So called "wavefront" laser treatments have been very oversold. One should generally not correct lenticular aberrations (deriving from the lens inside the eye) with treatments on the cornea, apart from regular astigmatism. This is because the major sphere/cylinder treatment on the cornea will alter the optics so much that the calculated correction for any lenticular aberrations will be put in the wrong place. The reason that some of the laser platforms have better results with their wavefront treatments than their standard ones is that the latter was so bad! These older treatments left the cornea oblate. See below for a fuller explanation of this. Probably the best customised treatments are those based on the cornea, to correct any aberrations there.
Here is a good recent article by Dr Guy Kezirian from Cataract and Refractive Surgery Today - Europe Edition Feb 2011 - Click Here
We are using the T-CAT (topography based) and Prolate modalities of the WaveLight laser with outstanding results
A "normal " laser ablation 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 LASEK or LASIK, corrects the eye in the same way as spectacles.
The more modern laser profiles aim to keep the cornea prolate to reduce spherical aberration. This laser profile is often known as "wavefront optimised"
The main idea is to keep the asphericity of the preoperative cornea, which is a prolate shape, and to also treat any large corneal irregularities. In most people this latter is quite small, and the most important thing is to take more tissue off the edge of the ablation to keep this prolate shape. An oblate shape is like a hamburger bun, with the periphery steeper than the centre. It is not a good idea to treat aberrations that come from the lens inside the eye ("lenticular aberrations") as this article explains.
Another article on keeping the natural prolate shape of the cornea is here
Below is an explanation of what is meant by wavefront:
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:
- In most eyes this is very "small" print. Glasses do not correct higher order aberrations although gas permeable contact lenses do.
- 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.
- 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.
- 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.
- 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:
- "Cornea Spoon" "Cornea Spoon"
- "Regular Astigmatism" "Regular Astigmatism"
- "Irregular Astigmatism" "Irregular Astigmatism"
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:
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.
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 is performed by an instrument called an aberrometer. The aberrometer 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)
How Does It Work?
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.
Aberrations that cannot be corrected by simple spherocylindrical systems, such as spectacles or contact lenses. They are caused by minute misalignments of the eye's optical components and include, in order of visual significance, spherical aberration, coma, higher-order astigmatism and others. Theoretically, an ablation that removes aberrations increases visual contrast and the spatial detail of images seen by the eye.
Numbers that describe wavefront aberrations. These numbers are used to generate an ablation pattern (treatment profile) for the excimer 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.