Recent studies have compared wavefront-guided laser in situ keratomileusis (LASIK) with conventional LASIK for the surgical correction of refractive error.1–4 Wavefront-guided systems increase the predictability of visual outcomes and may reduce the amount of induced aberrations compared with conventional LASIK surgery.4–11 One important question is whether there are differences in visual outcomes between the available wavefront-guided systems.
To date, there have been few reported studies analyzing 2 wavefront-guided systems in the same patient with respect to visual outcomes and higher-order aberrations (HOAs).12,13 In this prospective randomized trial of wavefront-guided LASIK for myopia with or without astigmatism, we compared the results of bilateral laser treatments in which 1 eye was randomly assigned to treatment with the LadarVision CustomCornea system (Alcon Laboratories, Inc.) and the fellow eye was treated with the Star S4 CustomVue system (Visx).
PATIENTS AND METHODS
Forty-four patients (88 eyes) were recruited and enrolled at the John A. Moran Eye Center, Department of Ophthalmology and Visual Science, University of Utah, between February 2004 and May 2005. All patients were between the ages of 21 and 54 years with stable myopia between −1.00 diopter (D) and −8.25 D and astigmatism between 0.00 D and 2.50 D. The University of Utah Hospital Institutional Review Board approved the research protocol in accordance with the tenets of the Declaration of Helsinki. All patients provided informed consent after they received an explanation of the procedure, including all risks and benefits.
All patients had a preoperative discussion of relevant medical history, including history of herpetic eye disease and family history of keratoconus. They all met the U.S. Food and Drug Administration guidelines for both lasers before their inclusion in the study. Patients were excluded if they had a history of clinically significant lens opacity, previous corneal or intraocular surgery, thin corneas, keratoconus, unstable refraction, or autoimmune disease; were pregnant or breastfeeding; or were on immunosuppressive therapy. Contact lenses were discontinued 2 weeks before screening for patients with soft contact lenses and 6 weeks before screening for patients with rigid gas-permeable contact lenses. Manifest refraction and wavefront measurements were repeated on 2 separate visits to ensure refractive stability.
All patients had a preoperative examination including uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), tonometry, slitlamp examination of the anterior segment, and dilated fundus evaluation. Manifest and cycloplegic refractions were performed. Corneal topography and thickness were measured using the Orbscan II (version 3.0, Bausch & Lomb). Multiple ultrasound measurements of corneal pachymetry were also taken to ensure a residual stromal bed of at least 250 μm after surgery.
Eligible patients were scheduled for bilateral custom LASIK. The right eye was randomly assigned to treatment with the CustomCornea or CustomVue laser system. The fellow eye was assigned treatment with the other laser. The correction target was based on manifest refraction and wavefront analysis, with emmetropia being the target in all patients. No patient had LASIK enhancement at any time during the study.
All eyes received preoperative wavefront analysis with the WaveScan (version 3.07 [Zernike], Advanced Medical Optics). In addition, eyes treated with the CustomCornea system had imaging with the LADARWave Hartmann-Shack wavefront system (Alcon) immediately before surgery. Before analysis and treatment with the CustomCornea system, the eye was dilated with tropicamide 1% and phenylephrine 2.5%. Each eye was analyzed 5 times with the CustomCornea aberrometer. The scan with measures of lower-order aberrations that most closely matched the patient's manifest refraction was selected as the basis of treatment.
Previously established Moran Laser Center wavefront-guided LASIK nomograms were followed for both the CustomCornea and the CustomVue treatments. The nomograms were generated with Datagraph-med refractive outcomes software (version 3.20a, Ingenieurbüro Pieger GmbH) using a minimum of 50 eyes for each wavefront platform with a minimum follow-up of 3 months.
All LASIK flaps were created with a Hansatome microkeratome (Bausch & Lomb) using a new blade in each eye. The flap size (8.5 or 9.5 mm) and thickness (160 or 180 μm) were calculated in each eye. Flaps were equivalent in size and thickness bilaterally; a superior hinge was used in all eyes. If the 8.0 mm maximum intended ablation diameter exceeded the flap diameter, the hinge and flap were shielded during ablation.
After surgery, each eye received 1 drop of gatifloxacin 0.3% (Zymar), prednisolone acetate 1% (Pred Forte), and ketorolac tromethamine 0.4% (Acular LS); the gatifloxacin and prednisolone acetate were continued 4 times a day for 1 week.
Patients were seen 1 day; 1 week; and 1, 3, and 6 months after surgery. At all visits, the UCVA and BSCVA were tested using a standard Snellen eye chart. Visual acuity was recorded in both Snellen notation and logarithm of the minimum angle of resolution (logMAR) format.
Higher-order aberrations, including coma, Z(3,1), trefoil, Z(3,3), and spherical aberration, Z(4,0), were measured using the WaveScan. Undilated scans of both eyes were taken preoperatively and 3 months postoperatively regardless of the wavefront platform used for treatment.
Primary outcome measures were UCVA, BSCVA, and manifest refraction. The secondary outcome measure was HOAs.
There were no violations in the randomization; all patients were analyzed as originally assigned. The randomization protocol was generated before the trial and known only to the study coordinator. Postoperative measurements were made by masked examiners with the exception of a few postoperative visits of several patients in whom measurements were performed by the surgeon due to schedule conflicts. After the study was completed, the results were compiled and the data unmasked for statistical analysis.
Refractive error, visual acuity, and HOAs were treated as continuous variables and analyzed for significance by paired t tests or the Wilcoxon rank sum test.
Thirty-nine of the 44 patients (78 eyes) completed the study for final analysis. One patient was excluded from the study due to an incomplete flap. Four patients were excluded because their eyes were insufficiently dilated and could not be tracked by the LADARWave despite a well-calibrated eye tracker and sufficient time and strength of the drug for pupil dilation. The remaining 39 patients had uneventful bilateral LASIK. The sample population consisted of 17 women and 22 men with a mean age of 35.1 years ± 8.6 (SD) (range 21 to 54 years). All patients attended the 3-month follow-up, and 34 (87%) attended the 6-month follow-up. There were no complications. Table 1 shows the preoperative comparisons.
Efficacy and Stability
Tables 2 and 3 show the visual acuity outcomes at 3 months and 6 months. Figures 1 and 2 show the cumulative UCVA at 6 months and the refractive stability, respectively. Two eyes treated with the CustomCornea system and 1 eye treated with the CustomVue system had 20/10 visual acuity at 6 months.
At 3 months, all 39 eyes in the CustomVue group and 33 of 39 eyes (85%) in the CustomCornea group were within ±0.50 D of emmetropia (P<.005). In addition, 30 eyes (77%) in the CustomVue group and 28 eyes (72%) in the CustomCornea group were within ±0.25 D of emmetropia (P>.25).
Figure 3 shows the scattergram of the closed correlation between the attempted and achieved spherical equivalent for both laser platforms. At 6 months, 31 (91%) of 34 eyes in the CustomVue group and 27 (79%) of 34 eyes in the CustomCornea group were within ±0.50 D of emmetropia (P<.1); 30 (88%) and 17 (50%), respectively, were within ±0.25 D (P<.001).
At 3 months, 7 eyes (18%) in the CustomVue group lost 1 line of BSCVA compared to 12 eyes (31%) in the CustomCornea group. Twenty eyes (51%) in both groups maintained equal BSCVA. Twelve eyes (31%) in the CustomVue group and 6 eyes (15%) in the CustomCornea group gained 1 line of BSCVA. One eye in the CustomCornea group gained 2 lines, while no eye in the CustomVue group gained 2 lines (Figure 4).
Twenty four (61.5%) of 39 patients completed CustomVue WaveScan analysis at the 3-month follow-up (Figure 5). Of these, the total root-mean-square (RMS) HOAs increased 8.3% in the CustomVue group and 22.7% in the CustomCornea group (P = .080); coma increased 16.3% in the CustomVue group and 1.5% in the CustomCornea group (P = .915); trefoil decreased by 17.1% in the CustomVue group and increased 7.6% in the CustomCornea group (P = .108); spherical aberration increased 26.3% in the CustomVue group and 10.5% in the CustomCornea group (P = .221). The total RMS values for these endpoints changed similarly in absolute amount without significant statistical differences.
This randomized prospective trial provides additional support that both laser systems are safe and effective for wavefront-guided refractive surgery. Although both lasers provided the majority of patients with 20/20 acuity or better, there was a significant difference in the primary outcome measure of UCVA in CustomVue-treated eyes compared with CustomCornea-treated eyes, representing a 2- to 3-letter difference on the standard Snellen eye chart. More eyes treated with the CustomVue laser had 20/20 or better acuity, both corrected and uncorrected, at 3 and 6 months. In addition, more CustomVue-treated eyes were within ±0.50 D and ±0.25 D of emmetropia at 3 and 6 months.
Two earlier studies compare the CustomCornea and CustomVue laser systems. Slade12 found that in 25 patients treated with 1 laser system in each eye, after 1 month, 92% of CustomCornea eyes and 72% of CustomVue eyes were within ±0.50 D of the attempted correction. However, when their Visx nomogram was applied, 8 of 10 CustomCornea eyes and all 10 CustomVue eyes were within ±0.50 D of the attempted correction. In contrast, our CustomVue results could not have been favorably affected by the previous use of a surgeon-specific nomogram. Before this study, a corresponding wavefront nomogram for each laser had been established based on a minimum of 50 eyes with 3 months postoperative follow-up for both CustomCornea and CustomVue platforms. Awwad et al.13 prospectively treated patients with the CustomCornea or the CustomVue platform in 93 eyes, each patient having surgery with only 1 laser. They found similar UCVA and predictability between lasers 3 months after surgery. They also found that more CustomCornea eyes could read the 20/12.5 chart than CustomVue eyes, a pattern we identified in 6% of our CustomCornea group. Durrie and Stahl,14 in a randomized study of the CustomCornea system and the Zyoptix custom platform (Bausch & Lomb), found a higher proportion of CustomCornea patients attained 20/12.5 UCVA (47% versus 10%).
The secondary outcomes of HOAs (coma, trefoil, and spherical aberration) showed no significant differences between laser groups based on mean change, as a percentage or absolute, from preoperative values. Qualitatively, however, the CustomVue platform minimized increases in total RMS compared with the CustomCornea, whereas the CustomCornea platform showed less increase in total spherical aberration and coma. Furthermore, the CustomVue platform successfully reduced trefoil, whereas the CustomCornea platform tended to increase trefoil. We speculate the reduction in trefoil aberration in the CustomVue eyes despite the larger induction of spherical aberration and coma compared with CustomCornea eyes explains the favorable change in the total HOA for the CustomVue laser. Interestingly, on average, total HOA increased with both systems—8% with the CustomVue system and 23% with the CustomCornea system. This provides additional support that wavefront-based LASIK, with this prevalent technology, does not broadly “treat” HOAs. We acknowledge there is a likely advantage over conventional LASIK, in which 2.3- to 5.5-fold HOA increases have been noted in symptomatic patients.15
The CustomCornea uses an ablation zone of 9.0 mm and the CustomVue, an 8.0 mm ablation zone. Furthermore, the CustomCornea removes more tissue than the CustomVue platform, which relates to the ablation profiles. Thus, the CustomCornea induces less spherical aberration than the CustomVue, but at the cost of a steeper central topography that sometimes resembles a central island. On the other hand, there was a gradual myopic regression in the sphere from −0.096 to −0.171 D in the CustomVue eyes and from −0.292 D to −0.308 D in the CustomCornea eyes during the 3 to 6 postoperative months. This regression in the CustomVue eyes may be attributable to the platform's smaller ablation zone. Theoretically, increased corneal ablation limits the possibilities for future enhancements, increases the probability of keratectasia,16 and may intensify the compensatory corneal biomechanical response.17 Although it expends more tissue, our data and data collected by others18,19 suggest that increasing the size of an ablation zone lowers the percentage of postoperative spherical aberration and coma.
There were several limitations to our study. Only 24 of 39 patients had WaveScan analysis at 3 months, limiting our wavefront-data analysis. Furthermore, we did not track objective measurements such as contrast sensitivity, vector analysis, effective optical zone, and subjective symptoms such as glare or halos. Another limitation was the lack of standardized preoperative and postoperative patient satisfaction questionnaires, which may have added valuable information to this comparative analysis of the 2 lasers.
Furthermore, all postoperative wavefront analyses were performed with the WaveScan. This may introduce bias as the LADARWave Hartmann-Shack wavefront system was used to treat the CustomCornea group and the WaveScan instrument was used to analyze both groups postoperatively. In a comparison of 8 wavefront reactor systems using standardized model test eyes, Campbell20 showed the systems did not give consistent results for the same test eye. This suggests these sensitive instruments do not necessarily measure the same tissue in the same way, and using 1 instrument to measure the effects of another's treatment profile may therefore introduce bias. To minimize the effect of such limitations, preoperative and postoperative wavefront data were collected on the CustomVue platform for all patients regardless of the wavefront platform used for treatment. Thus, comparisons between preoperative and postoperative data were generated using data from only the CustomVue platform.
Differences exist between the laser systems in their acquisition process and laser platforms. The CustomCornea uses an acquisition process performed immediately before surgery. In addition, surgical tracking with the CustomCornea requires a minimum pupil diameter of 7.0 mm. Four of the original 44 patients were unable to be dilated to 7.0 mm despite the use of multiple mydriatic agents with a well-calibrated eye tracker and were subsequently excluded from the study. The CustomCornea, using a scanning-spot 60 Hz laser, requires a longer ablation period, especially in cases of high myopia, which may increase stromal dehydration and secondary heterogeneous ablation. This may explain the less favorable UCVA and BSCVA outcomes in the CustomCornea eyes with larger induction of total HOA, leading to higher percentage of eyes with a loss of 1 line of BSCVA in our series. Conversely, the CustomVue uses a broad-beam 10 Hz laser, resulting in an accelerated ablation time. Finally, the WaveScan instrument conveniently allows for wavefront acquisition several days before the surgery, unlike the LADARWave system, for which acquisition is recommended after dilation shortly before the actual procedure.
At the time of the study, both systems were using Hartmann-Shack wavefront platforms, although different wavefront refractor systems from different manufacturers may have poor compatibility.20 Since we completed our study, Visx introduced the Fourier wavefront software and Alcon upgraded the CustomCornea alignment software, resulting in enhanced wavefront acquisition and decreased subjectivity. Data in the present study also predate the routine use of iris-registration software, which permits compensation for cyclotorsional errors. Such upgrades could improve visual results.
With the CustomVue system, accommodation during the acquisition process causes pupil constriction, which alters wavefront measurements. In addition, there is occasional cylinder variability in both power and axis, causing inconsistent results. Furthermore, at the time of this study, the CustomVue did not use iris registration. This can cause the wavefront to be quite different in alignment due to cyclotorsional changes.21,22 Perhaps the recently approved iris-registration software will help eliminate this variable.
In summary, this study reinforces that both the CustomCornea and the CustomVue are safe and effective wavefront-based excimer laser systems. Both platforms led to a small rise in total HOAs. The CustomVue system reduced trefoil and induced less of an increase in total HOAs than the CustomCornea platform, whereas the CustomCornea platform increased trefoil but induced less of an increase in spherical aberrations and coma. Finally, the CustomVue platform led to a greater percentage of patients with a UCVA of 20/20 or better in 6-month follow-up in this contralateral-eye study.
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