Tracking, centration, and ablation
For surface ablation, the exposed Bowman layer should be inspected and found to be smooth, uniformly dry, and free of debris and residual epithelial islands. For LASIK, the flap must be lifted and reflected, and the stromal bed must be uniformly dry before treatment. Fluid or blood accumulation on the stromal bed should be avoided, as it can lead to an irregular ablation.
Excimer lasers in current use employ open-loop tracking systems, which have improved clinical outcomes. The tracker uses video technology to monitor the location of an infrared image of the pupil and to shift the laser beam accordingly.
The laser is centered and focused according to the manufacturer’s recommendations. Tracking systems, although effective, do not lessen the importance of keeping the reticule centered on the patient’s entrance pupil. If the patient is unable to maintain fixation, the illumination of the operating microscope should be reduced. If decentration occurs and the ablation does not stop automatically, the surgeon should immediately stop the treatment until adequate refixation is achieved. It is still important for the surgeon to monitor for excessive eye movement, which can result in decentration despite the tracking device.
The change in illumination and in patient position (ie, from sitting to lying down) can cause pupil centroid shift and cyclotorsion. In most patients, the pupil moves nasally and superiorly when it is constricted. Registration is a technique in which a fixed landmark is used at the time of aberrometry and treatment to apply the ablation to the correct area of the cornea; it relies on iris landmarks and not on the pupil for laser centration (Fig 5-10). Once the patient confirms that the fixation light of the excimer laser is still visible and that he or she is looking directly at it, ablation begins. Neither tracking nor iris registration is a substitute for accurate patient fixation. It is important to initiate stromal ablation promptly, before excessive stromal dehydration takes place. During larger-diameter ablations, a flap protector may be needed to shield the underside of the LASIK flap near the hinge from the laser pulses. In addition, it is important to remove the excessive fluid that can accumulate during treatment, especially in patients undergoing high corrections.
Figure 5-10 Excimer laser ablation of the stromal bed. Note the faint blue fluorescence of the stromal bed from the laser pulse (arrows). The rectangular shape of the exposure by this broad-beam laser indicates that the laser is correcting the cylindrical portion of the treatment. (Photograph is enhanced to visualize fluorescence; the surgeon usually sees minimal or no fluorescence through the operating microscope.)
(Courtesy of Roger F. Steinert, MD.)
Donnenfeld E. The pupil is a moving target: centration, repeatability, and registration. J Refract Surg. 2004;20(5):S593–S596.
Moshirfar M, Chen MC, Espandar L, et al. Effect of iris registration on outcomes of LASIK for myopia with the VISX CustomVue platform. J Refract Surg. 2009;25(6):493–502.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.