Changes of posterior corneal astigmatism and tilt after myopic laser in situ keratomileusis

Purpose. The purpose of this study was to assess the changes of posterior corneal astigmatism and tilt after laser in situ keratomileusis (LASIK) and to correlate these changes with the amount of correction and the residual stromal bed thickness. Methods. This prospective nonrandomized (self-controlled) comparative trial included 57 eyes from 14 females and 15 males, whose mean age (± standard deviation [SD]) at the time of surgery was 33 ± 9 years (range, 19-53), with a spherical equivalent (SEQ) of -1.00 to -15.50 (mean, -5.07 ± 2.81) diopters (D). All LASIK procedures were accomplished with the Keratom II Coherent-Schwind excimer laser and the Moria Model One microkeratome (150-μm head). Subjective refractometry, Orbscan slit-scanning corneal topography analysis, 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]) or myopic astigmatism (n = 22; sphere, 0.00 to -9.75 D [mean, -4.75 ± 2.36]; cylinder, -0.75 to -3.50 D [-1.68 ± 0.86]). Intended ablation depth ranged from 12 to 108 (mean, 48 ± 22) μm. Topographic raw data were decomposed into a set of Zernike polynomials as published in detail previously, and parameters for detection of asymmetric mechanical deformation of the cornea were derived. Posterior corneal astigmatism and tilt before and after LASIK were compared, and changes in these variables were correlated with the SEQ change (ΔSEQ) and the residual corneal bed thickness (RBT). Results. The RBT after LASIK ranged from 186 to 373 (mean, 280 ± 42) μm. Overall, astigmatism (0.19 ± 0.07 D/0.22 ± 0.13 D; p = 0.80) and tilt (3.58 ± 0.35°/3.65 ± 0.48°; p = 0.61) did not change significantly by 3 months after LASIK. In eyes with RBT ≤250 μm, the average change in astigmatism (0.05 ± 0.11 versus 0.01 ± 0.13 D; p = 0.46) and tilt (0.21 ± 0.45° versus 0.04 ± 0.55°; p = 0.30) was not greater than in eyes with RBT >250 μm. Change in astigmatism (p = 0.19) and tilt (p = 0.56) did not correlate with the RBT during LASIK. Conclusions. Zernike decomposition of topographic height data discloses that no significant asymmetric mechanical deformation of the posterior corneal curvature occurs after myopic LASIK. Further studies with long-term follow-up are needed to clarify whether this symmetry of the posterior corneal surface can indeed be preserved over time after LASIK if the RBT is <250 μm.