Cyclotorsion and Centroid Shifts Observed Before, During LASIK and PRK

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Significant changes in cyclotorsion and centroid shifts occurred before surgery as well as intraoperatively with both LASIK and PRK, according to a study in the Journal of Refractive Surgery.¹

The researchers measured intraoperative cyclorotation and centroid shift in 63 eyes of 34 patients (mean age, 34 years; range, 20-56 years) undergoing either LASIK (n = 30 eyes) or PRK (n = 33 eyes). Preoperatively, the investigators obtained an iris image of each eye with the Visx WaveScan Wavefront System (Abbott Medical Optics Inc.) with iris registration, and they used the Visx Star S4 (Abbott Medical Optics Inc.) to measure cyclotorsion and pupil centroid shift at the beginning of the refractive procedure and after the flap’s creation or epithelial removal.

The mean change in intraoperative cyclorotation was 1.48 ±1.11º in LASIK eyes and 2.02 ±2.63º in PRK eyes. The direction of cyclorotation changed by more than 2º in 21% of eyes after flap creation in the LASIK group and in 32% of eyes after epithelial removal in the PRK group. The respective mean intraoperative shift in the x axis and y axis was 0.13 ±0.15 mm and 0.17 ±0.14 mm, respectively, in LASIK eyes, and 0.09 ±0.07 mm and 0.10 ±0.13 mm, respectively, in PRK eyes. Intraoperative centroid shifts greater than 100 μm in either the x axis or y axis occurred in 71% of LASIK eyes and 55% of PRK eyes.

These data suggest that it may be advantageous to engage iris registration immediately prior to ablation to provide a reference point representative of the eye’s position at the initiation of laser delivery, the researchers concluded.

1. Narváez J, Brucks M, Zimmerman G, et al. Intraoperative cyclorotation and pupil centroid shift during LASIK and PRK. J Refract Surg. 2012;28(5):353-358.

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