Overview
To the refractive surgery patient in year 2000, achieving an uncorrected visual acuity of 20/20 after refractive surgery was considered a success. Ongoing research in this field is focused on further improving these results. Realizing that 20/20 does not represent perfect vision is important because many young healthy adults have visual acuities of 20/15 to 20/12. If optical aberrations in the eye could be eliminated, the theoretical limit of foveal acuity would be 20/12 for a small pupil and up to 20/5 for a dilated pupil.
Using conventional laser ablation profile, patients with high corrections frequently complained about night vision problems and sometimes monocular diplopia after refractive surgery. Some patients reported a decrease in functional vision after laser-assisted in situ keratomileusis (LASIK) when the target contrast decreased or when light intensity decreased.
Image degradation occurs after conventional laser treatment on the cornea because optical aberrations are induced after conventional refractive surgery, especially at night when the pupil is dilated. Optical aberrations are increased because the normal cornea is prolate in shape (steeper in the center), but it becomes oblate (steeper in the periphery) after successful conventional laser ablation profile.
Future goals in LASIK surgery are 3-fold: (1) to create postoperative eyes with higher quality 20/20 vision (ie, less glare, less halo) at night, (2) to create eyes with vision exceeding 20/20 (ie, super vision), and (3) LASIK may be used to correct refractive errors and aberrations in eyes that had previous less-than-optimal refractive procedures (ie, radial keratotomy [RK], astigmatic keratotomy [AK], automated lamellar keratoplasty [ALK], photorefractive keratectomy [PRK], LASIK).
If corneal aberrations and irregularities could be reduced after LASIK, then halo, glare, and monocular diplopia also should be reduced.
Properties of light
Light possesses properties of a particle and a wave. If light is considered as waves, then light waves travel like ocean waves in one direction. Wavefront is a term describing the surface connecting the points on a propagating wave that are of equal phase. A point source in produces a spherical wavefront, as shown in the image below. When it passes from the tear film on the cornea to the retina, they are bent and distorted by all the structures in the eye. The optical aberration produced by each individual’s eye is as distinct as each individual’s fingerprints.
A point source of light is represented by the red dot. Spherical waves are traveling from left to right, as indicated by the black arrow. Blue lines indicate the surfaces of the wave that are of equal phase. After the wavefronts enter the eye, they are distorted.
Limitation of current process
Currently, the optical characteristic of the eye is described with only 3 numbers when measured with a phoropter, as follows: sphere, cylinder, and axis. The following explain why describing the optical characteristic of the eye with 3 numbers is too generic and too simple.
Etiology
Optical aberrations are classified into monochromatic and chromatic types. Monochromatic aberrations can be subdivided further into spherical refractive error, cylindrical refractive error, spherical aberration, coma, and higher order aberrations. Since biological structures in the eye contain imperfections, these aberrations are not accounted for by spherical and cylindrical refractive error.
As an example, consider spherical aberration. It causes light rays at the edge of a convex lens to be focused in front of the focus of the central rays. Spherical aberration increases as the fourth power of the pupil size; therefore, visual acuity decreases in dark conditions (see the image below.) The refractive power across the eye is inhomogeneous. It varies with spatial location. Assuming that the center of the visual axis has a spherical refraction of 4.0, the middle ring has a spherical refraction of 5.5, and the outer ring has a spherical refraction of 6.0.
This diagram illustrates spherical aberration. The refractive power of the eye varies with spatial location. In this example, the power at the visual axis is 4 diopters. Under bright light conditions, the constricted pupil is indicated by the dotted circle. Under dim light conditions, the pupil is indicated by the dashed circle; the power is the weighted average of 4 and 5.5. Under dark conditions, the pupil is fully dilated and represented by the solid red circle; the power of the eye is the weighted average of 4, 5.5, and 6.
Current manifest or cycloplegic refraction measures a weighted average of these numbers across the different spatial locations and, therefore, is incomplete. This accounts for the phenomenon termed night myopia. The refraction of the eye becomes more myopic under dark conditions. Programming the excimer laser using only 3 numbers (ie, sphere, cylinder, axis) to treat refractive error is incomplete and too generic. Custom ablation can reduce the other optical aberrations and create a more perfect optical system.