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Potential Quality of the Patient
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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. 5.1.1 Interactivity and IntegrationThe 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 need to be managed to induce best interactivity and integration of the patient. This will be done in detail in chapter 6.4. 5.1.2 Life SituationTaking 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 age will help to improve patient satisfaction. The author suggests that the patient should qualify his importance about the different fields of vision. 5.1.3 Eye ProfilesEye 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 19: 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. | Figure 20: Treated Zone and Spherical Zone |
5.1.3.1 Potential Quality of Myopic Eyes1. thickness in the centre of the cornea2. major centration difficulties· nystagmus 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. 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. 5.1.3.2 Potential Quality in Hyperopic Eyes1. radius of the cornea2. depth of the eyeball3. any factors influencing centration· unknown centre· missing orientation· 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 21: Largest Diameter of Treated ZoneAny 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.
Eye's accomodation to near vision decreases strongly with age. The relationship between age and loss of accomodation is very predictible. As the deviation from target correction is still high, individual near vision examination does not make much sense. This feature might seem somewhat buerocratic to some surgeons, but is to the benefit of the patients. Moreover, it does not take the decision away from the ophthalmologist, it rather provokes well discussed decisions. | | | |
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