title: Corneal Astigmatism Measurement by Ray Tracing Versus Anterior Surface-Based Keratometry in Candidates for Toric Intraocular Lens Implantation.
abstract: OBJECTIVE
  To compare corneal astigmatism derived from total corneal refractive power (CATCRP), a ray-tracing method, with that derived from simulated keratometry (CASimK), an anterior surface-based method, in candidates for toric intraocular lens (IOL) implantation.
  
  
  METHODS
  Reliability analysis.
  
  
  METHODS
  In 1 eye of 200 consecutive patients (mean age 71 ± 9 years) with cataract, a Pentacam HR (Oculus) was used to measure CATCRPand CASimK. Eyes with CATCRP>1 diopter (D) were defined as candidates for toric IOL implantation. The estimation difference between CATCRPand CASimKwas analyzed by calculating the arithmetic, absolute, and vector differences between the 2 methods.
  
  
  RESULTS
  Seventy-seven candidates were identified. In the candidates, CASimKwas <1 D in 22% of cases and underestimated the astigmatism magnitude, on average, by -0.15 ± 0.34 D. The mean absolute and vector difference between CATCRPand CASimKwas 0.31 ± 0.29 D and 0.30 ± 0.29 D × 180 degrees, respectively. The absolute and vector differences between the 2 methods were >0.50 D in 21 of 77 (27.2%) and 25 of 77 (32.5%) eyes, respectively; the proportions were significantly (P < .001) higher than in the eyes with CATCRP≤1 D (0.8% and 9.8%, respectively). The difference in steep meridian alignment between CATCRPand CASimKwas >5 degrees in 22.1% and >10 degrees in 2.6% of cases.
  
  
  CONCLUSIONS
  The difference between CATCRPand CASimK, as calculated by Pentacam HR, is greater in candidates for toric IOL implantation than in the general population. That difference considerably influences the candidate and toric IOL power selection in a large proportion of cases.
authors.affiliation: Department of Surgical Sciences - Ophthalmology Institute, University of Turin, Turin, Italy.
authors.forename: Ugo
authors.initials: U
authors.lastname: de Sanctis
authors.affiliation: Department of Surgical Sciences - Ophthalmology Institute, University of Turin, Turin, Italy.
authors.forename: Piero
authors.initials: P
authors.lastname: Donna
authors.affiliation: Department of Surgical Sciences - Ophthalmology Institute, University of Turin, Turin, Italy. Electronic address: rachepenna@gmail.com.
authors.forename: Rachele R
authors.initials: RR
authors.lastname: Penna
authors.affiliation: Department of Mathematical Sciences - Politecnico, University of Turin, Turin, Italy.
authors.forename: Maria Isabella
authors.initials: MI
authors.lastname: Calastri
authors.affiliation: Department of Surgical Sciences - Ophthalmology Institute, University of Turin, Turin, Italy.
authors.forename: Chiara M
authors.initials: CM
authors.lastname: Eandi
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001251:Astigmatism / Q000175:diagnosis* / Q000601:surgery; D003315:Cornea / Q000473:pathology*; D019781:Corneal Topography / Q000379:methods*; D005260:Female; D006801:Humans; D019654:Lens Implantation, Intraocular / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011474:Prosthesis Design; D012029:Refraction, Ocular / Q000502:physiology*; D015203:Reproducibility of Results; D014792:Visual Acuity