title: Difficult lens power calculations.
abstract: OBJECTIVE
  Although cataract extraction seems to be feasible without major technical obstacles, the surgical technique has changed completely, and patients are no longer satisfied with good spectacle-corrected vision but anticipate complete visual rehabilitation after cataract surgery, without correction. To fulfill this desire, toric or accommodative intraocular lenses are of increasing popularity, and the intraocular lens power calculation after keratorefractive surgery has been improved.
  
  
  RESULTS
  In this review article, we provide an overview of different mathematical strategies of calculating the intraocular lens power with standard formulas and with new algorithms, such as paraxial or numeric ray-tracing. These enhanced techniques may improve the validity of lens power calculation due to reduction of the prediction error, especially in cases with high or excessive corneal astigmatism and after refractive laser surgery. Furthermore, a new calculation scheme for the determination of bitoric eikonic intraocular lenses allows a distortion-free imaging in astigmatic eyes. The biometric determinants for the different formulas and calculation schemes are discussed in detail.
  
  
  CONCLUSIONS
  In difficult cases, standard calculation schemes are overemployed and new mathematical algorithms are necessary to adequately address these problems. Ray-tracing algorithms and other complex mathematical computation schemes are of increasing interest and will more and more replace conventional calculation formulas for determination of intraocular lens power.
authors.affiliation: Department of Ophthalmology, University of Erlangen-Nürnberg, Erlangen, Germany. achim.langenbucher@augen.imed.uni-erlangen.de
authors.forename: Achim
authors.initials: A
authors.lastname: Langenbucher
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Haigis
authors.forename: Berthold
authors.initials: B
authors.lastname: Seitz
mesh_terms: D000065:Accommodation, Ocular; D000465:Algorithms; D002387:Cataract Extraction; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer; D019654:Lens Implantation, Intraocular; D007910:Lenses, Intraocular; D055095:Optics and Photonics; D018903:Photorefractive Keratectomy; D011474:Prosthesis Design; D054140:Refractive Surgical Procedures