44 Avoiding Unexpected Outcomes with Toric Intraocular Lenses Our ability to provide excellent unaided acuity after cataract surgery improved dramatically with the introduction of toric intraocular lenses (IOLs). Nevertheless, despite accurate keratometry, precise alignment, and complex calculations, the refractive outcome after toric IOL implantation is not always predictable. Choosing the correct toric IOL for patients is challenging, as we have to consider the magnitude and axis of the toric cylinder required. To avoid unexpected astigmatic outcomes, we need to consider which devices should be used to measure the cornea, how to interpret the measurements, what methods to use to predict the required cylinder, and what techniques to use to accurately align the toric IOL axis. The first practical device able to measure the corneal curvature accurately was developed by Louis Emile Javal in the late 19th century. The optics were described initially by Helmholtz and even earlier by Ramsden. Essentially if the size of an object reflected in the cornea is known and the image size can be measured, the radius of curvature of the anterior cornea can be accurately determined. Today we have many different devices capable of measuring the corneal curvature. These include manual keratometers, optical biometers, and topographers based on placido videokeratography or Scheimpflug imaging devices. The challenge we face is that different devices can provide different measurements for the same patient. The best way to understand the utility of multiple devices is to embrace the concept of primary and secondary instruments introduced by Warren Hill,1 who used the analogy of a pilot selecting an altitude indicator to determine his orientation but due to the critical nature of the activity always considers secondary instruments to confirm or validate the primary instrument. Predicting residual astigmatism for a specific patient is critical to the outcome, and so we should use these same principles. My personal practice is to use three different devices: an optical biometer, a topographer, and a manual keratometer. I rely on the optical biometer as my primary instrument for the magnitude and axis of astigmatism but use the Javal type kerometer (Gm 300; CSO, Milan, Italy) and Pentacam (Oculus, Optikgeräte, Arlington, WA) to confirm the Lenstar (Haag-Streit, Mason, OH) or IOL Master (Carl Zeiss Meditec, Jena, Germany) measurement. If they do not correspond, then the correct measurement is located somewhere within the triangle of agreement. It is feasible to combine the results of different devices mathematically, and I have been able to demonstrate that the mean or median of the measurements from multiple devices does improve the prediction of residual astigmatism compared with a single device. The one scenario in which I do consider the cylinder in the patient’s spectacle correction is when all devices provide different values. The predicted residual astigmatism (predRA) is the sum of the predicted corneal vector of the toric IOL cylinder plus the corneal astigmatism obtained from our keratometer and method of prediction. The predRA is in error when it differs from the actual refraction. This can be expressed as a median value or the percentage of cases with an error of predRA less than 0.5 D. Even more informative, however, is the centroid value, which is the mean or median vector value reflecting both the axis as well as the magnitude of the error. Louis Émile Javal, the noted 19th century ophthalmologist, acknowledged that he could not account for total ocular astigmatism by simply measuring the power of the anterior cornea. This phenomenon is known as Javal’s rule and is thought to be due to the posterior cornea contributing on average 0.5 D against the rule astigmatism. Doug Koch, in his Innovators Lecture in 2012, noted that posterior corneal astigmatism increased with increasing amounts of with-the-rule (WTR) astigmatism but showed no relationship with against-the-rule (ATR) astigmatism measured from the anterior corneal surface. Ignoring the posterior cornea results in an overcorrection of ∼ 0.5 D in patients with WTR astigmatism and an undercorrection of ∼ 0.3 D in patients presenting with ATR astigmatism. Commonly used methods to account for the contribution of the posterior cornea include a nomogram introduced by Doug Koch and Li Wang or actually measuring the posterior corneal radius with a Scheimpflug device such as the Pentacam or Galilei instruments (Ziemer, Port, Switzerland; Alton, IL).2–4 There are several different calculators available. The Alcon (Fort Worth, TX) calculator uses a fixed ratio in calculating the corneal vector of the cylinder power of the toric IOL. This can be adjusted for the posterior cornea using the Baylor nomogram. The Holladay Calculator (Holladay Consulting, Bellaire, TX) uses the effective lens position (ELP) to calculate the corneal vector of the toric IOL and can be adjusted by the Baylor nomogram or direct measurements of the posterior cornea. The alternative is to go online to the Asia-Pacific Association of Cataract and Refractive Surgeon (APACRS) or American Society of Cataract and Refractive Surgery (ASCRS) Web sites and use the Barrett Toric Calculator that I developed, which is also available on the Lenstar. The Barrett Toric Calculator uses the Universal II formula to predict the required spherical equivalent IOL power. The calculator derives the posterior corneal curvature based on a theoretical model proposed to explain the behavior of the posterior cornea. The toric IOL cylinder power required to correct the corneal astigmatism, including the posterior corneal astigmatism, is calculated from the predicted effective lens position using vector math for each eye. In a study conducted at the Ein-Tal Eye Center (Tel Aviv, Israel), the most accurate prediction of residual astigmatism was achieved with the Barrett Toric Calculator in combination with the Lenstar. In a subsequent study that I performed with Adi Abulafia in Perth, Australia, we analyzed the outcome in 54 eyes that had a toric IOL implanted. We compared the results using preoperative versus postoperative keratometry K value, the intended versus the actual axis of alignment, and different calculators to identify the relative contribution of each of these factors to the errors in predicted residual astigmatism. The results of the study demonstrate that errors in estimating surgically induced astigmatism (SIA) adversely impact the predictably of toric outcomes, and utilizing the centroid value for SIA offers significant improvement. Similarly eliminating errors in axis alignment offers further improvement but the impact is less. The most important benefit, however, that can be obtained in improving toric outcomes is to use an improved calculator. A double angle plot of the x and y values for each vector demonstrates that the Alcon and Holladay calculators result in significant unintended ATR astigmatism. Adjusting the measured K’s with the Baylor nomogram improved the percentage of cases with an error in predRA within 0.5 D to 50%, as did utilizing the Pentacam 4.5 mm equivalent K-reading (EKR) value. Both these modifications, however, proved to be less accurate than the Barrett Toric Calculator, with which 70.4% of cases had an error in predRA within 0.5 D and these differences were statistically significant (Fig. 44.1). An error in alignment of 1 degree reduces the effective correction of a toric IOL by an estimated 3%. Although clinically the impact of misalignment appears to be less than not taking into account factors such a posterior corneal astigmatism there are several techniques that can be considered to improve alignment.
Devices Used to Measure the Cornea
Interpreting Measurements from Multiple Devices
Methods to Predict the Required Cylinder
Techniques to Accurately Align the Toric IOL Axis
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