Aberrations of the eye

Aberration of normal eyes

In the normal population, the dominant aberrations are second-order spherocylindrical focus errors, known as refractive errors. Higher-order aberrations constitute a relatively small component, roughly 10% of the total. These higher-order aberrations increase with age, and mirror symmetry exists between right and left eyes.

Several studies report compensation of corneal aberrations by the crystalline lens: corneal spherical aberration is usually positive, while the young lens exhibits negative spherical aberration. Evidence also shows compensation for astigmatism and horizontal coma. The accommodative response alters lens shape and wavefront pattern; most eyes show positive spherical aberration when unaccommodated, trending toward negative spherical aberration with accommodation.

High order aberrations

Among numerous higher-order aberrations, only spherical aberration, coma, and trefoil are of clinical interest. Spherical aberration, a fourth-order aberration, produces halos around point images and exacerbates myopia in low light (night myopia); it is commonly increased after myopic LASIK and surface ablation, and its effect rises with the fourth power of pupil diameter. Coma, a third-order aberration, is common in patients with decentred corneal grafts, keratoconus, and decentred laser ablations. Trefoil, also third-order, causes less image degradation than coma of similar RMS magnitude.

Assessment and quantitative expression of ocular aberrations

Many techniques measure ocular aberrations, the most common being Shack-Hartmann aberrometry; other methods include Tscherning systems, ray tracing, and skiascopy. Quantitative comparisons use root mean square (RMS), calculated by squaring the difference between the aberration and mean value and averaging across the pupil area. Different aberrations with equal RMS may affect vision differently, so RMS error is unrelated to visual performance. The majority of eyes have total RMS values less than 0.3 μm.

Aberration maps are commonly classified as sums of Zernike polynomials, a set of basis functions. Each aberration may be positive or negative and induces predictable image alterations. Vision scientists use the first 15 Zernike polynomials, sufficient to describe common aberrations; the most important coefficients affecting visual quality are coma, spherical aberration, and trefoil. Zernike polynomials are expressed in polar coordinates (ρ,θ); they are independent of one another, with a mean value of zero across the pupil, and each coefficient gives the RMS error for that mode.

However, coefficients are valid only for the specific pupil diameter measured. The subscript n denotes the order of aberration; superscript m is the angular frequency, indicating how many times the wavefront pattern repeats.

Management

Low-order aberrations (hyperopia, myopia, regular astigmatism) are correctable by eyeglasses, soft contact lenses, and refractive surgery. Neither spectacles, soft contact lenses, nor routine keratorefractive surgery adequately corrects higher-order aberrations; significant higher-order aberration usually requires a rigid gas-permeable contact lens for optimal visual rehabilitation. Customized wavefront-guided corneal laser treatments aim to reduce existing aberrations and prevent new ones, but actual results with wavefront-guided LASIK show that it cannot remove higher-order aberrations and may increase them, though less than conventional LASIK.

Larger ablation zones with transition zones produce less pronounced aberrations. Aspherical intraocular lenses (IOLs) compensate for positive corneal spherical aberrations; they may give better contrast sensitivity, but their effect on distance visual acuity is doubtful. Conventional IOLs provide better depth of focus and near vision, linked to residual spherical aberration.

Did You Know?

• A wavefront is a surface over which an optical disturbance has a constant phase; rays and wavefronts are complementary approaches to light propagation.
• For light to converge to a perfect point, the emerging wavefront must be a perfect sphere centered on the image point.