gatinel d et al. comparison of corneal and total ocular aberrations before and after myopic lasik. j...

8/16/2019 Gatinel D Et Al. Comparison of Corneal and Total Ocular Aberrations Before and After Myopic LASIK. J Refract Surg. May 2010

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333Journal of Refractive Surgery • Vol. 26, No. 5, 2010

Comparison of Corneal and Total Ocular Aberrations Before and After Myopic LASIK

Damien Gatinel, MD, PhD; Pierre-Alexandre Adam, MD; Slim Chaabouni, MD;Jacques Munck, OD; Maud Thevenot, OD; Thanh Hoang-Xuan, MD; Stefan Pieger, Dipl.Ing;Masanao Fujieda, MSE; Harkaran S. Bains, BSc(Hons)

From AP-HP Bichat Claude Bernard Hospital (Gatinel, Adam, Chaabouni,Munck, Thevenot, Hoang-Xuan), Rothschild Foundation (Gatinel, Adam,Chaabouni, Munck, Thevenot), and Center for Expertise and Research in

Optics for Clinicians (CEROC) (Gatinel), Paris, France; and NIDEK Co Ltd,Gamagori, Japan (Pieger, Fujieda, Bains).

Messrs Pieger and Fujieda are employees of NIDEK Co Ltd. Mr Bains is a con-sultant to NIDEK Co Ltd. The remaining authors have no financial interest inthe materials presented herein.

Correspondence: Harkaran S. Bains, BSc(Hons), 17927-80 Ave, Edmonton,AB, Canada, T5T 0S6. Tel: 780.418.8354; Fax: 780.418.8359; E-mail:[email protected]

Received: January 8, 2009; Accepted: May 5, 2009

Posted online: June 22, 2009

ABSTRACT

PURPOSE: To assess the compensation of total ocularand corneal wavefront aberrations after conventionalmyopic LASIK.

METHODS: This study comprised 57 eyes of 57 patients. Total and corneal aberrations were measured preopera-tively and 3 months postoperatively using the OPD-Scan(NIDEK Co Ltd) aberrometer. Total and corneal aberra-tions root-mean-square (RMS) was calculated out tothe 6th Zernike order for a 6.0-mm pupil diameter. Thepercentage increase postoperatively was defined by theratio of RMS pre- and postoperatively for each cornealand total eye group. The compensation between cornealand internal aberrations for a given aberration group wasdefined as: (corneal aberration group RMS total eyeaberration group RMS)/corneal aberration group RMS.

RESULTS: Postoperatively, higher order aberrations in-creased by a factor of 1.771.26 (total) and 2.472.25(corneal) (P.05). Coma aberration increased by a fac-tor of 2.432.61 (total) and 2.562.66 (corneal).Spherical aberration increased by a factor of 1.461.83(total) and 2.642.24 (corneal). The values of the ratioof compensation did not change significantly before andafter LASIK for individual aberrations (P.05).

CONCLUSIONS: Although myopic LASIK induced signifi-cant corneal aberrations, the level of partial compensa-tion of corneal aberrations by internal structures remainedunchanged. These results suggest that the previouslydescribed emmetropization that is effective during de- velopment may also be effective with acquired variationsin corneal shape. [ J Refract Surg . 2010;26:333-340.]doi:10.3928/1081597X-20090617-01

The human eye is affected by aberrations that candegrade retinal image quality. Recent studies havefound that in pre-presbyopic patients, the magnitude

of higher order aberrations for the cornea or the lens indi-vidually are larger than for the whole eye.1-4 Additionally,despite variation in size and shape, the average magnitude of

higher order aberrations of emmetropic eyes is similar to thatfound in mild to moderate myopes and hyperopes.5 Guirao andArtal2 propose a passive, simple geometric model for the ocu-lar compensation of aberrations. They suggest that the compo-nents of the eye are similar to an auto-compensating design,producing similar overall average optical quality for differentrefractive errors, despite large structural variations.2

The progressive lack of compensation over time may be ex-plained by changes in the lens shape and size throughout life.6,7 Whether a compensatory mechanism exists that operates overa period of months to account for the optical decoupling ofcorneal and lenticular higher order aberrations after cornealrefractive surgery remains unaddressed. The variations of the

total ocular and corneal higher order aberrations and visualperformance before and after LASIK have been investigatedpreviously.8-11 One study that measured corneal and internalhigher order aberrations on separate instruments reports par-tial compensation of higher order aberrations before and aftercorneal refractive surgery.12 In the current study, we useda combined ocular aberrometer and corneal topographer tocompare pre- and postoperative corneal (corneal aberrations)


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Copyright © SLACK Incorporated334

Corneal and Total Ocular Aberrations After LASIK/Gatinel et al

and total ocular aberrations (total aberrations) of eyesthat underwent myopic LASIK. These measurementswere used to determine whether the sudden decouplingof higher order aberrations due to LASIK unlocked acompensatory process that minimized the amount ofinduced aberrations after surgery.

PATIENTS AND METHODS

P ATIENT POPULATION

Consecutive patients undergoing conventional myo-pic LASIK by the same surgeon (D.G.) were included inthis study. Inclusion criteria were preoperative mani-fest refraction spherical equivalent (MRSE) 10.00diopters (D) and preoperative refractive astigma-tism 0.75 D. Soft contact lens wearers had to re-move their lenses 1 week prior to the first examination.

Patients wearing hard contact lenses prior to surgery werenot included. Eyes with topographic abnormalities suchas forme fruste keratoconus, keratoconus, pellucid mar-ginal degeneration, contact lens warpage, marked corne-al irregularity, and490 µm of central corneal thicknesspreoperatively were not included in this study. Patientswith a physiologic pupil diameter 6 mm under meso-pic conditions were excluded from participating.

Patients with intra- or postoperative complicationswere not included in this study. Postoperative cornealtopographies were evaluated by the surgeon (D.G.)to ensure a well-centered ablation and the lack of ir-regular corneal astigmatism. The criteria for evaluat-

ing ablation centration using corneal topography have been described previously.13

This study adhered to the tenets of the Declarationof Helsinki. Informed consent was obtained from allpatients after an explanation of the risks and benefitsof the study.

All eyes underwent a baseline ophthalmic examina-tion that included measurement of uncorrected visualacuity (UCVA), best spectacle-corrected visual acu-ity (BSCVA) (decimal notation), manifest refraction,cycloplegic refraction, slit-lamp microscopy, dilatedfunduscopy, ultrasound corneal pachymetry (ReichertOphthalmic Instruments, Depew, NY) and simulta-

neous corneal topography, wavefront aberrometryand pupillometry (infrared light) using the OPD-Scan(NIDEK Co Ltd, Gamagori, Japan). Postoperatively, allpatients underwent the same measurements as pre-operatively with the exception of dilated funduscopy(unless clinically warranted).

COMBINED A BERROMETRY AND CORNEAL MEASUREMENTS

Mean central keratometry, corneal, and total opticalaberrations were measured using the OPD-Scan, which

measures corneal topography and wavefront aberra-tions nearly simultaneously (within 0.20 seconds ofeach other) on the same optical axis without movingthe patient. The corneal topography is measured usingPlacido-disk technology and aberrometry is measuredusing the principle of skiascopic phase difference.14 Wavefront measurements are performed by scanningwith an infrared slit beam, and the reflected light iscaptured by an array of rotating photodetectors cov-ering 360° within the eye in 1° increments. With thistechnique, 1440 wavefront data points within a 6-mmpupil diameter are measured. A built-in eye trackeraccounts for eye movements that may occur dur-ing measurements. A quality check of the differencein the pupil centers between corneal and wavefrontmeasurements is included to ensure there is no shiftin pupil center between measurements. This quality

check alerts the operator to shifts in the pupil centerwith a color-coded message. Alignment in the x-y-zaxes and automated tracking ensures all measurementswere centered without pupillary shift between cornealand wavefront measurements. Internal wavefront andcorneal wavefront can be computed using OPD-Stationsoftware (NIDEK Co Ltd). The patient remained seatedin the same position for all measurements, hence thewavefront and corneal topography data can be directlycorrelated.

All measurements were acquired in a dark examina-tion room after 2 minutes of dark adaptation and wererepeated at least three times (with dark adaptation be-

tween each measurement). Both the total and cornealwavefronts were reconstructed using 6th order Zernikepolynomial decomposition for a 6-mm pupil, centeredon the corneal vertex.

The internal aberration calculations are proprietary.To simplify the description for the purposes of this paper,the wavefront of the internal eye was calculated usingthe corneal wavefront derived from corneal topogra-phy and whole eye wavefront measured with the OPD-Scan. The wavefront was converted to Zernike poly-nomials to calculate internal aberration coefficients bydetermining the difference between corneal aberrationsand whole eye aberrations. All aberrations were trans-

posed to vertex=0 (corneal vertex) prior to calculation.The computation of corneal aberration used in the cur-rent study has been described previously.15

SURGERY

All patients underwent bilateral conventional (notwavefront) myopic LASIK using the NIDEK EC-5000(NIDEK Co Ltd) excimer laser. All surgeries wereperformed by the same surgeon (D.G.) and outcomeswere targeted for emmetropia using a personalized


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nomogram. The eye undergoing surgery was preparedusing a povidone-iodine solution. Topical anestheticwas instilled on the cornea and a lid speculum wasinserted. The Hansatome microkeratome (Bausch &Lomb Inc, Rochester, NY) was used to create the cor-neal flap with a superior hinge. The corneal flap wasreflected back and the laser ablation was deliveredto the stroma using an optical zone of 6.0 mm and a7.0-mm transition zone. The NIDEK EC-5000 laserablation algorithm uses an expanding diaphragm androtating scanning slit delivery system to remove theappropriate volume of corneal tissue. Patients fixatedon a red fixation light, coaxial with the surgeons’ lineof sight, and the excimer laser beam. A 60-Hz activeinfrared eye tracker was used for all surgeries. Theflap was repositioned and the interface was irrigatedusing balanced salt solution. One drop each of topi-

cal fluoroquinolone, corticosteroid, and artificial tearswas instilled prior to discharging the patient. Patientsreceived topical fluoroquinolone antibiotic to use fourtimes per day for 5 days and corticosteroid drops twicea day for 21 days.

D ATA A NALYSIS

Although all surgeries were bilateral, only one eyeper patient was selected for this study based on a ran-domization schedule developed by a biostatistician.The following data were compared preoperatively and 3months postoperatively: MRSE, BSCVA, 3-mm centralmean keratometry, and RMS values of the total higher

order wavefront and corneal higher order wavefront.The following normalized Zernike terms out to

the 6th Zernike order were examined (excluding the0th=Z

00 piston term):

● total higher order aberrations group (all terms in-cluded in the 3rd, 4th, 5th, and 6th order),

● total coma group (7th=Z-13, 8th=Z

31, 17th=Z-1

5, 18th=Z

51

terms),● total trefoil group (6th=Z-3

3, 9th=Z

33, 16th=Z-3

5, 19th=Z

53

terms),● total spherical aberration group (12th=Z

40 and

24th=Z60 terms),

● total tetrafoil group (10th=Z-44, 14th=Z

44, 22nd=Z-4

6,

26th=Z64 terms), and● total higher order astigmatism (11th=Z-2

4, 13th=Z

42,

23rd=Z-26, 25th=Z

62, 39th=Z-2

8, 41st=Z2

8 terms).

These aberration groups were used to describe boththe corneal aberration and the total eye aberrations. Allwavefront measurements were reported for a 6-mmpupil diameter. The percentage increase (PI) postop-eratively was defined as the ratio between the RMSvalues before and after LASIK for each of the cornealand total eye aberration groups described above.

The ratio of compensation (RC) between corneal andinternal aberrations was quantified using the followingequation:

RC =(corneal aberration group RMS value total eye aberration group RMS)

_________________________________________________________

corneal aberration group RMS

When the total eye aberration group is equal to thecorneal aberration group, the RC value is 0 (no inter-nal compensation). When the total eye aberration groupvalue is null (eg, full compensation), the RC value is 1.

Statistical comparisons between the pre- and post-operative measurements were performed using theStudent t test (independent samples). A P value .05was considered statistically significant.

RESULTS

Fifty-seven eyes from 57 patients were included

in this study. Mean patient age was 33.24

9.09 years(range: 20 to 56 years). Measurements and clinicalexaminations were performed 2739 days before and7317 days after LASIK surgery.

Mean central (3 mm) keratometry changed from7.630.252 mm (range: 7.05 to 8.14 mm) preop-eratively to 8.410.46 mm (range: 7.65 to 9.40 mm)postoperatively (P .001). Mean preoperative spheri-cal equivalent refraction was 4.192.19 D (range:0.75 to 9.75 D), and mean postoperative sphericalequivalent refraction was 0.390.65 D (range: 2.25to 1.00 D) (P .00). Thirty-seven (64.9%) eyes hada postoperative MRSE between 0.50 and 0.50 D.

Fifty-one (89.5%) eyes had a postoperative MRSEwithin 1.00 D of the intended correction. Preopera-tively, mean astigmatism was 0.320.33 D (range:0 to 0.75 D). Mean postoperative astigmatism was0.310.27 D (range: 0 to 1.00 D) (P =.75).

Pre- and postoperative corneal higher order aber-rations and total higher order aberrations are shownin Table 1. The RMS of each corneal aberration groupincreased significantly after LASIK (P .05) (Table 1).The RMS of total trefoil and total higher order astig-matism did not increase significantly (P .05); how-ever, all other groups showed a statistically significantincrease in RMS postoperatively (P .05) (non-paired

Student t test) (Table 1). The ratio of compensation forvarious aberrations is shown in Table 2.

DISCUSSION

In our investigation of eyes that had undergone con-ventional myopic LASIK, partial compensation of in-duced corneal higher order aberrations by the internalaberrations of the eye was found. However, the differ-ences in the methods of measurement used in this studycompared to previous studies requires explanation.


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CORNEAL V ERTEX V ERSUS L INE OF SIGHT

REPRESENTATION OF W AVEFRONT A BERRATIONS

Historically, internal aberrations have been mea-sured using two separate instruments, a corneal topog-rapher and laboratory wavefront sensors or commer-cially available aberrometers. Using two instrumentswhere the patient or instrument has to be shifted intoposition between these successive measurements in-

troduces error due to the two different measurementaxes. This limitation remains unaddressed without acyclotorsion error detection module built into one ofthe measuring instruments that could verify the irisposition remains unchanged. To date, no combinedapparatus is available that addresses ocular cyclotor-sion in this manner when reporting internal aberra-tions. The OPD-Scan circumvents this limitation byusing one instrument for both corneal and whole eyemeasurements.

The American National Standards Institute (ANSI)recommends the representation of wavefront aber-rations relative to the line of sight (axis joining thefovea with the natural pupil center).16,17 The OPD-Scanreports all wavefront data using the vertex normalrather than the line of sight, which does not conformto the recommended “standard” proposed by ANSI.The vertex normal has been in use for well over two

decades in instruments such as autorefractors, auto-keratometers, and corneal topographers, yet the ANSIrecommendation is relatively recent. It seems themanufacturer elected to use the convention incorpo-rated in the majority of ophthalmic instruments. TheANSI recommended line of sight has one limitationthat remains unaddressed—the change in pupil centerthat can occur due to different pupil sizes. Using thevertex normal obviates this limitation, as this axis isindependent of pupil diameter.

TABLE 1

Change in Root-mean-square Aberrations of 57 Eyes That UnderwentConventional Myopic LASIK

MeanStandard Deviation

Aberration Group Preoperative (µm) Postoperative (µm) PI P Value

Total higher order aberration

Total eye 0.330.11 0.530.31 1.771.26 .00002

Corneal 0.850.51 1.711.64 2.472.25 .0003

Total coma

Total eye 0.1510.07 0.277-0.217 2.432.61 .000061

Corneal 0.3850.276 0.7150.777 2.562.66 .0030

Total trefoil

Total eye 0.2140.103 0.2490.167 1.421.22 .18

Corneal 0.4780.403 0.850.91 2.923.67 .0075

Total spherical aberration

Total eye 0.1080.066 0.231 0.178 1.461.83 .0000035

Corneal 0.3070.12 0.6880.467 2.642.24 .00000003

Total tetrafoil

Total eye 0.0750.062 0.100.10 2.132.66 .0536

Corneal 0.2840.241 0.6710.79 4.386.69 .00054

Total higher order astigmatism

Total eye 0.0680.11 0.0780.042 1.841.40 .51

Corneal 0.230.19 0.520.57 4.6613.38 .0005

PI = increase factor, defined as the ratio between the root-mean-square values before and after LASIK for each of the corneal and total eye aberration groups,

total eye = total ocular aberrations, corneal = total corneal aberrations

All wavefront measurements are reported for a 6-mm pupil diameter.

Note. P.05 denotes a statistically significant difference.

Total higher order aberration group, all terms included in the 3rd, 4th, 5th, and 6th Zernike order; total coma group comprises the 7th=Z -13 , 8th=Z 31 ,17th=Z -1

5 , and 18th=Z

51 Zernike terms; total trefoil group comprises the 6th=Z -3

3 , 9th=Z 3

3 , 16th=Z -3

5 , and 19th=Z

53 Zernike terms; total spherical aberration

group comprises the 12th=Z 40 and 24th=Z

60 Zernike terms; total tetrafoil group comprises the 10th=Z -4

4 , 14th=Z

44 , 22nd=Z -4

6 , and 26th=Z

64 Zernike terms;

and total higher order astigmatism comprises the 11th=Z -24 , 13th=Z

42 , 23rd=Z -2

6 , 25th=Z

62 , 39th=Z -2

8 , and 41st=Z 2

8 Zernike terms.


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To allow direct calculation of the internal aberra-tion, corneal and total aberrations must be plottedon a common axis to avoid calculation errors. In thecurrent study, both the total and corneal aberrationswere acquired while the eye was focused at infin-ity and calculated with regard to the vertex normal;no realignment procedure was required before com-paring corneal and total aberrations. However, sys-tematic parallax between the overlying cornea andthe aberrations at the pupil plane may occur due to

this measuring principle. The anticipated effect ofthis misregistration would be elevated coma values,which warrants further investigation. Therefore, thecoma values generated in this study must be inter-preted with caution.

Unlike OPD-Scan measurements, most aberrometersthat evaluate wavefront measure the wavefront slopefrom which the pupil center or line of sight is deter-mined. However, this is ambiguous, as locating thepupil center depends on the pupil diameter and thecenter can deviate with differing pupil diameters.17,18 Furthermore, coma aberration that is common in mostocular structures displaces the chief ray, which con-

siderably changes the wavefront plot depending on thedefinition of the line of sight (axis joining the fovea tothe pupil center vs average of a bundle of rays recom-mended by the Optical Society of America) used.18,19 The ambiguity of plotting wavefront on the line ofsight is obviated in the current study by using a singleinstrument measuring corneal and total aberrations onthe same optical axis. Although the vertex normal maychange after refractive surgery, it remains independentof pupil diameter.

In 2002, Salmon and Thibos20 investigated the lackof compensation of reporting corneal wavefront dataat the vertex normal and whole eye wavefront on theline of sight. They found the overall magnitude of cor-neal and internal aberrations was slightly lower whenthere was no compensation.20 For example, the maxi-mum mean difference between compensation and lackof compensation of higher order aberrations was 0.08 µmRMS for corneal aberrations and 0.05 µm RMS for in-ternal aberrations.20 To determine the magnitude of

difference from reporting wavefront aberrations at thevertex normal and not the line of sight, we calculatedthe internal wavefront aberration at both locations todetermine the difference for high, moderate, and lowrefractive errors representative of our patient popula-tion (Fig). The differences in the Figure are clinicallynegligible.

A BERRATION COMPENSATION, PREVIOUS STUDIES, AND

CLINICAL IMPLICATIONS

Laser in situ keratomileusis is a corneal procedurethat does not induce changes in the crystalline lensyet produces significant changes in optical aberrations

and corneal power. If we hypothesize that no changesother than those incurred on the anterior corneal sur-face occur after LASIK (eg, no change in the internalaberrations postoperatively), the net increase in totalaberrations should be equal to the increase in cornealaberrations. Thus, the balance between corneal and in-ternal aberrations would be disrupted. Disruption ofthe balance between corneal and internal aberrationsoccurs with age in unoperated eyes.1,7 For example,the internal optical aberrations increase three-fold

TABLE 2

Pre- and Postoperative Values of the Ratio of Compensation Between Cornealand Total Eye Aberrations

Aberration Group Preoperative Ratio Postoperative Ratio P Value*

Total higher order aberration 0.510.25 0.590.28 .09

Total coma 0.440.40 0.400.50 .31

Total trefoil 0.290.59 0.360.77 .27

Total spherical aberration 0.620.20 0.640.19 .39

Total tetrafoil 0.550.60 0.650.55 .17

Total higher order astigmatism 0.610.38 0.690.28 .15

All wavefront measurements are reported for a 6-mm pupil diameter.

Note. P.05 denotes a statistically significant difference.

Total higher order aberration group, all terms included in the 3rd, 4th, 5th, and 6th Zernike order; total coma group comprises the 7th=Z -13 , 8th=Z

31 ,

17th=Z -15 , and 18th=Z

51 Zernike terms; total trefoil group comprises the 6th=Z -3

3 , 9th=Z 3

3 , 16th=Z -3

5 , and 19th=Z

53 Zernike terms; total spherical aberration

group comprises the 12th=Z 40 and 24th=Z

60 Zernike terms; total tetrafoil group comprises the 10th=Z -4

4 , 14th=Z

44 , 22nd=Z -4

6 , and 26th=Z

64 Zernike terms; and

total higher order astigmatism comprises the 11th=Z -24 , 13th=Z

42 , 23rd=Z -2

6 , 25th=Z

62 , 39th=Z -2

8 , and 41st=Z 2

8 Zernike terms.


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between ages 20 and 70 years, whereas corneal aberra-tions increase only mildly.7

We found that corneal aberrations were higher thanthe total aberrations both pre- and postoperatively. Adecrease in the magnitude of the total eye aberrationsindicates some degree of compensation by the internaloptics, as corneal and internal aberrations combine togive total eye coefficients. The results of our study in-dicate that internal aberrations continue to reduce theimpact of the induced corneal wavefront changes afterLASIK. Our results concur with those of Marcos et al12

who measured total ocular and corneal aberrations be-fore and after LASIK surgery. Total aberrations weremeasured using a laser ray-tracing technique; whereascorneal aberrations were obtained from corneal eleva-tion data measured using a corneal topographer withcustom software.12 Marcos et al found that the totalspherical aberration increased less than the sphericalaberration induced on the anterior corneal surface.12 Incontrast to our findings, the same analysis for postop-erative LASIK 3rd order aberrations showed no statis-

tically significant difference between corneal and totalaberrations.12 This discrepancy may be due to the lim-ited sample size in the Marcos et al study12 (14 eyes inthe Marcos study vs 57 eyes in the current study) andto the difference in the wavefront acquisition method(eg, one instrument in the current study vs two separateinstruments in the Marcos study). Additionally, therewas an approximate 38-fold difference in the numberof points used in our study (1440 points) compared toMarcos et al (37 points) to plot the total higher orderaberrations of the eye; mathematical data smoothing

between points may account for the differences. Alter-nately, the differences can be due to reporting aberra-tions based on ANSI standards (Marcos et al) versusnon-ANSI standards discussed above.

The posterior corneal spherical aberration is nega-tive in young eyes.21 Marcos et al12 attributed the in-crease in the internal compensation of spherical aber-ration observed after myopic LASIK to changes in theposterior curvature of the cornea due to biomechanicalremodeling. However, the accuracy of the posterior

A

C

B

D

Figure. A) Percent difference in the internal wavefront aberrations reported using the vertex normal or line of sight as the reference axis. B) Internal

wavefront aberration reported on the vertex normal (blue bar) or line of sight (red bar) for a low myope (3.95 D). C) Internal wavefront aberration

reported on the vertex normal (blue bar) or line of sight (red bar) for a moderate myope (7.48 D). D) Internal wavefront aberration reported on the

vertex normal (blue bar) or line of sight (red bar) for a high myope (11.23 D).


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corneal changes reported with slit-scanning topogra-phy after corneal surgery have been refuted by rotatingScheimpflug imaging, demonstrating instrument arti-fact.22-24 The refractive power of the whole cornea isdue to the variation in the refractive indices of air (1.0),cornea (1.376), and the aqueous (1.336).25 Due to thedifference in the indices of refraction and curvature,the anterior interface is 10 times greater than the pos-terior interface; hence the contribution of the posteriorsurface to the corneal aberration would be minimal.Dubbelman et al26 concluded that the posterior cornealsurface coma compensates approximately 6% of theanterior corneal surface coma.

Artal et al27 reported a higher compensation of in-ternal optics, particularly coma in hyperopic eyes,which usually have a larger angle kappa. They foundthat a displacement of the cornea with positive spher-

ical aberration induces positive coma, whereas thissame displacement for the lens, with a similar butnegative spherical aberration, induces negative coma,which nearly cancels that of the cornea.27 These re-sults support a simple passive mechanism for thecompensation. Such an auto-compensating designmay explain the similar overall optical quality ofhyperopes and myopes, despite the large scale ofstructural variations.

Our data show a consistent match in the magni-tude of aberrations pre- and postoperatively, whichsuggests that a process exists to proportionally in-crease internal aberration compensation for the eye.

This dual optimization strategy likely allows main-tenance of retinal image quality (and visual quality)despite the induced corneal aberrations. Some stud-ies have reported a recovery of contrast sensitivity at3 months after LASIK.28,29 Due to the surgically in-duced profile change of the anterior corneal surface,the angle of incidence of the light rays emergingfrom the posterior corneal surface and striking thecrystalline lens is modified compared to preopera-tively. During the course of writing this manuscript,one of the authors (D.G.) performed sample ray trac-ing calculations using myopic eyes and found that asthe cornea flattened and became more oblate (akin to

the changes induced after myopic LASIK), the calcu-lated internal aberrations tended to increase in thesame fashion as the current study (unreported data,

June 2008), supporting a passive mechanism. How-ever, this modeling was performed on a small samplesize after conclusion of the study and was not sub-mitted for publication. The change in position of thelight rays striking the lens may passively change theoptical properties of the internal optics and partiallyexplain our results. Perhaps an active mechanism

for this auto-compensation could also be at play (eg,a subtle shift, tilt, or curvature change of the physi-ologic lens), which would thus also make the eyerobust to structural variations such as LASIK-inducedcorneal aberrations. An alternate explanation may bea mild change in the gradient refractive index of thelens that allows active compensation of the inducedcorneal aberrations.6,30 However, a change in refrac-tive index generally does not occur so quickly. Theobservation that the ratio of internal compensationwas not statistically different before and after LASIKsupports both an active or passive mechanism or acombination. In both cases, the mechanisms behindsuch changes have yet to be determined.

Sufficient plasticity seems to exist in the opticaldesign of the eye that allows for this “emmetropiza-tion” to occur across the range of refractive error treated

in the current study. However, the range of inducedcorneal change over which this compensation occursremains unanswered in our study. One requirementof an auto-compensation mechanism is the presenceof an active feedback loop that enables changes overa relatively short temporal scale. There are examplesof such rapid ocular auto-compensating mechanisms.For example, retinal photoreceptors used the directionof incident light to realign themselves from the pupilperiphery to the pupil center within 10 days afterintraocular lens implantation in a patient with bilat-eral congenital cataracts for four decades.31,32

Some drawbacks of this study include the short fol-

low-up period, which does not account for completecorneal wound healing, and the lack of normativedata over the same time period. Ideally, a random-ized, contralateral study design treating one eye withLASIK and not treating the fellow eye would have

been provided conclusive data. In our experience, fewrefractive surgery candidates would accept or tolerateanisometropia over 3 or 4 months, and we would have

been faced with repeated requests for treatment of theuntreated eyes and a significant dropout rate. Physi-ologic “emmetropization” generally occurs before age40. In the current study, patients were presbyopic andpre-presbyopic; however, we did not dichotomize the

compensation of aberrations between these groups.This observation warrants further study. Furthermore,the effect of the natural decoupling of corneal and len-ticular spherical aberrations from the presbyopic yearsonwards requires further investigation

This is the first publication to measure corneal andocular optical aberrations on one instrument using thesame optical axis to show that the compensation forcorneal aberrations by the internal aberrations of theeye increases after myopic LASIK surgery. In our study,


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this effect was observed in all higher order aberrationgroups that were tested. This systematic compensa-tion suggests an automated mechanism and supportsthe concept of an optically robust design to develop-mental variation in ocular shape and geometry and toacquired variation in corneal shape.

AUTHOR CONTRIBUTIONS

Study concept and design (D.G.); data collection (D.G., P.A., S.C.,

J.M., M.T.); analysis and interpretation of data (D.G., P.A., S.C., J.M.,

T.H.X., S.P., M.F., H.S.B.); drafting of the manuscript (D.G., P.A.,

H.S.B.); critical revision of the manuscript (D.G., P.A., S.C., J.M., M.T.,

T.H.X., S.P., M.F., H.S.B.); statistical expertise (D.G.); supervision

(D.G., J.M.)

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gatinel d et al. comparison of corneal and total ocular aberrations before and after myopic lasik. j...

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