Aberrations of the eye

The appearance of visual complaints such as halos, glare and monocular diplopia after corneal refractive surgery has long been correlated with the induction of optical aberrations.

Several mechanisms may explain the increase in the amount of higher-order aberrations with conventional excimer laser refractive procedures: a change in corneal shape toward oblateness or prolateness (after myopic and hyperopic ablations respectively), insufficient optical zone size and imperfect centration.

[3] High order aberrations increase with age and mirror symmetry exists between the right and the left eyes.

Besides, there is strong evidence of compensation for aberrations between the cornea and intraocular optics in cases of astigmatism (horizontal/vertical) and horizontal coma.

[5] The accommodative response of the eye results in changes to the lens shape and substantially affects the wavefront aberration pattern.

Spherical aberration is the cause of night myopia and is commonly increased after myopic LASIK and surface ablation.

In brighter conditions, the pupil constricts, blocking the more peripheral rays and minimizing the effect of spherical aberration.

As the pupil enlarges, more peripheral rays enter the eye and the focus shifts anteriorly, making the patient slightly more myopic in low-light conditions.

Trefoil (a third-order aberration) produces less degradation in image quality compared with coma of similar RMS magnitude.

The superscript m is called the angular frequency and denotes the number of times the Wavefront pattern repeats itself.

List of Zernike modes and their common names:[11]Low order aberrations (hyperopia, Myopia and regular astigmatism), are correctable by eyeglasses, soft contact lenses and refractive surgery.

Neither spectacles nor soft contact lenses nor routine keratorefractive surgery adequately corrects high order aberrations.

Significant high order aberration usually requires a rigid gas-permeable contact lens for optimal visual rehabilitation.

[6] The wavefront map of the eye may be transferred to a Lasik system and enable the surgeon to treat the aberration.

Perfect alignment of the treatment and the pupil on which the Wavefront is measured is required, which is usually achieved through iris feature detection.

[2] Actual results with Wavefront guided LASIK showed that not only it cannot remove HOA but also the optical aberrations are increased.

[13] An upcoming systematic review will seek to compare the safety and effectiveness of wavefront excimer laser refractive surgery with conventional excimer laser refractive surgery, and will measure differences in residual higher order aberrations between the two procedures.

Although Aspherical IOLs may give better contrast sensitivity, it is doubtful, whether they have a beneficial effect on distance visual acuity.

The small improvement in depth of focus with the conventional IOLs enhances uncorrected near vision and contribute to reading ability.