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Rotational eye tracker can compensate for cyclotorsion
J Cataract Refract Surg. 2008;34(10):1720-1726.
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A dynamic rotational eye tracker can help compensate for cyclotorsion and enhance the accuracy of excimer laser ablation during LASIK, particularly in eyes with high astigmatism and those undergoing wavefront- and topography-guided treatment.
Cyclotorsion is the rotation of one eye around its visual axis. Misaligned laser ablation may induce irregular astigmatism and diminish visual outcomes. Two degrees or more of torsional error is likely to induce higher-order aberrations.
Manual and static iris registrations compensate for cyclotorsion before laser ablation. Cyclotorsion also occurs during ablation, but static eye trackers do not detect it.
The study included 245 eyes of 137 patients who underwent LASIK for myopia with or without astigmatism. Researchers acquired iris images with the Zywave Aberrometer (Bausch & Lomb) and compared them with those captured with the Zyoptix 100 Hz excimer laser (Bausch & Lomb).
The mean magnitude of cyclotorsion — the difference between maximum and minimum positions — was 2.181°.
Eyes with mixed cyclotorsion had the highest level of cyclotorsion, at a mean of 2.67° during ablation. The mixed cyclotorsion group could benefit most from dynamic rotational eye tracking, the study author said.
This study further documents the importance of preventing misalignment of an excimer laser ablation by measuring and treating cyclotorsion. Effective treatment of astigmatism and most higher-order aberrations is dependent upon accurate delivery of excimer laser energy to the desired axis. As we have reported (Donnenfeld, The importance of cyclotorsional registration and pupil centroid shift compensation on excimer laser photoablation, ASCRS, 2006), significant cyclotorsion occurs between sitting up and lying down, which can have a significant effect on residual refractive error. The first laser that measured cyclotorsion and realigned the ablation was the VISX Star S4 with iris registration. This study shows the advantage of using a Bausch & Lomb laser that has the ability to track cyclotorsion and adjust treatment not only at the beginning of surgery, but also while the procedure is taking place.
Better patient outcomes are dependent upon accurately treating astigmatism. Precise registration allows surgeons to treat irregular corneas such as decentered ablations, forme fruste keratoconus and small ablation zones. This is improbable with a pupil tracking system and impossible with a wavefront optimized ablation. More importantly, registration will prevent the development of higher-order aberration in “normal eyes” with pupil centroid shift and cyclotorsion.
Cyclorotation management is an integral part of state-of-the art custom wavefront-guided laser vision correction. With registration there is a precise application of treatment. The energy is applied to exactly the right area of the cornea, and an improved result is achieved. Further improvement in registration systems that track cyclotorsion at the level of the cornea should provide even better outcomes in the future.
– Eric D. Donnenfeld, MD
OSN Cornea/External Disease Section Editor