Preoperative Planning and Laser Programming
An important part of preoperative planning is programming the laser with the appropriate refraction. Often, the patient’s manifest and cycloplegic refractions differ, or the amount and axis of astigmatism differ between the topographic evaluation and refractive examination. Thus, it may be unclear which refractive data to enter into the laser. The surgeon’s decision about whether to use the manifest or the cycloplegic refraction is based on his or her individual nomogram and technique. The manifest refraction is more accurate than the cycloplegic refraction in determining cylinder axis and amount. If the refractive cylinder is confirmed to differ from the topographic cylinder, lenticular astigmatism or posterior corneal curvature is assumed to be the cause. In this case, the laser is still programmed with the axis and amount of cylinder noted on refraction. The surgeon should take particular care to check the axis obtained on the refraction with the value programmed into the laser. Entering an incorrect value is a potential source of error, particularly when converting between plus and minus cylinder formats. Before each surgery, the surgeon and the technician should review a checklist of information, confirming the patient’s name, the refraction, and the eye on which surgery is to be performed. In wavefront procedures, the treatment should correspond to the patient’s refraction, and adjustments may be required to compensate for accommodation.
For many laser models, the surgeon also must enter the size of the optical zone and indicate whether a blend of the ablation zone should be performed. The blend zone is an area of peripheral asphericity designed to reduce the possible undesirable effects of an abrupt transition from the optical zone to the untreated cornea (see Fig 5-1B). A prolate blend zone reduces the risk of glare and halo after excimer laser photoablation.
Special considerations for wavefront-guided techniques
Several wavefront mapping systems and wavefront-guided lasers are available commercially. Wavefront mapping systems are unique to the specific wavefront-guided laser used. Calibration should be performed according to the manufacturer’s specifications.
For wavefront-guided ablations, the wavefront maps are taken with the patient sitting up at an aberrometer under scotopic conditions; the mapping results are then applied to the cornea in the laser suite with the patient lying down under the operating microscope. Some systems require pupillary dilation to capture wavefront data. The wavefront refraction indicated on wavefront analysis is then compared with the manifest refraction. If the difference between them exceeds 0.75 D, the manifest refraction and the wavefront analysis may need to be repeated. The data are either electronically transferred to the laser or downloaded to a portable drive and then transferred to the laser. Unlike conventional or wavefront-optimized excimer laser treatment, in which the manifest or cycloplegic refraction is used to program the laser, wavefront-guided laser treatment uses programmed wavefront data to create a custom ablation pattern.
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.