1. Range of analytical approaches. In the January 2001 issue of JCRS, several authors described their methods for analyzing change in corneal astigmatism following laser in situ keratomileusis surgery. Several of these methods are commonly and effectively used in peerreviewed scientific studies to characterize astigmatic changes. 2. Analyzing astigmatic change in intraocular lens (IOL)-based surgery is more complex than in corneal refractive surgery. One issue is simply the transition of terminology and standards for reporting outcomes from corneal laser refractive surgery to toric-IOL-based surgery, which was recently addressed by a joint editorial in the Journal of Cataract & Refractive Surgery and the Journal of Refractive Surgery. A larger problem is complexity of measurement and analysis. With corneal refractive surgery, we evaluate changes in two measurable entities: refraction and anterior corneal curvature. With IOL-based surgery, however, parameters required to analyze astigmatic change after cataract surgery include: (a) refraction; (b) anterior corneal astigmatism; (c) posterior corneal astigmatism, which still eludes consistently accurate measurement; (d) IOL alignment, which must be rigorously measured; (e) effective IOL toricity at corneal plane, which can be calculated but not directly measured; and (f) IOL tilt, which induces astigmatism with nontoric and toric IOLs. 3. There are four clinical scenarios in which cataract surgery alters ocular astigmatism. They are: (a) nontoric IOL without corneal relaxing incisions, (b) nontoric IOL with corneal relaxing incisions, (c) toric IOL without corneal relaxing incisions, and (d) toric IOL with corneal relaxing incisions. Since both the cornea and IOL have an astigmatic component (tilt in the case of a spherical IOL), for each of these scenarios we are interested in the corneal, IOL-induced, and total ocular astigmatism (which is typically characterized by the manifest refractive astigmatism, but objective measures of ocular astigmatism can also be used).

Pursuing perfection in intraocular lens calculations: IV. Rethinking astigmatism analysis for intraocular lens-based surgery: Suggested terminology, analysis, and standards for outcome reports.

Purpose To evaluate and compare the accuracy of 2 toric intraocular lens (IOL) calculators with or without a new regression formula. Setting Ein-Tal Eye Center, Tel-Aviv, Israel, and the Lions Eye Institute, Nedlands, Western Australia, Australia. Design Retrospective case series. Methods A new regression formula (Abulafia-Koch) was developed to calculate the estimated total corneal astigmatism based on standard keratometry measurements. The error in the predicted residual astigmatism was calculated by the Alcon and Holladay toric IOL calculators with and without adjustments by the Abulafia-Koch formula. These results were compared with those of the Barrett toric calculator. Results Data from 78 eyes were evaluated to validate the Abulafia-Koch formula. The centroid errors in predicted residual astigmatism were against-the-rule with the Alcon (0.55 diopter [D]) and Holladay (0.54 D) toric calculators and decreased to 0.05 D ( P x -axis], P  = .776 [ y -axis]) and 0.04 D ( P x -axis], P  = .726 [ y -axis]) with adjustments by the Abulafia-Koch formula. The Alcon and the Holladay toric calculators had a higher proportion of eyes within ±0.50 D of the predicted residual astigmatism with the Abulafia-Koch formula (76.9% and 78.2%, respectively) than without it (both 30.8%). There were no significant differences between the results of the Abulafia-Koch-modified Alcon and the Holladay toric calculators and those of the Barrett toric calculator. Conclusion Adjustment of commercial toric IOL calculators by the Abulafia-Koch formula significantly improved the prediction of postoperative astigmatic outcome. Financial Disclosure Dr. Abulafia received a speaker's fee from Haag-Streit AG. Dr. Barrett has licensed the Barrett Toric Calculator to Haag-Streit AG. Dr. Koch is a consultant to Alcon Laboratories, Inc., Abbott Medical Optics, Inc., and Revision Optics, Inc. Dr. Hill is a paid consultant to Haag-Streit AG and Alcon Laboratories, Inc. None of the other authors has a financial or proprietary interest in any material or method mentioned.

New regression formula for toric intraocular lens calculations

PURPOSE
To evaluate the accuracy of central corneal power measurements by Scheimpflug imaging (Pentacam) for eyes that had corneal refractive surgery.


METHODS
This study comprised two groups: a pilot group of 100 eyes with prior hyperopic or myopic LASIK that did not have cataract surgery, and a test group of 41 eyes with prior radial keratotomy (RK) and cataract surgery. In the pilot group, Pentacam images and refraction were taken preoperatively and 3 months after LASIK. The historical method was used to compute the theoretical postoperative keratometry (K) -reading and then compared to the measured equivalent K-reading (EKR) from the Pentacam. The EKR is the same value measured by standard keratometry or topography on the front surface, adjusted for the effect of the back surface power difference from normal. In the test group of RK eyes, the postoperative refraction and EKR were measured 3 months after cataract surgery. The Holladay IOL Consultant Program was used to back-calculate the theoretical K-reading. The EKR measurements were then compared to the back-calculated corneal power.


RESULTS
The optimal zone sample size was determined to be 4.5 mm for the pilot group. The mean prediction error for this group was -0.06+/-0.56 diopters (D) (range: -1.63 to +/-1.34 D). Using the 4.5-mm zone determined in the pilot group, the EKR value for the test group of 41 RK eyes had a mean prediction error of -0.04+/-0.94 D (range: -1.84 to +/-2.27 D).


CONCLUSIONS
When historical refractive data are not available, Scheimpflug imaging with the Pentacam provides an alternative method of measuring the central corneal power in eyes that previously received corneal refractive surgery.

Corneal Power Measurements Using Scheimpflug Imaging in Eyes With PriorCorneal Refractive Surgery

Purpose To evaluate the repeatability of automatic measurements by a new anterior segment optical coherence tomographer (AS-OCT) combined with Placido topography and their agreement with those provided by 2 rotating Scheimpflug cameras. Setting G.B. Bietti Foundation IRCCS, Rome, Italy. Design Evaluation of a diagnostic test instrument. Methods Unoperated eyes and eyes with previous myopic excimer laser surgery were analyzed. Three consecutive scans were acquired with an AS-OCT device (MS-39) and 1 with 2 rotating Scheimpflug cameras (Pentacam HR and Sirius). The following parameters were evaluated: simulated keratometry, posterior and total corneal power, total corneal astigmatism, corneal asphericity, thinnest corneal thickness, central epithelial thickness, corneal diameter, and aqueous depth. Repeatability was assessed using test–retest variability, the coefficient of variation (CoV), and the intraclass correlation coefficient; agreement was assessed by the 95% limits of agreement. Results The study comprised 96 unoperated eyes and 43 eyes with previous myopic excimer laser surgery. High repeatability was achieved in both groups, as shown by a CoV less than 1.0% for most parameters. The repeatability of epithelial thickness was slightly lower than that of the whole corneal thickness, although the CoV was still good (1.87% in unoperated eyes; 3.28% in post-refractive surgery eyes). Moderate repeatability was found for total corneal astigmatism measurements, with a CoV greater than 20.0%. Agreement with Scheimpflug cameras was high for aqueous depth and thinnest corneal thickness and moderate for most other parameters. Conclusion The high repeatability of automatic measurements by the new AS-OCT device supports its use in clinical practice.

Repeatability of automatic measurements by a new anterior segment optical coherence tomographer combined with Placido topography and agreement with 2 Scheimpflug cameras

PURPOSE To compare the prediction error in the calculation of toric intraocular lenses (IOLs) associated with methods that estimate the power of the posterior corneal surface (ie, Barrett toric calculator and Abulafia-Koch formula) with that of methods that consider real measures obtained using Scheimpflug imaging: a software that uses vectorial calculation (Panacea toric calculator: http://www.panaceaiolandtoriccalculator.com) and a ray tracing software (PhacoOptics, Aarhus Nord, Denmark). METHODS In 107 eyes of 107 patients undergoing cataract surgery with toric IOL implantation (Acrysof IQ Toric; Alcon Laboratories, Inc., Fort Worth, TX), predicted residual astigmatism by each calculation method was compared with manifest refractive astigmatism. Prediction error in residual astigmatism was calculated using vector analysis. RESULTS All calculation methods resulted in overcorrection of with-the-rule astigmatism and undercorrection of against-the-rule astigmatism. Both estimation methods resulted in lower mean and centroid astigmatic prediction errors, and a larger number of eyes within 0.50 diopters (D) of absolute prediction error than methods considering real measures (P < .001). Centroid prediction error (CPE) was 0.07 D at 172° for the Barrett toric calculator and 0.13 D at 174° for the Abulafia-Koch formula (combined with Holladay calculator). For methods using real posterior corneal surface measurements, CPE was 0.25 D at 173° for the Panacea calculator and 0.29 D at 171° for the ray tracing software. CONCLUSIONS The Barrett toric calculator and Abulafia-Koch formula yielded the lowest astigmatic prediction errors. Directly evaluating total corneal power for toric IOL calculation was not superior to estimating it. [J Refract Surg. 2017;33(12):794-800.].

Comparison of Methodologies Using Estimated or Measured Values of Total Corneal Astigmatism for Toric Intraocular Lens Power Calculation.

PURPOSE To evaluate and compare the accuracy of three toric intraocular lens (IOL) calculators using keratometry measurements derived from the anterior corneal curvature and direct measurements of the posterior corneal curvature. METHODS Postoperative corneal astigmatism was measured by the IOLMaster (Carl Zeiss Meditec, Jena, Germany) and Pentacam (Oculus Optikgerate, Wetzlar, Germany). The data were processed by the Alcon, Holladay, and Barrett toric IOL calculators. The error in predicted residual astigmatism (PredRA) was calculated by subtracting the PredRA from the postoperative subjective refraction by vector analysis. RESULTS The centroid errors in PredRA were against-the-rule (ATR) with the Alcon (0.56 diopters [D]) and Holladay (0.55 D) toric calculators using the IOLMaster (Carl Zeiss Meditec, Jena, Germany) measurements. The centroid errors in PredRA were lower when Pentacam (Oculus Optikgerate, Wetzlar, Germany) measurements were used (0.38 D, ATR). The Barrett toric calculator using the IOLMaster measurements had the lowest centroid errors in PredRA (0.02 D, P < .001) and achieved the most accurate results: 75.8% and 92.9% of eyes were within 0.50 and 0.75 D of the PredRA, respectively. CONCLUSIONS The prediction of the postoperative astigmatic outcome can be improved by using appropriate methods of adjustment for posterior corneal astigmatism.

Maria Franchina

Papers

New regression formula for toric intraocular lens calculations

Comparison of Methods to Predict Residual Astigmatism After Intraocular Lens Implantation.