Who Is a Candidate for Ablative (Laser) Refractive Surgery?

Ablative refractive surgery—using techniques such as LASIK, PRK, or TransPRK—is one of the most precise and predictable procedures in modern ophthalmology. However, not all patients are suitable candidates; proper patient selection is key to achieving safe and stable results.

Below are the fundamental criteria used to determine whether an individual is a candidate for this type of surgery.

1. Basic requirements: prescription and corneal topography

To evaluate a patient, two essential assessments are required:

a) The patient’s current prescription

The prescription must be stable. Surgery does not halt the progression of myopia (nearsightedness) or astigmatism; therefore, it is essential that the prescription has remained unchanged for at least one year.

b) Corneal topography or tomography

This assessment provides information on:

• The shape of the cornea

• Its thickness

• Its regularity

• Any signs of ectasia (bulging) or corneal weakness

Modern refractive surgery is based on mathematical models and physical equations that determine how many diopters can be safely removed from the corneal tissue.

2. What is a micron, and why does it matter?

Corneal thickness is measured in microns.

1 millimeter = 1,000 microns

A normal human cornea measures approximately 540–560 microns—that is, slightly more than half a millimeter.

This measurement is crucial because the laser removes microns of tissue to correct the patient’s prescription. The higher the number of diopters being corrected, the more tissue must be removed.

3. What corneal thicknesses are considered safe? As a rule of thumb:

Ideal cornea for refractive surgery:

≥ 550 microns

Potential candidates (requiring more careful assessment):

≥ 500 microns

Corneas requiring very cautious analysis—or which may not be suitable:

< 490 microns

These latter cases require a highly individualized evaluation: we must ask whether reshaping such a cornea will be safe in the long term, not just in the short term.

4. How many diopters can be safely corrected?

Modern equipment allows for more precise treatments, but there are still limits.

Generally speaking, once you exceed 8–9 diopters of myopia (depending on corneal thickness and the ablation profile), the viability of the surgery begins to be called into question.

It is important to understand that the surgery is like sculpting a piece of marble:

• The cornea is the marble.

• The prescription (refractive error) is the part that must be “carved away.”

We must evaluate how much “marble” is available.

If too much is removed, the remaining tissue may become weak, increasing the risk of corneal ectasia. Sculpted and Polished Marble

5. When laser surgery is NOT the first option

If the cornea is too thin, the prescription is too high, or the corneal shape is irregular, there are alternatives:

• Phakic intraocular lenses (ICL)

• Refractive lens exchange (for older patients)

• Combined techniques

However, the global trend—given the extraordinary precision of modern lasers—is to first evaluate whether the patient is a good candidate for ablative (laser) surgery, as it is a procedure that is:

• Highly predictable

• Very safe

• Characterized by a rapid recovery

• Associated with an excellent visual outcome

6. The priority: improving the patient’s life through science and safety

The primary goal is not to “perform surgery for the sake of it,” but rather to improve the patient’s visual quality of life, while always focusing on:

• Safety

• Scientific evidence

• Long-term stability

• Predictable results

Correctly determining who is a suitable candidate—and who is not—is just as important as the surgery itself.