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Phakic
intraocular lenses (IOLs) are a new
technology for the correction of high
refractive errors. They include both
anterior and posterior chamber varieties.
The main
anterior chamber IOLs under investigation
are:
The lens
is supported in an angle of the anterior
chamber and lies in front of the iris.
Complications include: Glaucoma, pupillary
distortion, and corneal damage
The lens
is put between the back of the iris
and the front of lens. Complications
associated with it are: Mainly cataract
formation. These lenses may be difficult
to remove because of adhesion formation.
Refractive
Lensectomy
Refractive
lensectomy, or clear lens extraction,
is the removal of the natural crystalline
lens for the treatment of high refractive
errors. A monofocal or multifocal
lens implant is inserted based on
the desired refractive outcome. Refractive
lensectomy has been used for the correction
of myopia, hyperopia, and presbyopia.
Refractive
lensectomy is essentially the same
surgical procedure as cataract extraction
and has the same complications, including:
Retinal
detachment remains a significant concern
when performing refractive lensectomy
on patients with high myopia. In a
group of 41 eyes with retinal detachment
after clear lens extraction, only
9 eyes achieved final visual acuity
of 20/60 or better. Retinal detachment
rates after refractive lensectomy
vary from 1.9% to 8.1%, depending
on the study and time to follow-up.
Patients
with high myopia have a higher incidence
of retinal detachment than the general
population. These patients account
for 42% of rhegmatogenous retinal
detachments, despite being only 10%
of the population. Patients with myopia
who undergo lens extraction and Nd:YAG
capsulotomy may further increase their
risk of retinal detachment.
Barraquer
et al. found a clear association between
Nd:YAG laser posterior capsulotomy
and retinal detachment (11% with YAG
capsulotomy vs. 5.5% without YAG capsulotomy)
in eyes undergoing refractive lensectomy.
Clinically significant posterior capsule
opacification requiring Nd:YAG capsulotomy
after refractive lensectomy ranges
from 8% to 61%, depending on the study
and time of follow-up.
Refractive
lensectomy is a viable option for
refractive correction at high extremes
of ametropia, but caution should be
exercised in cases of high axial myopia.
Refractive lensectomy is a good option
for patients who have corneas too
thin or irregular for corneal refractive
surgery. Furthermore, patients with
evidence of early nuclear sclerosis
may be better candidates for refractive
lensectomy if cataract extraction
would be anticipated in the next several
years.
Refractive
Surgery for Myopia, Myopic Astigmatism,
and Mixed Astigmatism Refractive surgical
options for the treatment of myopia
and myopic astigmatism include laser
surgeries, incisional surgeries, intrastromal
ring segments, phakic intraocular
lenses, and refractive lensectomy.
Bioptics, or a planned combination
of more than one refractive surgical
modality, is also gaining popularity.
For mixed astigmatism, several techniques
are being used, including astigmatic
keratotomy, photorefractive keratectomy,
and laser in situ keratomileusis.
Laser
Surgery
Photorefractive
Keratectomy
Photorefractive
keratectomy (PRK) was developed in
the late 1980s as the first laser
vision correction procedure. In October
1995, PRK became the first FDA-approved
laser treatment for the correction
of myopia and myopic astigmatism.
In PRK,
a surgeon uses a 193-nm argon fluoride
excimer laser to resculpt the surface
of the cornea to correct refractive
errors. In this procedure, the epithelium
is removed by one of several techniques,
including:
Following
the epithelial removal, the laser
reticule is centered over the entrance
pupil and the laser ablation is performed
on Bowman's membrane. The cornea is
irrigated with a balanced salt solution,
and a bandage contact lens is left
in place for 3-7 days, until the epithelium
regenerates. Most surgeons treat one
eye at a time because functional visual
acuity does not return until the epithelium
has healed.
Depending
on the type of laser used, PRK is
approved for the treatment of myopia
up to -13.0 D and astigmatism up to
-4.5 D. PRK is more predictable in
patients with a lower degree of myopia
(<6.0 D). Patients with a higher degree
of myopia who are treated with PRK
tend to have more regression of their
refractive effect , and more significant
haze.
To minimize
haze formation following PRK, surgeons
prescribe the use of topical steroids
for several months. In larger treatments,
the use of antimetabolites to prevent
haze formation may be beneficial.
Preliminary rabbit and human studies
suggest that a single intraoperative
application of topical mitomycin C
(0.2 mg/mL) may reduce corneal haze
associated with PRK. However, the
long-term safety of antimetabolite
use in refractive surgery has not
been established. Depending
on the study and the amount of myopic
correction, PRK has been successful
in achieving uncorrected visual acuity
of 20/40 or better in 67%-98% of patients,
with 48%-81% of patients achieving
20/20 uncorrected visual acuity. Long-term
refractive outcomes of PRK and LASIK
are similar.
Laser
Surgery
Laser
In Situ Keratomileusis
Since the introduction of laser in
situ keratomileusis (LASIK) in 1990,
there have been many reports describing its safety and efficacy. Uncorrected
visual acuity has been reported at 20/40 or better in 46.4%-100% of eyes,
depending on the study and degree of myopia. Higher refractive errors
have less predictable results, resulting in more under- and overcorrections.
Depending on the laser used, LASIK is approved by the FDA for treatments
of myopia up to -15.0 D and astigmatism up to -5 D. There have been reports,
however, of LASIK being used to treat myopic corrections of -25.0 D or
more.
In LASIK,
the microkeratome (laser or mechanical
types) suction ring increases intraocular
pressure to greater than 65-70 mm
Hg. This is confirmed by
The microkeratome
is used to make a corneal flap of
130-200 µm. Depending on the type
of microkeratome used, either a superior-
or nasal-hinged flap can be made.
The corneal flap is reflected back
toward the hinge, and the stromal
bed is dried. The laser reticule is
centered on the entrance pupil and
the excimer laser ablation is performed.
Balanced salt solution is irrigated
under the flap, which is then stretched
back into place and dried.
The flap
is inspected for lack of striae and
symmetry of the peripheral gutters.
If the flap or stromal bed is irregular,
laser treatment should not be performed.
The flap should be left to heal in
place, and a new flap can be cut in
6 months.
Postoperatively,
topical antibiotics and steroids are
used for 1-3 weeks. It may take up
to 1 month per diopter of correction
to achieve refractive stability. An
enhancement should not be considered
before 3 months, and in most cases
it is prudent to wait 6 months.
Advantages
of LASIK over PRK
Exclusion
criteria are the same as for PRK but
also include situations that make
flap creation difficult, including:
Poor
exposure (anterior buckles or deep-set
eyes) may interfere with the microkeratome
pass. Very steep or flat corneas increase
the risk of buttonhole formation or
free caps, respectively. Anterior
basement membrane dystrophy increases
the risk of epithelial defects and
subsequent lamellar inflammation.
In addition,
LASIK is not recommended for patients
at risk for ectasia, including those
with thin corneas, pellucid marginal
degeneration, or suspected keratoconus.
The current standard of care is that
250 µm of corneal tissue should be
left in the stromal bed to minimize
the risks of ectasia. However, there
have been reports of iatrogenic ectasia
even when the residual stromal bed
was of sufficient thickness. Therefore,
some surgeons recommend leaving up
to 250 µm in the stromal bed.
Laser
Surgery
Laser-assisted
Subepithelial Keratectomy (LASEK)
In recent years, LASIK has become the preferred choice for vision correction
because results demonstrate reduced postoperative discomfort and immediate
improved postoperative visual acuity. However, as reports of LASIK complications
surface, many surgeons and patients are indicating a preference for PRK.
Nevertheless, significant postoperative pain, slower visual recovery,
and haze remain deterrents to patient and surgeon acceptance of PRK.
Laser
epithelial keratomileusis (LASEK)
is a recent modification of PRK conceived
by Massimo Camellin, MD. LASEK may
reduce the incidence of postoperative
pain, speed visual recovery, and reduce
regression and haze when compared
to PRK.
In this
procedure, a trephine is used to make
an epithelial groove. A reservoir
is filled with an alcohol solution
and left on the eye for 30-60 seconds.
Then a microhoe is used to retract
a hinged epithelial flap. Laser treatment
is applied directly to Bowman's layer,
and the epithelium is replaced and
covered by a bandage contact lens.
If the epithelium is torn or lost,
the procedure is converted to a PRK
by removing the residual epithelium.
In LASEK,
the epithelial covering of the stroma
may reduce haze formation and improve
postoperative pain as compared to
PRK. The advantages of LASEK compared
to LASIK include:
It may
also be preferred in patients who
had previous glaucoma filtering surgery.
The few
published studies to date show encouraging
results of this new refractive procedure.
Scerrati et al. compared their results
from treating two groups of 15 patients
with either LASIK or LASEK. The results
in the LASEK group were superior to
those in the LASIK group when comparing
postoperative corneal topography, best
spectacle-corrected visual acuity, and
contrast sensitivity. Lee et al. studied
27 patients with low to moderate myopia
in which one eye was treated with LASEK
and the other with conventional PRK.
At 3-months' follow-up, no between-eye
differences in epithelial healing time,
uncorrected visual acuity, or refractive
error was found. The LASEK eyes, however,
had lower pain scores and corneal haze
than the PRK eyes. |
