### Abstract

Purpose: To determine the keratometric index based on actual measurements of the anterior and posterior corneal surfaces using a rotating Scheimpflug camera (Pentacam, Oculus) and evaluate the accuracy of this keratometric index in estimating total and posterior corneal powers. Setting: Departments of Ophthalmology, Taipei Medical University Hospital and Taipei City Hospital, Taipei, Taiwan. Methods: The right eye of 221 subjects was measured with the Pentacam system. The radius of the best-fit sphere for the anterior corneal surface (r_{ant}) and posterior corneal surface (r_{post}), mean radius of simulated keratometry (r_{simK}), and central corneal thickness were obtained. The ratio of r_{ant} to r_{post} (AP ratio) and keratometric index were calculated in each eye. Results: The means for r_{ant}, r_{post}, r_{simK}, and AP ratio were 7.75 mm ± 0.28 (SD), 6.34 ± 0.28 mm, 7.75 ± 0.27 mm, and 1.223 ± 0.034 mm, respectively. These parameters were normally distributed. The mean calculated keratometric index (N_{cal}) was 1.3281 ± 0.0018. Using the keratometric indices of 1.3281 (N_{cal}), 1.3315 (Gullstrand schematic eye), and 1.3375 (conventional), the mean arithmetic and absolute estimation errors for the total corneal power were, 0.00 ± 0.24 diopter (D) and 0.17 ± 0.17 D, 0.43 ± 0.23 D and 0.45 ± 0.21 D, and 1.21 ± 0.24 D and 1.21 ± 0.24 D, respectively. The total corneal power was predicted to within ±0.50 D of the actual value in 95.0%, 60.2%, and 0.9% of eyes, respectively. The mean arithmetic and absolute estimation errors for the posterior corneal power using an AP ratio of 1.223 (this study) or 1.132 (Gullstrand schematic eye) were 0.00 ± 0.17 D and 0.13 ± 0.12 D and 0.47 ± 0.18 D and 0.47 ± 0.17 D, respectively. The posterior corneal power was estimated to within ±0.50 D of the actual value in 97.7% and 60.2% of eyes, respectively. Conclusion: Using the Pentacam-derived keratometric index improved the prediction accuracies of total and posterior corneal powers.

Original language | English |
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Pages (from-to) | 137-145 |

Number of pages | 9 |

Journal | Journal of Cataract and Refractive Surgery |

Volume | 34 |

Issue number | 1 |

DOIs | |

Publication status | Published - Jan 2008 |

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### ASJC Scopus subject areas

- Ophthalmology

### Cite this

*Journal of Cataract and Refractive Surgery*,

*34*(1), 137-145. https://doi.org/10.1016/j.jcrs.2007.09.033