Static cyclotorsion registration during excimer laser surgery: reliability and repeatability

To measure the repeatability of static cyclotorsion registry by the WaveLight EX500 excimer laser platform and its reliability in terms of the percentage of errors during different phases of the surgery.

Experimental, prospective, descriptive study. Candidates for LASIK surgery were evaluated with a Placido-based topographer for iris registration. Once in the operating room and after placing the blepharostat (phase 1), 3 consecutive cyclotorsion measurement attempts were performed using the excimer laser. Valid measurements or the number of errors were noted. Then, after the flap creation (phase 2) and after the flap lifting (phase 3) the first valid cyclotorsion measurement up to a maximum of three attempts was noted. The number and percentage of errors in every phase were calculated. The intraclass correlation coefficient (ICC) for absolute agreement was also calculated.

135 eyes of 80 subjects were included. Three consecutive measurement errors were found in 2.9%, 20.7%, and 23% of the eyes in phases 1, 2, and 3, respectively. In Phase 1, the mean range of the 3 measurements was 0,7 ° with a maximum difference of 3.5 °. The ICC between the 3 measurements was 0.960. The maximum difference between the measurements of Phases 1, 2, and 3 was calculated as 5.5 °. Twenty percent of patients with a valid measurement in Phases 1 or 2 were unable to achieve a valid measurement in Phase 3.

Static cyclotorsion measurements are highly repeatable. Registry errors are more likely to occur after flap lifting.