Higher-order aberrations

Thursday, December 17, 2009

Higher-order aberrations - Lasik Eye Surgery

Higher-order aberrations are visual problems that cannot be diagnosed using a traditional eye exam, which tests only for acuteness of vision. Severe aberrations can cause significant vision impairment. These aberrations include starbursts, ghosting, halos, double vision, and a number of other post-operative complications.

There have always been concerns about LASIK because of its tendency to induce higher-order aberrations. The advancement of the LASIK technology has helped reduce the risk of clinically significant visual impairment after surgery. There is a correlation between pupil size and aberrations. Effectively, the larger the pupil size, the greater the risk of aberrations. This correlation is the result of the irregularity between the untouched part of the cornea and the reshaped part. Daytime post-lasik vision is optimal, since the pupil is smaller than the LASIK flap. But at night, the pupil may expand such that light passes through the edge of the LASIK flap into the pupil which gives rise to many aberrations, including the appearance of halos surrounding sources of light. There are other currently unknown factors in addition to pupil size that also may lead to higher order aberrations.

In extreme cases in which ideal procedures were not followed by ophthalmologists, and before key advances, some people could suffer debilitating symptoms such as serious loss of contrast sensitivity in poor lighting situations.

Over time, most attention has shifted from other aberrations and centered on spherical aberration. LASIK and PRK tend to induce spherical aberration, because of the tendency of the laser to undercorrect as it moves outward from the center of the treatment zone. This is primarily an issue for major corrections. There are theories that posit that if the lasers were simply programmed to adjust for this tendency, no significant spherical aberration would occur. In eyes with few higher order aberrations, wavefront-optimized LASIK (rather than wavefront-guided LASIK) may well be the future.

Higher order aberrations are measured in micrometers (µm) on the wavescan taken during the pre-op examination, while the smallest beam size of lasers approved by the U.S. Food and Drug Administration lasers is about 1000 times larger, at 0.65 mm. Thus imperfections are inherent in the procedure and a reason why patients experience halo, glare, and starburst, even with small naturally-dilated pupils in dim lighting.

Source: Lasik eye surgery at wikipedia.org