J CATARACT REFRACT SURG - VOL 33, JANUARY 2007

ARTICLES

Determining corneal power using

Orbscan II videokeratography for intraocular

lens calculation after excimer laser

surgery for myopia

Mujtaba A. Qazi, MD, Irwin Y. Cua, MD, Cynthia J. Roberts, PhD, Jay S. Pepose, MD, PhD

PURPOSE: To assess the accuracy of Orbscan II slit-scanning videokeratography for intraocular lens(IOL) calculation in eyes with previous photorefractive surgery for myopia.

SETTING: Private practice, St. Louis, Missouri, USA.

METHODS: Corneal power (K) was measured by manual keratometry, Placido-based videokeratogra-phy (Atlas), slit-scanning videokeratography (Orbscan II), and contact lens overrefraction in 21 post-photoablation eyes having cataract surgery. Postoperative data collected after phacoemulsificationwere used to back-calculate corneal power (BCK). The BCK values were statistically compared at 3.0to 6.0 mm central Orbscan II curvature and power measurements, including total axial power, totaltangential power, total mean power, and total optical power. Similar comparisons were made to Atlascurvature at the 0.0 to 10.0 mm zones.

RESULTS: The mean corneal power after refractive surgery based on BCK values using the Holladay 2formula (BCK H2) was 39.35 diopters (D) G 2.58 (SD). The mean manual value (40.52 G 1.95 D) andAtlas-based values were statistically higher than BCK H2 values (P<.001). The mean corneal power cal-culated from historical data was 39.33 G 2.70 D (P Z .83 to BCK H2; n Z 19) and from contact lensoverrefraction, 41.38 G 3.11 D (P Z .19; n Z 5). Orbscan II parameters (n Z 21) of the total meanpower (3.0 mm, 39.10 G 2.63 D), total tangential power (3.0 mm, 39.11 G 2.60), total axial power(5.0 mm, 39.19 G 2.55 D), and total optical power (3.0 mm, 39.08 G 2.78 D; 4.0 mm, 39.39 G 2.76 D)were statistically similar to both the historical and BCK H2 values (P>.11). If used prospectively, 80.9% ofeyes would have been within G0.50 D of the targeted refraction using a 4.0 mm total optical power,76.2% using a 5.0 mm total axial power, and 42.1% using the historical method.

CONCLUSION: The Orbscan II 5.0 mm total axial power and 4.0 mm total optical power can be used tomore accurately predict true corneal power than the history-based method and may be particularlyuseful when pre-LASIK data are unavailable.

J Cataract Refract Surg 2007; 33:21–30 Q 2007 ASCRS and ESCRS

Manual and automated keratometry are the most fre-quently used techniques for measuring central corneal

refractive power (K) to calculate intraocular lens (IOL)

power in cataract surgery. Although these instruments offer

good accuracy when applied to healthy, unoperated cor-

neas with regular astigmatism, their use in measuring cor-

neal power in patients with irregular astigmatism1 or who

have had keratorefractive surgery2–12 can lead to significant

residual postoperative refractive errors. These ‘‘surprises’’

Q 2007 ASCRS and ESCRS

Published by Elsevier Inc.

stem partly from the fact that the manual keratometer aver-ages measurements from only 4 discrete paracentral points

on the anterior cornea and that it assumes a spherical cen-

tral cornea, with the posterior cornea having a radius of

curvature 1.2 mm smaller than its anterior counterpart.

Eyes that have had keratorefractive laser surgery have

altered corneal asphericity13 and variations in the relation-

ship between the anterior and posterior corneal radii of

curvature.14,15

0886-3350/07/$-see front matterdoi:10.1016/j.jcrs.2006.08.026

21

CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

The literature is replete with strategies to better deter-

mine corneal power in eyes that have had refractive surgery

including photorefractive keratectomy (PRK) and laser insitu keratomileusis (LASIK). Among the most commonly

used methods are refraction-based calculations,3–11,16–19

rigid gas-permeable contact lens (RGP) overrefrac-

tion,1,3,8,10,11,17,18,20 and computerized videokeratography

(CVK).1–4,6,9–11,14,16,18,21–26 The latter 2 techniques be-

come particularly important when the preoperative refrac-

tive surgical history is unavailable. Srivannaboon et al.23

describe using slit-scanning videokeratography to detectthe change in corneal refractive power after LASIK for

myopia. They note that the total optical power map of the

Orbscan topography system (Bausch & Lomb) provides

K-values that correlate closely with changes in manifest

refraction produced by LASIK.

The present study evaluated the use of CVK parameters

of corneal power for calculating IOL power in a myopic

LASIK cohort. Predictability of the CVK power measure-ments was compared to ‘‘actual’’ post-keratorefractive sur-

gery corneal power derived from back-calculating corneal

power (BCK) from refractive data collected at least 1 month

after uneventful phacoemulsification. Back-calculated cor-

neal power was statistically compared to manually derived

corneal power, corneal power calculated from preoperative

refractive surgery data (historical method) and contact lens

overrefraction, Placido disk–based topography (Atlas), andOrbscan-derived corneal power at 3.0 to 6.0 mm central

diameters.

PATIENTS AND METHODS

Preoperative Testing

Standard preoperative testing, including manifest refraction,tonometry, and slitlamp biomicroscopy, was performed on post-keratorefractive surgery patients scheduled for phacoemulsifica-tion over a 12-month period. Eyes were excluded if they had

Accepted for publication August 13, 2006.

From a private practice (Qazi, Pepose) and the Department ofOphthalmology and Visual Sciences (Qazi, Pepose), WashingtonUniversity School of Medicine, St. Louis, Missouri, and the Depart-ment of Ophthalmology and Biomedical Engineering Depart-ment (Roberts), Ohio State University, Columbus, Ohio, USA;St. Luke’s Medical Center (Cua), Quezon City, Philippines.

Supported in part by the Midwest Cornea Research Foundation,St. Louis, Missouri, USA.

Dr. Pepose received research and travel support from Bausch &Lomb. Dr. Roberts is a consultant to Bausch & Lomb. No otherauthor has a financial or proprietary interest in any material ormethod mentioned.

Corresponding author: Jay S. Pepose, MD, PhD, Pepose VisionInstitute, 16216 Baxter Road, Suite 205, Chesterfield, Missouri63017, USA. E-mail: jpepose@peposevision.com.

J CATARACT REFRACT SURG22

best spectacle-corrected visual acuity (BSCVA) worse than 20/30after phacoemulsification, a history of flap complications, or sig-nificant systemic or ocular co-morbidity including diabetes melli-tus, retinopathy or maculopathy, glaucoma, and optic neuropathy.

Patients included in the study had myopic PRK or LASIKa mean of 52.2 months before their cataract surgery. Manual kera-tometry (Reichert), Atlas topography, and Orbscan II slit-scanningvideokeratography were prospectively obtained in 21 eyes of 16patients. The axial length (AL) was measured using ultrasound bi-ometry (Echograph Axis-II, Quantel Medica) in contact mode (15)and immersion mode (6).

Pre-LASIK or pre-PRK refractive history was available for 19eyes. Before cataract surgery RGP overrefraction3 was performedin 5 eyes, all with a BSCVA of 20/70 or better. Refraction was per-formed before and after placement of the contact lens. The cornealpower was estimated using the following formula:

KZBCþDþ ðOR� SEÞ

where K is the corneal power, BC is the base curve of rigid contactlens, D is the contact lens power, OR is the spherical equivalent ofcontact lens overrefraction, and SE is the spherical equivalentwithout the contact lens.

Orbscan simulated K-values, mean power at the 3.0 mm and5.0 mm zones, anterior axial power, total anterior power, total tan-gential power, total mean power, and total optical power mapswere evaluated for each eye. The ‘‘Tools’’ menu was used to obtain‘‘analyze area – statistics’’ for 3.0, 4.0, 5.0, and 6.0 mm of the centralcornea (Bausch & Lomb. Orbscan II, Orbscan IIz, Operator’s Man-ual, version 3.12, March, 2002). The numerical display menu wasselected for the Atlas to determine corneal power at the 0.0, 1.0,and 2.0 to 10.0 mm zones. These data were also statistically com-pared to mathematical derivation of central corneal power basedon a technique by Smith et al.27 (Atlas power at 0 mm � (376/337.5) � 4.90 diopters [D]) and Wang et al.18 modification ofthe previous method (Atlas power at 0 mm� [376/337.5]�6.1 D).

Intraocular Lens Power Calculation

Intraocular lens power was determined using the HolladayIOL Consultant (Holladay Consulting), which includes a modein which IOL power calculation can be modified in cases of previ-ous refractive surgery. In all cases, the boxed marked ‘‘PreviousRefractive Surgery’’ was checked. This allowed for IOL powercalculation using the historical method, RGP overrefractiontechnique, and surgeon-entered corneal power values. Once his-torical refractive data were entered, the Holladay IOL Consultantused the pre-refractive surgery K-values in the Holladay 2 formulawhen calculating the effective lens position (ELP). When pre-LASIK K-values were unavailable, a default preoperative K of43.86 D was used to calculate the ELP rather than the flatterpostoperative K-values, which can significantly offset the ELP.25

Because of this advantage, the Holladay 2 formula was prospec-tively used to calculate IOL power in the present study.

The IOL power was calculated with all applicable techniquesincluding manually derived corneal power, historical method,contact lens overrefraction, and 4.0 mm Orbscan-derived totaloptical power. The flattest K-value was then used in the Holladay2 formula to determine the power of the implanted IOL, generallytargeting a �0.50 to �2.00 D refractive outcome depending onwhether the patient wanted better distance or near vision. Orbs-can white-to-white (WTW) corneal diameter measurements,ultrasound-based anterior chamber depth (ACD), and lens thick-ness (LT) were included in the Holladay 2 IOL calculations.

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CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

Uneventful small-incision phacoemulsification with in-the-bagposterior chamber IOL implantation was performed by 1 of 2surgeons (M.A.Q., J.S.P.) using standard techniques.

Back-Calculation of Corneal Power

Uncorrected visual acuity, BSCVA, and manifest refractionwere measured at least 1 month after cataract surgery. Post-pha-coemulsification refractive data were entered into the HolladayIOL Consultant to determine the BCK values, which correct for re-sidual postoperative refractive error without altering other preop-erative biometry values such as AL, WTW, ACD, and LT. The BCKvalues were calculated using the Holladay 2 (BCK H2). These werestatistically compared to the historical method, contact lens over-refraction, manually derived corneal power, and Atlas-derivedand Orbscan-derived corneal curvature and power.

Given that adjusting corneal power by 1.00 D changesthe IOL power by almost the same value28 and that every1.00 D change in IOL power affects the spectacle planerefraction by approximately 0.70 D,29 the error in corneal power(Kmethod � KBCK H2) for each technique was multiplied by 0.7 tocalculate the amount of residual post-phacoemulsification SEerror if Orbscan parameters had been used prospectively in theHolladay 2 formula to calculate IOL power.

RESULTS

Demographics

Sixteen patients (5 women) with a mean age of 60.7

years G 8.1 (SD) (Table 1) who had uneventful phacoemul-

sification after myopic LASIK (18 eyes) or PRK (3 eyes)

were included in the study. There were 8 right eyes and 13

left eyes. Historical data were unavailable for 2 eyes. Themean manifest refraction spherical equivalent (MRSE)

before keratorefractive surgery was �5.88 G 2.55 D. The

Table 1. Demographic data of eyes with cataract that had previous

excimer-based keratorefractive vision-correction surgery.

Characteristic n Mean G SD

Age (y) 21 60.7 G 8.1LASIK (eyes) 18/21 dPRK (eyes) 3/21 dPre-LASIK/PRK SE (D) 19 �5.88 G 2.55Pre-LASIK/PRK K (D) 19 44.04 G 1.36Interval between LASIK & phaco (mo) 21 52.20 G 16.2Pre-phaco SE (D) 21 �1.02 G 1.76Pre-phaco UCVA (20/) 21 94.8 G 80.0Pre-phaco BSCVA (20/) 21 33.4 G 12.2AL (mm) 21 26.15 G 1.10White-to-white (mm) 21 11.75 G 0.41Ultrasound ACD (mm) 21 3.69 G 0.45Ultrasound LT (mm) 20 4.37 G 0.46

ACD Z anterior chamber depth; AL Z axial length; BSCVA Z best spec-

tacle-corrected visual acuity; K Z corneal power; LASIK Z laser in situ

keratomileusis; LT Z lens thickness; PRK Z photorefractive keratectomy;

SE Z spherical equivalent; UCVA Z uncorrected visual acuity

J CATARACT REFRACT SURG

MRSE after keratorefractive surgery but before cataract for-

mation was �1.02 G 1.76 D. The mean AL was 26.15 G1.10 mm (median 25.91 mm, range 23.94 to 28.83 mm).

Refractive Outcomes

Patients were evaluated a mean of 3.7 G 5.8 months

after cataract surgery and had a mean postoperative MRSEof �0.95 G 0.79 D. The mean predicted postoperative SE

for the IOLs implanted, based on the Holladay 2 formula,

was �0.71 G 0.61 D, indicating these eyes were a mean of

�0.24 G 0.75 D more minus postoperatively than pre-

dicted. The paired Student t test comparing actual post-

cataract surgery SE and targeted SE for IOLs implanted gave

a value of 0.15. There was a low correlation (R2!0.001)

between AL and the difference between the predicted SEand the SE after phacoemulsification (SEactual postoperative –

SEH2 predicted postoperative).

Comparison to Back-Calculated Corneal Power

Table 2 shows the mean values for corneal power ob-

tained from various methods, including their deviation

from BCK H2 values. Central corneal power calculated us-

ing the historical method (Pearson [r] Z 0.90) correlated

more closely than RGP overrefraction-based values (r Z0.45) with BCK H2 values. Although the difference be-tween BCK H2 and contact lens overrefraction-derived

K-values was not statistically different because of the large

standard deviation (G3.11 D) and small sample size of the

latter technique, all 5 eyes in which contact lens overrefrac-

tion was available would have had a hyperopic outcome

after cataract surgery outcome if contact lens overrefraction

data had been prospectively used to calculate IOL power.

Figure 1 shows the close correlation between BCK valuesand measured K-values. The historical method calculated

corneal power values closest to those of the BCK method

and in several cases provided values flatter than the BCK

H2 method, minimizing the risk for a hyperopic refractive

outcome.

Several parameters (Table 3) from the Orbscan II were

not statistically different from ‘‘actual’’ corneal power (BCK

H2, 39.34 G 2.57 D). These include the central 3.0 mmtotal mean power, 5.0 mm total axial power, 6.0 mm total

axial power, 3.0 mm total tangential power, 3.0 mm total

optical power, and 4.0 mm total optical power. The central

4.0 mm total axial power, because its values were statisti-

cally flatter on average than those of the BCK H2 method,

would provide the fewest hyperopic outcomes after cataract

surgery and thus may offer an alternative measure if

surgeons are particularly interested in a myopic refractiveresult. Figure 2 shows the distribution of central corneal

power measured via different Orbscan II curvature and

power maps. Ranges for the difference between K-values

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CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

Table 2. Comparison of corneal power derived from method and that derived by the BCK H2 method.

Mean G SD

Method nCorneal

Power (D)Variance from BCK H2 (D)

(Method � BCK H2) P Value*

BCK H2 21 39.34 G 2.57 d dAverage manual K 21 40.52 G 1.95 C1.03 G .78 !.01Historical 19 39.33 G 2.70 C0.06 G 1.21 .82†

Contact lens 5 41.32 G 3.11 C1.97 G 2.78 .18†

Smith et al.27 10 39.75 G 2.40 C0.64 G 0.81 .03Wang et al.18 10 38.56 G 2.41 �0.56 G 0.81 .06†

BCK H2 Z back-calculated K-value using the Holladay 2 IOL formula

*K-value determined by method versus K-value determined by BCK H2†K-value from method not statistically different from BCK H2 value

provided by select Orbscan II maps and BCK H2 are as

follows: total mean power 3.0 mm,�1.30 to C1.65 D; total

mean power 4.0 mm, �0.78 to C1.78 D; total axial power

4.0 mm,�1.52 to C1.24 D; total axial power 5.0 mm,�1.32to C1.42 D; total tangential power 3.0 mm, �1.42 to

C1.76 D; total optical power 3.0 mm, �1.66 to C1.78 D;

and total optical power 4.0 mm, �1.10 to C2.15 D.

Corneal power values derived solely from Placido-based

keratography (Table 4) were significantly steeper than the

BCK values P!.01).

The median and mean calculated SE after phacoemul-

sification, if K-values from the method were prospectivelyused to calculate IOL power using the Holladay 2 formula,

were between �0.25 D and plano for 3.0 mm total mean

power, 5.0 mm total axial power, 3.0 mm total tangential

power, and 3.0 mm total optical power (Table 5). Of these,

use of 5.0 mm total axial power would result in a greater

percentage of eyes with a post-phacoemulsification

J CATARACT REFRACT SURG24

refractive error within G0.50 D (76.2%) and G1.00 D

(100.0%) of the targeted refractive error. Use of the

4.0 mm total optical power, while providing a slightly

plus median and mean residual refractive error, wouldalso result in a high percentage of eyes within G0.50 D

(80.9%) and G1.00 D (95.2%) of target. The parameters

with the least hyperopic calculated residual refractive error

were the 4.0 mm total axial power (upper range C0.87 D)

and 5.0 mm total axial power (upper range C0.99 D).

Statistical Comparison of Orbscan Total PowerMethods

Statistical testing of equivalence of individual Orbscan

total power values to BCK H2 values was completed

through the generation of 95% confidence intervals (CIs)

about the mean difference (Orbscan total power value –

BCK H2). If the 95% CI was completely contained within

Figure 1. Correlation of measured or calculated

corneal power with BCK H2 from the Holladay 2 IOL

formula.

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CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

Table 3. Comparison of BCK H2 and Orbscan parameters (N Z 21).

Mean G SD

Method Corneal Power (D) Variance from BCK H2 (D) (Method � BCK H2) P Value BCK H2*

Average simulated K 40.81 G 2.33 C1.46 G 0.73 !.001Mean power 3.0 mm 41.12 G 2.33 C1.78 G 0.69 !.001Mean power 5.0 mm 42.53 G 1.76 C3.19 G 1.21 !.001Total mean power 3.0 mm 39.10 G 2.63 �0.24 G 0.68† .12z

Total mean power 4.0 mm 39.64 G 2.46 C0.29 G 0.73 .09z

Total mean power 5.0 mm 40.52 G 2.11 C1.18 G 0.96 !.001Total axial power 4.0 mm 38.88 G 2.65 �0.47 G 0.64† .003Total axial power 5.0 mm 39.19 G 2.55 �0.16 G 0.64† .26z

Total axial power 6.0 mm 39.52 G 2.48 C0.22 G 0.70 .22z

Total tangential power 3.0 mm 39.11 G 2.60 �0.24 G 0.71† .15z

Total tangential power 4.0 mm 39.94 G 2.37 C0.59 G 0.91 .007Total tangential power 5.0 mm 41.04 G 2.04 C1.69 G 1.27 !.001Total optical power 3.0 mm 39.07 G 2.78 �0.27 G 0.78† .13z

Total optical power 4.0 mm 39.39 G 2.76 C0.04 G 0.72 .77z

Total optical power 5.0 mm 39.90 G 2.68 C0.56 G 0.71 .002

BCK H2 Z back-calculated K-value using the Holladay 2 IOL formula

*K-value determined by method versus K-value determined by BCK H2†Mean K-value of method flatter than mean BCK H2 valuezMean K-value of method not statistically different from BCK H2 value

the equivalence limits (�0.50, 0.50), the Orbscan total

power value was concluded to be equivalent to the BCK

H2 value. Although several Orbscan total power values

were not statistically different from BCK H2 values and

correlated highly with BCK H2 values, only mean total axial

power 5.0 mm and mean total optical power 4.0 mmprovided a 95% CI within G0.50 D of the mean BCK H2

value (Table 6). A stepwise multivariate regression analysis

identified the 5.0 mm total axial power as providing the

highest predictive value for BCK H2 (P!.0001). Three

assessments of statistical power were completed using a

clinically meaningful difference of 0.50 D (between method

J CATARACT REFRACT SUR

and BCK H2) and 3 representative standard deviations

(0.65, 0.70, and 0.75); the statistical power of this study

was 91%, 87%, and 82%, respectively, assuming a 2-sided

paired test with a Z 0.05 for a sample size of 21.

DISCUSSION

Manual keratometry, automated keratometry, and

Placido disk–based topography are the most frequently

used techniques for measuring central power in normal,

unoperated corneas. Most manual keratometers and topog-

raphy systems use a corneal index of refraction of 1.3375,

Figure 2. The analysis tool on the Orbscan was used to

calculate the average power of the central 3.0 mm of

several total power maps, which use slit-scanning

technology to incorporate the contributions of both

posterior and anterior corneal surfaces.

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CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

Table 4. Comparison of BCK H2 and Atlas axial map K-values averaged from central regions of the cornea (N Z 10).

Mean G SD

Method Corneal Power (D)Variance from BCK H2 (D)

(Method � BCK H2) P Value*

Average simulated K 40.25 G 1.93 C1.14 G 0.77 .001Axial power 0 mm 40.09 G 2.16 C0.97 G 0.71 .002Axial power 3.0 mm 40.21 G 1.19 C1.09 G 0.74 .001Axial power 4.0 mm 40.52 G 1.95 C1.34 G 0.87 .006

BCK H2 Z back-calculated K-value using the Holladay 2 IOL formula

*K-value determined by method versus K-value determined by BCK H2 comparing K determined by method to BCK H2

which is based on the assumption that the posterior cornea

has a radius of curvature 1.2 mm less than its anterior coun-

terpart, its power is negative due to the difference in index

of refraction between the cornea and the aqueous, and the

central 2.0 to 3.0 mm corneal zone is spherical. Laser abla-

tion changes this fixed ratio as well as the central corneal

sphericity. For these reasons, manual keratometry is inac-

curate in estimating corneal power in eyes that have hadlaser refractive surgery.2–11

Computerized videokeratography is based on com-

puter analysis of Placido disk images projected onto the

corneal surface. Computerized videokeratography mea-

sures more than 5000 points over the entire cornea and

more than 1000 points within the central 3.0 mm.4 This ad-

ditional information can provide greater accuracy than

manual keratometry for measuring corneal power in nor-mal eyes2 and in eyes after radial keratotomy (RK).25 How-

ever, Seitz and Langenbucher6 showed that topography

J CATARACT REFRACT SURG26

may overestimate true corneal power in patients with pre-

vious PRK as the ratio of anterior to posterior curvature has

been surgically altered. Therefore, the default index of re-

fraction used by these instruments is no longer accurate

in determining the true power of the cornea. The present

study confirms that axial map parameters from the Atlas

topography system, which are based on direct measurement

of only the anterior surface curvature, overestimate centralcorneal power by a mean of 1.00 D or greater (Table 4).

Calculations not based on topography have, therefore,

been developed to provide alternative approaches for esti-

mating central corneal power after PRK and LASIK. These

include refraction-derived keratometric value calculation

and hard or RGP overrefraction. The refraction-based or

historical technique requires availability of K-values and

the SE before laser refractive surgery. The advantages ofthe contact lens overrefraction technique include detection

of vision loss from corneal surface irregularities and the use

Table 5. Calculation of the residual post-phacoemulsification SE refractive error if K-values based on each method were prospectively used in the Holladay

2 formula to calculate IOL power.

Calculated Residual SE Refractive Error (D)

Method

PearsonCorrelationto BCK H2 Median Mean G SD Range

WithinG0.50 Dof Target

(%)

WithinG1.00 Dof Target

(%)

Manual K 0.94 C0.78 C0.82 G 0.69 � 0.50 to C3.96 33.3 71.4Historical 0.90 �0.15 C0.04 G 0.85 � 0.71 to C2.27 42.1 89.4RGP 0.45 C0.87 C1.70 G 2.06 C0.60 to C5.36 0.0 40.0Wang et al.18 0.95 �0.53 � 0.39 G 0.57 � 1.18 to C0.50 50.0 90.0Total mean power 3.0 mm 0.97* �0.19 �0.17 G 0.48 � 0.89 to C1.16 66.6 95.2Total mean power 4.0 mm 0.96 C0.17 C0.20 G 0.52 � 0.45 to C1.24 66.6 90.4Total axial power 4.0 mm 0.97* �0.31 �0.33 G 0.45 � 1.07 to C0.87* 61.9 90.4Total axial power 5.0 mm 0.97* �0.03* �0.11 G 0.44* � 0.93 to C0.99 76.2 100.0*Total tangential power 3.0 mm 0.96 �0.17 � 0.17 G 0.50 � 0.99 to C1.23 71.4 95.2Total tangential power 4.0 mm 0.94 C0.37 C0.42 G 0.63 � 0.56 to C1.81 47.6 85.7Total optical power 3.0 mm 0.96 �0.13 �0.19 G 0.54 � 1.16 to C1.25 66.6 86.7Total optical power 4.0 mm 0.97* C0.06 C0.03 G 0.50 � 0.75 to C1.50 80.9* 95.2

BCK H2 Z back-calculated K-value using the Holladay 2 IOL formula; RGP Z rigid gas permeable

*Metric(s) with best value in each column

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CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

Table 6. Calculation of 95% confidence level of mean central corneal power of select Orbscan total curvature and power maps. The 2 parameters with

a mean corneal power closest to emmetropia and with their 95% CIs within G0.50 D of the BCK H2 value are the 5.0 mm total axial power and 4.0 mm total

optical power.

Orbscan n Lower CL Mean Mean Upper CL Mean P Value

TAP 5.0 mm 21 �0.449 �0.160 0.1296 .26TAP 6.0 mm 17 �0.143 C0.216 0.5743 .22TTP 3.0 mm 21 �0.562 �0.236 0.0898 .15TOP 3.0 mm 21 �0.623 �0.270 0.0823 .13TOP 4.0 mm 21 �0.282 C0.046 0.3743 .77

CLZ confidence level; TAP Z total axial power; TOP Z total optical power; TTP Z total tangential power

of the physiologic optical pathway. Its main limitation is

that the patient must have good enough visual acuity to

obtain an accurate refraction. Zeh and Koch20 advise that

contact lens overrefraction would be acceptable in patients

with preoperative best corrected visual acuity of 20/70 or

better. We were able to obtain reliable contact lens overre-fraction measurements in only 5 patients in this cohort, for

whom the deviation from BCK H2 ranged from C0.85 to

C6.93 D, similar to findings reported by Wang et al.18 in

a myopic LASIK cohort. Therefore, although it served as

a corroborative measure for a few eyes in this cohort, con-

tact lens overrefraction was not used to prospectively

program corneal power into IOL calculation formulas.

Several topographic maps can be used to estimate cen-tral corneal power in eyes after refractive surgery. The def-

initions and descriptions of these metrics are given below

and can be reviewed in several sources14 (Orbscan II, Orbs-

can IIz, Operator’s Manual, version 3.12). When inter-

preting corneal topographic maps, it is important to

differentiate power, which is a functional representation,

from curvature, which describes shape. Although axial,

tangential, and mean maps are labeled ‘‘power’’ and ex-pressed in units of diopters, all 3 maps represent power

only in the central paraxial region of the cornea. Outside

this region, where the angle of incidence relative to the cor-

neal surface becomes large, they are representations of cur-

vature.30 After refractive surgery for myopia, the paraxial

region is likely larger than it was preoperatively as the flat-

tened ablation zone retains a small angle of incidence over

a larger region of the central cornea.The axial power map measures the curvature at a cer-

tain point on the corneal surface in a meridional direction

relative to the center. Tangential power maps measure the

instantaneous, local curvature at a certain point on the

corneal surface in a meridional direction relative to several

adjacent points along the same meridian. Axial and tangen-

tial curvatures are related in that the axial value at a certain

point equals the average tangential curvature along thespecified meridian from the map center to the point of in-

terest. Anterior axial and anterior tangential maps use the

difference in index of refraction between cornea and air

J CATARACT REFRACT SURG

in the conversion to diopters, in contrast to keratometric

axial maps and keratometric tangential maps, which use

the composite index of refraction of 1.3375 for the cornea.

Mean power (SE) also represents curvature and is the aver-

age of the 2 principal curvatures at each surface point. Total

optical power is based on ray tracing of parallel incidentrays with a Snell’s law refraction at both the anterior and

posterior corneal surfaces and is therefore a functional rep-

resentation of corneal power over the entire map. Total ax-

ial power and total tangential power are the simple addition

of the anterior and posterior dioptric values of the respec-

tive axial map and tangential map, as though the incident

rays were normal over the entire corneal surface. Neither

is an accurate functional representation of power, exceptin the central paraxial region.

Standard topographers use smoothing and extrapola-

tion algorithms to determine the power and elevation of

the corneal center because there are no mires in the center

of the cornea from which to derive data directly. The Orbs-

can II videokeratography system combines Placido disk and

slit-scanning technology to directly measure elevation and

curvature of both the anterior and posteriorcorneal surfaces(Orbscan II, Orbscan IIz, Operator’s Manual, version 3.12).

The measurement of both elevation and curvature data

from slit images and Placido images allows the Orbscan II

to take advantage of both technologies.14 Some authors

suggest that a Scheimpflug camera also offers improved

mapping of the corneal center by maintaining a fixed point

on the vertex of the cornea. However, there are sensitivity

issues when calculating curvature from elevation datarather than directly measuring it with a Placido.31 We look

forward to peer-reviewed studies comparing and applying

this technology to post-LASIK IOL calculations (J. Holladay,

MD, ‘‘IOL Calculation After Refractive Surgery,’’ Supple-

ment to Cataract and Refractive Surgery Today, January

2005, pages 1–4).

A limitation of Orbscan elevation maps is that intracor-

neal opacities may obscure imaging of the posterior corneaand introduce artifacts into total corneal power calcula-

tions. Additionally, the reliability of posterior corneal mea-

surements with the Orbscan has not been fully established,

- VOL 33, JANUARY 2007 27

CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

particularly for post-refractive surgery eyes in which im-

age-detection software may have difficulty identifying the

posterior aspect of the corneal slit beam because of the

change in contrast (Roberts CJ, et al. IOVS 2003;

44:ARVO E-Abstract 2543).31–33 However, the anterior

surface is heavily weighted in the total power calculationsbecause of the 10-fold greater index of refraction difference

at the anterior corneal surface versus the posterior corneal

surface. Therefore, potential errors of the posterior surface

are minimized in the conversion to diopters.

Cheng and Lam26 incorporated Orbscan-derived ante-

rior (central 5.0 mm) and posterior (central 10.0 mm) radii

of curvature into a Gaussian optics formula to calculate

post-LASIK corneal power in 121 eyes. They describe closecorrelation between historical-based values and their

method (r Z 0.97; P!.001). Sonego-Krone et al.14 deter-

mined that direct measurement of central corneal power af-

ter myopic LASIK using total mean power and total optical

power Orbscan maps provides accurate post-refractive sur-

gery corneal power independent of historical refractive

data. The total optical power map (Figure 3) applies

ray-tracing analysis and Snell’s law refraction through theanterior, and then posterior, corneal surface to calculate

the cornea’s total effective power.1,14,23

Our study confirms the utility of total mean power and

total optical power Orbscan parameters, as well as total

axial power (4.0 and 5.0 mm) and total tangential power

(3.0 mm), to calculate IOL power after LASIK and PRK

(Table 3 and Figure 2). The 5.0 mm total axial power

Figure 3. The Orbscan total optical power map from a post-LASIK patient.

The mean total optical power for the central 3.0 mm of this cornea was

obtained using the ‘‘Stats’’ feature found under the ‘‘Tools’’ menu. The

‘‘Statistics’’ mode for the ‘‘Analyze Area’’ submenu was then selected.

The cursor was dragged from the central cornea to create a 1.5 mm radius

central circle of analysis. An internal calculation by the Orbscan software,

applying ray tracing to both the anterior and posterior corneal surfaces,

provided a mean total optical power value for this area of interest.

J CATARACT REFRACT SURG -28

map (Table 5: median residual refractive error �0.03 D;

mean �0.11 G 0.44 D; range �0.93 to �0.99 D; 76.2%

within G0.50 D) came closest to achieving the targeted

refractive outcome, with the least statistical probability of

deviating from the target and simultaneously avoiding a

hyperopic outcome. Use of the 4.0 mm total axial powermap (median �0.31 D; mean �0.33 G 0.45 D; range

�1.07 to C0.87 D; 61.9% within G0.50 D) offers the least

likelihood of obtaining a hyperopic refractive outcome.

The 4.0 mm total optical power map (median C0.06 D;

mean C0.03 G 0.50 D; range �0.75 to C1.50 D; 80.9%

within G0.50 D) was the only other metric that would

result in a greater percentage of post-phacoemulsification

eyes within G0.50 D of the targeted refraction.Formulas used for post-RK and post-LASIK eyes with

good efficacy include the Haigis, Binkhorst, Holladay 2,

Hoffer Q, and SRK/T,7,8,10,18,34,35 particularly after adjust-

ment for calculations of ELP.36 Aramberri37 points out

that if post-refractive surgery K-values are used by third-

generation theoretical formulas to calculate ELP, a hyper-

opic refractive outcome can result. He describes a double-K

method in which K-values before refractive surgery areused to calculate ELP and then K-values after refractive sur-

gery are used in a vergence formula to calculate IOL power.

The Holladay 2 formula uses parameters of ACD, corneal

diameter (WTW), and LT to estimate the ELP. The Holladay

IOL Consultant also includes a mode in which K-values be-

fore refractive surgery, if available, or a default preoperative

K-value of 43.86 D, if pre-refractive surgery keratometry

is unavailable, are used when calculating the ELP in post-keratorefractive eyes.25 Hence, the Holladay 2 formula with

the Holladay IOL Consultant was prospectively used to cal-

culate IOL power in our study, including in 2 eyes for which

historical data were not available, to assist with calculation

of ELP.

In general, the lowest corneal power value and a myo-

pic target should be selected to minimize the risk for hyper-

opia after cataract surgery.10,16 We used this approach withthe Holladay 2 formula, and only 1 of 21 eyes had a hyper-

opic refractive outcome (C0.75 D SE). In using BCK values

to determine true corneal power, we assumed that the

residual refractive error after cataract surgery stemmed

from inaccurate measurement of the central corneal power.

Recently, we converted completely from contact to immer-

sion ultrasonography and laser interferometry to measure

AL, and our preliminary refractive results in phacoemulsi-fication after LASIK show even tighter standard deviations

using the topographic metrics described herein.

Fourteen eyes (66.6%) and 18 eyes (85.7%) in the pres-

ent study were within G0.50 D and G1.00 D of the in-

tended target, respectively. The mean post-cataract surgery

SE was �0.96 G 0.79 D overall but was within �0.24 G0.75 D of the intended target. These results compare

VOL 33, JANUARY 2007

CORNEAL POWER DETERMINATION FOR IOL CALCULATION USING ORBSCAN II AFTER LASER REFRACTIVE SURGERY

favorably to those reported using the double-K method

(mean post-cataract surgery SE C0.13 G 0.62 D), in which

a greater percentage of patients had a hyperopic result,37

and those reported using intraoperative autorefractive ret-

inoscopy (n Z 6; 83% within G1.00 D).38 Rosa et al.39

used a regression formula with correcting factors as a func-tion of AL to recalculate post-refractive surgery corneal

power so that pre-LASIK refractive data were not required.

With this approach, in the SRK/T formula, 47.4% of eyes

were within G1.00 D of emmetropia. Feiz et al.40 applied

a nomogram that adjusted IOL power based on post-LASIK

keratometry and the refractive change induced by LASIK.

Using their method, 63.2% and 84.2% of eyes were within

G0.50 D and G1.00 D of the intended target, respectively.However, as with the historical method, the preoperative

refraction data required for the Feiz-Mannis method is

not always available.

CONCLUSION

In patients with a history of keratorefractive surgery,

including LASIK and PRK, slit-scanning computerized

videokeratography that analyzes both the anterior and

posterior corneal surfaces more accurately predicts true

corneal power than manual keratometry or maps that

analyze the anterior surface alone. This study shows the

utility of parameters extracted from central Orbscan totalpower maps, particularly the averaged total axial power

in the central 5.0 mm of the cornea and the total optical

power in the central 4.0 mm, combined with the Holladay

2 IOL calculation formula in the Holladay IOL Consultant

software for preventing hyperopic refractive surprises after

phacoemulsification with IOL implantation. This method

offers the advantage of not relying on the availability of

pre-LASIK/PRK data or the accuracy of refraction-based cal-culations and, in our series, achieved a better refractive out-

come than the history method. Future studies are planned

to compare K-values derived from Orbscan total power

maps to values of other mathematical derivations of corneal

power such as those proposed by Masket and Masket,41

Cheng and Lam,26 Feiz and Mannis,11 among others,42–44

and to recalculate the Orbscan II posterior elevation directly

from the slit images using a more advanced software version.

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