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:
- The proper
control of the collagen shrinkage temperature
- The collagen stability
- The keratocyte
reaction
