Abstract:  Objective To evaluate refractive changes and associated factors after phototherapeutic keratectomy (PTK) using the WaveLight EX500 excimer laser. Design Retrospective case series. Participants 11 Patients (14 eyes) who underwent PTK using the WaveLight EX500 excimer laser at the Department of Ophthalmology, Beijing Tongren Hospital, between April 2023 and April 2024. Methods The patient's medical records were reviewed. The associations between ΔSEQ and clinical variables, including ablation depth (AD), optical zone (OZ), and other factors, were analyzed. Multivariate linear regression was performed to identify independent predictors of refractive change. Main Outcome Measures The change in spherical equivalent (SEQ)at 3 months postoperatively (ΔSEQ). Results The mean AD in all 11 patients (14 eyes) was 41.50±30.68 μm, and the mean OZ was 6.75±0.91mm. At 3 months postoperatively, the SEQ changed from -1.77±3.17 D to -2.37±3.35 D， with ΔSEQ of -0.60±1.06 D, indicating a trend toward myopic shift; however, the difference in SEQ between preoperative and postoperative measurements was not statistically significant (P=0.054). Correlation analysis showed a significant association between ΔSEQ and AD (r=-0.733, P=0.003), while no significant correlations were found with OZ, age, preoperative central corneal epithelial thickness, or preoperative SEQ. Multivariate linear regression demonstrated that both AD and OZ were independently associated factors of ΔSEQ (both P<0.05). The regression equation was: ΔSEQ=-2.739-0.033×AD+0.521×OZ, with good model fit (R2=0.687). No postoperative complications were observed during follow-up. Conclusion The spherical equivalent decreased at 3 months after PTK using the WaveLight EX500 excimer laser, indicating an overall trend toward a myopic shift. Ablation depth and optical zone diameter were independently associated with postoperative refractive change, with greater ablation depth associated with a greater myopic shift and a larger optical zone associated with a smaller magnitude of myopic shift.

Key words:  Phototherapeutic keratectomy, Myopic shift