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
Email: sloanrush@suddenlink.net
Phone: (806) 382-6849
Fax: (806) 743-2471
Abstract:
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.
Introduction:
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 postoperative 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 (http://www.acrysoftoriccalculator.com) 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
indicated.
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
accuracy.
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.
Results:
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.
Discussion:
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
cases.
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.
Acknowledgements:
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
Autorefractor
Corneal Topography
Zeiss IOL Master
Manual Keratometry
Means [95% Confidence Intervals]
0.77 [0.66-0.88]
0.83 [0.72-0.93]
0.84 [0.73-0.95]
1.18 [1.07-1.29]