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J.C. Rabin, S. Linnemeyer, M. Foxworth, D. Ivan; A New Metric for Quantifying the Impact of Higher Order Aberrations . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1189.
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© ARVO (1962-2015); The Authors (2016-present)
Wavefront–guided correction of optical aberrations beyond sphere and cylinder can enhance vision in normal and aberrated eyes. While numerous metrics of wavefront error exist, a single index predictive of visual performance is desirable for clinical application. We describe a new metric which quantifies aberration as a ratio relative to normal values. The aberration ratio predicts performance in aberrated eyes.
Retrospective analysis of visually normal eyes (n=133) was conducted to quantify the relation between higher order aberrations (HOA; RMS in microns; WaveScan WavefrontTM System) and pupil size (wavefront diameter; WFD in mm). Log MAR visual acuity (VA) and contrast sensitivity (Cambridge Research Systems, Ltd. sine–wave grating and PrecisionVision® letter CS) were evaluated as a function of HOA in visually normal patients, keratoconus and PRK.
In normal eyes HOA increases with the square of WFD: HOA = 0.006 x WFD2.1 (r2=0.47, p<0.0001). This equation calculates the expected HOA for a specific WFD, such that a measured HOA can be expressed relative to the expected value as an aberration ratio: measured HOA/expected HOA. In keratoconus the log of the aberration ratio (LAR) is predictive of visual performance accounting for 40% of the variability in high spatial frequency CS for gratings (12 cycles/deg; r2=0.4, p<0.0001) and letter stimuli (20/50 letters; r2=0.35, p<0.0001); and 20% of the variability in VA (r2=0.21, p<0.001). Using data from normal and aberrated eyes, a linear model was derived to predict CS from LAR: Log CS = 1.7 – 0.6(LAR). The model suggests that CS changes by a factor of 2X for each 3X change in HOA.
The LAR quantifies optical aberrations as a ratio relative to normal values. It is predictive of performance in aberrated eyes, and can be used to estimate the impact of HOA on contrast perception.
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