الفهرس 
Causes and Sources of Vision Blurring In The Human EyeOnly 14 pages are availabe for public view
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Abstract
Refractive surgery is becoming one of the most popular surgeries performed nowadays, and as the number of candidates undergoing such procedures continue to grow tremendously, a not to neglect number of dissatisfied patients evolves dramatically in a way that can’t be neglected, therefore the term ”unhappy 6/6 patients” was postulated to refer to the group of patients who can actually see 6/6 on the high contrast acuity chart but are having troubles with low contrast vision and night vision as well, this has led to the researchers paying attention to a new refractive entity; the aberropia, at which the patient may have been corrected on the low order aberration level but neglected at his own high order aberrations side, or even ended up with induced, new visually disabling high order aberrations.
High order aberrations with clinical interest are mainly spherical aberrations, coma and trefoil; spherical aberrations are the cause of night myopia, it results in halos around point image. Double vision has been correlated to total coma in general and horizontal coma in specific. Starburst showed significant correlation with spherical aberration and total aberration for the scotopic pupil size.
The asphericity quotient, or Q value is used to describe the curvature of the ellipsoid corneal shape, The average Q value of human cornea is -0.26. Myopic refractive surgery makes the cornea oblate, Such cornea will have a positive Q value and more of spherical aberrations.In aspheric treatment mode, prolateness of the cornea is preserved as much as possible. The excimer laser beam profile is designed in such a way to ablate more tissue in the periphery so that disturbance of Q value is less.
Larger attempted corrections, which resultes in deeper ablations and greater changes in the corneal power, are correlated with greater amounts of induced corneal HOAs; therefore an extremely large amount of myopic correction should be avoided, as myopic correction of 9 diopters or more may lead to a considerable magnitude of induced HOAs of the cornea.
Accurate ocular alignments are important for good results in the refractive surgery, dynamic eye movements and fixation errors by the patient can also influence the actual profile of corneal tissue removed during the treatment. Two types of eye movements that are relevant to refractive surgery procedures have been defined; namely torsional and fixational movements, pupil centroid shift and iris tilting are another confusing problems for centration of ablation, all can influence ablation centration and registration on the cornea hence the final post-operative visual outcome, pupils decentered from the corneal apex by more than 0.4 mm or irises tilted by more than 6 degrees can only be detected by topographic examination such as that using the Orbscan device. The use of dynamic eye trackers along with iris registration technologies can compensate for such movements along with pupil centroid compensation and improve the final visual outcome after the surgery.
The ideal visual axis would be expected to intercept the cornea nasal to the line of sight and temporal to the coaxially sighted corneal reflex, this ensures the closest to perfect centre of ablation at which the surgeon must manually lock the eye tracker prior to laser treatment. The surgeon must manually align the center of the patient’s pupil with the laser center axis and engage the eye-tracker. Inaccuracies in such step will result in a statically decentered ablation profile and a subsequent induction of HOAs.
Fractional Clearance is defined as the ratio of optic zone diameter to pupillary diameter, it has a significant effect on higher order aberrations after laser refractive surgery, avoiding ablations at FC less than 1 at least in higher myopic patients, along with OZ diameters that overlap the pupil by 15% or more could help minimize the difference between preoperative and postoperative HOA.
Choice of the ablation algorithm is also a crucial factor, patients with RMS HOAs of 0.3µm or higher should benefit more of the wavefront guided ablation than wavefront optimized profile and the opposite for those with RMS less than 0.3µm.
Another commonly missed factor is patient education, for better centration, making patients familiar with the sound and the procedure by videos and with the illumination and fixation light of the excimer laser before ablation is important especially for those with high myopia.
Tangential maps are used to measure decentration, treating decentration is done using topography guided customized ablation profiles, The topographically driven custom contoured ablation pattern (C-CAP) method (Visx Inc.) enables surgeons to perform individualized treatments .precisely controlled by size, depth, and location, it is possible to reshape the cornea to address the previously difficult- to-treat laser decentration, The accompanying software allows the surgeon to plan and simulate any ablation algorithm before the C-CAP treatment, with excellent post-operative improvements of symptoms, also, the topographically guided custom ablation treatment (T-CAT)(Alcon) has shown very good results at treating steep central islands and decentered ablation by fitting the height data to an aspheric shape function and in a second step subtracted from the original data set. The residual height information is exported to the excimer laser system and an ablation profile reshaping the cornea to a best fit asphere is generated.
Cataract surgery has evolved from a sight-saving procedure to a refractive procedure in which quality of vision is of crucial importance and visual acuity alone cannot be considered to be the sole criterion of surgical success, conventional IOLs degrade image quality by increasing HOAs, such as spherical aberration, decreasing spherical aberration with aspheric IOLs improves retinal image quality and mesopic contrast sensitivity at low spatial frequencies. An aspheric IOL generates a negative spherical aberration to compensate for the positive corneal spherical aberration, but IOL decentration could limit, cancel, or turn into disadvantages the benefits of aspheric IOLs, the advantages of asphericity are lost when IOL decentration is greater than 0.5 mm, on the other hand, best-corrected eyes with spherical IOLs should perform better at near tasks than best corrected eyes with aspheric IOLs, tolerance to defocus is higher with spherical IOLs than with aspheric IOLs, this is due to the fact that the effect of spherical aberration on depth of field is inversely related to the image quality of the eye.
Selection of IOLs depend on many factors, pupil size, patient’s visual requirements, how hyperopic is the patient, previous corneal refractive surgery and capsular support should be put into consideration in order to choose the optimum IOL for each specific patient; myopic lasik patients or those to which quality of vision is crucial benefit from aspheric IOLs with negative SA, while patients who require more depth of field or those with a likelihood of IOL decentration will benefit from aspheric IOLs with zero SA, but hyperopic lasik patients will benefit from spherical IOLs with positive SA.