الفهرس 
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Abstract
The femtosecond laser (IntraLase) is used to create flaps in LASIK. It uses an infrared wavelength (1053 nm) to deliver closely spaced, 3 μm spots that can be focused to a preset depth to photodisrupt tissue within the corneal stroma. The most common indication for LASIK surgery in the present times is myopia, although it is also being increasingly utilized to treat hypermetropia, and astigmatism. Other indications in which LASIK may be performed are anisometropia, induced refractive errors after other surgical procedures such as penetrating keratoplasty, radial keratotomy or cataract surgery. LASIK has also been undertaken to correct presbyopia. In certain cases, LASIK is absolutely contraindicated and should not be undertaken. Refractive instability is important to document, as results are not predictable in such eyes and the patient may become dissatisfied due to requirement of glasses soon after surgery. Conditions such as ectatic corneal diseases like keratoconus, Terrien’s and pellucid marginal degeneration may be aggravated by LASIK and lead to severe ectasia and decrease in the best corrected visual acuity. LASIK should also not be performed in forme-fruste keratoconus or subclinical cases of keratoconus. Further, the posterior corneal elevation preperatively should be greater than at least 40 μm . Patients with a thin cornea (corneal thickness less than 490μm) should not undergo LASIK since there is not enough cornea available to ablate and correct the refractive error.
Certain medical conditions, such as autoimmune diseases (e.g., lupus, rheumatoid arthritis) immunodeficiency states (e.g., HIV) and diabetes may prevent proper healing after a refractive procedure and are therefore also considered relative contraindications. If the patient has a history of keloid formation, then PRK or LASEK is to be avoided. Although safety with PRK has been reported in keloid formers, LASIK may be safer in such patients given the minimal wound healing response. In conclusion, the safety and effectiveness of the excimer laser have not been established in patients with unstable or worsening myopia or astigmatism, higher degree of refractive error, diseased or abnormal corneas and any previous insults to the cornea. In such cases, LASIK should not be performed. Preoperative evaluation for refractive surgery follows a structured sequence that includes patient interview followed by a complete ophthalmologic examination. The aim of preoperative evaluation is to answer three broad questions in addition to generating specific refractive data for the actual treatment; 1) Is it possible to safely perform refractive surgery in the patient; 2) What is the risk of possible complications, given the
patient specifics; and 3) Is it possible to meet the expectations that the patient has from the surgery?
The goal of refractive surgery is to reduce the dependence on glasses and contact lenses in a safe and effective way. Postoperative vision can be invariably improved further with additional optical correction. In presbyopes, additional near vision correction is required after adequate distance vision correction. Patients who expect perfect distance vision or presbyopes who expect equally good distance and reading
vision may not be satisfied with the surgical outcome. In the counseling session, the surgeon first should review the results of the physical examination with the patient. Then any questions that the patient may have should be answered. Patients are then asked if there are any more questions that they would like
answered.
It’s also important to explain in details how the procedure is performed including the preoperative work up and postoperative care and medications, its benefits, side effects and complications. It is the patient¢s decision to have refractive surgery, but she or he should be very well educated and informed on all surgical options even possible future options that may be currently under investigations The patient should be well informed about the complications. The surgeon should emphasize that a specific refractive outcome is purely a statistic and not guaranteed for any one patient.
The patient should be informed in simple words about the possible complications associated with LASIK procedure either being vision-threatening or not. These include, without limitation, either vision threatening or non-vision threatening ompliations. Corneal topography is the study of the anterior corneal surface detecting minor surface irregularities and asymmetry and offering both diagnostic and prognostic
valuable clinical data. The first classification system for normal corneal topography in emmetropic eyes using photokeratoscopic data was done by Knoll (1961). He classified corneas into four groups based on central asymmetry and the amount of peripheral flattening along the horizontal meridian.
Computerized videokeratoscopy provides a colour-coded map of corneal surface. The diopteric powers of the steepest and the flattest meridian and their axes are also calculated and displayed. More recently, non placido-based corneal topography has been used to measure corneal surface elevation and depression. It must be noted that these units calculate and not directly measure curvature. Orbscan is currently the most common instrument in clinical practice that provides a reliable and reproducible data of the anterior corneal surface, posterior corneal surface, keratometry, and pachymetry values with three-dimensional presentations and all LASIK candidates must be evaluated by this method preoperatively to detect an early keratoconus.
Advantages of the Pentacam include the following: (1) high resolution of the entire cornea, including the center of the cornea; (2) ability to measure corneas with severe irregularities, such as keratoconus, that may not be amenable to Placido imaging; and (3) ability to calculate pachymetry from limbus to limbus. The Pentacam can also provide corneal wavefront analysis to detect higher-order aberrations.
Pachymetry measurements become very important in the preoperative LASIK assessment of patients with high corrections and borderline corneal thickness and in patients being evaluated for enhancement surgery. Overestimation of corneal pachymetry can lead to inadvertent thinning of the stromal bed beyond 250 mm and, theoretically, may increase the risk of keratectasia. Underestimation of corneal pachymetry can lead to the exclusion of patients who may be candidates for primary procedures or enhancements. The evaluation of pupil size should be carried out under Photopic & mesopic or scotopic light conditions. The data are used to select an appropriate ablation zone diameter. After refractive surgery, visual quality can be significantly influenced by the size of the pupil, so the size of the treatment zone is usually set so that the effective optic zone diameter is greater than the scotopic pupil diameter. This is necessary to minimise the risk of glare, ghosting and haloing after treatment.
The advent of wavefront measurement technology enables the quantification of ocular higher-order aberrations, which are typically described mathematically using “Zernike polynomials” which describe only deviations from an idealized optical system, not the optical system itself. The concept of correcting ocular higher-order aberrations by excimer laser surgery “wavefront-guided ablation” would theoretically improve the image quality of the eye and therefore, improve visual outcome.
Corneal hysteresis is an important measure of the biomechanical properties of the cornea, it is an indicator of viscous damping in the cornea during inward and outward applanation pressure events that is the ability of the tissue to absorb and dissipate energy, a property that is determined by the visco-elastic property of the corneoscleral shell. There have been numerous reports of iatrogenic ectasia developing after excimer laser surgery, possibly resulting from biomechanical weakening of the cornea. Moreover,
the biomechanical properties of the cornea may affect not only the refractive outcomes after keratorefractive surgery such as LASIK, leading to unpredictability for this surgical technique, but also the measurement of the intraocular pressure (IOP) especially in eyes undergoing LASIK. Thus, it is of clinical importance to assess the biomechanical properties of the cornea.