LASIK (Laser-Assisted in Situ Keratomileusis) is a common surgical procedure that permanently alters the shape of the cornea, the clear, dome-shaped front surface of the eye. By precisely reshaping the corneal tissue, LASIK changes how light enters the eye, allowing it to focus correctly on the retina. This process uses advanced laser technology to reduce dependence on glasses or contact lenses.
Understanding Refractive Errors
The cornea is responsible for two-thirds of the eye’s total focusing power. Vision problems, known as refractive errors, occur when the cornea’s shape is imperfect, preventing light rays from refracting properly to form a sharp image on the retina. LASIK is designed to correct three primary types of these errors by modifying the corneal curvature.
Myopia (nearsightedness) occurs when the cornea is too steeply curved or the eyeball is too long, causing light to focus in front of the retina. This results in distant objects appearing blurry. Conversely, hyperopia (farsightedness) happens when the cornea is too flat or the eyeball is too short, leading to the light focusing theoretically behind the retina, which causes close-up objects to appear blurry.
The third common issue is astigmatism, where the cornea has an uneven, football-like curvature instead of a perfectly spherical shape. This irregular shape causes light to scatter, leading to blurred or distorted vision at any distance.
Creating the Protective Corneal Flap
Reshaping the cornea begins with a preparatory step to access the underlying tissue, called the stroma. A thin, hinged flap must first be created on the surface. This flap consists of the epithelium, the cornea’s protective outer layer, and a small portion of the anterior stroma.
The flap is temporarily lifted, exposing the deeper stromal bed where vision correction will take place. Creating this flap allows for faster healing and less post-operative discomfort compared to earlier surface-based procedures. Two main methods are used to create this precise cut.
The traditional method employs a mechanical microkeratome, which uses a high-precision oscillating blade. The modern, bladeless approach uses a femtosecond laser, which delivers ultra-short pulses of infrared light to create microscopic bubbles at a programmed depth. These interconnected bubbles separate the tissue, allowing the surgeon to lift a flap with highly predictable thickness and geometry.
The femtosecond laser technique allows for greater customization of the flap’s diameter and angle, leading to more uniform thickness than flaps created by a microkeratome. Creating a more precise and often thinner flap has expanded the number of patients who can safely undergo the LASIK procedure.
The Excimer Laser Mechanism
Once the protective flap is moved aside, the excimer laser performs the actual vision correction on the exposed corneal stroma. This specialized device emits a cool, ultraviolet light at a wavelength of 193 nanometers. This specific wavelength possesses enough energy to break the molecular bonds within the corneal collagen tissue without generating heat or damaging adjacent cells. This process is known as photoablation, where the pulsed light energy instantly vaporizes microscopic amounts of tissue with extreme precision.
The amount of tissue removed for a single diopter of correction for myopia is typically very thin, often measuring only 10 to 12 microns. A single pulse removes approximately 0.25 micrometers of tissue, demonstrating the laser’s ability to sculpt the cornea precisely.
A sophisticated computer-guided system maps the eye’s unique imperfections, often using wavefront technology, to create a personalized treatment plan. The laser then follows this custom map, determining the exact pattern and depth of tissue removal needed to correct the refractive error.
To correct myopia, the laser removes tissue from the center of the corneal bed, which flattens the cornea and reduces its focusing power. Conversely, to correct hyperopia, the laser removes tissue from the periphery of the exposed area, causing the central cornea to steepen and increase its focusing power. Astigmatism correction involves a targeted, uneven pattern of tissue removal to make the surface uniformly spherical.
Procedure Completion and Stabilization
Immediately after the excimer laser finishes reshaping the underlying stromal tissue, the surgeon carefully repositions the corneal flap back over the treated area. The surgeon then smooths the flap into place, ensuring there are no wrinkles or bubbles underneath. This flap adheres naturally to the stromal bed without the need for any stitches.
The cornea’s strong collagen structure allows the flap to immediately secure itself through capillary attraction and epithelial suction. Within the first 24 to 48 hours, the epithelial cells along the outer edges of the flap begin to regenerate, forming a natural seal that stabilizes the flap. This rapid initial healing acts as a protective bandage for the newly sculpted tissue, which contributes to the quick visual recovery common with LASIK.