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M.P. Weikert, L. Wang, D.D. Koch; Calculated Manifest Refraction as a Function of Pupil Size: Evaluation of the VISX® Wavescan WavefrontTM System . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2599.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To evaluate the accuracy of the calculated refraction produced by the VISX Wavescan Wavefront system, a Hartmann-Shack aberrometer, and to determine its dependence on pupil size. Methods: Pre- and post-LASIK wavefront data were collected from the Baylor College of Medicine VISX® Wavescan WavefrontTM system database from January 2000 to May 2002 (n=333). The clinical manifest refractions (MR) and the refractions calculated from the wavefront data (WR) for pupil sizes of 3, 4, 5, and 6 mm were recorded for the pre-operative visit, as well as the 1-week (n=144), 1-month (n=79), and 3-month (n=68) post-operative visits. For each time period, the subjective MR was compared to the calculated WR with respect to spherical equivalent, vector cylinder magnitude, and vector cylinder axis, using double angle analyses. For each comparison, the parameter of the calculated WR was subtracted from that of the clinical MR. Results: The magnitude of the mean difference in spherical equivalent (ΔSE) between the subjective MR and the calculated WR was less than 0.25D (standard deviation 0.50D) for all pupil sizes and time periods, both pre- and post-operatively. At 1 week and 1 month, there was no statistically significant difference between the pre- and post-LASIK mean MR and WR ΔSE's (p>0.16). At 3 months, the mean WR ΔSE was approximately 0.20D greater than the mean MR ΔSE for all pupil sizes (p<0.05). The mean ΔSE decreased with decreasing pupil size for all time periods, indicating a hyperopic shift of the wavefront refraction, possibly due to the loss of spherical aberration with smaller pupils. The mean difference in vector cylinder magnitude (Δ|Cyl|) was less than 0.12D for all pupil sizes and time periods. Though the pre- and post-LASIK mean Δ|Cyl| remained statistically equal (p>0.1), the standard deviation increased with decreasing pupil size for all time periods. This change was gradual from 6 to 4 mm (range 0.34 to 0.78D), but increased dramatically with 3-mm pupils (range 0.76 to 1.46D). Conclusions: The mean spherical and cylindrical refractive data obtained with the VISX Wavescan correlated well with the clinical manifest refraction for all pupil sizes and post-LASIK time periods. However, the standard deviations for the difference between the MR and WR spherical and cylindrical data were significantly large. This variability increased with decreasing pupil size. Further work is required to correlate the differences in sphere and cylinder with the magnitude and distribution of higher order aberrations, since the MR encompasses all aberrations and WR only considers defocus and astigmatism.
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