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Laser refractive surgery
is widely used for correcting the
refraction
and
astigmatism
of human eyes, allowing the patients to get rid of
glasses
or contact lenses.
Nowadays, these operations are applicable for myopic and
hyperopic cases from -15 to +6 D, including cases complicated by
cylinder up to 5 D. However, in more complex cases, when
aberrations of the third order or higher cannot be neglected
[
see Zernike
polynomials
],
or when no clinical correction data are available, the standard
operations cannot be carried out. In such cases, wavefront-guided
operations, also known as
customized ablation operations,
may be the way out.
The
aberration
data obtained with
MultiSpot aberrometers are used to calculate
customized operations
(two of the instruments are under clinical tests at Tambov and
Cheboksary branches of the S. Fyodorov Eye Microsurgery State
Institute). High repeatability and accuracy of measurements
permit treating cases caused by aberrations up to 6th order.
In cooperation with the
operation laser manufacturer,
Physics
Instrumentation Center, our group develops new
algorithms to calculate the operation profile, taking into
account all aberrations measured,
cornea topography
data (if available), and specific laser characteristics
affecting the operation procedure. An original algorithm
controlling the laser shot sequence provides a unique
possibility to reproduce the operational profile with a
deviation not exceeding 1 um within the optical zone.
In complex cases, the
surgeon can individually adjust the magnitudes of different
aberration groups (like astigmatism, coma, spherical etc.)
[
see Zernike
polynomials
]
or eliminate them totally.
The algorithm takes into
account the laser beam parameters, the dependence of the
ablation on the incident angle, and the
biophysical response of the
cornea. A special
attention has been paid to keep the
conic constant intact,
yet the surgeon can vary this parameter in order to diminish the
overall ablation depth.
Since 2003, several
hundreds of customized operations have been carried out. A
significant improvement of the post-operational vision quality
has been observed.
The poster
Utility of
dynamic aberrometry for acuity measurements and testing
(PDF, 1.55 Mb) contains more detailed information.
In
Fig. 1,
a schematic of LASIK
(Laser-Assisted In
Situ Keratomileusis)-type surgery is shown. Other types of the
laser refractive operations, which can be calculated from the
wavefront data, include
PRK (Photorefractive
Keratectomy), LASEK
(Laser Epithelial Keratomileusis), and Epi-LASIK.
Fig. 2
is an example of calculation of a wavefront-guided refractive
operation. The case is complicated by a large coma, which
prevents standard lens correction and reliable measurements with
the use of an autorefractor.
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