Alteration in corneal biomechanics can also be achieved by collagen shrinkage. Heating collagen to a critical temperature of 58^°-76^° C causes it to shrink, inducing changes in the corneal curvature. Thermokeratoplasty treatments such as conductive keratoplasty (CK) are generally avoided in the central cornea because of scarring but can be used in the midperiphery to cause local collagen contraction with concurrent central corneal steepening (see Chapter 10, Figure 10-3).

Three dif­fer­ent types of ­laser–­tissue interactions are used in keratorefractive surgery: photoab­ lative, photodisruptive, and photothermal. Photoablation, the most impor­tant ­laser–­tissue interaction in refractive surgery, breaks chemical bonds by using excimer (from “*exci*ted di*mer*”) ­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 ­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 electro­ magnetic spectrum has very low tissue penetrance and thus is suitable for operating on the surface of a 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 non- mutagenic, 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 for 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 corneal flaps for LASIK and for the SMILE procedure. It may also be used to cre­ ate channels for intrastromal ring segments and for lamellar or penetrating keratoplasty. It uses a 1053-nm infrared beam that ­causes photodisruption, in which tissue is transformed into plasma, and the subsequent high pressure and temperature generated by this pro­cess lead to rapid tissue expansion and the formation of microscopic cavitation ­bubbles within the corneal stroma. Contiguous ­laser pulses allow creation of the corneal flap, lenticule of tissue, channel, or keratoplasty incision.

Photothermal effects are achieved by focusing a holmium:YAG laser with a wavelength of 2.13 μ m 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 steepening of the cornea. This technique is FDA approved for low hyperopia but is not commonly used because of regression of effect.

All photoablative procedures result in the removal of corneal tissue. The amount of tissue removed centrally for myopic treatments using a broad-­beam ­laser is estimated by the Munnerlyn formula:

Clinical experience has confirmed that the effective change is in­de­pen­dent of the initial curvature of the cornea. The Munnerlyn formula highlights some of the prob­lems and limitations of ­laser vision correction. The amount of ablation increases by the square of the optical zone, but the complications of glare, halos, and regression increase when the optical zone decreases. To reduce ­these adverse effects, the optical zone should be 6 mm or larger.

With surface ablation, the laser treatment is applied to the Bowman layer and the anterior stroma; in contrast, LASIK combines an initial lamellar incision with ablation of the underlying cornea, typically in the stromal bed (see Chapter 4 for further details of surgical technique). Flap thickness ranges from ultrathin (80–100 μ m) to standard (120–180 μ m). The thickness and diameter of the LASIK flap depend on instrumentation, corneal diameter, corneal curvature, and corneal thickness.

Treatments for myopia flatten the cornea by removing central corneal tissue, whereas ­those for hyperopia steepen the cornea by removing a doughnut-­shaped portion of mid- peripheral tissue. Some ­lasers use a multizone treatment algorithm to conserve tissue by creating several concentric optical zones to achieve the total correction required. This method can provide the full correction centrally, while tapering peripheral zones can re­ duce symptoms created by abrupt transitions between ablated and nonablated cornea. It also allows higher degrees of myopia to be treated (Fig 1-22).

Care must be taken to ensure that enough stromal tissue remains after creation of the LASIK flap and ablation to maintain adequate corneal structure. The historical standard has been to leave a minimum of 250 μ m of tissue in the stromal bed, although the exact amount of remaining tissue required to ensure biomechanical stability is not known and likely varies among individuals. See Chapters 2 and 5 for further discussion of these issues.