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Evaluation of refractive error measurements obtained
by three different aberrometers
Radha Ram, BALi Wang, MD, PhDMitchell P. Weikert, MD, MS
Disclosure: No authors have any financial interest in this subject.
Introduction
• Accurate refractive error measurements are a crucial part of the ophthalmologic exam.
• Refractive error measurements not only influence proper prescriptive lenses, but they also serve an important role in preparing for refractive surgery and calculating IOL power in patients undergoing cataract extraction.
• Refractive eye surgery along with technology measuring refractive error have seen rapid advancements in recent years.
• The purpose of this study was to compare refractive error measurements obtained with three different aberrometers in eye screenings for refractive surgery.
Methods
• Used subjective manifest refraction (MR) as the gold standard
• Measured spherical equivalent, sphere, and cylinder
• Employed 3 aberrometers:1. Hartmann-Shack2. Ray tracings3. Dynamic skiascopy
• Analyzed 79 normal eyes Inclusion criteria
No history of refractive or other surgeries
Best spectacle-corrected visual acuity (BSCVA) of 20/25 or better
Average age: 41 years (Range 18 - 76 years)
AMO WaveScan WaveFront systemHartmann-Shack
Tracey iTrace 4.2 Wavefront AberrometerRay tracings
Nidek OPD-Scan IIDynamic skiascopy
Instruments
Methods
• Bland-Altman plots Plots graphed the difference between 2 measurements against their mean.
95% limits of agreement (LoA) were calculated as the mean difference +/- 1.96 standard deviation of the differences (SD). Indicating 95% of the differences will lie between the 95% LoA
• Vector Analysis for Astigmatism Double-angle plots graphed differences between astigmatism from MR and 3 aberrometers.
Results
Bland-Altman plots of differences between MR SE and aberrometer SE vs. mean of MR SE and aberrometer SE
Vector differences between astigmatism measured by MR and aberrometers
MR – Hartmann-ShackMean difference: 0.10 D @ 114o +/- 0.48 D
MR – Ray tracingsMean difference: 0.15 D @ 1270 +/- 0.61 D
MR – Dynamic skiascopy Mean difference: 0.03 D @ 1390 +/- 0.48 D
Percentages of eyes within certain errors of the MR values
Cylinder
77%
97% 97%
70%
91%98%
83%
95%99%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
+/-0.5 +/- 1.0 +/-1.5
Difference from MR cylinder (D)
Total percentage of eyes
Sphere
69%
90%96%
65%
86%92%
78%
92%96%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
+/-0.5 +/- 1.0 +/-1.5
Difference from MR sphere (D)
Total percentage of eyes
Spherical Equivalent
62%
92%96%
62%
83%
95%
77%
91%96%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
+/-0.5 +/- 1.0 +/-1.5
Difference from MR SE (D)
Total percentage of eyes
Conclusions
• These results suggest that in normal eyes, the Hartmann-Shack, multiple ray tracings, and dynamic skiascopy technologies produce globally similar results within common refractions but may vary in some details.
• Overall, there was excellent agreement between the measurements of the 3 devices, as demonstrated by their mean differences, 95% LA values, and aggregate analysis of astigmatic values.
• Since the MR was based on the dynamic skiascopy refraction, the dynamic skiascopy may have greater agreement with MR. However, dynamic skiascopy also correlated well with the Hartmann-Shack aberrometer.
• With regard to vector analysis, the ray tracings instrument had the greatest standard deviation when compared to the other two technologies.
• These findings can help clinicians better interpret measurements of healthy, phakic eyes based on their available aberrometer.
Conclusions
• Limitations Relatively small number of patients MR was based on dynamic skiascopy refraction Although reproducibility has been documented in other
studies, the reproducibility was not analyzed in this study
• Future Directions Further work is required to improve accuracy for
measuring SE, sphere, and cylinder in post-operative, aphakic, and pseudophakic eyes.
Further studies are required to compare higher-order aberration measurements.
References
Bland JM and DG Altman. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1:307-310.
Kim DS, J Navaez, J Krassin, and K Bahjri. Comparison of the Visx WaveScan and Nidek OPD-Scan Aberrometers. Journal of Refractive Surgery 2009; 25:429-434.
Lawless MA and C Hodge. Wavefront’s role in corneal refractive surgery. Clin Experiment Ophthalmol. 2005;31:1114-1127.
Rozema JJ, DEM Van Dyck, and MJ Tassignon. Clinical comparison of 6 aberrometers Part 2: Statistical comparison in a test group. J Cataract Refract Surg 2006; 32:33-44.
Wang L, M Misra, IG Palikaris, and DD Koch. A comparison of a ray-tracing refractometer, autorefractor, and computerized videokeratography in measuring pseudophakic eyes. J Cataract Refract Surg 2002; 28:276-282.
Wang L, N Wang, and DD Koch. Evaluation of refractive error measurements of the WaveScan WaveFront system and the Tracey wavefront aberrometer. J Cataract Refract Surg 2003; 29:970-979.
