Laser-Assisted In Situ Keratomileusis (LASIK) is a widely performed refractive surgery that corrects vision problems such as nearsightedness, farsightedness, and astigmatism. The underlying principle of LASIK involves permanently reshaping the cornea, the clear, dome-shaped front surface of the eye, so that light is correctly focused onto the retina. While often used generically, LASIK represents a family of procedures distinguished by the technology used to prepare the cornea and the precision of the correction. Understanding the differences among these techniques is important for anyone considering laser vision correction.
Blade-Assisted vs. All-Laser Flap Creation
The foundational step of a traditional LASIK procedure involves creating a thin, hinged flap on the outermost layer of the cornea, which is then lifted to allow an excimer laser to reshape the underlying tissue. The primary distinction between the two main types of LASIK begins with the tool used for this initial flap creation.
Blade-assisted LASIK, often referred to as conventional LASIK, uses a mechanical surgical instrument called a microkeratome. This device employs an oscillating metal blade to make a precise cut to create the corneal flap. This method is recognized for its speed and remains a cost-effective option. Flaps created this way tend to be meniscus-shaped, meaning they are slightly thinner in the center and thicker toward the periphery.
In contrast, all-laser LASIK, or Femto-LASIK, utilizes a femtosecond laser to create the corneal flap. The laser emits ultra-short pulses of light that create microscopic gas bubbles within the corneal tissue at a pre-programmed depth, a process called photodisruption. These bubbles connect to form the precise separation plane, which the surgeon then gently lifts.
This technique allows for superior precision and consistency in flap dimensions, creating a more uniform, planar shape. Surgeons can precisely customize parameters like flap thickness and diameter, which reduces the risk of certain flap-related complications. All-laser creation has become the preferred standard for many surgeons, despite typically carrying a higher cost.
Wavefront-Guided Customization
Regardless of the flap creation method, the next phase involves using the excimer laser to reshape the corneal tissue. Standard LASIK corrects vision based on the patient’s existing eyeglass or contact lens prescription, which measures vision errors in relatively broad increments, typically 0.25 diopters. This conventional method effectively corrects basic refractive errors, known as lower-order aberrations.
Wavefront-guided LASIK, or custom LASIK, elevates the level of personalized correction using a wavefront analyzer. This device sends light into the eye and measures how it reflects back from the retina, creating a highly detailed, three-dimensional map of the visual system. This map captures not only lower-order aberrations but also higher-order aberrations (HOAs).
HOAs are subtle imperfections in the eye’s optics that are not corrected by standard lenses. They can contribute to visual symptoms such as glare, halos, and difficulty with night driving. The wavefront map guides the excimer laser to apply a highly personalized treatment profile, often with a precision of 0.01 diopters. This customization aims to reduce the induction of HOAs and potentially improve visual quality and night vision compared to non-customized treatments.
Surface Ablation Techniques
A distinct category of laser vision correction procedures, known as surface ablation techniques, bypasses the creation of a permanent corneal flap altogether. The most common example is Photorefractive Keratectomy (PRK), which predates LASIK and remains a suitable option for patients who are not ideal candidates for flap-based surgery, such as those with thin corneas or who have certain occupational risks.
In PRK, the outermost layer of the cornea, the epithelium, is completely removed before the excimer laser reshapes the underlying stromal tissue. Since no flap is created, the integrity of the deeper corneal structure is better maintained. The trade-off for this structural benefit is a significantly longer recovery period, as the corneal epithelium must completely regrow over the treated area, typically taking several days to a week.
Variations of PRK, such as LASEK and Epi-LASIK, were developed to attempt to preserve or partially lift the epithelial layer, but they generally result in a similar surface ablation and prolonged recovery profile. Although the initial discomfort and visual recovery are slower compared to LASIK, the final visual outcomes are comparable.
Small Incision Lenticule Extraction
Small Incision Lenticule Extraction (SMILE) represents a newer evolution in laser vision correction, differing significantly from both flap-based LASIK and surface ablation. This procedure uses only a femtosecond laser to perform the entire correction, eliminating the need for the large hinged flap characteristic of LASIK.
The femtosecond laser first creates a small, lens-shaped disc of corneal tissue, called a lenticule, inside the intact cornea. The laser then creates a tiny keyhole incision, typically less than four millimeters wide, on the corneal surface. The surgeon extracts the pre-cut lenticule through this small incision, which changes the shape of the cornea and corrects the refractive error.
The minimally invasive nature of SMILE is its primary advantage. The small incision size disturbs fewer of the corneal nerves responsible for tear production, potentially leading to a lower incidence of long-term dry eye syndrome compared to LASIK. The absence of a large flap also means the procedure may better maintain the biomechanical stability of the cornea, positioning SMILE as a compelling choice for certain patients.