title: Refractive error assessment: influence of different optical elements and current limits of biometric techniques.
abstract: OBJECTIVE
  To identify and quantify sources of error on refractive assessment using exact ray tracing.
  
  
  METHODS
  The Liou-Brennan eye model was used as a starting point and its parameters were varied individually within a physiological range. The contribution of each parameter to refractive error was assessed using linear regression curve fits and Gaussian error propagation analysis. A MonteCarlo analysis quantified the limits of refractive assessment given by current biometric measurements.
  
  
  RESULTS
  Vitreous and aqueous refractive indices are the elements that influence refractive error the most, with a 1% change of each parameter contributing to a refractive error variation of +1.60 and -1.30 diopters (D), respectively. In the phakic eye, axial length measurements taken by ultrasound (vitreous chamber depth, lens thickness, and anterior chamber depth [ACD]) were the most sensitive to biometric errors, with a contribution to the refractive error of 62.7%, 14.2%, and 10.7%, respectively. In the pseudophakic eye, vitreous chamber depth showed the highest contribution at 53.7%, followed by postoperative ACD at 35.7%. When optic measurements were considered, postoperative ACD was the most important contributor, followed by anterior corneal surface and its asphericity. A MonteCarlo simulation showed that current limits of refractive assessment are 0.26 and 0.28 D for the phakic and pseudophakic eye, respectively.
  
  
  CONCLUSIONS
  The most relevant optical elements either do not have available measurement instruments or the existing instruments still need to improve their accuracy. Ray tracing can be used as an optical assessment technique, and may be the correct path for future personalized refractive assessment.
authors: Instituto Superior Tecnico, Lisbon, Portugal. filomenajribeiro@gmail.com
authors: Filomena
authors: F
authors: Ribeiro
authors: Antonio
authors: A
authors: Castanheira-Dinis
authors: Joao Mendanha
authors: JM
authors: Dias
mesh_terms: D000867:Anterior Chamber / Q000502:physiology*; D001082:Aqueous Humor / Q000502:physiology*; D058452:Axial Length, Eye / Q000503:physiopathology; D001699:Biometry / Q000379:methods*; D003315:Cornea / Q000502:physiology*; D006801:Humans; D007908:Lens, Crystalline / Q000502:physiology*; D008954:Models, Biological; D009010:Monte Carlo Method; D019591:Pseudophakia / Q000503:physiopathology; D011680:Pupil / Q000502:physiology; D012030:Refractive Errors / Q000175:diagnosis*; D014822:Vitreous Body / Q000502:physiology*