domingo, 25 de noviembre de 2018

What is LASIK?

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LASIK is a surgical procedure where cornea of the eye is reshaped in order to reduce a person’s dependency on glasses or contact lenses. The acronym LASIK stands for Laser In Situ Keratomileusis because an excimer laser (type of an ultraviolet laser) is used. It is similar to other corrective procedures such as photorefractive keratectomy, but provides several additional benefits – primarily quicker recovery period.

Like any other procedure, the results of LASIK surgery are determined on the basis of certain parameters that include efficacy, predictability, stability, safety and patient satisfaction. It is crucial that the surgeon evaluate the patient’s expectations of LASIK surgery and to make sure that they are realistic.

Fundamentals of LASIK

The LASIK procedure involves increasing the eye’s intraocular pressure to at least 65 mmHg with a suction device. Then a mechanical microkeratome (a blade device) or a laser keratome (a laser device) is used to create a corneal flap at least 6 mm in diameter and 150 microns thick.

This flap is then folded back revealing the stroma, the middle section of the cornea, which are reshaped by the pulses from a computer-controlled excimer laser beam. The size of the optical zone and the depth of the laser ablation represent key determinants of the correction achieved. The amount of the tissue the laser will remove is calculated ahead of time.

Once the reshaping is finished, the surgeon replaces and secures the flap without the need of the stitches. The cornea will hold the flap in place without any special intervention. Patients can have mild postoperative discomfort for up to 6 hours following LASIK treatment, during which time they should rest with their eyes closed.

LASIK represents an outpatient procedure that takes only 10 to 15 minutes per eye, thus it can be done on one or both eyes during the same session. Eye drops that numb the surface of the eye are used as the anesthetic, and the procedure is done in the awake state.

General medical history, ophthalmic examination and several special investigations are required to adequately assess pre-operative patient’s suitability for LASIK. Patients at a risk of developing intraoperative or postoperative complications need to be identified prior to the operation.

Refractive errors treatable with LASIK

In order to achieve clear vision, the eye’s cornea and lens must properly refract light rays, so images can be focused clearly on the retina. Otherwise the images will be blurry, which can be caused by a difference between the curvature of the cornea and the length of the eye. This is also known as refractive error.

Traditionally, three major types of refractive errors have been described: myopia, hyperopia and astigmatism. Wavefront analysis of human eyes reveal additional irregularities that are classified into complex subgroups, often simplified as lower and higher order aberrations.

In myopia (also known as nearsightedness), the secondary focal point is anterior to the retina, so affected individuals have more difficulty observing distant objects as clearly as near objects. The prevalence of physiologic myopia is roughly 25% in general population. LASIK corrects myopia by removing tissue in the center of the cornea and flattening it, thus decreasing the refractive power of the eye.

In hyperopia (also known as farsightedness), the secondary focal point is posterior to the retina, hence those persons have difficulty seeing nearby objects as clearly as distant ones. Although it affects more people than myopia (approximately 40%), it is less visually significant due to the substantial compensation by accommodation. LASIK corrects this condition by removing a ring of tissue around the center of the cornea, thereby making it steeper.

Astigmatism is a distortion of the image on the retina due to the variability of the curvature of the cornea. Depending on the symmetry of principal meridians in this condition, astigmatism can be regular or irregular. LASIK can correct regular astigmatism via removal of the tissue from the steeper side of the cornea, but it is contraindicated in eyes with irregular astigmatism.

Sources

http://umm.edu/health/medical/ency/articles/lasik-eye-surgery
http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/SurgeryandLifeSupport/LASIK/default.htm
http://memo.cgu.edu.tw/cgmj/3103/310303.pdf
Azar DT, Koch D. LASIK (Laser in Situ Keratomileusis): Fundamentals, Surgical Techniques, and Complications. CRC Press, 2002; pp. 1-20.
Sharma N, Vajpayee RB, Sullivan L. Step by Step LASIK Surgery. CRC Press, 2005; pp. 3-30.