How Laser Eye Surgery Corrects Your Vision

The cornea is on average 525 microns thick, or about half a millimeter. It is made of three stacked layers, separated by two membranes. Two of these layers are involved in the laser procedure. The third, the deepest, is never touched.

The epithelium, the surface layer

The epithelium is the outermost layer of the cornea, and is about 50 microns thick. It is a non-keratinized stratified squamous epithelium, with an architecture close to that of the skin of the body, but thinner and more regular. It has five to seven stacked cell layers, organized into three types. A single basal layer of cuboidal cells rests on Bowman’s membrane. Above it come two to three layers of so-called “wing” (intermediate) cells, then two to three layers of flattened surface cells.

Only the basal cells are mitotically active. They divide, and their daughter cells gradually migrate toward the surface, differentiating successively into wing cells and then surface cells, before being shed into the tear film. Complete renewal of the epithelium takes about seven days in the absence of injury.

TransPRK relies on this natural regeneration: the excimer first ablates the epithelium to expose the stroma, then sculpts the stroma. After the laser, the corneal surface closes over in 24 to 72 hours as epithelial cells migrate in from the periphery, and full stratification of the epithelium is completed over one to two weeks.

In LASIK and SMILE too, the epithelium heals, but only along the “side cut”, the peripheral cut line at the edge of the flap (LASIK) or of the micro-incision through which the lenticule is extracted (SMILE). This peripheral healing also takes a few days.

The stroma, the deep and strong part

The stroma makes up about 90% of the thickness of the cornea. It is composed of highly organized collagen fibers, which provide both the transparency and the mechanical strength of the whole. Unlike the epithelium, the stroma does not regenerate after ablation.

The laser changes the corneal curvature at the level of the stroma. Because the stroma does not heal by returning to its original shape, the new curvature is maintained over time. This is what makes refractive surgery long-lasting. All four techniques (LASIK, TransPRK, SMILE, PresbyLASIK) change the shape of the stroma; they differ only in how it is accessed.

The endothelium, the inner surface, never touched

The endothelium is the deepest layer of the cornea. It acts as a pump: it keeps the cornea dehydrated, and therefore transparent. No laser refractive surgery touches the endothelium. The laser never goes that deep into the cornea.

The two membranes separating the layers are Bowman’s membrane (between the epithelium and the stroma) and Descemet’s membrane (between the stroma and the endothelium). They act as transitions but have no direct bearing on the principle of laser treatment.

Ref.: Bourges JL et al., eds., « La cornée, Rapport de la Société Française d’Ophtalmologie », Elsevier-Masson, 2015. Wilson SE et al., « Epithelial Basement Membrane Wound Healing », Experimental Eye Research, 2017. Ehlers N et al., « Morphological evaluation of normal human corneal epithelium », Acta Ophthalmologica, 2010.

Two complementary lasers are used at the Institut Voltaire for refractive surgery. They do not have the same function and do not act on the same layers of the cornea.

The operating room at the Institut Voltaire. On the left, the Zeiss Visumax 800 femtosecond laser (ultra-precise cuts: the LASIK flap, the SMILE lenticule); on the right, the Schwind Amaris 750S excimer laser (sculpting the stroma by photoablation).

The femtosecond laser: Visumax 800 (Zeiss)

The femtosecond is a cutting laser with micrometer precision. Its pulses last a millionth of a billionth of a second, hence its name. They create a clean tissue separation inside the cornea, without any mechanical incision. The cornea is never touched by an instrument.

The Institut Voltaire is equipped with the Visumax 800, the latest generation of Zeiss femtosecond laser. It is used in two techniques:

• In LASIK, it cuts a corneal flap, a thin surface layer about 110 microns thick, which is then lifted to expose the stroma to the excimer treatment. The cut takes 8 to 10 seconds per eye, at low pressure and painlessly.
• In SMILE, it performs the entire treatment, sculpting a lenticule inside the stroma, which is then extracted through a peripheral micro-incision.

The excimer laser: Schwind Amaris 750S

The excimer is the laser that sculpts the cornea. It emits ultraviolet light at a precise wavelength (193 nanometers), which removes corneal tissue by photoablation: the light instantly breaks the molecular bonds of the outermost layers of the tissue, which are vaporized without significant heat transfer. The surrounding corneal tissue stays cold.

The Schwind Amaris 750S is the excimer platform used at the Institut Voltaire. Its firing rate and its eye-tracking system allow a fast treatment (about 2.5 seconds per diopter of myopia treated) that stays aligned with the eye at all times, including during involuntary microsaccades of the gaze.

The excimer is used in LASIK, TransPRK and PresbyLASIK.

Summary: which laser for which technique

Understanding aberrometry and customized treatment

Does the refractive surgery laser burn the cornea?

No. This fear is common and understandable; the word laser immediately suggests heat. Excimer photoablation, however, is not combustion: the ultraviolet light breaks the molecular bonds of the surface layers of the tissue, which are vaporized instantly, without significant heat transfer. The cornea itself stays cold throughout the laser treatment.

You may notice a slight smell during the laser treatment, a bit like burnt dust. It comes from the reaction of the beam with the surrounding air, not from your cornea.

Can the refractive surgery laser burn the skin of the face?

No. This question comes up fairly often in consultation, from patients who have in mind the scene in Final Destination where a laser eye operation turns into a disaster and burns the patient’s face. That is a movie scenario, unrelated to the physical reality of the refractive surgery laser.

The laser beam is focused on a very precise spot on the cornea, and its energy is entirely absorbed by the first layers of tissue. It can neither escape the surgical field nor travel any distance. During the procedure, the eye is positioned under the laser by a fixation and eye-tracking system, and the rest of the face is covered by a sterile surgical drape. Your skin is never exposed to the beam. No physical mechanism would allow the laser to burn a surface located outside its focal point.

Does the laser go inside the eye?

Never. Laser refractive surgery acts only on the cornea, the outer, transparent membrane of the eye. The laser does not penetrate inside the eye, and touches neither the lens, nor the retina, nor the optic nerve.

Is the correction achieved long-lasting?

Yes. The stromal tissue removed does not regenerate, so the new shape given to the cornea is long-lasting. The corrected vision is most often maintained for decades.

There are rare cases of regression, mainly in highly farsighted patients and with older generations of lasers. A secondary enhancement can then correct them.

Does the laser slow the progression of myopia?

No. The laser reshapes the cornea to make vision clear without optical correction. It does not change the length of the eye, which, in nearsighted people, is the parameter that may still change in young adults. That is why I generally wait until your prescription is globally stable before operating, without waiting for a specific length of time. Don’t worry about micro-variations, which are natural fluctuations.

Does the eye remain intact after surgery?

Yes. No implant and no intraocular lens is placed in LASIK, TransPRK, SMILE or PresbyLASIK. The laser reshapes the cornea, and that is all. The eye keeps its internal architecture.

The laser corrects most refractive errors, but it has its limits. The usual thresholds at the Institut Voltaire are about -10 diopters for myopia (sometimes -12 depending on the cornea), +7 to +8 diopters for hyperopia, and -6 diopters for astigmatism.

Beyond these, or if the cornea is too thin or too irregular, other options exist. The phakic implant (ICL, implantable collamer lens) is a lens placed inside the eye, behind the iris, which corrects high refractive errors without touching the cornea. When this option is best suited to your case, I refer you to a specialized colleague.

My role is to identify beforehand when laser is possible, and when it is not. I would rather not operate than take a risk with your eyes.

If laser is not possible