Because of the numerous refractive surgery terms that are
used almost exclusively in the successful understanding of refractive disorders,
first presenting a glossary of terms to assist the reader is essential.
Glossary of terms
Ablation - Removal. In excimer laser surgery, a
frequency of energy causes corneal molecules to detach from one another from
their points of attachment.
AK - Abbreviation for astigmatic keratotomy
ALK - Abbreviation for automated lamellar keratectomy
ArF - Abbreviation for argon fluoride
Astigmatic keratotomy - Surgical procedure in which
microscopic incisions are positioned in the peripheral cornea to create a more
spherical shape
Astigmatism - A refractive condition where the surface
of the cornea is not spherical. A distorted image is formed because light images
focus on 2 separate points in the eye.
Automated lamellar keratoplasty - An incisional
refractive surgery method used in low-to-moderate myopia. An automated
microkeratome placed on the eye removes, in an oscillating shaving motion, a
thin layer of cornea that is microns thick. Subsequent removal of a thinner
underlying corneal stroma is performed with stitchless replacement of the
initial superficial cap.
Excimer - Abbreviation for excited dimer
LASEK - Abbreviation for laser epithelium
keratomileusis
Laser - Abbreviation for light amplification by the
stimulated emission of radiation
Laser in situ keratomileusis - A refractive laser
procedure combining the use of ALK and photorefractive keratectomy in reshaping
the central cornea to treat refractive errors. An automated microkeratome,
similar to that used in ALK, is used to fashion a flap with a hinge. Subsequent
ablation is performed on the corneal stromal bed with stitchless replacement of
the corneal flap.
Laser epithelium keratomileusis - A modification of
the epithelial debridement performed in photorefractive keratectomy. In this
procedure, the epithelium is detached with the use of an alcohol solution
(usually 20%) to lyse DNA bonds (Cleave basement membrane) and to soften the
epithelium to facilitate rolling over into a flap. Ablation is performed
subsequently with replacement of the epithelial flap.
LASIK - Abbreviation for laser in situ keratomileusis
Light amplification by stimulated emission of radiation
- Laser light is composed of one color (wavelength) traveling in one direction,
and each light wave is traveling in step with the adjacent wave, making the
laser light more powerful by a factor of millions.
Oblate - Shape of the cornea after conventional laser
ablation profile where it is steeper in the periphery
PARK - Abbreviation for photoastigmatic refractive
keratectomy
Pachymetry - Optical or ultrasonic procedure to
measure the corneal thickness
Photoastigmatic refractive keratectomy - Although
similar to photorefractive keratectomy, the ablation profile is specific for
astigmatism.
Photorefractive keratectomy - Surgical procedure using
an excimer laser to fashion the central cornea to treat refractive errors
Prolate - Normal corneal shape; steeper in the center
PRK - Abbreviation for photorefractive keratectomy
Topography - Used to measure the low or high areas of
a plane
Wavefront - Describes the surface that connects all
the points on a propagating light wave that are of equal phase
Wavelength - The distance between the top of one wave
and the top of the next wave. In the case of an excimer laser, this is measured
in nm (eg, argon fluoride has a wavelength of 193 nm).
Astigmatism and the advent of photoastigmatic refractive
keratotomy
Astigmatism, a refractive condition where the surface of the
cornea is not spherical, can decrease visual acuity by forming a distorted image
because light images focus on 2 separate points in the eye. Clinicians and
surgeons have searched constantly for the most successful device or procedure to
treat this refractive error. Nonsurgical devices include spectacles and contact
lenses. To date, these devices are being improved continuously to address the
complex problem of astigmatism. Initial surgical approaches include astigmatic
keratotomy, compression sutures, and wedge resection. Recent surgical procedures
involve the use of the excimer laser in PARK and LASIK with or without wavefront-guided
technology.
PRK is the application of ultraviolet high-energy photons
(193-nm wavelength) of the ultraviolet range generated by an argon fluoride
excimer laser to the anterior corneal stroma to change its curvature and, thus,
to correct a refractive error. The physical process of remodeling by PRK is
called photoablation. This surgical procedure reshapes the central cornea to a
flatter shape for people who are nearsighted and a more curved surface for
people who are farsighted. Several techniques are being used to correct for
astigmatism.
Device description
Two different methods of energy delivery are available by the
excimer laser device, a large circular beam and a scanning slit or spot.
The earlier devices initially used large area ablation. To
date, some manufacturers still use large area ablation in their modern devices.
The circular laser beam passes through a diaphragm that slowly enlarges to
deliver more cumulative energy in the center and less in the periphery. Some
laser-induced irregularities (central islands) have been reported in these large
area systems. This method results in a shorter operating time to deliver the
necessary laser pulses versus a system that uses a scanning slit system. The
following manufacturers use circular beam lasers: Schwind (Coherent Medical Inc,
Palo Alto, Calif), Summit (Waltham, Mass), and VISX (Santa Clara, Calif).
The scanning slit or spot is an alternative method of energy
delivery by the excimer laser. A smaller beam passes through a beam-shaping
aperture, delivering a pattern of more pulses centrally than peripherally and
resulting in greater corneal tissue ablation centrally. Less total energy is
delivered at the corneal surface; therefore, a less powerful laser device may be
used. In principle, this system is more effective in providing different
ablation patterns in the treatment of astigmatism, irregular astigmatism, and
hyperopia.
The use of scanning laser technology with its small moving
beam has resulted in reduced thermal heating. This is visualized in a study that
showed the different areas of plume production after each area of ablation
following movement of the scanning beam. Central islands have not been reported
in these systems. The smaller ablation size of the scanning laser consequently
results in a longer operating time. Maintaining fixation has always been a
problem for these scanning lasers, especially with the longer operating time,
which results from more ablations by the smaller beams. Moreover, precise
pulse-to-pulse registration of the scan is necessary to achieve a smooth and
accurate final pattern. Automatic tracking devices are provided standard in
these devices.
Manufacturers of scanning slit systems include the following:
Autonomous (Orlando, Fla), LaserSite (Orlando, Fla), Meditec (Aesculap-Meditec,
Heroldsberg, Germany), Nidek (Fremont, Calif), Novatec (Carlsbad, Calif), and
Technolas (Chiron Vision Corp, Irvine, Calif). Novatec's claim to fame is its
use of solid-state laser crystals that obviates the need for argon fluoride gas
to create its shorter ultraviolet beam. Presently, the Food and Drug
Administration (FDA) has approved only the Summit and the VISX laser systems for
commercial use within the United States. All the other systems currently are
being used in other countries.
History of the Procedure:
The history of
surgical treatment for astigmatism dates back to the late 1800s. Certain
milestones in the development of this procedure can be attributed to several
individuals, and a number of parallel procedures were in development at certain
time periods.
In 1885, Schiotz performed limbal incision in the steep
meridian to reduce iatrogenic astigmatism. Faber performed anterior transverse
incisions to reduce idiopathic astigmatism. Lucciola also performed
nonperforating corneal incisions to correct astigmatism. In 1894, Bates
postulated that corneal incisions made at right angles to steeper meridians
might correct astigmatism. Later, Lans showed that flattening in the meridian
perpendicular to a transverse incision was associated with steepening in the
orthogonal meridian and that a greater effect may be achieved with deeper and
longer incisions.
In the 1940s, Sato began an extensive investigation of radial
and astigmatic keratotomy. Fyodorov is responsible for presenting several
nonperforating anterior keratotomy patterns.
Modern techniques for astigmatic keratotomy are attributed to
the works of Nordan, Thornton, and Nichamin. Nordan advocated a simple method of
straight transverse keratotomy, with a target correction of 1-4 diopters (D).
Thornton proposed a technique that included up to 3 pairs of arcuate incisions
in varying optical zone sizes and with consideration of age and timing after
surgery, respectively. Nichamin developed an extensive nomogram for astigmatic
keratotomy at the time of cataract surgery, although this technique has been
modified by the use of a limbal relaxing incision during cataract surgery.
Consequently, Troutman, who fancied wedge resection for reduction of postcorneal
transplant astigmatism, developed another technique of astigmatism reduction.
Increased interest in using lasers to ablate tissue occurred
in the late 1980s. The excimer laser initially was developed to etch out
inscriptions on microchips. The postulated application of controlled ablation on
corneal tissue led to its use in refractive correction.
In the late 1990s, wavefront aberrometry promised both
physicians and patients the potential to achieve the so-called supervision.
Initially used by astronomers, this wavefront technology reduced unwanted
wavefront distortions in the creation of land-based telescopes.
Problem:
Refractive errors (ie, myopia,
hyperopia, astigmatism) can decrease visual acuity. Astigmatism is a more
challenging entity because it is determined by regularity, amount, and
orientation. It also is more difficult to treat than myopia or hyperopia.
The quest to treat astigmatism began with the use of
nonsurgical devices, including spectacles and contact lenses. These nonsurgical
devices were followed by surgical techniques involving astigmatic keratotomy,
compression sutures, and wedge resection. Newer surgical procedures include the
use of intracorneal ring segments, PARK, LASIK, and LASEK, with or without
wavefront-guided technology.
Frequency:
The frequency of astigmatism has
a wide range of values as presented in modern literature. Naturally occurring
(idiopathic) astigmatism is common. Surgically induced (iatrogenic) astigmatism
is less common yet more problematic.
Clinically detectable refractive astigmatism reportedly is
present in as many as 9 out of 10 eyes. However, refractive astigmatism in most
of these eyes would not be clinically significant. The incidence of clinically
significant astigmatism has been reported to be 7.5-75%, a wide range that
primarily depends on the specific study and an author's definitions. Studies
have estimated that approximately 44% of the population has more than 0.50 D of
astigmatism, 10% of the population has more than 1.00 D, and 8% of the
population has 1.50 D or more.
Etiology:
Aside from the previously
mentioned idiopathic astigmatism that is present, iatrogenic astigmatism may
result after surgery. Visually significant refractive astigmatism is fairly
common after different kinds of ophthalmic surgery, including cataract
extraction, lamellar or penetrating keratoplasty, other corneal and anterior
segment surgeries, and trabeculectomy.
Reportedly, astigmatism of at least 1.00 D often results
after extracapsular cataract extraction, and astigmatism of at least 3.00 D is
present in as many as 20% of cases with 10-mm incisions. Even
phacoemulsification procedures, using the clear cornea technique, reportedly
cause postoperative astigmatism, thereby guiding the cataract surgeon as to the
proper placement of the corneal approach. High astigmatism usually results after
penetrating keratoplasty.
Pathophysiology:
The means of ablation of
the excimer laser seem to be photochemical in type. This removal of tissue is
called photochemical ablation or ablative photodecomposition. Photochemical
ablation is an extremely confined tissue interaction centered on the fact that
every photon created by the ArF excimer laser has 6.4 eV of energy, which is
sufficient to split covalent bonds.
The intramolecular bonds of uncovered organic macromolecules
are split when a sufficient number of high-energy, 193-nm photons are absorbed
in a brief period. The resulting fragments rapidly expand and are ejected from
the exposed surface at supersonic velocities observed under high-resolution
magnification as the plume effect. This is the reason why only the irradiated
organic materials are affected and the adjacent areas are not affected.
Clinical:
A patient with astigmatism may
complain of shadowing, bending, loss of contrast, and distortion. Astigmatism is
believed to be the most common cause of ametropia. In mild cases, it may cause
blurring of vision and ghosting. In more advanced cases of untreated
astigmatism, amblyopia may be noted. Astigmatism may occur naturally
(idiopathic) or secondary to surgical procedures (iatrogenic), such as cataract
extraction and penetrating keratoplasty. Several clinical procedures may be
performed to detect astigmatism. These procedures include automated and/or
manifest refraction, keratometry, Placido ring reflections, corneal topography,
and wavefront aberrometry.
