COMPARISON BETWEEN CORNEAL ABLATION DEPTH MEASURED BY ULTRASOUND, SCHEIMPFLUG AND OPTICAL COHERENCE PACHYMETRY AND ITS CORRELATION WITH REFRACTIVE OUTCOME IN MYOPIC LASER IN SITU KERATOMILEUSIS

Abstract

Laser in situ keratomileusis (LASIK) is currently one of the most widely performed ophthalmic surgical procedures in the world. LASIK has produced excellent results worldwide and has been approved by the United States Food and Drug Administration (FDA) for the treatment of myopia, hyperopia, and astigmatism. Corneal thickness is a key factor at all stages of a refractive correction. The volume of tissue removal determines the refractive change, and corneal thickness provides structural support. Ablations deeper than planned may lead to overcorrections and inadequate residual corneal thickness, which may increase the risk of postoperative keratectasia. Several instruments are available to measure the corneal thickness with varying degrees of accuracy. USP pachymetry is commonly used as it is easy to use and relatively inexpensive and has been considered the gold standard for CCT measurement. Disadvantages of USP include the need to anesthetize the cornea, cornea–probe contact, corneal indentation and the possible compression effect during measurement, and corneal surface disturbance. There is also the risk for corneal epithelial damage and transmission of infections. In addition, measurements can vary as a result of probe misalignment or decentering and the probe may not be perpendicularly aligned or may be inaccurately positioned because of a lack of fixation and gaze control. Until recently, there was no possibility to measure corneal thickness during refractive corneal surgery. With the integration of optical coherence tomography (OCT) into excimer laser systems this has changed which allows the detection of very fine structures in the eye through an interferometric principle. The significant advantages of online pachymetry are the high resolution in micrometer range, no requirement of contact with the cornea and the continuous measurement of corneal thickness during the surgical procedure Our study included thirty four eyes with mild to moderate myopia or myopic astigmatism (simple and compound) ranging from –1.50 to – 6.00 D of spherical myopia, with up to –2.00 D of refractive astigmatism. All eyes had BCVA of 20/20, and Residual stromal bed no less than 300 μm. We compared three methods of measurement of corneal ablation depth namely: ultrasound, Scheimpflug imaging and optical coherence pachymetry in myopic LASIK, and assessed its correlation to refractive outcome. Preoperative examination included refractio