Study Population

This retrospective study comprised 137 eyes in 137 keratoconic patients (97 men and 40 women, mean age ± standard deviation [SD]: 36.9 ± 12.2 years) with good-quality scans of corneal tomography measured with a rotating Scheimpflug imaging instrument (Pentacam HR; Oculus, Wetzlar, Germany). The patients were recruited in a continuous cohort. Only 1 eye per subject was selected randomly for statistical analysis. Some of the subjects were those in our preceding reports on anterior and posterior corneal elevation in eyes with keratoconus. The sample size in this study offered >99% statistical power at the 5% level in order to detect a correlation of 0.700. Keratoconus was diagnosed by 1 experienced clinician (K.K.) with evident findings characteristic of keratoconus (eg, corneal topography with asymmetric bow-tie pattern with or without skewed axes), and at least 1 keratoconus sign (eg, stromal thinning, conical protrusion of the cornea at the apex, Fleischer ring, Vogt striae, or anterior stromal scar) on slit-lamp examination. Pellucid marginal degeneration having inferior corneal thinning with ectasia above the area of thinning and no inflammatory signs on slit-lamp biomicroscopy was excluded from the study. Eyes with other corneal diseases and eyes with previous ocular trauma or surgery were also excluded from the study. The study population was divided into 4 subgroups, grade 1 (44 eyes), grade 2 (38 eyes), grade 3 (16 eyes), and grade 4 (39 eyes) keratoconus groups, according to the Amsler-Krumeich classification, based on astigmatism, corneal power, corneal transparency, and corneal thickness, obtained using the rotating Scheimpflug imaging instrument and slit-lamp biomicroscopy. The patients who wore RGP and soft contact lenses were asked to stop using them for 3 and 2 weeks before this assessment, respectively. This retrospective review of the data was approved by the Institutional Review Board at Kitasato University and followed the tenets of the Declaration of Helsinki. Our Institutional Review Board waived the requirement for informed consent for this retrospective study.

Assessment of Corneal Astigmatism

The magnitude and the axis orientation of anterior, posterior, and total corneal astigmatism on the central 15-degree ring (equal to the 3.0-mm ring) around the corneal apex were automatically measured with the Scheimpflug imaging system (Pentacam HR, software version 1.20; Oculus) by experienced examiners, who were masked to the clinical condition of the subjects. The readings were taken as recommended in the instruction manual of the instrument. In brief, the patient’s chin was placed on the chin rest and the forehead against the forehead strap. The patient was asked to open both eyes and stare at the fixation target on the black background in the center of the blue fixation beam. After attaining perfect alignment, the instrument automatically took 25 Scheimpflug images within 2 seconds. Image quality was checked, and for each eye only 1 examination with a high quality factor was recorded.

Anterior corneal astigmatism is defined as the difference in simulated keratometry between the flattest and steepest meridians, calculated by using the standard keratometric index (1.3375) and the radius of anterior corneal curvature. Posterior corneal astigmatism is defined as the difference in keratometry between the flattest and steepest meridians, calculated by using the refractive index of the cornea (1.376), and the aqueous humor (1.336), and the radius of posterior corneal curvature. Total corneal astigmatism is defined as the difference in total corneal refractive power between the steepest and flattest meridians, calculated by ray tracing through the anterior and posterior corneal surfaces according to Snell’s law.

Anterior and total corneal astigmatism was classified as “with-the-rule” (WTR) when the steep meridian was within 60–120 degrees, and as “against-the-rule” (ATR) when the steep meridian was within 0–30 degrees or 150–180 degrees. Otherwise, the remaining astigmatism was classified as oblique astigmatism. Since the dioptric power of the posterior corneal surface was negative, posterior corneal astigmatism was classified as WTR when the steep meridian was within 0–30 degrees or 150–180 degrees, and as ATR when the steep meridian was within 60–120 degrees. Otherwise, the remaining astigmatism was classified as oblique astigmatism, as described previously.

In addition, in order to confirm the repeatability of the measurements, anterior, posterior, and total corneal astigmatism measurements were made at the same time of day on 2 consecutive days in 20 keratoconic eyes. The repeatability of the 2 measurements was evaluated using Bland-Altman plots, as described previously.

Statistical Analysis

All statistical analyses were performed using a commercially available statistical software (Ekuseru-Toukei 2015; Social Survey Research Information Co, Ltd, Tokyo, Japan). After normal distribution of the data was confirmed with the Kolmogorov-Smirnov test, the Pearson correlation coefficient was calculated in order to assess the relationship of the 2 variables. One-way analysis of variance (ANOVA) was used to assess the relationship of each corneal astigmatism with the Amsler-Krumeich classification. The Cohen kappa coefficient was used to assess the agreement between the axis orientations (WTR, ATR, and oblique astigmatism) of anterior and posterior corneal astigmatism. The results are expressed as mean ± SD, and a value of P < .05 was considered statistically significant.