Surgical techniques

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

bandwhereby 4 crescent-shaped polymethylmethacrylate

segments, each 4.5 mm long and 600 µm wide, are inserted in

the sclera 22.5 mm from the limbuscan 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 patients 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



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


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


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 34 days for myopia

and 45 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.


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.


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 612 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


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 612 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 2448 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 130160 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 23 days compared with 514 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).


LASIK is a relatively new procedure, and most reported

data are only for a mean follow-up of 612 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.


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 surgeons

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

56 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 36 months. Patient satisfaction

is very high with H-PRK because of the improvement

in both distance and near vision28


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



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,3138 stromal scarring from injury,32 infection

or surgery, elevated corneal lesions, keratoconus and

irregular astigmatism (Table 5). The excimer lasers 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.



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



Refractive Surgery

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.




Wavefront LASIK advances refractive surgery

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

Surgical techniques