title: Corneal changes after laser in situ keratomileusis: measurement of corneal polarization magnitude and axis.
abstract: OBJECTIVE
  Laser in situ keratomileusis (LASIK) involves ablation of the corneal stroma, which may induce a change in birefringence. The purpose of this study was to determine the effect of LASIK on corneal birefringence by measuring corneal polarization magnitude (CPM) and axis (CPA).
  
  
  METHODS
  Cohort study.
  
  
  METHODS
  In this prospective study, we measured the change in CPM and CPA before and after LASIK with a scanning laser polarimeter ([SLP] GDx-VCC; Laser Diagnostic Technologies, San Diego, California). Scans were completed on 23 subjects before and 3 months after LASIK. 14 normal controls were tested twice during the same time interval. Change in CPM, CPA, corneal thickness, and corneal curvature measurements were compared between LASIK and normal subjects.
  
  
  RESULTS
  At baseline, the mean (95% confidence interval) values of CPM, CPA, corneal thickness, and corneal curvature measurements of the total population (n = 37) were 41.6 nm (36.6, 46.5); 31.5 degrees (25.7, 37.3); 548.4 microm (540.0, 556.7); and 7.6 mm (7.5, 7.7), respectively. There were no significant differences in baseline values between normal and LASIK subjects. The reproducibility, measured as the average standard deviation of CPM and CPA measurements in 30 normal control eyes, was 1.95 nm (1.43, 2.48) and 1.69 degrees (0.92, 2.46), respectively. Mean CPA, corneal thickness, and corneal curvature measurements were significantly different in patients after LASIK (all P <.0001). Mean absolute values of the change in both CPM and CPA were significantly greater in LASIK patients (4.8 nm [3.3, 6.4], and 10.4 degrees [6.8, 14.1], respectively) than in normal subjects (2.43 nm [1.53, 3.33], and 1.64 degrees [1.15, 2.14], respectively; both P < or =.05). The absolute value of change in CPA was linearly associated with the absolute value of change in both corneal thickness (R(2) = 0.46) and corneal curvature (R(2) = 0.44).
  
  
  CONCLUSIONS
  LASIK causes a measurable change in corneal birefringence as measured by the CPM and CPA that may be related to loss of corneal tissue. Comparison of SLP measurements before and after LASIK requires eye-specific compensation to adjust for the change in corneal birefringence.
authors.affiliation: Hamilton Glaucoma Center and the Department of Ophthalmology, University of California, San Diego, California 92093, USA.
authors.forename: Raymund
authors.initials: R
authors.lastname: Angeles
authors.forename: Teresa
authors.initials: T
authors.lastname: Abunto
authors.forename: Christopher
authors.initials: C
authors.lastname: Bowd
authors.forename: Linda M
authors.initials: LM
authors.lastname: Zangwill
authors.forename: David J
authors.initials: DJ
authors.lastname: Schanzlin
authors.forename: Robert N
authors.initials: RN
authors.lastname: Weinreb
mesh_terms: D000328:Adult; D001718:Birefringence; D015331:Cohort Studies; D003315:Cornea / Q000502:physiology* / Q000601:surgery; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D007834:Lasers; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D011446:Prospective Studies; D012029:Refraction, Ocular / Q000502:physiology*