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 1900, Terrien reported the use of cautery to correct the severe astigmatism associated with Terrien marginal degeneration, and in 1928, Knapp used cautery to improve the visual acuity of patients with keratoconus.
In 1975, Gasset and Kaufman proposed a modified technique known as thermokeratoplasty to treat keratoconus. They theorized that the hot cautery used in prior reports caused collagen necrosis. Their goal was to apply heat to the cornea in a controlled fashion, in order to shrink collagen fibers without causing necrosis. It is now known that the optimal temperature for avoiding stromal necrosis while still obtaining corneal collagen shrinkage is approximately 58°–76°C. Human collagen fibrils can shrink by almost two-thirds when exposed to temperatures in this range, as the heat disrupts the hydrogen bonds in the supercoiled structure of collagen. In the cornea, the maximal shrinkage is approximately 7%. When higher temperatures are reached (>78°C), tissue necrosis occurs.
In 1984, Fyodorov introduced a technique of radial thermokeratoplasty that used a handheld heated Nichrome needle designed for deeper thermokeratoplasty. The handheld probe contained a retractable 34-gauge wire heated to 600°C. A motor advanced the wire to a preset depth of 95% of the corneal pachymetry for a duration of 0.3 second. Fyodorov used different patterns to treat hyperopia and astigmatism. However, excessive heating of the cornea resulted in necrosis and corneal remodeling, and regression of treatment and unpredictability limited the success of this technique.
, FyodorovS, SandersDR. Radial thermokeratoplasty for the correction of hyperopia.1990;6(6):404–412.