title: Comparison of axial and tangential topographic algorithms for contact lens fitting after LASIK.
abstract: OBJECTIVE
  Computerized videokeratography (CVK) has become the standard method for measuring the corneal curvature after refractive surgery. However, few resources exist on the use of CVK data for postoperative contact lens fitting, and no recommendations exist on the selection of the best topographic algorithm relevant to gas-permeable (GP) lens fitting. This study assessed the optimal use of topographic data to fit spherical GP contact lenses on patients who have undergone laser in situ keratomileusis.
  
  
  METHODS
  A retrospective analysis of CVK maps from eight post-LASIK eyes fitted with spherical GP lenses was performed in the Contact Lens Service of the Department of Ophthalmology at Case Western Reserve University and University Hospitals of Cleveland. Axial and tangential maps from the Dicon CT 200 (version 3.50) or Humphrey Atlas (version A8) topographer were analyzed. Averaged dioptric curvatures from different locations (inferior, superior, and temporal) at various distances (2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, and 4.0 mm) from the vertex normal of each eye were compared with the base curves of the prescribed GP contact lenses.
  
  
  RESULTS
  One-way analysis of variance, Pearson correlation analysis, and paired t tests showed that the best topographic predictors of a successful GP base curve were the average curvatures at the 4.0 mm distance on the axial maps (r=0.8078, P=0.05) and at the 2.0 mm distance on the tangential maps (r=0.9738, P=0.0002). The mean dioptric powers of the GP base curve, axial map 4.0 mm curvature, and tangential map 2.0 mm curvature were 41.50 diopters (D), 42.65 D, and 42.67 D, respectively.
  
  
  CONCLUSIONS
  To simplify and guide GP fitting after LASIK, the average dioptric curvature 4.0 mm from the vertex normal on axial maps or 2.0 mm from the vertex normal on tangential maps are the best predictors of accurate GP base curve selection.
authors.affiliation: Department of Ophthalmology, University Hospital of Cleveland, Cleveland, OH 44106, USA. loretta.szczotka@uhhs.com
authors.forename: Loretta
authors.initials: L
authors.lastname: Szczotka-Flynn
authors.forename: Binoy R
authors.initials: BR
authors.lastname: Jani
mesh_terms: D000465:Algorithms; D003262:Contact Lenses, Extended-Wear; D003315:Cornea / Q000033:anatomy & histology*; D019781:Corneal Topography / Q000379:methods*; D005500:Follow-Up Studies; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D011184:Postoperative Period; D017755:Prosthesis Fitting / Q000379:methods; D012189:Retrospective Studies