This chapter is the application of the developed approach to apply Total Quality Management in LASIK surgery. First, patients wishes will be analysed. Then the Model of Meyer is used to apply Total Quality Management in LASIK surgery.6.1 True Quality Characteristics of Refractive Corneal Surgery
As earlier mentioned, application of TQM is strongly connected to the wishes of the customers. It 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 to what 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 order 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.6.1.1 Motivation of the Patients
The 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.184.108.40.206 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 questionnaire
About 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.
Surprisingly 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.
Contrary to expectation, PERK patients were less likely to be anxious, depressed or to function poorly in either their social lives or jobs.
motivation for surgery
For 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.
Seeing 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 completely 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.220.127.116.11 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.18.104.22.168 Type of Patient Who Elects Surgery
The 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 situation 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.
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.
incompatible with contact lenses and upset with glasses:
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 cosmetic2, 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 22.214.171.124.
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.
Besides 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.126.96.36.199 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 surgery
42% 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 vision
In 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 lifestyle
In 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 satisfaction
Further 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.188.8.131.52 Complaints of Bothered Contact Lens Patients after Surgery
Haverbeke 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.6.1.3 Functions of Refractive Surgery
Having 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.
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.6.1.4 Requirements for Refractive Surgery
As 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 introduced for judging different kinds of refractive surgical techniques. The discussed complaints of patients who had undergone surgery will help 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 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 184.108.40.206.
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 (figure 18) have been ordered according to basic importance.
Stability is divided into long term stability and into the absence of daily refractive changes induced by surgery. 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.
The third column points out the variables to fulfil the requirements. 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 (last column). 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 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.6.2 Potential Quality of the Patient
The patient influences the result of the surgery in three different
areas: his behaviour during his stay in the clinic, his personal
situation and his eye profile.
6.2.1 Interactivity and Integration
The interactivity with other patients and the ability of integration can effect the outcome of the surgery. However, both depend strongly on the ability of the institution in contact and specification, as discussed in the Model of Meyer. The process needs to be managed to induce best interactivity and integration of the patient. This will be done in detail in chapter 220.127.116.11.2 Life Situation
Taking into account the patients life situation before undergoing surgery is of upper importance to ensure best satisfaction after surgery. The ophthalmologist needs to listen carefully to the individual situation and arguments which concern the Refractive Surgery. For our circumstances quality of vision can be divided into quality of near vision, quality of far vision and quality of vision in difficult circumstances (dark, rainy, blended). However, Refractive Surgery is somewhat focused on quality of best far vision without taking a patient's circumstances into account due to the common focus on Snellen vision examination. The need for near vision and excellent vision at night has been neglected. It is important to know which vision will be regarded most important when undergoing surgery.
In many cases the patient might need to choose between having unchanged good night vision but still wearing glasses, or to be emmetropic but having very poor night vision.
Myopic patients bear the advantage that they do not need presbyopic correction as quickly as emmetropic patients. Often they can see until their mid-sixties without adjusted glasses for near vision. A patient having one diopter of myopia can handle his life until the mid fifties without glasses, instead a emmetropic person will need glasses to read his newspaper by the age of 45. Moreover, a high myopic person using glasses has a higher dioptric range than a contact lens wearer or a emmetropic person due to the Galilee lens configuration. This accustomed advantage will be lost after Refractive Surgery. Moreover, near vision from 0.2m to 2m is a very common working position in today's world of information and computing. Taking into account how much time the patient spends in near vision each day in respect to the patient's age3 will help to improve patient satisfaction. The author suggests that the patient should qualify his importance about the different fields of vision.6.2.3 Eye Profiles
Eye profiles differ in each case. In regard to Refractive Surgery there are various factors which limit the potential quality of surgery. In the first case there exist some contraindications for undergoing LASIK. These will be listed in the last chapter and not further discussed in this thesis. Anisometropia and Ambliopia must be dealt with similar to the use of contact lenses. However, as surgery is not reversible, sufficient simulation with contact lenses must be done before final surgery. For further reading see [Zei95] and [LFH96]. Correct treatment of regular astigmatism depends more on the laser and the knowledge of the surgeon than on the patients profile. This will be discussed in the chapter Potential Quality of the Clinic. Besides these circumstances, each eye has different characteristics which can limit surgery and potential outcome of the surgery, especially if ametropia is high. Furthermore, the factors which determine the potential outcome depend on whether the person is myopic or hyperopic.
The following two diagrams show the induced changes in magnification and success in accommodation due to moving spectacle correction to correction at corneal plane (either contact lenses or Refractive Surgery). The first picture reveals that in myopia, magnification is increased significantly just as a result of changing spectacle correction to the corneal plane. This magnification is equal to a gain of two lines at 5 diopters. In hyperopic correction the effect is opposite. Whereas in magnification the myope wins and the hyperope loses, in success of accommodation the myope loses and the hyperope wins. However, the latter effect only seems to be significant in high emmetropia.
Figure 14: Magnification and Success of Accommodation [AH93, LFH96]
Of major importance will be the understanding of the optical zone. However, the optical zone in Refractive Surgery has different meanings. This often leads to the misconception that the optical zone treated by laser equals the obtained zone (fig.20). The treated zone has the largest diameter, the actually best corrected zone will be somewhat smaller. The difference between the two zones depends on the kind of treatment, e.g. myopic or hyperopic, and the ablation software. In LASIK wound healing does not smallen the optical zone, but the corset function of the outer flap will smoothen and therefore smallen the spherical optical zone. The quality of vision, in particular night vision, will be determined by the achieved size of the spherical zone. In many studies authors do not clearly distinguish between the size of the spherical zone and the zone of treatment. Only in spherical myopic ablation they are identical but in aspheric (multizone) and hyperopic ablation the inner spherical zone is the most determining factor with regard to quality of vision. Definitions can become even more confused, if taking into account the 'fill in' for central islands. However, to keep things simple and correct the author suggests distinguishing between the diameter of the whole treated zone and the diameter of the obtained spherical zone. The obtained spherical zone will be the major quality indicator for quality of vision at night. The diameter of the treated zone determines the needed flap size. The difference between both zones can indicate an additional improvement of vision. In single zone myopic treatment the size is practically the same, in hyperopic and in aspheric (multizone) treatment the diameters can differ of up to 4mm. The diameter of the treatment zone should be about 1.5mm smaller than the diameter of the flap. The flap can barely be centred like the laser. Moreover, the hinge of the flap decreases the actual diameter of the flap.18.104.22.168 Potential Quality of Myopic Eyes
1.thickness in the centre of the cornea
2.major centration difficulties
In myopic treatment the thickness of the cornea in the centre is of upper importance. It is the cornea's spot where most tissue must be ablated to correct myopia. Unfortunately, it is also the cornea's thinnest point with a thickness of only530µm in the average eye. The long Barraquer experience tells us that in no circumstances should more than one third of the cornea's thickness be removed. Moreover, at least 400µm should be left to keep the static's of the cornea unchanged and ensure stable refraction. This is to say that in the centre about 130µm can be ablated to change refraction. This will be enough for about 10 diopters with a spherical zone of about 6mm. For people with myopia above 10 diopters the spherical zone must be decreased to allow higher dioptric ablation without ablating more than 130µm. However, a smaller spherical optical zone will cause a loss in the quality of night vision.
So far, it has been common to limit ablation to 100µm allowing ablation of about 8 diopters with optical zones of 6mm. This rule is quite simple and on the safe side for most eyes. Nevertheless, with this rule of thumb it can happen that a patient with a very thin cornea of about 430µm would be ablated much below 400µm, taking the risk of corneal instability. More likely, patients with very thick corneas of 570µm and high myopia will be operated on with small zones losing night vision, although they have sufficient potential to be treated with larger spherical zones. With today's ablation software the ophthalmologist would have to try various optical zones before surgery and see how much would be ablated.
In the author's point of view, software needs to be changed in the way that additional to the programmed diopters the thickness must be entered and that the software calculates the largest possible spherical diameter securing that 400µm are always left. The software could be designed so that a second person authorised by the laser manufacturer must confirm the cornea's thickness of each patient to insure that the thickness is measured well and has not been invented.4
The absence of centration difficulties is the second potential quality of the patient's eye in myopia is. Decentration will cause irregular astigmatism and result in a significant loss of best corrected vision, although this can be retrieved to a certain extent, everything should be done to prevent decentration. Nystagmus make it difficult to hold centration a damaged iris can make it difficult to find the centre.22.214.171.124 Potential Quality in Hyperopic Eyes
1.radius of the cornea
2.depth of the eyeball
3.any factors influencing centration
asymmetric iris (corectopia)
4. thickness of cornea 2.5 mm of centre (above 9 Diopters)
The proper correction of hyperopia requires a flap size of 8.75mm to 9.00mm, as the zone of treatment will be around 8.25mm to 8.50mm for an optical spherical zone of 6.0mm. However, in high hyperopia the eye is often very small, which makes it difficult to use common microkeratomes. Only few microkeratome can be fixated properly. The radius of the hinged flap in small eyes will barely be above 8mm. Centration is even more important and difficult. Ablation in hyperopia is deeper at the periphery (6mm) of the cornea and the amount of tissue ablated is much higher in hyperopia than in myopia, therefore surgery will be twice as long. The thickness of the cornea in hyperopia will hardly be a problem as the cornea is much thicker in the periphery than in the centre.
The following calculation (fig.21) concentrates on the decreased area to be treated due to the hinge of the flap. Calculations are simple, however they reveal the real size of the stroma to be treated without ablating the back of the flap. Suggesting a cut with a diameter of 9mm and a hinge with an arc of 300,, that is to say (l=2r sin (a/2)) with a 2.33mm wide hinge, will only leave a diameter of d'=8.84mm. However, taking full advantage of this "hinged diameter" the cut must be a little of the center torwards nasal side to fully center the zone of treatment. In reality this will be difficult to manage. So if the flap has been centred the diameter available for treatment is only d''=h+h, here d''=8.52mm.
Figure 14: Largest Diameter of Treated Zone
Any circumstances which do not allow proper centration of the ablation make surgery in these conditions impossible. Even it will take all the experience of the refractive surgeon. Treatment of hyperopia in small eyes should only be done by the most experienced surgeons. Detailed discussion about optical zones will be done in the following chapters.
The principal factors determining the potential quality of the patient have been discussed. Now with the focus on outcome quality, the different factors need to be matched utilising all potentials to allow and provoke best individual outcome.6.3.1 Optical Zone And Pupil Size
An important goal of Refractive Surgery will always be to obtain large corrected optical zones for best quality of vision in all circumstances. However, there exist different factors limiting this goal. In the LASIK procedure, this will be the maximum possible flap size and in myopic correction the thickness of the cornea in its center.
It will be very useful to know the minimum diameter of spherical optical zone to be corrected to obtain best vision in all circumstances. It seems obvious that, if the full corrected zone is smaller than the pupil size, entering light will be broken in different ways and quality of vision will drop. However, pupil size can be 8mm in size (in excited dark circumstances), so adding an additional millimetre of tolerance would suggest that the full corrected zone must be about 9mm. Even neglecting the fact that this would require a flap size of at least 10mm in diameter, the 130µm ablation would already be exceeded with 4 diopters.
Fortunately, the pupil is only about 3.5mm in diameter in usual daily circumstances. Moreover, even the emmetropic eye only has a spherical zone of about 4mm, so the cornea becomes aspheric anyway. The cornea is continuously losing about 3 diopters from the 4mm zone to the 9mm zone. Again, the pupil barely opens more than 6mm for reasons of vision.
These observations lead to the conclusion that an obtained spherical zone of less than 4mm will probably reduce the quality of vision significantly. It also suggests that a strong refractive edge somewhere between 4mm and 6mm might reduce the quality of vision when the pupil size exceeds the 4mm range. Taking into account the fact that in LASIK the corset function of the flap will produce a kind of transition zone, the intended spherical corrected zone should be 6mm. The refractive edge would be just outside the 6mm zone and the corset function of the flap already induces a aspheric zone at about 5mm. Maybe, a 5.5mm spherical zone with a smoothing zone to 6.5mm might be an improvement. A 6mm spherical zone will allow correction of about 10 diopters of myopia not exceeding the 130µm ablation in respect of the average 530µm cornea thickness. For more details about the amount of ablation see the following subchapter.
The experience with PRK treated patients has shown that night vision strongly improved when enlarging the spherical zone from 4mm to 5mm. Less than 5% of patients treated with spherical zones of 5mm reported that halos were of significance compared to about 18% treated with 4mm zones. [LFB93]. However, an intended PRK optical zone will turn out somewhat smaller compared with LASIK zones. Woundhealing at the edge in PRK makes the achieved spherical zone smaller.
It still seems surprising that with a treated zone of about 5mm or even 6mm best quality of vision is reached, although the pupil can open up to 8mm. However, the human vision only works in sharp, colour "camera" conditions with sufficient illumination (>02 cd m-2). In these perfect conditions the pupil is smaller than 3mm. This vision is called photopic. In surroundings where illumination is between 102 to 10-3 cd m-2 vision is received mesoptic, below 10-3 cd m-2 vision is received skotopic then the pupil is opened to its maximum. Sharp vision and the sensitivity to ametropia is only possible when cones are responsible for vision, its to say at photopic vision [Sch95]. In mesopic condition vision reception is shared by cones and rods. Rods do not allow sharp and fixed vision. Although in lower light conditions some cones still allow sharp and coloured vision, it will be most unlikely that these cones with low sensitivity can detect the few misguided rays caused by the aspheric refraction in the outer cornea periphery. The density of cones is highest in the center of the retina, this area, below the angle of 30, is called fovea. At 100 density of cones has already dropped to less than 15% of its maximum.
Roberts and Koester [RK93] examined the optical zone diameters to produce a glare free field in computer simulation. They suggest that the optical zone must be based on the postoperative corneal curvature because that determines the magnification of the pupil. The minimal optical zone diameter of uniform power was determined in respect to chamber depth and for various myopic and hyperopic situations.
The results of this simulation have been:
Glare free at 00 will be reached when the uniform corrected zone is equal to the postsurgical entrance pupil. Obtaining a glare free field at increasing angle will need a greater uniform optical zone than the postsurgical entrance pupil. With an increasing chamber depth the glare free field will decrease. The postsurgical entrance pupil will appear smaller after myopic surgery and bigger after hyperopic surgery. In theory this means that in myopic surgery the radius of uniform correction can be slightly smaller than intended and in hyperopic surgery the radius of uniform correction must be somewhat larger. A myopic patient of 10 diopters perfectly treated with a uniform zone of a diameter of 3.82mm will be glare free at 00, as with the same real pupil size before surgery, this is to say a former entrance pupil of 4mm. In contrast a hyperopic patient with a preoperative entrance pupil of 4mm whose cornea was to be stepened from 43 to 59 diopters would require a 4.11mm optical zone for glare free vision at the fovea.
However, in the opinion of the author it does not make sense to adjust the ablation zone to the amount of ametropia, if maximal difference is less than 0.5mm in diameter in a range of 23 diopters. The simulation might even provoke an adjustment of the treated zone in regard to chamber depth. However, the chamber depth does not change the minimum diameter needed to obtain a glare free field at the fovea. The simulation is based on a full uniform eye model. Nevertheless, the human eye is aspheric above a 4mm uniform zone anyway, therefore creating a 7mm uniform zone would be far above the target.
The analysis of the typical anatomy of a healthy cornea, the short discussion about the more complex vision reception in human eyes and the experience with PRK treated patients in relation to treated diameter and complaints in night vision, lead to the conclusion that a uniform spherical zone of 4 mm with an aspheric smooth outflow of about 2 diopters difference at the 6mm zone will allow best quality of vision. A smaller optical zone will decrease the quality of vision at night. A much larger uniformal zone will do no harm, but is unnecessary. If the cornea's thickness allows this perfect correction, this should be the assured quality level. The quality target and even the quality standard must be somewhat above this assured quality level. If the spherical optical zone of the laser is only 4mm, the corset function of the flap will reduce the reached uniform zone.
Moreover, any decentration will induce an area in the 4mm pupil zone of undercorrection. Amano and others [ATS94] found that a decentration of up to 0.5mm had no significant influence on the visual function when the pupil is below 4mm in diameter. However, at 4mm due to decentration, not all the pupil will be covered with a uniformal zone. In their PRK study where centration is somewhat easier than in LASIK, the mean decentration was 0.51+- 0.31mm. The quality standard in respect to a uniform optical zone should be 4.5mm ensuring that in common minor decentration a 4mm entrance pupil is still fully covered with an uniformal zone. Visual dysfunction due to major decentration above 0.5mm cannot be solved by enlarging the optical diameter. Keeping decentration below 0.5mm will be a different quality factor and discussed elsewhere. The corset function of the flap must also be taken into account. A treated uniform zone of 4.5mm with a refractive edge at 4.5mm would most likely be lessened by the flap.
Therefore the uniform treated zone must be even larger, knowing that the untreated eye already is aspheric in the outer zone, a uniform spherical treatment of 6mm will not change the amount of natural aspherity. The corset function of the flap will then smoothen the left refractive edge at 6mm. However, it will be important that the refractive gap will occur at the periphery, the illumination at an entrance pupil of 6mm will not allow sharp vision to notice this refractive gap126.96.36.199 Corrected Diopters In Respect To Presbyopia
The typical Snellen test is focused on near vision, due to this
fact many ophthalmologist don't really care about a proper correction
of near vision. Disregarding the fact that many people will never
really depend on perfect far vision but almost everyone will depend
on correction of near vision at some age.
For the satisfaction of the patient above the age of 40, it should be a must to question him whether he prefers to use glasses for near vision or for driving a car and other far vision circumstances. However, this question should be supported by a short questionnaire to help him reach the best decision. Taking into account this questionnaire and the age of the patient, a myopic offset from emmetropia should be calculated to allow accustomed near vision without glasses. It must be mentioned that in patients with very thin corneas, a reduced target refraction will help to reduce loss of night vision. As earlier mentioned, the myopic offset can usually be calculated in relation to age. Only if near vision, even without additional glasses, is essential, should the myopic offset be measured. The following table [LFH96] will give the needed offset in relation to age, if the patients prefers to be without glasses at near vision or if he wants total correction for perfect far vision.
This table reveals that a patient treated at the age of 45, with
a myopic offset of 1 diopter will need to use glasses again for
near vision 10 years later, then still needing opposite correction
for perfect far vision. Taking into account the total offset of
3 diopters already at the age of 45 will mean that the patient will
not need a second pair of glasses at the age of 55. A good compromise
would be an offset of 2 diopters allowing that vision is perfect
at near vision to the mid fifties and even allowing sufficient handling
without glasses to get around for the rest of their life. If the
patient is ambliope with best corrected vision above 0.7 in both
eyes, it might be useful to correct one eye to emmetropia and the
other eye with an offset of 3 diopters, even at the age of 35. However,
this situation should be simulated with contact lenses. Thereby
the decision of which eye will get the myopic offset must also be
taken. The best sequence for decision making is shown in the flowchart
5.3.3Matchment Scheme For High Myopic Correction
Here, the focus will be on the relation between the size of spherical optical zone and the depth of ablation. In other words the individual matchment of parameters for myopic patients to allow best possible night vision without violating the stability of the eye.
If individual thickness allows full target correction at 6mm this will be the optimal choice to keep best corrected vision after surgery. However, if degree of myopia or a thin cornea do not allow full target correction the patient should be informed that with a smaller zone his quality of night vision might suffer. He then might choose between good night vision but still needing spectacles or a reduced quality of night vision. However, the spherical corrected zone should never be smaller than 4.5 mm. 4mm might just be sufficient, but only if centration is perfect. Changing the diameter of spherical ablation from 6mm to 4.5mm will allow about an additional 2/3 of the possible correction. For instance, if the patients cornea is only 490µm at its center, 90µm can be ablated without risk. This will allow a uniform ablation of 8 diopters at 6mm or 15 diopters at 4.5mm. Regarding the patient's initial target refraction change will be 12 diopters, he might choose between a full correction and diminished night vision or still wearing glasses of 4 diopters. Today's patients are barely asked to answer this question, most ophthalmologists will choose a smaller diameter. Even if they were asked, no patient would really know, if he depends on good night vision. The proposed questionnaire in respect to one's lifestyle can help to answer the question. In respect to the importance of night vision the diameter can be decreased, if 6mm diameter can not be used for the reason of cornea's thickness.6.3.4 Clinical Quality Indicators
Knowing the true quality characteristics of the patient, having
matched the individual potentials to reach the best patient satisfaction,
clinical quality indicators can are now selected to observe and
control the desired quality. The quality indicators will be divided
into integral and specific quality indicators (fig.25, fig. 26).
Figure 14: Integral Quality Indicators
The integral quality indicators most quickly reveal whether surgery has been done perfectly or only a fair outcome has been reached. They clearly demonstrate if clinical standards are reached or not. However, they do not explain the reason for any abnormality. Here, the specific quality indicators will give further information on what has not been perfect, if quality standard has not been reached.
Figure 14: Specific Quality Indicators
6.3.5 Satisfaction of Patient
Parallel to the clinical measurement the patient should be asked about his subjective opinion. The questionnaire should be filled out computer-assisted and data should be processed automatically to keep staff from time consuming statistical work and to guarantee complete statistical process control. At least once that is to say six months after surgery the patient must be asked about his experienced satisfaction. However, if data is processed automatically patients should be asked systematically at all postsurgical controls to observe any changes. Taking into account latest psychological findings in the design of questionnaires will be essential in obtaining representative data. Figure 27 outlines important points of interests to be asked in the questionnaire to qualify patient's satisfaction. First the patient should be asked about his general satisfaction of having undergone Refractive Surgery. There he should compare his reached satisfaction with his expectations before surgery. Then a question should ask him about any recommendations, advantages and disadvantages of having undergone this surgery. It is important to ask this unguided question in the beginning, that is to say before asking any specific questions. Now the patient can give marks about specific areas of his vision. A second set of questions is focused on the transitional phase shortly after the surgery. The opened question about complaints and recommendations should be repeated. Asking this question twice, that is to say before and after specific questioning will give a good representation about the importance of complaints.6.3.6 Statistical Process Control
The quality indicators should be measured and processed at all stages
of the process, before, during and after the surgery. If quality
indicator are fully processed, statistical process control (management)
becomes possible. On the short run the clinical quality indicators
reveal any offsets. Processed data of clinical quality indicators
can quickly reveal trends, shifts or even cycles. Defects can be
realised as soon as possible. On the long run the clinical quality
indicators and the data about patient satisfaction allow even greater
improvements. SPC helps to locate the bottleneck’s. They processed
data points out the most important areas to be managed.