LASIK (Laser-Assisted in Situ Keratomileusis) is a widely practiced surgical procedure that reshapes the cornea to correct common vision problems. This treatment allows many people to reduce or eliminate their reliance on glasses or contact lenses. However, LASIK is not universally available; its effectiveness and long-term safety depend on the patient meeting a rigorous set of clinical and anatomical standards. Every potential candidate must be thoroughly evaluated against specific physical limitations and health requirements before undergoing the procedure.
Refractive Error Boundaries
The primary limit for LASIK involves the strength of the patient’s prescription, measured in diopters. These boundaries exist because correcting vision requires the precise removal of corneal tissue (photoablation). A higher diopter value means a greater volume of tissue must be removed from the corneal stroma, which dictates the maximum safe correction.
For myopia (nearsightedness), the upper boundary for treatment is typically -10 to -12 diopters. Treating prescriptions beyond this range requires an excessively deep ablation profile, which compromises the cornea’s structural integrity. High myopic corrections also demand a wide treatment zone, sometimes limited by the patient’s pupil diameter or the laser platform’s capabilities.
Hyperopia (farsightedness) requires reshaping the cornea to be steeper by removing tissue in a ring pattern. Maximum corrections usually fall between +4 and +6 diopters. Higher corrections can lead to less predictable outcomes and side effects, such as spherical aberration.
Astigmatism, caused by an irregular corneal shape, is usually limited to about 5 or 6 diopters. When astigmatism combines with high myopia or hyperopia, the total tissue removal increases, pushing the patient toward the biomechanical safety margin. Exceeding these refractive boundaries risks post-LASIK ectasia, a condition where the thinned cornea bulges outward.
Minimum Corneal Thickness
A critical physical constraint for LASIK eligibility is the initial thickness of the patient’s cornea. The average cornea measures about 550 microns, and this is precisely measured pre-operatively using pachymetry. The LASIK procedure involves creating a thin, hinged flap and then using a laser to remove tissue from the underlying corneal bed.
The amount of remaining tissue after photoablation determines long-term mechanical safety. Surgeons require a minimum residual stromal bed (RSB) thickness, typically 250 to 300 microns, to prevent the cornea from succumbing to the eye’s internal pressure. This RSB acts as a structural floor; if the calculated ablation depth violates this minimum, the patient is disqualified.
This biomechanical limit creates an inverse relationship between initial corneal thickness and the correctable prescription. For example, a patient with a thin cornea but a low prescription might be eligible because the shallow ablation preserves the RSB. Conversely, a patient with a thick cornea but a very high prescription might be ineligible if the extensive tissue removal violates the RSB rule. This calculation ensures the cornea retains sufficient tensile strength to prevent long-term instability.
Systemic Health and Stability Requirements
A person’s overall systemic health and vision stability are crucial for determining LASIK suitability. Candidates must be at least 18 years old, ensuring the eye has completed its natural growth. The refractive prescription must also have remained stable for a minimum of one year preceding the surgery.
Certain systemic medical conditions compromise the post-operative healing process and serve as contraindications. Autoimmune disorders, such as severe lupus or rheumatoid arthritis, impair the body’s ability to repair corneal tissue efficiently. Uncontrolled diabetes is also a barrier because it can affect microvascular integrity, delay wound healing, and cause unpredictable fluctuations in refractive error.
Specific ocular conditions also limit the procedure. Patients with severe dry eye syndrome may experience worsened symptoms, as LASIK temporarily disrupts corneal nerves regulating tear production. Progressive eye diseases, including advanced glaucoma or cataracts, must be managed before considering LASIK. Furthermore, hormonal changes during pregnancy and nursing cause temporary shifts in refractive error, requiring surgery to be delayed until vision stabilizes post-weaning.
Alternative Corrective Procedures
For individuals who fall outside the safety limits for standard LASIK, several other surgical options exist.
Photorefractive Keratectomy (PRK)
PRK is often recommended for patients with thinner corneas or those at high risk of eye trauma. Unlike LASIK, PRK does not create a permanent corneal flap. Instead, the surgeon removes the surface layer of the cornea before performing the laser ablation directly on the underlying stroma.
Small Incision Lenticule Extraction (SMILE)
SMILE is a minimally invasive method used primarily for moderate to high myopia and astigmatism. This procedure uses a femtosecond laser to create a lens-shaped piece of tissue (lenticule) inside the cornea. The lenticule is then removed through a small keyhole incision. This technique disrupts fewer corneal nerves than traditional LASIK, potentially reducing post-operative dry eye incidence.
Implantable Collamer Lens (ICL)
The ICL is often the most suitable option for patients with extremely high prescriptions or very thin corneas. This procedure involves placing a permanent, artificial lens inside the eye, typically behind the iris. The ICL works with the eye’s existing structure, allowing for the correction of refractive errors far beyond the limits of laser-based tissue removal procedures.