Stability of clinical aberrometry measurements

Abstract

Purpose: To evaluate the stability of clinical monochromatic aberrometry measurements over a wide range of time-scales. Method: Two experienced operators measured monochromatic aberrations in eight normal eyes using a clinical Shack-Hartmann aberrometer (COAS by Wavefront Sciences, Inc.). A fixation target and a standard chin-rest, or a supplemental bite-bar attachment, were used to stabilize head and eye position and rotation. Five repeated measurements were taken within one test (5 frames, t< 1 secon d). After re-alignment each test was repeated five times (t< 20 minutes). These tests were repeated at the same time on five consecutive days. Also, subjects were tested at different times throughout a single day. Result: We compared variability of wave front RMS error (excluding defocus and astigmatism) across repeated measurements. Standard deviations for different time scales were: 0.009 mm (t<1sec), 0.015 mm (t<20min), 0.025 mm (t<1 day), 0.021 mm (t<1 wk). These within-eye variability estimates are small compared to the between-eye variability of our subjects (SD=0.045 mm) and the known population range of 0.52 mm. The fact that RMS SDs across the full day were larger (0.025 mm) than those for multiple trials at the same time on five different da ys (0.021 mm) may indicate a possible small diurnal fluctuation. Bite-bar and chin rest data were almost identical. Conclusion: Although aberrometry measurements on individual eyes are relatively stable when compared to between eye differences, MTF analysis shows that perfect wavefront corrections based upon a single aberrometry measure made at one time, may provide less successful outcomes at other times of the day or on other days.