title: Astigmatic Vector Analysis of Posterior Corneal Surface: A Comparison Among Healthy, Forme Fruste, and Overt Keratoconic Corneas.
abstract: To determine novel diagnostic parameters for keratoconus, and to assess the correlation between anterior and posterior corneal surfaces based on vectorial astigmatism analyses.
  
  
  
  Retrospective case-control study.
  
  
  
  Six hundred and ninety-eight eyes of 698 patients were enrolled in the study. Healthy corneas, or controls (C, n = 264), were compared to keratoconic corneas, further categorized as forme fruste (FFKc, n = 212) and overt keratoconus (Kc, n = 222). Corneal measurements were obtained from a Scheimpflug-based tomographer. Vectorial analyses were conducted in accordance with the method proposed by Thibos.
  
  
  
  Posterior corneal astigmatic power vector (APV) >0.23 diopter (D) yielded a test for overt Kc with sensitivity and specificity rates of, respectively, 81% and 77%, indicating a positive likelihood ratio (LR+) of 3.5 and a negative likelihood ratio (LR-) of 0.25. Posterior corneal overall blur vector (Blur) >6.45 D yielded a test slightly less sensitive and specific, with rates of 75% and 72%, respectively, associated to LR+ of 2.7 and LR- of 0.35. The highest (Spearman ρ) correlation coefficients between anterior and posterior corneal astigmatisms were associated with Blur, being 0.93 for Kc, 0.87 for C, and 0.81 for FFKc. The astigmatism vectors along the 45-degree (J45) and 0-dregree meridians (J0) and APV most often presented higher coefficient values for Kc and FFKc than for C (P = .01).
  
  
  
  Posterior corneal vectors APV and Blur constitute objective supplemental parameters for the diagnosis of Kc. Anterior and posterior corneal surfaces correlate in all groups, although it was not possible to accurately predict posterior astigmatism from anterior astigmatism.
authors.affiliation: Department of Ophthalmology and Otorhinolaryngology, University of São Paulo, São Paulo, Brazil. Electronic address: giulianofreitas@hotmail.com.
authors.forename: Giuliano de Oliveira
authors.initials: Gde O
authors.lastname: Freitas
authors.affiliation: Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil.
authors.forename: Renato
authors.initials: R
authors.lastname: Ambrósio
authors.affiliation: Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil.
authors.forename: Isaac
authors.initials: I
authors.lastname: Ramos
authors.affiliation: Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil.
authors.forename: Bernardo
authors.initials: B
authors.lastname: Lopes
authors.affiliation: Department of Ophthalmology and Otorhinolaryngology, University of São Paulo, São Paulo, Brazil; Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil.
authors.forename: Bruno de Freitas
authors.initials: Bde F
authors.lastname: Valbon
authors.affiliation: Department of Ophthalmology, Federal University of Minas Gerais, Belo Horizonte, Brazil.
authors.forename: Cristiane
authors.initials: C
authors.lastname: Botteon
authors.affiliation: Department of Ophthalmology and Otorhinolaryngology, University of São Paulo, São Paulo, Brazil; Department of Ophthalmology, Federal University of São Paulo, São Paulo, Brazil.
authors.forename: Milton Ruiz
authors.initials: MR
authors.lastname: Alve
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000175:diagnosis* / Q000503:physiopathology; D016022:Case-Control Studies; D004108:Dilatation, Pathologic / Q000175:diagnosis / Q000503:physiopathology; D004728:Endothelium, Corneal / Q000473:pathology*; D019573:Epithelium, Corneal / Q000473:pathology*; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D007640:Keratoconus / Q000175:diagnosis* / Q000503:physiopathology; D008297:Male; D012372:ROC Curve; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D041623:Tomography, Optical Coherence