Photorefractive Keratectomy (PRK) is a type of laser eye surgery used to correct vision problems, including astigmatism. Astigmatism is a refractive error caused by an irregular curvature of the eye’s front surface (the cornea or sometimes the lens). This irregularity prevents light from focusing onto a single point on the retina, resulting in blurred or distorted vision. PRK is a surface-based procedure that addresses this curvature issue by precisely reshaping the eye’s optics, offering a path to reduced reliance on corrective lenses.
Understanding Astigmatism and PRK
Astigmatism occurs when the cornea is shaped more like an American football or the back of a spoon, rather than the smooth, spherical curve of a baseball. This uneven surface causes incoming light rays to refract at different angles, creating multiple focal points instead of one clear image on the retina. The degree of irregularity is measured in diopters, which quantifies the amount of vision correction needed.
Photorefractive Keratectomy (PRK) is a surface ablation procedure that predates LASIK. During the PRK operation, the surgeon first removes the outermost layer of the cornea, called the epithelium, which is a thin, regenerative protective layer. This action exposes the underlying corneal tissue, which is then reshaped using an excimer laser to correct the refractive error. The epithelium naturally regrows over the treated area in the days following the surgery.
The Mechanism of Astigmatism Correction
PRK corrects astigmatism by sculpting the cornea into a more symmetrical shape. Prior to the procedure, the eye’s surface is precisely mapped using advanced imaging techniques like topography or wavefront analysis. This mapping provides the laser with a detailed blueprint of the irregular curvature.
The excimer laser is then guided by this data to selectively remove microscopic amounts of tissue from the exposed corneal surface. It targets the steep or irregular meridians of the cornea, flattening them to match the flatter meridians. The laser vaporizes tissue one cell layer at a time, creating a uniformly curved surface that allows light to converge properly on the retina.
The goal of this process is to transform the asymmetric corneal shape into a near-perfect sphere. By achieving this more even curvature, the light entering the eye is bent equally in all directions, eliminating the multiple focal points that cause distorted vision. A protective bandage contact lens is placed over the eye afterward to aid the healing of the removed epithelial layer.
Effectiveness and Limitations
PRK effectively reduces or eliminates mild to moderate astigmatism. Many patients achieve vision of 20/20 or 20/40, significantly reducing the need for glasses or contact lenses. Success rates for treating mild-to-moderate astigmatism often fall in the range of 90% to 98% of patients achieving excellent visual outcomes.
The procedure has limitations concerning the degree of astigmatism it can safely correct. PRK is typically approved to treat astigmatism up to about 4.0 diopters, though treatments up to 5.0 diopters may be performed depending on the patient’s corneal characteristics. If the irregularity is severe or the patient has thin corneas, the required tissue removal may compromise the eye’s structural stability, leading to a risk of residual astigmatism—a minor remaining irregularity that may require a small prescription.
PRK vs. LASIK for Astigmatism Correction
Both PRK and LASIK use the excimer laser to reshape the cornea and correct astigmatism, but they differ in how they access the tissue. The defining difference is that PRK performs the correction on the corneal surface, whereas LASIK involves creating a thin, hinged flap of corneal tissue before reshaping the layer underneath. This flap-free approach is a major factor in choosing PRK for astigmatism treatment in certain patients.
PRK is often the preferred choice for individuals with corneas considered too thin for a LASIK flap, as it preserves more underlying tissue, maintaining greater structural integrity. It is also recommended for those with a high risk of eye trauma, such as contact sports, because there is no corneal flap that could be dislodged. The longer recovery period of PRK, due to the time needed for the surface layer to regrow, is often considered a trade-off for the increased stability it provides.