title: Correction of low levels of astigmatism.
abstract: OBJECTIVE
  To reevaluate the analysis of the correction of astigmatism.
  
  
  METHODS
  Academia, industry, and private practice.
  
  
  METHODS
  Evaluation of diagnostic test or technology.
  
  
  METHODS
  Astigmatic refractive surgery outcomes are based on vector methods, including the correction index (also known as the correction ratio), which is the ratio of the surgically induced astigmatism to the target induced astigmatism (TIA). Mean correction indices substantially greater than 1 have been reported for astigmatic corrections less than 1.00 diopter (D) and as representing systematic overcorrection. We hypothesize that this reflects a limitation of the correction index rather than systematic flaws in treatments. The theoretical mathematic behavior of the correction index was analyzed, accounting for variability in astigmatism measurement. Then, the impact of cylinder measurement variability on the mean correction index was modeled. A Monte Carlo simulation was performed and calculated 10 000 values of correction index for various values of TIA. Finally, correction indices from published and unpublished studies of refractive lasers were compared with the simulations.
  
  
  RESULTS
  The mean correction index is always greater than 1 for the case of a perfect refractive correction; however, for astigmatic corrections less than 1.00 D, the mean correction index increases sharply because the measurement variability is similar in magnitude to TIA. Almost all previous studies show the predicted increase in the correction index for low astigmatic corrections.
  
  
  CONCLUSIONS
  The correction index is a useful vector-based metric for the evaluation of refractive procedures, but mean values greater than 1 should be anticipated for lower astigmatic treatments and do not necessarily represent systematic overcorrection.
  
  
  BACKGROUND
  Dr. Bullimore is a consultant to Alcon Surgical, Inc., Carl Zeiss Meditec AG, Digital Vision Systems, Essilor, Innovega, Inc., and Paragon Vision Sciences, Inc. Dr. Spooner is a consultant to Alcon Surgical, Inc., Carl Zeiss Meditec AG, Digital Vision Systems, Thru-Focus Optics LLC, and i2eyediagnostics, Ltd. Dr. Dishler is a consultant to Carl Zeiss Meditec AG and Revision Optics, Inc. Dr. Sluyterman is an employee of Carl Zeiss Meditec AG.
authors.affiliation: From the College of Optometry (Bullimore), University of Houston, Houston, Texas, Gain Consulting Services (Spooner), San Francisco, California, and the Dishler Laser Institute (Dishler), Greenwood Village, Colorado, USA; Carl Zeiss Meditec AG (Sluyterman), Jena, Germany. Electronic address: bullers2020@gmail.com.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Bullimore
authors.affiliation: From the College of Optometry (Bullimore), University of Houston, Houston, Texas, Gain Consulting Services (Spooner), San Francisco, California, and the Dishler Laser Institute (Dishler), Greenwood Village, Colorado, USA; Carl Zeiss Meditec AG (Sluyterman), Jena, Germany.
authors.forename: Greg
authors.initials: G
authors.lastname: Spooner
authors.affiliation: From the College of Optometry (Bullimore), University of Houston, Houston, Texas, Gain Consulting Services (Spooner), San Francisco, California, and the Dishler Laser Institute (Dishler), Greenwood Village, Colorado, USA; Carl Zeiss Meditec AG (Sluyterman), Jena, Germany.
authors.forename: Georg
authors.initials: G
authors.lastname: Sluyterman
authors.affiliation: From the College of Optometry (Bullimore), University of Houston, Houston, Texas, Gain Consulting Services (Spooner), San Francisco, California, and the Dishler Laser Institute (Dishler), Greenwood Village, Colorado, USA; Carl Zeiss Meditec AG (Sluyterman), Jena, Germany.
authors.forename: Jon G
authors.initials: JG
authors.lastname: Dishler
mesh_terms: D001251:Astigmatism / Q000175:diagnosis / Q000503:physiopathology / Q000601:surgery*; D001699:Biometry; D003198:Computer Simulation; D003315:Cornea / Q000503:physiopathology*; D006801:Humans; D019654:Lens Implantation, Intraocular; D009010:Monte Carlo Method; D018918:Phacoemulsification; D019591:Pseudophakia / Q000503:physiopathology; D012029:Refraction, Ocular / Q000502:physiology; D014792:Visual Acuity / Q000502:physiology