Posterior corneal curvature changes after myopic laser in situ keratomileusis

Objective: To assess the posterior corneal power and asphericity changes after myopic laser in situ keratomileusis (LASIK) and to correlate these changes 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-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 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 108 (mean, 48 ± 22) μm. Topographic raw data were decomposed into a set of Zernike polynomials as published in detail previously, and parameters potentially indicative for detection of a "mild keratectasia" were derived. Main Outcome Measures: Posterior central corneal power and asphericity before and after LASIK were compared, and changes of these variables were correlated with the SEQ change (ΔSEQ) and the residual corneal bed thickness (RBT). Results: The mean RBT after LASIK was 280 ± 42 μm. Overall, change of posterior power (-6.28 ± 0.22 D/-6.39 ± 0.23 D, P = 0.02) was statistically significant, and change of asphericity (0.98 ± 0.07/1.14 ± 0.20, P < 0.0001) was highly significant. In eyes with RBT ≤ 250 μm, 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 μm (P = 0.003). The change of posterior corneal power correlated significantly with ΔSEQ (P = 0.004) and the RBT (P = 0.002). Conclusions: Increased negative keratometric diopters and oblate asphericity of the posterior corneal curvature suggest that mild "keratectasia" of the cornea may be common early after LASIK. Further studies with longer follow-up are required to clarify whether this biomechanical deformation is progressive and whether a residual bed thickness of >250 μm can completely prevent it.