Forum»Higher-order aberrations
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Administrator   Firstname: Marc Gender:  Joined: 28 Aug 2004 Posts: 39598 Chats: 11125 Location: Hennef  | Higher-order aberrations are visual problems not captured in a traditional eye exam which tests only 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 correctable 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:[2] 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 a significant issue for only 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. However, higher order aberrations are measured in µm (micrometers) as indicated on the wavescan taken during the pre-op examination, but the smallest beam size of all FDA approved lasers is 0.65 mm(millimeters). The lasers are supposed to remove imperfections that are measured 1000 times smaller than its smallest beam. This is one of the reasons patients experience halo, glare, and starburst even with a small naturally dilated pupils in dim lighting. |
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