Factors that Affect the Dynamic Cyclotorsion Component during Femtosecond Laser in Situ Keratomileusis

Abstract

Objective: To analyze the factors that affect the dynamic cyclotorsion component (DCC) during femtosecond laser in situ keratomileusis (FS-LASIK). Methods: In this retrospective study, we analyzed 500 patients (1 000 eyes) who underwent bilateral FS-LASIK for correction of myopic astigmatism from June 2015 to July 2016 at Eye Hospital, Wenzhou Medical University. Ablation was performed using the SCHWIND AMARIS (750S) excimer laser system, and the magnitude of dynamic cyclotorsion was recorded during the operation. Differences of DCC between left and right eyes were compared with paired t-tests. The factors that affect DCC were analyzed using multivariate linear regression. The main factors were age, eye (right or left), gender, spherical equivalent (SE), pre-ablation spherical and astigmatism, ablation zone, ablation time, central ablation depth, and temperature and humidity in the laser cavity. The cut-off points were measured using receiver operating characteristic curves. Results: The mean DCC amplitude was 1.61°±0.90°, with 1.50°±0.85° in right eyes and 1.71°±0.94° in left eyes (t=-4.85, P<0.001). The mean dynamic cyclortorsion degree (absolute value) was 1.44°±0.93°, with 1.32°±0.91° in right eyes and 1.56±0.95° in left eyes (t=-4.87, P<0.001). Ablation time (β=0.022, P<0.001), age (β=-0.026, P<0.001), eye (right or left, β=-0.213, P<0.001), and humidity in the laser cavity (β=-0.018, P=0.008) were statistically significant factors for the DCC amplitude. Eye (right or left, β=-0.238, P<0.001), age (β=-0.019, P=0.001), and humidity in the laser cavity (β=-0.016, P=0.025) were significant factors for the absolute degree of DCC. Conclusions: Ablation time, age, eye (right or left), and humidity in the laser cavity can affect the DCC during binocular FS-LASIK. Key words: femtosecond laser; laser in situ keratomileusis; dynamic cyclotorsion component