Document 6478060

Transcription

Document 6478060

Refractive Management Volume1: Module 1
Refractive Management
Volume 1: Module 1
Please note: The complete product is available online at
http://www.aao.org/education/clinicalupdates.cfm
This PDF is provided as an additional benefit for your convenience.
Refractive Surgery for Myopia, Myopic
Astigmatism, and Mixed Astigmatism
Nicole J. Anderson, MD, Elizabeth A. Davis, MD, David R. Hardten, MD
Objectives
Upon completion of this module, the ophthalmologist should be able to
1. Identify and describe current surgical techniques used to correct myopia, myopic
astigmatism, and mixed astigmatism, including laser correction, incisional techniques,
intrastromal corneal rings, phakic IOLs, and refractive lensectomy.
2. Describe the advantages and disadvantages to each surgical technique for the correction of
myopia, myopic astigmatism, and mixed astigmatism.
3. Identify clinical situations where one refractive surgical technique for the correction of
myopia, myopic astigmatism, or mixed astigmatism may be preferred to another.
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.
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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
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manual scraping
rotating brush removal
laser ablation followed by manual scraping (laser-scrape)
laser ablation (transepithelial)
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).1-5 Patients with a higher degree of myopia who are treated with PRK tend to have more
regression of their refractive effect3,6 and more significant haze.6-8
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.9,10 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.11-16 Long-term refractive outcomes of PRK and
LASIK are similar.46
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With some patients, PRK may be preferred to LASIK. These patients include those with
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anterior basement membrane dystrophy (ABMD)
corneas too thin for LASIK
small, deep-set orbits
superficial corneal scars
very steep or flat keratometry values
anterior scleral buckles
glaucoma, after trabeculectomy
optic nerve disease
insufficient corneal thickness
a risky occupation or activity
suspected keratoconus
Contraindications to PRK are
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an unstable refraction
evidence of keratoconus or pellucid marginal degeneration
irregular astigmatism on topography
autoimmune disease
severe dry eye or blepharitis
neurotrophism
certain medications (ie, isotretenoin, sumatriptan)
unrealistic expectations
age less than 18 to 21
Relative contraindications to PRK are
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a history of herpes simplex or zoster
pregnancy or lactation
immunosupression
advanced glaucoma
uncontrolled diabetes
keloid formation
high myopia or astigmatism
thin corneas
large pupils
Complications of PRK are4,6,7,13,15,17-22
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under- or overcorrection
haze or scaring
glare
halos
loss of contrast
irregular astigmatism
decentered ablations
central islands
infectious and noninfectious keratitis
reduced corneal sensation
reactivation of herpes simplex keratitis
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Laser Surgery
Laser In Situ Keratomileusis
Since the introduction of laser in situ keratomileusis (LASIK) in 1990,23 there have been many
reports describing its safety and efficacy.24-26 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.27 Higher refractive
errors have less predictable results, resulting in more under- and overcorrections.26,28 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.29
In LASIK, the microkeratome suction ring increases intraocular pressure to greater than 65–70 mm
Hg. This is confirmed by
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manual palpation
pupil dilation
subjective patient response of diminished vision
a variety of tonometers
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 (Video 1). 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 include
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ability to treat a broader range of myopia
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more rapid visual recovery
less postoperative haze
less postoperative discomfort
easier enhancement procedure
reduced need for long-term steroids
Exclusion criteria are the same as for PRK but also include situations that make flap creation
difficult, including
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deep-set eyes
very steep or flat corneas
anterior basement membrane dystrophy
glaucoma filtering surgeries
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.30,31 Therefore, some surgeons recommend leaving up to 300 µm in the stromal
bed.32,33
Complications
Complications of LASIK can be divided into intraoperative, early postoperative, and late
postoperative.
Intraoperative Complications
Intraoperative complications are estimated to occur in 0.68%–2.1% of cases.34-36 They can be
related to the microkeratome or the laser.
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Microkeratome-related complications include34-40
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free caps
buttonhole flaps
irregular, thin, or incomplete flaps
displaced flaps
epithelial defects
anterior segment perforation
intraoperative bleeding
Laser complications include39
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inadequate laser homogeneity
decentration of laser ablation
laser malfunction
central islands
incorrect ablation
under- and overcorrection
Early postoperative complications include36,38,39
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flap dislocation
flap edema
flap striae
epithelial defects
dry eye
interface debris
diffuse lamellar keratitis
infectious keratitis
Late Postoperative Complications include36,38,39,41
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epithelial ingrowth
night glare and halos
irregular astigmatism
late corneal haze
corneal ectasia
visual aberrations, including loss of contrast sensitivity
Certain complications can be prevented by careful patient selection. For instance, buttonhole flaps
can be minimized by choosing a smaller ring size for steep corneas (>46 D) or performing PRK.
Free caps are more common with excessively flat corneas (<41.0 D). The postoperative
keratometry value should be considered when planning LASIK because excessively flat corneas
(<34.0 D) increase the risk of visual aberrations.
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A postoperative complication unique to LASIK is diffuse lamellar keratitis (Sands of the Sahara
syndrome, or DLK). DLK is an inflammatory condition in which white blood cells collect in the
interface in a shifting sands' appearance.42 DLK is almost always present on the first postoperative
day. Treatment with hourly topical steroids should be instituted. If the cells coalesce on the central
cornea (stage III), the flap should be lifted and irrigated. DLK is multifactorial and has been linked
to43
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sterilizers
epithelial defects
glove powders
endotoxins
residual cleaning solutions on the instruments
infections
Severe or improperly treated cases can result in persistent haze, scarring, and flap melting.
The most common postoperative complication of LASIK is the dry eye syndrome. Factors that have
been implicated in postoperative dryness include27
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neurotrophic epitheliopathy secondary to nerve severance with the microkeratome
aqueous tear deficiency
poor tear film coverage of the altered corneal surface
Laser Delivery Patterns
These include broad-beam, scanning-slit, and flying-spot.
Broad-Beam Lasers
Broad-beam lasers deliver a laser beam of a particular diameter through a diaphragm that can
expand or contract to modulate the beam size. Typically, the beam begins small and expands as
the laser is delivered. The main advantage of broad-beam lasers is a shortened operative time,
which results in even stromal hydration throughout the ablation. The main disadvantage is that
broad-beam lasers treat all corneas the same and do not take into account corneal asymmetry.
Older broad-beam lasers resulted in central islands because the emitted laser plume masked the
cornea from successive laser pulses. New laser software addresses this by applying more
treatment to the central cornea.
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Scanning Excimer Lasers
Scanning excimer lasers, including scanning-slit and flying-spot lasers, provide a much smoother
ablation than broad-beam lasers. In addition, the profile can produce aspheric ablations and larger
diameter ablations. Scanning lasers can achieve any ablation profile, which is an advantage for
irregular or asymmetric corneas.
Outcomes
LASIK outcomes continue to benefit from advancements in technology. Eye-tracking devices rely on
infrared lasers or cameras to follow small eye movements and move the laser ablation beam
accordingly. Preliminary studies have shown better uncorrected visual acuity, best-corrected visual
acuity, and centration in certain patient groups (Hardten DR, McCarty TM, Lindstrom RL, et al.
Unpublished data, 2002) with eye-tracking devices. Larger ablation and blend zones may reduce
the incidence of glare and halos. Scanning lasers allow the ability to treat irregular asymmetric
corneas. Customized corneal ablation is in the forefront. Customized ablation can be guided by
topography or by wavefront mapping.
Wavefront analysis is able to detect refractive errors at multiple points over the entrance pupil of the
eye. It takes into account the whole optical system of the eye and determines how it deviates from a
normal wavefront. Wavefront-guided ablation will allow surgeons to customize an ablation for an
individual visual system. It will allow for the correction of irregular astigmatism and for the treatment
of higher order aberrations and LASIK-induced optical aberrations.
Preliminary results with wavefront-guided ablation suggest reduced higher order aberrations and
improved visual acuity as compared to results from standard excimer laser surgery. Wavefront
sensors are currently available in the United States for diagnostic purposes only. It may be several
years before wavefront-guided ablation will be approved for use in the United States.
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Laser Surgery
Laser-assisted Subepithelial Keratectomy
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,35,44-46 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.47-51
Laser epithelial keratomileusis (LASEK) is a recent modification of PRK conceived by Massimo
Camellin, MD (Video 2). LASEK may reduce the incidence of postoperative pain, speed visual
recovery, and reduce regression and haze when compared to PRK. 52,53
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
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eliminating flap complications
minimizing risks of corneal ectasia
LASEK may be preferred to LASIK in patients with
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thin corneas and high corrections
deep-set eyes
steep or flat corneas
risky occupations
suspected keratoconus
It may also be preferred in patients who had previous glaucoma filtering surgery.
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The few published studies to date show encouraging results of this new refractive procedure.52-56
Scerrati et al.55 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.53 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.
Incisional Surgery
Radial Keratotomy
Radial keratotomy (RK) is an incisional procedure popularized in the 1970s by Fyodorov. This
procedure is performed by making deep radial incisions in the paracentral cornea with a diamond
blade (Figure 1). The effect of these incisions is to cause bulging of the peripheral cornea and
corresponding flattening of the central cornea.
Figure 1. Here, eight radial incisions are made in the cornea during radial keratotomy.
Factors that affect surgical outcome include
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iameter of the central clear zone
number of incisions
depth of incisions
patient age
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The results of RK are best in patients with low to moderate myopia (up to -6.0 D).57 The Prospective
Evaluation of Radial Keratotomy (PERK) study was a multicentered study aimed at evaluating longterm stability after RK. At 10 years following surgery, 38% of patients were within ±0.5 D and 67% of
patients were within ±1.0 D of the intended correction. Uncorrected visual acuity was 20/20 or better
in 53% of patients and 20/40 or better in 85% of patients.
Complications
Intraoperative complications of RK include58,59
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microperforation
macroperforation
anterior lens perforation
invasion of the optical zone
intraoperative bleeding
Postoperative complications include60-65
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glare
epithelial ingrowth
endophthalmitis
late traumatic ruptured incisions
infectious and noninfectious keratitis
under- and overcorrection\
induced astigmatism
endothelial cell loss
diurnal fluctuation in vision
difficulty with contact lens fitting
One of the most common problems with RK is the instability of the postoperative refraction.
Postoperative diurnal fluctuation in vision and progressive hyperopia are common after RK. In the
PERK study, 43% of eyes had a hyperopic shift of greater than 1.0 D between 6 months and 10
years following surgery. The hyperopic shift was statistically associated with the diameter of the
clear zone, with smaller optical zones inducing more hyperopia.60
Treatment of consecutive hyperopia after RK has proved challenging. Purse-string and interrupted
suturing have been described but have not been predictable in most cases.66,67 Photorefractive
keratectomy (PRK) has also been tried but is associated with haze.68 Laser in situ keratomileusis
(LASIK) after RK is complicated by flap-splitting at the incision sites and epithelial ingrowth into the
incisions.69,70 Because of the risk of haze associated with PRK and the flap-associated problems
with LASIK, some surgeons are using PRK with intraoperative mitomycin-C in the treatment of postRK hyperopia.
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Few surgeons today perform RK because of advances in excimer laser technology. However, RK
has some distinct advantages over modern forms of refractive surgery, including
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incisions that do not directly involve the optical zone
more long-term data on this procedure than on other refractive surgical options
equipment that is less expensive than that used in laser surgery
Surgeons in communities that do not have access to laser technology may continue to offer RK as a
method of correcting mild to moderate myopia. Although today excimer laser surgery is preferred
over RK, when surgeons are limited by cost or laser accessibility, RK may still have a small but
limited role in the correction of myopia.
Incisional Surgery
Astigmatic Keratotomy
Figure 2. In astigmatic keratotomy, transverse or arcuate incisions are used to flatten the steep corneal
meridian and to steepen the flat meridian.
Astigmatic keratotomy (AK) is an incisional method of reducing corneal astigmatism (Figures 2, 3).
Transverse or arcuate AK incisions are placed in the steep corneal meridian to flatten it and to
steepen the flat meridian (coupling). The coupling ratio is defined as the amount of flattening in the
steep meridian compared to the amount of steepening in the unincised flat meridian. The coupling
ratio depends on the incision's
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Figure 3. A diamond blade is used to make incisions in astigmatic keratotomy.
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length
type
depth
location
Astigmatic keratotomy can achieve a coupling ratio near 1, so the spherical equivalent remains
unchanged. Therefore, AK is a good option for patients with 1.5 to 3.0 D of astigmatism whose
spherical equivalent is near plano.
Several nomograms for AK determine the cutting parameters based on the primary determinants of
refractive effect. Factors that influence the refractive outcome include71-73
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size of optical zone
length of incision
depth of incision
number of incisions
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Incisions are typically placed between a 5-mm-diameter and an 8-mm-diameter zone. Incisions
closer to the visual axis cause a greater refractive effect. However, closer incisions (<5.0 mm optical
zone) can cause glare, halos, ghosting, and irregular astigmatism.73
Incision length varies from 30° to 90° for arcuate incisions and 2–3 mm for transverse incisions.
Depth of the incisions is typically 80%–90% of corneal thickness. Other important factors in the
refractive effect achieved are gender, age, and race.74-76 Axis alignment and degree of preoperative
astigmatism may also cause variations in the astigmatic correction achieved.77
AK remains an alternative to laser ablation for the correction of naturally occurring astigmatism or
after intraocular surgery such as penetrating keratoplasty and cataract surgery. Large-optical-zone
AK, which is commonly termed limbal relaxing incisions, is commonly combined with cataract
surgery to reduce concomitant astigmatism at the time of small-incision phacoemulsification.
Incisional Surgery
Automated Lamellar Keratoplasty
The importance of automated lamellar keratoplasty (ALK), a technique that has now been largely
abandoned, was twofold:
1. It demonstrated that removal of central corneal tissue would result in corneal flattening and
the correction of myopia.
2. The invention of the automated microkeratome by Luis Antonio Ruiz allowed the creation of
a corneal lamellar flap dissection. In this technique, the microkeratome makes two refractive
cuts into the anterior to mid stroma, producing a disk of stromal tissue. The disk is excised
and the overlying flap is repositioned. The thickness of the excised corneal tissue
determines the amount of correction.
The ALK technique has been largely abandoned due to its poor predictability, a small effective
optical zone, irregular astigmatism, and regression.78-80 New generation microkeratomes and the
accuracy of laser ablation has made LASIK a much more predictable procedure.80
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Intrastromal Corneal Ring Segments
Intrastromal corneal rings (Intacs, Addition Technology, Fremont, CA) were approved by the US
Food and Drug Administration in April 1999. Intacs consists of two 180° polymethylmethacrylate
(PMMA) ring segments that are inserted into the midperipheral cornea at two-thirds corneal depth
(Figure 4). A 1.2-mm corneal incision is made, and a lamellar dissecting instrument is rotated in
each direction to create two intrastromal channels (Video 3). Ring thicknesses of 0.21 mm and
0.25-0.45 mm (in 0.5-mm increments) are available and are inserted into the channels (Video 4).
Increasing ring thickness produces a shorter arc length and further central corneal flattening.81,82
Figure 4. Two arc-shaped polymethylmethacrylate (PMMA) ring segments
The prolate aspheric shape of the cornea is maintained in this procedure because the central
corneal tissue is not treated. Maintenance of corneal sphericity may be beneficial in reducing
spherical aberrations, glare, and contrast loss.83,84
In the phase II and III clinical trials, 97% of patients had 20/40 or better uncorrected visual acuity
and 76% had 20/20 or better uncorrected visual acuity at 24 months' follow-up.85,86 Intacs are
approved only for the treatment of mild myopia (-1.0 to-3.0 D spherical equivalent) with little or no
astigmatism (<1.0 D).
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Advantages
An advantage of Intacs over laser ablation is that the original rings can be replaced with a different
ring size at a later time to revise the correction. Furthermore, the refractive effect may be reversible
after removal of the ring segments. In the phase II and III FDA clinical trials, 4.7% of eyes had
explantation of their ring segments. At 3 months following explantation, 86% of these eyes returned
to within ±0.5 D and 95% returned to within ±1.0 D of preoperative spherical equivalent refraction.86
Reasons for lens removal include86-88
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glare
halos
night vision problems
infection
induced astigmatism
patient dissatisfaction
Advantages of Intacs over LASIK
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maintenance of the prolate shape of the cornea to reduce spherical aberration
preservation of the central corneal tissue
reversibility
adjustability
LASIK, however, still remains the most common choice among refractive surgeons for the
correction of mild myopia because of
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ease
rapid visual recovery
ability to do bilateral simultaneous surgery
ability to treat astigmatism
predictability
Because only 5 different sizes of Intacs are available, essentially only 5 prescriptions are available,
whereas with excimer laser correction, an unlimited degree of precision is available.
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Complications
Complications of Intacs include87,89-91
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over- or undercorrection
induced astigmatism
epithelial defects
corneal thinning
infectious keratitis
epithelial inclusion cysts
segment migration
neovascularization
channel haze
glare
halos
anterior or posterior perforation
Mean induced astigmatism in the FDA trials was 0.13 D at 12 months. The astigmatism was more
frequently with-the-rule than against-the-rule and appeared to increase with segment thickness.91
Phakic Intraocular Lenses
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
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the Artisan lens (Ophtec USA Inc./Allergan, Boca Raton, FL)
the Baikoff NuVita MA20 lens (Bausch and Lomb Surgical, Irvine, CA)
The two main posterior chamber IOLs under investigation are
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the Implantable Contact Lens (ICL, STAAR Surgical, Monrovia, CA)
the Phakic Refractive Lens (PRL, Medennium, Inc/CibaVision, Atlanta, GA
Artisan Lens
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The Artisan anterior chamber lens was designed by Jan Worst and has been used in the
Netherlands for 12 years. The Artisan lens is an 8.5-mm one-piece PMMA lens with a 6.0-mm optic.
It is an iris-supported IOL, with fixation to the peripheral iris stroma.92-98 The optic is convexconcave, which ensures vaulting over the natural lens after insertion. Fixation of the lens to the iris
is achieved by lifting a fold of iris stroma through an opening in the haptics in a process called
enclavation (Video 5).
Clinical investigation of the Artisan lens in the United States reported that 97% of eyes were 20/40
or better uncorrected at 6 months, where the postoperative goal was ±0.5 D of emmetropia,
monovision eyes were eliminated, and postoperative cylinder was <1.0 D.99
Compared to LASIK, the Artisan lens may provide100
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improved contrast sensitivity
lower enhancement rates
ability to remove or exchange the lens
Complications include101-103
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iridocyclitis
iris atrophy
lens dislocation
pupil ovalization
decreased corneal endothelial cell density
The incidence of cataract formation is very low because of the position of the lens in the anterior
chamber away from the anterior surface of the crystalline lens. The lens is made in powers ranging
from -5 to -20 D.
NuVita MA20 Lens
The NuVita MA20 anterior chamber lens (formerly the Baikoff ACIOL) is a PMMA lens with a fourpoint fixation into the anterior chamber angle (Figure 5).104 There is little difference between
implantation of this lens and other anterior chamber lenses used for the correction of aphakia. This
lens is sized by measuring the horizontal white-to-white limbal diameter and adding 0.5–1.0 mm.
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Lens size is important to prevent late ovalization of the pupil or migration of the implant. The lens is
available in powers ranging from -7.0 to -20.0 D and sizes ranging from 12.0 to 13.5 mm.
Figure 5. The NuVita MA20 is an anterior chamber intraocular lens.
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Complications are105
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pupil ovalization
glare
iridocyclitis
implant migration
In one study, endothelial cell loss of 12% occurred at 2 years following implantation of the NuVita
lens.105
Implantable Contact Lens
The Implantable Contact Lens (ICL) is a foldable collamer posterior chamber lens made from a
mixture of hydrogel and collagen polymer (collagen 0.3%/2-hydroxyethyl- methacrylate).106 It is very
thin (50 µm at the optical zone), permeable, and hydrophilic. It is placed through a 3-mm clear
corneal incision between the iris and the natural crystalline lens (Videos 6, 7). The lens is 10.8–13
mm long, with optic sizes ranging from 4.5 to 5.5 mm. The myopic powers range from -3.0 to -20 D.
It has been found in several studies to be a safe and effective treatment of myopic refractive errors,
with 67%–81% of patients falling within ±1.0 D of intended correction postoperatively.107-109
Complications specific to this lens include106,109-111
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cataract formation
pupillary entrapment
decreased endothelial cell density
crystalline lens touch
increased aqueous flare
pupillary block
In addition, natural crystalline lens transmittance of light appears to decrease over time.111
Furthermore, peripheral posterior chamber phakic intraocular lens and crystalline lens touch has
occurred in up to 60% of patients in one study.111 Trace trabecular meshwork pigmentation has
been found, but this has not been correlated with increased IOP or pigmentary glaucoma.112
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Phakic Refractive Lens
The Phakic Refractive Lens (PRL) is a one-piece hydrophobic silicone plate-haptic posterior
chamber IOL. It is foldable and can be inserted through a 3.5-mm clear corneal incision. It is
designed to be independent of intraocular support for fixation and floats on an aqueous fluid layer
over the natural crystalline lens. The lens powers to treat myopia range from -3 to -20 D.
Advantages
Phakic IOLs offer several advantages over incisional or laser surgeries. The preservation of corneal
sphericity may improve contrast sensitivity and diminish optical aberrations. In patients with high
myopia, large ablations and small ablation zones may increase visual aberrations and place the
patient at risk for corneal ectasia. Patients who are not good candidates for LASIK may be
candidates for phakic IOLs. However, large pupil size and insufficient anterior chamber depth can
be limiting factors for patient selection for these lenses.
Disadvantages
Disadvantages of phakic IOLs include
•
•
•
•
•
pupil ovalization in some models
cataract formation
iritis
complications related to any intraocular surgery
Preoperative or intraoperative peripheral iridectomies are required in all patients.
Currently, phakic IOLs are in phase III FDA clinical trials and approval is anticipated in the next
couple of years.
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
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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, including113-117
•
•
•
•
•
•
endophthalmitis
secondary glaucoma
cystoid macular edema
posterior capsular opacification
loss of accommodation
retinal detachment
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.118 Retinal detachment rates after
refractive lensectomy vary from 1.9% to 8.1%, depending on the study and time to follow-up.114116,119
Patients with high myopia have a higher incidence of retinal detachment than the general
population.120 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.116 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.114,115,121
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.
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Bioptics
Bioptics, popularized by Roberto Zaldivar, is the planned combination of phakic or aphakic
intraocular lens surgery with corneal surgery to correct large refractive errors.
Typically, the maximum IOL power is used, and the undercorrection is corrected by corneal ablation
surgery (PRK or LASIK). The surgeries can be staged with the lens surgery performed first, followed
later by PRK or LASIK. Alternatively, the LASIK flap can be made at the time of the lens surgery
and lifted several weeks later for laser ablation.
Bioptics can be performed with either phakic IOLs or clear lens extraction. Preliminary results with
these techniques have been encouraging for high myopia.122-126 Bioptics is especially useful in
patients with high myopia, as traditional IOL calculations can be less accurate secondary to long
axial lengths, posterior staphylomas, reduced accuracy of lens power calculations, and vertex
distance adjustments.
Bioptics is often preferable to laser ablation alone because of the reduced risk of visual aberrations,
contrast loss, glare, and halos that are associated with extremely large myopic excimer laser
ablations. In addition, the increased tissue ablation and smaller optical zones necessary with large
myopic corrections decrease the predictability and stability of laser refractive surgery.
Potential complications of bioptics are complications of the corneal and intraocular lens surgery.
Mixed Astigmatism
In mixed astigmatism, one focal line is projected in front of the retina while the other focal line is
projected behind the retina. Therefore, the spherical equivalent is near plano. Several surgical
techniques can be used to treat mixed astigmatism, including incisional surgery, such as astigmatic
keratotomy (AK), and laser surgery, such as photorefractive keratotomy (PRK) and laser in situ
keratomileusis (LASIK).
In AK, transverse or arcuate incisions are placed in the steep corneal meridian to flatten the steep
meridian and steepen the flat meridian (coupling). The coupling ratio is defined as the amount of
flattening in the steep meridian compared to the amount of steepening in the unincised flat meridian.
The coupling ratio depends on the incision's
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•
•
•
•
length
type (arcuate or tranverse)
depth
location
as well as on the number of incisions.
Astigmatic keratotomy can achieve a coupling ratio near 1, meaning the spherical equivalent
remains unchanged.127,128 It is, therefore, an ideal procedure for patients with mixed astigmatism.
Both PRK and LASIK are also used for the correction of mixed astigmatism. Many excimer lasers
now have software programs for treating it. Previously, the surgeon could decide to treat mixed
astigmatism in one of four ways:
1. Negative cylinder ablation flattens the steep axis first, with a myopic cylinder ablation. The
resulting hyperopia is treated with a spherical hyperopic ablation.
2. Positive cylinder ablation steepens the flat axis first and then flattens both axes with a
myopic spherical ablation.
3. Cross-cylinder approach treats half the cylinder in the steep meridian and half in the flat
meridian. The remaining spherical equivalent is then treated.
4. Bitoric ablation treats the entire cylindrical correction, leaving no spherical equivalent to
treat. A negative cylinder ablation is used to flatten the steep axis and then a positive
cylinder ablation steepens the flat axis. Because there is no compensatory spherical
ablation, more corneal tissue is preserved.
Some authors have found no difference in outcomes among positive cylinder, negative cylinder, and
bitoric ablations.129 Other authors have found a reduced frequency of reablation with bitoric
ablations compared with monotoric ablations.130 Furthermore, bitoric ablations have the advantage
of removing less tissue and possibly improving optics.131
In comparing LASIK and AK for the treatment of mixed astigmatism, Chayet and colleagues
achieved a 91% decrease in the amount of preexisting astigmatism after treating it with bitoric
LASIK.132 This compares with only a 72% cylinder reduction after the same authors treated mixed
astigmatism with AK.128 Other authors have found no significant difference in outcomes between AK
and LASIK.133
The current trend is toward treating mixed astigmatism with LASIK because of the availability and
improvements in laser software. In the FDA clinical trials of VISX's (VISX Inc, Santa Clara, CA)
mixed astigmatism software, 99.1% of eyes treated achieved uncorrected visual acuity of 20/40 or
better and 61.28% achieved 20/20 or better at 6 months' posttreatment.
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