Excessive induction of spherical aberration after hyperopic LASIK is a main cause of reduction in final visual quality.1 New advances in hyperopic LASIK are focused on developing new algorithm designs, resulting in fewer higher-order aberrations induced during surgery. Because a negative increase in corneal asphericity is expected after hyperopic LASIK,2 one strategy of these new algorithms is to avoid induced changes in corneal asphericity to maintain spherical aberration unchanged or even to customize the postoperative value of corneal asphericity to control the postoperative spherical aberration, thus obtaining advantages in presbyopic LASIK treatments.3
The purpose of this study was to evaluate whether the use of a new-generation aspheric-customized ablation profile in cases of moderate hyperopia may provide advantages with respect to a wavefront-optimized non-customized ablation profile, both designed to minimize postoperative changes in corneal asphericity and hence in spherical aberration induction.
Patients and Methods
This retrospective case series comprised 40 eyes of 24 consecutive patients (mean age: 38.6 ± 9 years; range: 20 to 49 years) that underwent LASIK to correct moderate hyperopia. The first consecutive 24 eyes received wavefront-optimized LASIK and the following consecutive 16 eyes received aspheric-customized LASIK. The study was designed and performed according to the tenets of the Declaration of Helsinki. Exclusion criteria were: age younger than 21 years, preexisting eye disease, previous eye surgery, preoperative topographic irregularities, a calculated postoperative corneal stromal bed below 300 µm at the thinnest corneal area, and cylinders greater than 2.00 D.
A complete preoperative ophthalmological study following a standard protocol was performed for each case. It included a wavefront analysis under cycloplegia for a 6-mm pupil diameter (LADARWave; Alcon Laboratories, Inc., Fort Worth, TX). The study also included corneal asphericity measurement, represented as the Q-factor value that corresponds to the asphericity of the central anterior cornea, performed by corneal topography (Oculus Keratograph; Oculus Optikgeräte, Wetzlar, Germany). The common standard 6-mm pupil diameter was chosen for analysis of results because most patients had a preoperative scotopic value of approximately this diameter.
The WaveLight Allegretto 400-Hz excimer laser platform (Alcon Laboratories, Inc.) was used in all cases with the Hansatome microkeratome (Bausch and Lomb, Rochester, NY). The wavefront-optimized algorithm of the platform places more pulses than a standard non-optimized treatment in the peripheral area to compensate for energy loss and reflections. This theoretically provides a nearly 100% optical zone and a minimized transition zone. At the same time, the natural aspheric shape of the cornea is preserved and the induction of spherical aberration is minimized. The aspheric-customized ablation profile allows the surgeon to schedule the intended postoperative value of corneal asphericity, customization of optical zone, and transition zone. In this manner, the postoperative spherical aberration could be theoretically calculated before the surgery knowing the expected change in corneal asphericity.2
All of the interventions were performed according to standard protocols. The ablation was centered on the visual axis (defined as 80% of the distance of the photopic corneal intersection of line of sight and the coaxially sighted corneal light reflex) in all cases. Hyperopic patients have a larger average angle kappa compared to myopic patients, and the centration of refractive surgery also remains controversial. We chose the visual axis as the centration point based on previous studies that have shown good results with this centration strategy.4 The optical zone was 6.5 mm with a transition zone of 1.25 mm in all cases.
The standard procedure suggested by the manufacturer was used in the wavefront-optimized LASIK group, whereas a customized technique consisting of the following steps was used in the aspheric-customized LASIK group:
In each case, the necessary laser ablation was calculated with the software platform for the standard treatment.
Aspheric-customized ablation profile treatment was scheduled by first introducing the preoperative Q-factor values for each corneal meridian in the F-CAT algorithm provided by the laser platform and then introducing a target of postoperative Q-factor of 0 in all cases. Because the negative spherical aberration induced by hyperopic ablation is directly related to postoperative changes in negative direction of the corneal asphericity,2,5 we attempted to counteract this effect by programming a postoperative Q-factor of 0.
The target sphere was modified to ablate the same number of microns as calculated for standard treatment in step 1.
In addition to the necessary postoperative visits, all patients underwent an examination identical to the preoperative one 6 months after the surgery.
All statistical analysis was done with the help of a standard spreadsheet. Statistically significant differences between data samples were determined using the two-tailed Student's t test. A P value of .05 or less was considered statistically significant. We tested for independence of the predictability and safety distributions between treatments by means of the two-tailed Fisher's exact test.
Surgery was performed without significant complications in all cases. At 6 months postoperatively, none of the eyes showed unanticipated abnormal indications (Figure 1). No patients needed re-treatment for residual ammetropia. No preoperative parameters presented statistically significant differences between groups (Table 1).
(A) The cumulative uncorrected distance visual acuity (UDVA) 6 months postoperatively in both groups. (B) The cumulative corrected distance visual acuity (CDVA) 6 months postoperatively in both groups. (C) The change in CDVA from the preoperative examination to the 6-month postoperative examination in terms of the number of Snellen lines changed. (D) The relationship between the attempted spherical equivalent refraction and the achieved spherical equivalent refraction in both groups. (E) The distribution of postoperative spherical equivalent refraction in both groups 6 months postoperatively. (F) The distribution of refractive astigmatism in both groups 6 months postoperatively. AS = aspheric-customized ablation profile; WF = wavefront-optimized non-customized ablation profile; D = diopters
Preoperative and Postoperative Outcomes of a Wavefront Optimized Non-customized Ablation Profile (WF) and a Customized Aspheric Ablation Profile (AS)
Outcomes of both ablation profiles showed statistically indistinguishable values (P > .05) except for postoperative spherical aberration and Q-factor values (P < .05). The aspheric-customized profile achieved a good predictability in obtained postoperative Q-factor (−0.04). This reduction in postoperative corneal asphericity with the aspheric-customized profile is reflected in the induced spherical aberration as shown in Table 1 and Figure 2. Induced spherical aberration was significantly lower using the aspheric-customized profile (P < .05). In addition, using the aspheric-customized profile allows better prediction of the final spherical aberration based on the sphere to be corrected, is reflected in the R2 values of Figure 2. When the postoperative Q-factor was calculated using paraxial or not paraxial Munnerlyn equations,2,6 expected postoperative values of Q = −0.08 and Q = 0.102 for the wavefront-optimized LASIK group and Q = 0.012 and Q = 0.26 for the aspheric-customized LASIK were obtained. The value obtained with the paraxial Munnerlyn equation was similar to that obtained with the aspheric-customized profile in our case series.
Induced (postoperative minus preoperative) spherical aberration (microns) for both studied groups versus corrected spherical equivalent (diopters). AS = aspheric-customized ablation profile; WF = wavefront-optimized non-customized ablation profile; D = diopters
Regarding efficacy, safety, and safety index, both algorithm results are similar to published results achieved with other systems for the correction of hyperopic refractive errors.7–9 In particular, the mean efficacy index (ratio of mean postoperative uncorrected distance visual acuity to mean preoperative corrected distance visual acuity) was 20/23.53 ± 0.20 for the wavefront-optimized LASIK group and 20/20.83 ± 0.14 for the aspheric-customized LASIK group, with differences between groups statistically not significant (P = .29). One patient in the wavefront-optimized LASIK group lost two or more lines of CDVA and no lines were lost in the aspheric-customized LASIK group.
In this study, outcomes with the aspheric-customized profile were good and promising. Results suggest that an aspheric-customized ablation profile achieves safe and predictable ablations on the cornea. The results are statistically compatible with an improvement in postoperative corneal asphericity and postoperative spherical aberration, without altering procedural safety with respect to a wavefront-optimized non-customized ablation profile.
Because the negative spherical aberration caused by hyperopic ablation is directly related to negative postoperative changes in the corneal asphericity, we have attempted to counteract this effect by programming a postoperative Q-factor of 0 in all patients for the aspheric-customized method. Other theoretical studies have proposed a postoperative Q-factor between −0.45 and −0.60 as an ideal option.6 However, as can be seen in the range and standard deviation values in Table 1, the aspheric-customized profile is not exact and a marked negative tendency exists in some cases. To prevent an excessive value of final negative spherical aberration, a Q-factor of 0 was scheduled in all cases, which is the most positive one allowed by the platform.
Despite the fact that there were no differences in safety index, efficacy, or safety between both treatments, these findings do not imply equal visual quality with both ablation profiles. An excessive negative spherical aberration is known to decrease visual performance, in particular contrast sensitivity at night and halos.10 Such a reduction in induced spherical aberration could lead to improved postoperative visual quality. This could be especially important in selected cases where negative spherical aberration exists preoperatively.
A possible limitation of the current study is that the results are applicable only to the used platform and cannot be inferred to other platforms or treatment strategies. Also, patients with relatively high hyperopia are not common, which has necessarily limited the final number of patients included. Future studies are warranted to evaluate this ablation profile in larger samples and analyze additional visual functions such as contrast sensitivity or halos.
- Durrie DS, Smith RT, Waring GO 4th, Stahl JE, Schwendeman FJ. Comparing conventional and wavefront-optimized LASIK for the treatment of hyperopia. J Refract Surg. 2010;26:356–363. doi:10.3928/1081597X-20090617-07 [CrossRef]
- Jiménez JR, Anera RG, Díaz JA, Pérez-Ocón F. Corneal asphericity after refractive surgery when the Munnerlyn formula is applied. J Opt Soc Am A Opt Image Sci Vis. 2004;21:98–103. doi:10.1364/JOSAA.21.000098 [CrossRef]
- Amigo A, Bonaque S, López-Gil N, Thibos L. Simulated effect of corneal asphericity increase (Q-factor) as a refractive therapy for presbyopia. J Refract Surg. 2012;28:413–418. doi:10.3928/1081597X-20120518-04 [CrossRef]
- Okamoto S, Kimura K, Funakura M, Ikeda N, Hiramatsu H, Bains HS. Comparison of myopic LASIK centered on the coaxially sighted corneal light reflex or line of sight. J Refract Surg. 2009;25(10 suppl):S944–S950. doi:10.3928/1081597X-20090915-09 [CrossRef]
- Manns F, Ho A, Parel JM, Culbertson W. Ablation profiles for wavefront-guided correction of myopia and primary spherical aberration. J Cataract Refract Surg. 2002;28:766–774. doi:10.1016/S0886-3350(01)01322-0 [CrossRef]
- Díaz JA, Anera RG, Jiménez JR, Jiménez del Barco L. Optimum corneal asphericity of myopic eyes for refractive surgery. J Modern Opt. 2003;50:1903–1915. doi:10.1080/09500340308235245 [CrossRef]
- Llorente L, Barbero S, Merayo J, Marcos S. Total and corneal optical aberrations induced by laser in situ keratomileusis for hyperopia. J Refract Surg. 2004;20:203–216.
- Alió JL, El Aswad A, Vega-Estrada A, Javaloy J. Laser in situ keratomileusis for high hyperopia (>5.0 diopters) using optimized aspheric profiles: efficacy and safety. J Cataract Refract Surg. 2013;39:519–527. doi:10.1016/j.jcrs.2012.10.045 [CrossRef]
- Oliver KM, O'Brart DP, Stephenson CG, et al. Anterior corneal optical aberrations induced by photorefractive keratectomy for hyperopia. J Refract Surg. 2001;17:406–413.
- Rocha KM, Vabre L, Harms F, Chateau N, Kreuger RR. Effects of Zernike wavefront aberrations on visual acuity measured using electromagnetic adaptive optics technology. J Refract Surg. 2007;23:953–959.
Preoperative and Postoperative Outcomes of a Wavefront Optimized Non-customized Ablation Profile (WF) and a Customized Aspheric Ablation Profile (AS)a
|Parameter||Preoperative||6 Months Postoperative|
|WF Group||AS Group||WF Group||AS Group|
|No. of eyes||24||16||24||16|
|SEQ (D)||3.66 ± 0.61 (+2.75 to +5.00)||4.05 ± 0.59 (+2.75 to +5.13)||0.08 ± 0.56 (−0.75 to +1.25)||0.21 ± 0.44 (−0.50 to+1.00)|
|UDVA||20/45.45 ± 0.30 (0.08 to 0.61)||20/41.67 ± 0.20 (0.10 to 0.60)||20/21.05 ± 0.24 (0.40 to 1.20)||20/20.41 ± 0.20 (0.50 to 1.20)|
|CDVA||20/17.86 ± 0.13 (0.80 to 1.20)||20/19.61 ± 0.17 (0.50 to 1.20)||20/18.69 ± 0.15 (0.80 to 1.20)||20/20 ± 0.17 (0.50 to 1.20)|
|Safety index||–||–||0.96 ± 0.10||0.99 ± 0.04|
|SA (µm)||0.20 ± 0.11 (−0.07 to 0.37)||0.23 ± 0.14 (−0.04 to 0.40)||−0.39 ± 0.23 (−0.76 to 0.01)||0.04 ± 0.18 (−0.34 to 0.29)b|
|Coma (µm)||0.18 ± 0.12 (0.04 to 0.50)||0.20 ± 0.13 (0.04 to 0.55)||0.64 ± 0.34 (0.18 to 1.20)||0.59 ± 0.25 (0.06 to 0.93)|
|OA (µm)||0.23 ± 0.08 (0.11 to 0.37)||0.18 ± 0.09 (0.07 to 0.42)||0.35 ± 0.11 (0.15 to 0.55)||0.40 ± 0.15 (0.19 to 0.72)|
|CRD (µm)||276 ± 123 (10 to 529)||307 ± 152 (60 to 560)||283 ± 95 (120 to 481)||333 ± 162 (58 to 642)|
|Keratometry (D)||42.92 ± 1.53 (40.55 to 46.40)||43.30 ± 1.32 (41.10 to 43.35)||45.43 ± 1.80 (42.70 to 49.10)||46.60 ± 2.00 (43.55 to 49.10)|
|Q-factor||−0.07 ± 0.11 (−0.27 to 0.20)||0.01 ± 0.20 (−0.23 to 0.40)||−0.52 ± 0.22 (−1.00 to −0.12)||−0.04 ± 0.25 (−0.64 to 0.40)b|