Keratoplasty for corneal curvature alteration
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Keratoplasty is an opthalmic practice that corrects corneal curvature and hence is an effective therapy for hyperopia and hyperopic astigmatism. The process utilizes laser energy to heat and shrink the peripheral and paracentral corneal collagen, level the peripheral cornea and steepen the central cornea.

Types of keratoplasty

1. Laser thermal keratoplasty (LTK) uses laser energy to heat the cornea and enlarge its curvature. However, this process has the disadvantage of weakening of the refractive effect after sometime. However, LTK and associated thermal refractive surgical methods may see a better day with more advanced research about the effect of heat on corneal collagen and more high-tech heat implementing techniques. There is also hope for LTK, since off late there is a growing interest in wave front-based therapy.

2. Conductive keratoplasty (CK) uses low-energy high radiofrequency currents to steepen the cornea, by heating and shrinking the peripheral collagen. CK is considered a more secure option due to its more stable heat distribution, compared to LTK. Both the processes however, have the common advantage of an unharmed central cornea.

A short history of keratoplasty

The process of keratoplasty that uses heat to change the corneal curvature was pioneered by Gayet in 1879. At that time, he used cautery as a mode of treatment. This process was prevalent until Castroviejo established penetrating keratoplasty in 1936.

Lans, in 1898, implemented a treatment for astigmatism in rabbit eye by altering the corneal curvature by employing electrocautery to the peripheral cornea. Three reports of effective results of this process came up by 1933.

However, in 1964 thermal keratoplasty came to the forefront. This was when Stringer and Parr discovered the shrinkage temperature of corneal collagen in the range between 55–58°C. Over the past three decades, several non-laser and laser devices have been tested. In the early 1970s, a thermostatically controlled electric probe was done with mixed results.

However, this process is now used only as a surgical accessory during keratoplasty. Then came an ineffective 1.6 MHz radiofrequency probe (the Los Alamos probe) and a retractable wire probe. This was followed by the discovery of CO2 lasers and the holmium:yttrium-aluminum-garnet (Ho:YAG) solid-state laser. The former led to various risks but the latter showed promising results.

Ho:YAG laser appliances ensure proper means to prevent overheating of the cornea beyond the appropriate shrinkage temperature. This if ensured aids in collagen relaxation and the lesion healing process. What’s more, this laser beam with a corneal access depth of 480–530 µm, results in least harm to the surrounding tissues from stromal heating.

The effect of heat on corneal curvature

The application of heat causes the collagen to shrink up to one third of its original length. Proper increase of corneal collagen fiber temperature causes contraction and consequent flattening of the region where heat is applied.

There are two ways of applying heat to the cornea. They are:

Central heating: Heat applied to the 4 mm-diameter zone of the cornea has three results: A central corneal flattening reduction of the refractive capacity of the cornea and a hyperopic shift.

Peripheral heating: This kind of heating of the cornea shows a beltlike effect of peripheral flattening. This is accompanied by distinct collagen stress marks caused by every stromal burn. This in turn causes central steepening and improvement in the refractive strength of the eye. Moreover, optimum central steepening and simultaneous myopic shift occurs with an increase in peripheral burns and a decrease of the optical area outside the 4.5 mm-diameter.

We can also look at the effects of heat according to the conditions it will address:

Astigmatism therapy: When heat is applied for astigmatism, correction there is peripheral rise of temperature in the cornea along a single meridian, which is the flatter meridian. The heat application also causes central steepening along the meridian of therapy.

A combined therapy for hyperopia and astigmatism: A composite and complex procedure for hyperopia combined with astigmatism is also possible with application of heat to the cornea. This process can be tailormade for treatment of a greater part of the cornea or areas nearer to the visual axis down the flattest meridian of the cornea.

The disadvantage of corneal collagen contraction is that its results wane off with time in both humans and animals. This is probably caused since corneal fibroblasts keep producing fresh collagen.

At least three key factors are believed to play a role in achieving adequate refractive results with the application of heat to the cornea:

  1. The proper control of the collagen shrinkage temperature
  2. The collagen stability
  3. The keratocyte reaction
 
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