Until recently, most wavefront-guided excimer laser treatments have been driven by Hartmann-Shack aberrometry. Hartmann-Shack aberrometry works by reflecting a ray of infrared laser light off of the retina and sampling the emerging beam over the pupillary zone with a grid array of lenslets. Aberrometric data are then derived from a function of the difference between the measured position of the emergent beam and its reference position based on a neutral wavefront at each point sampled. Measurement fidelity for Hartmann-Shack systems is limited by the density of the sampling array, and the measurement range is limited by spot cross-over. Spot cross-over is a term used to describe the situation in which the emergent beam is deviated beyond the sampling area of the reference sensor and into the sampling area of the neighboring sensor, resulting in a failed scan acquisition. This limits the application of Hartmann-Shack systems in the highly aberrated eyes that would benefit most from wavefront-guided treatment.
Ragazzoni1 described pyramidal aberrometry in 1996. Pyramidal aberrometry in the eye is also based on sampling the emergent beam from infrared light reflected off of the retina over the pupillary zone. An oscillating pyramidic optical component placed at the focal plane splits emergent light into four images of the pupil. These images are captured through relay optics by a charged coupled device camera. Differences in light intensity between corresponding loci on these four images are used to derive aberrometric information. Measurement fidelity is only limited by the pixel density of the charged coupled device camera, and spot cross-over does not occur. Theoretical advantages for pyramidal aberrometry include greater sampling density and a higher dynamic range than Hartmann-Shack aberrometry.
We set out to evaluate measurement repeatability in routine clinical use and clinical results in myopic wavefront-guided laser in situ keratomileusis (LASIK) using the first commercially available system based on pyramidal aberrometry. To the best of our knowledge, this article is the first published data on treatment guided by pyramidal aberrometry.
Patients and Methods
We conducted a retrospective analysis of anonymized data from consecutive cases of myopic wavefront-guided LASIK (≤ 10.00 diopters [D] sphere; ≤ 4.00 D cylinder) performed by a single surgeon (BDA) at Moorfields Eye Hospital between November 2017 and January 2019.
We extracted additional data from consecutive wavefront scans acquired during preoperative and postoperative examination for the first 100 eyes treated for measurement repeatability analysis.
We studied data collected electronically in the course of routine clinical practice as part of a continuous review of laser vision correction accuracy approved by the Clinical Audit and Effectiveness Committee at Moorfields Eye Hospital NHS Foundation Trust. The study and consent procedures adhered to the tenets of the Declaration of Helsinki.
Aberrometry
We performed Peramis (SCHWIND eye-tech-solutions GmbH) pyramidal aberrometry as a first step in preoperative and postoperative examinations. We uncoupled aberrometry from topography measurement, selecting aberrometry only rather than combined aberrometry and topography measurement, and performed aberrometry before any other scans or manifest refraction to minimize acquisition time and the possible influence of fatigue on measurement repeatability. Three consecutive scans were acquired in mesopic lighting conditions for first the right and then the left eyes by a single optometrist (HH) according to a standardized operating procedure, including standardized oral instructions to each patient. We instructed patients to keep their forehead and chin in contact with the rests, avoid head tilt, keep their focus relaxed (looking through rather than at the fixation target), and blink whenever they felt like doing so, but to keep their eyes wide open between blinks.
Treatment
We determined eligibility for LASIK using standard criteria.2,3 We selected patients for wavefront-guided treatment if the aberrometric acquisition diameter was greater than 5 mm on all scans and greater than 5.5 mm on the scan selected for treatment planning in each eye. Eyes not meeting these criteria were treated with conventional myopic LASIK and were excluded from analysis. We exported the scan with the largest acquisition diameter and a green light quality indicator for the iris cyclotorsional registration image for treatment planning in SCHWIND CAM software. We used a 6.5-mm optical zone throughout.
After importing aberrometric and topographic data, we performed nomogram adjustments to the target sphere in treatment planning software with reference to the manifest refraction spherical equivalent as previously described.4 No adjustments were entered for the target cylindrical correction.
Throughout the study period, we performed wavefront-guided LASIK using Intralase iFS femtosecond laser (J&J Vision) flap creation, 8.5-mm flap diameter, 100 to 110 µm flap thickness, and the SCHWIND Amaris 1050RS excimer laser.
Data Archiving and Analysis
We archived anonymized data extracts on an Excel (Microsoft Corporation) spreadsheet for analysis and filtered outlying values using plausibility limits to screen for data entry errors.
In the subset of 100 eyes studied for measurement repeatability, we calculated 95% limits of repeatability (95% LoR) from the standard deviation within measures (Sw) derived from a random effects analysis of variance applying the formula: 95% LoR = 1.96*SQRT(2)*Sw.5 We calculated 95% LoR for spherical equivalent values normalized to a 5-mm pupil for the following variables preoperatively and postoperatively: sphere, cylinder, coma, trefoil, spherical aberration, and RMS-HOA.
We compared pupil diameters throughout the aberrometry scan acquisition sequence as a surrogate measure of accommodation control and measurement fatigue during scanning.
For the first 100 eyes, we derived limits of agreement (LoA) and bias, or mean difference, values for measured aberrometric and manifest refraction spherical equivalent values preoperatively and postoperatively using Bland-Altman plots.6
Aberration terms were reported as equivalent defocus (D) using a linear conversion between RMS wavefront variance (µm) and equivalent defocus (D)7: D = 16.SQRT(3).µ/P2, where D = dioptric spherical equivalent; µ = RMS wavefront variance in microns; and P = analysis diameter.
We summarized treatment results for myopic wavefront-guided LASIK using standard outcome reporting.8
Results
A total of 81% of eyes eligible for myopic LASIK had a mesopic pupil size and aberrometry scan acquisition diameter of greater than 5.5 mm, and were treated with wavefront-guided LASIK.
Mean preoperative and postoperative values for aberrometric indices and 95% LoR for the first 100 eyes were tabulated (Table 1). To one decimal place, we found 95% LoA for sphere, cylinder, and HoA indices at 0.3 D, 0.2 D, and 0.1 D, respectively, implying that differences between 19 of 20 consecutive measures would not exceed this value.
There was a trend toward a reduction in pupil size at the end of the measurement sequence (Figure A, available in the online version of this article) but this was not reflected in any trend to changes in the mean measured sphere (Table 1).
On average, the preoperative aberrometric refraction spherical equivalent was approximately 0.2 D less myopic than manifest refraction spherical equivalent. Again, this implies good control over accommodation during pyramidal aberrometry (Figure BA, available in the online version of this article). We observed a trend (R2 = 0.2; Kendall's Tau = −0.22; P = .001) toward overestimation of myopic outcomes versus manifest refraction values in postoperative examination (Figure BB).
Outcomes for 265 consecutive eyes (133 patients; age: 36.2 ± 8.9 years) treated with myopic wavefront guided LASIK using pyramidal aberrometry are summarized in Figure 1. Three months after surgery, 95% of eyes were within ±0.50 D of the intended refraction spherical equivalent target. Uncorrected distance visual acuity in 96% of 232 eyes with a plano refraction target outcome was 20/20 or better. A total of 97% of eyes had refraction cylinder of 0.50 D or less after surgery. No eyes lost one or more lines of corrected distance visual acuity.
Discussion
This study was initiated to investigate measurement repeatability data and treatment results for a pyramidal aberrometer in routine myopic LASIK. Our results show good spherical equivalent measurement repeatability in pyramidal aberrometry. Treatment results of wavefront-guided myopic LASIK using this pyramidal aberrometry system demonstrated efficient, safe, and predictable refractive outcomes in routine clinical practice.
Although data were analyzed retrospectively, these data were archived prospectively in a well-structured clinical database based on United Kingdom national recommendations.9 Data acquisition and aberrometry in particular were also based on standard operating procedures. Our aberrometric results are reported as spherical equivalent dioptric values (D) at a standardized 5-mm pupil diameter. As described by Thibos et al,7 this format is more clinically intuitive than aberrometric results expressed in microns, and has the advantage of normalizing RMS expressions of wavefront variance in microns by pupil area.
Against these strengths, this study is non-comparative and references the existing literature to evaluate results in relation to measurement repeatability versus manifest refraction and treatment outcomes. We also did not use a patient-reported outcome measure in addition to standard reporting in routine clinical practice. We are therefore unable to comment on possible benefits of wavefront-guided versus conventional treatment for subjective visual outcomes.
The existing literature on measurement repeatability for aberrometers in routine clinical practice is limited by variations in methodology and expression of aberration terms. But our data suggest measurement precision (repeatability) for the pyramidal aberrometer used here is similar to that for Hartmann-Shack aberrometers10,11 used in leading contemporary wavefront-guided LASIK systems (Table A, available in the online version of this article). Pyramidal aberrometry avoids problems with spot cross-over inherent in Hartmann-Shack systems when imaging more irregular corneas, and may therefore have advantages for therapeutic treatment of irregular astigmatism. This is an important area for further study.
There are more than 300 publications on wavefront-guided laser surgery in the scientific literature. This is a technology in evolution, and existing studies report variable and conflicting outcomes and conclusions.12 Studies of earlier systems13,14 have failed to demonstrate a clear advantage of wavefront-guided over conventional treatment for low to moderate myopia and myopic astigmatism. No statistically significant differences were observed regarding safety, efficacy, or predictability among groups.13,14 To define patient groups for whom wavefront-guided laser surgery may offer an advantage, other studies are stratified eyes by RMSHOA scores. Results for wavefront-guided and conventional LASIK were similar for eyes with less than 0.30 µm preoperative RMS-HOA at the same pupil sizes. For eyes with a preoperative RMS-HOA greater than 0.30 µm, wavefront-guided treatment resulted in lower aberration scores postoperatively.15,16
Correction of HOAs could lead to an improvement in contrast sensitivity and visual acuity17,18 and a reduction in visual quality problems including glare and halos after treatment.19,20 These side effects have been attributed to the increased HOAs, induction of positive spherical aberration, and decreased corneal asphericity that are associated with the ablation profile of traditional LASIK refractive surgery, with some studies reporting superior night vision performance and a reduction of glare symptoms after wavefront-guided LASIK.16,17 Schallhorn et al21 observed a significant improvement of night driving visual performance after wavefront-guided correction compared to conventional treatment, but aberration compensation in conventional LASIK treatment based on mean induced aberrations has improved in later laser systems since these results were published. Our findings and work by Thibos et al7 suggest that equivalent defocus for total HOAs in normal eyes is less than 0.30 D. If they exist, differences between results for contemporary wavefront-guided systems and conventional LASIK are small, and may not be picked up in analyses restricted to visual acuity or spherical equivalent refraction data.
Both our data and previous results for Hartmann-Shack aberrometers12,23–26 suggest better measurement repeatability for aberrometric sphere and cylindrical refraction than for manifest refraction data. Aberrometric precision for cylinder terms in particular is superior to manifest refraction.23,24 Our good astigmatic outcomes (Figure 1) indicate that enhanced measurement precision for astigmatism may confer some advantages for wavefront-guided treatment in routine clinical practice.
Wavefront-guided treatment does not require data transcription other than for nomogram adjustments, protecting from human error during treatment programming. This may also be an important advantage in routine clinical practice, particularly in high volume treatment settings.
The standard measurement for refractive outcomes,8 including those for investigations of wavefront-guided LASIK, remains subjective manifest refraction. Previous investigators23,24 have highlighted the difference between measurement repeatability (precision) and accuracy, aligning defocus measurements correctly with visual acuity. Both refraction modalities are likely to have some bias (systematic undercorrection or over-correction versus the true value). Nomograms derived from regression analysis applying a modification to the target sphere based on a weighted difference between the manifest and aberrometric refraction have previously been shown to improve spherical equivalent manifest refraction results4 and were used in this study. Our analyses suggest a small (0.2 D) uniform trend to underestimation of manifest refraction spherical equivalent myopia by pyramidal aberrometry in preoperative patients (Figure BA). In postoperative pyramidal aberrometry, we observed a weak but statistically significant trend toward overestimation of myopia in comparison with manifest refraction spherical equivalent (Figure BB). It is important to consider this in relation to wavefront-guided enhancement LASIK treatments using this system, and to modulate the refraction target sphere with reference to the pre-enhancement manifest refraction spherical equivalent.
Our data demonstrate that pyramidal aberrometry can be applied safely and effectively as a basis for treatment programming in routine myopic LASIK. Pyramidal aberrometry systems may have advantages over Hartmann-Shack aberrometry, including a higher dynamic range and greater measurement fidelity. Differences between results for normal eyes undergoing wavefront-guided and conventional LASIK are small, but incremental gains are important in the quest for optimized outcomes. Future research will determine whether pyramidal aberrometry is superior to Hartmann-Shack systems for the measurement and treatment of irregular astigmatism and eyes with higher starting levels of HOAs.
References
- Ragazzoni R. Pupil plane wavefront sensing with an oscillating prism. J Mod Opt. 1996;43(2):289–293. doi:10.1080/09500349608232742 [CrossRef]
- Watson SL, Bunce C, Allan BD. Improved safety in contemporary LASIK. Ophthalmology. 2005;112(8):1375–1380. doi:10.1016/j.ophtha.2005.02.012 [CrossRef]
- Chan C, Saad A, Randleman JB, et al. Analysis of cases and accuracy of 3 risk scoring systems in predicting ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2018;44(8):979–992. doi:10.1016/j.jcrs.2018.05.013 [CrossRef]
- Allan BD, Hassan H, Ieong A. Multiple regression analysis in nomogram development for myopic wavefront laser in situ keratomileusis: improving astigmatic outcomes. J Cataract Refract Surg. 2015;41(5):1009–1017. doi:10.1016/j.jcrs.2014.08.042 [CrossRef]
- Bland JM, Altman DG. Measurement error. BMJ. 1996;312(7047):1654. doi:10.1136/bmj.312.7047.1654 [CrossRef]
- McAlinden C, Khadka J, Pesudovs K. Statistical methods for conducting agreement (comparison of clinical tests) and precision (repeatability or reproducibility) studies in optometry and ophthalmology. Ophthalmic Physiol Opt. 2011;31(4):330–338.
- Thibos LN, Hong X, Bradley A, Cheng X. Statistical variation of aberration structure and image quality in a normal population of healthy eyes. J Opt Soc Am A Opt Image Sci Vis. 2002;19(12):2329–2348. doi:10.1364/JOSAA.19.002329 [CrossRef]
- Waring GO 3rd, Reinstein DZ, Dupps WJ Jr, et al. Standardized graphs and terms for refractive surgery results. J Refract Surg. 2011;27(1):7–9.
- Royal College of Ophthalmologists. Refractive data set. Accessed November 20, 2019. https://www.rcophth.ac.uk/standards-publications-research/audit-and-data/clinical-data-sets/refractive-surgery-dataset
- López-Miguel A, Maldonado MJ, Belzunce A, Barrio-Barrio J, Coco-Martín MB, Nieto JC. Precision of a commercial Hartmann-Shack aberrometer: limits of total wavefront laser vision correction. Am J Ophthalmol. 2012;154(5):799–807.e5. doi:10.1016/j.ajo.2012.04.024 [CrossRef]
- Prakash G, Jhanji V, Srivastava D, et al. Single session, intra-observer repeatability of an advanced new generation Hartmann-Shack aberrometer in refractive surgery candidates. J Ophthalmic Vis Res. 2015;10(4):498–501. doi:10.4103/2008-322X.176893 [CrossRef]
- Fares U, Suleman H, Al-Aqaba MA, Otri AM, Said DG, Dua HS. Efficacy, predictability, and safety of wavefront-guided refractive laser treatment: metaanalysis. J Cataract Refract Surg. 2011;37(8):1465–1475. doi:10.1016/j.jcrs.2011.02.029 [CrossRef]
- Phusitphoykai N, Tungsiripat T, Siriboonkoom J, Vongthongsri A. Comparison of conventional versus wavefront-guided laser in situ keratomileusis in the same patient. J Refract Surg. 2003;19(2 suppl):S217–S220.
- Dougherty PJ, Bains HS. A retrospective comparison of LASIK outcomes for myopia and myopic astigmatism with conventional NIDEK versus wavefront-guided VISX and Alcon platforms. J Refract Surg. 2008;24(9):891–896.
- Durrie DS, Slade SG, Marshall J. Wavefront-guided excimer laser ablation using photorefractive keratectomy and sub-Bowman's keratomileusis: a contralateral eye study. J Refract Surg. 2008;24:77–84.
- Wallau AD, Campos M. One-year outcomes of a bilateral randomised prospective clinical trial comparing PRK with mitomycin C and LASIK. Br J Ophthalmol. 2009;93(12):1634–1638. doi:10.1136/bjo.2008.152579 [CrossRef]
- Williams D, Yoon GY, Porter J, Guirao A, Hofer H, Cox I. Visual benefit of correcting higher order aberrations of the eye. J Refract Surg. 2000;16(5):S554–559.
- Rocha KM, Vabre L, Chateau N, Krueger RR. Enhanced visual acuity and image perception following correction of highly aberrated eyes using an adaptive optics visual simulator. J Refract Surg. 2010;26(1):52–56.
- Lackner B, Pieh S, Schmidinger G, et al. Glare and halo phenomena after laser in situ keratomileusis. J Cataract Refract Surg. 2003;29(3):444–450. doi:10.1016/S0886-3350(02)01816-3 [CrossRef]
- Hammond SD Jr, Puri AK, Ambati BK. Quality of vision and patient satisfaction after LASIK. Curr Opin Ophthalmol. 2004;15(4):328–332. doi:10.1097/00055735-200408000-00009 [CrossRef]
- Schallhorn SC, Tanzer DJ, Kaupp SE, Brown M, Malady SE. Comparison of night driving performance after wavefront-guided and conventional LASIK for moderate myopia. Ophthalmology. 2009;116(4):702–709. doi:10.1016/j.ophtha.2008.12.038 [CrossRef]
- Lee HK, Choe CM, Ma KT, Kim EK. Measurement of contrast sensitivity and glare under mesopic and photopic conditions following wavefront-guided and conventional LASIK surgery. J Refract Surg. 2006;22(7):647–655.
- Pesudovs K, Parker KE, Cheng H, Applegate RA. The precision of wavefront refraction compared to subjective refraction and autorefraction. Optom Vis Sci. 2007;84(5):387–392. doi:10.1097/OPX.0b013e31804f81a9 [CrossRef]
- Thibos LN, Hong X, Bradley A, Applegate RA. Accuracy and precision of objective refraction from wavefront aberrations. J Vis. 2004;4(4):329–351. doi:10.1167/4.4.9 [CrossRef]
- MacKenzie GE. Reproducibility of sphero-cylindrical prescriptions. Ophthal Physiol Opt. 2008;28(2):143–150.
- Visser N, Berendschot TT, Verbakel F, Tan AN, de Brabander J, Nuijts RM. Evaluation of the comparability and repeatability of four wavefront aberrometers. Invest Ophthalmol Vis Sci. 2011;52(3):1302–1311. doi:10.1167/iovs.10-5841 [CrossRef]
Measurement Repeatability in Pyramidal Aberrometry Before and After Myopic Wavefront-Guided LASIK (N = 100 Eyes)a
| Variable | Measurement 1, Mean ± SD | Measurement 2, Mean ± SD | Measurement 3, Mean ± SD | 95% LoR |
|---|
| Preoperative | | | | |
| SE | −4.625 ± 2.087 | −4.582 ± 2.102 | −4.566 ± 2.110 | 0.325 |
| Cylinder | 0.533 ± 0.466 | 0.531 ± 0.492 | 0.543 ± 0.477 | 0.183 |
| Coma | 0.109 ± 0.069 | 0.117 ± 0.079 | 0.116 ± 0.081 | 0.079 |
| Trefoil | 0.084 ± 0.070 | 0.096 ± 0.076 | 0.100 ± 0.080 | 0.085 |
| SA | 0.063 ± 0.068 | 0.063 ± 0.063 | 0.066 ± 0.069 | 0.059 |
| Total HOA | 0.218 ± 0.063 | 0.230 ± 0.076 | 0.240 ± 0.072 | 0.094 |
| Postoperative | | | | |
| SE | −0.530 ± 0.529 | −0.505 ± 0.546 | −0.487 ± 0.566 | 0.273 |
| Cylinder | 0.266 ± 0.337 | 0.268 ± 0.327 | 0.275 ± 0.373 | 0.159 |
| Coma | 0.153 ± 0.102 | 0.158 ± 0.110 | 0.169 ± 0.117 | 0.100 |
| Trefoil | 0.065 ± 0.085 | 0.066 ± 0.089 | 0.060 ± 0.089 | 0.092 |
| SA | 0.055 ± 0.072 | 0.055 ± 0.079 | 0.049 ± 0.085 | 0.069 |
| Total HOA | 0.256 ± 0.098 | 0.275 ± 0.108 | 0.279 ± 0.119 | 0.113 |
Comparison of LoR for Aberrometers Used in Leading Contemporary Wavefront-Guided LASIK Platformsa,b
| Parameter | Peramis | iDesign | Zywave |
|---|
| Sphere | 0.33 | 0.7 | 0.33 |
| Cylinder | 0.18 | 0.21 | 0.28 |
| Coma | 0.08 | 0.06 | 0.10 |
| Trefoil | 0.09 | 0.07 | 0.11 |
| SA | 0.06 | 0.05 | 0.06 |
| Total HOA | 0.09 | 0.07 | 0.11 |