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Test–Retest Reliability of Clinical Shack-Hartmann Measurements

From the School of Optometry, Indiana University, Bloomington, Indiana.

PURPOSE. To evaluate the stability of clinical monochromatic aberrometry measurements over a wide range of time scales.

METHODS. Monochromatic aberrations in four normal eyes were measured with a clinical Shack-Hartmann aberrometer. A chin rest or a supplemental bite bar attachment was used to stabilize head and eye position. Five repeated measurements were taken within one test (5 frames, t < 1 second) without realignment. With realignment between each measurement, aberration measurements were repeated five times (t < 1 hour) on each day, at the same time of day on five consecutive days, and again on 5 days at monthly intervals. A control experiment studied the effect of systematically misaligning the eye to determine whether fixation errors can account for the variation in the repeated measurements.

RESULTS. Variability of wavefront root mean square (RMS) error (excluding defocus and astigmatism) was tracked across repeated measurements. Variances for different time scales were: 8.10 x 10^-5 µm² (t < 1 second), 3.24 x 10^-4 µm² (t < 1 hour), 4.41 x 10^-4 µm² (t < 1 week), 9.73 x 10^-4 µm² (t < 1 year). Bite bar and chin rest data were almost identical. Rotational fixation error up to 3° accounts for only part of the variability.

CONCLUSIONS. Increased variability in aberration maps between days and months indicates biological fluctuations that are large enough to prevent achievement of "perfect vision," even in the unlikely event that spherical and astigmatic refractive errors are corrected perfectly. However, lack of stability does not justify withholding treatment. A lasting benefit of aberration correction is expected despite temporal variability.

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