title: New approach for correction of error associated with keratometric estimation of corneal power in keratoconus.
abstract: OBJECTIVE
  The aim of this study was to obtain the exact value of the keratometric index (nkexact) and to clinically validate a variable keratometric index (nkadj) that minimizes this error.
  
  
  METHODS
  The nkexact value was determined by obtaining differences (ΔPc) between keratometric corneal power (Pk) and Gaussian corneal power ((Equation is included in full-text article.)) equal to 0. The nkexact was defined as the value associated with an equivalent difference in the magnitude of ΔPc for extreme values of posterior corneal radius (r2c) for each anterior corneal radius value (r1c). This nkadj was considered for the calculation of the adjusted corneal power (Pkadj). Values of r1c ∈ (4.2, 8.5) mm and r2c ∈ (3.1, 8.2) mm were considered. Differences of True Net Power with (Equation is included in full-text article.), Pkadj, and Pk(1.3375) were calculated in a clinical sample of 44 eyes with keratoconus.
  
  
  RESULTS
  nkexact ranged from 1.3153 to 1.3396 and nkadj from 1.3190 to 1.3339 depending on the eye model analyzed. All the nkadj values adjusted perfectly to 8 linear algorithms. Differences between Pkadj and (Equation is included in full-text article.)did not exceed ±0.7 D (Diopter). Clinically, nk = 1.3375 was not valid in any case. Pkadj and True Net Power and Pk(1.3375) and Pkadj were statistically different (P < 0.01), whereas no differences were found between (Equation is included in full-text article.)and Pkadj (P > 0.01).
  
  
  CONCLUSIONS
  The use of a single value of nk for the calculation of the total corneal power in keratoconus has been shown to be imprecise, leading to inaccuracies in the detection and classification of this corneal condition. Furthermore, our study shows the relevance of corneal thickness in corneal power calculations in keratoconus.
authors.affiliation: *Department of Optics, Pharmacology and Anatomy, Grupo de Óptica y Percepción Visual (GOPV), University of Alicante, Alicante, Spain; †Department of Ophthalmology (OFTALMAR), Medimar International Hospital, Alicante, Spain; and ‡Fundación para la Calidad Visual (FUNCAVIS), Alicante, Spain.
authors.forename: Vicente J
authors.initials: VJ
authors.lastname: Camps
authors.forename: David P
authors.initials: DP
authors.lastname: Piñero
authors.forename: Esteban
authors.initials: E
authors.lastname: Caravaca-Arens
authors.forename: Dolores
authors.initials: D
authors.lastname: de Fez
authors.forename: Rafael J
authors.initials: RJ
authors.lastname: Pérez-Cambrodí
authors.forename: Alberto
authors.initials: A
authors.lastname: Artola
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000465:Algorithms; D001699:Biometry; D003315:Cornea / Q000503:physiopathology*; D019781:Corneal Topography; D005260:Female; D006801:Humans; D007640:Keratoconus / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D016011:Normal Distribution; D012029:Refraction, Ocular / Q000502:physiology*