الفهرس 
Optics of AstigmatismOnly 14 pages are availabe for public view
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Abstract
urgical correction of natural and induced astigmatism has advanced rapidly in the last decade. Laser-assisted in situ keratomileusis (LASIK), surface ablation procedures such as photo-astigmatic refractive keratectomy (PARK), Laser-assisted subepithelial keratomileusis (LASEK), and Epi-polis laser in situ keratomileusis (Epi-LASIK), Astigmatic keratotomy (AK), cataract or clear lens extraction surgery with toric intraocular lens (IOL) implantation, phakic toric intraocular lenses, photo-thermal keratoplasty techniques like conductive keratoplasty (CK), or a combination of these procedures are the main techniques employed.(32)
A thorough understanding of the types of astigmatism, the relationship between refraction, keratometry, and computerized corneal topography, the effect of contact-lens-induced molding, and the laser delivery systems are all critical elements in achieving successful astigmatic treatment with the refractive surgery. (17)
As with any form of astigmatic correction, the key to obtaining propitious outcomes hinges on proper centration with the axis of astigmatism, in that relatively small degrees of misalignment may lead to a profound loss of effect. The lesson that ”Less is more” should be considered while manipulating the corneal surface for the best postoperative visual recovery. Undercorrection is always much tolerated by patients and treatable than overcorrection. (83)
The current approach to astigmatic correction using the excimer laser involves ablating tissue from the cornea in a cylindrical fashion, by differential removal of tissue in the frontal plane of the cornea in one of the two major meridians. Actual tissue removal can occur by flattening the steep axis or steepening the flat axis, or splitting the ablation on the two meridians. Several techniques have been developed to create this toric ablation including: Minus cylinder format, Plus cylinder format, Cross-cylinder technique, Bitoric technique, and Customized wavefront ablation patterns. (15)
Multiple studies have demonstrated the safety and efficacy but also variability of using PARK to correct myopic astigmatism. With current nomograms, 44% to 94% of cases of astigmatism can be corrected, and 55% to 93% of eyes have an uncorrected visual acuity of 20/40 or better. Scanning slit PRK with Multipass technique is preferred for the treatment of patients with high myopia and large pupil, who are suspected to develop night glare and halos postoperatively. (19)
Regression, corneal haze, and functional symptoms secondary to optical aberrations are possible complications of astigmatism correction with excimer laser using PARK, LASEK, or LASIK. The main cause of these complications lies in the high dioptric gradient induced by commonly used ablation strategies in the transition area along the steepest topographical meridian, the effectiveness of different lasers or nomograms and axis misalignment. (20)
LASIK is currently the most popular keratorefractive procedure performed worldwide with generally excellent outcomes. However, LASIK correction of astigmatism can result in incomplete resolution, worsening, or shift in axis. Many factors can contribute to the outcome of LASIK correction of astigmatism. Among these factors is the effectiveness of different lasers or nomograms. Incomplete correction, worsening, or shift in axis may occur following rotational ocular shift or drift during laser ablation. Other factors such as cyclotorsion between the sitting and supine position and the effect of the lamellar cut may contribute to astigmatism alteration. Induced astigmatism is another factor that can contribute to outcome refractive correction of astigmatism. Nevertheless, LASIK correction of astigmatism is generally less effective and less precise than pure myopic LASIK. The use of a cylindrical optical or elliptical ablation zones has been shown to provide better results in the treatment of myopic astigmatism (55).
Results of topography assisted customized ablation have been encouraging in regular astigmatism and decentered ablations but require refinement with irregular astigmatism (54). Newly refined diagnostic technology, such as wavefront sensing, and more sophisticated spot laser delivery systems with eye tracking provide the refractive surgical team greater flexibility in tackling challenging optical abnormalities. However, disagreement between refractive and topographic determination of the astigmatic axis can still be considered a prognostic factor for the conventional LASIK correction of myopic astigmatism. (55)
LASIK is safe but also limited alternative for the correction of hyperopic astigmatism. Early regression of the astigmatic effect in the first 6 months seems to be the primary disadvantage of this technique in hyperopic patients. LASIK guided by wavefront technology-linked techniques is a promising way of optimizing astigmatic treatment. (111)
The goal of modern surface ablation techniques is to avoid the potential complications of lamellar surgery while minimizing the traditional disadvantages associated with conventional PRK such as pain, slow healing, and scar formation. Epi-LASIK addresses these requirements and can be very good alternative to almost other photoablative procedures. The differences in the speed of visual recovery and early postoperative pain between LASIK and Epi-LASIK are no longer clinically significant, and patients readily perceive the advantages of blade-free laser vision correction. (61)
Similar to PARK, one can expect variability in low astigmatic corrections with LASEK from asymmetric ablations in different regions of the cornea or meridional differences in corneal wound healing. Wavefront technology may improve these limitations, and further studies comparing conventional LASEK with wavefront LASEK may better explain this variability. (65)
The need to manage pre-existing astigmatism has become a requisite aspect of modern phacosurgery. Experience with keratorefractive surgery has proved that astigmatism of as little as 0.75 diopters (D) may leave a patient symptomatic with visual blur, ghosting, and halos. Fortunately, techniques have emerged that afford the refractive lens surgeon the ability to effectively, safely, and reproducibly reduce cylinder error to acceptable levels of 0.50Dor less. There are three basic methods of effecting astigmatism reduction: using the cataract incision to counteract preexisting astigmatism, performing astigmatic keratotomy, or using a toric intraocular lens (IOL). (85)
LRIs have become the most popular way to manage astigmatism at the time of cataract surgery. A recent study concluded that LRIs in combination with a temporal clear corneal incision provided superior astigmatic outcomes to that of ”on-axis” surgery. from a surgical standpoint, LRIs compete with laser vision correction for addressing corneal astigmatism. However, 2.50 to 3.00 D is the upper limit of correction for LRIs. PRK or LASIK is probably more predictable for treating higher amounts of corneal astigmatism. (85)
Yet another important and increasingly popular alternative is that of Bioptics. In this approach, in a staged manner, one may treat both residual spherical and astigmatic error following implant surgery. (91)
Toric IOLs offer another tool for cataract and refractive surgeons to reduce astigmatism. The main advantages of this method of astigmatism reduction are that it requires no extra tools or special skills, requires no extra incisions into the cornea or limbus, and is reversible. With newer lens designs and increased use of these lenses, instability and axis rotation are reduced, it may also allow the correction of astigmatism in the phakic patient. With the advent of multifocal lenses, it is not a stretch to imagine the toric technology now available to be merged with presbyopia correcting lenses, truly allowing for spectacle independence. (82)
Finally, Conductive Keratoplasty has also recently been described as a means by which residual hyperopia and hyperopic astigmatism may be effectively reduced following cataract surgery. The corneal steepening procedure employs radiofrequency energy, therefore involves no laser, tissue cutting, tissue removal, or change to central cornea, thus avoiding many complications observed with other refractive techniques. The efficacy, stability, and safety of CK for treating hyperopic astigmatism are satisfying. Careful patient selection in terms of age (should always be older than 40) and the attempted correction not higher than C1.75 D of cylinder is the key to success when treating hyperopic astigmatism with CK. (8)
The various procedures for the surgical management of high levels of astigmatism have had their limitations. Combined procedures, such as AK plus PARK, AK plus myopic LASIK, Toric IOL plus LASIK, AK with cataract surgery, and a procedure termed keratolenticuloplasty have been tried in these cases to improve efficacy and predictability. (91)