The theoretical effect of measured wavefront diameter on estimating peripheral wavefront data

To compare the difference between wavefront data measured in the periphery and that calculated from estimations using a smaller measured region.

A total of 32 eyes were measured using the Alcon LADARWave aberrometer (Alcon Laboratories Inc, Fort Worth, Tex). Aberrations were measured through a dilated pupil to the 6th order at a 6.5-mm diameter. Data were then "clipped" to 5.0, 5.5, and 6.0 mm, and each of these smaller diameter wavefronts was used to estimate the larger diameter wavefront coefficients. The measure of interest was the mean and range of error between the estimated and measured values, and how they varied for the different diameters of known data.

The differences in both the mean and range of error were inversely correlated to the diameter of the region of known data. As this region grew smaller, the region estimated grew larger, with resultant larger mean errors. At 5 mm, the estimated zone was an annulus of 0.75-mm width; in this case the estimated higher order aberrations were 60% higher than those measured, with a range of 40% lower to 300% higher. At 5.5 mm and 6.0 mm, these differences were much smaller, but statistically significant. The differences were greatest when significant peripheral aberrations were measured, as can occur with post-laser in situ keratomileusis eyes.

Basing a treatment on an estimation of the peripheral wavefront may lead to an inaccurate ablation profile. The inaccuracy increases as the size of the measured wavefront decreases relative to the planned ablation zone. The inaccuracy is also greatest when significant peripheral aberrations are present. Refractive surgery systems should only treat optical zones that are smaller than or equal to the diameter of the measured wavefront.