder, SE, sex, predicted ablation depth, CCT, or OZ diameter. As expected from the preoperative inclusion criteria, there was a statistically significant difference in age, with PRK patients being younger ( P = .02).
| PRK Group Mean ± SD (Range) | LASIK Group Mean ± SD (Range) | P Value a | |
|---|---|---|---|
| Age (y) | 30.4 ± 8.4 (21 to 54) | 34.7 ± 7.1 (25 to 58) | .02 |
| Sphere (D) | −3.47 ± 1.45 (−0.48 to −5.9) | −2.96 ± 1.37 (−0.85 to −6.9) | .126 |
| Cylinder (D) | −1.01 ± 1.23 (−0.25 to −5.8) | −0.99 ± 1.18 (−0.10 to −5.86) | .94 |
| SE (D) | −3.99 ± 1.26 (−2.04 to −6.52) | −3.46 ± 1.45 (−1.28 to −7.4) | .102 |
| CCT (μm) | 544.4 ± 34.4 (481 to 617) | 557.7 ± 22.03 (513 to 618) | .052 |
| Ablation depth (μm) | 85.3 ± 19.5 (49 to 116) | 79.0 ± 25.2 (38 to 131) | .233 |
| Optical zone (mm) | 6.69 ± 0.44 (6 to 7.5) | 6.69 ± 0.37 (6 to 7.5) | .954 |
a Statistically significant P < .05, t test (PRK vs LASIK treatment groups).
Follow-Up
In the PRK group, all patients completed the 1-year examination. In the LASIK group, 2 patients (4 eyes) did not complete the 3-month to 1-year follow-up. At the 1-month visit, both had UDVA of 20/20 in each eye. No eye lost a line of CDVA.
Three patients (4 eyes) in the PRK group and 2 patients (3 eyes) in the LASIK group were not available for the 1-year wavefront measurements and these were excluded from the aberrometry analysis.
Retreatments
One patient in the PRK group who was treated for −6.52 SE required a retreatment to his left eye at 11 months postoperatively of −1.48 SE and was excluded from the 1-year analysis. Final UDVA in this eye was 20/20. In the LASIK group, 1 patient who was treated for −7.38 SE in the left eye needed a retreatment of −0.875 SE at 5 months postoperatively and this eye was excluded from the 1-year analysis. Final UDVA in this eye was 20/20. Neither patient lost a line of CDVA.
Complications
No intraoperative complications occurred in the study population. In the PRK group, 3 eyes of 2 patients developed grade 2 corneal haze at 1 and 3 months postoperatively respectively. The haze responded to topical steroid treatment and fully resolved. No postoperative complications occurred in the LASIK group.
Efficacy
At 1 year, mean ± standard deviation (SD) logMAR UDVA was −0.05 ± 0.08 (20/18) in the PRK group and −0.04 ± 0.09 (20/18) in the LASIK group ( P = .786).
Thirty-two of 37 eyes (86.5%) in the PRK group and 30 of 37 eyes (81.2%) in the LASIK group had UDVA of 20/20 or better ( P = .75) ( Figure 1 ). Three of 37 eyes (8%) in the PRK group and none in the LASIK group had supranormal UDVA of 20/12 or better. The efficacy index (ratio of postoperative UDVA to preoperative CDVA) was 1.67 for PRK and 0.86 for LASIK.
Predictability
At 1 year, the mean ± SD MRSE was −0.26 ± 0.3 D in the PRK group and −0.16 ± 0.34 D in the LASIK group ( P = .222). Thirty-three of 37 eyes (89.2%) were within ±0.50 D and 36 of 37 eyes (97.3%) were within ±1.00 D of the plano target in the PRK group. Thirty-two of 37 eyes (86.5%) were within ±0.50 D and 37 of 37 eyes (100%) within ±1.00 D of the plano target in the LASIK group ( Figure 2 ). The SE attempted vs achieved correction is shown in Figure 3 for both groups. Mean ± SD refractive astigmatism at 1 year was 0.47 ± 0.51 D in the PRK group and 0.46 ± 0.28 D in the LASIK group ( P = .826) ( Figure 4 ).
Stability
One patient (2.7%; 1/37) in each group had change in postoperative SE refraction of >0.50 D from 3 to 12 months.
Safety
One eye (3%; 1/37) in the PRK group lost 1 line of CDVA and 1 eye (3%; 1/37) in the LASIK group lost 2 lines of CDVA ( Figure 5 ). The safety index (ratio of postoperative CDVA to preoperative CDVA) at 1 year was 1.84 for PRK and 1.12 for LASIK.
