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LASIK |
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Potential Quality of the Patient
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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. 5.1.1 Optical Zone And Pupil SizeAn 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 gap. 5.1.2 Corrected Diopters In Respect To Presbyopia | age | myopic offset | | at 45 years | 1 diopter | | at 48 years | 1.5 diopter | | at 50 years | 2 diopters | | at 55 years | 2.5 diopters | | at 60 years | 3 diopters |
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| Figure 22: Myopic Offset in Relation to Age |
| Figure 23:Myopic Offset for Presbyopia |
5.1.3 Matchment Scheme For High Myopic Correction| Figure 24: Matchment Scheme for High Myopia |
5.1.4 Clinical Quality IndicatorsKnowing 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). | quality indicator | unit | target | standard | inspection | assured | | no loss of stereoptic vision | | none | none | few | few | | change in BCV contact lenses | Snellen, lines | 0 | 0 | -1 | -2 | | spectacles | | expected gain/loss due to magnification | expected -0 | expected -1 | expected-2 | | uncorrected vision (only BCV patients of 20/20 or more and target correction= emmetropia) | Snellen,20/20 or more20/25 or more10/20 | 20/20 | 20/25 | 10/20 | 10/20 |
Figure 25: 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. | quality indicator | unit | target | standard | inspection | assured | | precision | | | | | | | offset from target correction (6month posts.) | diopters | 0 | <+1, >-2 | >+1, <-2 | <+2, >-3 | | centration (only measured when target is center of entrance pupil): | mm | 0 | <0.3 | >0.5 | 1.0 | | induced astigmatism | diopters, angle (for discovering systematic faults) | 0 | <0.5 | >0 | <1 | | night vision, contrast sensivity | | | | | | | interface visible | not visible,barely visiblevisible | not visible | barely visible | visible | - | | faults | occurrence, why | none | none | any | | | fully cut flap, lost flap, lost suction while cutting, others | | | | | |
Figure 26: Specific Quality Indicators 5.1.5 Satisfaction of PatientParallel 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. | Figure 27: Satisfaction of Having Undergone Surgery |
5.1.6 Statistical Process ControlThe 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.
The growing aspherity at the periphery might have its natural purpose to stimulate a greater zone of rods to better detect movement at side angles. Moving dangerous objects can be observed quicker. Once noticed the head can then be turned towards the moving object for sharp identification. For this purpose the direction of asperity will rather be unimportant, an asperity of +1 or -1 diopter will stimulate a very similar area of rods. However, negative aspherity widens the range of visual field. A -1aspherity will therefore be better than a +1 aspherity allowing that a moving (dangerous) object will be detected at an even greater angle. If aspherity becomes bigger, the light of a moving object will be distributed to a larger area of rods and the partial light received by each single rod will not be enough to detected at all. | | | |
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> TQM in Lasik > Outcome Quality
