title: Corneal ray tracing versus simulated keratometry for estimating corneal power changes after excimer laser surgery.
abstract: OBJECTIVE
  To evaluate whether the refractive changes induced by excimer laser surgery can be accurately measured by corneal ray tracing performed by a combined rotating Scheimpflug camera-Placido-disk corneal topographer (Sirius).
  
  
  METHODS
  Private practices.
  
  
  METHODS
  Evaluation of diagnostic test.
  
  
  METHODS
  This multicenter retrospective study comprised patients who had myopic or hyperopic excimer laser refractive surgery. Preoperatively and postoperatively, 2 corneal power measurements--simulated keratometry (K) and mean pupil power--were obtained. The mean pupil power was the corneal power calculated over the entrance pupil by ray tracing through the anterior and posterior corneal surfaces using Snell's law. Agreement between the refractive and corneal power change was analyzed according to Bland and Altman. Regression analysis and Bland-Altman plots were used to evaluate agreement between measurements.
  
  
  RESULTS
  The study evaluated 72 eyes (54 patients). The difference between the postoperative and preoperative simulated K values underestimated the refractive change after myopic correction and overestimated it after hyperopic correction. Agreement between simulated K changes and refractive changes was poor, especially for higher amounts of correction. A proportional bias was detected (r = -0.77; P<.0001), and the 95% limits of agreement (LoA) were -0.15 -0.14 × ±0.62 diopters (D). The difference between the postoperative and preoperative mean pupil power showed an excellent correlation with the refractive change (r(2) = 0.98). The mean pupil power did not overestimate or underestimate the refractive change. The 95% LoA ranged between -0.97 D and +0.56 D.
  
  
  CONCLUSIONS
  Corneal ray tracing accurately measured corneal power changes after excimer laser refractive surgery.
  
  
  BACKGROUND
  Dr. Calossi is consultant to Costruzione Strumenti Oftalmici. Dr. Carones is consultant to Wavelight Laser Technologie AG. No other author has a financial or proprietary interest in any material or method mentioned.
authors.affiliation: From the Studio Oculistico d'Azeglio (Savini), Bologna, the Department of Physics (Optics and Optometry) (Calossi), University of Florence, Florence, Sekal Microchirurgia Rovigo (Camelin), Rovigo, Carones Ophthalmology Center (Carones), Milan, and Studio Oculistico Fantozzi (Fantozzi), Pescia, Italy; the School of Optometry and Ophthalmology and Eye Hospital (Savini), Wenzhou Medical University, Wenzhou, Zhejiang, China; the Department of Ophthalmology (Hoffer), University of California, Los Angeles, and St. Mary's Eye Center, Santa Monica, California, USA. Electronic address: giacomo.savini@alice.it.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Savini
authors.affiliation: From the Studio Oculistico d'Azeglio (Savini), Bologna, the Department of Physics (Optics and Optometry) (Calossi), University of Florence, Florence, Sekal Microchirurgia Rovigo (Camelin), Rovigo, Carones Ophthalmology Center (Carones), Milan, and Studio Oculistico Fantozzi (Fantozzi), Pescia, Italy; the School of Optometry and Ophthalmology and Eye Hospital (Savini), Wenzhou Medical University, Wenzhou, Zhejiang, China; the Department of Ophthalmology (Hoffer), University of California, Los Angeles, and St. Mary's Eye Center, Santa Monica, California, USA.
authors.forename: Antonio
authors.initials: A
authors.lastname: Calossi
authors.affiliation: From the Studio Oculistico d'Azeglio (Savini), Bologna, the Department of Physics (Optics and Optometry) (Calossi), University of Florence, Florence, Sekal Microchirurgia Rovigo (Camelin), Rovigo, Carones Ophthalmology Center (Carones), Milan, and Studio Oculistico Fantozzi (Fantozzi), Pescia, Italy; the School of Optometry and Ophthalmology and Eye Hospital (Savini), Wenzhou Medical University, Wenzhou, Zhejiang, China; the Department of Ophthalmology (Hoffer), University of California, Los Angeles, and St. Mary's Eye Center, Santa Monica, California, USA.
authors.forename: Massimo
authors.initials: M
authors.lastname: Camellin
authors.affiliation: From the Studio Oculistico d'Azeglio (Savini), Bologna, the Department of Physics (Optics and Optometry) (Calossi), University of Florence, Florence, Sekal Microchirurgia Rovigo (Camelin), Rovigo, Carones Ophthalmology Center (Carones), Milan, and Studio Oculistico Fantozzi (Fantozzi), Pescia, Italy; the School of Optometry and Ophthalmology and Eye Hospital (Savini), Wenzhou Medical University, Wenzhou, Zhejiang, China; the Department of Ophthalmology (Hoffer), University of California, Los Angeles, and St. Mary's Eye Center, Santa Monica, California, USA.
authors.forename: Francesco
authors.initials: F
authors.lastname: Carones
authors.affiliation: From the Studio Oculistico d'Azeglio (Savini), Bologna, the Department of Physics (Optics and Optometry) (Calossi), University of Florence, Florence, Sekal Microchirurgia Rovigo (Camelin), Rovigo, Carones Ophthalmology Center (Carones), Milan, and Studio Oculistico Fantozzi (Fantozzi), Pescia, Italy; the School of Optometry and Ophthalmology and Eye Hospital (Savini), Wenzhou Medical University, Wenzhou, Zhejiang, China; the Department of Ophthalmology (Hoffer), University of California, Los Angeles, and St. Mary's Eye Center, Santa Monica, California, USA.
authors.forename: Marco
authors.initials: M
authors.lastname: Fantozzi
authors.affiliation: From the Studio Oculistico d'Azeglio (Savini), Bologna, the Department of Physics (Optics and Optometry) (Calossi), University of Florence, Florence, Sekal Microchirurgia Rovigo (Camelin), Rovigo, Carones Ophthalmology Center (Carones), Milan, and Studio Oculistico Fantozzi (Fantozzi), Pescia, Italy; the School of Optometry and Ophthalmology and Eye Hospital (Savini), Wenzhou Medical University, Wenzhou, Zhejiang, China; the Department of Ophthalmology (Hoffer), University of California, Los Angeles, and St. Mary's Eye Center, Santa Monica, California, USA.
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Hoffer
mesh_terms: D000328:Adult; D003198:Computer Simulation; D003315:Cornea / Q000502:physiology*; D048988:Corneal Surgery, Laser / Q000379:methods*; D019781:Corneal Topography; D005260:Female; D006801:Humans; D006956:Hyperopia / Q000503:physiopathology / Q000601:surgery*; D054018:Lasers, Excimer / Q000627:therapeutic use*; D008297:Male; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D012029:Refraction, Ocular / Q000502:physiology; D012189:Retrospective Studies; D014792:Visual Acuity / Q000502:physiology