Keratorefractive surgical procedures aim to alter the refractive power of the cornea by changing its shape. Various methods are used to alter corneal curvature, including incising or removing corneal tissue or implanting artificial material into the cornea. Procedures that change the character of the corneal collagen have also been developed. This chapter focuses on 2 such procedures: corneal collagen shrinkage and corneal crosslinking (CLL).
The idea of using heat to alter the shape of the cornea was first proposed by Lans, a Dutch medical student, in 1898. When Lans used electrocautery to heat the corneal stroma, he noticed astigmatic changes in the cornea. In 1975, Gasset and Kaufman proposed a modified technique known as thermokeratoplasty to treat keratoconus. In 1984, Fyodorov introduced a technique of radial thermokeratoplasty that used a handheld, heated Nichrome inoculating needle designed for deeper thermokeratoplasty. The handheld probe contained a retractable 34-gauge wire heated to 600°C. For a duration of 0.3 second, a motor advanced the wire to a preset depth of 95% of the corneal thickness. Fyodorov used different patterns to treat hyperopia and astigmatism. However, excessive heating of the cornea resulted in necrosis, scarring, and variable corneal remodeling; regression and unpredictability of treatment limited the success of this technique. It is now known that the optimal temperature for avoiding stromal necrosis while still obtaining corneal collagen shrinkage is approximately 58°–76°C.
In the 1990s, numerous lasers were tested for use in laser thermokeratoplasty (LTK) but only the holmium:yttrium-aluminum-garnet (Ho:YAG) laser reached commercial production. The Ho:YAG laser produces light in the infrared region at a wavelength of 2100 nm and has corneal tissue penetration to approximately 480–530 μm. A noncontact system slit-lamp delivery system was used to apply 8 simultaneous spots at a frequency of 5 Hz and a pulse duration of 250 μsec. The system was approved for the temporary correction of 0.75–2.50 D of hyperopia with less than 1.00 D of astigmatism. Interest in LTK waned, primarily because of the significant refractive regression that frequently occurred. Few LTK units remain in clinical use.
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.