**Purpose**:
To evaluate the accuracy of IOL power calculation using adjusted corneal power according to the posterior/anterior corneal curvature radii ratio.

**Methods**:
Nine hundred twenty-eight eyes from 928 reference subjects and 158 eyes from 158 cataract patients who underwent phacoemulsification surgery were enrolled. Adjusted corneal power of cataract patients was calculated using the fictitious refractive index that was obtained from the geometric mean posterior/anterior corneal curvature radii ratio of reference subjects and adjusted anterior and predicted posterior corneal curvature radii from conventional keratometry (K) using the posterior/anterior corneal curvature radii ratio. The median absolute error (MedAE) based on the adjusted corneal power was compared with that based on conventional K in the Haigis and SRK/T formulae.

**Results**:
The geometric mean posterior/anterior corneal curvature radii ratio was 0.808, and the fictitious refractive index of the cornea for a single Scheimpflug camera was 1.3275. The mean difference between adjusted corneal power and conventional K was 0.05 diopter (D). The MedAE based on adjusted corneal power (0.31 D in the Haigis formula and 0.32 D in the SRK/T formula) was significantly smaller than that based on conventional K (0.41 D and 0.40 D, respectively; *P* < 0.001 and *P* < 0.001, respectively). The percentage of eyes with refractive prediction error within ± 0.50 D calculated using adjusted corneal power (74.7%) was significantly greater than that obtained using conventional K (62.7%) in the Haigis formula (*P* = 0.029).

**Conclusions**:
IOL power calculation using adjusted corneal power according to the posterior/anterior corneal curvature radii ratio provided more accurate refractive outcomes than calculation using conventional K.

^{1–3}Of these three components, corneal power can be obtained from corneal thickness and anterior and posterior corneal curvature radii. However, in the past, clinical data regarding the posterior corneal curvature radius were not available. Thus, corneal refractive power was traditionally estimated from anterior corneal measurements, keratometry (K), using a fictitious refractive index of the cornea under the assumption that the posterior/anterior corneal curvature radii ratio is constant.

^{4}Conventional K has become the gold standard for corneal power in IOL power calculation of cataract surgery.

^{5–7}Tamaoki et al.

^{5}demonstrated that IOL power calculation using real corneal power values obtained from both the anterior and posterior corneal curvature radii improved refractive outcomes in eyes with posterior keratoconus. In contrast, there was no improvement in the accuracy of IOL power calculation in normal eyes when corneal power measurements from a Scheimpflug camera were applied directly.

^{8}

^{9}Reference subjects were selected based on the results of a single Scheimpflug examination as previously described.

^{9}Exclusion criteria included (1) age younger than 30 years, (2) a history of previous ocular surgery, (3) presence of corneal disease (such as keratoconus) or other corneal pathology that could affect Scheimpflug measurement, or (4) use of a contact lens within 3 weeks prior to measurement.

^{9}

^{10}Postoperative uncorrected distance visual acuity, BCVA, objective refraction measured with an autorefractor/keratometer (KR-8100; Topcon, Tokyo, Japan), and subjective refraction were measured at visits between 4 and 8 weeks after cataract surgery.

^{2}:

*D*is the dioptric power of the anterior corneal surface,

_{A}*n*is the refractive index of the cornea (1.376),

_{cornea}*r*is the mean anterior corneal curvature radius,

_{cornea}*D*is the dioptric power of the posterior corneal surface,

_{P}*n*is the refractive index of the aqueous (1.336),

_{aqueous}*R*is the geometric mean posterior/anterior corneal curvature radii ratio,

_{PA}*D*is the dioptric power of the total cornea,

_{Total}*T*is the mean central corneal thickness, and

*n*is the fictitious refractive index of the cornea for a single Scheimpflug camera.

_{c}*r*is the adjusted anterior corneal curvature radius,

_{A}*n*is the fictitious refractive index of the cornea for a single Scheimpflug camera,

_{c}*D*is the dioptric power of K measurements of the IOLMaster,

_{IOLMaster}*r*is the predicted posterior corneal curvature radius, and

_{P}*R*is the posterior/anterior corneal curvature radii ratio.

_{PA}^{4}

*a*

_{0},

*a*

_{1}, and

*a*

_{2}constants for the Haigis formula were calculated with linear regression analysis using the back-calculated effective lens position in order to obtain zero mean arithmetic error in IOL power prediction.

^{11}The back-calculated effective lens position was defined as the postoperatively calculated effective lens position based on preoperative K, AL, implanted IOL power, and postoperative refraction.

^{1,11,12}The Haigis constant optimization Excel spreadsheet (Microsoft, Inc., Redmond, WA, USA) was used to calculate the data-adjusted SRK/T A-constant.

^{13}

^{9,11,14}This calculator enables easy calculation of the predicted refraction and IOL power using the Haigis, Hoffer Q, and SRK/T formulae with adjusted corneal power based on the posterior/anterior corneal curvature radii ratio (Fig. 1).

*t*-tests were performed to compare mean absolute error. χ-Square tests were performed to determine whether there was a statistically significant difference in the ratio of refractive prediction error. A post hoc power analysis using the χ-square tests option of G*power (version 3.1.9.2; Franz Paul, Kiel, Germany) was conducted to determine study power.

*P*values less than 0.05 were considered statistically significant.

^{9}The geometric mean posterior/anterior corneal curvature radii ratio was 0.808. Mean anterior corneal curvature radius was 7.71 mm, and mean central corneal thickness was 0.565 mm. Based on these results, the fictitious refractive index of the cornea for a single Scheimpflug camera was calculated as 1.3275. Of 158 cataract patients, the mean age of subjects was 68.5 ± 11.0 years (range, 30–88 years). There were 89 females (56.3%) and 87 left eyes (55.1%). The mean adjusted corneal power was 44.35 ± 1.78 diopter (D), and the mean difference between adjusted corneal power and conventional K was 0.05 ± 0.21 D. Preoperative K, ACD, and AL measured via IOLMaster and anterior and posterior corneal curvature radii using a single Scheimpflug camera are shown in Table 1.

*P*< 0.001 and

*P*< 0.001, respectively). The percentage of eyes that achieved a postoperative refractive prediction error within ±0.50 D from the preoperative predicted refraction was 62.7%, when conventional K was used in the Haigis formula. This percentage improved significantly to 74.7% when adjusted corneal power was applied to the Haigis formula (

*P*= 0.029). In a post hoc power analysis, the calculated effect size from the percentage of eyes within ±0.50 D of the refractive prediction error was 0.248. The effect size of 0.248 and α of 0.05 with 158 patients led to a power of 0.88.

*P*< 0.05; Fig. 3). In the SRK/T formula, all MedAEs predicted using the true net power and total corneal refractive power were significantly greater than that predicted using conventional K, except the true net power and total corneal refractive power of the 2.0-, 3.0-, and 4.0-mm apex zone (Fig. 4).

*P*< 0.001 and

*P*< 0.001, respectively) in the upper and lower 25% of the data group. In addition, the percentage of eyes with refractive prediction error within ±0.50 D calculated using adjusted corneal power was significantly greater than that obtained using conventional K in both the Haigis and SRK/T formulae in the upper and lower 25% of the data group (Tables 3, 4).

^{15}demonstrated a significant association between the refractive prediction error and posterior corneal curvature when single Scheimpflug imaging was not included in IOL power calculation.

^{16,17}and demonstrated that using the variable keratometric index could improve refractive outcomes after multifocal IOL implantation.

^{18,19}IOL power calculation using adjusted corneal power calculated with the variable keratometric index also improved refractive outcomes in eyes with a history of previous refractive corneal surgery after cataract surgery.

^{20–22}

^{6–8,15}Whang et al.

^{23}found that K readings from the IOLMaster are appropriate for IOL power calculation, but K readings from other devices are not. In addition, the true net power map and refractive power map at various zones and rings of the single Scheimpflug camera were significantly different from the K reading from the IOLMaster.

^{6}K readings are usually taken from a 3.0- or 2.5-mm-diameter midperipheral area, but not from the most important central area.

^{24–26}Ray-tracing IOL power calculations in normal eyes showed comparable accuracy compared to the conventional formula,

^{24,25}although the refractive outcomes obtained by applying the true net power and total corneal refractive power to IOL power calculation were worse than those obtained using conventional K in this study. In addition, in eyes with prior myopic ablation, ray-tracing IOL power calculation provided accurate refractive outcomes similar to those in eyes without prior surgery.

^{26}Ray-tracing IOL power calculation does not use the fictitious refractive index for corneal power calculation.

^{26}On the other hand, the method used in this study relies on the introduced fictitious refractive index of the cornea for a single Scheimpflug camera to adjust IOLMaster K according to the posterior/anterior corneal curvature radii ratio. Therefore, the refractive outcomes from ray-tracing IOL power calculation are more accurate than the method used in this study. However, devices that perform ray-tracing IOL power calculation are not available in all eye clinics, and many ophthalmologists are most familiar with conventional formulae. Thus, we sought to reduce error caused by the conventional method by creating a method that would be useful in clinics where IOL power calculation via optical ray tracing are not available.

^{27,28}Similarly, refractive outcomes might become less accurate in eyes that deviate from the average posterior/anterior corneal curvature radii ratio. In fact, the magnitude of improvement in MedAE using adjusted corneal power increased up to 0.13 D in the Haigis formula and 0.14 D in the SRK/T formula in the upper and lower 25% of data when patients were divided into two groups according to the lower (25th) and upper quartile (75th) of the posterior/anterior corneal curvature radii ratio. In contrast, there was no improvement in MedAE in the middle 50% of the data. Consistent with this study, direct application of corneal power derived from both the anterior and posterior corneal measurements to IOL power calculation improved refractive outcomes in eyes with posterior keratoconus that deviated from the average posterior/anterior corneal curvature radii ratio

^{5}but not in normal eyes.

^{6,7,15}Thus, IOL power calculation using the adjusted corneal power developed in this study might have clinical benefit, especially in eyes that deviate from the average posterior/anterior corneal curvature radii ratio.

^{9,29,30}Thus, a prospective study with a large number of cataract eyes that have multiple measurements of a single Scheimpflug camera will be needed to address whether IOL power calculation using adjusted corneal power based on the posterior/anterior corneal curvature radii ratio is superior to that using conventional K.

**M. Kim**, None;

**Y. Eom**, None;

**H. Lee**, None;

**Y.-W. Suh**, None;

**J.S. Song**, None;

**H.M. Kim**, None

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