title: Validity of the keratometric index: evaluation by the Pentacam rotating Scheimpflug camera.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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 (rant) and posterior corneal surface (rpost), mean radius of simulated keratometry (rsimK), and central corneal thickness were obtained. The ratio of rant to rpost (AP ratio) and keratometric index were calculated in each eye.
  
  
  RESULTS
  The means for rant, rpost, rsimK, 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 (Ncal) was 1.3281 +/- 0.0018. Using the keratometric indices of 1.3281 (Ncal), 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.
  
  
  CONCLUSIONS
  Using the Pentacam-derived keratometric index improved the prediction accuracies of total and posterior corneal powers.
authors.affiliation: Department of Ophthalmology, Taipei Medical University Hospital, Taipei, Taiwan.
authors.forename: Jau-Der
authors.initials: JD
authors.lastname: Ho
authors.forename: Ching-Yao
authors.initials: CY
authors.lastname: Tsai
authors.forename: Ray Jui-Fang
authors.initials: RJ
authors.lastname: Tsai
authors.forename: Li-Lin
authors.initials: LL
authors.lastname: Kuo
authors.forename: I-Lun
authors.initials: IL
authors.lastname: Tsai
authors.forename: Shiow-Wen
authors.initials: SW
authors.lastname: Liou
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000869:Anterior Eye Segment / Q000033:anatomy & histology*; D003315:Cornea / Q000502:physiology*; D003941:Diagnostic Techniques, Ophthalmological; D004728:Endothelium, Corneal / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D008297:Male; D008433:Mathematics; D008875:Middle Aged; D010781:Photography / Q000379:methods*; D012029:Refraction, Ocular / Q000502:physiology*; D015203:Reproducibility of Results