The Effect of Flap Elevation on the Ocular Cyclotorsion in Customized Laser Ablation

Objectives: Corneal refractive surgery aims to correct refractive errors. Proper corneal alignment is crucial. Eye-tracking technologies, specifically designed to address cyclotorsion using iris registration, help reduce the effects of cyclotorsion during surgery. The timing of iris registration can influence the efficacy of these technologies. This study compared cyclotorsion measurements before and after flap elevation/epithelium removal in FemtoLASIK and alcohol-assisted (aa) PRK. Methods: This retrospective cohort study was conducted at Einaim Medical Centers, Israel, and included patients who underwent refractive surgery using the VISX Star S4 IR excimer laser. Cyclotorsion measurements were obtained pre- and post-flap elevation or removal using the Wavescan™ and VISX Star S4 iris registration systems. Patients' data were collected and analyzed retrospectively. Results: Overall, 152 eyes of 86 patients were included. In the FemtoLASIK group, 73 eyes from 45 patients were analyzed. For pre-flap lift, 34.2% had incyclotorsion and 65.8% had excyclotorsion, with a mean cyclotorsion of 2.3 ± 1.5 degrees. For post-flap lift, the mean cyclotorsion was 2.8 ± 1.9 degrees, showing a significant difference (p = 0.01). In the aa-PRK group, 79 eyes from 41 patients were analyzed. For pre-flap removal, 45.6% had incyclotorsion and 53.2% had excyclotorsion, with a mean cyclotorsion of 2.6 ± 1.8 degrees. For post-flap removal, the mean cyclotorsion was 2.5 ± 2.1 degrees, with no significant difference (p = 0.47) and a mean change of 1.6 ± 1.2 degrees. A total of 15.2% of eyes in the aa-PRK group and 13.6% in the LASIK group exhibited more than 3 degrees of cyclotorsional difference before and after flap lift or epithelial removal. Conclusions: Cyclotorsion occurs after flap lift/removal. To minimize residual astigmatism, iris registration should be performed post-flap elevation.