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True Quality Characteristics of Refractive Corneal Surgery
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As earlier mentioned, application of TQM is strongly connected to the wishes of the customers. It is is for quality characteristics to become the results of engineers' mental possessions or they turn out to be indeed a good deal of guesswork as towhat the customer might like. In ophthalmology, as in any medical field, clinical standards are traditionally set by authorities. Patients have rarely influenced these norms directly. As refractive surgery has an elective character, the patient's opinion is even more important. However, so far, refractive surgery has been technology pushed. Now as refractive surgery leaves the phase of technological possibility, the patients' opinion should be analysed more systematically. A patient undergoing refractive surgery is investing money and pain to become less dependent on prosthetic devices in oder to improve individual circumstances. These individual circumstances may be improving appearance or getting rid of the annoying disadvantages caused by glasses or contact lenses; in some cases the patient might even improve his best corrected vision. However, the outcome of the investment is not certain, the quality of uncorrected vision improvement is uncertain. And what is more, he is taking the risk, that his best corrected vision might even be worse after surgery, especially in difficult conditions, e. g. at night or in rainy conditions. Looking out for true quality characteristics means finding typical patient groups, their major motivations for undergoing surgery and common complaints which are not covered by usual vision testing. By knowing the patients major motivations, the true functions of refractive surgery will be developed and the standard requirements of engineering together with patient's complaints will be used to build the capital requirements of refractive surgery. Investigations cited, have analysed the behaviour towards refractive surgery in respect to particular techniques, such as radial keratotomy and superficial PRK techniques. Nevertheless, we try develop the true quality characteristics independently, this is to say not to refractive technique in particular, but for refractive surgery as a whole. It will be assumed that the motivations for undergoing refractive surgery will not initially depend on the technique.
5.1.1 Motivation of the PatientsThe typical motivation pattern for refractive surgery will most likely differ in each country, depending on the practical existence of alternatives (glasses or contact lenses), cultural beliefs and typical working conditions. A high myopic poor Chinese farmer, not having access to contact lenses or glasses, working in rainy and muddy conditions, will certainly be easier to satisfy than a typical European who has full access to glasses and contact lenses and working indoors. Even within western countries, motivation can differ a great deal, depending on unquestioned cultural values. Most of the investigations into patient motivation have been done in the US and although these will not represent the world's ametropic patients, they can help to reveal typical motivation patterns amongst western ametropes.
5.1.1.1 Results from the PERK Study in 1984 [PERK84p.1187ff.]The American National Eye Institute Prospective Evaluation of Radial Keratotomy has been a multicenter clinical trial (PERK). Part of the evaluation has been a questionnaire data collection to find out about motivation patterns for undertaking refractive surgery. Before, this evaluation, most ophthalmologists had postulated that people elect surgery for occupational reasons, cosmetic reasons or for reasons of psychological imbalance. methods and questionnaireAbout 400 persons undergoing RK on one eye were asked to fill out a questionnaire answering 140 questions. The questionnaire took about 30 minutes to complete. The questionnaire included items concerning general demographic characteristics, the candidate's reasons for wanting surgery, perceptions of visual ability, role functioning, anxiety and depression. Results have been compared to a rand sample study in order to know whether PERK patients represent typical myopes. resultsdemographic characteristicsSurprisingly there have been slightly more men undertaking the surgery in respect to the women/men ration in the rand study. In comparison with the rand study PERK patients were on average slightly younger, 53% between 25 and 34. The most striking demographic finding has been the high proportion of highly educated and high socio-economic status male volunteers. psychological findingsContrary to expectation, PERK patients were less likely to be anxious, depressed or to function poorly in either their social lives or jobs. motivation for surgeryFor 58% of the men and 73% of the women the most important reason for having surgery was "not to depend on lenses". 13% wore only eyeglasses and had never tried contact lenses. 34% wore both and 53% had tried contact lenses and had returned to wearing only glasses alone. Almost all of the women (94%) had at least tried contact lenses compared with only 81% of men.. The failure rate of 60% was the same, regardless of sex. People were asked what they liked and disliked about glasses and contact lenses. As a result of pre-tests, various likes and dislikes were listed for glasses and contact lenses (easy to look after, hurt nose, easy to lose etc.) The average number of likes in glasses was 1,6 and of dislikes 8.2. 60% of the study patients said the only advantage of glasses is to see better. An average of 3.5 likes and 5.7 dislikes was reported by those who wore contact lenses; women reported both more likes and dislikes. The most frequently mentioned likes were (1) to look better (81%) (cosmetic!), (2) to participate in sports (79%) and to improve vision (62%). Some differences existed between patients' sexes: women more often ticked to look better (89%) and comfort (42%) and men choosing that participating in sports becomes easier (79%). Most who had stopped wearing contact lenses answered that lenses had become inconvenient or bothersome. PERK's comment (shortened)There is no evidence that persons who want radial keratotomy are psychologically unbalanced, nor that their main motivations are cosmetic or occupational. They simply dislike wearing corrective lenses. Patients dislike lenses in two interrelated dimensions. The first aspect is a true fear of being unable to see. Expressions like "What happens, if I am caught in a burning building, ..., or an accident, lose my eyeglasses or contact lenses and then must encounter life threatening obstacles with poor vision?, "Seeing well at all time, without depending on lenses" reflect this dimension. The second motivating aspect, which relates to the dislike of lenses, is the inconvenience of corrective lenses. The following comments referring to eyeglasses and contact lenses were made: references to eyeglasses distorting vision (39%), hurting nose or ears (75%), cost (34%) being a hassle to clean (41%) and interfering with the patient's participation in sports (60%). Contact lenses being costly (46%), being difficult to care for (50%), and being a hassle to put in (65%). While patterns of likes and dislikes for both eyeglasses and contact lenses, are remarkably similar for the two sexes, the few differences that do exist suggest that women have tried harder to use contact lenses, perceive some cosmetic advantages to them, and are more willing to put up with the problems they create. In contrast, men cite dislikes of both eyeglasses and contact lenses and specify difficulties in sports or other physical activities. author's opinionSeeing health as something more than just the absence of illness, suggests that patients undergoing aesthetic or cosmetic surgery do not have to lack self confidence or to have a weak mental character, as has been commonly thought. Why not spend money to achieve good vision without any prosthetic device to restore one's own original look. Direct eye communication is not to be underestimated and can only gain when the eye is released from glasses. Intercultural studies have shown that the average amount ofvisual eye contact in conversations differs from country to country. Not surprisingly these results show a correlation to the country's general acceptance of refractive surgery and the common dislike of glasses. The results of the PERK study clearly indicate that people undergoing refractive surgery have a mental health of above average. The authors of the PERK study deny that there are strong cosmetic reasons for under going refractive surgery. However, what would the main motivation be, if contact lenses did not exist? Figures that 87% have at least tried wearing contact lenses, women even 94%, and the major advantage of contact lenses "to look better" (81%) suggests that for most patients refractive surgery has a strong cosmetic function. Roughly multiplying these figures reveals that for about 70% of the PERK patients the absence of glasses in order "to look better" (appearance) is of importance. Anyway, why do most people wear contact lenses in public and glasses at home? Moreover, it needs to be questioned, whether patients want to accept their true motivations themselves. When patients asked in a questionnaire, where many "good reasons" have been of choice, why should someone still mark cosmetic reasons? In the author's opinion, the cosmetic function is the major reason for the majority of patients undergoing refractive surgery. As most people compare refractive surgery with their problems in wearing contact lenses, the initial cosmetic reason for surgery seems forgotten. For people who have tried contact lenses at least once, the author suggest two different motivation patterns. Both start with the change from glasses to contact lenses for cosmetic reasons in youth. Getting used to many other advantages of contact lenses which they have never really thought of before (even better vision, no rain drops on the eyeglasses etc.) eyeglasses become even less liked. The cosmetic reason might disappear with age. When soft contact lenses start bothering the eye after five or teen years, people look for refractive surgery. Either they still do not like wearing glasses because of "looking worse" (appearance) or because they have got used to the additional benefits of contact lenses in comparison to eyeglasses. A completly different motivation pattern must be looked out for in people who have never tried contact lenses. Cosmetic reasons will probably not be of importance initially, as they have never tried contact lenses before. They do not like spending the additional time necessary to care for contact lenses, and maybe these patients applying for refractive surgery are highly rational, being aware of the long term costs of wearing contact lenses. It seems that many of them are very pragmatic patients and do not mind taking the "risk" of surgery for the advantage of not having to depend on prosthetic devices.
5.1.1.2 Attitudes of Successful Contact Lens Wearers Toward Refractive Surgery [MP96p.128ff.]Mary Migneco and Jay Pepose recently surveyed 133 successful contact lens wearers as to whether they would be interested in undergoing refractive surgery. They had to be older than 21 years and were requested to read information regarding both radial keratotomy and excimer photorefractive keratectomy and complete a questionnaire. The study brought four major results. First, the major reason for undergoing surgery would be the inconvenience of contact lenses. Second, the great majority (77%) would consider refractive surgery, however, the probability that they would undergo surgery at the near future is quite low. Most patients seem to have adopted a "wait and see" attitude and do not represent "early adapters". Third, the decision to have surgery is highly connected to the financial cost, 68% of the patients would not spend more than $500 per eye regardless of the current cost of $1750 per eye. Patients highly underestimated the long term costs of contact lenses. Fourth, people's caution was also the result of their fear of possible side effects mentioned in the information paper, which had been given to them before answering the questionnaire. Interesting to note, however, that a multivariate analysis failed to find a relation between interest in refractive surgery and gender, degree of myopia, or type of contact lens in current or past use.
5.1.1.3 Type of Patient Who Elects SurgeryThe following diagram shows the proportion of patients who had undergone refractive surgery in Washington University Eye Center between October 1993 and March 1994 [MP96]. Although this small sample will not represent statistically the patients undergoing surgery, it will reveal the basic dimensions of interest. Comparing the results with the PERK study of 1984, the proportion seems similar. The author has not collected his own data about the proportion of people wearing spectacles, successful contact lens wearers and contact lens failures. Suggesting that about 50% of the ametropic population belongs to the spectacle wearers, 30% to the successful contact lens wearers and 20% to the contact lens failures, would reveal that the probability of a 'contact lens failure patient' undergoing surgery is about 15 times higher than that of an ordinary spectacle wearer. The probability of a successful contact lens wearer would still be five times higher than that of a spectacle wearer. It seems somewhat obvious that a bothered patient is more likely to want to undergo refractive surgery than a happy contact lens wearer, however, depending on the success of refractive surgery this situatation might change in the future. Any sign that the proportion of successful contact lens wearers undergoing refractive surgery is growing, will indicate that refractive surgery is becoming more accepted.
| Figure 15: Type of Patients ("spectacle wearers" refers to patients that have never worn contact lenses; "contact lens failures" are defined as patients who either changed back to eyeglasses or keep on using contact lenses, although they feel discomfort etc.) | Depending on the condition of the ametropic patient there are five basic categories for adoptions to refractive surgery: ametropia without correction, difficult ametropia, incompatibility with contact lenses, successful spectacle wearers and successful contact lens wearers. Although motivation will differ in each patient, there will be typical motivation patterns in respect to the category of adoption. Interpreting the investigations analysed above and personal experience with myopic patients and myopic friends, the author suggests the following motivation patterns (fig.16). ametropia without correction: "the poor"These patients could wear glasses or contact lenses, but do not so for financial reasons or for missing infrastructure. They will be most likely found in third world countries. difficult ametropia "things can only get better"These patients have refractive problems which can not be corrected properly with glasses or contact lenses. Visual acuity is low and best corrected vision is somewhat below 1. This usually refers to a high myopic patients (above 12 D.), strong compound astigmatism, high hyperopia and certain cases of anisometropia. Although the potential satisfaction after surgery is expectedly high, the settings for surgery will differ in each case and surgical techniques often work at their limits. Their major motivation is to improve their total vision after surgery, followed by the relief from heavy, ugly and often face distorting glasses.
| Figure 16: Motivation Patterns for Choosing Refractive Surgery | incompatible with contact lenses and upset with glasses: "the bothered"These patients achieve best correction with contact lenses and even glasses, but have been upset by these. Often they have tried all kinds of contact lenses from hard to disposable soft lenses. Intolerance in contact lenses can be right from the beginning or the result of ten years' (soft!) contact lens wear. Although there are cases of people who had to change to contact lenses for reasons other than cosmetic, the primary motivation for wearing contact lenses will be somewhat cosmetic (often at the age of puberty), as described earlier. Once they have got used to the additional benefits of contact lenses these patients do not like to return to glasses even if the original aesthetic reasons have become rather unimportant. successful contact lens wearing: "the happy"These patients have been successful wearing contact lenses. If they are wearing soft contact lenses for more than ten years, they have most probably been very strict in cleaning procedures and respecting daily wearing time. They might consider refractive surgery in the future, but are in no hurry to undergo surgery, they will wait until the surgery has been further developed and until the prices have fallen. See 6.1.1.2. wearing glasses: "the brave"These patients have always worn glasses more or less satisfactorily. Older patients tend to belong to this group, as in their youth glasses were the only choice. They have passed the time when cosmetic reasons would give the initiative for change to contact lenses. The attitude towards refractive surgery will most likely differ according to the major reason why they never have thought of wearing contact lenses. Some neglect contact lenses for fear of negative side effects in contact lenses. Refractive surgery would therefore be the last thing they would do. Others, do not really mind wearing glasses, and thinking of costs and the additional time needed for cleaning contact lenses has hindered them from changing to contact lenses. Some of these pragmatic patients might be interested in surgery to get rid of the glasses purely for their "little" but annoying functional inconveniences. For instance, in doing sports, having a shower or during sexual relations. Unfortunately, there have been no studies to find out about patient's satisfaction which correlates the subjective satisfaction variables with clinical measurements as measured visual acuity or treated optical zone.
5.1.2 Complaints and ObservationsBesides motivation for surgery in getting to know true quality characteristics, it is important to analyse the success of refractive surgery. Results and complaints of patients who have already undergone refractive surgery will now be taken into account. Analysing complaints after surgery is a very difficult task. Complaints depend on the type of refractive surgery and its importance in respect to the former expectations. It seems somewhat obvious that patients who even gained in (clinical) best corrected visual acuity will be much less demanding than a happy contact lens wearer. In the latter, refractive surgery has much less to compensate and any side effects of surgery will much easier overweight the original motivation and disappoint the patients. Thinking in terms of TQM, similar complaints will have a different weighting. It is to be expected that a happy contact lens wearer will be frustrated when complaints occur after surgery.
5.1.2.1 Findings in Radial Keratotomy Patients after Surgery [PMA+94]Until the beginning of the nineties, radial keratotomy was the only recognised refractive procedure for low myopia. The investigations which have been done in measuring complaints and patient satisfaction after surgery, refer to this technique. Although the complaints of LASIK patients will be different in quantity and quality, these studies reveal typical observations of patients undergoing any refractive corneal procedures. Power and others [PMA+84] studied patients' opinions two years after surgery. 88 of 101 patients who had undergone radial keratotomy returned the sent out questionnaire. 60% of the patients had been wearing contact lenses in addition to or instead of glasses. Unfortunately the definition of contact lens failure was not used for classification. However, all patient types were covered and the sample was not limited to a specific type of patient. Results for motivation can be interpreted as in the former mentioned studies. 84% were satisfied with the overall results, 70.5% even were extremely satisfied and 76.5% of the patients felt that the benefits "definitely" outweighed the "costs". 86.4% would undergo the surgery again and 81.8% would recommend it to a friend or relative. However, 14.8% claimed overall dissatisfaction, 8 of 88 patients were extremely dissatisfied. For closer analysis results will be grouped: concerning the surgical procedure, changes in vision, changes in lifestyle and changes in appearance. Results will be presented shortly and then discussed by the author after each group. results of the surgery42% of the patients said that the amount of time required to prepare for, participate in and recover from operation was more than the they had expected undergoing RK. Doing refractive surgery by means of LASIK, the time of surgery and time of recovery will be much less. changes in visionIn quantity 85% of the patients reported that their vision was greatly improved, 9.2% said there was somewhat of an improvement, 1.1% said there was no change, 1.1% said that vision was somewhat worse and 3.4% claimed vision was much worse. Spoken in characteristic 85% reported "some subjective visual problems" postoperatively (glare, starbursts, fluctuations in vision, awareness of incisions, scaring and regression of the effect). 2.2% said ghost images, eye dryness, astigmatism, headaches or squinting were problematic. Forty-two patients specified the length of time their problems persisted with half of them bothered beyond one year postoperatively. Nearly all of these patients mentioned glare or starbursts as continuing problems. About 40% reported still wearing correction lenses. At the first glance results of vision seem quite promising. However, it is questionable whether an improvement in vision which can be reached with glasses may allow that more than three people in a hundred lose in vision. Moreover, this loss in vision can barely be improved by any kind of lenses. Unfortunately, the cited study is not very specific when it comes to complaints. If 85% of patients report "some subjective visual problems", their will at least exist a "high potential of improvement" following the very optimistic style of this study. Concerning these visual problems which will barely be detected by the common Snellen test, patients undergoing LASIK will suffer much less from these problems. Glare and starbursts result from the small optical zone in RK. Daily fluctuations are a result of the weakening of the cornea by incisions in RK. In accordance with the first chapter these unexpected effects can be classified by system theory. A naive patient will hardly think of these undesired effects before surgery. They reveal necessary requirements for future techniques as LASIK. changes in lifestyleIn quantity 50.6% reported no change in lifestyle, 43.5% reported improvement, 5.7% reported that their lifestyles were somewhat worse and no one claimed that lifestyle was much worse due to surgery. Patients were also asked to report changes in specific areas of lifestyle. 82.3% reported that dealing with inconveniences of glasses became better, 62.8% noticed increased confidence in appearance, in driving 31.4%, in performance in sports activities. 34.9% experienced a reduced embarrassment and 24.9% a reduced fear. 10% of the patients rated confidence in driving worse or much worse. Many answers given in open-ended question fell into three categories: easier participation in recreational activities, getting up at night without fear and the participation in sexual activities without the annoyance of glasses. The analysis of changes in lifestyle can be very informative. It can show if expectations have been fulfilled and what kind of positive unexpected improvements arise. It also gives light to typical complaints, and changed behaviour can reveal any losses of visions not covered by clinical examinations. The most mentioned motivations for undergoing surgery in this sample, to improve vision and avoiding the inconveniences of glasses (weight, fogging , etc.) have also been mentioned most after surgery. "Surprisingly" appearance improved after surgery which has not been mentioned strongly as a motivation before surgery. The author, however suggests that appearance was not thought of before surgery because most patients had already used contact lenses and therefore "solved" their appearance problem already. Although only 4.5% had answered that vision had become worse, 10% answered that confidence in driving became worse. This fact reveals that at least 10% lost quality of vision. appearance and satisfactionFurther analysis showed that increased confidence in appearance was significantly correlated to overall satisfaction (r=0.52, P<.0005). The authors of that study seemed somewhat "relaxed" that the correlation between vision and satisfaction was even higher (r=0.77). Frankly, it is nothing but surprising that quality of vision comes first, this is to say, before appearance. Any patient who has significantly lost vision will hardly be satisfied if his appearance has improved. Contrary to the authors of that study, the author of this thesis comes to the conclusion that improvement in appearance is a major motivation for undergoing surgery. Improving unaided vision is the means to achieve this, and for some patients it might also be the major motivation itself. Keeping best corrected vision will be essential even to maintain satisfaction high, and improving overall best corrected vision must be the challenge for ophthalmologists society.
5.1.2.2 Complaints of Bothered Contact Lens Patients after SurgeryHaverbeke investigated the reasons and the complaints of bothered contact lens patients in undergoing RK in 1992 [Hav92]. In contrast to the mentioned American studies, the sample of this European study has been limited to contact lens wearers. The vast majority of the patients were bothered by their contact lenses. Moreover, Haverbeke is convinced that from the optical viewpoint contact lenses are a better mode of correction than radial keratotomy. However, all patients preferred RK to contact lens wear. Before surgery one third had been wearing rigid lenses and the other two thirds soft contact lenses. Reasons for undergoing surgery were Intolerance (73%) and Discomfort (31%) only two patients had had no problems with their contact lenses. Concerning the complaints of the patients, although no one has regretted surgery, impaired night vision has been mentioned by half of the patients, glare by 10%, suboptimal visual acuity 7%, astigmatic blur, headache 5% increased sensitivity 4% and near vision problems 3%. As already mentioned complaints will depend on the kind of refractive surgery used. However, impaired night vision, glare and starbursts have been the most common complaints in refractive surgical techniques due to the limited optical corrected zone.
5.1.3 Functions of Refractive SurgeryHaving discussed motivation and the complaints of patients undergoing refractive surgery different functions of refractive surgery to the patient can now be defined. The functions should be defined independently and systematically to allow easy classification. If functions can be ordered in sequence according to the general benefit improvement of the patients, it could help to weight the importance of each case and to match expectations with possible outcome. The scheme developed here will hold these requirements. Although each case will be unique and personal benefits might differ from the scheme, the scheme can help to find out if the expected benefit will justify surgery in special conditions, e.g. low potential quality of the patient , discussed later on. When describing the functions of refractive surgery, it can be in relation to no correction, to spectacles and to contact lenses. Defining the functions of refractive surgery on the base that there are no alternatives would be most promising but unrealistic. The following approach takes spectacles and contact lenses into account, as refractive surgery must compete with all alternatives available. The scheme of functions is defined in words understood by patients. The use of ophthalmologic terms or norms would limit patients true quality indicators and judge patients need at this stage. Often used rules of thumb expressed in limits of diopters or of visual acuity do not take into account the special circumstances of each patient. The following scheme must be read downwards when analysing a patient's case, the first function which applies to the patient will be the strongest justifying motivation in undergoing refractive surgery. Functions following the first important and applying function will be positive "side-effects" when undergoing surgery.
| Figure 17: Functions of Refractive Surgery | To give some examples, for a high myopic patient of about ten diopters who does not tolerate contact lenses, refractive surgery will function with point three. His best corrected vision with spectacles will most likely improve undergoing refractive surgery, due to the distorting effects of the spectacles at this magnitude and due to a higher resolution after surgery. The same patient tolerating contact lenses perfectly but "suffering" from having to get up after sexual relations to get out his contact lenses, refractive surgery would work by function eight, as the contact lenses already supply best vision. In a child suffering from high hyperopic refraction unable to tolerate contact lenses, refractive surgery can work with function one. In cases like this refractive surgery can help to safe the ability processing vision in young years, if other methods fail. A myopic youngster, 21 years, not tolerating contact lenses only "wanting to improve his chances finding his future spouse" would classify for function seven. Function five and six might not be self explanatory for the typical western patients, they refer to poor conditions in third world countries. For instance, regarding a poor (in terms of capital) Chinese farmer refractive surgery will work with the function five or six.
5.1.4 Requirements for Refractive SurgeryAs refractive surgery is in most cases of selective nature and competes with very successful and common alternatives, it must hold much higher requirements than any other surgery. In the second chapter the system approach was already shortly introduced for judging different kinds of refractive surgery techniques. The discussed complaints of patients who had undergone surgery will also serve in building a scheme of requirements. The requirements can be divided into two areas: minimal risk at surgery, and the successful outcome in the long run. The magnitude of the potential risk at surgery can be divided between the probability of occurrence, and the amount of the possible negative effect, some risks might even been eliminated. For instance, the loss of power at the moment the microkeratome is in use, can have fatal effects on the patients cornea, even if the internal power supplies starts working after a couple of seconds. A power bridging grant can eliminate this risk. However, eliminating potential risks in relation to equipment failure are discussed in 6.5.2. Hints in handling typical problems at surgery are given in 6.4.2.2. The requirements for successful outcome will be discussed by the means of standard engineering requirements. The vocabulary has been introduced in 2.3. The requirements (list) has been ordered according to basic importance. Stability is divided into long term stability and into the absence of daily refractive changes induced by surgery. The requirement of long term stability is further divided into refractive and non refractive stability. In addition to the loss of refractive stability refractive surgery might induce eye diseases such as a keratoconus. The second major requirement block concerns robustness after surgery. Might surgery limit good vision to a sunny, 20o C condition? In older techniques there have been three typical areas where refractive surgery has strongly weakened vision: night vision, height (poor vision while mountain climbing!, because of slow refractive "adjustment") and lowered stability to injuries. Deviation from target must be specified according to deviation from intended refractive correction (diopters) and to loss of best corrected vision due to decentration. The requirements concerning the behaviour in the transitional phase seem somewhat less important. However, they strongly influence the willingness and ability of the patient to undergo refractive surgery.
| Figure 18: Requirements for Refractive Surgery | Certain limits of these variables can be used as adequate substitute quality indicators. For instance, the size of optical zone treated may represent the quality of night vision. The used term "kind of effect" means what kind of technique has been used to correct refraction. As already explained in second chapter, principally the intrastromal keratomileusis seems to have best prediction. It directly changes the refraction, this is to say without changing the biostatics of the eye, and the refraction will not depend on wound healing. Once the best technique has been chosen, in our case LASIK, the requirements can only be satisfied further by few parameters (underlined). The amount/depth of tissue left after surgery strongly determines the stability. The size of the treated zone determines night vision. However, the bigger the optical zone less tissue will be left. The original thickness of the cornea limits the kind of tissue which can be removed. The thickness of the cornea in each eye is therefore a major potential quality indicator. This fact will be thoroughly analysed in 6.2.3. The original radius of the cornea to be treated somewhat influences the later reached refraction. Flat corneas tend to be overcorrected with standard laser ablation software [Bar96]. Ablation software could take the cornea's radius into account to bring changed refraction closer to intended refraction.
In cases of anisometropia, eyeglasses for both eyes differ significantly in thickness and shape, therefore one eye often appears much bigger than the other.
In the beginning of refractive surgery, occupational reasons were thought to be of major motivation. Illustrated by the cook either losing his contact lenses or having foggy eyeglasses while looking in the kettle.
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