Three different types of laser–tissue interactions are used in keratorefractive surgery: photoablative, photodisruptive, and photothermal. Photoablation, the most important laser–tissue interaction in refractive surgery, breaks chemical bonds using excimer (from “excited dimer”) lasers or other lasers of the appropriate wavelength. Laser energy of 4 eV per photon or greater is sufficient to break carbon–nitrogen or carbon–carbon tissue bonds. Argon-fluoride (ArF) lasers are excimer lasers that use electrical energy to stimulate argon to form dimers with fluorine gas. They generate a wavelength of 193 nm with 6.4 eV per photon. The 193-nm light is in the ultraviolet C (high ultraviolet) range, approaching the wavelength of x-rays. In addition to having high energy per photon, light at this end of the electromagnetic spectrum has very low tissue penetrance and thus is suitable for operating on the surface of tissue. This laser energy is capable of great precision, with little thermal spread in tissue; moreover, its lack of penetrance or lethality to cells makes the 193-nm laser nonmutagenic, enhancing its safety. (DNA mutagenicity occurs in the range of 250 nm.) Solid-state lasers have been designed to generate wavelengths of light near 193 nm without the need to use toxic gas, but the technical difficulties in manufacturing these lasers have limited their clinical use.
The femtosecond laser is approved by the US Food and Drug Administration (FDA) for creating flaps for LASIK and for the SMILE procedure. It may also be used to create channels for intrastromal ring segments and for lamellar keratoplasty and PKP. It uses a 1053-nm infrared beam that causes photodisruption, a process by which tissue is transformed into plasma, and the subsequent high pressure and temperature generated lead to rapid tissue expansion and formation of microscopic cavities within the corneal stroma. Contiguous photodisruption allows creation of the corneal flap, lenticule of tissue, channel, or keratoplasty incision.
Photothermal effects are achieved by focusing a Ho:YAG laser with a wavelength of 2.13 μm into the anterior stroma. The beam’s energy is absorbed by water in the cornea, and the resulting heat causes local collagen shrinkage and subsequent surface flattening. This technique is FDA approved for low hyperopia but not commonly used.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.