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For most patients a "normal " laser ablation will be quite sufficient. This corrects the "sphere" (short or longsight ) and the "cylinder" (any regular astigmatism). Spectacles also correct sphere and cylinder. However, ordinary soft contact lenses only correct the sphere. Hence if you are, for example, -4.0 / -0.5 x 180, this means that you are -4 in the vertical axis and -4.5 in the horizontal axis. Spectacles will correct both, but a soft contact lens will be set at "half-way" between the 2 extremes i.e. in this case the contact lens would be -4.25. With small astigmatism this is not noticed by the patient, but with larger astigmatism then you need toric soft contact lenses or hard gas permeable lenses. Hence the normal excimer laser, whether PRK or LASIK. corrects the eye in the same way as spectacles. However, perhaps more important than minor optical aberrations is to keep the cornea prolate to reduce spherical aberration. This is explained in detail on the following page, also reached from the blue link button above. In a number of lasers the ablation algorithms have been altered to have the option of keeping the cornea more prolate. This is true with the new Technolas 217 laser, the Allegretto, the Mel-80 (see nice article re prolate ablation and the Mel 80 here) and the Nidek EC-5000. I am using the Nidek laser which has the "OATZ" option (Optimised Aspheric Transition Zone ) or the "CATZ" option (Customised Aspheric Transition Zone). OATZ is for regular corneas and the CATZ is for irregular one (often for re-treatments in problem eyes)
Wavefront technology This is advertised by the various manufacturers as "SuperVision" or some similar phrase. It means that there is an analysis of the "higher order" optical errors that are not corrected by just lasering the sphere and cylinder. It is certainly a useful tool but the following should be remembered: 1. In most eyes this is very "small" print. Glasses do not correct higher order aberrations although gas permeable contact lenses do. 2. It is most applicable to unusual or problem eyes with irregularly shaped corneas. e.g. mild Keratoconus, irregular astigmatism following previous surgery complications etc. The other major reason to use it is to reduce the aberrations in low light after surgery to pre-operative levels or better. 3. It takes off more tissue so is most applicable to lower myopes. It is possibly best used with PRK rather than LASIK because the cutting of the flap will itself induce some random aberrations. no matter how perfect the surgery. The "bow-string" tension of the corneal collagen fibres is released, with the cornea flattening centrally and steepening peripherally. Hence any pre-surgical detailed analysis is intrinsically flawed. Any custom ablation will only be taking off a few microns more in various places on the cornea, so mechanical flap changes, which can be a magnitude higher at least, can swamp any proposed customised lasering, however much the laser manufacturers would wish otherwise. One suggestion with lasik is to cut the flap, analyse after one week and then do the laser ablation. I don't think too much of this suggestion. This is why surface ablation (PRK/LASEK) may be better for such ablations, as there is no flap to worry about, However, even here, we are dealing with a biological system rather than a piece of plastic and there can still be changes as the cornea heals. In particular the very active epithelium can change the post-operative state a lot. 4. Prof. Iohannis Pallikaris, the inventor of LASIK, has posed the question "Do we really want SuperVision anyway?". We have evolved to have a certain level of acuity and this gives us a bigger depth of field. Higher order aberrations change with accommodation and what is correct for distance is not correct for near. It may be very tiring to have to keep accommodating ones eyes all the time. 5. Studies on US air force pilots at the "Topgun" school who have "supervision" of 6/4 have shown that they tend to have positive coma, one of the higher order aberrations that wavefront can eliminate. Hence more studies need to be done to determine exactly what we want these wavefront lasers to do Some eyes have "irregular astigmatism" that is not properly corrected by a regular laser. This is probably no more than 5-10% of the population. Regular astigmatism is seen in the following example where the eye is steeper in the vertical axis than the horizontal axis. This is called "with the rule" astigmatism as it is present in most people.. The cornea is shaped like the back of a spoon h held horizontally:
Corneal topography machines make a picture of such an eye with steeper parts having "hotter" colours than flatter parts. A corneal topography picture of this spoon would look like the following:
Regular Astigmatism (click to enlarge )
Such an eye can be easily lasered with a standard PRK or LASIK. This would change the cornea from a desert spoon shape into a flatter soup spoon for the correction of myopia and into a steeper soup spoon for the correction of hyperopia. However, an eye with mild keratoconus is like such a spoon tilted downwards and this is a type of Irregular Astigmatism, as seen in the following topography picture:
Irregular Astigmatism (click to enlarge)
This is the sort of cornea that would need to be done in a customised way as the astigmatism is asymmetrical. There are two elements to analysing such an eye: a) The shape of the cornea i.e. the corneal topography b) The passage of light through the system i.e. wavefront analysis. This measures the total optical performance of the eye system - cornea, lens, retinal shape etc. An eye with perfect performance for distance will have the light rays going in and out of the eye being parallel. To best analyse an eye the experts seem to think that corneal topography + wavefront analysis are both necessary.
Wavefront Analysis Wavefront analysis is performed by an instrument called an aberometer. The aberometer can detect optical errors at a fine level. Wavefront technology assesses every ray of light that enters the eye and then determines what changes will produce the clearest image. Therefore, wavefront analyzers precisely measure the overall refractive error of the entire eye, including any aberrations caused by the tear film, anterior and posterior cornea, lens, vitreous and retina. Remember that corneal topography systems can define corneal irregularities, but they cannot detect aberrations in other parts of the eye. The wavefront sensor measures the refraction to submicron
levels, or about 0.01 D. When a refraction is measured with today's conventional
subjective tests, the accuracy is only to within 0.25 to 0.50 D.
Hartmann-Shack Wavefront Analyzer (this
is the system used in the Technolas 217 and VISX Lasers) Light travels in flat sheets called wavefronts. The irregularities or aberrations in the cornea and the lens of the eye wrinkle the light waves and create wavefront errors or distortions, as the light rays enter and exit the eye. That's the scientific principle that this technology uses. The wavefront analyzer aims light rays from a single laser beam into the eye and focuses them on the retina. As they are reflected back out from the retina these light rays are subjected to possible aberrations as they travel through the eye's optics. If the eye has no irregularities these light rays will come out of the eye in a plane wavefront, or a straight line. However, if the eye has irregularities, also called higher order aberrations, the wavefront emerges not in a straight line, but with a unique shape specific for that eye.
This wavefront of light then passes through a tiny array of lenses, called the lenslet array, in the wavefront analyzer. The analyzer measures wavefront deviation of the reflected light, and the image created by the lenslet array is captured by a video camera. You can think of the wavefront maps created almost as a fingerprint of the eye. In a normal eye, the video image shows small dots of light in a symmetrical grid, aligned in a highly uniform pattern. In an eye with significant aberrations, however, the dots are blurred and the pattern appears distorted. The system then compares the pattern seen in the eye being analyzed to an ideal pattern with no optical aberrations to generate a series of equations that describe the aberrations, called Zernike polynomials, for that particular eye. This system is fine for relatively normal eyes but can have difficulties with very deformed eyes. The small dots are deviated so much that the system cannot tell which original dot to assign it to. Putting in more receptors only makes the problem worse. For these eyes you need a combination of corneal topography and wavefront analysis combined. Higher-Order Aberrations Zernike Polynomials I am at present using the Nidek-EC5000 system for customised treatments and the following presentation by this company explains a lot about this system: Nidek Custom Ablation. Nidek use a different and very original system to analyse the optics of the eye. This is explained in more detail in the presentation but essentially uses a machine called an "Optical Path Difference" (OPD) analyser. This measures the refraction at nearly 1500 points simultaneously and combines this information with corneal topography to make up the "shot data" for the laser. However, perhaps more important than minor optical aberrations is to keep the cornea prolate to reduce spherical aberration. This is explained in detail on the following page, also reached from the blue link button above.
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