Determination of the ablation center in LASIK surgery: comparison of three difference maps with anterior segment tomographic images.

PURPOSE: To identify a more rational method for determining the actual ablation center from three different maps with anterior segment tomography after FS-LASIK.

METHODS

This retrospective cohort study enrolled 56 cases after FS-LASIK to correct myopia and myopic astigmatism. The preoperative and postoperative differential subtraction maps based on corneal thickness, anterior tangential curvature, and anterior elevation topographies were adopted for the measurements of the actual ablation center. Measurements were manually performed by three different observers, and the inter-observer reliability was evaluated using the intraclass correlation coefficient (ICC). Bland–Altman plots and Bonferroni multiple comparisons were used to evaluate agreement among the measurements with the above difference maps.

RESULTS

The mean ablation center in coordinates measured from the corneal thickness, curvature, and elevation difference maps was (0.129 ± 0.223, 0.094 ± 0.322) mm, (0.101 ± 0.145, −0.028 ± 0.186) mm, and (0.079 ± 0.128, −0.048 ± 0.153) mm, respectively. Statistical significance was found in measurements of the y direction that compared the corneal thickness with curvature (p = 0.018) and elevation difference maps (p = 0.004). However, no statistically significant difference was found in the x direction. The intended ablation center is precisely the pupil center. The mean distance of the actual ablation center from the entrance pupil center was 0.366 ± 0.207 mm in the thickness difference map, which was statistically higher than 0.218 ± 0.135 mm in curvature and 0.190 ± 0.110 mm in the elevation difference map (p < 0.001). The observers’ ICC was 0.868, 0.929, and 0.931 for thickness, curvature, and elevation maps, respectively.

CONCLUSION

Measurements of actual ablation centers based on the tangential curvature and anterior elevation difference maps showed good agreement, rather than the corneal thickness map.