LASIK (Laser-Assisted In Situ Keratomileusis) is a surgical procedure designed to correct common vision problems, known collectively as refractive errors. Modern LASIK technology can effectively correct astigmatism, often eliminating or significantly reducing a person’s dependence on glasses or contact lenses. This refractive surgery addresses the irregular shape of the eye’s front surface that causes light to focus incorrectly. Understanding how LASIK achieves this correction requires a look at the nature of astigmatism, the precise mechanisms of the laser, and the factors that determine a person’s eligibility for the treatment.
Understanding Astigmatism
Astigmatism is a refractive error that results from an imperfectly curved cornea or, less commonly, the lens inside the eye. In an eye without this condition, the cornea, which is the clear front surface, is shaped like a perfectly round sphere, similar to a basketball. With astigmatism, however, the cornea has an oval or oblong shape, more like a football.
This irregular curvature means that when light enters the eye, it is refracted, or bent, unevenly. Instead of focusing light onto a single point on the retina at the back of the eye, the light rays focus at multiple points. This scattering of light is what causes vision to appear distorted, blurry, or stretched out at all distances. The degree of this focusing error is measured in diopters, and it often occurs alongside nearsightedness or farsightedness.
How LASIK Corrects Astigmatism
The core of LASIK’s ability to correct astigmatism lies in its capacity to precisely reshape the irregularly curved cornea using a specialized light beam. The procedure begins with the creation of a thin, hinged flap on the cornea’s outermost layer, often accomplished with an ultrafast femtosecond laser. This flap is gently lifted to expose the underlying corneal tissue, known as the stroma.
Before the laser is applied, the surgeon creates a highly detailed, three-dimensional map of the eye’s surface. Modern techniques, such as wavefront-guided or topography-guided LASIK, capture thousands of data points to identify the exact degree and location of the corneal irregularities that cause the astigmatism. This mapping provides the excimer laser with a personalized treatment plan for the unique contours of the patient’s eye.
The excimer laser then delivers pulses of cool ultraviolet light to the exposed stromal tissue, removing microscopic amounts of tissue. For astigmatism correction, the laser ablates the steeper areas of the cornea to create a more uniformly spherical surface. This sculpted shape allows incoming light to converge on a single focal point on the retina, correcting the blur. Once the reshaping is complete, the corneal flap is repositioned, where it adheres naturally without stitches.
Limitations and Eligibility for Astigmatism Correction
While LASIK is effective, not every degree or type of astigmatism is treatable with the procedure. Most modern laser platforms are approved to correct astigmatism up to 5.0 or 6.0 diopters, though some technologies can extend this range. The treatable range is dependent on the specific laser system used and the surgeon’s clinical judgment.
Corneal Thickness Requirements
A primary factor determining eligibility is the patient’s corneal thickness. The excimer laser removes tissue to correct the refractive error, and a certain amount of corneal tissue must remain to maintain the structural integrity of the eye. Surgeons must ensure a sufficient residual stromal bed, often aiming for at least 250 to 300 microns, to prevent corneal weakening. Consequently, patients with thin corneas, especially those with high degrees of astigmatism requiring significant tissue removal, may not be candidates for LASIK.
Prescription Stability and Type
The stability of the astigmatism prescription is another criterion for treatment. Candidates must have a stable prescription, meaning it has not changed significantly for at least one year prior to the procedure. Furthermore, the type of astigmatism matters; LASIK is most effective for regular astigmatism, where the steep and flat curves are perpendicular. Irregular astigmatism, caused by corneal scarring or disease, presents a greater challenge and may not be suitable for standard LASIK treatment.
Other Surgical Options for Astigmatism
For individuals who are not eligible for LASIK due to thin corneas, high refractive errors, or other eye health issues, several other surgical options exist to correct astigmatism. Photorefractive Keratectomy (PRK) is a surface ablation technique that is a common alternative. PRK uses the same excimer laser to reshape the cornea but does not involve creating a corneal flap, making it suitable for patients with corneas that are too thin for LASIK.
Another option, particularly for high degrees of astigmatism or for patients who may also have developing cataracts, is the implantation of a Toric Intraocular Lens (Toric IOL). This procedure, often part of a Refractive Lens Exchange, involves removing the eye’s natural lens and replacing it with an artificial lens that has built-in astigmatism correction. This approach can correct the refractive error from inside the eye, which is particularly beneficial when the astigmatism is partially or entirely due to the natural lens.