Laser-Assisted In Situ Keratomileusis (LASIK) is a popular procedure that permanently reshapes the cornea to correct common vision problems. The surgery uses a laser to adjust the focusing power of the eye, reducing or eliminating the need for glasses or contact lenses. While it has a high success rate, LASIK is not suitable for everyone, as candidacy depends on anatomical and health limitations. These limits maximize the safety and long-term stability of the visual outcome for the patient.
Refractive Error Limits
The extent of vision correction LASIK can safely achieve is directly tied to the patient’s prescription strength, measured in diopters (D). Exceeding certain thresholds for nearsightedness, farsightedness, or astigmatism is the most common reason for disqualification. This is because a higher prescription requires the removal of a greater volume of corneal tissue.
For myopia (nearsightedness), the maximum treatment range is up to -12.00 D, though many surgeons prefer to treat no more than -8.00 D to -10.00 D. Hyperopia (farsightedness) has a much narrower correction window, limited to prescriptions up to +4.00 D to +6.00 D. Correcting hyperopia involves making the cornea steeper, a process that is less predictable and prone to regression over time.
Astigmatism (irregular curvature of the cornea) can be corrected by LASIK up to about 6.00 D, depending on the pattern and corneal thickness. Pushing beyond these refractive limits increases the risk of complications, such as compromised corneal stability and the induction of visual aberrations like glare or halos. Conserving corneal tissue for long-term health is the primary factor limiting the treatment of stronger prescriptions.
Corneal Thickness and Stability Requirements
Safe LASIK surgery relies on having sufficient corneal tissue to maintain the eye’s structural integrity after the procedure. The cornea’s thickness is measured through a test called pachymetry and is a primary determinant of candidacy. The average central corneal thickness is around 550 microns.
During LASIK, a thin hinged flap is created, and the underlying tissue is reshaped with the laser. A minimum amount of untouched corneal tissue, known as the residual stromal bed, must remain under the flap to prevent the cornea from weakening and bulging, a condition called ectasia. Guidelines recommend leaving at least 250 to 270 microns of stromal tissue after the laser ablation.
Patients must also demonstrate refractive stability, meaning their prescription should not have changed significantly (less than 0.5 D) for at least one to two years prior to surgery. Pre-existing conditions that affect corneal structure, such as keratoconus, are absolute contraindications. Performing LASIK on an unstable cornea can accelerate the disease and cause irreversible vision loss.
Systemic Health Contraindications
A patient’s overall health can preclude them from undergoing LASIK due to concerns about impaired healing and increased complication risk. Autoimmune disorders, including rheumatoid arthritis and systemic lupus erythematosus, are contraindications because they interfere with the eye’s ability to heal post-surgery. These conditions can cause unpredictable corneal remodeling or severe dry eye syndrome.
Poorly managed diabetes also poses a risk due to its negative impact on wound healing. Chronic dry eye syndrome often leads to disqualification, as LASIK can temporarily worsen tear production and lead to persistent discomfort. Pregnancy and nursing are also contraindications because associated hormonal fluctuations can cause temporary, unstable changes in prescription.
Alternative Refractive Procedures
Individuals who do not meet LASIK candidacy requirements have several effective alternatives. Photorefractive Keratectomy (PRK) is recommended for those with thin corneas, as it involves reshaping the surface of the cornea without creating a flap. Although PRK has a longer initial recovery period, it preserves more of the deeper corneal tissue, making it a safer option for borderline cases.
Patients with high prescriptions may be better suited for Implantable Collamer Lenses (ICLs). This procedure involves placing a permanent lens inside the eye, behind the iris and in front of the natural lens, without removing any corneal tissue. For older patients, Refractive Lens Exchange (RLE) is an option that replaces the natural lens with an artificial one to correct the refractive error.