Dr Moïse Tourabaly · Key points
The laser corrects vision by removing a thin layer of cornea. Beneath the treated area, a sufficient reserve must be kept so that the cornea stays strong.
LASIK. The flap uses up thickness even before the laser. On a cornea of normal thickness, the reserve remains sufficient.
LASIK. On a thin cornea, flap and laser leave too thin a reserve: a risk of weakening (ectasia). It is to avoid this, and not out of preference, that LASIK is sometimes ruled out.
PRK. The laser works on the surface, without a flap: it spares this thickness.
PRK. Without a flap, it preserves more of the available thickness: that is why it comes up so often when the cornea is thin.
The stronger the correction, the more tissue the laser removes. When the cornea is too thin for the laser, a phakic implant (ICL) is usually offered; a suspicious topography or keratoconus rules out the laser. The assessment decides.
Educational diagram · Dr Moïse Tourabaly, ophthalmologist. Illustrative proportions, not to scale: actual thicknesses are measured at the assessment. It does not replace a consultation.