Keratometry in Alcon AcrySof Toric IOL s What K s to Use astigmatism

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					    Pre-operative Keratometry and the Alcon AcrySof

                          Toric Intraocular Lens

Sloan W. Rush, MD
Texas Tech University Health Sciences Center, Dept of Ophthalmology & Visual
Sciences, Lubbock, TX

Jay C. Bradley, MD
Texas Tech University Health Sciences Center; Dept of Ophthalmology & Visual
Sciences, Lubbock, TX

J. Avery Rush, MD
Panhandle Eye Group L.L.P., Amarillo, TX

*J.A. Rush is a speaker for Alcon. All authors do not have any financial interest to
disclose and no financial support was provided for this study.

Manuscript word count: 2128

Corresponding Author:
Sloan W. Rush, MD
Address: 3601 4th St, STOP 7217, Lubbock, TX 79430-7217
Phone: (806) 382-6849
Fax: (806) 743-2471

Purpose: To compare the accuracy of different preoperative keratometric methods for

implantation of the Alcon AcrySof Toric Intraocular Lens (AASTIOL).

Methods: One hundred consecutive cases (65 subjects) with AASTIOL implantation by

the same surgeon were enrolled into the study for retrospective analysis. For each subject,

the following data was collected: current glasses prescription, preoperative and

postoperative refraction, manual keratometry, and automated keratometry using an

autorefractor, a corneal topographer, and a Zeiss IOL Master. The calculated preoperative

astigmatism was retrospectively determined for each subject using vector analysis.

Absolute cylinder errors were then calculated by crossing the cylinder vectors of the

resulting preoperative calculated astigmatism with the predicted cylinder for the various

keratometric methods.

Results: One-way analysis of the variance (ANOVA) was used to compare the means of

the absolute cylinder errors for the four preoperative keratometric techniques. Absolute

cylinder errors (in diopters with [95% confidence intervals]) from least to greatest are as

follows: autorefractor 0.77 [0.66-0.88], corneal topography 0.83 [0.72-0.93], Zeiss IOL

Master 0.84 [0.73-0.95], and manual keratometry 1.18 [1.07-1.29]. Automated

keratometry techniques yielded cylinder errors significantly lower (p<0.05) than manual

keratometry. There was no statistical difference in cylinder errors between methods of

automated keratometry (p=0.5555).

Conclusions: No single preoperative method of keratometry for predicting astigmatism

was found to be extremely accurate in all cases, although methods of automated

keratometry were superior to manual keratometry. Further investigation is necessary to
determine the keratometric method to achieve most accurate and reliable postoperative

refractive outcomes with AASTIOL use.

       Refractive outcomes in cataract surgery patients have become increasingly

important as additional methods for astigmatism correction become available.1 Routine

astigmatic correction with cataract surgery is becoming the standard of care for many

surgeons.2-3 Prior studies have used incisional techniques to obtain the desired post-

operative refractive result4-5, but with the advent of toric intraocular lenses (IOL),

astigmatic correction has become more predictable.6-8 Many toric lens implants have been

described in the literature including the Artisan toric IOL for phakic correction of

astigmatism9-10 and post-penetrating keratoplasty,11 Staar toric IOL for pseudophakic

astigmatism correction,12 Nidek toric IOL,13 and Microsil silicone toric IOL.14 Toric IOL

implantation has become progressively more common in the United States since Food &

Drug Administration (FDA) approval and widespread availability of the Alcon AcrySof

toric IOL (Alcon Laboratories, Inc, Fort Worth, Texas).

       Numerous calculation protocols have been proposed to determine the toric IOL

power for implantation in cataract surgery15-16. For the Alcon AcrySof toric IOL

(AASTIOL), the AASTIOL calculator ( can be

used to obtain the recommended cylinder power (either 1.50, 2.25, or 3.0 diopters in the

lens plane) and axis of the IOL implant with input of the following: IOL spherical power

(using the surgeon’s routinue technique and formula), anticipated surgically induced

astigmatism (SIA) with incision location, and keratometry values (diopter value with axis

of orientation in both the steep and flat axis). Previous investigators have looked at both

the biometric spherical IOL calculation17 and the effects of SIA on toric IOL selection18,

displaying that both are surgeon-dependent to varying degrees. The ideal method for
obtaining pre-operative keratometry in patients undergoing toric IOL implantation has

not been previously investigated, although manual keratometry was used and

recommended based on the original FDA AASTIOL studies.

       Many methods are available to the practitioner for preoperative keratometry

including manual keratometry and automated keratometry by an autorefractor, a corneal

topographer, or the Zeiss IOL Master. The purpose of this study is to compare these

various keratometric methods with regards to postoperative refractive outcomes when

applied to the AASTIOL calculator.

Materials and Methods:

       Sample data from a 100 consecutive cases (65 subjects) was collected in a

retrospective fashion from patients that had phacoemulsification with implantation of the

AASTIOL. Exclusion criteria included a history of prior ocular trauma, prior ocular

surgery of any kind, keratoconus by history or by topography, anticipated Snellen visual

acuity potential <20/40 post-operatively due to any reason (in order to ensure more

accurate subjective pre- and postoperative refraction), or any intra- or postoperative

complication. The AASTIOL calculator was used to determine which toric IOL was


       To maintain consistency, the same surgeon (J.A.R.) performed all cases with

identical surgical technique using a clear cornea (superior 90 or temporal 180 degrees)

2.4 millimeter sutureless incision and performed all postoperative refractions (done for all

subjects at the 3 week postoperative interval). Prior to each case, the same surgeon used a

Mendez degree gauge (Katena Eye Instruments, Denville, NJ) to mark the 180 degree
axis with the patient sitting upright and used this orientation to mark the axis of

implantation intraoperatively just prior to toric IOL implantation. For each subject, the

following measurements were obtained: lensometer reading (Zeiss Meditech, San

Leandro, CA) of the patient’s current glasses, preoperative manifest refraction (Reichert

phoropter, Depew, NY), manual keratometry (Marco, Jacksonville, FL), automated

keratometry using an autorefractor (Zeiss Meditech, San Leandro, CA), corneal

topography (Tomey, Phoenix, AZ), or the Zeiss IOL Master (Zeiss, Jena, Germany), and

postoperative manifest refraction. Manual keratometry was performed by technicians

with greater than ten years experience and history of consistent postoperative refractive

outcomes. Particular attention was given to both pre- and postoperative manifest

refractions in order ensure that occult cylinder was not present.

       All astigmatic measurements for the preoperative keratometric methods were

rounded to the nearest whole number in terms of axis, to the nearest quarter diopter for

refractions and glasses prescriptions, to the nearest eighth diopter for manual

keratometry, and exactly as appeared on the objective print-out for each the autorefractor,

the corneal topographer, and the IOL Master. Although a prior study showed that

computation of cylinder power accounting for vertex distance from the spectacle plane to

the corneal plane19 had a negligible effect on the results, old glasses prescriptions and

pre- and postoperative refractions were adjusted to account for this issue to ensure


       Several authors have advocated sophisticated mathematical methods for

combining and calculating astigmatic vectors (i.e. a quantity with both magnitude and

direction) across a population sample.20,21 In this study, the absolute cylinder error across
various keratometric methods was analyzed and, by convention, all astigmatic vector

quantities were formulated in the following fashion: magnitude of plus cylinder power in

diopters and direction of axis in degrees 0-180.

       Irrespective of the keratometric method used for the AASTIOL calculator, the

actual calculated preoperative cylinder was retrospectively calculated using vector

analysis as previously described.22,23 The vector of the toric IOL power in the corneal

plane with the axis of placement used during surgical implantation was added to the

amount of SIA and the residual postoperative astigmatism as determined at the 3 week

postoperative refraction. The AASTIOL power at the corneal plane is 1.03, 1.55, or 2.06

D according to the manufacturer depending on which of the three models is used. SIA

was calculated using vector analysis of the difference between preoperative keratometric

readings and postoperative astigmatism vertexed to the corneal plane as previously

described.22,23 Absolute cylinder errors in diopters were calculated by crossing the

cylinder vectors of the resulting preoperative calculated astigmatism with the predicted

cylinder for each of the four keratometric methods.

       This particular method of vector analysis of the keratometric data is useful only

for comparison of means across the sample population and that the calculated absolute

cylinder error does not correspond with the postoperative clinical outcome. For example,

if a patient has a calculated pre-operative cylinder of 2.00 D at 180 degrees and a

particular keratometric method predicts 2.00 D at 165 degrees, it would appear that the

keratometric measurement was fairly accurate. When using vector analysis, the difference

between these two cylinders yields an absolute cylinder error of 1.04 D due to the 15

degree axis differential. This resulting absolute error does not correspond to a 1.04 D
error in anticipated post-operative residual astigmatism and does not correlate with

expected post-operative visual acuity but does however provide the most useful

mathematical expression for comparison of means over the population sample.


       Collected data was analyzed using one-way analysis of the variance (ANOVA)

with statistical software (SAS Institute, Inc, North Carolina) to compare the means of the

absolute cylinder errors for the four different preoperative keratometric techniques.

Absolute cylinder errors (in diopters with [95% confidence intervals]) are presented in

Table 1 from least (most accurate method) to greatest (least accurate method). When

pooled together, there was strong statistical difference between the four methods

(p<0.0001). For automated keratometry using an autorefractor, a corneal topographer,

and a Zeiss IOL Master, cylinder errors were significantly lower (p<0.05) and found to be

more accurate than manual keratometry when analyzed in pairs alone (student’s T-test).

There was no statistical difference in cylinder errors between the autorefractor, corneal

topography, and Zeiss IOL Master (p=0.5555). Only nine of one hundred eyes had more

or within 0.25 diopters of astigmatism postoperatively than preoperatively. No significant

IOL rotation was found in any patient enrolled in this study.

       In subjects that had >1 diopter absolute cylinder error for the autorefractor (n=30),

both corneal topography and Zeiss IOL Master were significantly more accurate

(p<0.05). In subjects with >1 diopter absolute cylinder error for corneal topography

(n=29), both the autorefractor and Zeiss IOL Master were significantly more accurate

(p<0.05). In subjects with >1 diopter absolute cylinder error for Zeiss IOL Master (n=30),
both corneal topography and the autorefractor trended towards better accuracy but did not

reach statistical significance (p=0.06 in both cases). Manual keratometry was found to be

less accurate in the above mentioned subsets. In cases where the autorefractor, corneal

topographer, and Zeiss IOL Master averaged >1 diopter of absolute cylinder error on one

eye, it did not appear to increase the risk (by use of odds ratios) for subsequent inaccurate

keratometric measurement on the fellow eye in the 35 cases in which both eyes were

included in the study.


         No single preoperative keratometric method for predicting astigmatism was found

to be extremely accurate in all cases, although the three automated methods

(autorefractor, corneal topography, and Zeiss IOL Master) were each found to be superior

on average to manual keratometry. Although they can include a significant lenticular

component, the old glasses prescription and preoperative refraction may be useful

clinically if significant discrepancy exists among obtained keratometric methods.

However, they should not be used alone for AASTIOL selection. Use of a corneal

topographer able to measure the posterior corneal surface may also prove useful in select


         Some inaccuracies observed in this study may be attributable to rotational

instability or inaccurate initial placement of the toric IOL. Since several recent studies

have confirmed the FDA findings of excellent rotational stability of the AASTIOL24-25,

these factors likely did not significantly affect the refractive outcome. Occasionally,

significant discrepancies occur among various preoperative keratometric methods.
Accuracy in postoperative outcomes may be improved by selectively implanting

AASTIOL only in patients with regular topography and highly consistent keratometric

measurements among various methods. However, based on the data above, the accuracy

of each automated keratometric methods is skewed in a minority of cases. Performing

various automated keratometric methods on AASTIOL candidates could potentially

avoid inaccuracies from any one particular method, thus further enhancing accuracy. The

data in this study suggests that larger than anticipated residual postoperative astigmatism

in one eye may not necessarily indicate a poor keratometric reliability or increased risk of

residual postoperative astigmatism in the fellow eye. Comparison of preoperative

keratometric data from more than one modality is already performed by many surgeons

in a subjective fashion in hopes of improving refractive outcomes. Future investigation is

necessary to determine the optimal method of preoperative assessment to achieve more

accurate and reliable postoperative results.

       Manual keratometry has been considered the “gold standard” keratometric

method and is currently recommended by Alcon for use with the AASTIOL calculator.

This modality is more subjective and operator-dependent than other methods and may be

subject to significant error on some patients. In this study, experienced technicians were

used to obtain manual keratometry but these measurements were found to be less

accurate than automated methods. Physician-obtained manual keratometry may yield

more accurate results, but this is often not employed due to time constraints and for the

sake of efficiency. Manual keratometry measures only the paracentral 3-4 mm of cornea

of 2 sets of 2 orthogonal points while assuming spherocylindrical optics. When used
alone, this testing modality may provide less accurate values in corneas with higher

asphericity or other significant topographic abnormalities.

       This study showed that with AASTIOL implantation the vast majority (>90%) of

patients had significant reduction in cylinder power postoperatively. Toric IOL

implantation shows marked improvement over incisional techniques (limbal relaxing

incisions or astigmatic keratotomy) with both predictability and potential complications

and is now the first line choice in correcting low to moderate amounts of astigmatism at

the time of cataract surgery. Toric IOL implantation has the added advantage in that it

does not require additional corneal incisions and is reversible in the event of a

postoperative refractive surprise. Incisional techniques remain important for patients with

high astigmatism and may be combined with toric IOL implantation.

       SIA due to wound construction has been proven to be a significant factor in the

postoperative outcome and is largely surgeon dependent. Prior studies have shown

variability in the significance of incision location with respect to amount of residual

astigmatism observed post-operatively.18,22,23 In patients with smaller amounts of

astigmatism, incision locations will often affect the toric IOL recommendation by the

AASTIOL calculator. Patients with higher amounts of astigmatism beyond what the

AASTIOL is able to correct at this time likely benefit from on-axis incision placement.

As with incisional refractive techniques, some patients with toric IOL implantation may

achieve a reduction in overall postoperative astigmatism but at the expense of a less

desirable or oblique cylinder axis. Further study is needed to determine if this affects

visual outcome or patient satisfaction related to the AASTIOL.
       The authors’ experience with the AASTIOL is that virtually all of the patients are

satisfied postoperatively. Other than keratometry, all of the data entered into the

AASTIOL calculator is surgeon-dependent to some extent and, therefore, achieving more

accurate and reliable preoperative keratometric data is key to optimize postoperative

outcomes. Large discrepancies between multiple methods of preoperative keratometric

data should indicate to the clinician that these patients may have a lower likelihood of

predictable postoperative results, and appropriate preoperative counseling should be

performed in all cases accordingly.


The authors would like to thank the office staff of Dr. J. Avery Rush for helping with

data collection and Dr. David L. McCartney for instructive critique.

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Table 1: Absolute cylinder errors (in diopters) over various keratometric methods.

                Method                           Means [95% Confidence Intervals]
              Autorefractor                               0.77 [0.66-0.88]
           Corneal Topography                             0.83 [0.72-0.93]
            Zeiss IOL Master                              0.84 [0.73-0.95]
           Manual Keratometry                             1.18 [1.07-1.29]

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