Refractive Surgery Feature Story

H uman eyes are capable of six types of movement: x/y lateral shifts, z levelling, horizontal and vertical rotations, and cyclotorsion (rotation around the optical axis). Current laser technology for refractive surgery creates corneal alterations to correct refractive errors more accurately than in the past.1 Ablation profiles are based on the removal of tissue lenticules through sequential laser pulses that ablate a small amount of corneal tissue to compensate for refractive errors. However, the quality of vision after laser refractive surgery can deteriorate significantly, especially under mesopic and low-contrast conditions.2 The induction of optical aberrations, such as spherical aberration and coma, are related to loss of visual acuity and quality.3 To balance existing aberrations, customized treatments have been developed using either wavefront measurements of the whole eye4 obtained with HartmannShack wavefront sensors5 or corneal topography-derived wavefront analysis.6,7 Topography-guided,8 wavefrontdriven,9 wavefront-optimized,10 asphericity-preserving, and Q-factor-based11 profiles have been advanced as solutions to address aberrations. Schwiegerling and Snyder measured eye motion in patients undergoing LASIK using a video technique to determine centration and variance of the position of the eye during surgery. They found a standard deviation in eye movements in all eyes of greater than 100 μm.12 Taylor et al determined the accuracy of an eye-tracking system designed for laser refractive surgery;13 the system demonstrated an accuracy of 60 μm for an intact cornea and 100 μm for a cornea with a thin flap removed. Bueeler et al investigated the lateral alignment accuracy needed in wavefront-guided refractive surgery to improve the optics to a desired level in a percentage of normally aberrated eyes.14 To achieve the diffraction limit in 95% of normal eyes with a 7-mm pupil, a lateral alignment accuracy of 70 μm or better was required. An accuracy of 200 μm was sufficient to reach the same goal with a 3-mm pupil. Bueeler and Mrochen quantified the parallax error associated with localizing corneal positions by tracking the subjacent entrance pupil center by means of optical ray tracing in a schematic model eye.15 They found that tracking error can amount to 30% of the detected lateral shift (or more for eye trackers mounted closer than 500 mm to the eye). Thus, if the eye tracker registers a lateral shift of the entrance pupil of 200 μm away from the tracking reference axis, the point of interest located on the cornea would essentially be 260 μm away from this reference axis. A laser pulse fired at that moment would be systematically displaced by 60 μm. Measuring rotation when the patient is upright16 but performing the refractive treatment when the patient is supine may lead to ocular cyclotorsion,17,18 resulting in mismatching of the applied versus the intended profiles.19,20 Recent technologies, such as the Amaris TotalTech laser (Schwind eye-tech-solutions, Kleinostheim, Germany), can facilitate measurement of and potentially compensate for static cyclotorsion that occurs when the patient moves from the upright to the supine position during the procedure21 by quantifying the cyclorotation occurring between wavefront measurement and laser refractive surgery22 and compensating for it.23-25 Further, measuring and compensating for ocular cyclotorsion during refractive treatments with the patient supine may reduce optical noise of the applied versus the intended profiles.26-28 Although many studies have discussed the implicaAstigmatism Management: Centration Techniques