Photoablative lasers can be subdivided into broad-beam lasers, scanning-slit lasers, and flying spot lasers. Broad-beam lasers have larger-diameter beams and slower repetition rates and rely on optics or mirrors to create a smooth and homogeneous multimode laser beam of up to approximately 7 mm in diameter. These lasers have very high energy per pulse and require a small number of pulses to ablate the cornea. Scanning-slit lasers generate a narrow-slit laser beam that is scanned over the surface of the tissue to alter the photoablation profile, thus improving the smoothness of the ablated cornea and allowing for larger-diameter ablation zones. Flying spot lasers use smaller-diameter beams (approximately 0.5–2.0 mm) that are scanned at a higher repetition rate; they require use of a tracking mechanism for precise placement of the desired pattern of ablation. Broad-beam lasers and some scanning-slit lasers require a mechanical iris diaphragm or ablatable mask to create the desired shape in the cornea, whereas the rest of the scanning-slit lasers and the flying spot lasers use a pattern projected onto the surface to guide the ablation profile without masking. The majority of excimer lasers in current clinical use some form of variable or flying spot ablation profile.
Wavefront-optimized and wavefront-guided laser ablations
Because conventional laser treatment profiles have small blend zones and create a more oblate corneal shape postoperatively following myopic corrections, they are likely to induce some degree of higher-order aberration, especially spherical aberration and coma. These aberrations occur because the corneal curvature is relatively more angled peripherally in relation to laser pulses emanating from the central location; thus, the pulses hitting the peripheral cornea are relatively less effective than are the central pulses.
Wavefront-optimized laser ablation improves the postoperative corneal shape by taking the curvature of the cornea into account and increasing the number of peripheral pulses; this approach minimizes the induction of higher-order aberrations and often results in better-quality vision and fewer night-vision concerns due to maintenance of a more prolate corneal shape. As in conventional procedures, the patient’s refraction alone is used to program the wavefront-optimized laser ablation. This technology does not directly address preexisting higher-order aberrations; however, recent studies have found that the vast majority of patients do not have substantial preoperative higher-order aberrations. It also has the advantage of being quicker than wavefront-guided technology and avoids the additional expense of the aberrometer.
In wavefront-guided laser ablation, information obtained from a wavefront-sensing aberrometer (which quantifies the aberrations) is transferred electronically to the treatment laser to program the ablation. This process is distinct from those in conventional excimer laser and wavefront-optimized laser treatments, in which the subjective refraction alone is used to program the laser ablation. The wavefront-guided laser attempts to treat both lower-order (ie, myopia or hyperopia and/or astigmatism) and higher-order aberrations by applying complex ablation patterns to the cornea to correct the wavefront deviations. The correction of higher-order aberrations requires non–radially symmetric patterns of ablation (which are often much smaller in magnitude than ablations needed to correct defocus and astigmatism). The difference between the desired and the actual wavefront is used to generate a 3-dimensional map of the planned ablation. Accurate registration is required to ensure that the ablation treatment actually delivered to the cornea matches the intended pattern. Such registration is achieved by using marks at the limbus before obtaining the wavefront patterns or by iris registration, which matches reference points in the natural iris pattern to compensate for cyclotorsion and pupil centroid shift. The wavefront-guided laser then uses a pupil-tracking system, which helps maintain centration during treatment and allows accurate delivery of the customized ablation profile.
The results for both wavefront-optimized and wavefront-guided ablations for myopia, hyperopia, and astigmatism are excellent, with well over 90% of eyes achieving 20/40 or better uncorrected distance visual acuity (UCVA; also called uncorrected distance visual acuity, UDVA). Although most visual acuity parameters are similar between conventional and customized treatments (including both wavefront-optimized and wavefront-guided treatments), the majority of recent reports demonstrate improved vision quality when customized treatment profiles are used. Outcomes with wavefront-optimized treatments are similar to those of wavefront-guided treatments for most patients, with the exception of patients with substantial preoperative higher-order aberrations.
Topography-guided laser ablations
Topography-guided lasers are similar in concept to wavefront-guided lasers, but they link the treatment to the corneal topography rather than to the total wavefront data. Although experience is still early, these instruments may offer significant benefit in the treatment of highly aberrated eyes, such as eyes with previous RK or PKP.
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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.