LASIK (Laser-Assisted In Situ Keratomileusis) is a surgical procedure that uses a laser to reshape the cornea, the clear front surface of the eye. Astigmatism is a common imperfection in the curvature of the cornea or the lens, which causes light to focus unevenly on the retina. Modern LASIK technology is highly effective at correcting astigmatism, often alongside nearsightedness (myopia) or farsightedness (hyperopia), allowing many individuals to achieve clear vision without corrective lenses.
Understanding the Mechanics of Astigmatism
Astigmatism is a type of refractive error that results in blurred or distorted vision at any distance. This blurring occurs because the cornea, which is normally shaped like a sphere, is shaped more like a football (oval or toric). This uneven shape means the curvature is steeper in one meridian than it is in the meridian perpendicular to it.
When light enters an eye with astigmatism, the irregular surface causes the light rays to bend, or refract, differently along the different axes. Instead of focusing light to a single, sharp point on the retina, the light is scattered across two different focal points. This dual focus is what the brain interprets as distorted, shadowed, or stretched images, making tasks like reading or driving challenging.
The Specific LASIK Approach for Correcting Irregular Curvature
The fundamental mechanism of LASIK involves using a precise excimer laser to sculpt the cornea’s shape. Correcting astigmatism requires a specialized application of this technology, moving beyond the simple flattening or steepening used for myopia and hyperopia. The surgeon first creates a thin, hinged flap in the outer layer of the cornea to access the underlying tissue (stroma).
Once the flap is lifted, the excimer laser performs a “toric ablation” pattern. This pattern is highly customized and designed to remove varying amounts of tissue across the different meridians of the cornea. The goal of this targeted removal is to smooth the irregular, oval shape into a more uniform, spherical surface.
The precision of this treatment is achieved through sophisticated diagnostic technology, specifically corneal mapping. This mapping generates a detailed, three-dimensional image of the cornea’s unique curvature, providing the exact coordinates and depth of tissue removal needed. The excimer laser follows these programmed instructions, ablating more tissue along the steeper axis and less along the flatter axis. By making the corneal surface symmetrical, the laser ensures that light is uniformly refracted to a single focal point directly on the retina, correcting the blurred vision.
Essential Factors for Determining Candidacy
While LASIK is highly effective for astigmatism, not every person with the condition is a suitable candidate for the procedure. A thorough pre-operative evaluation determines eligibility by assessing eye health and the severity of the refractive error. One limitation relates to the degree of astigmatism, measured in diopters. Most modern lasers can safely correct up to 5.00 to 6.00 diopters, but individuals with a higher severity may require alternative surgical options.
Corneal thickness is a particularly important factor, as the procedure inherently involves the removal of tissue. A minimum residual stromal bed of at least 250 micrometers must remain after the laser ablation and flap creation to maintain the structural integrity of the eye. Because higher degrees of astigmatism require the removal of more tissue, a person must have an adequately thick cornea to qualify safely. Surgeons typically prefer a pre-operative thickness of at least 500 micrometers.
Candidacy also depends on the stability of the patient’s prescription, which must have remained unchanged for at least one year prior to surgery. This stability ensures that the surgical correction targets a permanent refractive error. Furthermore, a comprehensive eye health screening is necessary to rule out contraindications such as severe dry eye, uncontrolled autoimmune diseases, or progressive corneal diseases like keratoconus, which could compromise healing or the long-term stability of the results.