### Materials and Methods

### Study population

### Main outcome measures

_{k}= 1.3375). The total corneal power, or true net power, was calculated using the Gaussian total corneal power with the Gullstrand eye model without regard for corneal thickness in the 4.0-mm zone [16]. Anterior, posterior, and total corneal powers were calculated in a single Scheimpflug camera based on the following equations: where

*r*is the radius of the anterior corneal surface, and

_{ant}*r*is the radius of the posterior corneal surface.

_{post}### Statistical analysis

*t*-tests were performed to compare anterior, posterior, and total mean corneal powers; cylinder powers; and flat meridians. Student's

*t*-tests were also performed to compare the mean magnitudes of the vector differences of the astigmatism and the mean absolute corneal power and the flat meridian differences between the simulated K and total cornea between keratoconus and healthy eyes. Linear regression and Pearson's correlation analyses were performed to evaluate correlations between corneal power, cylinder power, and flat meridian among anterior and posterior corneal surfaces and the total cornea. Chi-square tests were performed to compare the proportions of anterior and posterior corneal astigmatism according to the flat meridian between eyes with keratoconus and healthy eyes. Results were considered statistically significant at a

*p*-value <0.05.

### Results

*p*< 0.001 and

*p*< 0.001, respectively). The mean anterior, posterior, and total corneal cylinder powers (±SD) of the keratoconus group (4.37 ± 1.73, 0.95 ± 0.39, and 4.36 ± 1.74 cylinder diopters [CD], respectively) were also greater than those of the control group (1.10 ± 0.68, 0.39 ± 0.18, and 0.97 ± 0.63 CD, respectively). The mean central corneal thickness (±SD) of the keratoconus group (509.8 ± 37.5 µm) was significantly smaller than that of the control group (567.8 ± 30.2 µm) (Table 1).

^{2}= 0.240 and

*p*= 0.004 in the keratoconus group, R

^{2}= 0.592 and

*p*< 0.001 in the control group) (Fig. 3) and negatively correlated with the absolute flat meridian difference between the simulated K and total cornea in both groups (R

^{2}= 0.370 and

*p*< 0.001 in the keratoconus group, R

^{2}= 0.592 and

*p*< 0.001 in the control group) (Fig. 4).

*p*= 0.002) (Table 3 and Fig. 5A, 5B), although the mean absolute corneal power difference between the simulated K and total cornea was not significantly different between the two groups. The magnitude of the vector difference between the astigmatism of the simulated K and total cornea was a maximum of 2.42 CD in the keratoconus group and 0.77 CD in the control group. In contrast, the mean absolute flat meridian difference between the simulated K and total cornea of the keratoconus group, 2.3 ± 3.8 degrees, was significantly smaller than that of the control group, 9.2 ± 12.6 degrees (

*p*= 0.005) (Table 3). The percentage of eyes with a flat meridian difference between the simulated K and total cornea >10 degrees in the keratoconus group, 9.1%, was significantly smaller than that in the control group, 30.3% (

*p*= 0.030).

### Discussion

_{k}= 1.3375), is imprecise in eyes with keratoconus, and that overestimation was observed in most cases compared to the true net power determined with the Gaussian equation. There was also a significant difference in corneal power between the conventional keratometric approach and the Gaussian equation in normal healthy eyes [19,20,21]. The results of this study were similar to the results of previous studies [16,19,20,21]. In this study, the anterior corneal power, which was estimated with a keratometric index of 1.3375, was significantly larger than the total corneal power, which is the true net power, in both groups. Park et al. [22] reported that the IOL power calculation that used the conventional keratometric approach was inaccurate and showed a hyperopic shift in patients with posterior keratoconus. This hyperopic shift occurs because the conventional keratometric approach ignores the posterior corneal surface and overestimates the corneal power.