Monday, January 17, 2011

Wavefront-guided and topography-guided customized LASIK: What do they mean and what's the big deal?

In recent years, the word 'customized' has become ubiquitous in LASIK treatments. If you're not doing customised treatments, if you're only doing standard treatments, you're out. But aren't all treatments customised, since they're customised according to the refractive error you have? Let me explain.

There are 2 things that so called customised treatments aim to control for, that so called standard treatments do not. One is ocular 'aberrations'. Most of us know that vision is affected by short or long sightedness, or astigmatism. But our vision is also affected by other irregularities of the cornea and lens, called aberrations. The chief ones which cause problems are 'coma' and 'spherical aberration'. When present in large degrees, they can cause glare and halos especially when the pupil is large at night. So these 'customised' treatments aim to treat the particular aberrations of the individual eye.

Now, when most people talk about 'wavefront-guided', they are talking about 'whole eye' wavefront. But whole eye wavefront is affected by a few different things. It is affected by pupil diameter, it is affected by the state of focus of the eye (accomodation), and it constantly changes as the eye ages, primarily due to changes in the shape of the lens. Furthermore, most measuring systems for whole eye wavefront only measure several hundred points at most through the pupil.

Topography-guided LASIK, or more precisely 'corneal wavefront guided' LASIK, is LASIK based only on measurements of the cornea.This is advantageous, because the cornea does not change as much in shape throughout life-definitely less so than the lens. It does not change in response to pupil shape, nor does it change in response  to accommodation. Topographers measure corneal shape right out to the periphery, and typically measure in excess of 20,000 points. In most cases, corneal aberrations account for a majority of whole eye aberrations. It would thus appear that topography or corneal wavefront guided LASIK is the way to go.

Customised treatments also take your corneal curvature into account when delivering the treatment. Why is this important? Well, imagine yourself standing in the sun at noon, at the equator. Your shadow would be tiny, directly under yourself. If you were standing in London in winter at noon, your shadow would be much longer, because the sun's rays are coming at an angle. In a similar way, the laser's rays strike the cornea at an angle at the periphery of the cornea. If a cornea is of relatively standard shape, that's not a problem, because the laser has been programmed to treat relatively 'standard' eyes. However, if your cornea is more curved than normal, ie steeper at the sides, then the laser would strike your corneal periphery at a larger angle than normal and work less effectively. You could end up with a smaller optical zone than expected. You could end up with lots of night glare and halos.

So, go for customized LASIK if you can. Better still, go for Topography Guided/Corneal Wavefront Guided LASIK for best visual quality.

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