What is LASIK?
LASIK is a surgical procedure in which the cornea of the eye is reshaped to reduce a person’s dependence on glasses or contact lenses. LASIK is formed from the initials of the English word Laser-Assisted in Situ Keratomileusis, and the Turkish equivalent of this word can be said as the in situ shaping of the laser-accompanied cornea. It is similar to other corrective procedures such as photorefractive keratectomy but has a few additional benefits. Like other procedures, the results of LASIK surgery are determined on the basis of certain parameters including efficacy, predictability, stability, safety and patient satisfaction. In addition, it is very important for the surgeon to evaluate the patient’s expectations from LASIK surgery and to make sure that they are realistic.
The LASIK procedure involves increasing the intraocular pressure of the eye to at least 65 mmHg with a suction device. A mechanical microkeratome (a blade device) or a laser keratome (a laser device) is then used to create a corneal flap with a diameter of at least 6 mm and a thickness of 150 microns. This wing is then folded back to reveal the stroma, the central part of the cornea that is reshaped by pulses from a computer-controlled laser beam. The size of the optical region and the depth of laser ablation represent the main determinants of the correction achieved. However, the amount of tissue to be removed by the laser is calculated in advance. After the reshaping is finished, the surgeon replaces and fixes the flap without the need for stitches. The cornea holds the wing in place without any special intervention. Patients may experience mild postoperative discomfort for up to 6 hours following LASIK treatment and during this time they should rest with their eyes closed.
LASIK represents an outpatient procedure that takes 10 to 15 minutes in just one eye, so it can be done in one or both eyes in the same session. Eye drops that numb the eye surface are used as anesthetic and the procedure is performed while awake. General medical history, ophthalmic examination and a few special investigations are required to adequately evaluate the patient’s suitability for LASIK before the operation. Patients at risk of developing intraoperative or postoperative complications should be identified before the operation.
Fracture Errors That Can Be Treated With LASIK
To achieve a clear vision, the cornea and lens of the eye must properly refract light rays so that images can be clearly focused on the retina. Otherwise, the images will be blurry, which may be due to the difference between the curvature of the cornea and the length of the eye. This is also known as breakage error. Traditionally, three main types of refractive errors have been defined as myopia, hyperopia and astigmatism. Frontal wave analysis of human eyes reveals additional irregularities that are classified into complex subgroups, often simplified as lower and higher order aberrations.
In myopia (also known as myopia) the secondary focus is on the front of the retina, so affected individuals have more difficulty observing distant objects as clearly as near objects. The prevalence of physiological myopia is roughly 25% in the general population. LASIK corrects myopia by lifting and straightening the tissue in the center of the cornea, thus reducing the refractive power of the eye. In hyperopia (also known as farsightedness) the secondary focus is on the back of the retina, so these individuals have difficulty seeing nearby objects as clearly as distant objects. Although it affects more people than myopia (about 40%), it is visually less important because it is significantly compensated through housing. LASIK corrects this by removing a ring of tissue around the center of the cornea, thus making it more erect. Astigmatism is the distortion of the retinal image due to the variability of the curvature of the cornea. In this case, astigmatism may be regular or irregular depending on the symmetry of the main meridians. LASIK can correct normal astigmatism by removing tissue on the steeper side of the cornea, but it is contraindicated in eyes with irregular astigmatism.
LASIK: Star Rains, Shading, Halos and Double Vision Problems
Laser in situ keratomileusis (LASIK) is the most commonly used surgical technique to correct refractive errors. It can be used in correction of myopia, hyperopia and astigmatism by reshaping the cornea with excimer laser. The efficacy and safety of this procedure are well established in the literature. However, although the technology has many benefits, the high degree of visual aberration has disturbed most fracture procedures. This is a term used for visual problems that cannot be diagnosed with a conventional eye exam but can permanently affect the quality of vision. Higher order aberrations include star bursts (flare), ghosting, halos, double vision, and others.
As mentioned earlier, the quality of vision can be permanently affected by vision deviations after surgery. The main reasons for the formation of high-grade aberrations mentioned above are subclinical decentration (less than 1.0 mm) and wide area laser ablation profiles. When the pupils are dilated and results in a diameter exceeding the optical treatment zone, the light beams refracted by the untreated peripheral cornea are not focused in the same location as the central beams. In turn, retinal blur circles, a phenomenon also known as a negative clearing effect, is formed. Due to the oval area of laser treatment with inherently smaller optic zone in the steep meridian, such symptoms are more pronounced after the treatment of cylindrical defects. Also, the correction of high refractive errors is often linked to increased aberrations, as the worn and intact cornea differ in their respective refractive indices. After LASIK, the flattened shape of the cornea is a dominant factor in functional vision reduction and high-grade aberrations.
Preoperative measurement with an infrared pupillometer or using the Rosenbaum proximity card scale is crucial in preoperative screening of eligible patients. If the pupil diameter is larger than 6 mm, the patient should be informed about possible night vision disorders after LASIK. Still, even these patients reported improvement in halos and nighttime flare a few months after surgery. Some corneal light scattering may be the result of interfacial debris, cells, or uneven collagen accumulation, but it is often difficult to distinguish. Therefore, the development of software to allow larger ablation diameter and measures to prevent decentralization and central islands help to reduce most symptoms. Light miotics are highly effective in managing these deviations and can be particularly helpful when driving at night. Topical dilute brimonidine, pilocarpine, and artificial pupils and colored contact lenses are two useful approaches to this problem. Adequate surface lubrication can be a relief, and large diameter tetra curved rigid gas permeable contact lenses can be used for adequate visual improvement.
Post-LASIK Optical Quality Measurement
Optical quality after laser vision correction is difficult to objectively assess, but is required due to the occurrence of higher refractive errors. The double crossing technique has been shown as one of the most accurate tools for this purpose. Records and analyzes images of a point source projected onto the retina after retinal reflection and double passage through the ocular environment.
The only device available in the market based on the double pass technique is the Optical Quality Analysis System II (OQAS II). It allows precise assessment of the effect of deviations and loss of ocular transparency by measuring various optical quality parameters such as the Modulation Transfer Function (MTF), the Lens Dispersion Index (OSI) and the Strehl ratio. MTF evaluates the relationship between the contrast in the image generated by the system (ie the human eye) and the original contrast of the scene observed. In the absence of contrast distortion, the contrast in the image will be the same as the contrast in the object. MTF represents a spatial frequency function, since the contrast reduction is greater for high spatial frequencies. OSI provides information about the relevant forward dispersion that affects vision, and the higher the index value the higher the amount of intraocular scattering. The Strehl ratio is defined as the ratio of the peak diffraction intensity pattern of an anomalous image to that of an aberrant image at its peak.