title: Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia.
abstract: OBJECTIVE
  Because keratometry readings may no accurately reflect the refractive changes after keratorefractive surgery for myopia, better methods for the assessment of corneal curvature in the postsurgical cornea are needed.
  
  
  METHODS
  We developed a procedure to calculate the average central power (ACP) of the cornea within the entrance pupil from videokeratography. Videokeratograph-derived keratometry-style readings (average K; K) and calculated ACPs, as well as the differences between the two values, were compared in four groups: normal corneas (n = 30), corneas with regular astigmatism (n = 30); post-radial keratotomy corneas (RK, n = 85), and post-excimer laser photorefractive keratectomy corneas (PRK, n = 63). Intraocular lens (IOL) powers calculated by using K or ACP in the Sanders-Retzlaff-Kraff formula were compared.
  
  
  RESULTS
  In the groups with normal corneas or regular astigmatism, none of the eyes showed a difference between K and ACP > 0.25 D. However, six (7%) of the RK eyes and 16 (25%) of the PRK eyes had differences > 0.55 D; in these eyes, the disparity between IOL powers calculated by using K and IOL powers calculated by using ACP was > 0.5 D.
  
  
  CONCLUSIONS
  These results suggest that basing the calculation of IOL powers on keratometry readings in patients who have undergone RK. PRK, or possibly other refractive procedures may result in a residual refractive error in some eyes. In particular, patients undergoing surgery involving a small optical zone or large attempted correction or both, as well as those with low postoperative keratometry readings, may be at risk for this problem if IOL placement becomes necessary in later years.
authors.affiliation: Lions Eye Research Laboratories, LSU Eye Center, Louisiana State University Medical Center School of Medicine, New Orleans 70112, USA.
authors.forename: N
authors.initials: N
authors.lastname: Maeda
authors.forename: S D
authors.initials: SD
authors.lastname: Klyce
authors.forename: M K
authors.initials: MK
authors.lastname: Smolek
authors.forename: M B
authors.initials: MB
authors.lastname: McDonald
mesh_terms: D003315:Cornea / Q000473:pathology / Q000503:physiopathology* / Q000601:surgery; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D007646:Keratotomy, Radial; D054018:Lasers, Excimer; D007910:Lenses, Intraocular; D009216:Myopia / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D018903:Photorefractive Keratectomy; D011680:Pupil / Q000502:physiology*; D012029:Refraction, Ocular / Q000502:physiology