WAVEFRONT ANALYSIS (The new revolution in laser surgery)
Summary:
The goal of refractive surgery (= chirurgy to eliminate glasses) is correctin the optical imperfections of the eye. So far we were limited to sferocilindric corrections. Myopia / hyperopia and astigmatism could be corrected. However, there are more optical aberrations in the eye, so far not treatable. They even obsolete, by laser treatment of the sferocylindric abnormalities that, the other aberrations increase, with some loss of contrast (mainly at night): Radius figures around lights / less visibility in darkness.
By identifying and treating the average aberrations of the optical system of the eye, the laser was recently refined and we can get a better quality of vision than before. This means: a sight that should hardly inferior to the best vision with glasses or contact lenses. Historically, less good vision after laser treatment, especially at night. Now a decent nightvision is achieved, without too much disturbing figures radius around lights (typical after previous lasertreatment, especially with higher refraction zeker bij hogere refractie-abnormalities (-7 and more)). We hoped to achieve super vision.
More info:
The detecting of aberrations is done by "wavefrontanalysis".
1- WHAT IS WAVEFRONTANALYSIS ?
A wavefront must be viewed as a snapshot of a light beam through an optical system: If there were no obstacles, the beam would be equidistant at any point: "the front of the bundle is straight or planar". With such an eye you should see much better. The resolution would be directly proportional to the distance between the two receptors in the eye. This should allow a vision of 30/10. In other words: you can see on a distance of 30 meter what a normal person sees at 10 m! Such a view is standard with hawks. Eagles even see 50 / 10! This is not possible in humans, given our receptors are not so fine!
When a part of the beam encounters an obstacle (a turbid or thinner zone), this part of the bundle will be delayed and the front of the beam will not be straight but bent, because of a delay of a part of the beam! This interferes with the beam!
Until recently wavefronts were only studied in astronomy to measure atmospheric turbulence, that disrupts the light from the stars. It is succeed to improve the images of the telescope via the measured turbelences. There are even possibilities in hardware: the mirrors of the telescope can be changed to compensate the atmosferic turbulence. We should also try this with the eye, by the grinding of the cornea.
First we must try to determine the turbulence. We speak of "aberrations". We do this with "wavefrontrefraction" or "aberrometry". Try to imagine that the old computertests took an average of the anomalies on the surface of the pupil. With wavefront refraction the deviations from various points in the pupil are measured, much more precise than before!
2- HOW CAN YOU DECLARE OPTICAL ABERRATIONS?
In an een optical perfect eye, free from aberrations, a beam, will go straight to the retina (in a uniform lead). People say that the beam forms a "parallel beam" and that the wafront is planar.
In a normal eye (even if visibility is 10/10) the optical imperfections will bend the light and they will disturb the original planar wavefront. This disturbance is numerically expressed by the RMS index of the eye. When the RMS index is less than 0.25 is, the aberrations are negligible. When the RMS-index is higher than 0.25, the ability of the eye reduces to perceive fine details. The RMS-index is a very important quantitative parameter in the wave-front-analysis. Also the patterns, that are established, can learn us something about the quality of the eye !
3- WHICH KIND OF ABERRATIONPATTERNS ARE THERE?
The optical aberrations of the eye may be divided into 2 categories:
a) 'lower order aberraties': those are the sferocylindric abnormalities, already corrected in the concentional refractive surgery (= myopia / hyperopia / astigmatism). When a patient sees less, these abnormalities are responsible for at least 80% of the problem. The typical "low order"- eyedisorder are well known: myopia (= to longe eyes)/hyperopia (to short eyes) / astigmatism (ovoid eyes)
b) 'higher order aberraties': these are small irregular variations, which they share with decreased visual acuity estimated at 7 %.
Five species can be distinguished:
"coma" or asymmetric astigmatism: This can be caused by a slight clouding or surface irregularities in the cornea, lens or vitreous ! Perfect smooth corneas, lenses and retinas does not excist !
sferic aberrations: the side of the cornea and the lens break more than the center. When the pupil increases at night, the sides can be more involved, thise manifests itself as "night myopia"
chromatic aberration: blew light breaks more than red light. Blew light gives more contrastloss.
prismatic aberration
axial decentration: the axis of the cornea and the lens, are not right in the center of the pupil
4- WHAT DO THEY ESTABLISH WITH CONVENTIONAL LASER TREATMENT ?
Classic laser treatment can easily eliminate the low order aberrations. However, the high order aberrations increases. How is this possible?
The sharped surface is less smoother than before! Luckily tears can compensate a lot. Unfortunately the quality of the tear film is less good the first month after treatment! This explains why the contrast increases, as the problem with dry eyes spontaneous disappears !
We often see little opacities, that decrease after a few months. This makes that the light scatters more into the eye.
The side of the eye is more myopic, than the center. What are the consequences of the increased high order aberrations?
5- IT CAN BE BETTER!
It can be done better, when we solve above problems. The surface (that will be treated) should be as smooth as possible, so that after the treatment there are no additional higher order aberrations arise!
The stronger breaking side (= more myopic), which occurs with everyone, must be treated: "asferic grinding" !
Opacities should be avoided
You can do even better: You can detect and treat individual disorders and cloudnesses. You can imagine that no one is equal even if you have the same sferocylindric correction!
6- WHAT CAN THEY DO ?
The first 3 problems (smooth surface / aspheric grinding/ clouding) are already fixed in the latest laser devices, that are on the market since 2002. That's why we decided to buy the newest laser. What does this new laser do better:
You get a smoother ground surface, when the laser works with a finer (=1 mm) beam.
The stronger breaking side, is better prepared by expanding the optical zone and the use of special nomograms.
Turbidity should be avoided because the ground surface is smoother.
With these techniques, in addition to the sferocylindric correction, there is also something done on the average aberrations. There is an attempt to avoid the laser induced aberrations. That way the contrast and the nightvision get better, than previous laser treatments. Only for high myopic (above -6) the nightview can be difficult, compared with glasses or contact lenses!
This new technology is called "wavefront inspired" or "wavefront optimized" laser !
7- WHAT ABOUT THE TREATMENT OF INDIVIDUAL ABERRATIONS ?
The treatment of individual aberrations stays difficult. It's technically not so easy to adjust the laserpattern to the wave front analysis. You must have a "aberrometer" that is very trustworthy. You also have to develop a software, that can convert the measured deviation to a cutting pattern. Thereafter a sophisticated laser must apply the cutting pattern on the eye. This required a small laser, that can grind every individually item an a good eye-tracker, who follows the eye so the laser knows where to shoot. We call this technique "wavefront guided laser". Only then a planair wavefront can be created.
Now they try to realize the "wavefront guided", but it takes a tremendous effort of doctor and patient (measurements are necessary after dilating the pupil) and they don't recieve a better view than the "wavefront inspired lasertreatment. Supervision is not immediately expected and will probably never been reached:
a) supervision: not for now
The latest generation of lasers can't follow the accuracy of the aberrometry: lasers take per shot an area of 1 mm weg over a depth of 0.25 µ. The aberrrometry can determine depth deviations of 0.05 µ between different points, within the surface of 1 mm! In the future probably more points in the area of 1 mm can be measured!
Healing after laser can give opacities. You might predict some, were it not that individual healing is very variable. We therefore have to look for medications, so the woundhealing always evoluates normal. Magnifiek healing (with too much scarring turbidity) should be avoided, just as too weak healing !
b) supervision will probably never be reached:
The aberrations change from moment to moment, because of blinking (that changes the corneaform), focus to read = accommodate (changes the form of the lens) the changing of swelling of the coroid under the retina (because of an effort or position-change), that replaced the retina.
Because the aberrations are constantly changing, we need to measure the aberrations several times during a similar survey and content ourselves with determining an average aberration! By growing older the aberration average certainly changes, because of the developing of cloudiness in the lens ("cataract"), vitreous (flakes) and cornea (by traumatic scars and age turbidity).
Even if we reach an almost perfection it will decrease with the age!
8 - TREATING INDIVIDUAL ABERRATIONS POSSIBLY NOT DONE?
With lasertreatment you can only grind the cornea.
The cornea must compensate for abnormalities elsewhere in the eye. We must make the cornea "worser" to compensate the other abnormalities. Mostly the lens. Know that thel ens changes when you get older (= cataract krijgen) and someday must be replaced with a cataractoperation. The retouched cornea isn't useful anymore, it is even a disadvantage!!
9 - DECISION:
In this state of science it is clear that we should choose 'wavefront inspired' laser, rather than "wavefront guided" laser, even if this seems very attractive! We already apply this type of laser treatment for 2 years!