wavefront's role in corneal refractive surgery

$: Wavefront's role in corneal refractive surgery$
Clinical and Experimental Ophthalmology

2005;

33

: 199–209

Perspective

__________________________________________

Perspective

Wavefront’s role in corneal refractive surgery

Michael A Lawless

FRANZCO

1,2,3

and Christopher Hodge

BAppSci(Orth) DOBA

3

1

Northern Sydney Health,

2

Department of Ophthalmology, University of Sydney, and

3

The Eye Institute, Sydney, New South Wales, Australia

A

BSTRACT

In the 5 years since the first wavefront-based LASIK treatmenton normal eyes, the ophthalmology profession has had toconfront a new language based on astronomy, optics andmathematics. Over this time wavefront technology has beenused for diagnosis and treatment, and its application has madethe profession define what is meant by good vision, and deter-mine whether, with psychophysical and psychometric tests, itis possible understand how an individual perceives the world.The clinical application of wavefront technology has forcedophthalmologists and vision scientists with an engineering biasto talk to those with a biological bias, and to appreciate thatif you try and change the corneal shape, its biological, anatom-ical and optical properties exist within a complex external eyeenvironment. This perspective article demonstrates that wave-front analysis is a useful diagnostic tool, and that wavefront-based corneal refractive surgery is an improvement overconventional techniques. Its use by an ophthalmologist is aclinical decision specific to an individual patient.

Key words:

aberrometry, cornea, optics, refractive surgery,wavefront.

I

NTRODUCTION

Laser

in situ

keratomileusis (LASIK) and photorefractivekeratectomy (PRK), and its surface variants advanced surfacelaser ablation and laser epithelial keratomileusis, are acceptedby much of society and the ophthalmology profession as asurgical alternative to glasses and contact lenses.

1–5

This isdespite the reality that corneal refractive surgery will gener-ally increase higher order wavefront errors while attemptingto decrease lower order errors (myopia and astigmatism).

6,7

Wavefront error is the fundamental description of theoptical quality of an optical system.

8

The human eye is onesuch optical system, and the fact that its optical quality isaffected by monochromatic aberrations has been recognizedsince the middle of the nineteenth century.

9

In 1999 Seiler performed the first LASIK procedures withan algorithm based on wavefront information.

10

In 2005 patients and surgeons in many parts of the worldhave a choice of aberrometers as diagnostic tools and achoice of wavefront-based or conventional corneal lasersurgery. The purpose of this article is to try and make senseof these choices.

Aberration originates from the Latin ‘ab-erratio’, whichmeans going off track or deviating. Aberration is the differ-ence that exists between an ideal image and that achieved inan optical system. There are three sources of image blur inthe human eye:

1

Light scatter: this occurs when incident light passesthrough the cornea and lens.

2

Diffraction: particularly at small pupil sizes of 2 mm orless.

3

Optical aberrations: these can be either chromatic ormonochromatic.Wavefront aberrometry as a diagnostic test, and custom-

ised wavefront LASIK as a treatment, specifically deal withmonochromatic aberrations and exclude scatter, diffractionand chromatic aberrations.

As ophthalmologists we are used to examining patientsand detecting an optical error that can be corrected byprescribing glasses. These are low order or defocus errors,described as sphere and regular astigmatism. We also havean understanding of what we consider to be irregularastigmatism, and traditionally we would think of this asirregularity on topography or retinoscopy, leading to reducedbest corrected acuity. Total higher order aberrations are justforms of irregular astigmatism, although individual compo-nents such as spherical aberration are rotationally sym-metric. They can be severe, as in keratoconus with clinicallysignificant irregular astigmatism, or they can be subtle andnot detected by conventional techniques.

Higher order monochromatic aberrations can be measuredand named. We need to understand how these aberrationsare measured, what the individual components are and howthey impact on a person’s vision.

W

HAT

DO

ABERROMETERS

MEASURE

?

Wavefront sensors (i.e. aberrometers) measure the distortionof a light wave as it is altered by passing through the optics

�

Correspondence:

Dr Michael A Lawless, The Eye Institute, 3/270 Victoria Avenue, Chatswood, NSW 2067, Australia. Email: [email protected]

200 Lawless and Hodge

of the eye. A plane wave of monochromatic light will bedistorted by optical imperfections. As mentioned previ-ously, wavefront sensors do not measure light scatter (fromstromal haze or corneal scars), chromatic aberrations ordiffraction phenomena. Their effects on vision have to beassessed by other means. A useful way to think of distortionsin a wavefront is to think of the path length of parallelrays entering the pupil and projecting toward the retina. Aslight enters the eye from the air, its speed is retardedaccording to the refractive index of the material along itspath to the retina. The arrival time is also influenced by thedistance it has to travel. These two factors, refractive indexand linear path variations, are measured with a wavefrontsensor.

A map can be made that shows the relevant retardationthat a plane wave undergoes as it traverses the optics of theeye.

11

Clinicians are now used to seeing this informationdisplayed as a Zernike polynomial expansion.

12

In order toparcel the wavefront error into individual building blocks, aset of normalized Zernike polynomials are best fit to themeasured wavefront error. The coefficient for each Zernike

term reveals that term’s relative contribution to the totalroot mean square (RMS) error (Fig. 1). In the normal ame-tropic eye, defocus (i.e. myopia or hyperopia) is by far thelargest aberration, followed by astigmatism. These are loworder terms. The Zernike pyramid is useful (Fig. 2, Table 1).As we go down the rows from the top, we go from low orderto high order. Low order encompasses the top three rows’piston, tilt, tip, and sphere and astigmatism. Row three (i.e.sphere and astigmatism) is what we would normally measureand prescribe in spectacles. The fourth row is called thirdorder aberrations, and it continues from there. Anythingbeyond lower order is lumped under the term higher orderaberrations. As you can see from the diagram, they haveindividual names such as coma, and spherical aberration. Inany row a function with a negative value of the index ‘f’ is arotated form of the function with the same but positivenumber for ‘f’.

When interpreting data we need to know whether thewavefront refers to total aberrations, higher order aber-rations or individual higher order aberrations. In the normalametropic eye, higher order aberrations are a relatively small

Figure 1.

Graphical representation of root mean squared wave-front error.

Figure 2.

Zernicke pyramid. Z(r

n

,f

θ

) = Z

fn

where f is angularfrequency and n is radial order. See Table 1.

Table 1.

Zernicke pyramid. See Fig. 2.

Plot index Term Binomial representation Polynomial (

ρ

,

θ

)

1 Tip (1,1) 2

ρ

sin(

θ

)2 Tilt (1,1) 2

ρ

cos(

θ

)3 Defocus (2,0)

√

3 (2

ρ

2

∠

1)4 Astigmatism (2,2)

√

6

ρ

2

sin(2

θ

)5 Astigmatism (2,2)

√

6

ρ

2

cos(2

θ

)6 Vertical coma (3,1)

√

8 (3

ρ

3

∠

2

ρ

)sin(

θ

)7 Horizontal coma (3,1)

√

8 (3

ρ

3

∠

2

ρ

)cos(

θ

)8 Trefoil (3,3)

√

8

ρ

3

sin(3

θ

)9 Trefoil (3,3)

√

8

ρ

3

cos(3

θ

)10 Spherical aberration (4,0)

√

5(6

ρ

4

∠

6

ρ

2

+

1)11 Secondary astigmatism (4,2)

√

10 (4

ρ

4

∠

3

ρ

2

)sin(2

θ

)12 Secondary astigmatism (4,2)

√

10 (4

ρ

4

∠

3

ρ

2

)cos(2

θ

)13 Tetrafoil (4,4)

√

10

ρ

4

sin(4

θ

)14 Tetrafoil (4,4)

√

10

ρ

4

cos(4

θ

)

Wavefront’s role in corneal refractive surgery 201

component, comprising about 10% of the eye’s total aber-rations. This varies between individuals. Figure 3a shows a2-D wavefront of a normal ametropic eye with a low amountof higher order aberrations (0.14

µ

m) and Fig. 3b shows asubtle form fruste keratoconic eye with a larger amount(0.42

µ

m) of higher order aberrations. Both images are fordata at a 6 mm pupil size. In an eye which is emmetropic(i.e. has no sphere or astigmatic error), higher order aber-rations can still be present. It is relevant to know what thepupil size was when aberrometry was performed, and atwhat pupil size data is presented, as higher order aberrationsincrease with increased pupil size.

At some point we have to stop measuring. Most aberro-meters measure to the sixth order, although fourth order isprobably all that is required to be clinically relevant both indiagnosis and treatment. You can measure up to the eighthand ninth order and beyond, but are measuring smaller andless relevant higher order aberrations, so there has be somearbitrary cut off for clinical purposes. For example, theAlcon LADARWave measures to sixth order, but displaysinformation to fourth order (Fig. 4). There are good and badthings about representing low and high order aberrations bythe Zernike polynomial expansion pyramid, and we will gointo the subtleties later.

How do aberrometers work?

Before trying to interpret Zernike polynomials and othermethods of displaying wavefront information, first let uslook at how specific aberrometers work.

There are four different principles of wavefront analysis;

1

Outgoing reflection aberrometry (Shack-Hartmann aber-rometry).

2

Retinal imaging aberrometry (Tscherning and ray tracingaberrometry).

3

Ingoing adjustable refractometry (spatially resolvedrefractometer).

4

Double pass aberrometry (slit skiascopy).

1

. Outgoing reflection aberrometry (Shack-Hartmann aberrometry)

This is used by LADARWave (Alcon Surgical, Fort Worth,TX, USA), WASCA (Meditech, Jena, Germany), WaveScan(VISX, Santa Clara, CA, USA) and Zyoptix (Bausch andLomb Surgical, Rochester, NY, USA).

Shack-Hartman aberrometers rely on an ingoing rayfrom a diode laser entering the eye as a thin beam focusedto a point on the retina. An array of tiny lenses focuses theimage of the reflected wavefront as it returns out of the eye.

Figure 3.

Higher order aberrations represented by 2-D image.(a) Untreated eye with low degree of high order aberrations.Phoropter refraction: –3.00/–0.75

×

180. Wavefront refraction:–2.89/–0.60

×

174. (b)

Untreated eye with high degree of highorder aberrations. Phoropter refraction: –0.25/–0.50

×

9. Wave-front refraction: –0.84/–0.63

×

51.

Figure 4.

Untreated eye with low degree of high order aberra-tions. Phoropter refraction: –3.50/–0.50

×

15. Wavefront refrac-tion: –2.95/–0.87

×

2. (a) Higher order aberrations represented by2-D image. (b) Higher order aberrations broken down to fourthorder. Note the scale difference for low versus high order aberra-tions.


With this array of lenses, the outgoing wave of light isdivided into many focused beams, generating multipleimages of the same retinal spot of light. These are detectedby charge couple device (CCD) sensors that determine thedisplacement of each spot of reflected light from its corre-sponding lenslet axis. Mathematical integration of thisinformation yields the shape of the aberrated wavefront.The wavefront can then be analysed at the level of the eye’sexit pupil.

13,14

2

. Retinal imaging aberrometry (Tscherning and ray tracing aberrometry)

The WaveLight (WaveLight Technologies, Erlangen, Ger-many) and Schwind aberrometers (Schwind, Kleinosterm,Germany) are examples of this type. They use a collimatedlaser beam source that illuminates a mask with regular matrixpinholes, which forms a bundle of thin parallel rays that areprojected into the eye. These rays form a retinal spotpattern that is distorted according to the eye’s aberrations.The retinal spot pattern is imaged through a small apertureonto a CCD camera by indirect ophthalmoscopy. The devi-ation of all spots from their ideal positions are measured atthe level of the eye’s exit pupil, and from these values theoptical aberrations are displayed.

The Tracey retinal ray tracing technology utilizes asimilar principle. Individual low intensity laser beams arerapidly fired in a specific sequence onto the retina. TheCCD camera captures the retinal image, and individuallyobtained sample points can be summed to measure theaberrations present when compared to the ideal pattern.

15

3

. Ingoing adjustable refractometry (spatially resolved refractometer)

This method of wavefront sensing is based on Scheiner’sprinciples and described by Smirnoff in 1961.

16

This systemis not commercially available. Peripheral beams of incominglight are subjectively redirected toward a central target tocancel the ocular aberrations from that peripheral point.Thirty-seven testing points are manually directed by theobserver to overlap the central target in defining the wave-front aberration pattern. The limitation of this technique,apart from not being commercially available, is the length oftime required for subjective alignment of the aberratedspots.

4

. Double pass aberrometry (slit skiascopy)

Nidek (Gamagori, Japan) has this commercially availablesystem as the ARK-10 000 Optical Path Differences Scan-ning System, known as the OPD-Scan. The retina isscanned with an infrared slit beam, and the reflected light iscaptured by an array of rotating photo detectors over a 360

°

area to define the wavefront pattern. This is sometimescalled slit skiascopy because it is based on the principle ofskiascopic phase difference analysis.

17

It should be evident that wavefront analysis is not a‘newer’ version of corneal topography or auto-refraction, buta measuring device that takes all elements of the opticalsystem into consideration. Given the acceptance of thesedevices by ophthalmologists as diagnostic and treatmenttools, there is a surprising paucity of information in thepeer-reviewed literature. Fernandes de Castro

et al.

measured99 eyes of 55 subjects with the LADARWave, WaveScan,and Zywave aberrometers and compared these to cyclo-plegic refraction.

18

In these structurally normal eyes, therewas excellent reproducibility of each wavefront device.There were differences between devices and betweencycloplegic refraction and wavefront determined refrac-tions. The LADARWave and Zywave instruments tended toslightly overstate refractive error, while WaveScan under-stated. Even though these are all Shack-Hartmann wave-front devices, the technical specifications of each device aredifferent, and this would account for some of the differ-ences. It is not quite as simple as that though. Why is it notjust a simple matter of determining the lower order defocusand astigmatism terms measured with aberrometers andapplying them to obtain the refraction? That is what you dowith a phoropter or an auto refractor. The problem ispatients with higher order aberrations can try and neutralizetheir refractive effect by using some combination of sphereand cylinder. A patient with significant coma, for example,might find vision improves with the addition of somecylinder, even though they have no measurable lower orderastigmatism. To accurately reproduce a cycloplegic refrac-tion, if that is what you want to do, the effects of higherorder aberrations need to be accounted for in determiningthe wavefront derived spherocylindrical refraction.

19,20

Thedifferent companies have taken individual approaches tothis issue. It is an issue which is commercially sensitive andnot in the public domain.

Wang

et al

.

21

evaluated the accuracy and repeatability ofthe WaveScan and Tracey systems, and compared them tomanifest refractions. They also compared these devices todata available for standard auto-refractors, and concludedthat the WaveScan and Tracy systems were comparableto auto-refractors in both repeatability and accuracy. Allwere comparable to manifest refraction. WaveScan wasunable to measure a number of eyes, particularly those thathad had previous corneal refractive surgery. They presumedthat this was due to the larger higher order aberrationscausing cross-over effects, and therefore precluded themachine from obtaining reproducible, useful measurements.

C

ORRELATION

OF ABERRATIONS WITH VISUAL PERFORMANCE

Glasser et al. performed an in vitro optical measurement inhuman lenses at different ages.22 They determined that the


young healthy crystalline lens has negative spherical aber-ration, which increases with age, becoming positive aroundthe age of 40 years. In young people the lens compensatesfor some corneal aberrations, and it seems possible that thiscompensation mechanism is less effective as we age.

Amano et al. demonstrated that ocular coma increaseswith age, mainly because of increased corneal coma.23 Theyalso found that spherical aberrations increased with age, butthis was mainly because of an increase in spherical aber-ration in the internal optics, presumably the lens.

As higher order aberrations, particularly spherical aber-rations, increase with pupil size, relative miosis in olderpeople might be a protective phenomenon.24

Miller et al. analysed Shack-Hartman wavefront images innormal subjects, dilated normal subjects, pseudophakicpatients and LASIK patients.25 The patients with LASIK andintraocular lens surgery (IOL) surgery had statistically sig-nificant elevations of fourth order spherical aberration forpupil sizes greater than 5 mm, compared to normal subjects.IOL patients had more net increase in higher order aberra-tions than LASIK patients. Vilarrodona et al. also revealedthat higher order aberrations increase in pseudophakiacompared to normal subjects, but the profile was differentdepending on the IOL type.26 Similar conclusions weredrawn by Taketani et al.,27 who found increased sphericalaberration in pseudophakic eyes when a 6.0 mm pupil wasanalysed, but this was not evident at 4 mm.

Mester et al. took this further and analysed higher orderaberrations in eyes with a standard IOL in one eye com-pared to a modified anterior surface (Tecnis Z9000, Phar-macia) IOL in the other eye.28 The Tecnis Z9000 lens has amodified prolate anterior surface and aims to decreasespherical aberration. Spherical aberration in the Z9000 eyeswas not statistically significantly different from controls,while the contralateral pseudophakic eyes had significantamounts of spherical aberration. There was no differencebetween the eyes in high contrast visual acuity, but therewas a trend toward an improvement in contrast sensitivityunder mesopic conditions with the modified prolate anteriorsurface IOL.

Increased higher order aberrations in pseudophakic eyesare mainly due to spherical aberrations. Keratoconus, adisease resulting in irregular astigmatism, has an overallincrease in higher order aberrations, but in particular, coma-like aberrations are much worse than in normal subjects.Madea et al. analysed mild keratoconus and keratoconussuspect patients because of the difficulty of obtaining meas-urements with Shack-Hartman wavefront sensors inadvanced keratoconus.29 Increased higher order aberrationsin keratoconus were confirmed by Shar et al. with increasedcoma at both 3 mm and 5 mm pupil sizes.30 Clear cornealtransplant patients with technically successful grafts alsohad significantly increased coma and spherical aberrationcompared to normal subjects. This is understandable assubtle irregular astigmatism is present even in the bestcorneal transplants.

Different pathological states (pseudophakia, kerato-conus, post-penetrating keratoplasty) have increased higherorder aberrations. Higher order aberrations are just measure-ments of irregular astigmatism, so of course they wouldincrease. You would assume therefore that the poor visionnoted by keratoconic patients is due to coma, and that thesometimes poor night vision experienced by pseudophakicpatients is due to spherical aberration. Are these reasonableassumptions? What evidence is there that higher orderaberrations correlate with visual symptoms? The issue isbeing investigated. Some evidence is confusing; for example,that reported by Gimbel et al.31 They looked at 20 eyes ofpost PRK and LASIK patients who had significant visualsymptoms described as ‘reductions in quality of vision’. Thesepatients underwent retreatment based on wavefront measure-ments and had a net improvement or resolution of subjectivevisual symptoms, but no correlation could be drawnbetween this and reduced higher order aberrations.

Mrochen et al. demonstrated that small decentrations inPRK patients would significantly increase wavefront aber-rations.32 Because of this study they looked at whethertracking, and presumably better centration, correlated withan improvement or less induction of higher order aber-ration.33 Interestingly they found that tracking did not leadto an improvement in defocus (i.e. sphere and cylinder) butdid lead to significantly better Snellen visual acuity, and lessinduced coma and spherical aberration. Tracking decreasedthe rate of subclinical decentration and appeared to improvethe optical and visual outcomes, although not necessarilyimproving the refractive outcomes after PRK. If changes aresubtle they may not be noticed.

Pallikaris et al. performed LASIK keratectomies in 15myopic eyes, but did not perform a laser ablation.34 Flapcreation alone statistically significantly increased totalhigher order wavefront aberrations, and in particular, comaand spherical aberrations at both 4 mm and 6 mm pupils.There was however, no change in the patient’s refractiveerror or in their corrected visual acuity. Again, subtlechanges that may not be evident to the patient or clinician.

Applegate et al. performed corneal first surface wavefrontanalysis to try and predict visual performance, in bothnormal subjects and a variety of corneal conditions.35 Visualperformance was quantified by measuring contrast sensitiv-ity, and high and low contrast acuities at 3 mm and 7 mmpupils. They demonstrated a statistically significant correla-tion between all three measures of visual performance andcorneal wavefront error. In addition all relationships werestronger for the 7 mm diameter pupil compared to the 3 mmpupil. The conclusion is that regardless of the cause, corneaswith increased wavefront errors show a quantifiable decreasein visual performance, and it is pupil size dependent. Apple-gate et al. went on to try and determine how each mode ofthe Zernike polynomial could affect both high and lowcontrast logMAR visual acuity.36 They drew interestingconclusions:


1 For an equal amount of RMS error, not all coefficients ofthe Zernike polynomial induce equivalent losses in highand low contrast logMAR acuity.

2 Wavefront error concentrated near the centre of theZernike pyramid (e.g. coma and spherical aberrations)adversely affects visual acuity more than those modesnear the edge of the pyramid (e.g. trefoil and tetrafoil).

3 Large changes in chart appearance are not reflected inequally large decreases in visual performance, that is,patients could correctly identify highly aberrated letters,so had reasonable Snellen acuity, but poor visualperformance.

4 Interactions between modes complicate weighting eachZernike mode for visual impact.In a separate report, Applegate et al. demonstrated that

aberrations interact to either increase or decrease acuity,depending on which aberrations are mixed and their relativeproportions.37 For example, they conducted an experimentwhereby a certain amount of spherical error (low order ordefocus) when combined with a small amount of sphericalaberration could actually improve acuity over that measuredwith the individual component aberrations alone. Thismeans that the total amount of RMS wavefront error can bemisleading. The same total RMS error may not mean thesame degree of visual impairment between individuals. It iswhat makes up the RMS error and the interactions of theindividual components that are important.

Chalita et al. used the LADARWave device to examine105 eyes of 58 patients who had previously undergoneLASIK.38 Their aim was to evaluate wavefront analysis andcorrelate it with visual symptoms. They examined informa-tion at pupil sizes of 5 mm and 7 mm, and as expectedshowed a statistically significant increase in higher orderaberrations with the larger pupil size. They were able todraw some conclusions. Double vision was associated withhorizontal coma at both pupil sizes, and total coma wasassociated with double vision for both 5 mm and 7 mmpupils. No association was found between vertical coma andvisual symptoms, suggesting that not only is the amount ofcoma important, but also its orientation.

Haloes revealed a trend of association, which was notstatistically significant, with spherical aberrations for scotopicpupil sizes, and glare was significantly associated both withspherical aberration and total higher order aberrations. Star-burst also showed a significant correlation with sphericalaberration and total aberrations for larger pupils.

There were some unusual findings, for example whentrying to correlate pupil diameter under scotopic conditionswith the visual symptom of double vision, they found signif-icant negative correlation. One could infer that doublevision is correlated with small pupils. It may be that doublevision due to horizontal coma at a small pupil size is moresensitive to the central asymmetry that represents doublevision than at a larger pupil size.

Generally the authors observed a strong correlation ofmost of the reported visual symptoms with one or moreof the higher order aberrations analysed at a scotopic pupil

size. How the individual higher order aberrations interact toaffect visual performance, as postulated by Applegate et al.,could not be determined from their study.

SURGICAL CORRECTION OF HIGHER ORDER ABERRATIONS

Once you have an accurate wavefront measurement device,you then need a wavefront laser interface; that is, a methodof taking the information and applying it to the cornea. Youalso need a laser delivery system with a small scanning spotand a fast and accurate eye tracker. Two studies have lookedat the spot size required to theoretically eliminate mosthigher order aberrations. Huang and Arif simulated correc-tions of wavefront aberrations from second to eighth orderto determine the effect of laser spot size on the outcomeof aberration correction.39 Gaussian and Tophat beams of0.6–2.0 mm full width half maximum diameters weremodelled. They found that a 2 mm or smaller beam wasadequate for spherocylindrical corrections, a 1 mm orsmaller beam was adequate for correction up to fourth orderterms, and a 0.6 mm or smaller beam was adequate forcorrection up to sixth order. Guirao et al. reached a similarconclusion showing that a 1 mm beam was small enough toproduce customised ablation up to fifth order.40

In 2004 algorithms for LASIK and surface ablation incor-porate wavefront information. The FDA has approved threedifferent systems for wavefront-based treatment, and indi-vidual surgeons use the technology either occasionally orcommonly. In our practice in 2004, wavefront-based treat-ment is the standard treatment and used whenever techni-cally possible, which proves to be approximately 65% oflaser treatments. Where is the information to support thispractice? Seiler was the first to treat eyes using wavefrontinformation.10 Fifteen eyes of 15 patients had LASIK, usingTscherning aberrometer information up to fourth orderhigher order aberrations. The ablation pattern was derivedfrom the reconstructed wavefront aberration map. Additionalfactors were used that considered changes in the ablationrate due to the laser/tissue interaction, corneal curvature andwound healing.41 Treatment was delivered with the Alle-gretto scanning spot laser with a spot diameter of 1.0 mm,and the procedure was centred on the entrance pupil. Onemonth data was included in this report, and showed achange in total aberrations ranging from a decrease of 4% toan increase of 130%. The average was an increase in higherorder aberrations of 40%. They noted that for individualeyes, as best corrected visual acuity improved, there was arelative decrease in higher order aberrations.41

After this normal eye LASIK study, there were threestudies using wavefront information to treat the complica-tions of previous corneal refractive surgery.

Mrochen et al. had three patients with large decentrationsof the optical zone between 1.5 mm and 2 mm scheduledfor wavefront guided LASIK.42 The Tscherning aberrometerwas used, but was unable to capture useful information in


one eye with a particularly large decentration. The othertwo had information that could be captured, and weretreated with an algorithm using information up to fourthorder aberrations. These two eyes improved their unaidedand best corrected visual acuity. The RMS wavefront errordecreased 61% for total aberrations and 33% for higherorder. The optical zone was enlarged as determined bycorneal topography, and both patients had a resolution ofmonocular diplopia and haloes 3 months after surgery. Theaberrations noted preoperatively were particularly horizon-tal tilt, horizontal coma and secondary astigmatism. Theseare all asymmetric aberrations and would be major compo-nents of irregular astigmatism in a horizontally decentredlaser ablation, as expected in these two patients.

Gimbel et al. used the Nidek (OPD-scan) and treated 20eyes of 19 patients with previous LASIK or PRK whocomplained of poor ‘quality of vision’.31 The 20 eyes showedan improvement or resolution of visual symptoms followingwavefront guided custom ablation treatment. The post-operative RMS higher order aberration values were variable,and not always related to improved visual function.

Carones et al. treated seven consecutive abnormal eyeswith previous PRK and LASIK.43 They used an AlconLADARWave and LADARVision system, and reported areduction in total higher order aberrations (up to sixthorder) ranging between 1% and 48% with an average reduc-tion of 26%. Spherical aberration decreased the most andcoma decreased least. There was some correlation betweensymptoms and specific higher order aberrations. Subjec-tively reported improvement in haloes at night correlatedwith reduced spherical aberration, and for eyes with ghostimages, improvement was correlated with reduced coma.Improved monocular diplopia correlated with reducedsecondary astigmatism.

These three reports are early, with small numbers, usedifferent aberrometers and lasers, have individually con-structed algorithms, and did not correlate higher orderaberrations with specific pupil sizes. Despite these limita-tions, the therapeutic use of customised treatment basedon wavefront information seems useful where it can beapplied given the physical constraints of reliably measur-ing the aberrations and having enough residual cornealthickness.

The American FDA has approved three systems forwavefront based corneal surgery. The Alcon LADARWaveLADARVision system was approved in October 2002, theVISX Star S4 active track and wavescan approved in May2003, and the Bausch and Lomb Technolas 217 Zyopticssystem in October 2003.

The data included in these three studies is voluminousand can be viewed on the FDA website at http://www.fda.gov.

In general terms wavefront based LASIK with these threesystems gave similar refractive results to conventionalLASIK. There were less induced higher order aberrationscompared to conventional LASIK, but not an improvementin total higher order aberrations compared to preoperatively.For the Bausch and Lomb system there was a reduction inthird order aberrations by 16%, but an increase in fourthorder, especially spherical aberrations, of 70%. The chanceof reducing higher order aberrations was directly correlatedwith the magnitude of the specific higher order aberrationpresent prior to treatment. These results were presented at apupil size of 6.0 m (Table 2).

For the Alcon system, coma increased by 22%, trefoildecreased by 11% (both third order terms), and sphericalaberration increased by 22% with data presented at a pupilsize of 6.5 mm. The visual significance of the changes isdifficult to determine. An RMS aberration of 0.1 µm isroughly equivalent to the blurring effect of 0.08 D for a6 mm pupil, so percentage increases do not mean much ifthe starting figure is small (Table 3).

The FDA panel recommendations confirmed thatwavefront-based LASIK caused less induction of higherorder aberrations than conventional LASIK. They alsonoted that for the Alcon system, wavefront LASIK demon-strated slightly superior optical quality (reduced mono-chromatic aberrations) compared with conventional LASIK.Minor improvements were also noted in visual acuity andcontrast sensitivity relative to conventional LASIK.

Despite this, the FDA noted that the accuracy of thecorrection for myopia is still the primary determination ofuncorrected image visual quality, and study data did notsupport improved functional performance (activities of dailyliving such as reading and driving) or satisfaction ratesin patients with wavefront-guided LASIK as compared toconventional LASIK.

Table 2. Change from baseline in wavefront aberration RMS at 6 month visit for matched conventional and Zyoptix eyes (wavefrontanalysis diameter = 6.0 mm)†

Aberration Zyoptix (n = 40) Conventional (n = 39) Size (µm) Change (%) Size (µm) Change (%)

Total RMS –3.51 –81 ↓ –3.40 –78 ↓Higher order 0.03 14 ↑ 0.17 45 ↑Second order –3.67 –85 ↓ –3.59 –82 ↓Third order –0.05 –16 ↓ 0.09 30 ↑Fourth order 0.14 70 ↑ 0.17 84 ↑Fifth order 0.02 28 ↑ 0.00 1 ↑

†Data obtained from the FDA website (http://www.fda.gov). RMS, root mean square.


So wavefront LASIK seems at least as good as conven-tional LASIK, is better on some objective psychophysicalmeasures, but these improvements may or may not benoticed by patients.

We reported 3 month data on our first cases of LASIKusing the LADARWave and LADARVision system,44 testedbetween January and March 2003. A battery of psycho-physical tests and psychometric analysis allowed us to makea judgement that this treatment was an improvement overconventional LASIK in our hands, and that it should beoffered to all patients who qualified for the procedure.Psychophysical tests showed an improvement in contrastsensitivity over all spatial frequencies from 3 to 18 cycles perdegree (Fig. 5). This was in contrast to previous reports45

that conventional LASIK caused a depression in contrastsensitivity function, and that recovery could take at least6 months. At 4 mm we found no statistically significantdifference before and after surgery for spherical aberrationsor for total higher order aberrations. Coma increased from

0.06 µm to 0.09 µm (P = 0.041). At a pupil size of 6 mmhigher order aberrations were more apparent. There was astatistically significant increase in total higher order aber-rations from 0.35 µm to 0.43 µm (P = 0.019), a small butstatistically significant increase in spherical aberration from0.14 µm to 0.18 µm (P = 0.003) and a greater increase incoma from 0.19 µm to 0.30 µm (P = 0.005). The subjectivevision index increased from 66.62 to 87.63, a level whichbrings these postoperative patients very close to the levelthat normal emmetropes perceive their vision to be.46

Our study demonstrated that some psychophysical testssuch as contrast sensitivity and best corrected Snellen acuitycould improve, and patients perceived an improvement intheir visual quality after wavefront-based treatment. Thiswas despite an increase in third and fourth order, and otherhigher order aberrations. These increases in higher orderaberrations were less than would be produced by conven-tional LASIK, but still were an increase, despite an improve-ment in some psychophysical and psychometric tests.Clearly the impact of higher order aberrations on visualperformance is still under investigation.

Thompson et al. have recently reported their visual out-comes in eyes undergoing aberrometry guided LASIK com-pared to standard LASIK.47 They used their own purposebuilt ingoing adjustable refractometry system, and a Nideklaser. The wavefront-based LASIK patients achieved Snellenvisual acuity and refractive results equivalent to those ofstandard LASIK, but had better mesopic vision. They alsosubanalysed patients, and demonstrated that improvementsin visual performance were a result of the aberrometry guidedlaser treatment, and not simply due to increased optical andtransition zones diameters.

WHERE TO NOW?

Should all corneal refractive surgery be based on wavefrontinformation? Not yet; at least not for all systems, surgeonsor patients. Aberrometers will need to demonstrate that theycan collect information accurately from both normal andabnormal eyes. Collecting this information will continue to

Table 3. Change in total wavefront error and in higher order aberrations for spherical myopic eyes treated with wavefront-guidedCustomCornea LASIK and conventional LASIK with the LADARVision 4000 system using manifest refraction†

Aberration 3 month mean values 6 month mean values CustomCornea (n = 138) Conventional (n = 47) CustomCornea (n = 139) Conventional (n = 50) Size (µm) Change (%) Size (µm) Change (%) Size (µm) Change (%) Size (µm) Change (%)

Total RMS –3.90 –80 –3.30 –69 –3.88 –79 –3.21 –67Higher order 0.10 27 0.31 77 0.08 20 0.33 82Coma 0.07 31 0.15 71 0.05 22 0.17 78Trefoil –0.01 –8 0.09 52 –0.02 –11 0.07 38Spherical aberration 0.04 24 0.21 96 0.04 22 0.23 108Secondary astigmatism 0.06 83 0.07 92 0.05 73 0.07 105Tetrafoil 0.07 108 0.09 124 0.05 81 0.09 119

†Data obtained from the FDA website (http://www.fda.gov). RMS, root mean square.

Figure 5. (a) Photopic and (b) scotopic single eye CSV–1000 Econtrast sensitivity results (green) before, (red) 1 month after and(blue) 3 months after CustomCornea wavefront LASIK. (�), 194 cm;(�), 219 cm; (�), 244 cm; (�), 269 cm; (�), 294 cm.


be operator dependent. Disturbance of the tear film withpoor blinking or topical medication will influence the dataobtained.

If good data are obtained they have to be over a largeenough area to make a difference. If data are collected at apupil size of 4 mm, then that is all that can be effectivelytreated. The bigger the pupil size at the time of aberrometrythe more information obtained. This is not topography. It ispupil size dependent. Figure 6 demonstrates a typical 3-Dsurgical plan with wavefront treatment to 6.5 mm and ablend zone to 9 mm using information captured from apharmacologically dilated eye. The effect of mydriatics andaccommodation on aberrometry measurements are beingevaluated. Delivering treatment will be constrained by thethings that influence excellent outcome in corneal refractivesurgery. These include individual nomogram adjustment,meticulous surgical technique, an understanding of the bio-mechanics of the cornea43 and structural limitations dictatedby corneal thickness, and anterior and posterior cornealcurvature.48,49

The surgeon will have to make a judgement that thetissue removed with wavefront-based treatment is thecorrect operation in an individual eye, and the possibleimprovement in visual quality will not be offset by anincreased risk of ectasia or a limited ability to perform anenhancement.

Wavefront-based treatment is simply an operation wherebythe corneal shape is being changed in order to produce anaberration free eye. Corneal analysis may not be essential incollecting information, but will have a role in determiningthose patients suitable for treatment, and may have a role inhelping determine correct algorithms. Will this all be toodifficult? The outcome will be influenced by many factors,including the limited precision and predictability of theablation, epithelial hyperplasia and stromal remodelling,changes in the thickness distribution of the tear film, cornealbiomechanics and variations in ocular aberrations with age

and accommodation.50 Some manufacturers have alteredtheir algorithms to incorporate features that improve spher-ical aberrations in a generic way. This is similar to the designof the Technis and other higher order aberrations IOLs. Itis not individualized to the particular eye of a particularpatient, but is a standard feature to limit the exacerbation ofspherical aberration. This would seem to be an improvementover both conventional IOLs and conventional laser algo-rithms, and may be useful as an intermediate step, but doesnot make full use of wavefront information from the indi-vidual patient.

Let us take one of the common arguments that thistechnology will not achieve its potential. Corneal surfacehealing is both a friend and an enemy in corneal refractivesurgery. It is a friend in that smoothing of the epitheliumthins over bumps or islands and thickens to fill divots orrelative depressions.51 This is a helpful process if irregulari-ties occur in the ablation pattern or in the creation of aLASIK flap. It is unhelpful if we are measuring higher orderaberrations and treating them at a submicron level. Epithelialsmoothing will overwhelm the accuracy of the treatmentperformed. Our ability to pharmacologically manipulateepithelial healing is limited, but there are quantitativemodels of corneal surface smoothing formulated to allowthis to be incorporated into treatment algorithms.52 Willsurface ablation allow wavefront treatments to achieve theirpotential? Will the more precise flaps achieved with femto-second lasers allow LASIK to gain an edge?53 These ques-tions are still be answered.

In order to move forward, we need to better understandhow higher order aberrations impact on visual performance,and come up with a measurement more appropriate thanRMS numbers. They are inadequate for this task. RMS is justone of many single value metrics designed to summarizeoptical quality in a single number. What are needed aresingle value metrics that correlate highly with measuredvisual performance. The visual Strehl ratio, as defined byApplegate,8 is an example. Visual Strehl is not solely basedon the optics of the system. It is based on both the opticaltransfer function and the neural transfer function (Fig. 7).Campbell has described metrics such as ‘subjective sharp-ness factor’ and ‘point spread quality’ based in part on the

Figure 6. 3-D treatment plan with LADARWave.

Figure 7. Mathematical definition of visual Strehl.


modulation transfer function.54 It would be useful to have asingle metric, but because different tasks in life presentdifferent visual demands, it is anticipated that differentmetrics may be needed for different tasks.

To move forward we need to know what we are measur-ing and how this impacts on visual performance. As conven-tional LASIK and surface ablation are good procedures andacceptable to most patients, improvements will necessarilybe subtle, and subtle improvements require sophisticatedmeasurement techniques to know whether a step forward isactually a step in the right direction.

The late Dr Charles Kellman said that ‘clinicians measurebut patients decide’, and it may be that patients in the questfor safe accurate surgical alternatives to glasses and contactlenses will seek out custom ablation prior to our ability toprove that it should be the standard of care.

REFERENCES

1. Fraenkel GE, Webber SK, Sutton GL, Lawless MA, Rogers CM.Toric laser in situ keratomileusis for myopic astigmatism usingan ablatable mask. J Refract Surg 1999; 15: 111–17.

2. Waring GO, Carr JD, Stulting RD, Thompson KP, Wiley W.Prospective randomized comparison of simultaneous andsequential bilateral laser in situ keratomileusis for the correctionof myopia. Ophthalmology 1999; 106: 732–8.

3. Davidorf JM, Zaldivar R, Oscherow S. Results and complica-tions of laser in situ keratomileusis by experienced surgeons.J Refract Surg 1998; 14: 114–22.

4. Van Gelder RN, Steger-May K, Pepose JS. Correlation ofvisual and refractive outcomes between eyes after same-sessionbilateral laser in situ keratomileusis surgery. Am J Ophthalmol2003; 135: 577–83.

5. Lawless MA. Refining visual quality assessment in refractivesurgery. J Cataract Refract Surg 1999; 25: 1031–2.

6. Martinez CE, Applegate RA, Klyce SD, McDonald MB,Medina JP, Howland HC. Effect of pupillary dilation on cornealoptical aberrations after photorefractive keratectomy. ArchOphthalmol 1998; 116: 1053–62.

7. Oshika T, Klyce SD, Applegate RA, Howland HC,El Danasoury MA. Comparison of corneal wavefront aber-rations after photorefractive keratectomy and laser in situkeratomileusis. Am J Ophthalmol 1999; 127: 1–7.

8. Applegate RA. Xxxx. In: ISRS/AAO, eds. Subspecialty Day:Refractive Surgery Comes of Age. San Francisco: AmericanAcademy of Ophthalmology, 2003; 167–70.

9. Helmholtz H. Handbuch der Physiologischen Optik. Leipzig:Leopold Voss, 1867; 137–47.

10. Seiler T, Mrochen M, Kaemmerer M. Operative correction ofocular aberrations to improve visual acuity. J Refract Surg 2000;16: S619–22.

11. Klyce SD. Basics of wavefront. In: ISRS/AAO, eds. SubspecialtyDay: Refractive Surgery Comes of Age. San Francisco: AmericanAcademy of Ophthalmology, 2003; 165–6.

12. Thibos LN, Applegate RA, Schwiegerling JT, Webb R. Stand-ards for reporting the optical aberrations of eyes. J Refract Surg2002; 18: S652–60.

13. Thibos LN. Principles of Hartmann-Shack aberrometry. JRefract Surg 2000; 16: S563–5.

14. Applegate RA, Thibos L, Hilmantel G. Optics of aberroscopyand super vision. J Cataract Refract Surg 2001; 27: 1093–107.

15. Molebny VV, Panagopoulou SI, Molebny SV, Wakil YS,Pallikaris IG. Principles of ray tracing aberrometry. J RefractSurg 2000; 16: S572–5.

16. Schmirnoff MS. Measurement of the wave aberration of thehuman eye. Biofizica 1961; 6: 687–703.

17. Assouline M. Optical aberrations. In: Buratto L, Brint SF, eds.Custom LASIK: Surgical Techniques and Complications. Thorofare:Slack Inc, 2003; 453–5.

18. Fernandez de Castro LE, Sandoval HP, Al Sarraf O, Vroman DT,Solomon KD. Relationship between cycloplegic andwavefront-derived refraction. J Refract Surg 2003; 19: S677–81.

19. Guirao A, Williams DR. A method to predict refractiveerrors from wave aberration data. Optom Vis Sci 2003; 80:36–42.

20. Campin JA, Gray GP, Liedel KK, Pettit GH. Impact of pupilaperture, wavefront origin and higher order aberrations on thecomputation of sphero-cylindrical powers in measuredwavefronts. Invest Ophthalmol Vis Sci 2002; 43: E–Abstract 2046.

21. Wang L, Wang N, Koch DD. Evaluation of refractive errormeasurements of the Wavescan Wavefront system and theTracey Wavefront aberrometer. J Cataract Refract Surg 2003;29: 970–9.

22. Glasser A, Campbell MC. Presbyopia and the optical changesin the human crystalline lens with age. Vision Res 1998; 38:209–29.

23. Amano S, Amano Y, Yamagami S et al. Age-related changes incorneal and ocular higher-order wavefront aberrations. Am JOphthalmol 2004; 137: 988–92.

24. Artal P, Berrio E, Guirao A, Piers P. Contribution of the corneaand internal surfaces to the change of ocular aberrations withage. J Opt Soc Am A Opt Image Sci Vis 2002; 19: 137–43.

25. Miller JM, Anwaruddin R, Straub J, Schwiegerling J. Higherorder aberrations in normal, dilated, intraocular lens, and laserin situ keratomileusis corneas. J Refract Surg 2002; 18: S579–83.

26. Vilarrodona L, Barrett GD, Johnson B. High-order aberrationsin pseudophakia with different intraocular lenses. J CataractRefract Surg 2004; 30: 571–5.

27. Taketani F, Matsuura T, Yukawa E, Hara Y. High-order aber-rations with Hydroview H60M and AcrySof MA30BA intra-ocular lenses: comparative study. J Cataract Refract Surg 2004;30: 844–8.

28. Mester U, Dillinger P, Anterist N. Impact of a modified opticdesign on visual function: clinical comparative study. J CataractRefract Surg 2003; 29: 652–60.

29. Maeda N, Fujikado T, Kuroda T et al. Wavefront aberrationsmeasured with Hartmann-Shack sensor in patients with kera-toconus. Ophthalmology 2002; 109: 1996–2003.

30. Shah S, Naroo S, Hosking S et al. Nidek OPD-scan analysisof normal, keratoconic, and penetrating keratoplasty eyes.J Refract Surg 2003; 19: S255–9.

31. Gimbel HV, Sofinski SJ, Mahler OS, Van Westenbrugge JA,Ferensowicz MI, Triebwasser RW. Wavefront-guided multipoint(segmental) custom ablation enhancement using the NidekNAVEX platform. J Refract Surg 2003; 19: S209–16.

32. Mrochen M, Kaemmerer M, Mierdel P, Seiler T. Increasedhigher-order optical aberrations after laser refractive surgery: aproblem of subclinical decentration. J Cataract Refract Surg2001; 27: 362–9.


33. Mrochen M, Eldine MS, Kaemmerer M, Seiler T, Hutz W.Improvement in photorefractive corneal laser surgery resultsusing an active eye-tracking system. J Cataract Refract Surg2001; 27: 1000–6.

34. Pallikaris IG, Kymionis GD, Panagopoulou SI, Siganos CS,Theodorakis MA, Pallikaris AI. Induced optical aberrationsfollowing formation of a laser in situ keratomileusis flap. JCataract Refract Surg 2002; 28: 1737–41.

35. Applegate RA, Hilmantel G, Howland HC, Tu EY, Starck T,Zayac EJ. Corneal first surface optical aberrations and visualperformance. J Refract Surg 2000; 16: 507–14.

36. Applegate RA, Sarver EJ, Khemsara V. Are all aberrationsequal? J Refract Surg 2002; 18: S556–62.

37. Applegate RA, Marsasck JD, Ramos R, Sarver EJ. Interactionbetween aberrations to improve or reduce visual performance.J Cataract Refract Surg 2003; 29: 1487–95.

38. Chalita MR, Xu M, Krueger RR. Correlation of aberrationswith visual symptoms using wavefront analysis in eyes afterlaser in situ keratomileusis. J Refract Surg 2003; 19: S682–6.

39. Huang D, Arif M. Spot size and quality of scanning lasercorrection of higher-order wavefront aberrations. J CataractRefract Surg 2002; 28: 407–16.

40. Guirao A, Williams DR, MacRae SM. Effect of beam size onthe expected benefit of customized laser refractive surgery.J Refract Surg 2003; 19: 15–23.

41. Mrochen M, Kaemmerer M, Riedel P, Seiler T. Why do wehave to consider the corneal curvature for the calculation ofcustomised ablation profiles? Invest Ophthalmol Vis Sci 2000; 41:S689.

42. Mrochen M, Krueger RR, Bueeler M, Seiler T. Aberration-sensing and wavefront-guided laser in situ keratomileusis:management of decentered ablation. J Refract Surg 2002; 18:418–29.

43. Carones F, Vigo L, Scandola E. Wavefront-guided treatmentof abnormal eyes using the LADARVision platform. J RefractSurg 2003; 19: S703–8.

44. Lawless MA, Hodge C, Rogers CM, Sutton GL. Laser in situkeratomileusis with Alcon CustomCornea. J Refract Surg 2003;19: S691–6.

45. Chan JW, Edwards MH, Woo GC, Woo VC. Contrast sensi-tivity after laser in situ keratomileusis. One-year follow-up.J Cataract Refract Surg 2002; 28: 1774–9.

46. Fraenkel G, Comaish F, Lawless MA et al. Development of aquestionnaire to assess subjective vision score in myopesseeking refractive surgery. J Refract Surg 2004; 20: 10–19.

47. Thompson KP, Staver PR, Garcia JR, Burns SA, Webb RH,Stulting RD. Using InterWave aberrometry to measure andimprove the quality of vision in LASIK surgery. Ophthalmology2004; 111: 1368–79.

48. Roberts C. Biomechanics of the cornea and wavefront-guidedlaser refractive surgery. J Refract Surg 2002; 18: S589–92.

49. Comaish I, Lawless MA. Progressive post-LASIK keratectasia:biomechanical instability or chronic disease process? J CataractRefract Surg 2002; 28: 2206–13.

50. Manns F, Ho A, Parel JM, Culbertson W. Ablation profiles forwavefront-guided correction of myopia and primary sphericalaberration. J Cataract Refract Surg 2002; 28: 766–74.

51. Wilson SE, Mohan R, Hong JW, Lee JS, Choi R. The woundhealing response after laser in situ keratomileusis and photore-fractive keratectomy: elusive control of biological variabilityand effect on custom laser vision correction. Arch Ophthalmol2001; 119: 889–96.

52. Huang D, Tang M, Shekhar R. Mathematical model of cornealsurface smoothing after laser refractive surgery. Am J Ophthal-mol 2003; 135: 267–78.

53. Kezirian GM, Stonecipher KG. Comparison of the IntraLasefemtosecond laser and mechanical keratomes for laser in situkeratomileusis. J Cataract Refract Surg 2004; 30: 804–11.

54. Campbell CE. Improving visual function diagnostic metricswith the use of higher-order aberration information from theeye. J Refract Surg 2004; 20: S495–503.

wavefront's role in corneal refractive surgery

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