title: Placido disk-based topography versus high-resolution rotating Scheimpflug camera for corneal power measurements in keratoconic and post-LASIK eyes: reliability and agreement.
abstract: OBJECTIVE
  To compare the repeatability/reproducibility of measurement by high-resolution Placido disk-based topography with that of a high-resolution rotating Scheimpflug camera and assess the agreement between the two instruments in measuring corneal power in eyes with keratoconus and post-laserin situkeratomileusis (LASIK).
  
  
  METHODS
  One eye each of 36 keratoconic patients and 20 subjects who had undergone LASIK was included in this prospective observational study. Two independent examiners worked in a random order to take three measurements of each eye with both instruments. Four parameters were measured on the anterior cornea: steep keratometry (Ks), flat keratometry (Kf), mean keratometry (Km), and astigmatism (Ks-Kf). Intra-examiner repeatability and inter-examiner reproducibility were evaluated by calculating the within-subject standard deviation (Sw) the coefficient of repeatability (R), the coefficient of variation (CoV), and the intraclass correlation coefficient (ICC). Agreement between instruments was tested with the Bland-Altman method by calculating the 95% limits of agreement (95% LoA).
  
  
  RESULTS
  In keratoconic eyes, the intra-examiner and inter-examiner ICC were >0.95. As compared with measurement by high-resolution Placido disk-based topography, the intra-examiner R of the high-resolution rotating Scheimpflug camera was lower for Kf (0.32vs0.88), Ks (0.61vs0.88), and Km (0.32vs0.84) but higher for Ks-Kf (0.70vs0.57). Inter-examiner R values were lower for all parameters measured using the high-resolution rotating Scheimpflug camera. The 95% LoA were -1.28 to +0.55 for Kf, -1.36 to +0.99 for Ks, -1.08 to +0.50 for Km, and -1.11 to +1.48 for Ks-Kf. In the post-LASIK eyes, the intra-examiner and inter-examiner ICC were >0.87 for all parameters. The intra-examiner and inter-examiner R were lower for all parameters measured using the high-resolution rotating Scheimpflug camera. The intra-examiner R was 0.17vs0.88 for Kf, 0.21vs0.88 for Ks, 0.17vs0.86 for Km, and 0.28vs0.33 for Ks-Kf. The inter-examiner R was 0.09vs0.64 for Kf, 0.15vs0.56 for Ks, 0.09vs0.59 for Km, and 0.18vs0.23 for Ks-Kf. The 95% LoA were -0.54 to +0.58 for Kf, -0.51 to +0.53 for Ks and Km, and -0.28 to +0.27 for Ks-Kf.
  
  
  CONCLUSIONS
  As compared with Placido disk-based topography, the high-resolution rotating Scheimpflug camera provides more repeatable and reproducible measurements of Ks, Kf and Ks in keratoconic and post-LASIK eyes. Agreement between instruments is fair in keratoconus and very good in post-LASIK eyes.
authors.affiliation: Department of Surgical Sciences, Eye Clinic, University of Torino, Via Juvarra 19, Torino 10122, Italy.
authors.forename: Rachele R
authors.initials: RR
authors.lastname: Penna
authors.affiliation: Department of Surgical Sciences, Eye Clinic, University of Torino, Via Juvarra 19, Torino 10122, Italy.
authors.forename: Ugo
authors.initials: U
authors.lastname: de Sanctis
authors.affiliation: Department of Surgical Sciences, Eye Clinic, University of Torino, Via Juvarra 19, Torino 10122, Italy.
authors.forename: Martina
authors.initials: M
authors.lastname: Catalano
authors.affiliation: Department of Surgical Sciences, Eye Clinic, University of Torino, Via Juvarra 19, Torino 10122, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Brusasco
authors.affiliation: Department of Surgical Sciences, Eye Clinic, University of Torino, Via Juvarra 19, Torino 10122, Italy.
authors.forename: Federico M
authors.initials: FM
authors.lastname: Grignolo
keywords: Placido disk-based topography; Scheimpflug camera; corneal power measurement; keratoconus; post-LASIK