corneal power estimation for intraocular lens power calculation after corneal laser refractive...

$: Corneal power estimation for intraocular lens power calculation after corneal laser refractive surgery in Chinese eyes$
ARTICLE

Corneal power estim
ation for intraocularlens power calculation after corneal laser
refractive surgery in Chinese eyesHaiying Jin, MD, Gerd U. Auffarth, MD, Haike Guo, MD, Peiquan Zhao, MD

Q 2012 A

Published

SCRS an

by Elsev

PURPOSE: To develop and evaluate an algorithm for corneal power estimation in intraocular lens(IOL) power calculation after corneal laser refractive surgery in Chinese eyes.

SETTING: Guangdong Eye Institute, Guangdong Academy of Medical Sciences, Guangzhou, China.

DESIGN: Prospective comparative case series.

METHODS: Corneal parameters in Chinese eyes and German eyes were measured using a rotatingScheimpflug camera. Corneal power was simplified as a corrective algorithm: KC Z 1.114 � KMC K2 (KM Z measured K reading; K2 Z KP � KA � KP � CT/1.376; KA Z anterior corneal power;KP Z posterior corneal power; CCT Z central corneal thickness). The variation and change in K2induced by refractive surgery were analyzed in Chinese eyes. The corrective algorithmwas identifiedas KC Z 1.114 � KM � 6.20. The method was evaluated in Chinese cataract cases after refractivesurgery using the Haigis formula.

RESULTS: No difference in anterior corneal radius (RA) or CCT between Chinese eyes and Germaneyes were found; however, the posterior corneal radius (RP), RA/RP ratio, keratometric index, and K2were different. The mean K2 was �6.23 diopters (D) G 0.24 (SD) in Chinese eyes and �6.12G 0.23 D in German eyes (P<.01). The mean change in K2 induced by refractive surgerywas �0.02 G 0.06 D. The median absolute prediction error in IOL power calculation was 0.43 D(range 0.01 to 1.80 D).

CONCLUSION: The algorithm was a relatively reliable method in IOL power calculation after cornealrefractive surgery in Chinese eyes.

Financial Disclosure: No author has a financial or proprietary interest in any material or methodmentioned.

J Cataract Refract Surg 2012; 38:1749–1757 Q 2012 ASCRS and ESCRS

It is well accepted that converting the anterior cornealradius to corneal power using the keratometric indexis a major source of error in intraocular lens (IOL)power miscalculation after corneal laser refractive sur-gery.1–5 The proposed solution to this error is to eval-uate the corneal refractive surfaces separately5 usingdevices that can measure both corneal surfaces. Thesedevices include a rotating Scheimpflug camera (Penta-cam, Oculus, Inc.),6,7 a dual Scheimpflug analyzer(Galilei, Ziemer Ophthalmics AG),8,9 a vertical slit-light scanning device (Orbscan, Bausch & Lomb,Inc),10,11 and optical coherence tomographers.12

When these devices are not available, a simplifiedcorrective algorithm is adopted as an alternativeapproach to calculate corneal power using anteriorcorneal power and an offset value as follows3,13:

d ESCRS

ier Inc.

KCZ1:114�KM þmean K2

K2ZKP �KA �KP �CCT1:376

where KC is the corneal power calculated by the cor-rective algorithms, KM is the measured keratometry(K) reading, K2 Z KP � KA � KP � CCT; KA is the an-terior corneal power, KP is the posterior cornealpower, and CCT is the central corneal thickness.

Different values for the offset (mean K2) in the algo-rithm have been advocated by different researchers.The value has been reported to be �4.9 D,3 �4.98 D,14

and �6.062 D.15 Wang et al.3 first reported a valueof �6.1 D using back calculation. We confirmedWang et al.’s result by direct corneal measuring using

0886-3350/$ - see front matter 1749http://dx.doi.org/10.1016/j.jcrs.2012.06.048

http://dx.doi.org/10.1016/j.jcrs.2012.06.048

1750 CORNEAL POWER ESTIMATION FOR IOL POWER AFTER REFRACTIVE SURGERY IN CHINESE EYES

a rotating Scheimpflug camera in white (German) eyesand found that the algorithm (KC Z 1.114� KM � 6.1)was reliable for post-refractive cases because of thesmall variation in K2 and small change in the value in-duced by refractive surgery.13

Previous studies16–18 have shown that ocular di-mensions (anterior chamber depth, anterior chamberwidth, corneal diameter, corneal vault, and axiallength [AL]) vary between Asian eyes and whiteeyes. Given the ethnic differences in ocular dimen-sions, it is postulated that corneal geometric parame-ters (anterior and posterior curvatures, cornealthickness), which are also a component of ocular ante-rior dimensions, may vary between different ethnicgroups. According to Gaussian optics, the K2 value isdetermined by corneal parameters. Thus, we hypothe-sized that K2 may be different in Chinese eyes, result-ing in a different algorithm for corneal powerestimation.

This study had 2 aims. The first was to compare cor-neal parameters between Chinese eyes and white eyesand to identify the mean K2 value in the correctivealgorithm for Chinese eyes using a relatively largenumber of patients. The second aim was to evaluatethe reliability of the algorithm in IOL power calcula-tion after corneal refractive surgery in Chinese eyes.

Submitted: March 12, 2012.Final revision submitted: June 4, 2012.Accepted: June 5, 2012.

From the Department of Ophthalmology (Jin, Zhao), Xinhua Hospi-tal Affiliated to Shanghai Jiaotong University School of Medicine,Shanghai, and Guangdong Eye Institute (Jin, Guo), GuangdongAcademy of Medical Sciences, Guangzhou, China; International Vi-sion Correction Research Centre (Auffarth), University of Heidel-berg, Germany.

Supported in part by stipends according to the Landesgraduierten-gesetz of the State Baden-W€urttemberg, Germany; National NaturalScience Foundation of China (81100655); Foundation for the Re-turned Overseas Chinese Scholars, State Education Ministry([2011]508); Scientific Research Foundation of the State HumanResource Ministry for Returned Chinese Scholars ([2011]508);Shanghai Leading Academic Discipline Project (S30205); and theState Education Ministry Medical Scientific Research Foundationof Guangdong Province (A2011030), China.

Presented in part at the Asia-Pacific Academy of Ophthalmology,Sydney, Australia, March 2011, and at the 4th Joint Meeting ofKorea-China-Japan Ophthalmologists, Seoul, Soth Korea,November 2011.

Corresponding author: Peiquan Zhao, MD, Department of Ophthal-mology, Xinhua Hospital Affiliated to Shanghai Jiaotong UniversitySchool of Medicine, Shanghai, China. E-mail: [email protected].

J CATARACT REFRACT SURG -

PATIENTS AND METHODS

Corneal parameters were measured in eyes of consecutiveChinese patients. Exclusion criteria included corneal disease,previous corneal surgery, dry eye with poor tear film, andretinal disease. Eyes were measured using a rotatingScheimpflug camera (Pentacam Oculus HR, version 1.11).Corneal parameters, including the anterior corneal radius(RA), posterior corneal radius (RP), RA/RP ratio, anteriorcorneal power (KA), posterior corneal power (KP), centralcorneal thickness (CCT), and KM, were obtained. The kerato-metric index, corneal optical power (KO), and K2 were calcu-lated separately according to the previous study.13

A refractive group comprising Chinese eyes was pro-spectively studied. Laser in situ keratomileusis (LASIK)was performed by the same surgeon (G.H.). Corneal topog-raphy was measured preoperatively and 12 monthspostoperatively.

The study also compared corneal parameters betweenChinese eyes and eyes of consecutive white patients (Ger-man) from the International Vision Correction Research Cen-tre, University of Heidelberg, Germany; the white eyes wereused as controls after the balance of age and sex between the2 groupswas confirmed. Themeasurementswere performedusing the same type of rotating Scheimpflug camera (version1.09) that was used to assess the Chinese eyes.

In the refractive group, the changes in KP and K2 inducedby refractive surgery were analyzed separately by compar-ing the KP and K2 values before refractive surgery and thevalues after refractive surgery using the paired t test. TheKP and K2 values before refractive surgery and the values af-ter refractive surgerywere correlated using linear regression.The KO and KM values after refractive surgery were corre-lated using linear regression to generate a regression formulafor corneal power estimation.

Two methods to estimate corneal power for post-refractive cases using KM were developed in this study.The first was the corneal power calculated by the correctivealgorithm using the mean K2 value as follows:

KCZ1:114�KM � 6:2

where KC is the corneal power calculated by the correctivealgorithm and �6.20 is the mean K2 value in the Chineseeyes in this study. The second was a regression formula asfollows:

KRZ1:06�KM � 4:05

where KR is the corneal power calculated from the measuredK reading using the regression formula.

To evaluate the method of corneal power estimation inChinese eyes, cataract patients who had corneal laser refrac-tive surgery were studied. The KM value was obtained withan Orbscan IIz device (Bausch & Lomb-Obtek, Inc.). The ALwas measured using an optical biometer (IOLMaster, ver-sion 5.4, Carl Zeiss Meditec AG). The Haigis formula19 wasused for IOL power calculation after optimization for Chi-nese eyes. Corneal power values were calculated using thefollowing methods: corrective algorithm for Chinese eyes(1.114 � KM � 6.2), Maloney method (1.114 � KM � 4.9),and Wang-modified Maloney method (1.114 � KM � 6.1).3

The regression formula developed in this study (1.06 � KM� 4.05), the Shammas no-history method (1.14 � KM�6.8),20 and the KM andK values calculated by the calibratedindex of 1.3280 for Chinese eyes in this study (K1.3280) wereused to calculate IOL power to compare the accuracy of

VOL 38, OCTOBER 2012

mailto:[email protected]

mailto:[email protected]

Table 1. Comparison of corneal parameters between Chinese eyes and German eyes.

Parameter

German (n Z 205) Chinese (n Z 352)

P ValueMean G SD 95% Mean G SD 95% CI

RA (mm) 7.73 G 0.26 7.22, 8.24 7.70 G 0.24 7.22, 8.17 .44KA (D) 48.70 G 1.64 45.49, 51.91 48.85 G 1.54 45.83, 51.87 .45RP (mm) 6.42 G 0.25 5.93, 6.91 6.30 G 0.24 5.83, 6.77 !.01KP (D) �6.24 G 0.24 �6.71, �5.77 �6.36 G 0.25 �6.85, �5.87 !.01RA/RP 1.205 G 0.027 1.152, 1.258 1.223 G 0.028 1.168, 1.278 !.01K2 (D) �6.12 G 0.23 �6.57, �5.67 �6.23 G 0.24 �6.70, �5.76 !.01CCT (mm) 548.2 G 38.0 473.7, 622.7 549.4 G 30.9 488.8, 609.0 .72KM (D) 43.71 G 1.48 40.81, 46.61 43.85 G 1.38 41.15, 46.55 .45KO (D) 42.57 G 1.46 39.71, 45.43 42.62 G 1.36 39.95, 45.29 .99Keratometric index 1.3287 G 0.0010 1.3267, 1.3307 1.3280 G 0.0011 1.3258, 1.3302 !.01

CCTZ central corneal thickness; CIZ confidence interval; KOZ corneal optical power; K2ZKP�KA�KP�CCT in [m]/1.376; KAZ anterior corneal power;KM Z measured keratometry reading; KP Z posterior corneal power; RA Z anterior corneal radius; RP Z posterior corneal radius

1751CORNEAL POWER ESTIMATION FOR IOL POWER AFTER REFRACTIVE SURGERY IN CHINESE EYES

different corneal power estimation methods. The predictionerror was defined as the difference between the predictedpostoperative refraction and the actual postoperative refrac-tion. The absolute prediction error was defined as absolutevalue of the prediction error. The median value for theabsolute prediction error was calculated. The mean absoluteprediction error (MAE) and standard deviation (SD) werealso calculated, although statistically the absolute predictionerror in general is not normally distributed. The MAE isgiven to simplify comparison with the literature, in whichthe MAE is often used.

Statistical Analysis

After the normal distributions of investigated corneal pa-rameters were confirmed using Kolmogorov-Smirnov tests,independent t tests were used to compare corneal parame-ters between Chinese eyes and German eyes. Paired t testswere used to assess the change in each parameter inducedby refractive surgery. Statistical analysis was performedusing SPSS software (version 15.0, SPSS, Inc.); a P valueless than 0.05 was considered statistically significant.

Table 2. Comparison of corneal parameters before and after laser corne

Parameter

Before Refractive Sugery (n Z 102)

Mean G SD 95% CI

RA (mm) 7.80 G 0.26 7.29, 8.31KA (D) 48.27 G 1.56 45.21, 51.32RP (mm) 6.37 G 0.25 5.88, 6.86KP (D) �6.30 G 0.25 �6.79, �5.81RA/RP ratio 1.2250 G 0.0281 1.1700, 1.2801K2 (D) �6.20 G 0.24 �6.67, �5.73CCT (mm) 554.3 G 27.9 499.6, 609.0KM (D) 43.32 G 1.40 40.58, 46.06KO (D) 42.06 G 1.37 39.37, 44.75Keratometric index 1.3277 G 0.0010 1.3257, 1.3297

CCTZ central corneal thickness; CIZ confidence interval; KOZ corneal optical poZ measured keratometry reading; KP Z posterior corneal power; RA Z anterior c


RESULTS

Patients

The evaluation of corneal parameters in Chineseeyes comprised 352 eyes of 352 patients. The variationand change in K2 induced by refractive surgery wereanalyzed in 102 Chinese eyes. Corneal parameterswere compared between Chinese eyes and 205 eyesof 205 consecutive white German patients. Intraocularlens power calculation after corneal refractive surgery(LASIK or photorefractive keratectomy) was evalu-ated in 42 Chinese eyes with cataract.

Comparison of Corneal Parameters: Chinese EyesVersus German Eyes

Themean age of Chinese patients was 46.05 yearsG21.05 (SD) (range 21 to 89 years). The mean age ofGerman patients was 45.29 G 16.06 years (range

al refractive surgery in Chinese eyes.

After Refractive Surgery ( n Z 102)

P ValueMean G SD 95% CI

8.73 G 0.49 7.77, 9.69 !.0143.23 G 2.37 38.58, 47.88 !.016.37 G 0.24 5.90, 6.84 .57

�6.29 G 0.24 �6.76, �5.82 .451.3705 G 0.0634 1.2462, 1.4948 !.01�6.23 G 0.23 �6.68, �5.78 !.01468.5 G 41.4 387.4, 549.6 !.0138.81 G 2.13 34.64, 42.98 !.0137.10 G 2.24 32.71, 41.49 !.011.3217 G 0.0026 1.3166, 1.3268 !.01

wer; K2ZKP�KA�KP�CCT (m)/1.376; KAZ anterior corneal power; KM

orneal radius; RP Z posterior corneal radius


Figure 1. The K2 values in German, Chinese, and Chinese refractivepatients (K2 Z measured keratometry reading).

Figure 2. Top: Correlation between post-refractive KP and pre-refractive KP. Bottom: Correlation between post-refractive K2 andpre-refractive K2. The green lines represent the post-refractivedata/pre-refractive data (Z1.0) (K2 Z measured keratometryreading).


25 to 81 years). No difference in age was found be-tween the 2 groups (PZ.42, independent t test). Therewere 183 women in the Chinese group and 95 womenin the German group. No difference in the sex ratiowas found between the 2 groups (PZ.20, Pearsonchi-square).

Table 1 compares the corneal parameters betweenChinese eyes and German eyes. No significant differ-ence in RA, KA, CT, KM, or KO was found betweenthe 2 groups; however, statistically significant differ-ences in RP, KP, the RA/RP ratio, the keratometricindex, and K2 were found. The posterior cornealsurface was steeper in Chinese eyes than in Germaneyes. The difference in the RP value between Chineseeyes and German eyes was statistically significant,causing a more negative power of posterior cornealsurface in Chinese eyes and a higher RA/RP ratio inChinese eyes (all P!.01, independent t test). The kera-tometric index was significantly lower in Chinese eyesthan in German eyes (P!.01, independent t test). Themean K2 value was significantly lower in Chinese eyesthan in German eyes (P!.01, independent t test).

Changes in KP and K2 Induced by Refractive Surgery

Table 2 compares the corneal parameters before andafter laser corneal refractive surgery in Chinese eyes.The change in posterior corneal curvature inducedby refractive surgery (RP and KP) was not statisticallysignificant. The postoperative K2 value was compara-ble to the preoperative K2 value (P!.01, paired t test).The mean change in K2 induced by refractive surgerywas�0.02G 0.06D (range�0.22 to 0.13D), whichwasnot clinically significant. The independent t test


showed no significant difference in K2 values in Chi-nese eyes between the normal group and the refractivegroup; however, the K2 values in Chinese eyes in thenormal group and in the refractive group were lowerthan those in German eyes (Figure 1). Relatively highcorrelations were found between pre-refractive sur-gery and post-refractive surgery KP and K2 values(post-refractive KP Z 0.92 � pre-refractive KP � 0.50,r Z 0.96, P!.01; post-refractive K2 Z 0.93 � pre-refractive K2 � 0.48, r Z 0.96, P!.01) (Figure 2).

Variations in KP and K2
Normal distributions of KP and K2 values with
a small variation in normal eyes and in the refractivegroup were observed (Tables 1 and 2 and Figure 3).Figure 4 shows scatterplots of the difference betweenindividual K2 values and the mean value (�6.20 D);


Figure 3. A: Normal distribution of K2 in normal Chinese eyes(n Z 352). B: Normal distribution of K2 before refractive surgeryin Chinese eyes (nZ 102).C: Normal distribution of K2 after refrac-tive surgery in Chinese eyes (n Z 102). The solid curves representthe Gaussian distribution (K2 Z measured keratometry reading).


the scatterplots indicate that using an offset value of�6.20 D generated a low error in corneal power esti-mation due to the small variation in the K2 value.According to the above outcomes, the offset value inthe corrective algorithm for Chinese eyes was identi-fied to be �6.20 D.


Correlation Between KO and KM After RefractiveSurgery

There was a high correlation between KM and KO af-ter refractive surgery (KOZ 1.06�KM� 4.05, rZ 0.996,P!.001) (Figure 5). This was used to generate theregression formula (KR) for corneal power estimation.


Figure 4. Top: Distribution of the difference between individual K2

and the offset value (�6.20 D) in normal Chinese eyes. Bottom:Distribution of the difference between individual K2 and the offsetvalue (�6.20 D) in refractive cases. The dashed lines representthe 95% confidence interval (K2 Z measured keratometryreading).

Figure 5. Correlation between KO and KM after corneal refractivesurgery. The red line represents the linear regression and the greenline, KC Z 1.114 � KM � 6.2 (KM Zmeasured keratometry reading;KO Z corneal optical power).


Intraocular Lens Power Calculation After CornealRefractive Surgery in Chinese Eyes

Table 3 shows the median values for the predic-tion errors of different corneal power estimationmethods in Chinese eyes. The 3 methods thatgenerated the highest percentage of prediction errorwithin G1.00 D were Jin-modified Maloney method(KC), Wang-modified Maloney method, and Jin re-gression method (KR). The Shammas no-historymethod, Maloney method, K1.3280, and KM generateda lower percentage of prediction error within G1.00 D.The KC in Chinese eyes, the Wang-modifiedMaloney method, and KR also tended to avoida postoperative hyperopic refraction; the mean pre-diction errors of the 3 methods were closest to zero(Figure 6).


DISCUSSION

The explosive growth of laser refractive surgery andthe aging of patients after refractive surgery have ledto increasing numbers of patients with previous laserrefractive surgery presenting for cataract surgery.The problems are more noticeable in the Chinesepopulations, which have a much higher proportionof myopia than white, Hispanic, and African Ameri-can populations.21,22

Two main sources of error in IOL power calculationhave been identified. One is the erroneous estimationof corneal power due to altered corneal geometry.1–4

The other source is the inaccurate prediction of IOLposition (effective lens position [ELP]) by using thecorneal radius after refractive surgery. The latter canbe avoided by using the Aramberri double-K modi-fication of theoretical formulas or the Haigis for-mula.5,13,19,23,24 Separate evaluation of the anteriorcorneal surface and the posterior corneal surfaceto calculate corneal power has also been shown tobe an effective resolution to corneal power miscalcula-tion6–12; however, instruments that can measure bothcorneal surfaces are not commonly available. An alter-native, the Maloney method has been proposed andwas modified by Wang et al.3 The advantage of thismethod is that it is relatively simple because it isa no-history technique that requires a KM value only.Themethod resolves corneal optical power into 2parts.The first is the measurable anterior power, which canbe obtained from conventional topography by trans-forming anterior corneal curvature to anterior cornealpower (1.114 � KM). The second part is an offset value(K2), which is critical in the estimation but is not


Table 3. Absolute prediction errors of different corneal power estimation methods (Haigis formula).

Method Median (D) Mean G SD (D) Range (D)

Prediction error (%)

P Value*G0.5 D G1.0 D G1.5 D G2.0 D

1.114 � KM � 6.2 (Jin-modified Maloney) 0.43 0.61 G 0.47 0.01, 1.80 54.8 83.3 92.9 100 d

1.114 � KM � 6.1 (Wang-modified Maloney) 0.44 0.62 G 0.49 0.03, 1.90 52.4 73.8 95.2 100 .841.06 � KM � 4.05 (Jin regression) 0.46 0.66 G 0.50 0.02, 1.80 52.4 71.4 90.5 100 .251.114 � KM � 4.9 (Maloney) 1.33 1.37 G 0.76 0.01, 3.17 11.9 35.7 61.9 78.6 !.0011.14 � KM � 6.8 (Shammas no history) 0.52 0.68 G 0.55 0.02, 2.28 47.6 71.4 92.9 97.6 .227K1.3280 0.86 0.97 G 0.71 0.01, 2.51 31.0 57.1 73.8 90.5 .001KM 1.75 1.87 G 0.91 0.04, 3.60 7.1 14.3 38.1 59.5 !.001

K1.3280 Z keratometric power calculated by calibrated keratometric index 1.3280; KM Z measured K reading*Paired t test comparing with the algorithm (1.114 � KM � 6.2)


measurable using conventional topography. DifferentK2 values (�4.9 D, �4.98 D, �6.062 D, and �6.1 D)have been reported.3,14,15 Using the Pentacam rotatingScheimpflug camera for direct corneal measurement,we confirmed that �6.1 D is a reliable value in thealgorithm for white eyes and found that the K2 valuehad a very small variation and changed little after laserrefractive surgery.13

It has been well recognized that the incidence ofmyopia is higher in Asian eyes21 and that Asian eyesare significantly more myopic than white eyes.22

Moreover, the ocular dimensions of Asian eyes wereshown to be different from those in white eyes, withAsian eyes having longer ALs and smaller anteriordimensions.16–18 As a result, an IOL power calculationmethod that is dependent on ocular dimensions willrequiremodificationwhen used in Asian eyes. The dif-ferences in IOL power calculation between different

Figure 6. Prediction errors (actual postoperative refraction � calcu-lated postoperative refraction) of different corneal power calculationmethods combinedwithHaigis formula for cataract patients after re-fractive surgery (KM Z measured keratometry reading).


ethnic groups may involve a variation in the kerato-metric index in normal eyes and the corneal powerestimation algorithm after refractive surgery as wellas the prediction of ELP due to the difference of oculardimensions. The latter can be solved by optimizationof IOL power calculation formulas, especially the Hai-gis formula, which uses 3 coefficients to optimize theELP prediction algorithm.19

Several studies of IOL power calculation aftercorneal refractive surgery inAsian eyes have been pub-lished6,11,25–27; however, to date, a simplified methodfor corneal power estimation has not been evaluatedin Asian eyes specifically. Therefore, subsequent re-search was performed based on our previous study.13

The geometric and refractive parameters in Chineseeyes andGerman eyes were compared using a rotatingScheimpflug camera (Pentacam). The results con-firmed our hypothesis. Differences in the RP, KP, RA/RP ratio, keratometric index, and K2 values werefound. The posterior corneal curvature was steeper inChinese eyes (mean 6.30 G 0.24 mm) than in Germaneyes (mean 6.42G 0.25mm), causing the posterior cor-neal surface power to bemore negative in Chinese eyes(mean �6.36 G 0.25 D) than in German eyes (mean�6.24 G 0.24 D). The keratometric index was lowerin Chinese eyes (mean 1.3280 G 0.0011) than in Ger-man eyes (mean 1.3287 G 0.0010). The ethnic differ-ences in corneal parameters in our study can beexplained by racial differences in ocular dimensions.Asian eyes have smaller anterior dimensions (shal-lower anterior chambers, smaller corneal diametersand anterior chamber widths, lower corneal vaults)than white eyes.17,18 Therefore, the steeper posteriorcorneal surface in Chinese eyes in our study might bea manifestation of this ocular dimensional differencebecause there is a close geometric relationship betweencorneal curvatures, corneal diameter, and cornealvault.27 We also found that there was no difference inanterior corneal curvature and corneal thickness be-tween Chinese eyes and German eyes.



Our results are compatible with those in previousstudies. Shimmyo et al.28 found no significant differ-ence in anterior corneal curvature between white,Asian, Hispanic, and African American eyes. Severalresearchers have compared the CCT and revealedthat Asian, white, and Hispanic eyes have a similarCCT (approximately 550 mm).28,29 However, after re-viewing the published literature in PubMed (May2012), we believe that ours is the first study to comparethe posterior corneal surface, RA/RP ratio, keratomet-ric index, andK2 value between ethnic groups. The dif-ferences we found should be of clinical importance inthe calibration of the keratometric index, in the furtherstudy of corneal optics and ocular dimensions, and inthe optimization of the ELP prediction algorithms inAsian eyes, a topic not addressed in the present study.

The mean K2 value was identified to be �6.20 D inChinese eyes, which is different from that in whiteeyes (�6.10D). This is a result of thedifference in cornealparameters between the 2 ethnic groups. As shown inourprevious study,13 the reliabilityof using theK2valueis basedon2preconditions; 1 is the small variation inK2,and the other is the small change in K2 induced byrefractive surgery. In the current study, the variationin K2 was very small in normal cases and refractivecases. The 1.96� SD of the variation was 0.47 D in nor-mal cornea, 0.47 D before refractive surgery, and 0.45 Dafter refractive surgery. The error of KC is derived fromthe variation in K2; thus, using a value of �6.20 D forcorneal power estimation could keep the error towithinG0.50D inmore than 95% of the eyes (Figure 4).

Laser refractive surgery induced change in posteriorcorneal curvature and the K2 value in 102 eyes. Thechange in posterior corneal curvature was not statisti-cally significant. The K2 value after refractive surgerywas comparable to the value after refractive surgery(paired t test); however, the change was only �0.02G 0.06 D (range �0.22 to 0.13 D). There was no differ-ence in the K2 value between normal corneas and post-refractive corneas (independent t test), confirming thatthe change in K2 induced by refractive surgery has noclinical significance and can be disregarded. The KP

and K2 values before refractive surgery and those afterrefractive surgery were highly correlated. The slopeswere 0.92 and 0.93, respectively. This confirms thatthe influence of laser refractive surgery on KP and K2

can be ignored. The results are in accordance withfindings in our previous study of white eyes, in whicha relatively small number of eyes (48 eyes) was stud-ied.13 This reconfirms that adopting an offset valuefor corneal power estimation is reliable in IOL powercalculation after corneal refractive surgery.

Based on our results, the corrective algorithm forChinese eyes was determined to be as follows:


KC Z1.114 � KM � 6.2. The algorithm was clinicallyevaluated for cataract patients after laser refractive sur-gery and compared with the previously publishedmethods. The corrective algorithms with an offsetvalue of�6.20D or�6.10D and the regression formulaproduced the lowest calculation errors in Chinese eyes.The Shammas no-history method, Maloney method,KM, and the calibrated K value using the index of1.3280 tended to underestimate IOL power in Chineseeyes after refractive surgery. No difference was foundusing the offset value of �6.20 D and �6.10 D. Thereasons might be that a 0.10 D difference between the2 algorithms can be regarded as a systematic differenceand may introduce less bias in IOL power calculationand that the number of cases (42 eyes) was relativelysmall to show the difference between the 2 algorithms.Nevertheless, we suggest using the offset value of�6.20 D in the corrective algorithm for Chinese eyesbased on statistical analysis.

A limitation is that ours was not a large population-based study comparing corneal parameters betweenethnic groups; however, the samples of Chinese eyesand German eyes were balanced for age and sex. Webelieve that the difference in corneal parameters inthis study can be attributed to the differences in ethnicocular dimensions. An offset value of �6.20 is appro-priate for Chinese eyes.

In conclusion, we found a difference in corneal re-fractive parameters of RP, KP, RA/RP ratio, and kerato-metric index between Chinese eyes and white eyes.The offset value was �6.20 in the corrective algorithmfor Chinese eyes and was different than that for whiteeyes. The modified algorithm could provide reliableIOL power calculation after corneal refractive surgeryin Chinese eyes.

VO

WHAT WAS KNOWN

� In eyes after myopic laser refractive surgery, a simplifiedcorrective algorithm (1.114 � KM � 6.1) that transformsmeasured K reading (KM) to corneal power was relativelyreliable in IOL power calculation for white eyes. However,the reliability of the method has not been evaluated in Chi-nese eyes.

WHAT THIS PAPER ADDS

� The corneal optical parameters RP, KP, the RA/RP ratio, andkeratometric index were different between Chinese eyesand German eyes, resulting in a modified algorithm(1.114 � KM � 6.2) for Chinese eyes.

� The modified method was relatively reliable for IOL powercalculation after laser refractive surgery in Chinese eyes.

L 38, OCTOBER 2012


REFERENCES1. Wang L, Hill WE, Koch DD. Evaluation of intraocular lens power

prediction methods using the American Society of Cataract and

Refractive Surgeons post-keratorefractive intraocular lens

power calculator. J Cataract Refract Surg 2010; 36:1466–1473

2. Hoffer KJ. Intraocular lens power calculation after previous laser

refractive surgery. J Cataract Refract Surg 2009; 35:759–765

3. Wang L, Booth MA, Koch DD. Comparison of intraocular lens

power calculation methods in eyes that have undergone LASIK.

Ophthalmology 2004; 111:1825–1831

4. Jin H, Rabsilber T, Ehmer A, Borkenstein AF, Limberger I-J,

Guo H, Auffarth GU. Comparison of ray-tracing method and

thin-lens formula in intraocular lens power calculations.

J Cataract Refract Surg 2009; 35:650–662

5. Seitz B, Langenbucher A. Intraocular lens power calculation in

eyes after corneal refractive surgery. J Refract Surg 2000;

16:349–361

6. Kim SW, Kim EK, Cho B-J, Kim SW, Song KY, Kim T-I. Use of

the Pentacam true net corneal power for intraocular lens calcu-

lation in eyes after refractive corneal surgery. J Refract Surg

2009; 25:285–289

7. Borasio E, Stevens J, Smith GT. Estimation of true corneal

power after keratorefractive surgery in eyes requiring cataract

surgery: BESSt formula. J Cataract Refract Surg 2006;

32:2004–2014

8. Savini G, Carbonelli M, Barboni P, Hoffer KJ. Repeatability of

automatic measurements performed by a dual Scheimpflug

analyzer in unoperated and post-refractive surgery eyes.


9. Wang L, Mahmoud AM, Anderson BL, Koch DD, Roberts CJ.

Total corneal power estimation: ray tracing method versus

Gaussian optics formula. Invest Ophthalmol Vis Sci 2011;

52:1716–1722. Available at: http://www.iovs.org/content/52/3/

1716.full.pdf. Accessed June 21, 2012

10. Qazi MA, Cua IY, Roberts CJ, Pepose JS. Determining corneal

power using Orbscan II videokeratography for intraocular lens

calculation after excimer laser surgery for myopia. J Cataract

Refract Surg 2007; 33:21–30

11. Cheng ACK, Lam DSC. Keratometry for intraocular lens power

calculation using Orbscan II in eyes with laser in situ keratomi-

leusis. J Refract Surg 2005; 21:365–368

12. Tang M, Li Y, Avila M, Huang D. Measuring total corneal power

before and after laser in situ keratomileusis with high-speed

optical coherence tomography. J Cataract Refract Surg 2006;

32:1843–1850

13. Jin H, Holzer MP, Rabsilber T, Borkenstein AF, Limberger I-J,

Guo H, Auffarth GU. Intraocular lens power calculation after

laser refractive surgery; corrective algorithm for corneal power

estimation. J Cataract Refract Surg 2010; 36:87–96

14. Savini G, Barboni P, Zanini M. Intraocular lens power calculation

after myopic refractive surgery; theoretical comparison of differ-

ent methods. Ophthalmology 2006; 113:1271–1282

15. AwwadST,ManassehC, BowmanRW,CavanaghHD, Verity S,

Mootha V, McCulley JP. Intraocular lens power calculation after

myopic laser in situ keratomileusis: estimating the corneal re-

fractive power. J Cataract Refract Surg 2008; 34:1070–1076

16. Ip JM, Huynh SC, Robaei D, Kifley A, Rose KA, Morgan IG,

Wang JJ, Mitchell P. Ethnic differences in refraction and ocular

biometry in a population-based sample of 11–15-year-old

Australian children. Eye 2008; 22:649–656. Available at: http://

www.nature.com/eye/journal/v22/n5/pdf/6702701a.pdf. Ac-

cessed June 21, 2012


17. Ip JM, Huynh SC, Kifley A, Rose KA, Morgan IG, Varma R,

Mitchell P. Variation of the contribution from axial length and

other oculometric parameters to refraction by age and ethnicity.

Invest Ophthalmol Vis Sci 2007; 48:4846–4853. Available at:

http://www.iovs.org/content/48/10/4846.full.pdf. Accessed

June 21, 2012

18. Qin B, Tang M, Li Y, Zhang X, Chu R, Huang D. Anterior seg-

ment dimensions in Asian andCaucasian eyesmeasured by op-

tical coherence tomography. Ophthalmic Surg Lasers Imaging

2012; 43:135–142

19. Haigis W. The Haigis formula. In: Shammas HJ, ed, Intraocular

Lens Power Calculations. Thorofare, NJ, Slack, 2004; 41–57

20. Shammas HJ, Shammas MC, Garabet A, Kim JH, Shammas A,

LaBree L. Correcting the corneal power measurements for

intraocular lens power calculations after myopic laser in situ

keratomileusis. Am J Ophthalmol 2003; 136:426–432

21. Pan C-W, Ramamurthy D, Saw S-M.Worldwide prevalence and

risk factors for myopia. Ophthalmic Physiol Opt 2012; 32:3–16.

Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1475-

1313.2011.00884.x/pdf. Accessed June 21, 2012

22. Ip JM, Huynh SC, Robaei D, Rose KA, Morgan IG, Smith W,

Kifley A, Mitchell P. Ethnic differences in the impact of parental

myopia: findings from a population-based study of 12-year-old

Australian children. Invest Ophthalmol Vis Sci 2007; 48:2520–

2528. Available at: http://www.iovs.org/content/48/6/2520.full.

pdf. Accessed June 21, 2012

23. Aramberri J. Intraocular lens power calculation after corneal re-

fractive surgery: double-K method. J Cataract Refract Surg

2003; 29:2063–2068

24. Haigis W. Intraocular lens calculation after refractive surgery for

myopia: Haigis-L formula. J Cataract Refract Surg 2008;

34:1658–1663

25. ChengACK,HoT, LauS, LamDSC. Introducing a new ratio use-

ful in the estimation of preoperative corneal power in patients

with myopic LASIK based on postoperative corneal data. Cor-

nea 2009; 28:1–4

26. Kim J-H, Lee D-H, Joo C-K. Measuring corneal power for intra-

ocular lens power calculation after refractive surgery; compari-

son of methods. J Cataract Refract Surg 2002; 28:1932–1938

27. Lim K-L, Fam H-B. Relationship between the corneal surface

and the anterior segment of the cornea: an Asian perspective.


28. ShimmyoM,RossAJ,MoyA,Mostafavi R. Intraocular pressure,

Goldmann applanation tension, corneal thickness, and corneal

curvature in Caucasians, Asians, Hispanics, and African Amer-

icans. Am J Ophthalmol 2003; 136:603–613

29. Aghaian E, Choe JE, Lin S, Stamper RL. Central corneal thick-

ness of Caucasians, Chinese, Hispanics, Filipinos, African

Americans, and Japanese in a glaucoma clinic. Ophthalmology

2004; 111:2211–2219

VOL
38, OCTOBER 2012
First author:Haiying Jin, MD

Department of Ophthalmology(Jin, Zhao), Xinhua HospitalAffiliated to Shanghai JiaotongUniversity School of Medicine,Shanghai, China

http://www.iovs.org/content/52/3/1716.full.pdf


http://www.nature.com/eye/journal/v22/n5/pdf/6702701a.pdf

http://www.nature.com/eye/journal/v22/n5/pdf/6702701a.pdf


http://onlinelibrary.wiley.com/doi/10.1111/j.1475-1313.2011.00884.x/pdf

http://onlinelibrary.wiley.com/doi/10.1111/j.1475-1313.2011.00884.x/pdf



corneal power estimation for intraocular lens power calculation after corneal laser refractive...

Documents