Reliability of Corneal and Total Wavefront Aberration ... - CiteSeerX

Reliability of Corneal and Total Wavefront Aberration Measurements With the SCHWIND Corneal and Ocular Wavefront Analyzers Mike P. Holzer, MD; Martin Sassenroth; Gerd U. Auffarth, MD

ABSTRACT PURPOSE: To evaluate the reliability of the SCHWIND Corneal and Ocular Wavefront Analyzers. METHODS: This study comprised 115 eyes of 58 healthy volunteers (26 [44.8%] women and 32 [55.2%] men) with no corneal or lenticular pathologies and normal visual acuity. All eyes underwent three consecutive measurements by one examiner with the Corneal Wavefront Analyzer and the Ocular Wavefront Analyzer (Hartmann-Shack principle). Corneal wavefront errors were calculated using the topography data, a standard eye model, and ray tracing. The reliability was tested for 2nd, 3rd, and 4th order aberrations as mean values of the standard deviations for all measurements. RESULTS: Mean patient age was 23⫾2.1 years. The mean refraction was ⫺0.77⫾1.56 diopters (D) (range: ⫹3.33 to ⫺5.28 D). The repeatability test revealed a good reliability for both machines with a slightly better value for the Ocular Wavefront Analyzer for 3rd and 4th order higher order aberrations (P⬍.05, Wilcoxon test). CONCLUSIONS: Wavefront measurements of corneal and total optical aberrations performed with the Corneal Wavefront Analyzer and the Ocular Wavefront Analyzer have good reliability. Both measurements are recommended prior to any refractive surgery treatment. [J Refract Surg. 2006;22:917-920.]

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he ability to measure wavefront errors of the eye was an important step in improving treatment plans and surgical outcomes for refractive surgery patients.1 Today, the examination of wavefront errors of the eye is an important factor in the preoperative work-up of refractive surgery candidates. Customized treatments are gaining popularity and the success of these treatments relies on a good excimer laser system and accurate and reliable wavefront measurements.2,3 Another important field for wavefront measurements is intraocular lens (IOL) surgery and wavefront-corrected IOLs, such as aspheric IOLs.4 Total wavefront errors of the eye as well as specific corneal aberrations can be evaluated with different machines. Most wavefront analyzers are based on the Hartmann-Shack principle, including the SCHWIND Ocular Wavefront Analyzer (SCHWIND eye-tech-solutions, Kleinostheim, Germany). This wavefront sensor has a resolution of 210 µm and a maximum of 1452 measuring points.5 Corneal wavefront measurements can be obtained by calculating corneal topography data with ray tracing, which is the basic principle of the SCHWIND Corneal Wavefront Analyzer (SCHWIND eye-tech-solutions). The purpose of this study was to evaluate the reliability and repeatability of corneal and total wavefront measurements for the SCHWIND ESIRIS excimer laser, which were performed with the Corneal Wavefront Analyzer and Ocular Wavefront Analyzer (Figs 1 and 2). MATERIALS AND METHODS This study comprised 115 eyes of 58 healthy volunteers (26 women [44.8%] and 32 men [55.2%]) with no corneal or lenticular pathologies and normal visual acuity. Mean patient age was 23⫾2.1 years. Prior to enrollment, informed consent was obtained from all individuals. All eyes underwent three consecutive measurements unFrom International Vision Correction Research Centre (IVCRC), Department of Ophthalmology, Ruprecht-Karls-University of Heidelberg, Heidelberg, Germany. The authors have no financial or propriety interest in the materials presented herein. Correspondence: Mike P. Holzer, MD, Dept of Ophthalmology, RuprechtKarls-University of Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany. E-mail: [email protected]

Journal of Refractive Surgery Volume 22 November 2006

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Reliability of Wavefront Measurements/Holzer et al

B

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Figure 1. A) Corneal Wavefront Analyzer (SCHWIND eye-tech-solutions, Kleinostheim, Germany) for measurement of B) corneal topography and corneal wavefront errors. An example of corneal wavefront error is shown.

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Figure 2. A) Ocular Wavefront Analyzer (SCHWIND eye-tech-solutions, Kleinostheim, Germany) for measurement of B) total wavefront errors. The correct alignment of the eye prior to the measurement is shown.

der dim light conditions by one well-trained examiner (M.S.) with the Corneal Wavefront Analyzer and the Ocular Wavefront Analyzer. No medication was given to dilate the pupils, and no individual received topical drops or ointment. Special attention was paid to careful alignment of the pupil prior to the individual measurements. During the measurement with the Ocular Wavefront Analyzer, the patient fixates on the center of a grid, which is optically fogged by approximately 1.50 D to avoid patient accommodation during the examination. Corneal wavefront errors were calculated using the topography data from the Corneal Wavefront Analyzer, a standard eye model, and ray tracing. Corneal and total wavefront errors were calculated as root mean square (RMS) Zernike polynomials for a 6mm pupil and the data exported to Excel (Microsoft 918


Corp, Redmond, Wash) for further calculation of the reliability. The reliability was tested for 2nd, 3rd, and 4th order aberrations as mean values of the standard deviations for all measurements. Statistical analysis was performed using MedCalc software version 7.3.0.1 (MedCalc Software, Mariakerke, Belgium), and P⬍.05 was considered statistically significant. RESULTS The mean refraction was ⫺0.77⫾1.56 D (range: ⫹3.33 to ⫺5.28 D). Nineteen (16.5%) eyes were between ⫺5.30 and ⫺2.1 D, 93 (80.9%) eyes were between ⫺2.00 and ⫹2.00 D, and 3 (2.6%) eyes were between ⫹2.10 and ⫹3.30 D. The reliability test revealed good repeatability for both machines. The 2nd order defocus aberrations journalofrefractivesurgery.com

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TABLE

Reliability* of the Corneal and Ocular Wavefront Analyzers Wavefront

2nd Order Aberrations (µm)

Analyzer

Z(2,-2)

Z(2,0)

Z(2,2)

Z(3,-3)

3rd Order Aberrations (µm) Z(3,-1)

Z(3,1)

Z(3,3)

Z(4,-4)

4th Order Aberrations (µm) Z(4,-2)

Z(4,0)

Z(4,2)

Z(4,4)

Corneal

0.072

0.086†

0.087

0.033

0.048

0.033

0.040

0.031

0.016

0.037

0.022

0.027

Ocular

0.034†

0.102

0.047†

0.022‡

0.029‡

0.023‡

0.017‡

0.010§

0.012§

0.014§

0.015

0.013§

*Represented as mean values of standard deviations. †P⬍.0002, Wilcoxon test ‡P⬍.05, Wilcoxon test §P⬍.001, Wilcoxon test

Z(2,0) showed a better reliability when measured with the Corneal Wavefront Analyzer (P⬍.0002, Wilcoxon test). The other 2nd order aberrations were more reliable when measured with the Ocular Wavefront Analyzer. For 3rd and 4th order aberrations, the Ocular Wavefront Analyzer showed better reliability than the Corneal Wavefront Analyzer (P⬍.05 and P⬍.001, respectively, Wilcoxon test). No difference was noted between the two machines for 4th order Zernike Z(4,2) polynomial (Table). DISCUSSION Wavefront sensors are important devices for refractive surgery work-ups and calculation of customized treatments.1 Both systems tested in this study, the Corneal Wavefront Analyzer, which is used for calculation of corneal wavefront errors, and the Ocular Wavefront Analyzer, showed good reliability for the measurements performed. When tested for statistical significance, the Ocular Wavefront Analyzer had a better reliability than the Corneal Wavefront Analyzer for 3rd and 4th order aberrations. This could be related to the fact that the wavefront sensor is based on the Hartmann-Shack principle and measures a real wavefront map whereas the corneal wavefront map is created by calculating topography data with ray tracing and a standardized eye model. However, this difference in reliability seems irrelevant in clinical practice, and corneal wavefront-guided treatments with the ESIRIS excimer laser—including retreatments—show excellent refractive outcomes.2 In a study by Rodriguez et al,6 the authors found a good reliability for different wavefront measuring devices. They compared measurements of five young subjects using the Bausch & Lomb Zywave (Rochester, NY) Hartmann-Shack wavefront sensor, the laser ray tracing Tracey (Tracey Technologies, Houston, Tex), and their own laboratory laser ray tracing prototype. They found a good reliability but also emphasized the importance of good pupil alignment and dependence Journal of Refractive Surgery Volume 22 November 2006


of accurate measurements on noise and other sources. In a study by Cheng et al,7 the test-retest reliability over time of Hartmann-Shack measurements was evaluated. The authors measured four normal eyes on five consecutive days and on five days on a monthly basis. The variability over a short time frame was low, but increased over time. The reliability and good visual outcomes with different corneal topographers have also been reported.810 Chui and Cho8 reported a less reliable performance of the Keratron Scout topographer (Optikon, Rome, Italy) compared to the Humphrey Atlas 991 (Zeiss, Jena, Germany) and Medmont E300 (Medmont International Pty Ltd, Victoria, Australia). However, their study was conducted in children whereas our analysis included volunteers with a mean age of 23 years. In general, it seems important for the examiner to be well trained for the device used and its specifications. The current study showed a good repeatability and reliability in consecutive measurements of healthy young volunteers for corneal wavefront as well as total wavefront measurements performed with the SCHWIND Corneal and Ocular Wavefront Analyzers, respectively. REFERENCES

1. Mrochen M, Jankov M, Bueeler M, Seiler T. Correlation between corneal and total wavefront aberrations in myopic eyes. J Refract Surg. 2003;19:104-112. 2. Kramann C, Tehrani M, Dick BH. Laser in situ keratomileusis with a flying-spot excimer laser and a Carriazo-Barraquer microkeratome—outcomes after 6 months [German]. Klin Monatsbl Augenheilkd. 2004;221:35-39. 3. Canals M, Elies D, Costa-Vila J, Coret A. Comparative study of ablation profiles of six different excimer lasers. J Refract Surg. 2004;20:106-109. 4. Mester U, Dillinger P, Anterist N. Impact of a modified optic design on visual function: clinical comparative study. J Cataract Refract Surg. 2003;29:652-660. 5. Rozema JJ, Van Dyck DE, Tassignon MJ. Clinical comparison of 6 aberrometers, part 1: technical specifications. J Cataract Refract Surg. 2005;31:1114-1127. 6. Rodriguez P, Navarro R, Gonzalez L, Hernandez JL. Accuracy

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and reproducibility of Zywave, Tracey, and experimental aberrometers. J Refract Surg. 2004;20:810-817. 7. Cheng X, Himebaugh NL, Kollbaum PS, Thibos LN, Bradley A. Test-retest reliability of clinical Shack-Hartmann measurements. Invest Ophthalmol Vis Sci. 2004;45:351-360. 8. Chui WS, Cho P. A comparative study of the performance of different corneal topographers on children with respect to orthokeratology practice. Optom Vis Sci. 2005;82:420-427.

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9. Guell JL, Velasco F, Roberts C, Sisquella MT, Mahmoud A. Corneal flap thickness and topography changes induced by flap creation during laser in situ keratomileusis. J Cataract Refract Surg. 2005;31:115-119. 10. Rabsilber TM, Becker KA, Auffarth GU. Reliability of Orbscan II measurements in relation to refractive status. J Cataract Refract Surg. 2005;31:1607-1613.

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