Higher-order aberrations

Higher-order aberrations are visual problems not captured in a traditional eye exam which only tests for acuteness of vision. Severe aberrations can effectively cause significant vision impairment. These aberrations include starbursts, ghosting, halos, double vision, and a number of other post-operative complications listed below.

Concern has long plagued the tendency of refractive surgeries to induce higher-order aberration not correctible by traditional contacts or glasses. The advancement of LASIK technique and technologies has helped reduce the risk of clinically significant visual impairment after the surgery. One of the major discoveries was the 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. There are other currently unknown factors in addition to pupil size that also affect higher order aberrations.

In extreme cases, where ideal technique was not followed and before key advances, some people could suffer rather debilitating symptoms including serious loss of contrast sensitivity in poor lighting situations.

Over time, most of the attention has been focused 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 really only a significant issue for large corrections. There is some thought if the lasers were simply programmed to adjust for this tendency, no significant spherical aberration would be induced. Hence, in eyes with little existing higher order aberrations, "wavefront optimized" lasik rather than wavefront guided LASIK may well be the future. Regardless, most patients with even the low to medium corrections remain highly satisfied even with conventional LASIK, however patients requiring higher corrections often complain about night vision.

Wavefront-guided LASIK

Wavefront-guided LASIK is a variation of LASIK surgery where, rather than apply a simple correction of focusing power to the cornea (as in traditional LASIK), an ophthalmologist applies a spatially varying correction, using a computer-controlled high-power UV laser guided by measurements from a wavefront sensor. The goal is to achieve a more optically perfect eye, though the final result still depends on the physician's success at predicting changes which occur during healing. In older patients though, scattering from microscopic particles plays a major role and may exceed any benefit from wavefront correction. Hence, patients expecting so-called "super vision" from such procedures may be disappointed. However, while unproven, surgeons claim patients are generally more satisfied with this technique than with previous methods, particularly regarding lowered incidence of "halos", the visual artifact caused by spherical aberration induced in the eye by earlier methods.

Suorce: Wikipedia.org , the free encyclopedia