Evaluation of refractive error measurements obtained by three different aberrometers Radha Ram, BA...

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.