Changes in chord µ and chord α after FS-LASIK and PRK in low-to-moderate myopia: a comparative study with vector analysis

PURPOSE: To comparatively evaluate changes in chord µ and chord α after photorefractive keratectomy (PRK) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) in eyes with low-to-moderate myopia using linear change (Δ) and displacement vector (DV) analysis. METHODS: This retrospective comparative case series was conducted at Istanbul University, Istanbul Faculty of Medicine. A total of 102 eyes from 56 patients who underwent PRK or FS-LASIK for myopia ± astigmatism were examined. Topographic measurements were performed using the Pentacam AXL preoperatively and at postoperative months 1 and 6. Changes in chord µ and chord α values were evaluated using Δ and DV analyses. RESULTS: At 6 months, chord µ increased from 0.15 ± 0.10 mm to 0.23 ± 0.12 mm, and chord α from 0.35 ± 0.14 mm to 0.41 ± 0.14 mm, with Δ values of 0.08 ± 0.07 mm and 0.06 ± 0.13 mm, respectively (p = 0.013 and p = 0.020). The increases were greater in the PRK group (p = 0.044 and p = 0.018), whereas DV magnitudes did not differ between groups (p > 0.05). Δ chord α correlated positively with pupil diameter, whereas Δ chord µ correlated negatively with trefoil (0°) and vertical coma, and chord µ DV correlated negatively with horizontal coma. CONCLUSION: Significant increases in chord µ and chord α were observed after both PRK and FS-LASIK, with a greater increase in the PRK group. These findings may aid the interpretation of chord parameters in eyes that may later undergo multifocal intraocular lens implantation.