This chapter is an introduction into the field of Refractive Corneal Surgery. It starts off with the definition and purpose of Refractive Surgery, then the nature of Refractive Surgery is discussed briefly. Requirements from system theory will give a first set of criteria to estimate surgical techniques. After surgical terms are defined a new classification for refractive techniques is developed to improve better understanding. The history and the development of Refractive Surgery is outlined. The LASIK procedure, being today's most promising refractive surgical technique, is discussed and compared with other refractive techniques. An overview about the US market and the German market end this chapter.2.1Definition and Purpose
Refractive Corneal Surgery is a fairly new branch of ocular surgery. It aims at the total correction of significant refractive defects. José I. Barraquer coined the term "Refractive Corneal Surgery" in 1949 [Bar49] and in 1983 expressed its aim as the following:
"...,the purpose of Refractive Corneal Surgery is restoring the visual function to ametropes, without the aid of prosthetic devices, since they ---no matter how perfect or well tolerated they become, out of habit or resignation--- continue to represent a handicap (when not a risk) in the performance of the functions proper to a normal life." [Nor89p.177]
In 1967 [Bar67] he admitted that its employment at that time was limited to the prevention only of ametropias which had resulted from pathological or surgical processes, and the correction of those cases of ametropia in which the usual clinical procedures would not lead to a satisfactory result. Since then he has become aware of cases of severe monocular myopia and monocular aphakia, especially in children in which it has been at times impossible to resolve the problem of anisometropia, amblyopia and the subsequent loss of binocular vision by traditional methods.
He suggests that medicine and surgery must be oriented towards the establishment of normal organic functions without resorting to external prosthetic devices. He refers to the hindering effect of these in traditional fields of work, such as agriculture and mining, and also in more recent ones arising from modern scientific and technical development such as space exploration.
The social acceptance of Refractive Corneal Surgery differs from country to country. Actually it is quite astonishing how it has been neglected in some countries over the decades. It is somewhat true that the so-called developed countries tend to be more wary and think that vision should not be gambled with. However, in the author's point of view a large part of the emotional arguments against Refractive Corneal Surgery comes from never questioned cultural beliefs. Statements like2. Myopic Patient: "No one is going to touch my eyes".
2.1 An ophthalmologist asked about his opinion of operating on children with serious!! myopia, to allow the child's development of vital vision processing in young years claimed: "The doctor must be without any moral obligation to operate on a child's healthy cornea."
These statements show intolerance and even ignorance to innovative approaches. Asking oneself why hearing aids are socially much less tolerated than wearing glasses, and how avoiding them through "risky" operations is not even questioned, demonstrates the so-called "rational" beliefs of some authorities.
In the author's point of view adequate distribution and application of Refractive Corneal Surgery weighing both the risk and the benefits will help to maximise all patients satisfaction and the applied quality. Today’s uncritical application in some countries and dogmatic non application in other countries does not serve for the patients benefit.2.2 Nature of Refractive Corneal Surgery
The basic concepts about the cornea and how to change its shape are introduced.
About 70 % of the refraction in the human eye is done by the cornea, other parts by the fluids inside the eye and a reserved portion of refraction by the eye's adjustable lens to guarantee vision at both short and long distance. The human vision system seems to be designed rather for long distance than for short distance, as its working point without any additional power lies more in far vision. With age the feature of vision at short distances is lost slowly with a breakpoint between 40 and 48 years; people then need convex lenses to be able to read newspapers etc.. For instance children, can see sharply between 10 cm of up to infinity, adults in their thirties from about 20 cm to infinity, adults in their fifties from 40 cm to infinity and retired people over 70 only see sharply in infinity.
Farsightedness or nearsightedness means that the normal working point of the human vision system is not (close) at long distance for some reason. Maybe, for genetic reasons and/or alientening the eye for constant near vision in the case of nearsightedness. However, depending on how much the working point is out of position, the adjustable lens inside the eye can only cover a limited range within the boundaries of normal vision.
In emmetropia -working point in infinity- the refraction system of the eye combines the light rays from infinity exactly in the retina. The radius of the refraction system represents the degree of refraction achieved, depending on the length of the eye the optimal radius varies in each eye in order to produce emmetropia. So if the rays of infinity are not bounded that they actually meet the retina in a point, either the length of the eye must be changed -practically rather impossible- or the refraction must be changed so that the rays meet properly at the retina. This means the overall radius of the lens system must be changed. Therefore we will always talk about an optimal radius in relation to the size of the eye.
If the radius is too small, rays from infinity are bound before they even get to the retina. In this case the person can see close things without adjusting the lens, the working point is somewhere at near vision. This situation is called nearsigthedness (myopia). Moving the working point back to infinity means that the radius must be increased, so that the lens gets less steep and the rays are less broken. Refractive Corneal Surgery would try to flatten the cornea in this case.
Far-sightedness (hyperopia) means that the radius is too big, that means the rays are broken too loose, they combine behind the retina. In adding additional lenses the overall refraction radius becomes shorter and rays will be bounded in the retina. Here, Refractive Corneal Surgery would try to steepen the cornea.
It means that light rays are not broken uniformly. This can be due to the eye's lens or more often to a non-spherical cornea. In most cases astigmatism is regular, that is to say there exist two uniform axis: one at refractive minimum and the other at refractive maximum. Again, if the axis of the maximum is at vertical position and the axis of the minimum is at horizontal position, it is an (regular) astigmatism with the rule. Any regular astigmatism can be corrected with prosthetic devices. Irregular astigmatism is seldom and is either sign of some corneal disease, most likely a keratoconos, or due to corneal surgery. Irregular astigmatism is difficult to correct. Best results are obtained with rigid contact lenses.
Adjusting the eye length to the given refraction seems practically impossible, whereas changing the refraction power seems to be the only solution. As already mentioned the cornea takes the highest portion of refraction, moreover its refraction is stable. To change the refraction without additional lenses like glasses or contact lenses, the most likely answer would be to change the shape of the cornea.
The corneal tissue is highly watered and it consists of various cell layers. The two interface cell layers, the epithelium and the endothelium, separate the cornea from the outside and the inside of the eye respectively. The Bowman's layer, the second layer from the anterior of the eye following the epithelium, needs a special mention as it covers different functions which must be taken into account when applying Refractive Surgery.
Firstly, as the layer is more dense and less watered , it has somewhat of a "corset-function" and gives stability to the eyeball. Destroying this layer can change the stability of the eye which can cause incalculable refraction changes due to the ocular pressure "taking the opportunity once the corset is not there anymore". Until now no reliable theory has been found to calculate this undesired effect. Some ophthalmologists even want to use this corset-function in the future to cure astigmatism and even myopia by measuring and changing the partial tension on the Bowman's membrane [Zei95].
Secondly, like the bark of a tree, the Bowman's layer is highly sensitive and partial destruction causes woundhealing effects which are followed by pain and haze. The pain disappears a week after surgery and the haze usually last for three months. However, the most annoying circumstance is that the effects due to reoperation in superficial PRK are highly incalculable, with the result that the haze often does not disappear at all. The effects of the "bark-function" of Bowman's membrane in Refractive Corneal Surgery are always undesired and in reoperation they can be even fatal.2.2.3Shaping Ideas
Changing the curve of the cornea in its center of about six mm, one needs to know the different factors influencing the shape and refraction of the cornea:
1.the law of thickness
i ocular pressure acting on cornea
ii innercorneal tension release
the law of thickness
Changing the thickness of the cornea follows the idea that the cornea is a stable lens, removing tissue in the center or adding tissue on the periphery therefore flattens the cornea. This is called the law of thickness.
Instead of removing or adding tissue, the shape of the cornea can be changed by weakening the strength of the cornea outside the center. Then, either the ocular pressure leads to a different curve of the cornea center, or the release of innerconreal tension changes the shape of the cornea. Most procedures, explained in brief later on, take into account either one of these two factors.
The difficulty of the first approach is that changing the thickness of the cornea can induce a change in the eye's biostatics causing undesired additional refraction changes.
Changing the biostatics of the cornea by incisions, so that the ocular pressure then reshapes the cornea often gives incalculable surprises due to unknown individual wound healing effects or due to difficulties in accuracy during the operation.
New theories try to model innercorneal tensions in order to explain observations which can not be explained by the thickness law in relating to the Bowman's corset function [Zei95p.21ff]. Some scientists suspect that the refractive change due to superficial PRK procedure is rather a result of destroying the Bowman's layer than to applying the thickness law. This would explain the observation why the results above six diopters become so unpredictable. The radius of totally destroyed Bowman's layer rises expotencionally above six diopters by now used ablation programmes.
The reduced astigmatism of inserting a homogenous corneal ring indicates that astigmatism is rather due to inhomogenous tension of the cornea, than to partial inhomogenous thickness of the cornea. The corneal ring can not change inhomogenous thickness by homogenous circular tension but it can homogenise tensions. The tension theory therefore could explain the regular results in treating astigmatism and hyperopia with thickness law applications. Correcting refraction caused by tensions by applying the thickness law might be possible to some extent. However, superposition of the two effects seems limited. Putting the theory of tension release in practice would suggest that "intelligent" partial destruction of the Bowman's layer could change most refractional malfunction without getting deeper than 80µm. However, techniques depending on the change of cornea's biostatics will always lower the overall stability of the eye. Working with tension release, only the future will reveal, if the loss in stability will have secondary effects, as the hyperopic shift did after radial keratotomy.
In the author´s point of view, procedures working with the simple "mechanic" thickness law are most predictable in their outcome, as they do not depend on the cornea's biostatics changes and have the advantage of 30 years' experience and therefore no long term surprises are expected. Moreover, they hold a successful experience of over thirty years, and unexpected long term effects have not occurred in that time. However, it must be secured that the effects of the thickness law are not biased by statical changes. That is to say, that the Bowman's layer must be saved and at least 400µm of tissue must be left in the cornea's center. The superficial PRK function, although designed by the thickness law, does not meet this condition, as it destroys the Bowman's layer most likely changing eyes overall static’s. Tension release might have its place in correcting astigmatism caused by inhomogenious corneal tension. Figure 1 gives an overview of surgical correcting refraction.2.3System Theory
In a comparison with system theory, which sees the eye as a vision system and any surgery like an intended system change, we can easily find a first set of criteria for evaluating each approach. Although most of these criteria seem like essential requirements when just getting rid of glasses, they can change from essentials to achievable goals, if the person suffers strongly from ametropia.2.3.1Stability
In system theory, stability is the most vital criteria for any design, meaning that after certain stimulation the system returns to its equilibrium, at best the former working point. There exist many categories of stability. However, in our case this criteria will mean that after surgery and the healing period there will be no change in the achieved refraction. Neither in the long term, for instance, a hyperopic shift, nor daily changes may occur.2.3.2Deviation from Target
The second criteria for approaches in system control is the deviation from the target after the system is stable. In our case: How close does the patient get to emmetropia after the healing period?2.3.3Behaviour in Transitional Phase
The third criteria covers the transition period from the induced change to the newly reached stable position. The usual goals are a quick progression to the stable position, and only a small deviation from the target size until stability is reached. These are often two contrary goals. In our situation this means that the desired emmetropia is reached quickly after surgery and with limited overcorrection during this time of healing.2.3.4Robustness
The last criteria is robustness which stands for unchanged well functioning in changed, unusual condition. Will the refraction change while montainclimbing or doing deep sea diving?2.4Classification of Techniques
Many kinds of techniques have been developed in Refractive Corneal Surgery with the aim of correcting myopia, astigmatism and sometimes even hyperopia. Most techniques brought best results in correcting pure myopia. Firstly, flattening the cornea seems to be somewhat easier than steepening the cornea or adding tissue to it. Secondly, there are more myopic than hyperopic patients. However, it needs to be mentioned that in no circumstance is the cause of the refractive disease cured by corneal surgery, only their refractive symptoms are taken away. Is very likely that neglecting the additional side effects of the applied procedure, "regression" will occur in corrected myopic eyes as the myopic disease is not stopped by the surgical adjustment of the cornea. Nevertheless, on the long term, when technology will be fully mastered, hyperopic correction might even be more stable than myopic correction.
There have been various approaches in classifying refractive surgical procedures, such as following the historical development, sticking to similarities in the surgical procedure or following surgical intention of change. The first approach following the historical development does not seem to be very systematic. However, knowing the historical development in detail can help to optimise future developments, by learning from past experiences, checking old results either being fundamentally true or only limited to the techniques of those times. The second approach: sticking to similarities in the surgical procedure seems most obvious in first sight. Similar procedures need similar instruments. However, often this connection is only an historical accident, showing the creative origin of a new technique, for instance, the birth of a new idea in correcting ametropia by a kind of accidental mutation is such case. The third approach, following surgical intention of change, is the most systematic one. Here, techniques of the same category will have similar (principal) complications or defects. If techniques of the same categorisation do have different kind of complications, it will then be a sign that one of the techniques is fairly undeveloped and at investigational stage. Its side effects are most likely due to the kind of instrumentation used.
Classifying refractive corneal procedures by any of these three approaches will bring similar clusters and only some of the techniques will change the cluster. However, classification is somewhat setting the horizon and the way of thinking in the refractive surgical world. Following a prospective future orientation, the classification by similar surgical intention of change will be most adequate. It helps to point out built-in complications and automatically reveals complications caused by instrumentation. Moreover, this classification is independent of technology, fascilating quick technology changes.2.4.1Surgical Terms Following Waring's Classification
Waring classified refractive surgical techniques due to their similarities in surgery. Before developing a classification which follows the intention of change, Waring's approach will be used to become familiar with the terms commonly used in Refractive Surgery.22.214.171.124Lamellar Refractive Surgery
Early Keratomileusis: "Freeze and Grind It"
Epikeratoplasty: "Cap It"
Keratophakia: "Sandwich It"
This class is characterised by slicing the cornea in different horizontal layers and treating some of them to change the overall refraction. These techniques usually follow the "thickness law" thoroughly investigated by J.I. Barraquer. Corneal tissue can be replaced, added or subtracted. Techniques subtracting tissue have shown best results and are called keratomileusis. A short explanation of some of lamellar techniques will be given in the following historical round up. Lamellar techniques in theoretical terms seem very promising, as they directly change the shape of the cornea and do not depend on indirect biostatical effects. Anyhow, in the past, they were not widely used, due to their complicated surgical procedure and high surgeon's learning curve. With laser application, it seems to be that in the future, LASIK will overcome all former non lamellar techniques. The surgical procedure is maybe not so easy as in superficial PRK that it can be done by every average ophthalmologist, but it is still very straight forward.
Keratotomy ("cutting the cornea") follows the idea of changing the biostatics of the eyeball by making a partial thickness incision into the cornea. Most commonly the corneal center is flattened by making incisions in the periphery to correct myopia. Over time many patterns such as radial, trapezoidal or special T-Cuts have been developed. Radial Keratotomy (RK) makes incisions in certain meridians in the periphery to reduce its refractive power in the center of that meridian. RK was made famous by Russia's ophthalmologist Fyodorov in the late 70's [2.5.1]. However, the first long term results have usually shown an unexpected hyperopic shift.
The excision of a piece of cornea is called keratectomy1. It is done to change the refraction of the cornea. It involves the removal of a lenticular or a crescentic piece of tissue. Keratectomy takes part in all keratomileusis procedures. The excision of the cornea can either been done mechanically or by light.
The photo refractive keratectomy (PRK) with the excimer laser is the most common keratectomy. Originally scientists tried to replace the manual keratotomy incisions by using the laser. However, the results in using the laser for keratotomy were poor, as the laser always removed tissue. Instead ablating total areas of cornea with laser, doing photo refractive keratectomy has been revolutional in Refractive Corneal Surgery [2.5.3]. Strictly speaking PRK is a superficial keratomileusis procedure.126.96.36.199Penetrating Keratoplasty in Aspects of Refraction
The major reason for doing a penetrating keratoplasty is to replace the central portion of a scarred or distorted cornea by healthier donor tissue. Until 1980 the major clinical challenge was to produce a clear graft that would last a long time. However, for usual refractive corneal problems, this procedure seems too expensive and risky.
Applying heat shrinks the corneal stromal collagen and flattens the cornea in this area. Various sources of power have been tried, such as thermal probe, radio frequency current and even different lengths of laser waves. The holmium YAG laser seemed a promising thermokeratoplasty tool, however its results have been poor due to very unstable refractive results: over time at least 50% of refractive correction has disappeared.
Barraquer classified the surgical techniques into subtraction, addition, substitution, relaxation, retraction, compression and mixed techniques [Bar89]. This reflects a very systematic approach, independent of existing procedures. However, over and above this useful classification all techniques can be divided according whether their first intentions is to change the biostatics of the cornea or to directly change the shape of the cornea. This additional classification feature seems very useful in the author's point of view, as it reveals the theoretical limitations of the techniques. Any technique which primarily changes the biostatics of the cornea will lower the stability qualities of the eye. The dioptric value of the cornea will be less stable, resulting in daily changes, refractive changes in relation to height or long term refractive shifts. The lower stability quality of the cornea can even have more serious effects, for instance, accidents involving the eye will have greater consequences. Any technique following the thickness law, directly changes the shape of the cornea. If the biostatics of the eyes are not changed due to excess excision of tissue, the stability of the cornea will not be changed. The following diagram shows the classification. At the first stage, techniques are separated by either following the thickness law or not. At the second stage, the classification of Barraquer is applied. Compression has been not been included in the diagram, as it can be seen as a kind of virtual subtraction. Mixed techniques has also been left out, as the classification is used to demonstrate the basic ideas of changing refraction. At the third stage, techniques are separated by more detailed differences. Subtraction is differed into intrastromal and superficial keratomileusis according to the destruction of the Bowman's membrane, techniques of relaxation are divided into tension release, keratotomy and ectasia. Most common of the relaxation techniques are the keratotomies, ectasie has been used to correct hyperopia (Ruiz). As earlier mentioned, new theories of tension release in respect to the Bowman's membrane seem very promising. On the most detailed level, known techniques of each class are listed. Unfortunately, many techniques intended to be of single effect are of multiple effect for their limited instrumentation or because of using them above their limits. Some techniques even change the shape of the cornea in a different way as originally calculated.
Paths and frames of techniques, either not fully understood or not handled well, are marked with little lines. The only paths marked in the diagram are of the keratomileusis techniques, as firstly these keratomileusis techniques follow the law of thickness and secondly, subtraction of tissue is somewhat better handled than addition or replacement.2.5History and Development2
Nearsighted people have looked for ways to rid themselves of their
glasses for centuries. Tradition says that the ancient Chinese slept
with sandbags on their eyes to flatten their corneas. In the mid-1800's
century Dr. J. Ball advertised an eye cup with small spring-mounted
mallet that struck the cornea through the closed eyelid. "It
restores your eyesight and renders spectacles useless.”
The idea of using glasses to change the refraction probably appeared in the 13th century by putting a correcting reading stone right in front of the eyes. This basic idea has been refined to today's one-day disposable soft contact lenses.
J. Lans, working in Leiden, the Netherlands, published in 1898 the results of his experiments in rabbits, employing keratectomy, keratotomy and thermokeratoplasty to treat astigmatism. After studying radial incisions, he enunciated the basic principles underlying keratotomy: 1. The cornea flattens in meridian of the incision. 2. Some of the effect is lost as the incision heals. 3. The incisions must penetrate deeply into the cornea to obtain an effect. Studies on the management of astigmatism with corneal surgery were also conducted by Italian and German surgeons in the late 1800's.
In 1933, T. Sato working in Tokyo observed by accident the flattening of a patient's cornea due to an acute break in Descement's membrane developed by a keratoconus. This gave him the idea of making incisions in order to cure keratoconus, astigmatism and myopia. His work was based on many anterior and posterior radial incisions into the cornea. Sato operated on approximately 680 eyes between 1951 and 1959. Studies in the seventies on a sample size of his patients showed that 86% developed bullos keratopathy due to the posterior incisions. The endothelium was absent. Sato stopped doing his myopia surgery as soon as contact lenses were introduced into his practice, before the rash of oedematous corneas appeared!
B.S. Yenaliev, working in the Soviet Union, was aware of the oedema that resulted from Sato's posterior incisions, and confined his research to keratotomy incisions through the anterior cornea only. Between 1969 and 1977 he performed his anterior radial keratotomy in 426 eyes. In 1972, S. N. Fyodorov of Moscow began studying radial keratotomy. In 1974 Fyodorov began surgery on humans. One of his most important observations was that sixteen incisions gave almost the same results as did 20, 24 and 32. Fyodorov and his college Durnev correlated their observations into a formula used to improve the ability to predict outcome for individual patients. Besides the diameter, the radius of the cornea and the diameter of the optical zone, the formula contains a surgeon's practical coefficient. Fyodorov became famous, as he had the greatest impact in Refractive Corneal Surgery, for example, his assembly line operation were shown all over the world. P. Siva-Reddy of Hyderabad, India, using this Russian technique had only poor results due to very superficial incisions.
Radial keratotomy spread inside the US in the late 70's by the promotion of Dr. Leo Bores. The technique was modified by various American scientists. They changed the direction of incision making centrifugally and reduced the number of incisions to eight and four.
In 1989 A. Arcienegas, Barraquer Institute, and the engineer L. Amaya, in Colombia developed the first, prospective approach of Keratotomy to correct myopia and myopic compound astigmatism [AA89].They based their biomechanical approach on tissue relaxation. With the calculated formula they could expand the surgery into the medium high myopic range and increase predictability. Until that time, the progress of radial keratotomy depended only on retrospective observations. However, even they had to go back to retrospective observations and stick to cluster like surgery calculation. Principally their technique encounters the same known problems of relaxing technique; as for instance, the long term hyperopic shift.In the future, however, studies only based on retrospective dimension will be too inefficient and even unethical. Systematic bio-engineering with the aid of today's complex computer simulations can minimize unexpected surprises and even reduce animal experiments. The medical "do it and see what happens" mentality shouldn't be strained to much because of being happy-go-lucky and of missing prospective studies.2.5.2The Early Keratomileusis Experience
Keratomileusis is strongly sticked to the father of modern Refractive Corneal Surgery Jose I. Barraquer. Barraquer has made numerous contributions to ophthalmology in general and in Refractive Surgery in particular. He dedicated his life to the dream of correcting ametropia when virtually no one else could or would respond the same calling. He first announced that refraction could be changed by remodelling the radius of curvature of human cornea in his doctoral thesis in Spain in 1949 [Bar49]. He moved to Bogota in 1953, leaving the families' clinic in Barcelona. The law of thickness was introduced by him in 1964 [2.2.3]. Later, he established the Instituto Barraquer de America and until now he is the honorary President of the International Society of Refractive Surgery (ISRS). [Nor89, Bar96]
Long before the great rush of radial keratotomy he realised that the correction of refractive defects should not depend on hopefully placed incisions and cicatrical retraction of the wound, as in Sato's keratotomy. Instead he demanded a process that permitted a predetermined result of the greatest possible accuracy on an organ in permanent regeneration [Bar67p.31]. His methods always aimed for precise optical correction by means of exact mathematical surgical interventions. He designed and fashioned his own equipment in various fields of refractive keratoplasty. Moreover, he invented and named many of these procedures like keratophakia (intracorneal lens) and keratomileusis. After extensive experimentation with corneal grafts, plastic corneal inlays, and limbal alterations, Barraquer realised that more permanent, predictable surgery could be accomplished only by changing the shape of the cornea itself.188.8.131.52Keratomileusis Procedure
cinelado o tallado de la córnea.
Queratom'ileusis, palabra derivida del Griego
Queratos: cornea, mileusis: cincelar.
Jose I. Barraquer[Bar64p.27,p.48]
Keratomileusis refers to a any refractive corneal procedure that uses subtraction of tissue with refractive means.
The first successful keratomileusis technique goes back to the year 1964 and was determined by lamellar resection, extracorporally cutting of corneal tissue lenses in a lathe [Bar67, Bar64]. At that time this technique unlike other keratomileusis techniques (plane section in corneal bed, plane section extracorporally and others) allowed maximum precision by mathematical calculation. However, in applying this technique, the jelly like corneal tissue needed to be hardened, before resectioning on the lathe. Principally there were three ways of hardening the cornea [Bar67]: 1. freezing, 2. desiccation or 3. increasing the lineal velocity of cutting. Freezing the corneal tissue brought best results with less damage to tissue, therefore this keratomileusis technique is commonly known by freeze keratomileusis. In the beginning the calculations were done longhand in the operating room, but Barraquer moved quickly to procure the first programmable calculator to calculate the shape of excision and to shorten the surgical process and improve its outcome. He later advanced his technique by constructing a compressed air turbine cryolathe and a computer guided cryolathe3. He applied this technique with remarkable results up into the 90's on about 3000 (!!) high myopes.184.108.40.206The Microkeratome and It's "Flap"
Before innercorneal tissue can be reshaped either intrastromally or outside the corneal bed, the anterior layers of the cornea must be hinged.
Barraquer invented the microkeratome fulfilling this difficult task. His microkeratome consisted of a suction ring hardening the cornea by maintaining the intraocular pressure above 65 mm Hg to allow a smooth cut. The anterior layers of the cornea were either hinged (flap) or even totally capped. The prototypes were made of brass. Later the American Steinway company manufactured a stainless steel version.
The hinged flap and the use of the microkeratome have had a great impact on newer developed Keratomileusis techniques, in particular LASIK. Barraquer's protégé L. Ruiz further developed this microkeratome with a motor drive to simplify the cutting procedure. Today this microkeratome is sold by the Chiron company. Although this microkeratome takes all the Barraquer's experience into account, Ruiz named it Automated Corneal Shaper to avoid any association with its origin.220.127.116.11Various Definitions of Keratomileusis
As already mentioned earlier keratomileusis refers to any refractive
corneal procedure that uses subtraction of corneal tissue with refractive
aims. However, depending on the context, the word is used in different
manners. Sometimes, it stands only for the original procedure called
freeze keratomileusis. Most commonly any intrastromal subtraction
with a microkeratome is referred to as keratomileusis. The superficial
PRK is often not associated with keratomileusis, although it holds
the definition criteria.
The use of the microkeratome is strongly associated with keratomileusis due to the historical development. Some would easily call all procedures keratomileusis which take advantage of a microkeratome. Although the microkeratome is involved in intrastromal keratomileusis, not all the techniques using the microkeratome are keratomileusis procedures. For instance, Ruiz's technique of ectasie to correct hyperopia -using a microkeratome- is a non keratomileusis technique, as there is no resection of tissue and the law of thickness is not applied.
The following graphic summarises the different levels of keratomileusis. All procedures in the big circle are keratomileusis procedures. Further specification is reached by separating intrastromal procedures from superficial procedures. Absolutely non keratomileusis techniques such as the technique of ectasie and radial keratotomy are outside the circled ring.2.5.3The Photorefractive Experience
In practise excimer, photorefractive and Laser Refractive Surgery
seem to be different words for the same thing. Here, the Photorefractive
Surgery using an excimer laser is introduced.
Argon fluoride excimer laser were first used for the production of computer microchips. R. Srinivasan and others described a very exact ablation of plastic materials with the excimer laser in 1982 [SM82]. Parts of the technology used were developed in 1976 and the patent rights belonged to IBM. S. Trokel and R. Srinivasan described laser ablation on cow cornea in 1983 [TSB83]. T. Seiler first applied laser ablation on human cornea 1985. In the beginning he tried to use the laser to make the incisions used in keratotomy. As photoablation always involves the excision of tissue, results of laser keratotomy were poor. He realised that the excimer laser must bring better results when doing excision of whole areas of the cornea. Seiler was the first who applied laser ablation on a well functioning eye in 1987 [Sei86]. From then on, many scientists started to apply laser ablation for Refractive Corneal Surgery. The abreviation PRK stands for photorefractive keratectomy, that is to say the excision of corneal tissue by laser ablation for refractive means. Strictly speaking PRK only presents a surgical instrument for Refractive Corneal Surgery like the microkeratome. PRK can be used for superficial excision or intra stromal keratomileusis resulting in LASIK. However, since the beginning, PRK has been applied superficially. Therefore when speaking of PRK, most ophthalmologists think of the superficial PRK procedure.18.104.22.168Argon-Fluorine Excimer laser5
The excimer laser consists of an energy source, a delivery system and a controlling unit. The term excimer is derived from the words excited and dimer. Within the energy source, argon-fluorine (ArF) atoms or ions are excited through the application of electrical energy. These energised atoms emit photons of light. This beam then passes through a series of mirrors and prisms to enhance the optical qualities of the beam. Researchers have developed several systems to deliver the beam to the cornea in the desired profile. When correcting myopia the central portion of the cornea receives more ablation then the periphery. For instance, a diaphragm delivery system, gradually opens at pre-programmed intervals to control the diameter of the beam.
Two basic types of excimer lasers are sold, depending on the kind of delivery system: whole field or scanning. The whole field approach often use a simple diaphragm delivery system. The scanning approach requires more complicated delivery and control systems, but the energy source can be much smaller.
Typical characteristics are the frequency of pulse repetition (5 to 20 Hz), power (120 mJ/cm to 400 mJ/cm) and the possible diameter of treated optical zone (4 mm to 9 mm).
Lasers have been used in ophthalmology for more than 25 years due their four unique properties: tight direction, high intensity, coherence and purity. However, depending on the laser's wavelength tissue interaction differs. Photons produced by longer wavelengths are relatively low in energy, while those produced by shorter wavelengths are relatively high in energy. Therefore, tissue interactions can be photodisruptive, photothermal or photochemical.
The excimer laser is of relatively short wavelength, 193 nm. It is a UVC beam producing photochemical interactions cleaving the cornea's carbon to carbon bonds. This interaction is a form of chemical photodecomposition, a process more commonly known as photoablation. The photochemical interaction is the most precise tissue removal; as its effect is non thermal and non disruptive, and surrounding tissue is neither burned nor damaged. Each pulse removes only partial areas of tissue with a 0.25 µm depth. This precise interaction allows the removal of the cornea's tissue in it's optical center without diminishing the vision. Although the removal is discrete, the precise removal of 0.25 µm tissue steps ensures that all light wavelength's pass through without distortion.
The ablation process causes a reverse impulse of up to 107 Pa, these wave fronts might imitate the retina if fragile, operation therefore shouldn't be applied shortly after retinal surgery. However, other parts of the eyeball will not be affected, as the wave resistance hardly changes passing the different mediums.
Although the 193 nm excimer beam effectively removes corneal tissue we must consider the safety of this UVC radiation. About 95 % of the energy of the 193 nm beam is attenuated within the corneal tissue within 1 µm, and in most cases the nucleus will be not be reached. However, secondary radiation of about 200 to 300 nm could also perform mutagenicity. Numerical estimations show that this radiation is below a factor of two from the critical 10 µJ/cm2 mark. Over 300.000 treatments without diagnosed neoplasm confirm the safety in this aspect. 40-60 µm below the treatment zone there will be no keratocites. However, this effect occurs just as often by simple abrasion. Within weeks and months keratocities come back and after a year the normal proportion is reached.
Conceptually the superficial PRK procedure applies the thickness law by superficial keratectomy (excision). After patient selection and different examinations the patient will be treated ambulantly with local anaesthesia. Before laser ablation the epithelia will be removed either manually or with the laser. Then laser ablation is applied. After surgery the patient usually gets steroid and non-steroid antiinflamatopry agents to control wound healing. After three months vision is mostly recuperated and after a year refraction is stable. Haze and halos are typical during recuperation time.22.214.171.124Results of the Superfical PRK Procedure6
Results have been quite promising, however, its interpretation strongly depends whether the treatment is applied to patients, or people who just prefer to get rid of their glasses or the daily contact lens procedure.
Results can be measured either by individual satisfaction or by standardised measurements. Measurement of individual satisfaction is very important for future patient selection. Standardised measurements allow comparison, scientific progress and direct quality control. Discussion of this theme is one of the major topics of this work, a solution will be developed in the sixth chapter. Presentation of superficial PRK results will give a practical introduction to finding proper quality indicators.
Common standardised indicators are uncorrected vision, best glass corrected vision, best lens corrected vision or best corrected vision, either expressed in relative loss/gain of lines or in absolute index form. However, all these indicators only measure vision in certain conditions, which neglect many other common circumstances of vision. Some of the contradictions between patient satisfaction and these measurements are due to today's clinical testing.
The superficial PRK procedure seems to be splendid when measured by the improvement of the uncorrected vision index . Every person gains in uncorrected vision and often can handle daily life without vision aids after surgery. Changes in bestcorrected vision with common measurements seem to be insignificant in the medium time range, if superficial PRK is applied in proper patient selection. In the short range, during the first three months, sometimes until a year, best corrected vision is reduced significantly, due to haze from the healing process. Although life time experience so far does not exist, clinical approximation does not expect a loss of vision once the results have stabilised.
Deviation from emmetropia is another internationally common indicator. The surgery is rated successful, if after one year the deviation is in-between ± 1 dpt. from emmetropia.
Results from the only prospective study done worldwide, which was started in 1989, by Seiler in Berlin, are the following: The success rate depends strongly on the ablated tissue. Eyes with low myopia (0 to -3 dpt.) 97,6 % lie in-between the mentioned range. Myopia from -3.1 dpt. to -6 dpt. is rated successful in 91,8 %, myopia from - 6.1 dpt. to -9 dpt. only in 44,4 % and myopia above - 9 dpt. only in 25%. The diameter used in this study was 5 mm, in Seiler's opinion results will improve using a diameter of 6 mm, and predictable success will be possible for myopia of up to -7 dpt. Results from retrospective studies bring similar findings.
The dissatisfaction of patients concerning night vision originated from small treated zones, central islands, and haze or halos, which were uncovered by traditional vision testing. Optical zones smaller than the pupil cause difficulties in night vision particularly when back light (car driving at night) appears. Central islands, somewhat less ablated tissue in the eyes center, also cause undesired vision reduction. Haze and halos, due to the Bowman's bark function, similarly reduce night vision as small treated zones.
Men's vision somehow can correct poor retinal images unconsciously, this ability is called Stiles-Crawford-effect. However, in poor condition these compensation abilities don't work to the full extent and best corrected vision in reality might be lost by surgery without knowing. A technique which would "switch off" this compensation ability during vision testing, would be more adequate.