Posterior corneal curvature changes after myopic laser in situ keratomileusis

Objective: To assess the posterior corneal power and asphericity changes af ter myopic laser in situ keratomileusis (LASIK) and to correlate these chan ges with the amount of correction and the residual stromal bed thickness. Design: Prospective nonrandomized (self-controlled) comparative study. Participants: Fifty-seven eyes of 14 women and 15 men, mean age at the time of surgery 33 +/- 9 (range, 19-53) years with a spherical equivalent (SEQ) of -1.00 to -15.50 (mean, -5.07 +/- 2.81) diopters (D). Intervention: All procedures were accomplished with the Keratom II Coherent -Schwind excimer laser and the Moria Model One microkeratome (150-mum head) . Subjective refractometry, Orbscan slit scanning corneal topography analys is and pachymetry were performed before and 3 months after LASIK for myopia (n = 35, -1.00 to -15.50 D, mean -4.75 +/- 3.07 D) or myopic astigmatism ( n = 22, sphere 0.00 to -9.75 D, mean -4.75 +/- 2.36 D; cylinder -0.75 to -3 .50 D, mean -1.68 +/- 0.86 D). Intended ablation depth ranged from 12 to 10 8 (mean, 48 +/- 22) mum. Topographic raw data were decomposed into a set of Zernike polynomials as published in detail previously, and parameters pote ntially indicative for detection of a "mild keratectasia" were derived. Main Outcome Measures: Posterior central corneal power and asphericity befo re and after LASIK were compared, and changes of these variables were corre lated with the SEQ change (Delta SEQ) and the residual corneal bed thicknes s (RBT). Results: The mean RBT after LASIK was 280 +/- 42 mum. Overall, change of po sterior power (-6.28 +/- 0.22 D/-6.39 +/- 0.23 D, P = 0.02) was statistical ly significant, and change of asphericity (0.98 +/- 0.07/1.14 +/- 0.20, P < 0.0001) was highly significant. In eyes with RBT <less than or equal to> 2 50 mum, the average change of posterior central power (-0.20 +/- 0.10 D vs. -0.08 +/- 0.18 D) was significantly greater than in eyes with RBT >250 mum (P = 0.003). The change of posterior corneal power correlated significantl y with Delta SEQ (P = 0.004) and the RBT (P = 0.002). Conclusions: Increased negative keratometric diopters and oblate asphericit y of the posterior corneal curvature suggest that mild "keratectasia" of th e cornea may be common early after LASIK. Further studies with longer follo w-up are required to clarify whether this biomechanical deformation is prog ressive and whether a residual bed thickness of >250 mum can completely pre vent it. (C) 2001 by the American Academy of Ophthalmology.