Prolate Ablations

The original corneal shape is prolate, i.e. a dome like the side of a ball. When you re-shape a cornea for myopia, you create a bucket or divot like shape in the cornea, causing it to become oblate. This creates a completely different light entrance pattern at the periphery, causing night glare/halos and contrast sensitivity loss. Even if a wavefront laser pattern perfectly smooths the center cornea, the oblate overall shape of the ablation can still lead to contrast sensitivity losses and night glare/halo issues. A prolate ablation much more closely resembles the dome like shape of the natural cornea, especially in the area of pupil dilation, and therefore acts much like the normal eye with the additional advantage of corneal smoothening by the excimer laser. That is what accounts for the high 20/15 rate of this procedure. Further advantages are less central tissue removal, and excellent overall regression rates.

Source: Dr. Motwani San Diego Lasik Surgeon