Subjects

This prospective validity assessment was conducted at the Rotterdam Ophthalmic Institute, Rotterdam, Netherlands. Ninety-one eyes of 91 patients who underwent cataract surgery with implantation of a monofocal, non-toric IOL were enrolled. If both eyes of a patient were eligible for inclusion, the eye that was operated most recently was selected. Exclusion criteria were corneal diseases (eg, keratoconus, corneal scarring, pterygium), pseudoexfoliation syndrome, previous corneal or refractive surgery, contact lens wear, any significant comorbid ocular disease, or inability to fixate on the fixation target of the measurement devices. In addition, patients were excluded if complications occurred during the surgery that could influence the outcome variables, such as a capsular tear or zonular rupture causing tilt or decentration of the IOL. The study adhered to the tenets of the Declaration of Helsinki. Approval for the study was prospectively obtained from the institutional review board of the Erasmus Medical Center in Rotterdam, and informed consent was received from all subjects.

Surgical Procedure

A 2.2 mm self-sealing clear corneal incision or scleral tunnel incision was made with a standard dual-bevel slit knife (INTREPID 2.2; Alcon, Fort Worth, Texas, USA). Incisions were either placed superiorly (at 100 degrees) or at the steepest corneal meridian. After routine cataract extraction by phacoemulsification and bimanual cortex removal, a hydrophobic acrylic single-piece IOL (Acrysof SA60AT; Alcon) was inserted in the capsular bag using a standard Monarch III injector (Alcon). This IOL has a 6.0-mm-diameter monofocal non-toric optic, an overall length of 13.0 mm, and no haptic angulation.

Measurements

All measurements were performed approximately 4 weeks postoperatively. Refractive astigmatism was measured with the ARK-530A autorefractor (Nidek, Gamagori, Japan). Corneal astigmatism was measured with 2 devices. The Pentacam HR (software version 6.07r12) is a Scheimpflug imaging system that generates tomographic images of the anterior chamber and uses edge detection to find the curvatures of the anterior and posterior corneal surfaces. It offers several options to calculate total corneal astigmatism, including True Net Power, Equivalent K-readings, and Total Refractive Power. Of these, we used the Total Refractive Power option in the 3.0-mm zone, because this option uses ray tracing to combine the contributions of both corneal surfaces and therefore most realistically reconstructs total corneal power. Moreover, this option most closely resembles the total corneal astigmatism calculation method of the Cassini. The Cassini (software version 2.2.0 beta) analyzes specular reflections of 679 colored LEDs on the anterior corneal surface and reflections of 7 infrared LEDs on the posterior surface to calculate the curvatures of both corneal surfaces at the 3.0-mm central zone. Total corneal astigmatism is calculated using ray tracing.

For each measurement on each device, the subjects were asked to fixate on the fixation target of the instrument and blink before the measurement. We used the internal quality check of each device to decide whether or not the measurement was acceptable. For the Pentacam, measurements were accepted if the quality check indicated that the measurement was “OK.” For the Cassini, measurements were accepted if at least 6 of the 7 infrared reflections were captured, as indicated by the posterior surface quality indicator. If not acceptable, the measurement was repeated until it was acceptable or until no acceptable measurements could be obtained after several repeated attempts.

Initial Data Processing

Refractive astigmatism was converted to the corneal plane using the method described by Holladay and associates. To calculate anterior corneal surface astigmatism for the measurements with the Pentacam and the Cassini, anterior radii of curvature were converted to meridional power using a keratometric index of 1.3375. Total corneal astigmatism was calculated by the Pentacam and Cassini using ray tracing, in which Snell’s law is applied to calculate refraction of a large number of light “rays” when incident on the anterior and, subsequently, the posterior corneal surface. For the Pentacam, these values can be found under “Total Refractive Power” (TRP) (we used the values calculated at the 3.0-mm zone), whereas for the Cassini they can be found under “Total Corneal Astigmatism” (TCA).

Accuracy Determination

The accuracy of the measurements that took into account only the anterior corneal surface, as well as of those that took into account both corneal surfaces, was calculated as the difference of these measurements with the refractive astigmatism. The smaller the difference, the better the accuracy. The refractive astigmatism was considered the “gold standard” because it consists of the astigmatism of all refractive surfaces of the eye combined. Since a non-toric IOL was inserted into the eye, the only refractive surfaces that can be astigmatic are the anterior and posterior corneal surface. Thus, refractive astigmatism must equal the astigmatism that results from both corneal surfaces. To calculate the accuracy, all astigmatism measurements were decomposed into orthogonal X and Y components (ie, a vector) according to the method described by Holladay and associates. Accuracy was inversely related to the length of the vector denoting the difference between the “gold standard” vector (refractive astigmatism) and the vector representing the measurement of interest. This will be called the “difference vector length.”

Statistical Analysis

Statistical analysis was performed using Excel 2010 (Microsoft, Redmond, Washington, USA) and SPSS version 21 software (IBM, Armonk, New York, USA). Normality of the distribution of all outcome variables was checked by inspection of histograms and using the Kolmogorov-Smirnov test. All variables except astigmatic magnitude were normally distributed. Thus, difference vector lengths were presented as mean ± standard deviation. The statistical significance of the change in difference vector length owing to the use of total instead of anterior corneal astigmatism measurements was calculated using the 1-sample t test. P values < .05 were considered statistically significant.

Sample Size Calculation

The smallest change in difference vector length that might be clinically relevant was defined to be 0.125 D. Based on pilot data of 24 patients, the standard deviation of this change was estimated to be 0.24 D. Setting α to 0.05 and power (1-β) to 0.9, the minimum required sample size was calculated to be 39 eyes.