Surgical techniques
Surgical techniques to correct refractive errors can be
divided into 4 categories: incisional, thermal, lamellar and
intraocular (Table 2). Common incisional surgery includes
radial and astigmatic keratotomy (Fig. 6). For these procedures,
a thin, ultra-sharp diamond knife is used to create
partial-thickness incisions in the cornea to weaken it structurally,
allowing the intraocular pressure and biomechanical
forces to induce changes in its curvature.
With thermal techniques, heat is used to shrink collagen
in the periphery of the cornea; this results in the formation
of a bandlike constriction that steepens the centre of the
cornea while flattening the periphery for hyperopia correc-
tion. Use of the noncontact holmium:YAG laser is currently
the most common method of thermal delivery and
can correct mild hyperopia without involving the corneal
epithelium or the central visual axis.10
Lamellar procedures are the most widely used in refractive
surgery. Common procedures include PRK for myopia, photoastigmatic
refractive keratectomy (PARK), PRK for hyperopia
(H-PRK) and LASIK. The intrastromal ring can be used
to correct mild myopia with the insertion of 2 thin crescentshaped
polymethylmethacrylate segments into the periphery
of the cornea. The segments flatten the centre of the cornea;
the degree of correction is determined by the thickness of the
segments. The segments leave the central optical zone untouched,
which results in excellent quality of vision, and they
can be removed if the refractive effect needs to be reversed.
Early results indicate that the use of a scleral expansion
band—whereby 4 crescent-shaped polymethylmethacrylate
segments, each 4.5 mm long and 600 µm wide, are inserted in
the sclera 2–2.5 mm from the limbus—can reverse the effects
of presbyopia by restoring accommodation. The use of
this scleral expansion band can be combined with corneal refractive
surgery to correct other refractive errors.
Intraocular surgery can be used to correct severe myopia
or hyperopia. The surgery is performed in the same way as
cataract surgery if the natural lens is removed. If the natural
lens is clear and the patient has accommodation, an intraocular
implant (phakic refractive lens or implantable
contact lens) can be positioned in the anterior chamber,
fixed to the iris or placed in the posterior chamber behind
the iris, in front of the patient’s lens. The main advantages
of intraocular procedures are rapid rehabilitation, better
predictability of outcome and excellent quality of vision, especially
in people with severe refractive errors. Disadvantages
include loss of accommodation if the natural lens is
removed and complications that are associated with any intraocular
surgery (corneal decompensation, retinal detachment,
especially in people with severe myopia, and postoperative
endophthalmitis). The long-term risk of cataract
formation after intraocular lens implantation is unknown.11
Surgical techniques for myopia
The 2 most common surgical techniques for the correction
of myopia and myopic astigmatism are PRK and
LASIK.
Photorefractive keratectomy
PRK was the first and is the simplest laser refractive surgical
procedure performed in Canada. A lamellar procedure, it
relies on the unique ability of the excimer laser to remove
submicrometer amounts of tissue from the central region of
the cornea through photoablation (Fig. 7). The cornea is
thereby flattened and the refractive power of the eye decreased.
The higher the degree of correction and the larger
the treatment diameter, the more tissue that must be ablated.
Astigmatism up to 6.00 D can be corrected by preferentially
ablating the steepest meridian or by ablating an elliptical
shape that will correct both the myopia and the astigmatism.7
Procedure
On the day of surgery, the patient is re-examined to ensure
that the refractive error is stable. The patient is then
placed in a supine position. A topical anesthetic is applied,
and a lid speculum is used to open the eye being treated.
The patient is instructed to fix on a target so that the laser
beam can be directed at the centre of the pupil. Epithelium
is then removed mechanically (with a spatula or rotating
brush), chemically (with alcohol) or with the laser. After the
epithelium is removed, ablation with the laser is performed
while the patient fixes on a blinking light. With broadbeam
lasers the ablation starts in the centre of the cornea
and expands peripherally to produce central flattening of
the cornea. With scanning and flying-spot lasers, the ablation
is applied more randomly in an attempt to produce a
smoother ablation profile.
Many surgeons treat both eyes during the same session.
Some delay treatment of the second eye for a few days or
weeks until they are assured of a good result in the first eye.
The duration of treatment for each eye depends on the degree
of correction required, but it usually lasts less than 60
seconds.
Postoperatively, a bandage contact lens or eye patch is
applied, along with antibiotic and anti-inflammatory eye
drops. The patient is monitored closely to assess the healing
process and to detect infections early. Daily follow-up
visits to the surgeon is mandatory until the epithelium is
healed. Epithelium normally heals in 3–4 days for myopia
and 4–5 days for hyperopia. Follow-up in the first few
months, especially if topical steroids are used, is important
and can be performed by the ophthalmologist or co-managed
with an optometrist.
Outcomes
Results of PRK are generally excellent and are related to
the degree of correction required and the preoperative visual
acuity.7 Recent data from the University of Ottawa Eye
Institute12 on 408 eyes treated with a broad-beam laser
showed that, at 1 year postoperatively, 97% of the 286 eyes
with mild myopia achieved an uncorrected visual acuity
(the visual acuity measured when a person is not wearing
any spectacles or contact lens [Appendix 1]) of 20/40 (metric
6/12), which is the legal vision requirement for driving,
and 68% achieved an uncorrected visual acuity of 20/20
(6/6); 97% were within ±1.00 D of the intended correction.
Haze was minimal in this group; 68% of the patients had
no loss of BSCVA (best spectacle-corrected visual acuity
[Appendix 1]), and no patient lost more than 1 line on the
Snellen chart. Of the 122 eyes with moderate myopia
100% achieved an uncorrected visual acuity of 20/40 (6/12)
and 58% an uncorrected visual acuity of 20/20 (6/6); 92%
of the eyes were within ±1.00 D of the intended correction.
Most (78%) of the patients had no loss of BSCVA, 21%
lost 1 line on the Snellen chart, and 1% lost 2 lines. Follow-
up at 2 years showed findings that were not significantly
different from those at 1 year.
The most common causes of loss of BSCVA are irregular
astigmatism and haze. Patients with severe myopia have
had lower success rates and may require a second treatment
to achieve the desired results. Some surgeons have had improved
outcomes with a planned 2-stage approach,13 where
8.00 to 10.00 D is corrected at the first session and the remaining
refractive error corrected 1 to 2 months later.
Complications
Although PRK is very effective and most patients are
pleased with the results, there are potential complications
(Table 3).6,7,9 The most common is the sensation of glare
and haloes, especially at night, similar to that experienced
by people who wear contact lenses. This complication is
more common in patients with large pupils and a small ablation
zone (less than 6 mm). The prevalence is about
1%–5% at 6 months after PRK, and it decreases with
longer follow-up. Glare and haloes are caused primarily by
the spherical aberration from the centrally flattened cornea.
It worsens at night as the pupil dilates and more peripheral
light enters the eye through the peripheral transition zone.
Haloes and glare can also be associated with loss of contrast
sensitivity when the person is looking at low-contrast objects
in dim light. Fortunately, in most cases the symptoms
disappear with time. If glare and haloes persist, however,
retreatment with an ablation zone greater than 6.5 mm
may resolve the problem.
Although the result of PRK is very predictable (over
90% of eyes will be within ±1.00 D of the intended correction),
over- and undercorrection are still possible, especially
in people with severe myopia. Overcorrection to hyperopia
is not tolerated well by people who have myopia and presbyopia.
Early overcorrection may occur in older patients
with moderate or severe myopia. Sustained overcorrection
is uncommon but can occur if there is minimal healing after
PRK. Treatment of overcorrection involves stopping
the use of corticosteroids, applying a bandage contact lens
or scraping the epithelium to stimulate wound healing and
using a topically applied NSAID for 1 month. If the patient
remains symptomatic at 6–12 months, H-PRK or holmium:
YAG laser surgery are the best treatment options and give
good results.
Undercorrection is more common than overcorrection,
occurring in 4%–10% of eyes with mild myopia and in
25% of those with severe myopia. If undercorrection occurs,
patients who may be accustomed to being myopic will
experience at least some improvement of vision; however,
significant undercorrection is disappointing to patients.
Unlike overcorrection, undercorrection is due to a more
Refractive surgery
aggressive healing response, with refractive regression usually
accompanied by corneal haze. For persistent undercorrection,
with or without haze, retreatment with PRK is safe
and effective in improving visual acuity without increasing
haze.14 Currently, there is controversy over whether retreatment
should be done in the first few months after
surgery or after at least 6 months.15
Myopic regression because of a lack of long-term refractive
stability is possible after PRK. It usually occurs within
the first 6 months but can present up to 18 months postoperatively.
The risk factors for regression are correction of
severe myopia, presence of regression and haze after treatment
of the first eye, significant ultraviolet light exposure,
sun tanning, pregnancy, viral infection and use of an oral
contraceptive.16 The incidence of regression is currently
greatly reduced with newer ablation algorithms, and our
experience with a broad-beam laser suggests that the average
amount of regression at 2 years is only –0.30 D. If regression
occurs, topical steroid therapy will be started in
most cases and may have dramatic results in some patients.
17,18 For persistent regression, retreatment with PRK
and avoidance of risk factors are usually satisfactory.19
Other complications of PRK include surgically induced
astigmatism, decentration (off-centre ablation) and central
islands (areas of undercorrection within the treatment centre).
However, these are largely preventable and are currently
very rare with new laser technology and experienced
surgeons.20,21
Haze formation or scarring occur in 3%–17% of people
with severe myopia and can cause decreased BSCVA.22
Haze and its most severe form scarring result from an aggressive
healing response, in which there is increased production
of stromal extracellular matrix and collagen. Mild
haze typically presents 2 to 4 weeks after PRK, peaks between
1 and 3 months and gradually resolves in 1 to 2
years. It frequently is associated with myopic regression.
The risk factors for haze include correction of severe myopia,
delayed healing of the epithelium, prior haze formation
in the same or fellow eye, and unprotected exposure to
ultraviolet light. As a precaution all patients should wear ultraviolet-
protection sunglasses for at least 1 year postoperatively.
For patients with marked haze within the first
month after surgery, the use of topical steroids can help to
reduce the level of haze and the associated myopic regression.
If significant haze persists beyond 6–12 months, retreatment
with PRK is successful in some patients, especially
those with significant regression. However, these
patients are prone to haze formation after retreatment and
need to use topical steroids for a number of months after
retreatment. If significant haze develops before the second
eye is treated, other refractive surgery techniques such as
LASIK may be considered.
Infectious keratitis is the most devastating complication
of PRK, but fortunately it occurs in only 0.1%–0.2% of
cases. It usually appears 24–48 hours after surgery. Significant
corneal infection can cause scarring or perforation of
the cornea, which requires corneal transplantation. It is essential
to follow-up patients daily after PRK until epithelialization
is complete. Other reported but rare complications
include reactivation of herpetic keratitis, posterior
subcapsular cataract formation and increased intraocular
pressure owing to steroid use.7
Laser-assisted in situ keratomileusis
LASIK is a lamellar procedure that combines photoablation
using the excimer laser and an intrastromal surgical
technique that preserves the integrity of the outer layer of
the cornea.23 A microkeratome is a mechanical device that
uses a high-vacuum suction ring to fix on the globe and a
motorized blade to perform a partial-thickness cut in the
cornea (Fig. 8). The depth of the cut is controlled by the
microkeratome and is usually 130–160 µm, depending on
the degree of correction required. After the corneal flap is
made, the beam from the excimer laser is then applied to
the stromal bed in a fashion similar to that in PRK to correct
myopia, hyperopia or astigmatism. After the ablation,
the corneal flap is replaced.
The main advantages of LASIK over PRK are the rapid
recovery of vision (in 2–3 days compared with 5–14 days)
and less discomfort. The amount of haze and regression is
also less with LASIK, especially in people with severe myopia.
As a result, LASIK is most commonly used to treat
more severe myopia, but with experience it is being used to
treat all degrees of myopia. The rapid recovery of vision
has encouraged many people to have both eyes treated during
the same session. The main disadvantages of LASIK
are related to the intraoperative complications caused by
the microkeratome. Flap-related problems include debris
becoming trapped under the flap, epithelial ingrowth, misalignment,
wrinkling of the flap and diffuse lamellar keratitis
(“shifting sands of the Sahara”).
Outcomes
LASIK is a relatively new procedure, and most reported
data are only for a mean follow-up of 6–12 months.7 It was
originally used in patients with moderate or severe myopia
because LASIK is believed to cause less haze and regression.
Published results with LASIK are generally comparable
to those with PRK. In a recent study of PRK and
LASIK 115 patients with moderate to severe myopia
(–60.00 to –14.00 D) underwent LASIK.24 At 6 months
41% of the eyes were within ±1.00 D of the intended correction,
and 56% of the patients had a visual acuity of
20/40 (6/12) or better.24 Only 3.2% of the eyes had decreased
BSCVA and lost 2 or more lines on the Snellen
chart. The results were similar for the 105 patients who underwent
PRK; however, the PRK patients achieved improved
visual acuity more slowly. A longer follow-up is
necessary to address the issue of haze and regression.
Complications
Patients undergoing LASIK are susceptible to all the
complications of PRK, as well as flap-related complications
(Table 4).7,25,26 Most complications associated with LASIK
occur intraoperatively and are related to the microkeratome
device. Intraoperative complications occur in up to
3% of cases, and the rate is significantly related to the surgeon’s
experience with using a microkeratome.26 Incomplete
flap creation or failure of the microkeratome to stop
at the hinge results in a free, off-centre or irregularly
shaped flap. Such complications should be detected before
the laser beam is applied and should be managed by repositioning
the flap and deferring laser treatment. In most
cases, there will be no significant loss of BSCVA, and treatment
can be undertaken within 3 months. The most devastating
complication is intraocular penetration from a misassembled
microkeratome. Damage to the iris and lens has
been reported.7
Postoperative flap-related complications are also possible.
Loss of the corneal flap can result from incomplete adherence
of the flap to the stromal bed. Eye rubbing may
result in dislocation or loss of the flap. This severe complication
usually results in significant haze formation that may
require a corneal transplant. Other postoperative flap-related
complications include epithelialization of the stromal
bed, particulate matter becoming trapped between the stromal
bed and the epithelium, diffuse lamellar keratitis
(“shifting sands of the Sahara”),27 epithelium ingrowth, flap
dislocation and wrinkling of the flap. Flap-related complications
can result in loss of BSCVA and may require lifting
of the flap and débridement. Rarer complications of LASIK
are listed in Table 4.
Postoperative complications seen with PRK such as decentration,
central island, haze and regression can also happen
with LASIK. However, short-term data (up to 1 year
postoperatively) indicate that the incidence of haze and regression
are lower with LASIK.24
Surgical techniques for hyperopia
There are currently 2 major techniques available to correct
hyperopia: thermal keratoplasty with a holmium:YAG
laser, and a lamellar procedure using H-PRK or LASIK. As
mentioned earlier, long-term data indicate that thermal
keratoplasty is associated with significant regression in patients
with severe hyperopia and thus should be used only
for patients with mild hyperopia (up to +3.00 D).10 In contrast,
H-PRK has recently been shown to yield excellent results
and high refractive stability in patients with mild to
moderate hyperopia (up to +6.00 or +8.00 D).28,29 Promising
results have also been obtained with the use of H-PRK to
treat overcorrection of myopia from PRK and radial keratotomy
(unpublished data).
Hyperopic photorefractive keratectomy
H-PRK is similar to PRK for myopia, wherein the excimer
laser is used to remove small amounts of corneal
stroma. Instead of flattening the central cornea, in H-PRK
the excimer laser produces central steepening by ablating
tissue in the periphery of the cornea, with a central zone of
5–6 mm and an overall treatment zone out to 9 mm (Fig. 9).
Patients who undergo H-PRK are generally older and
more likely to have dry eyes, eyelid diseases and cataract
than those who undergo myopic PRK. Presbyopia is also
an important issue. Patients over 40 years need to understand
that reading glasses may still be necessary despite a
successful outcome. Re-epithelialization takes longer after
H-PRK than after myopic PRK because of the larger ablation
zone. Postoperative improvement in visual acuity and
Refractive surgery
the return of BSCVA may take 3–6 months. Patient satisfaction
is very high with H-PRK because of the improvement
in both distance and near vision28
Outcomes
In a study of H-PRK to treat mild hyperopia29 80% of the
eyes were within 0.50 D of the intended correction at 1 year
postoperatively, and 98% were within 1.00 D; 70% achieved
the preoperative BSCVA, with acuity not worse than 20/25
(6/8). The results were the same at 18 months. Regression
tended to occur within the first 6 months after surgery, but
refraction tended to stabilize between 6 and 12 months. At
18 months, 75% of the eyes had uncorrected visual acuity of
20/25 (6/8). There were no significant complications. The
use of H-PRK to treat moderate hyperopia is also effective,
but more initial overcorrection and regression is noted in the
first 12 months.28,30 Treatment of hyperopic astigmatism by
steepening the flatter axis is currently being evaluated.30
The use of LASIK to treat hyperopia offers the potential
of faster recovery of visual acuity and BSCVA, less discomfort
and more stability than that achieved with H-PRK.
Careful clinical studies are necessary to confirm these initial
impressions.
Complications
Possible complications from H-PRK are identical to
those from PRK for myopia. However, central haze is
much less likely to occur, because the deepest ablation zone
is located in the periphery of the cornea. Peripheral haze
may occur, but it usually does not decrease visual acuity.
Other uses of the excimer laser
Besides its usefulness in reshaping the cornea for refractive
correction, the excimer laser is also effective in treating
superficial corneal problems such as various types of
corneal dystrophy,31–38 stromal scarring from injury,32 infection
or surgery, elevated corneal lesions, keratoconus and
irregular astigmatism (Table 5). The excimer laser’s ability
to remove corneal tissue precisely and in a relatively atraumatic
manner makes it an ideal tool for treating these conditions.
The objective of phototherapeutic keratectomy is
to smooth the surface of the cornea to enable correction
with eyeglasses or contact lenses, and in some cases to
avoid or delay the need for corneal transplantation.
Relative contraindications include corneal abnormalities
with a poor healing response such as neurotrophic keratitis,
exposure keratitis and dry eyes. Haze with associated decreased
visual acuity could develop if the patient has an
overly aggressive healing response. Patients with a history
of herpetic keratitis are at risk of recurrence. Other complications
include induced hyperopia, irregular astigmatism
and infective corneal ulcer. Patients with corneal dystrophy
should understand that these conditions can recur after
treatment, just as they do after corneal transplantation.
Summary
Refractive surgery has undergone tremendous advances
over the past 20 years. The use of the excimer laser has revolutionized
the surgical treatment of refractive errors. We
can now achieve a level of accuracy in modifying the cornea
that was unattainable before the laser era. The results of refractive
surgery are gratifying, with over 90% of patients
achieving uncorrected visual acuity of 20/40 (6/12) or better.
Although refractive surgery techniques involving the
excimer laser are generally safe and effective, there are
some risks. These can be minimized through careful patient
selection with detailed history taking and physical examination
to detect possible contraindications. A careful
and meticulous surgical technique coupled with frequent
postoperative follow-up help to reduce the incidence and
severity of complications. The experience of the surgical
team also affects the outcome and the rate of complications,
especially with the use of LASIK. In the next millennium,
we can expect the laser technology to continue to
improve, with more compact machines, homogeneous
beam profile, lower maintenance costs, accurate eye tracking
and customized ablation, allowing the correction of irregular
astigmatism. A better understanding of the biological
response of the cornea to laser treatment may allow
modulation of the wound healing response for more precise
correction. The future of laser surgery for refractive
errors is to continue to improve the outcomes, minimize
complications and explore new techniques.
Competing interests: The University of Ottawa Eye Institute is a
beta test site for new hardware and software for excimer laser
surgery related to PRK for VISX Inc., Sunnyvale, Calif. Dr. Jackson
has received honoraria as a certified trainer for VISX and
travel assistance and speaker fees for presenting data at international
meetings.
While current excimer laser technology produces accurate and reliable results, data continue to accumulate on improvements that are being made with existing technology. Wavefront technology is one such rapidly advancing topic. At the meeting, Richard Lindstrom, MD,[1] Clinical Professor of Ophthalmology at the University of Minnesota, reported on the 6-month wavefront results using the VISX laser. The visual results of this study have been impressive, with 98% of patients achieving 20/20 vision or better and 71% achieving 20/15 or better. Furthermore, the majority of patients reported a reduction in night vision glare postoperatively compared with preoperative subjective complaints. Dr. Lindstrom made the point that the objective visual results as well as the excellent subjective results are, in part, related to the ability of this new technology to maintain the optical benefits of a prolate cornea. (A prolate cornea is one that is more steep centrally than peripherally. This is the true shape of the native cornea. Following nonwavefront myopic refractive surgery, the center is flattened, creating an oblate cornea, which may alter some of the higher order optical qualities of the cornea.)
According to a presentation by MacRae and colleagues,[2] another device that has increased in use and acceptance is the US Food and Drug Administration (FDA)-approved femtosecond laser (IntraLase, Irvine, California), which creates LASIK flaps in place of a standard microkeratome. The femtosecond laser uses a long wavelength that is not absorbed by the cornea but is delivered to the programmed depth. The spot size is 3 microns. This device creates a precise corneal flap with very high accuracy; in addition, it gives surgeons the ability to determine the orientation and thickness of the flap. This process is totally software driven, and the procedure itself appears to be safe and accurate. Ultimately, due to its consistency, this laser may prove to be more suited to create flaps when wavefront ablations are used. Standard microkeratomes may change higher order optical aberrations prior to the excimer ablation. Only further analysis will answer this question.
Mark Speaker, MD, also gave a talk on the femtosecond laser and its potential advantages. One such advantage is that loss of suction with the femtosecond laser is not as potentially devastating as with a standard microkeratome. Because the software stops the flap creation, any loss of suction is sensed, and the creation of the flap is immediately stopped. The software can then create the flap from the precise point at which suction was lost later that same day. Dr. Speaker also made the point that epithelial defects, which can be very problematic with regular microkeratomes, are essentially eliminated with this device. While the final visual acuity appears to be the same no matter which microkeratome is used, standard microkeratomes may result in better optical quality at postop day 1 after LASIK compared with the femtosecond.
The other subject that was discussed and debated in multiple forums at this meeting was the procedure LASEK (laser-assisted subepithelial keratomileusis) and its relationship to LASIK. Briefly, LASEK is a procedure where an epithelial flap is created and laser ablation is initiated subepithelially rather than intrastromally as in LASIK. Like photorefractive keratectomy (PRK), LASEK preserves more tissue for potential ablation, and it has been pointed out that the unwanted haze formation that is seen with PRK might be seen with LASEK. Dimitri Azar, MD, of the Massachusetts Eye and Ear Infirmary, made the point that LASEK may have a potential indication when wavefront ablations are used. Potentially, this technique could limit higher order optical aberrations compared with LASIK with creation of a standard microkeratome flap.
In a symposium called the "Innovator's Session," Daniele S. Aron-Rosa, MD,[3] presented comparison data on LASIK, LASEK, and PRK. In her experience, at postop day 1 and postop month 1, more patients preferred LASIK than LASEK. This difference was more pronounced at day 1. LASEK tended to be preferred by those patients with dry eye. This is likely related to preservation of the corneal innervation. She felt that patients preferred LASEK to PRK for comfort, and she reiterated the point that LASEK may eventually prove to be more suited to wavefront ablation.
The practice of performing refractive surgery on pediatric patients was also addressed. While the procedure has been performed internationally, it remains fairly uncommon in the United States. Most surgeons agree that it should not be a purely elective procedure in patients younger than 18 years of age. However, certain circumstances may warrant its consideration. Jonathan Davidorf[4] outlined his working criteria for evaluating pediatric patients for refractive surgery. One potential indication for its use would be on patients within the amblyopic age group after all conventional methods of amblyopia therapy have failed. In this situation, the procedure would be used to try to optimize visual potential and limit amblyopia during this critical period. Another scenario for possible use would be in patients outside of the amblyopic range (older than 8 years) in which a "functional improvement" in vision may be obtained in those who are spectacle and contact lens intolerant. He described a patient who received hyperopic LASIK and improved both functionally in school and driving after the procedure. Such cases would have to be treated with care. Whether LASIK can gain wide acceptance and what its indications would be for the pediatric population are yet to be determined; long-term results will need to be monitored closely.
SAN FRANCISCO – Millions of people have reduced their dependence on eyeglasses and contact lenses over the past several years with the refractive surgery procedure known as LASIK. LASIK can correct refractive errors such as nearsightedness, farsightedness and astigmatism. Now an enhanced version of LASIK, known as wavefront LASIK, is available. This improved system allows eye surgeons to customize the procedure for each eye, providing the possibility of even better vision.
Adapted from technology that allows land-based telescopes to produce images comparable to those generated by the Hubble Space Telescope, wavefront LASIK consists of a sensor and a laser. A wave of light from a laser beam is sent through the eye to the retina. This light is reflected back through the vitreous, the lens and the pupil. The sensor measures the irregularities at the front of the wave of light as it emerges from the eye. This produces a precise three-dimensional map of the eye's visual system, including the cornea's imperfections or aberrations.
The wavefront data is translated into a mathematical formula that the surgeon uses to program corrections into the laser, which vaporizes tissue to reshape the cornea to correct refractive errors. This new technology also corrects the higher-order aberrations that cause glare, haloes and blurry images. Higher-order aberrations are distortions in the visual system that can only be detected with wave
