Refractive surgery: prices in Paris

Excimer laser

Excimer laser photoablation for reshaping the cornea is, through techniques such as photorefractive keratectomy (PRK) and LASIK, the main surgical method for correcting refractive errors, currently accounting for 90% of indications.

Excimer laser photoablation techniques are effective for all mild to moderate refractive errors, including:

• Spherical errors (myopia, hyperopia, presbyopia)
• Spherocylindrical errors (compound myopic, hyperopic, or mixed astigmatism)
• Cylindrical errors (pure myopic and hyperopic astigmatism)

Light is a wave composed of particles called photons. The term “laser” is an acronym for Light Amplification by Stimulated Emission of Radiation. The electrical stimulation of a rare gas leads to the emission of a photon, whose wavelength depends on the gas used. For argon, used in the Excimer laser, the wavelength produced is 193 nm. The term “Excimer” comes from initial research on the argon laser, where it was thought that the excited molecules formed a dimer (excited dimers, hence “Excimer”).

The energy generated by the laser is absorbed by the cornea, resulting in tissue photoablation. The ablation profile allows for modification of the corneal curvature, thereby producing a refractive effect.

The optical zone refers to the corneal surface area where the laser treatment is applied, typically around 6 mm in diameter.

• Myopia is corrected by a maximal ablation profile at the center of the optical zone.
• Hyperopia is corrected by a maximal ablation profile at the periphery of the optical zone.
• Astigmatism is corrected by an asymmetric ablation profile, with different photoablation for each meridian of the cornea (flattening the steepest meridian for myopic astigmatism, steepening the flattest for hyperopic astigmatism).

There is an approximate correlation between the depth of photoablation and the amount of refractive error treated, expressed by the Munnerlyn formula:

P=1/3 (D x ZO2)

Where:

• : Maximum ablation depth (in microns)
• : Number of diopters treated
• : Diameter of the optical zone (in mm)

For a 6 mm optical zone, approximately 12 microns of corneal ablation are theoretically required to correct one diopter. In practice, in vivo, correcting one diopter results in an average photoablation of about 16 microns.

Femtosecond laser

The femtosecond laser, which emits ultra-short pulses, can be focused within the cornea to achieve tissue ablation by vaporization (photodisruption) beneath a stromal flap, unlike the Excimer laser, which, due to its shorter wavelength, acts on all tissue layers it traverses.

These pulses, lasting 10-15 seconds, have the advantage of producing no thermal effect while allowing for the delivery of very high power (power being defined as the amount of energy delivered per unit of time).

In ophthalmology, this laser is now mainly used for refractive surgery, most commonly during LASIK to create the superficial corneal flap with a hinge. It is also used in SMILE surgery to create an intrastromal corneal lenticule. Other ophthalmic applications include corneal graft cutting, relaxing corneal incisions, intracorneal rings, inlays, and cataract surgery.