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Why LASIK is called "flap and zap"

If you are having LASIK, please read the other pages in this section carefully. The following are results graphs for my last few months results. Most are taken at 3 months post op.


Running Audit of my results (using the Wavelight Eye-Q 400 Hz laser - March 2013 )

(results are the same for LASIK and LASEK)

Picture 1: Accuracy Levels: The middle line is spot on and the dotted lines are +/- 1 Dioptre. As you can see, this is a very accurate and safe laser, Both LASIK and LASEK results are in the myopia charts. 

Picture 2: Another Accuracy Graph: Driving standard is about -0.75 Dioptres. A patient who gets even one eye within half a Dioptre is usually very pleased. 

Picture 3: Astigmatism Results: There is usually a bit more scatter with astigmatism as there is an axis that varies as well as the size.

Picture 4: Safety for treating myopia (short sight): "No harm" is no loss of Best Corrected Spectacle Visual Acuity (BCSVA). One line loss is usually not noted by the patient line loss as a measure of safety. The chart plots how many lines gained or lost on the standard Snellen vision chart. Most stay the same. Of those who gain sharpness of vision, a lot do so because of magnification effects; glasses make the object size smaller, whereas contact lenses and Lasik do not.

The FDA counts the loss of 2 lines of vision as a complication. One line loss or gain can vary depending on the room the test is done in, the tiredness of the patient etc.

Picture 5: Defocus Equivalent: A technical graph of accuracy made by the numerical addition of the sphere and half the cylinder without taking into account the sign. ( +1 / -2 x 180 has a spherical equivalent of 0 but a defocus equivalent of 2). It is a truer measure of accuracy than the spherical equivalent used in other graphs. (sphere + half the cyl taking into account the sign).

Picture 6: Results for hyperopia (long sight): Wavelight Laser:

In hyperopia, the image size is bigger with glasses and smaller with contact lenses or Lasik/PRK. Hence there is a tendency, opposite to myopia, to lose sharpness of vision with contact lenses or Lasik because of the image size difference.

The average age of people presenting for hyperopic lasik is around 46y, whereas the average age for myopia is 33y. This is because the hyperopes can manage until middle age until their fails. There is nothing worse than a middle aged hyperope as they can see neither distance or near. Correspondingly, they are often amongst the happiest patients following refractive surgery.

I have also scanned in a recent article (April 2010) about this laser from an ophthalmic magazine called Ocular Surgery News.

Wavefront & Custom Laser

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:

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:

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

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.

Higher-Order Aberrations

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.

Zernike Polynomials

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.

Hubble Space Telescope

1. We have come to take the excellent performance of the Hubble Space Telescope for granted.

2 & 3. Soon after Hubble began sending images from space, scientists discovered that the telescope's primary mirror had a flaw called spherical aberration. The outer edge of the mirror was ground too flat by a depth of 4 microns (roughly equal to one fiftieth the thickness of a human hair). The flaw resulted in images that were fuzzy because some of the light from the objects being studied was being scattered. During the first Hubble Servicing Mission in December 1993 a crew of astronauts carried out the repairs necessary to restore the telescope to its intended level of performance. Although the two other servicing missions which have since been performed were at least as demanding in terms of complexity and work load, the First Servicing Mission captured the attention of both the professional community and the public at large to a degree that no other Shuttle mission has achieved. Meticulously planned and brilliantly executed, the mission succeeded on all counts. It will go down in history as o

The Wide Field and Planetary Camera 1 was replaced with a second-generation camera (Wide Field and Planetary Camera 2) and the High-Speed Photometer was replaced with COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR is not a science instrument but a corrective optics package that corrects the aberration for the three remaining scientific instruments: the Faint Object Camera (FOC), the Faint Object Spectrograph (FOS), and the Goddard High Resolution Spectrograph (GHRS).

4. Before and after the spherical aberration correction of Hubble

More on Hubble at the Space Telescope Science Institute

Laser Eye Surgery - an acceptable alternative to glasses and contact lenses?

From a talk given to the Manchester Society of Engineers in the University of Manchester Institute of Science and Technology (UMIST) on 9th May 2000 and printed in their journal. Also printed with variations in PULSE medical magazine for GP's on March 6th 1999


Recent high profile people having laser eye surgery rather than wear contact lenses or glasses, such as Richard Branson, Tiger Woods and Nicole Kidman, have made people realise that perhaps this is a technology that is coming of age. Many tens of thousands of people in the UK have had PRK (Photorefractive Keratectomy), or LASIK (Laser Assisted Keratomileusis), a mixture of laser and microsurgery, offers a quick and pain-free recovery. The ophthalmologist who first thought of this surgery, Prof. Steven Trokel in New York, did his first degree in engineering!

How It Works

This technology utilises an ultra-violet "excimer" laser of 193 nm to alter the shape of the cornea. This is an argon-fluoride laser with a very high photon energy of 6.5 eV. This cuts off about 0.25 microns per pulse. In the case of myopia (short sight), the cornea is flattened and in hyperopia (long-sight) it is steepened. The process might be thought of as "carving a contact lens" onto the eye. The cornea is about 550 microns (0.55 mm) thick and, in an average myope of 3 Dioptres (D), the laser takes off about 40 microns. The cornea is not weakened physically and the operation is not visible to the naked eye (and indeed it is often not possible to detect even using an operating microscope). The laser hardly raises the temperature of the cornea and hence does not cause any scarring due to collagen thermal damage.


No-one yet knows how to measure what the laser is doing in "real time" i.e. during the operation. People are not as identical as the inert materials of circuit boards, for which this type of laser has been used for many years in industry, and the laser may cut off a bit more or less from the cornea than predicted. People’s healing characteristics also vary. Hence, the bigger the prescription, the bigger the spread of results. As a rough guide, most low myopes (less than -6D) achieve within ½ D of aim and most higher myopes (-6 to -8D) achieve within 1D. Although it is less accurate for the higher myopes, the patients are often even more pleased, as they are effectively blind without glasses or contact lenses. (Try putting on a couple of +3.5 reading glasses, one on top of the other, to see what a -7D myope is like!). About 25% of the adult Caucasian population are myopic and 90% of these are -6D or less. PRK/LASEK is suitable for up to about 8D of myopia, 4D of astigmatism and 4D of hyperopia.


No operation has zero risk, including PRK/LASEK, despite the "street cred" of lasers. PRK is elective surgery on a healthy eye, so the criteria are more strict than operating on a diseased organ. No one, to my best knowledge, has been blinded by PRK and at least 50,000 have now been treated in the UK alone.

The commonest risk is loss of sharpness of vision that is not correctable with glasses. About 1% will lose 1 line of vision on the Snellen chart (the commonest eye chart) and about 1 in 1000 will lose 2 lines. This is mostly due to micro irregularities of the surface that cannot be corrected optically by the regular surface of glasses. Most patients will not notice the loss of one line of vision but will notice 2 lines of loss. Patients can also have a "touch-up" for residual refractive errors and most clinics do not charge extra for this. About 5% of the higher myopes will have such an "enhancement".

Patients have to understand that they could have some ghastly problem such as a corneal abscess and might end up needing a corneal graft. Such situations may be exceedingly rare, but if it happens to you then statistics are no consolation! The risk of such a disaster is probably about the same as when wearing a soft lens, which many patients will have used for some years.

How about LASIK?

LASIK (Laser in situ keratomileusis) is the other major alternative in laser eye surgery. This combines PRK with an older surgical procedure known as keratomileusis. In this, a powered microkeratome (a fancy sort of bacon slicer!) is attached to the cornea with a suction ring and a partial flap of about 160 microns is created with a "hinge" at one side. The excimer laser is then fired in the same way as in surface PRK and the flap is replaced. It has hence been called the "flap and zap" operation! The flap re-attaches initially by osmotic pressure and no sutures are needed. LASIK is a better surgical experience for the patient than PRK because, as the corneal epithelium is left almost intact, there is little pain and a faster visual rehabilitation. A LASIK patient at day one post-op will see what a PRK patient will see at 1-2 weeks. This has been called the "wow" factor of the surgery. Most refractive surgery in the USA is now LASIK. There are many detailed internet sites which will give you more on the surgical details. Over the years I have used Hansatome microkeratome and the Nideklaser. Also the Technolas 217 Nidek. I have also used the Lasersight laser and microkeratomes from Moria.

At present I am using the Wavelight 400Hz laser. I am impressed by this machine. It is very fast, accurate and the tracker is the best I have used. The whole philosophy of the company agrees with mine. i.e. Their "standard" treatment is "wavefront optimised" (ie prolate ablation - see later articles) and for the few cases that need them, there are full wavefront or topography based treatments. as well as Q adjusted.

What are the risks?

LASIK is clearly a "proper" surgical procedure and has more to go wrong then PRK. Perhaps the most important piece of advice for the patient contemplating LASIK is to choose the surgeon well! LASIK costs more than PRK, being about £800-1500 per eye as opposed to £700 -850

How does a person decide whether to opt for PRK/LASEK or LASIK?

The end results of surface laser and LASIK are the same. As they both use the same laser, the accuracy levels and numbers of people who lose sharpness of vision is also about the same. LASIK "gets there" faster and with less pain.. Which procedure to chose depends on each patient’s attitude to risk versus convenience. Neither procedure should be used for myopia greater than -10 Dioptres as the optical zones carved on the cornea are too small for low light vision. One eventually just "runs out of cornea"! If the cornea is thicker than average then one can do more treatment and, correspondingly, if it is thinner then one can do less. The corneal thickness is measured using an ultrasound probe and only takes a few seconds to do.


PRK and LASIK are now beyond the experimental stage and into the developmental stage. Having treated my brother (-6.5D)and my GP (-2.25 D), who are both delighted, I think that it is a reasonable alternative to glasses or contact lenses, particularly for the lower myope.

The Royal College of Ophthalmologists (Tel: 0207-935-0702, Fax: 0207-935-9398) has imformation for prospective patients at the College Web Site.