Ablation induced decentration in post-LASIK eyes

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Objective To study cases with visual complaints caused by ablation induced decentration after traditional laser in situ keratomileusis (LASIK) and wavefront-guided LASIK. To evalulate the distribution of higher-order aberrations and the degree of decentered ablation. Methods This was a retrospective case control study. Forty-eight eyes of 40 patients with double vision,starburst and other visual symptoms were checked with WaveScan (3.62 version) after LASIK. All postoperative cases were followed for more than 3 months. Their refractive errors were within ±1.0 D and stable except for overcorrection, under correction, and other eye diseases. The period of observation was (9.3±5.7)months (range: 4-26 months) for the wavefront-guided LASIK group, and (24.1±22.7)months (range: 5-72 months) for the traditional LASIK group. For this study, 28 eyes of 26 patients were selected whose dominant aberrations were vertical or horizontal comas due to decentered ablation after LASIK surgery. Higher-order aberrations in post-LASIK eyes were analyzed.The variations between the ablation center with the pupil center were measured and plotted on the horizontal (X-axis) and vertical (Y-axis) and the vector shift was drawn. The preoperative and postoperative results of the wavefront-guided LASIK group were compared (using a paired samples t test or Wilcoxon signed rank test). The postoperative results for the wavefront-guided LASIK group and tranditional LASIK group were also compared (using an independent samples t test or a Mann-Whitney test). Results The higher-order aberration root mean squares (RMS) value for the postoperative wavefront-guided eyes was (1.262±0.590)μm, which was 1.96 times greater than the average for preoperative eyes. The total coma RMS value of the postoperative wavefront-guided eyes was (0.686±0.315)μm, which was 2.61 times greater than the average for preoperative eyes. There was a 3.51 time increase in vertical coma and a 1.17 time increase in horizontal coma after LASIK.The horizontal coma after LASIK was significantly less than the vertical coma (Z=-2.803, P=0.004).The difference of total higher order aberrations and total coma between traditional LASIK and wavefront-guided LASIK was without any significance, however, the proportion of total coma in total higher order aberrations between the two groups was statistically significant (t=-1.548, P=0.007).Decentered ablation measurement: the mean ablation induced decentration was (0.65±0.38)mm for the right eye and (0.90±0.35)mm for the left eye horizontally (X-axis). The mean for the right eye was (0.80±0.30) and (0.53±0.34)mm for the left eye vertically (Y-axis). The mean ablation induced decentration vector was (1.11±0.25)mm for the right eye and (1.13±0.22)mm for the left eye. For all 28 eyes, the vector shift was ＞0.5 mm, and for 71% of the eyes, ablation centers differed from pupil centers ＞1.0 mm. The vector decentration for LASIK ablated eyes was 2.02 times higher than the population but with no visual symptoms. Right eyes had a more significant pupil shift than left eyes in the vertical direction (t=2.157, P=0.040). Conclusion Total higher-order aberrations and coma aberrations increased significantly in cases with visual symptoms caused by ablation induced decentration after LASIK surgery, with vertical coma increasing more than horizontal coma. The degree of decentration was moderate to severe in these patients. Therefore, decentration is one of the main factors that lead to distorted images after LASIK. Key words: Decentred ablation; Higher-order aberrations; Coma aberration; Keratomileusis,laser in situ